![emphasis on low-sulfur gasoline. The contribution of the alkylation process is critical in
the production of quality motor fuels including many of the “environmental” gasoline
blends. The process provides refiners with a tool of unmatched economy and efficiency,
one that will assist refiners in maintaining or strengthening their position in the production
and marketing of gasolines.
PROCESS CHEMISTRY
General
In the HF Alkylation process, HF acid is the catalyst that promotes the isoparaffin-olefin
reaction. In this process, only isoparaffins with tertiary carbon atoms, such as isobutane or
isopentane, react with the olefins. In practice, only isobutane is used because isopentane
has a high octane number and a vapor pressure that has historically allowed it to be blend-
ed directly into finished gasolines. However, where environmental regulations have
reduced the allowable vapor pressure of gasoline, isopentane is being removed from gaso-
line, and refiner interest in alkylating this material with light olefins, particularly propy-
lene, is growing.
The actual reactions taking place in the alkylation reactor are many and are relatively
complex. The equations in Fig. 1.4.1 illustrate the primary reaction products that may be
expected for several pure olefins.
In practice, the primary product from a single olefin constitutes only a percentage of
the alkylate because of the variety of concurrent reactions that are possible in the alkyla-
tion environment. Compositions of pilot-plant products produced at conditions to maxi-
mize octane from pure-olefin feedstocks are shown in Table 1.4.1.
Reaction Mechanism
Alkylation is one of the classic examples of a reaction or reactions proceeding via the car-
benium ion mechanism. These reactions include an initiation step and a propagation step
and may include an isomerization step. In addition, polymerization and cracking steps may
be involved. However, these side reactions are generally undesirable. Examples of these
reactions are given in Fig. 1.4.2.
Initiation. The initiation step (Fig. 1.4.2a) generates the tertiary butyl cations that
will subsequently carry on the alkylation reaction.
Propagation. Propagation reactions (Fig. 1.4.2b) involve the tertiary butyl cation
reacting with an olefin to form a larger carbenium ion, which then abstracts a hydride
from an isobutane molecule. The hydride abstraction generates the isoparaffin plus a
new tertiary butyl cation to carry on the reaction chain.
Isomerization. Isomerization [Eq. (1.4.12), shown in Fig. 1.4.2c] is very important in
producing good octane quality from a feed that is high in 1-butene. The isomerization
of 1-butene is favored by thermodynamic equilibrium. Allowing 1-butene to isomerize
to 2-butene reduces the production of dimethylhexanes (research octane number of 55
to 76) and increases the production of trimethylpentanes. Many recent HF Alkylation
units, especially those processing only butylenes, have upstream olefin isomerization
units that isomerize the 1-butene to 2-butene.
1.34 ALKYLATION AND POLYMERATION
UOP HF ALKYLATION TECHNOLOGY](https://image.slidesharecdn.com/2059224-250605002501-65b2728c/85/Handbook-Of-Petroleum-Refining-Processes-3-Ed-3rd-Edition-Meyers-Robert-A-38-320.jpg)
![UOP HF ALKYLATION TECHNOLOGY 1.35
Equation (1.4.13) is an example of the many possible steps involved in the isomeriza-
tion of the larger carbenium ions.
Other Reactions. The polymerization reaction [Eq. (1.4.14), shown in Fig. 1.4.2d]
results in the production of heavier paraffins, which are undesirable because they
reduce alkylate octane and increase alkylate endpoint. Minimization of this reaction is
achieved by proper choice of reaction conditions.
The larger polymer cations are susceptible to cracking or disproportionation reactions
[Eq. (1.4.15)], which form fragments of various molecular weights. These fragments can
then undergo further alkylation.
Isobutylene
CH3-C = CH2+CH3-CH-CH3 CH3-C-CH2-CH-CH3
CH3 CH3 CH3
CH3 CH3
Isobutane
Isobutane
Isobutane
Isobutane
(Isooctane)
2,2,4-Trimethylpentane
(1.4.1)
CH3-CH = CH2 + CH3-CH-CH3
CH3
CH3-CH-CH-CH2-CH3
CH3CH3
Propylene 2,3-Dimethylpentane
(1.4.4)
2-Butene
CH3-CH = CH-CH3 + CH3-CH-CH3 CH3- C-CH2-CH-CH3
CH3 CH3
CH3
2,2,4-Trimethylpentane
CH3
or CH3-CH-CH-CH-CH3
CH3CH3CH3
2,3,4-Trimethylpentane
(1.4.3)
CH2 = CH-CH2-CH3 + CH3-CH-CH3
CH3
CH3-CH-CH-CH2-CH2-CH3
CH3CH3
1-Butene 2,3-Dimethylpentane
(1.4.2)
FIGURE 1.4.1 HF alkylation primary reactions for monoolefins.
UOP HF ALKYLATION TECHNOLOGY](https://image.slidesharecdn.com/2059224-250605002501-65b2728c/85/Handbook-Of-Petroleum-Refining-Processes-3-Ed-3rd-Edition-Meyers-Robert-A-39-320.jpg)
![Hydrogen Transfer. The hydrogen transfer reaction is most pronounced with
propylene feed. The reaction also proceeds via the carbenium ion mechanism. In the
first reaction [Eq. (1.4.16)], propylene reacts with isobutane to produce butylene and
propane. The butylene is then alkylated with isobutane [Eq. (1.4.17)] to form
trimethylpentane. The overall reaction is given in Eq. (1.4.18). From the viewpoint of
octane, this reaction can be desirable because trimethylpentane has substantially
higher octane than the dimethylpentane normally formed from propylene. However,
two molecules of isobutane are required for each molecule of alkylate, and so this
reaction may be undesirable from an economic viewpoint.
PROCESS DESCRIPTION
The alkylation of olefins with isobutane is complex because it is characterized by simple
addition as well as by numerous side reactions. Primary reaction products are the isomer-
1.38 ALKYLATION AND POLYMERATION
(1.4.14)
(1.4.15)
+
C-C-C-C-C + C-C = C-C C12
+
C
C=C-C + C-C-C
C3H6 + 2iC4H10
Trimethylpentane
C3H8 + Trimethylpentane
C C C16
+ etc.
Polymerization
Cracking-Disproportionation
C12
+ C5
+ + C7
+
Hydrogen Transfer
C-C-C + C-C=C
C
C
(1.4.16)
(1.4.17)
C-C=C + C-C-C
C
C
Overall Reaction: (1.4.18)
FIGURE 1.4.2d HF alkylation reaction mechanism—other.
UOP HF ALKYLATION TECHNOLOGY](https://image.slidesharecdn.com/2059224-250605002501-65b2728c/85/Handbook-Of-Petroleum-Refining-Processes-3-Ed-3rd-Edition-Meyers-Robert-A-42-320.jpg)
![water, CaF2 is the desired end product. The effluent containing HF acid can be treated with
a lime [CaO-Ca(OH)2] solution or slurry, or it can be neutralized indirectly in a KOH sys-
tem to produce the desired CaF2 product.
The KOH neutralization system currently used in a UOP-designed unit involves a
two-stage process. As HF acid is neutralized by aqueous KOH, soluble potassium fluo-
ride (KF) is produced, and the KOH is gradually depleted. Periodically, some of the KF-
containing neutralizing solution is withdrawn to the KOH regenerator. In this vessel, KF
reacts with a lime slurry to produce insoluble CaF2 and thereby regenerates KF to KOH.
1.46 ALKYLATION AND POLYMERATION
FIGURE 1.4.5 UOP HF Alkylation process effluent management.
UOP HF ALKYLATION TECHNOLOGY](https://image.slidesharecdn.com/2059224-250605002501-65b2728c/85/Handbook-Of-Petroleum-Refining-Processes-3-Ed-3rd-Edition-Meyers-Robert-A-50-320.jpg)
Handbook Of Petroleum Refining Processes 3 Ed 3rd Edition Meyers Robert A
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- 5. ALKYLATION AND POLYMERIZATION P ● A ● R ● T ● 1 Source: HANDBOOK OF PETROLEUM REFINING PROCESSES
- 7. 1.3 CHAPTER 1.1 NExOCTANE™ TECHNOLOGY FOR ISOOCTANE PRODUCTION Ronald Birkhoff Kellogg Brown & Root, Inc. (KBR) Matti Nurminen Fortum Oil and Gas Oy INTRODUCTION Environmental issues are threatening the future use of MTBE (methyl-tert-butyl ether) in gasoline in the United States. Since the late 1990s, concerns have arisen over ground and drinking water contamination with MTBE due to leaking of gasoline from underground storage tanks and the exhaust from two-cycle engines. In California a number of cases of drinking water pollution with MTBE have occurred. As a result, the elimination of MTBE in gasoline in California was mandated, and legislation is now set to go in effect by the end of 2003. The U.S. Senate has similar law under preparation, which would eliminate MTBE in the 2006 to 2010 time frame. With an MTBE phase-out imminent, U.S. refiners are faced with the challenge of replacing the lost volume and octane value of MTBE in the gasoline pool. In addition, uti- lization of idled MTBE facilities and the isobutylene feedstock result in pressing problems of unrecovered and/or underutilized capital for the MTBE producers. Isooctane has been identified as a cost-effective alternative to MTBE. It utilizes the same isobutylene feeds used in MTBE production and offers excellent blending value. Furthermore, isooctane pro- duction can be achieved in a low-cost revamp of an existing MTBE plant. However, since isooctane is not an oxygenate, it does not replace MTBE to meet the oxygen requirement currently in effect for reformulated gasoline. The NExOCTANE technology was developed for the production of isooctane. In the process, isobutylene is dimerized to produce isooctene, which can subsequently be hydro- genated to produce isooctane. Both products are excellent gasoline blend stocks with sig- nificantly higher product value than alkylate or polymerization gasoline. Source: HANDBOOK OF PETROLEUM REFINING PROCESSES
- 8. 1.4 ALKYLATION AND POLYMERIZATION HISTORY OF MTBE During the 1990s, MTBE was the oxygenate of choice for refiners to meet increasingly strin- gent gasoline specifications. In the United States and in a limited number of Asian countries, the use of oxygenates in gasoline was mandated to promote cleaner-burning fuels. In addi- tion, lead phase-down programs in other parts of the world have resulted in an increased demand for high-octane blend stock.All this resulted in a strong demand for high-octane fuel ethers, and significant MTBE production capacity has been installed since 1990. Today, the United States is the largest consumer of MTBE. The consumption increased dramatically with the amendment of the Clean Air Act in 1990 which incorporated the 2 percent oxygen mandate. The MTBE production capacity more than doubled in the 5-year period from 1991 to 1995. By 1998, the MTBE demand growth had leveled off, and it has since tracked the demand growth for reformulated gasoline (RFG). The United States con- sumes about 300,000 BPD of MTBE, of which over 100,000 BPD is consumed in California. The U.S. MTBE consumption is about 60 percent of the total world demand. MTBE is produced from isobutylene and methanol. Three sources of isobutylene are used for MTBE production: ● On-purpose butane isomerization and dehydrogenation ● Fluid catalytic cracker (FCC) derived mixed C4 fraction ● Steam cracker derived C4 fraction The majority of the MTBE production is based on FCC and butane dehydrogenation derived feeds. NExOCTANE BACKGROUND Fortum Oil and Gas Oy, through its subsidiary Neste Engineering, has developed the NExOCTANE technology for the production of isooctane. NExOCTANE is an extension of Fortum’s experience in the development and licensing of etherification technologies. Kellogg Brown & Root, Inc. (KBR) is the exclusive licenser of NExOCTANE. The tech- nology licensing and process design services are offered through a partnership between Fortum and KBR. The technology development program was initialized in 1997 in Fortum’s Research and Development Center in Porvoo, Finland, for the purpose of producing high-purity isooctene, for use as a chemical intermediate. With the emergence of the MTBE pollution issue and the pending MTBE phase-out, the focus in the development was shifted in 1998 to the conver- sion of existing MTBE units to produce isooctene and isooctane for gasoline blending. The technology development has been based on an extensive experimental research program in order to build a fundamental understanding of the reaction kinetics and key product separation steps in the process. This research has resulted in an advanced kinetic modeling capability, which is used in the design of the process for licensees. The process has undergone extensive pilot testing, utilizing a full range of commercial feeds. The first commercial NExOCTANE unit started operation in the third quarter of 2002. PROCESS CHEMISTRY The primary reaction in the NExOCTANE process is the dimerization of isobutylene over acidic ion-exchange resin catalyst. This dimerization reaction forms two isomers of NExOCTANE™ TECHNOLOGY FOR ISOOCTANE PRODUCTION
- 9. NExOCTANE™ TECHNOLOGY FOR ISOOCTANE PROCUCTION 1.5 trimethylpentene (TMP), or isooctene, namely, 2,4,4-TMP-1 and 2,4,4-TMP-2, according to the following reactions: TMP further reacts with isobutylene to form trimers, tetramers, etc. Formation of these oligomers is inhibited by oxygen-containing polar components in the reaction mixture. In the Isobutylene 2 2,4,4 TMP-1 CH2= C - CH3 CH3 CH2 = C - CH2 - C - CH3 CH3 CH3 CH3 CH2 - C = CH2 - C - CH3 CH3 CH3 CH3 2,4,4 TMP-2 NExOCTANE process, water and alcohol are used as inhibitors. These polar components block acidic sites on the ion-exchange resin, thereby controlling the catalyst activity and increasing the selectivity to the formation of dimers. The process conditions in the dimer- ization reactions are optimized to maximize the yield of high-quality isooctene product. A small quantity of C7 and C9 components plus other C8 isomers will be formed when other olefin components such as propylene, n-butenes, and isoamylene are present in the reaction mixture. In the NExOCTANE process, these reactions are much slower than the isobutylene dimerization reaction, and therefore only a small fraction of these components is converted. Isooctene can be hydrogenated to produce isooctane, according to the following reaction: CH2 – C – CH2 – C – CH3 CH3 CH3 CH3 Isooctane Isooctene CH2 = C – CH2 – C – CH3 + H2 CH3 CH3 CH3 NExOCTANE PROCESS DESCRIPTION The NExOCTANE process consists of two independent sections. Isooctene is produced by dimerization of isobutylene in the dimerization section, and subsequently, the isooctene can be hydrogenated to produce isooctane in the hydrogenation section. Dimerization and hydrogenation are independently operating sections. Figure 1.1.1 shows a simplified flow diagram for the process. The isobutylene dimerization takes place in the liquid phase in adiabatic reactors over fixed beds of acidic ion-exchange resin catalyst. The product quality, specifically the distri- NExOCTANE™ TECHNOLOGY FOR ISOOCTANE PRODUCTION
- 10. bution of dimers and oligomers, is controlled by recirculating alcohol from the product recov- ery section to the reactors. Alcohol is formed in the dimerization reactors through the reaction of a small amount of water with olefin present in the feed. The alcohol content in the reactor feed is typically kept at a sufficient level so that the isooctene product contains less than 10 percent oligomers. The dimerization product recovery step separates the isooctene product from the unreacted fraction of the feed (C4 raffinate) and also produces a concentrated alco- hol stream for recycle to the dimerization reaction. The C4 raffinate is free of oxygenates and suitable for further processing in an alkylation unit or a dehydrogenation plant. Isooctene produced in the dimerization section is further processed in a hydrogenation unit to produce the saturated isooctane product. In addition to saturating the olefins, this unit can be designed to reduce sulfur content in the product. The hydrogenation section consists of trickle-bed hydrogenation reactor(s) and a product stabilizer. The purpose of the stabilizer is to remove unreacted hydrogen and lighter components in order to yield a product with a specified vapor pressure. The integration of the NExOCTANE process into a refinery or butane dehydrogenation complex is similar to that of the MTBE process. NExOCTANE selectively reacts isobuty- lene and produces a C4 raffinate which is suitable for direct processing in an alkylation or dehydrogenation unit. A typical refinery integration is shown in Fig. 1.1.2, and an integra- tion into a dehydrogenation complex is shown in Fig. 1.1.3. NExOCTANE PRODUCT PROPERTIES The NExOCTANE process offers excellent selectivity and yield of isooctane (2,2,4- trimethylpentane). Both the isooctene and isooctane are excellent gasoline blending compo- nents. Isooctene offers substantially better octane blending value than isooctane. However, the olefin content of the resulting gasoline pool may be prohibitive for some refiners. The characteristics of the products are dependent on the type of feedstock used. Table 1.1.1 presents the product properties of isooctene and isooctane for products produced from FCC derived feeds as well as isooctane from a butane dehydrogenation feed. The measured blending octane numbers for isooctene and isooctane as produced from FCC derived feedstock are presented in Table 1.1.2. The base gasoline used in this analy- 1.6 ALKYLATION AND POLYMERIZATION Dimerization Product Recovery Hydrogenation Reaction Stabilizer Isobutylene C4 Raffinate Alcohol Recycle Isooctane Hydrogen Fuel Gas Isooctene DIMERIZATION SECTION HYDROGENATION SECTION FIGURE 1.1.1 NExOCTANE process. NExOCTANE™ TECHNOLOGY FOR ISOOCTANE PRODUCTION
- 11. sis is similar to nonoxygenated CARB base gasoline. Table 1.1.2 demonstrates the signif- icant blending value for the unsaturated isooctene product, compared to isooctane. PRODUCT YIELD An overall material balance for the process based on FCC and butane dehydrogenation derived isobutylene feedstocks is shown in Table 1.1.3. In the dehydrogenation case, an isobutylene feed content of 50 wt % has been assumed, with the remainder of the feed NExOCTANE™ TECHNOLOGY FOR ISOOCTANE PROCUCTION 1.7 FCC ALKYLATION DIMERIZATION Hydrogen Isooctane Isooctene HYDROGENATION C4 C4 Raffinate NExOCTANE FIGURE 1.1.2 Typical integration in refinery. HYDROGE- NATION DEHYDRO Hydrogen Isooctane Isooctene DIMERIZATION iC4= NExOCTANE Butane HYDROGEN TREATMENT RECYCLE TREATMENT ISOMERI- ZATION DIB C4 Raffinate FIGURE 1.1.3 Integration in a typical dehydrogenation complex. NExOCTANE™ TECHNOLOGY FOR ISOOCTANE PRODUCTION
- 12. mostly consisting of isobutane. For the FCC feed an isobutylene content of 22 wt % has been used. In each case the C4 raffinate quality is suitable for either direct processing in a refinery alkylation unit or recycle to isomerization or dehydrogenation step in the dehy- drogenation complex. Note that the isooctene and isooctane product rates are dependent on the content of isobutylene in the feedstock. UTILITY REQUIREMENTS The utilities required for the NExOCTANE process are summarized in Table 1.1.4. 1.8 ALKYLATION AND POLYMERIZATION TABLE 1.1.1 NExOCTANE Product Properties FCC C4 Butane dehydrogenation Isooctane Isooctene Isooctane Specific gravity 0.704 0.729 0.701 RONC 99.1 101.1 100.5 MONC 96.3 85.7 98.3 (R ⫹ M) / 2 97.7 93.4 99.4 RVP, lb/in2 absolute 1.8 1.8 1.8 TABLE 1.1.2 Blending Octane Number in CARB Base Gasoline (FCC Derived) Isooctene Isooctane Blending BRON BMON (R ⫹ M) / 2 BRON BMON (R ⫹ M) / 2 volume, % 10 124.0 99.1 111.0 99.1 96.1 97.6 20 122.0 95.1 109.0 100.1 95.1 97.6 100 101.1 85.7 93.4 99.1 96.3 97.7 TABLE 1.1.3 Sample Material Balance for NExOCTANE Unit Material balance FCC C4 feed, lb/h (BPD) Butane dehydrogenation, lb/h (BPD) Dimerization section: Hydrocarbon feed 137,523 (16,000) 340,000 (39,315) Isobutylene contained 30,614 (3,500) 170,000 (19,653) Isooctene product 30,714 (2,885) 172,890 (16,375) C4 raffinate 107,183 (12,470) 168,710 (19,510) Hydrogenation section: Isooctene feed 30,714 (2,885) 172,890 (16,375) Hydrogen feed 581 3752 Isooctane product 30,569 (2,973) 175,550 (17,146) Fuel gas product 726 1092 NExOCTANE™ TECHNOLOGY FOR ISOOCTANE PRODUCTION
- 13. NExOCTANE TECHNOLOGY ADVANTAGES Long-Life Dimerization Catalyst The NExOCTANE process utilizes a proprietary acidic ion-exchange resin catalyst. This catalyst is exclusively offered for the NExOCTANE technology. Based on Fortum’s exten- sive catalyst trials, the expected catalyst life of this exclusive dimerization catalyst is at least double that of standard resin catalysts. Low-Cost Plant Design In the dimerization process, the reaction takes place in nonproprietary fixed-bed reactors. The existing MTBE reactors can typically be reused without modifications. Product recov- ery is achieved by utilizing standard fractionation equipment. The configuration of the recovery section is optimized to make maximum use of the existing MTBE product recov- ery equipment. High Product Quality The combination of a selective ion-exchange resin catalyst and optimized conditions in the dimerization reaction results in the highest product quality. Specifically, octane rating and specific gravity are better than those in product produced with alternative catalyst systems or competing technologies. State-of-the-Art Hydrogenation Technology The NExOCTANE process provides a very cost-effective hydrogenation technology. The trickle-bed reactor design requires low capital investment, due to a compact design plus once-through flow of hydrogen, which avoids the need for a recirculation compressor. Commercially available hydrogenation catalysts are used. Commercial Experience The NExOCTANE technology is in commercial operation in North America in the world’s largest isooctane production facility based on butane dehydrogenation. The project includes a grassroots isooctene hydrogenation unit. NExOCTANE™ TECHNOLOGY FOR ISOOCTANE PROCUCTION 1.9 TABLE 1.1.4 Typical Utility Requirements Utility requirements FCC C4 Butane dehydrogenation per BPD of product per BPD of product Dimerization section: Steam, 1000 lb/h 13 6.4 Cooling water, gal/min 0.2 0.6 Power, kWh 0.2 0.03 Hydrogenation section: Steam, 1000 lb/h 1.5 0.6 Cooling water, gal/min 0.03 0.03 Power, kWh 0.03 0.1 NExOCTANE™ TECHNOLOGY FOR ISOOCTANE PRODUCTION
- 14. NExOCTANE™ TECHNOLOGY FOR ISOOCTANE PRODUCTION
- 15. CHAPTER 1.2 STRATCO EFFLUENT REFRIGERATED H2SO4 ALKYLATION PROCESS David C. Graves STRATCO Leawood, Kansas INTRODUCTION Alkylation, first commercialized in 1938, experienced tremendous growth during the 1940s as a result of the demand for high-octane aviation fuel during World War II. During the mid-1950s, refiners’ interest in alkylation shifted from the production of aviation fuel to the use of alkylate as a blending component in automotive motor fuel. Capacity remained relatively flat during the 1950s and 1960s due to the comparative cost of other blending components. The U.S. Environmental Protection Agency’s lead phase-down pro- gram in the 1970s and 1980s further increased the demand for alkylate as a blending com- ponent for motor fuel. As additional environmental regulations are imposed on the worldwide refining community, the importance of alkylate as a blending component for motor fuel is once again being emphasized. Alkylation unit designs (grassroots and revamps) are no longer driven only by volume, but rather by a combination of volume, octane, and clean air specifications. Lower olefin, aromatic, sulfur, Reid vapor pressure (RVP), and drivability index (DI) specifications for finished gasoline blends have also become driving forces for increased alkylate demand in the United States and abroad. Additionally, the probable phase-out of MTBE in the United States will further increase the demand for alkylation capacity. The alkylation reaction combines isobutane with light olefins in the presence of a strong acid catalyst. The resulting highly branched, paraffinic product is a low-vapor-pres- sure, high-octane blending component. Although alkylation can take place at high temper- atures without catalyst, the only processes of commercial importance today operate at low to moderate temperatures using either sulfuric or hydrofluoric acid catalysts. Several dif- ferent companies are currently pursuing research to commercialize a solid alkylation cat- alyst. The reactions occurring in the alkylation process are complex and produce an alkylate product that has a wide boiling range. By optimizing operating conditions, the 1.11 Source: HANDBOOK OF PETROLEUM REFINING PROCESSES
- 16. majority of the product is within the desired gasoline boiling range with motor octane numbers (MONs) up to 95 and research octane numbers (RONs) up to 98. PROCESS DESCRIPTION A block flow diagram of the STRATCO effluent refrigerated H2SO4 alkylation project is shown in Fig. 1.2.1. Each section of the block flow diagram is described below: Reaction section. Here the reacting hydrocarbons are brought into contact with sulfu- ric acid catalyst under controlled conditions. Refrigeration section. Here the heat of reaction is removed, and light hydrocarbons are removed from the unit. Effluent treating section. Here the free acid, alkyl sulfates, and dialkyl sulfates are removed from the net effluent stream to avoid downstream corrosion and fouling. Fractionation section. Here isobutane is recovered for recycle to the reaction section, and remaining hydrocarbons are separated into the desired products. Blowdown section. Here spent acid is degassed, wastewater pH is adjusted, and acid vent streams are neutralized before being sent off-site. The blocks are described in greater detail below: Reaction Section In the reaction section, olefins and isobutane are alkylated in the presence of sulfuric acid cat- alyst. As shown in Fig. 1.2.2, the olefin feed is initially combined with the recycle isobutane. The olefin and recycle isobutane mixed stream is then cooled to approximately 60°F (15.6°C) by exchanging heat with the net effluent stream in the feed/effluent exchangers. 1.12 ALKYLATION AND POLYMERIZATION FIGURE 1.2.1 Block flow diagram of STRATCO Inc. effluent refrigerated alkylation process. STRATCO EFFLUENT REFRIGERATED H2SO4 ALKYLATION PROCESS
- 17. STRATCO EFFLUENT REFRIGERATED H2 SO4 ALKYLATION PROCESS 1.13 Since the solubility of water is reduced at lower temperatures, water is freed from the hydrocarbon to form a second liquid phase. The feed coalescer removes this free water to minimize dilution of the sulfuric acid catalyst. The feed stream is then combined with the refrigerant recycle stream from the refrig- eration section. The refrigerant recycle stream provides additional isobutane to the reac- tion zone. This combined stream is fed to the STRATCO Contactor reactors. The use of separate Contactor reactors in the STRATCO process allows for the segre- gation of different olefin feeds to optimize alkylate properties and acid consumption. In these cases, the unit will have parallel trains of feed/effluent exchangers and feed coa- lescers. At the “heart” of STRATCO’s effluent refrigerated alkylation technology is the Contactor reactor (Fig. 1.2.3). The Contactor reactor is a horizontal pressure vessel con- taining an inner circulation tube, a tube bundle to remove the heat of reaction, and a mix- ing impeller. The hydrocarbon feed and sulfuric acid enter on the suction side of the impeller inside the circulation tube. As the feeds pass across the impeller, an emulsion of hydrocarbon and acid is formed. The emulsion in the Contactor reactor is continuously cir- culated at very high rates. The superior mixing and high internal circulation of the Contactor reactor minimize the temperature difference between any two points in the reaction zone to within 1°F (0.6°C). This reduces the possibility of localized hot spots that lead to degraded alkylate product and increased chances for corrosion. The intense mixing in the Contactor reactor also pro- vides uniform distribution of the hydrocarbons in the acid emulsion. This prevents local- ized areas of nonoptimum isobutane/olefin ratios and acid/olefin ratios, both of which promote olefin polymerization reactions. Figure 1.2.4 shows the typical Contactor reactor and acid settler arrangement. A por- tion of the emulsion in the Contactor reactor, which is approximately 50 LV % acid and 50 LV % hydrocarbon, is withdrawn from the discharge side of the impeller and flows to the acid settler. The hydrocarbon phase (reactor effluent) is separated from the acid emul- sion in the acid settlers. The acid, being the heavier of the two phases, settles to the lower portion of the vessel. It is returned to the suction side of the impeller in the form of an emulsion, which is richer in acid than the emulsion entering the settlers. The STRATCO alkylation process utilizes an effluent refrigeration system to remove the heat of reaction and to control the reaction temperature. With effluent refrigeration, the hydrocarbons in contact with the sulfuric acid catalyst are maintained in the liquid phase. The hydrocarbon effluent flows from the top of the acid settler to the tube bundle in the FIGURE 1.2.2 Feed mixing and cooling. STRATCO EFFLUENT REFRIGERATED H2SO4 ALKYLATION PROCESS
- 18. Contactor reactor. A control valve located in this line maintains a back pressure of about 60 lb/in2 gage (4.2 kg/cm2 gage) in the acid settler. This pressure is adequate to prevent vaporization in the reaction system. In plants with multiple Contactor reactors, the acid settler pressures are operated about 5 lb/in2 (0.4 kg/cm2 ) apart to provide adequate pressure differential for series acid flow. The pressure of the hydrocarbon stream from the top of the acid settler is reduced to about 5 lb/in2 gage (0.4 kg/cm2 gage) across the back pressure control valve. A portion of the effluent stream is flashed, reducing the temperature to about 35°F (1.7°C). Additional vaporization occurs in the Contactor reactor tube bundle as the net effluent stream removes the heat of reaction. The two-phase net effluent stream flows to the suction trap/flash drum where the vapor and liquid phases are separated. 1.14 ALKYLATION AND POLYMERIZATION FIGURE 1.2.3 STRATCO Contactor reactor. FIGURE 1.2.4 Contactor reactor/acid settler arrangement. STRATCO EFFLUENT REFRIGERATED H2SO4 ALKYLATION PROCESS
- 19. The suction trap/flash drum is a two-compartment vessel with a common vapor space. The net effluent pump transfers the liquid from the suction trap side (net effluent) to the effluent treating section via the feed/effluent exchangers. Refrigerant from the refrigera- tion section flows to the flash drum side of the suction trap/flash drum. The combined vapor stream is sent to the refrigeration section. The sulfuric acid present in the reaction zone serves as a catalyst to the alkylation reac- tion. Theoretically, a catalyst promotes a chemical reaction without being changed as a result of that reaction. In reality, however, the acid is diluted as a result of the side reac- tions and feed contaminants. To maintain the desired spent acid strength, a small amount of fresh acid is continuously charged to the acid recycle line from the acid settler to the Contactor reactor, and a similar amount of spent acid is withdrawn from the acid settler. In multiple-Contactor reactor plants, the reactors are usually operated in parallel on hydrocarbon and in series/parallel on acid, up to a maximum of four stages. Fresh acid and intermediate acid flow rates between the Contactor reactors control the spent acid strength. The spent acid strength is generally monitored by titration, which is done in the labo- ratory. In response to our customer requests, STRATCO has developed an on-line acid ana- lyzer that enables the operators to spend the sulfuric acid to lower strengths with much greater accuracy and confidence. When alkylating segregated olefin feeds, the optimum acid settler configuration will depend on the olefins processed and the relative rates of each feed. Generally, STRATCO recommends processing the propylene at high acid strength, butylenes at intermediate strength, and amylenes at low strength. The optimum configuration for a particular unit may involve operating some reaction zones in parallel and then cascading to additional reaction zones in series. STRATCO considers several acid staging configurations for every design in order to provide the optimum configuration for the particular feed. Refrigeration Section Figure 1.2.5 is a diagram of the most common refrigeration configuration. The partially vaporized net effluent stream from the Contactor reactor flows to the suction trap/flash drum, where the vapor and liquid phases are separated. The vapor from the suction trap/flash drum is compressed by a motor or turbine-driven compressor and then con- densed in a total condenser. A portion of the refrigerant condensate is purged or sent to a depropanizer. The remain- ing refrigerant is flashed across a control valve and sent to the economizer. If a depropaniz- er is included in the design, the bottoms stream from the tower is also sent to the economizer. The economizer operates at a pressure between the condensing pressure and the compressor suction pressure. The economizer liquid is flashed and sent to the flash drum side of the suction trap/flash drum. A lower-capital-cost alternative would be to eliminate the economizer at a cost of about 7 percent higher compressor energy. Another alternative is to incorporate a partial con- denser to the economizer configuration and thus effectively separate the refrigerant from the light ends, allowing for propane enrichment of the depropanizer feed stream. As a result, both depropanizer capital and operating costs can be reduced. The partial condens- er design is most cost-effective when feed streams to the alkylation unit are high (typical- ly greater than 40 LV %) in propane/propylene content. For all the refrigeration configurations, the purge from the refrigeration loop is treated to remove impurities prior to flowing to the depropanizer or leaving the unit. These impu- rities can cause corrosion in downstream equipment. The main impurity removed from the purge stream is sulfur dioxide (SO2). SO2 is produced from oxidation reactions in the reac- tion section and decomposition of sulfur-bearing contaminants in the unit feeds. STRATCO EFFLUENT REFRIGERATED H2 SO4 ALKYLATION PROCESS 1.15 STRATCO EFFLUENT REFRIGERATED H2SO4 ALKYLATION PROCESS
- 20. The purge is contacted with strong caustic (10 to 12 wt %) in an in-line static mixer and is sent to the caustic wash drum. The separated hydrocarbon stream from the caustic wash drum then mixes with process water and is sent to a coalescer (Fig. 1.2.6). The coalescer reduces the carryover caustic in the hydrocarbon stream that could cause stress corrosion cracking or caustic salt plugging and fouling in downstream equipment. The injection of process water upstream of the coalescer enhances the removal of caustic carryover in the coalescer. Effluent Treating Section The net effluent stream from the reaction section contains traces of free acid, alkyl sulfates, and dialkyl sulfates formed by the reaction of sulfuric acid with olefins. These alkyl sul- fates are commonly referred to as esters. Alkyl sulfates are reaction intermediates found in all sulfuric acid alkylation units, regardless of the technology. If the alkyl sulfates are not removed, they can cause corrosion and fouling in downstream equipment. STRATCO’s net effluent treating section design has been modified over the years in an effort to provide more effective, lower-cost treatment of the net effluent stream. STRATCO’s older designs included caustic and water washes in series. Until recently, STRATCO’s standard design included an acid wash with an electrostatic precipitator fol- lowed by an alkaline water wash. Now STRATCO alkylation units are designed with an acid wash coalescer, alkaline water wash, and a water wash coalescer in series (Fig. 1.2.7) or with an acid wash coalescer followed by bauxite treating. Although all these treatment methods remove the trace amounts of free acid and reaction intermediates (alkyl sulfates) from the net effluent stream, the acid wash coalescer/alkaline water wash/water wash coa- lescer design and acid wash coalescer/bauxite treater design are the most efficient. Fractionation Section The fractionation section configuration of grassroots alkylation units, either effluent refrig- erated or autorefrigerated, is determined by feed composition to the unit and product spec- ifications. As mentioned previously, the alkylation reactions are enhanced by an excess 1.16 ALKYLATION AND POLYMERIZATION FIGURE 1.2.5 Refrigeration with economizer. STRATCO EFFLUENT REFRIGERATED H2SO4 ALKYLATION PROCESS
- 21. amount of isobutane. A large recycle stream is required to produce the optimum I/O volu- metric ratio of 7 : 1 to 10 : 1 in the feed to the Contactor reactors. Therefore, the fraction- ation section of the alkylation unit is not simply a product separation section; it also provides a recycle isobutane stream. To meet overall gasoline pool RVP requirements, many of the recent alkylation designs require an alkylate RVP of 4 to 6 lb/in2 (0.28 to 0.42 kg/cm2 ). To reduce the RVP of the alkylate, a large portion of the n-butane and isopentane must be removed. Low C5⫹ con- tent of the n-butane product is difficult to meet with a vapor side draw on the DIB and STRATCO EFFLUENT REFRIGERATED H2 SO4 ALKYLATION PROCESS 1.17 FIGURE 1.2.6 Depropanizer feed treating. FIGURE 1.2.7 Effluent treating section. STRATCO EFFLUENT REFRIGERATED H2SO4 ALKYLATION PROCESS
- 22. requires the installation of a debutanizer tower (Fig. 1.2.8). Typically, a debutanizer is required when the specified C5⫹ content of the n-butane product must be less than 2 LV %. A simpler system consisting of a deisobutanizer (DIB) with a side draw may suffice if a high-purity n-butane product is not required. The simplest fractionation system applies to a unit processing a high-purity olefin stream, such as an isobutane/isobutylene stream from a dehydrogenation unit. For these cases, a single isostripper can be used to produce a recycle isobutane stream, a low-RVP alkylate product, and a small isopentane product. An isostripper requires no reflux and many fewer trays than a DIB. Blowdown Section The acidic blowdown vapors from potential pressure relief valve releases are routed to the acid blowdown drum to knock out any entrained liquid sulfuric acid. Additionally, spent acid from the last Contactor reactor/acid settler system(s) in series is sent to the acid blowdown drum. This allows any residual hydrocarbon in the spent acid to flash. The acid blowdown drum also provides surge capacity for spent acid. The acidic vapor effluent from the acid blowdown drum is sent to the blowdown vapor scrubber. The acidic vapors are countercur- rently contacted with a circulating 12 wt % caustic solution in a six-tray scrubber (Fig. 1.2.9). TECHNOLOGY IMPROVEMENTS The following information is provided to highlight important design information about the STRATCO H2SO4 effluent refrigerated alkylation process. STRATCO Contactor Reactor The alkylation reaction requires that the olefin be contacted with the acid catalyst concur- rently with a large excess of isobutane. If these conditions are not present, polymerization 1.18 ALKYLATION AND POLYMERIZATION FIGURE 1.2.8 Fractionation system. STRATCO EFFLUENT REFRIGERATED H2SO4 ALKYLATION PROCESS
- 23. reactions will be promoted which result in a heavy, low-octane product and high acid con- sumption. Since the early days of alkylation, the Contactor reactor has been recognized as the superior alkylation reactor with higher product quality and lower acid consumption than those of competitive designs. However, STRATCO continues to modify and improve the Contactor reactor to further optimize the desirable alkylation reaction. Several of these improvements are listed next. The modern Contactor reactor has an eccentric shell as opposed to a concentric shell in older models. The eccentric shell design provides superior mixing of the acid and hydro- carbons and eliminates any localized “dead” zones where polymerization reactions can occur. The result is improved product quality and substantially lower acid consumption. The heat exchange bundle in the Contactor reactor has been modified to improve the flow path of the acid/hydrocarbon mixture around the tubes. Since this results in improved heat transfer, the temperature gradient across the reaction zone is quite small. This results in optimal reaction conditions. The heat exchange area per Contactor reactor has been increased by more than 15 per- cent compared to that in older models. This has resulted in an increased capacity per Contactor reactor and also contributes to continual optimization of the reaction conditions. The design of the internal feed distributor has been modified to ensure concurrent con- tact of the acid catalyst and olefin/isobutane mixture at the point of initial contact. The Contactor reactor hydraulic head has been modified to include a modern, cartridge- type mechanical seal system. This results in a reliable, easy-to-maintain, and long-lasting seal system. STRATCO offers two types of mechanical seals: a single mechanical seal with a Teflon sleeve bearing and a double mechanical seal with ball bearings that operates with a barri- er fluid. The STRATCO Contactor reactors can be taken off-line individually if any main- tenance is required. If seal replacement is required during normal operation, the Contactor reactor can be isolated, repaired, and back in service in less than 24 h. STRATCO EFFLUENT REFRIGERATED H2 SO4 ALKYLATION PROCESS 1.19 FIGURE 1.2.9 Blowdown system. STRATCO EFFLUENT REFRIGERATED H2SO4 ALKYLATION PROCESS
- 24. Process Improvements Several process modifications have been made to provide better alkylation reaction condi- tions and improve overall unit operations. Some of these modifications are as follows: Acid retention time in the acid settler has been reduced by employing coalescing media in the acid settler. The reduced retention time minimizes the potential for undesirable poly- merization reactions in the acid settler. Two stages of coalescing are employed to separate the hydrocarbon product from the acid phase. The first stage results in a 90 vol % H2SO4 stream that is recycled to the Contactor reactor. The second stage reduces the acid carry- over rate to only 10 to 15 vol ppm. This is at least a threefold decrease in comparison to simple gravity settling with a typical 50 to 100 vol ppm in the hydrocarbon stream. Fresh H2SO4 is continuously added to the unit, and spent H2SO4 is continuously with- drawn. In multiple-Contactor reactor units, the H2SO4 flows in series between the Contactor reactors. Thus, the acid strength across the unit is held at its most effective value, and the acid strength at any one location in the unit does not vary with time. This method of han- dling H2SO4 provides a very stable operation and continual acid strength optimization. To ensure complete and intimate mixing of the olefin and isobutane feeds before con- tacting with the acid catalyst, these hydrocarbon feeds are premixed outside the Contactor reactor and introduced as one homogeneous stream. Alkyl sulfates are removed in a fresh acid wash coalescer/warm alkaline water wash. Afterward, the net effluent stream is washed with fresh process water to remove traces of caustic, then is run through a coalescer to remove free water before being fed to the DIB tower. This system is superior to the caustic wash/water wash system which was imple- mented in older designs. The fractionation system can be designed to accommodate makeup isobutane of any purity, eliminating the need for upstream fractionation of the makeup isobutane. The alkylation unit is designed to take maximum advantage of the refinery’s steam and utility economics. Depending upon these economics, the refrigeration compressor and/or Contactor reactors can be driven with steam turbines (condensing or noncondensing) or electric motors, to minimize unit operating costs. STRATCO now employs a cascading caustic system in order to minimize the volume and strength of the waste caustic (NaOH) stream from the alkylation unit. In this system, fresh caustic is added to the blowdown vapor scrubber, from which it is cascaded to the depropanizer feed caustic wash and then to the alkaline water wash. The only waste stream from the alkylation unit containing caustic is the spent alkaline water stream. The spent alkaline water stream has a very low concentration of NaOH (⬍ 0.05 wt %) and is com- pletely neutralized in the neutralization system before being released to the refinery waste- water treatment facility. Since the cascading system maintains a continuous caustic makeup flow, it has the additional advantages of reduced monitoring requirements and reduced chance of poor treating due to inadequate caustic strength. H2SO4 ALKYLATION PROCESS COMPARISON The most important variables that affect product quality in a sulfuric acid alkylation unit are temperature, mixing, space velocity, acid strength, and concentration of isobutane feed in the reactor(s). It is usually possible to trade one operating variable for another, so there is often more than one way to design a new plant to meet octane requirements with a giv- en olefin feed. Going beyond the customary alkylation process variables, STRATCO has developed unique and patented expertise in separate processing of different olefin feeds. This tech- 1.20 ALKYLATION AND POLYMERIZATION STRATCO EFFLUENT REFRIGERATED H2SO4 ALKYLATION PROCESS
- 25. nology can improve product quality compared to alkylation of the same olefins mixed together. The two major H2SO4 alkylation processes are the STRATCO effluent refrigerated process and the autorefrigerated process by design; these two processes take different approaches to achieve product quality requirements. These design differences and their impacts on operability and reliability are discussed below. Cooling and Temperature Control The STRATCO effluent refrigerated process utilizes a liquid-full reactor/acid settler sys- tem. The heat of reaction is removed by an internal tube bundle. In the autorefrigerated process, the heat of reaction is removed by operating the reactor at a pressure where the acid/hydrocarbon mixture boils. The autorefrigerated reactor and acid settler therefore contain a vapor phase above the two mixed liquid phases. Both systems can be operated in the same temperature range. However, the STRATCO system is much easier to operate. Temperature control in the STRATCO effluent refrigerated process is simpler than that in the autorefrigerated process. The pressure of the refrigerant flash drum is used to con- trol the operating temperature of all the Contactor reactors in the reaction zone. The autorefrigerated process requires two or more pressure zones per reactor to control tem- perature and to maintain liquid flow between the reactor zones. Good control of the acid/hydrocarbon ratio in a sulfuric acid alkylation reactor is crit- ical to reactor performance. This is the area in which the STRATCO system has its largest operability advantage. Since the Contactor reactor system operates liquid-full, gravity flow is used between the Contactor reactor and acid settler. The Contactor/settler system is hydraulically designed to maintain the optimum acid-to-hydrocarbon ratio in the reactor as long as the acid level in the acid settler is controlled in the correct range. The acid/hydro- carbon ratio in the Contactor reactor can be easily verified by direct measurement. In con- trast, the autorefrigerated process requires manipulation of an external acid recycle stream in order to control the acid/hydrocarbon ratio in the reactor. As a result, the acid/hydro- carbon ratio in the different mixing zones varies and cannot be readily measured. The Contactor reactor/settler system is also designed to minimize acid inventory in the acid settler. Minimizing the unmixed acid inventory suppresses undesirable side reactions which degrade product quality and increase acid consumption. Quick, clean separation of the acid and hydrocarbon phases is much more difficult in the boiling autorefrigerated process. When operated at the same temperature, the effluent refrigerated system requires some- what greater refrigeration compressor horsepower than the autorefrigerated process because of resistance to heat transfer across the tube bundle. Mixing The topic of mixing in a sulfuric acid alkylation unit encompasses (1) the mixing of the isobutane and olefin feeds outside the reactor, (2) the method of feed injection, and (3) the mixing intensity inside the reactor. The best-quality alkylate is produced with the lowest acid consumption when ● The “local” isobutene/olefin ratio in the mixing zone is maximized by premixing the olefin and isobutane feeds. ● The feed is rapidly dispersed into the acid/hydrocarbon emulsion. ● Intense mixing gives the emulsion a high interfacial area. STRATCO EFFLUENT REFRIGERATED H2 SO4 ALKYLATION PROCESS 1.21 STRATCO EFFLUENT REFRIGERATED H2SO4 ALKYLATION PROCESS
- 26. In STRATCO’s effluent refrigerated process, all the isobutane sent to the reactors is pre- mixed with olefin feed, maximizing the “local” isobutane concentration at the feed point. The feed mixture is rapidly dispersed into the acid catalyst via a special injection nozzle. Mixing occurs as the acid/hydrocarbon emulsion passes through the hydraulic head impeller and as it circulates through the tube bundle. The tube bundle in the Contactor reactor is an integral part of the mixing system. The superior mixing in the Contactor reactor produces an emulsion with a high interfacial area, even heat dissipation, and uniform distribution of the hydrocarbons in the acid. Intense mix- ing reduces the temperature gradient within the Contactor’s 11,500-gal volume to less than 1°F. The result is suppression of olefin polymerization reactions in favor of the alkylation reaction. Good mixing is particularly important when the olefin feed contains propylene. In the autorefrigerated process, only a portion of the isobutane is premixed with the olefin feed. The “local” concentration of isobutane is therefore lower when the feeds first make contact with acid catalyst. The less intensive mixing in the autorefrigerated process can result in nonuniform distribution of the hydrocarbons in the acid. The desired finely dispersed hydrocarbon in acid emulsion cannot be easily controlled throughout the different reaction zones. As a consequence, the autorefrigerated alkylation process must be operated at a very low space velocity and temperature to make up for its disadvantage in mixing. Acid Strength The acid cascade system employed by STRATCO provides a higher average acid strength in the reaction zone than can usually be accomplished with large autorefrigerated reactors. The higher average acid strength results in higher alkylate octane with reduced acid consumption. STRATCO has recently completed pilot-plant studies that enable us to optimize the acid cas- cade system for different plant capacities. Large autorefrigerated reactors must be designed for lower space velocity and/or lower operating temperature to compensate for this difference. Isobutane Concentration and Residence Time in the Reactor Since the Contactor reactor is operated liquid-full, all the isobutane fed to the reactor is available for reaction. In the autorefrigerated process, a portion of the isobutane fed to the reactor is vaporized to provide the necessary refrigeration. The isobutane is also diluted by reaction products as it cascades through the reactor. To match the liquid-phase isobutane concentration in the STRATCO process, the deisobutanizer recycle rate and/or purity in the autorefrigerated process must be increased to compensate for the dilution and isobu- tane flashed. The DIB operating costs will therefore be higher for the autorefrigerated process unless other variables such as space velocity or temperature are used to compen- sate for a lower isobutane concentration. Research studies have shown that trimethylpentanes, the alkylate components which have the highest octane, are degraded by extended contact with acid. It is therefore desir- able to remove alkylate product from the reactor as soon as it is produced. STRATCO Contactor reactors operate in parallel for the hydrocarbons and approach this ideal more closely than the series operation of reaction zones in autorefrigerated reactors. Reliability One of the primary factors affecting the reliability of an alkylation unit is the number and type of mechanical seals required in the reaction zone. 1.22 ALKYLATION AND POLYMERIZATION STRATCO EFFLUENT REFRIGERATED H2SO4 ALKYLATION PROCESS
- 27. Each Contactor reactor has one mechanical seal. STRATCO offers two types of mechanical seals; a single mechanical seal with a Teflon sleeve bearing and a double mechanical seal with ball bearings that operates with a barrier fluid. The Contactor reac- tors can be taken off-line individually if any maintenance is required. If seal replacement is required during normal operation, the Contactor reactor can be isolated, repaired, and back in service in less than 24 h. The number of mechanical seals required for autorefrigerated reactor systems is high- er. An agitator for every reactor compartment and redundant acid recycle pumps are required. The dry running seals often used on autorefrigerated reactor agitators have a shorter expected life than STRATCO’s double mechanical seal. While special agitators are available which allow mechanical seals to be replaced without shutting down the reactor, many refiners’ safety procedures require the autorefrigerated reactor to be shut down for this type of maintenance. It is common practice to shut down the agitator and stop feed to a reactor chamber in the event of agitator seal or shaft problems. Product quality will then be degraded until the reactor can be shut down for repairs. Separate Processing of Different Olefin Feeds Olefin feed composition is not normally an independent variable in an alkylation unit. STRATCO has recently developed unique and patented expertise in the design of alkyla- tion units which keep different olefin feeds separate and alkylate them in separate reactors. By employing this technology, each olefin can be alkylated at its optimum conditions while avoiding the “negative synergy” which occurs when certain olefins are alkylated together. This know-how provides an advantage with mixtures of propylene, butylene, and amylene, and with mixtures of iso- and normal olefins. As a result, alkylate product qual- ity requirements can be met at more economical reaction conditions. COMMERCIAL DATA STRATCO alkylation technology is responsible for about 35 percent of the worldwide production of alkylate and about 74 percent of sulfuric acid alkylation production. Of the 276,000 bbl/day of alkylation capacity added from 1991 to 2001, about 81 percent is STRATCO technology. Capital and Utility Estimates Total estimated inside battery limit (ISBL) costs for grassroots STRATCO effluent refrig- erated alkylation units are shown in Table 1.2.1. Utility and chemical consumption for an alkylation unit can vary widely according to feed composition, product specifications, and design alternatives. The values in Table 1.2.2 are averages of many designs over the last several years and reflect mainly butylene feeds with water cooling and electrical drivers for the compressor and Contactor reactors. Steam and cooling water usage has crept up in recent years as a result of lower RVP targets for the alkylate product. The acid consumption given in the table does not include the con- sumption due to feed contaminants. More information on alkylate properties and STRATCO’s H2SO4 effluent refrigerated alkylation process is available at www.stratco.dupont.com. STRATCO EFFLUENT REFRIGERATED H2 SO4 ALKYLATION PROCESS 1.23 STRATCO EFFLUENT REFRIGERATED H2SO4 ALKYLATION PROCESS
- 28. REFERENCES 1. D. C. Graves, K. E. Kranz, D. M. Buckler, and J. R. Peterson, “Alkylation Best Practices for the New Millennium,” NPRA Annual Meeting in Baton Rouge, La., 2001. 2. D. C. Graves, “Alkylation Options for Isobutylene and Isopentane,” ACS meeting, 2001. 3. J. R. Peterson, D. C. Graves, K. E. Kranz, and D. M. Buckler, “Improved Amylene Alkylation Economics,” NPRA Annual Meeting, 1999. 4. K. E. Kranz and D. C. Graves, “Olefin Interactions in Sulfuric Acid Catalyzed Alkylation,” Arthur Goldsby Symposium, Division of Petroleum Chemistry, 215th National Meeting of the American Chemical Society (ACS), Dallas, Tex., 1998. 5. D. C. Graves, K. E. Kranz, J. K. Millard, and L. F. Albright, Alkylation by Controlling Olefin Ratios. Patent 5,841,014, issued 11/98. 6. D. C. Graves, K. E. Kranz, J. K. Millard, and L. F. Albright, Alkylation by Controlling Olegin Ratios. Patent 6,194,625, issued 2/01. 1.24 ALKYLATION AND POLYMERIZATION TABLE 1.2.1 Estimated Erected Costs (U.S., ±30%) Mid-1999 U.S. Gulf Coast basis Production Total erected costs, capacity, BPD $/bbl 5,000 5,000 12,000 4,500 20,000 4,000 TABLE 1.2.2 Estimated Utilities and Chemicals (per Barrel of Alkylate Production) Electric power, kW 15 Cooling water, gal 1370 Process water, gal 4 Steam, lb 194 Fresh acid, lb 13 NaOH, lb 0.05 STRATCO EFFLUENT REFRIGERATED H2SO4 ALKYLATION PROCESS
- 29. CHAPTER 1.3 UOP ALKYLENE™ PROCESS FOR MOTOR FUEL ALKYLATION Cara Roeseler UOP LLC Des Plaines, Illinois INTRODUCTION The UOP Alkylene process is a competitive and commercially available alternative to liq- uid acid technologies for alkylation of light olefins and isobutane. Alkylate is a key blend- ing component for gasoline having high octane, low Reid vapor pressure (RVP), low sulfur, and low volatility. It is composed of primarily highly branched paraffinic hydro- carbons. Changing gasoline specifications in response to legislation will increase the importance of alkylate, making it an ideal “clean fuels” blend stock. Existing liquid acid technologies, while well proven and reliable, are increasingly under political and regula- tory pressure to reduce environmental and safety risks through increased monitoring and risk mitigation. A competitive solid catalyst alkylation technology, such as the Alkylene process, would be an attractive alternative to liquid acid technologies. UOP developed the Alkylene process during the late 1990s, in response to the indus- try’s need for an alternative to liquid acid technologies. Early attempts with solid acid cat- alysts found some to have good alkylation properties, but the catalysts also had short life, on the order of hours. In addition, these materials could not be regenerated easily, requir- ing a carbon burn step. Catalysts with acid incorporated on a porous support had been investigated but not commercialized. UOP invented the novel HAL-100 catalyst that has high alkylation activity and long catalyst stability and easily regenerates without a high- temperature carbon burn. Selectivity of the HAL-100 is excellent, and product quality is comparable to that of the product obtained from liquid acid technologies. ALKYLENE PROCESS Olefins react with isobutane on the surface of the HAL-100 catalyst to form a complex mixture of isoalkanes called alkylate. The major constituents of alkylate are highly branched trimethylpentanes (TMP) that have high-octane blend values of approximately 1.25 Source: HANDBOOK OF PETROLEUM REFINING PROCESSES
- 30. 100. Dimethyl hexanes (DMH) have lower-octane blend values and are present in alkylate at varying levels. Alkylation proceeds via a carbenium ion mechanism, as shown in Fig. 1.3.1. The com- plex reaction paths include an initiation step, a propagation step, and hydrogen transfer. Secondary reactions include polymerization, isomerization, and cracking to produce other isoalkanes including those with carbon numbers which are not multiples of 4. The primary reaction products are formed via simple addition of isobutane to an olefin such as propy- lene, butenes, and amylenes. The key reaction step is the protonation of a light olefin on the solid catalyst surface followed by alkylation of an olefin on the C4 carbocation, form- ing the C8 carbocation. Hydride transfer from another isobutane molecule forms the C8 paraffin product. Secondary reactions result in less desirable products, both lighter and heavier than the high-octane C8 products. Polymerization to acid-soluble oil (ASO) is found in liquid acid technologies and results in additional catalyst consumption and yield loss. The Alkylene process does not produce acid-soluble oil. The Alkylene process also has minimal polymerization, and the alkylate has lighter distillation properties than alky- late from HF or H2SO4 liquid acid technologies. Alkylation conditions that favor the desired high-octane trimethylpentane include low process temperature, high localized isobutane/olefin ratios, and short contact time between the reactant and catalyst. The Alkylene process is designed to promote quick, intimate con- tact of short duration between hydrocarbon and catalyst for octane product, high yield, and efficient separation of alkylate from the catalyst to minimize undesirable secondary reac- tions. Alkylate produced from the Alkylene process is comparable to alkylate produced from traditional liquid acid technologies without the production of heavy acid-soluble oil. The catalyst is similar to other hydroprocessing and conversion catalysts used in a typical refinery. Process conditions are mild and do not require expensive or exotic metallurgy. 1.26 ALKYLATION AND POLYMERIZATION High Low Isobutane/Olefin Ratio C4 = C4 = C4 i-C4 i-C4 C12 – C20 C12 – C20 90 RON C5 – C7 Cracked Products 60-93 RON + C8 C8 TMP 100 RON Isomerized C8 DMH 60 RON + + + Low High Temperature Low High Contact Time Minor M i n o r M i n o r FIGURE 1.3.1 Reaction mechanism. UOP ALKYLENE™ PROCESS FOR MOTOR FUEL ALKYLATION
- 31. UOP ALKYLENE™ PROCESS FOR MOTOR FUEL ALKYLATION 1.27 Reactor temperature, isobutene/olefin ratio, contact time, and catalyst/olefin ratios are the key operating parameters. Feeds to the Alkylene unit are dried and treated to move impurities and contaminants such as diolefins, oxygenates, nitrogen, and sulfur. These contaminants also cause higher acid consumption, higher acid-soluble oil formation, and lower acid strength in liquid acid technologies. Diolefin saturation technology, such as the Huels Selective Hydrogenation Process technology licensed by UOP LLC, saturates diolefins to the corresponding monoolefin and isomerizes the 1-butene to 2-butene. The alkylate formed by alkylating isobutane with 2-butene is the preferred 2,2,3-TMP compared to the 2,2-DMH formed by alkylating isobutane with 1-butene. The olefin and isobutane (Fig. 1.3.2) are combined and injected into a carbon-steel ris- er reactor with continuous catalyst reactivation (Fig. 1.3.3) to maintain a constant catalyst activity and minimize catalyst inventory. This provides constant product quality, high yield, and high on-stream efficiency. Liquid-phase hydrocarbon reactants transport the cat- alyst around the reactor circuit where velocities are low relative to those of other moving catalyst processes. The reaction time is on the order of minutes for the completion of the primary reactions and to minimize secondary reactions. The catalyst and hydrocarbon are intimately mixed during the reaction, and the catalyst is easily disengaged from the hydro- carbon product at the top of the reactor. The catalyst is reactivated by a simple hydro- genation of the heavier alkylate on the catalyst in the reactivation wash zone. Hydrogen consumption is minimal as the quantity of heavy alkylate on the HAL-100 catalyst is very small. The reactivation process is highly effective, restoring the activity of the catalyst to nearly 100 percent of fresh. The liquid-phase operation of the Alkylene process results in less abrasion than in other catalyst circulation processes due to the lubricating effect of the liquid. Furthermore, the catalyst and hydrocarbon velocities are low relative to those in other moving catalyst processes. This minimizes the catalyst replacement requirements. Catalyst circulation is maintained to target catalyst/olefin ratios. A small catalyst slip- stream flows into a separate vessel for reactivation in vapor phase with relatively mild con- ditions to remove any last traces of heavy material and return the catalyst activity to essentially the activity of fresh catalyst. Alkylate from the reactor is sent to a downstream fractionation section, which is simi- lar to fractionation sections in liquid acid process flow schemes. The fractionation section recycles the unconverted isobutane back to the reactor and separates out the final alkylate product. Feed Pretreatment Reactor Section Fractionation Section Butamer Unit Propane n-Butane Alkylate H2 H2 Butane Feed Butane Feed Light Ends Light Ends Isobutane Recycle Olefin Feed optional FIGURE 1.3.2 Alkylene process flow scheme. UOP ALKYLENE™ PROCESS FOR MOTOR FUEL ALKYLATION
- 32. ALKYLENE PERFORMANCE HAL-100, the Alkylene process catalyst, has high acidity to promote desirable alkylation reactions. It has optimum particle size and pore distribution to allow for good mass transfer of reactants and products into and out of the catalyst. The catalyst has been commercially produced and demonstrates high physical strength and very low attrition rates in extensive physical testing. Catalyst attrition rates are several orders of magnitude lower than those experienced in other moving-bed regeneration processes in the refining industry. HAL-100 has been demonstrated in a stability test of 9 months with full isobutane recy- cle and showed excellent alkylate product qualities as well as catalyst stability. Performance responses to process parameters such as isobutane/olefin ratio, catalyst/olefin ratio, and process temperature were measured. Optimization for high performance, cata- lyst stability, and economic impact results in a process technology competitive with tradi- tional liquid acid technologies (Fig. 1.3.4). Typical light olefin feedstock compositions including propylene, butylenes, and amylenes were also studied. The primary temporary deactivation mechanism is the block- age of the active sites by heavy hydrocarbons. These heavy hydrocarbons are significant- ly lower in molecular weight than acid-soluble oil that is typical of liquid acid technologies. These heavy hydrocarbons are easily removed by contacting the catalyst with hydrogen and isobutane to strip them from the catalyst surface. These heavy hydro- carbons are combined in the total alkylate product pool and are accounted for in the alky- late properties from the Alkylene process. The buildup of heavy hydrocarbons on the catalyst surface is a function of the operat- ing severity and the feedstock composition. The reactivation conditions and the frequency of vapor reactivation are optimized in order to achieve good catalyst stability as well as commercially economical conditions. 1.28 ALKYLATION AND POLYMERIZATION Feed Pretreatment Section Olefin Feed Isobutane Recycle Alkylate Light Ends LPG Reactivation Wash Zone i-C4 / H2 Alkylene Reactor Reactivation Vessel Fractionation Section H2 FIGURE 1.3.3 Alkylene process flow diagram. UOP ALKYLENE™ PROCESS FOR MOTOR FUEL ALKYLATION
- 33. ENGINEERING DESIGN AND OPTIMIZATION The liquid transport reactor for the Alkylene process was developed by UOP based on extensive UOP experience in fluid catalytic cracking (FCC) and continuous catalyst regen- eration (CCR) technologies. Novel engineering design concepts were incorporated. Extensive physical modeling and computational fluid dynamics modeling were used to verify key engineering design details. More than 32 patents have been issued for the Alkylene process technology. The reactor is designed to ensure excellent mixing of catalyst and hydrocarbon with lit- tle axial dispersion as the mixture moves up the riser. This ensures sufficient contact time and reaction time for alkylation. Olefin injection nozzles have been engineered to mini- mize high olefin concentration at the feed inlet to the riser. The catalyst is quickly sepa- rated from the hydrocarbon at the top of the riser and falls by gravity into the reactivation zone. The catalyst settles into a packed bed that flows slowly downward in the upper sec- tion of the vessel, where it is contacted with low-temperature hydrogen saturated isobutane recycle. The heavy hydrocarbons are hydrogenated and desorbed from the catalyst. The reactivated catalyst flows down standpipes and back into the bottom of the riser. The reac- tor section includes separate vessels for reactivating a slipstream of catalyst at a higher temperature to completely remove trace amounts of heavy hydrocarbons. By returning freshly reactivated catalyst to the riser continuously, catalyst activity is maintained for con- sistent performance. The UOP Butamer process catalytically converts normal butane to isobutane with high selectivity, minimum hydrogen consumption, and excellent catalyst stability. When the Butamer process is combined with the Alkylene process, n-butane in the feed can be react- ed to extinction, thereby reducing the fresh feed saturate requirements. In addition, the UOP ALKYLENE™ PROCESS FOR MOTOR FUEL ALKYLATION 1.29 C5 C6-C7 C8 C9+ Product Distribution, LV-% 0 20 40 60 80 100 HF H2SO4 Alkylene RON 95.7 96.6 97.0 MON 94.2 93.6 94.2 Temp, °F 100 50 77 Temp, °C 38 10 25 FIGURE 1.3.4 Catalyst comparison: mixed 4 olefin feed. UOP ALKYLENE™ PROCESS FOR MOTOR FUEL ALKYLATION
- 34. increased isobutane concentration in the isostripper reduces the size of the isostripper and allows for a reduction in utilities consumption. A novel flow scheme for the optimal inte- gration of the Butamer process into the Alkylene process was developed. The two units can share common fractionation and feed pretreatment equipment. Synergy of the two units reduces the capital cost requirement for the addition of the Butamer process and reduces the operating cost. Table 1.3.1 illustrates the maximum utilization of the makeup C4 paraf- fin stream and the utilities savings. ALKYLENE PROCESS ECONOMICS The product research octane number can be varied according to the reaction temperature and the isobutane/olefin ratio. Additional refrigeration duty can be justified by higher product octane, depending on the needs of the individual refiner. Higher isobutane/olefin ratio requires higher capital and utilities. Mixed propylene and butylene feedstocks can also be processed with less dependence on operating temperature. However, the alkylate product octane is typically lower from mixed propylene and butylene feed than from buty- lene-only feed. Processing some amylenes with the butylenes will result in slightly lower octane. Most refiners have blended the C5 stream in the gasoline pool. However, with increasing restrictions on Reid vapor pressure, refiners are pulling C5 out of the gasoline pool and processing some portion in alkylation units. The three cases shown in Table 1.3.2 compare the economics of the Alkylene process with those of conventional liquid acid alkylation. The basis is 8000 BPSD of alkylate prod- uct from the Alkylene process. Case 1 is the Alkylene process, case 2 is an HF alkylation unit, and case 3 is a sulfuric acid unit with on-site acid regeneration. All cases include a Butamer process to maximize feed utilization. The Alkylene process has a yield advantage over liquid acid alkylation technologies and does not produce acid-soluble oil (ASO) by-products. In addition, the capital cost of the Alkylene process is competitive compared with existing technologies, and maintenance costs are lower. The HF alkylation unit requires HF mitigation capital and operating costs. The sulfuric acid alkylation unit requires regeneration or transport of large volumes of acid. Overall, the Alkylene process is a safe and competitive option for today’s refiner. SUMMARY Future gasoline specifications will require refiners to maximize the use of assets and rebal- ance refinery gasoline pools. The potential phase-out of MTBE will create the need for 1.30 ALKYLATION AND POLYMERIZATION TABLE 1.3.1 Alkyene Process Capital Costs Alkylene Alkylene ⫹ Butamer Total feed from FCC, BPSD 7064 7064 C4 paraffin makeup 9194 2844 C5⫹ alkylate, BPSD 8000 8000 C5⫹ alkylate RONC 95.0 95.0 USGC EEC, million $ 43.0 43.7 Utilities Base 0.96*Base UOP ALKYLENE™ PROCESS FOR MOTOR FUEL ALKYLATION
- 35. clean, high-octane blending components, such as alkylate, to allow refiners to meet pool requirements without adding aromatics, olefins, or RVP. Alkylate from the Alkylene process has excellent alkylate properties equivalent to those of HF acid technology, does not generate ASO, has better alkylate yield, and is a safe alternative to liquid acid tech- nologies. Recent developments propel the Alkylene process technology into the market- place as a viable option with technical and economic benefits. As the demand for alkylate continues to grow, new alkylation units will help refiners meet the volume and octane requirements of their gasoline pools. The Alkylene process was developed as a safe alternative to commercial liquid acid alkylation technologies. BIBLIOGRAPHY Cara M. Roeseler, Steve M. Black, Dale J. Shields, and Chris D. Gosling, “Improved Solid Catalyst Alkylation Technology for Clean Fuels: The Alkylene Process,” NPRA Annual Meeting, San Antonio, March 2002. UOP ALKYLENE™ PROCESS FOR MOTOR FUEL ALKYLATION 1.31 TABLE 1.3.2 Comparison of Alkylation Options Alkylene ⫹ HF ⫹ On-site regeneration Butamer Butamer H2SO4 ⫹ Butamer Total feed from FCC, BPSD 7064 7064 7064 C5⫹ alkylate, BPSD 8000 7990 7619 C5⫹ alkylate RONC 95.0 95.2 95.0 MONC 92.9 93.3 92.2 (R ⫹ M) / 2 94.0 94.3 93.6 C5⫹ alkylate D-86, °F 50% 213 225 21 90% 270 290 29 Utilities, $/bbl C5 ⫹ alkylate 174 0.70 1.32 Acid cost, $/bbl — 0.08 0.01 Catalyst cost, $/bbl 0.60 0.02 0.02 Metals recovery, $/bbl 0.03 0.00 0.00 Chemical cost, $/bbl 0.03 0.02 0.02 Variable cost of production, $/bbl 2.39 0.82 1.37 Fixed cost, $/bbl 1.97 2.43 3.53 Total cost of production, $/bbl 4.37 3.25 4.90 Estimated erected cost, million $ 43.5 40.5 63.3 UOP ALKYLENE™ PROCESS FOR MOTOR FUEL ALKYLATION
- 36. UOP ALKYLENE™ PROCESS FOR MOTOR FUEL ALKYLATION
- 37. CHAPTER 1.4 UOP HF ALKYLATION TECHNOLOGY Kurt A. Detrick, James F. Himes, Jill M. Meister, and Franz-Marcus Nowak UOP Des Plaines, Ilinois INTRODUCTION The UOP* HF Alkylation process for motor fuel production catalytically combines light olefins, which are usually mixtures of propylene and butylenes, with isobutane to produce a branched-chain paraffinic fuel. The alkylation reaction takes place in the presence of hydrofluoric (HF) acid under conditions selected to maximize alkylate yield and quality. The alkylate product possesses excellent antiknock properties and high-octane because of its high content of highly branched paraffins. Alkylate is a clean-burning, low-sulfur, low- RVP gasoline blending component that does not contain olefinic or aromatic compounds. The HF Alkylation process was developed in the UOP laboratories during the late 1930s and early 1940s. The process was initially used for the production of high-octane aviation fuels from butylenes and isobutane. In the mid-1950s, the development and con- sumer acceptance of more-sophisticated high-performance automotive engines placed a burden on the petroleum refiner both to increase gasoline production and to improve motor fuel quality. The advent of catalytic reforming techniques, such as the UOP Platforming* process, provided an important tool for the production of high-quality gasolines available to refiners. However, the motor fuel produced in such operations is primarily aromatic- based and is characterized by high sensitivity (that is, the spread between research and motor octane numbers). Because automobile performance is more closely related to road octane rating (approximately the average of research and motor octanes), the production of gasoline components with low sensitivity was required. A natural consequence of these requirements was the expansion of alkylation operations. Refiners began to broaden the range of olefin feeds to both existing and new alkylation units to include propylene and occasionally amylenes as well as butylenes. By the early 1960s, the HF Alkylation process had virtually displaced motor fuel polymerization units for new installations, and refiners had begun to gradually phase out the operation of existing polymerization plants. The importance of the HF Alkylation process in the refining situation of the 2000s has been increased even further by the scheduled phase-out of MTBE and the increased 1.33 *Trademark and/or service mark of UOP. Source: HANDBOOK OF PETROLEUM REFINING PROCESSES
- 38. emphasis on low-sulfur gasoline. The contribution of the alkylation process is critical in the production of quality motor fuels including many of the “environmental” gasoline blends. The process provides refiners with a tool of unmatched economy and efficiency, one that will assist refiners in maintaining or strengthening their position in the production and marketing of gasolines. PROCESS CHEMISTRY General In the HF Alkylation process, HF acid is the catalyst that promotes the isoparaffin-olefin reaction. In this process, only isoparaffins with tertiary carbon atoms, such as isobutane or isopentane, react with the olefins. In practice, only isobutane is used because isopentane has a high octane number and a vapor pressure that has historically allowed it to be blend- ed directly into finished gasolines. However, where environmental regulations have reduced the allowable vapor pressure of gasoline, isopentane is being removed from gaso- line, and refiner interest in alkylating this material with light olefins, particularly propy- lene, is growing. The actual reactions taking place in the alkylation reactor are many and are relatively complex. The equations in Fig. 1.4.1 illustrate the primary reaction products that may be expected for several pure olefins. In practice, the primary product from a single olefin constitutes only a percentage of the alkylate because of the variety of concurrent reactions that are possible in the alkyla- tion environment. Compositions of pilot-plant products produced at conditions to maxi- mize octane from pure-olefin feedstocks are shown in Table 1.4.1. Reaction Mechanism Alkylation is one of the classic examples of a reaction or reactions proceeding via the car- benium ion mechanism. These reactions include an initiation step and a propagation step and may include an isomerization step. In addition, polymerization and cracking steps may be involved. However, these side reactions are generally undesirable. Examples of these reactions are given in Fig. 1.4.2. Initiation. The initiation step (Fig. 1.4.2a) generates the tertiary butyl cations that will subsequently carry on the alkylation reaction. Propagation. Propagation reactions (Fig. 1.4.2b) involve the tertiary butyl cation reacting with an olefin to form a larger carbenium ion, which then abstracts a hydride from an isobutane molecule. The hydride abstraction generates the isoparaffin plus a new tertiary butyl cation to carry on the reaction chain. Isomerization. Isomerization [Eq. (1.4.12), shown in Fig. 1.4.2c] is very important in producing good octane quality from a feed that is high in 1-butene. The isomerization of 1-butene is favored by thermodynamic equilibrium. Allowing 1-butene to isomerize to 2-butene reduces the production of dimethylhexanes (research octane number of 55 to 76) and increases the production of trimethylpentanes. Many recent HF Alkylation units, especially those processing only butylenes, have upstream olefin isomerization units that isomerize the 1-butene to 2-butene. 1.34 ALKYLATION AND POLYMERATION UOP HF ALKYLATION TECHNOLOGY
- 39. UOP HF ALKYLATION TECHNOLOGY 1.35 Equation (1.4.13) is an example of the many possible steps involved in the isomeriza- tion of the larger carbenium ions. Other Reactions. The polymerization reaction [Eq. (1.4.14), shown in Fig. 1.4.2d] results in the production of heavier paraffins, which are undesirable because they reduce alkylate octane and increase alkylate endpoint. Minimization of this reaction is achieved by proper choice of reaction conditions. The larger polymer cations are susceptible to cracking or disproportionation reactions [Eq. (1.4.15)], which form fragments of various molecular weights. These fragments can then undergo further alkylation. Isobutylene CH3-C = CH2+CH3-CH-CH3 CH3-C-CH2-CH-CH3 CH3 CH3 CH3 CH3 CH3 Isobutane Isobutane Isobutane Isobutane (Isooctane) 2,2,4-Trimethylpentane (1.4.1) CH3-CH = CH2 + CH3-CH-CH3 CH3 CH3-CH-CH-CH2-CH3 CH3CH3 Propylene 2,3-Dimethylpentane (1.4.4) 2-Butene CH3-CH = CH-CH3 + CH3-CH-CH3 CH3- C-CH2-CH-CH3 CH3 CH3 CH3 2,2,4-Trimethylpentane CH3 or CH3-CH-CH-CH-CH3 CH3CH3CH3 2,3,4-Trimethylpentane (1.4.3) CH2 = CH-CH2-CH3 + CH3-CH-CH3 CH3 CH3-CH-CH-CH2-CH2-CH3 CH3CH3 1-Butene 2,3-Dimethylpentane (1.4.2) FIGURE 1.4.1 HF alkylation primary reactions for monoolefins. UOP HF ALKYLATION TECHNOLOGY
- 40. 1.36 ALKYLATION AND POLYMERATION TABLE 1.4.1 Compositions of Alkylate from Pure-Olefin Feedstocks Olefin Component, wt % C3H6 iC4H8 C4H8-2 C4H8-1 C5 isopentane 1.0 0.5 0.3 1.0 C6s: Dimethylbutanes 0.3 0.8 0.7 0.8 Methylpentanes — 0.2 0.2 0.3 C7s: 2,3-Dimethylpentane 29.5 2.0 1.5 1.2 2,4-Dimethylpentane 14.3 — — — Methylhexanes — — — — C8s: 2,2,4-Trimethylpentane 36.3 66.2 48.6 38.5 2,2,3-Trimethylpentane — — 1.9 0.9 2,3,4-Trimethylpentane 7.5 12.8 22.2 19.1 2,3,3-Trimethylpentane 4 7.1 12.9 9.7 Dimethylhexanes 3.2 3.4 6.9 22.1 C9⫹ products 3.7 5.3 4.1 5.7 C-C = C + HF C (1.4.5) (1.4.8) (1.4.7) (1.4.6) C-C-C C-C-C C C + C-C = C-C + HF C-C-C-C F F C-C-C-C + + + + + + iC4 iC4 iC4 C-C-C-C + C-C-C C C = C-C-C + HF C-C-C-C F C-C-C-C C-C-C-C + C-C-C C C = C-C + HF C-C-C F C-C-C C-C-C + C-C-C C FIGURE 1.4.2a HF alkylation reaction mechanism—initiation reactions. UOP HF ALKYLATION TECHNOLOGY
- 41. UOP HF ALKYLATION TECHNOLOGY 1.37 C=C-C-C + C-C-C = C (1.4.9) (1.4.11) (1.4.10) C-C-C-C-C-C + C-C-C C C + Dimethylhexane C + iC4 iC4 iC4 + C-C = C-C + C-C-C C C-C-C-C-C + C-C-C C C + Trimethylpentane C + + C C-C = C + C-C-C C C-C-C-C-C + C-C-C C C + Trimethylpentane C + + C C FIGURE 1.4.2b HF alkylation reaction mechanism—propagation reactions. C=C-C-C (1.4.12) (1.4.13) 2, 2, 4 -Trimethylpentane iC4 iC4 iC4 1-Butene C-C = C-C 2-Butene C-C-C-C-C C C + C C-C-C-C-C C C + C C-C-C-C-C C C + C 2, 3, 4 -Trimethylpentane C-C-C-C-C C C + C C-C-C-C-C C + C C C-C-C-C-C C + C C C-C-C-C-C C + C C 2, 3, 3 -Trimethylpentane FIGURE 1.4.2c HF alkylation reaction mechanism—isomerization. UOP HF ALKYLATION TECHNOLOGY
- 42. Hydrogen Transfer. The hydrogen transfer reaction is most pronounced with propylene feed. The reaction also proceeds via the carbenium ion mechanism. In the first reaction [Eq. (1.4.16)], propylene reacts with isobutane to produce butylene and propane. The butylene is then alkylated with isobutane [Eq. (1.4.17)] to form trimethylpentane. The overall reaction is given in Eq. (1.4.18). From the viewpoint of octane, this reaction can be desirable because trimethylpentane has substantially higher octane than the dimethylpentane normally formed from propylene. However, two molecules of isobutane are required for each molecule of alkylate, and so this reaction may be undesirable from an economic viewpoint. PROCESS DESCRIPTION The alkylation of olefins with isobutane is complex because it is characterized by simple addition as well as by numerous side reactions. Primary reaction products are the isomer- 1.38 ALKYLATION AND POLYMERATION (1.4.14) (1.4.15) + C-C-C-C-C + C-C = C-C C12 + C C=C-C + C-C-C C3H6 + 2iC4H10 Trimethylpentane C3H8 + Trimethylpentane C C C16 + etc. Polymerization Cracking-Disproportionation C12 + C5 + + C7 + Hydrogen Transfer C-C-C + C-C=C C C (1.4.16) (1.4.17) C-C=C + C-C-C C C Overall Reaction: (1.4.18) FIGURE 1.4.2d HF alkylation reaction mechanism—other. UOP HF ALKYLATION TECHNOLOGY
- 43. ic paraffins containing carbon atoms that are the sum of isobutane and the corresponding olefin. However, secondary reactions such as hydrogen transfer, polymerization, isomer- ization, and destructive alkylation also occur, resulting in the formation of secondary prod- ucts both lighter and heavier than the primary products. The factors that promote the primary and secondary reaction mechanisms differ, as does the response of each to changes in operating conditions or design options. Not all sec- ondary reactions are undesirable; for example, they make possible the formation of isooc- tane from propylene or amylenes. In an ideally designed and operated system, primary reactions should predominate, but not to the complete exclusion of secondary ones. For the HF Alkylation process, the optimum combinations of plant economy, product yield, and quality are achieved with the reaction system operating at cooling-water temperature and an excess of isoparaffin and with contaminant-free feedstocks and vigorous, intimate acid- hydrocarbon contact. To minimize acid consumption and ensure good alkylate quality, the feeds to the alky- lation unit should be dry and of low sulfur content. Normally, a simple desiccant-drying system is included in the unit design package. Feed treating in a UOP Merox* unit for mer- captan sulfur removal can be an economic adjunct to the alkylation unit for those applica- tions in which the olefinic feed is derived from catalytic cracking or from other operations in which feedstocks of significant sulfur content are processed. Simplified flow schemes for a typical C4 HF Alkylation unit and a C3-C4 HF Alkylation unit are shown in Figs. 1.4.3 and 1.4.4. Treated and dried olefinic feed is charged along with recycle and makeup isobutane (when applicable) to the reactor section of the plant. The combined feed enters the shell of a reactor–heat exchanger through several nozzles positioned to maintain an even tempera- ture throughout the reactor. The heat of reaction is removed by heat exchange with a large volume of coolant flowing through the tubes having a low temperature rise. If cooling water is used, it is then available for further use elsewhere in the unit. The effluent from the reactor enters the settler, and the settled acid is returned to the reactor. The hydrocarbon phase, which contains dissolved HF acid, flows from the settler and is preheated and charged to the isostripper. Saturate field butane feed (when applicable) is also charged to the isostripper. Product alkylate is recovered from the bottom of the col- umn. Any normal butane that may have entered the unit is withdrawn as a sidecut. Unreacted isobutane is also recovered as a sidecut and recycled to the reactor. The isostripper overhead consists mainly of isobutane, propane, and HF acid. A drag stream of overhead material is charged to the HF stripper to strip the acid. The overhead from the HF stripper is returned to the isostripper overhead system to recover acid and isobutane. A portion of the HF stripper bottoms is used as flushing material. A net bottom stream is withdrawn, defluorinated, and charged to the gas concentration section (C3-C4 splitter) to prevent a buildup of propane in the HF Alkylation unit. An internal depropanizer is required in an HF Alkylation unit processing C3-C4 olefins and may be required with C4 olefin feedstocks if the quantity of propane entering the unit is too high to be rejected economically as previously described. The isostripper overhead drag stream is charged to the internal depropanizer. Overhead from the internal depropaniz- er is directed to the HF stripper to strip HF acid from the high-purity propane. A portion of the internal depropanizer bottoms is used as flushing material, and the remainder is returned to the alkylation reactor. The HF stripper overhead vapors are returned to the internal depropanizer overhead system. High-purity propane is drawn off the bottom of the HF strip- per, passes through a defluorination step, and is then sent to storage. A small slipstream of circulating HF acid is regenerated internally to maintain acid purity at the desired level. This technique significantly reduces overall chemical con- UOP HF ALKYLATION TECHNOLOGY 1.39 *Trademark and/or service mark of UOP. UOP HF ALKYLATION TECHNOLOGY
- 46. sumption. An acid regenerator column is also provided for start-ups after turnarounds or in the event of a unit upset or feed contamination. When the propane or normal butane from the HF unit is to be used as liquefied petro- leum gas (LPG), defluorination is recommended because of the possible breakdown of combined fluorides during combustion and the resultant potential corrosion of burners. Defluorination is also required when the butane is to be directed to an isomerization unit. After defluorination, the propane and butane products are treated with potassium hydrox- ide (KOH) to remove any free HF acid that might break through in the event of unit misop- eration. The alkylation unit is built almost entirely of carbon steel although some Monel is used for most moving parts and in a few other limited locations. Auxiliary neutralizing and scrubbing equipment is included in the plant design to ensure that all materials leaving the unit during both normal and emergency operations are acid-free. ENGINEERING DESIGN The reactor and distillation systems that UOP uses have evolved through many years of pilot-plant evaluation, engineering development, and commercial operation. The overall plant design has progressed through a number of variations, resulting in the present con- cepts in alkylation technology. Reactor Section In the design of the reactor, the following factors require particular attention: ● Removal of heat of reaction ● Generation of acid surface: mixing and acid/hydrocarbon ratio ● Acid composition ● Introduction of olefin feed The proper control of these factors enhances the quality and yield of the alkylate product. Selecting a particular reaction system configuration requires careful consideration of the refiner’s production objectives and economics. The UOP reaction system optimizes processing conditions by the introduction of olefin feed through special distributors to pro- vide the desired contact with the continuous-acid phase. Undesirable reactions are mini- mized by the continuous removal of the heat of reaction in the reaction zone itself. The removal of heat in the reaction zone is advantageous because peak reaction temperatures are reduced and effective use is made of the available cooling-water supply. Acid Regeneration Section The internal acid regeneration technique has virtually eliminated the need for an acid regenerator and, as a result, acid consumption has been greatly reduced. The acid regener- ator has been retained in the UOP design only for start-ups or during periods when the feed has abnormally high levels of contaminants, such as sulfur and water. For most units, dur- ing normal operation, the acid regenerator is not in service. When the acid regenerator is in service, a drag stream off the acid circulation line at the settler is charged to the acid regenerator, which is refluxed on the top tray with isobutane. 1.42 ALKYLATION AND POLYMERATION UOP HF ALKYLATION TECHNOLOGY
- 47. The source of heat to the bottom of the regenerator for a C3-C4 HF Alkylation unit is super- heated isobutane from the depropanizer sidecut vapors. For a C4 HF Alkylation unit, the stripping medium to the acid regenerator is sidecut vapors from the HF stripper bottoms. The regenerated HF acid is combined with the overhead vapor from the isostripper and sent to the cooler. Neutralization Section UOP has designed the neutralization section to minimize the amount of additional efflu- ents such as offensive materials and undesirable by-products. Releasing acid-containing vapors to the regular relief-gas system is impractical because of corrosion and odor prob- lems as well as other environmental and safety concerns. The system is composed of the relief-gas scrubber, KOH mix tank, circulating pumps, and a KOH regeneration tank. All acid vents and relief valves are piped to this relief section. Gases pass up through the scrubber and are contacted by a circulating KOH solution to neutralize the HF acid. After the neutralization of the acid, the gases can be safely released into the refinery flare system. The KOH is regenerated on a periodic basis in the KOH regeneration tank by using lime to form calcium fluoride (CaF2) and KOH. The CaF2 settles to the bottom of the tank and is directed to the neutralizing basin, where acidic water from acid sewers and small amounts of acid from the process drains are treated. Lime is used to convert any fluorides into calcium fluoride before any waste effluent is released into the refinery sewer system. Distillation System The distillation and recovery sections of HF Alkylation units have also seen considerable evolution. The modern isostripper recovers relatively high-purity isobutane as a sidecut that is recycled to the reactor. This recycle is virtually acid-free, thereby minimizing unde- sirable side reactions with the olefin feed prior to entry into the reactor. A small rectifica- tion section on top of the modern isostripper provides for more efficient propane rejection. Although a single high-pressure tower can perform the combined functions of isostrip- per and depropanizer, UOP’s current design incorporates two towers (isostripper and depropanizer) for the following reasons: ● Each tower may be operated at its optimum pressure. Specifically, in the isostripper for this two-tower design, the relative volatilities between products increase, and the num- ber of trays required for a given operation are reduced in addition to improving separa- tion between cuts. ● This system has considerably greater flexibility. It is easily convertible to a butylene- only operation because the depropanizer may be used as a feed splitter to separate C3s and C4s. The two-tower design permits the use of side feeds to the isostripper column, should it be necessary to charge makeup isobutane of low purity. This design also per- mits the production of lower-vapor-pressure alkylate and a high-purity sidecut nC4 for isomerizing or blending and the ability to make a clean split of side products. ● The two-tower design permits considerable expanded capacity at low incremental cost by the addition of feed preheat and side reboiling. ● Alkylate octane increases with decreasing reaction temperature. During cooler weather, the unit may be operated at lower isobutane/olefin ratios for a given product octane, because the ratio is fixed by the product requirement and not by the fractionation requirements. The commensurate reduction in utilities lowers operating costs. UOP HF ALKYLATION TECHNOLOGY 1.43 UOP HF ALKYLATION TECHNOLOGY
- 48. ● Because of the low isostripper pressure in a two-tower system, this arrangement permits the use of steam for reboiling the isostripper column instead of a direct-fired heater, which is necessary in a single-tower system. In most cases, a stab-in reboiler system is suitable even for withdrawing a sidecut. Using a steam reboiler can be a considerable advantage when refinery utility balances so indicate, and it also represents considerable investment-cost savings. ● The two-tower system has proven its performance in a large number of operating units, and its flexibility has been proven through numerous revamps for increased capacity on existing units. ● The two-tower system also requires less overhead condenser surface, which lowers the investment required for heat exchange. ● Clean isobutane is available for flush, whereas only alkylate flush is available in the sin- gle-column operation. This clean-isobutane stream is also available to be taken to stor- age and is a time saver during start-ups and shutdowns. ● Although fewer pieces of equipment are required with the single tower, the large num- ber of trays and the high-pressure design necessitate the use of more tons of material and result in a somewhat higher overall cost than does the two-tower system. ● The regenerator column contains no expensive overhead system, and the internal HF regeneration technique results in improved acid consumption. ● Because a high-temperature differential can be taken on most cooling water, cooling- water requirements for the two-tower system are only about two-thirds those of the sin- gle-tower system. COMMERCIAL INFORMATION Typical commercial yields and product properties for charging various olefin feedstocks to an HF Alkylation unit are shown in Tables 1.4.2 and 1.4.3. Table 1.4.4 contains the detailed breakdown of the investment and production costs for a pumped, settled acid-alkylation unit based on a typical C4 olefin feedstock. ENVIRONMENTAL CONSIDERATIONS The purpose of operating an HF Alkylation unit is to obtain a high-octane motor fuel blending component by reacting isobutane with olefins in the presence of HF acid. In the UOP HF Alkylation process, engineering and design standards have been developed and improved over many years to obtain a process that operates efficiently and economically. This continual process development constitutes the major reason for the excellent product qualities, low acid-catalyst consumption, and minimal extraneous by-products obtained by the UOP HF Alkylation process. 1.44 ALKYLATION AND POLYMERATION TABLE 1.4.2 HF Alkylation Yields Olefin Required vol. Vol. alkylate feedstocks iC4/vol. olefin produced/vol. olefin C3-C4 1.28 1.78 Mixed C4 1.15 1.77 UOP HF ALKYLATION TECHNOLOGY
- 49. As in every process, certain minor process inefficiencies, times of misoperation, and periods of unit upsets occur. During these times, certain undesirable materials can be dis- charged from the unit. These materials can be pollutants if steps are not taken in the process effluent management and product-treating areas to render these by-product mate- rials harmless. In a properly operated HF Alkylation unit, the amount of additional effluent, such as offensive materials or undesirable by-products is minimal, and with proper care, these small streams can be managed safely and adequately. The potentially offensive nature of the streams produced in this process as well as the inherent hazards of HF acid has result- ed in the development of effluent management and safety procedures that are unique to the UOP HF Alkylation process. The following sections briefly describe these procedures and how these streams are safely handled to prevent environmental contamination. The refin- er must evaluate and comply with any pertinent effluent management regulations. An over- all view of the effluent management concept is depicted in Fig. 1.4.5. Effluent Neutralization In the Alkylation unit’s effluent-treating systems, any neutralized HF acid must eventual- ly leave the system as an alkali metal fluoride. Because of its extremely low solubility in UOP HF ALKYLATION TECHNOLOGY 1.45 TABLE 1.4.3 HF Alkylation Product Properties Propylene- Property butylene feed Butylene feed Specific gravity 0.693 0.697 Distillation temperature, °C (°F): IBP 41 (105) 41 (105) 10% 71 (160) 76 (169) 30% 93 (200) 100 (212) 50% 99 (210) 104 (220) 70% 104 (219) 107 (225) 90% 122 (250) 125 (255) EP 192 (378) 196 (385) Octanes: RONC 93.3 95.5 MONC 91.7 93.5 Note: IBP ⫽ initial boiling point; EP ⫽ endpoint; RONC ⫽ research octane number, clear; MONC ⫽ motor octane number, clear. TABLE 1.4.4 Investment and Production Cost Summary* Operating cost $/stream day $/MT alkylate $/bbl alkylate Labor 1,587 0.016 0.176 Utilities 6,609 0.066 0.734 Chemical consumption, laboratory 5,639 0.056 0.627 allowance, maintenance, taxes, and insurance Total direct operating costs 13,835 0.138 1.537 Investment, estimated erected cost (EEC), first quarter 2002 $27,800,000 *Basis: 348,120 MTA (9000 BPSD) C5 ⫹ alkylate. Note: MT ⫽ metric tons; MTA ⫽ metric tons per annum; BPSD ⫽ barrels per stream-day. UOP HF ALKYLATION TECHNOLOGY
- 50. water, CaF2 is the desired end product. The effluent containing HF acid can be treated with a lime [CaO-Ca(OH)2] solution or slurry, or it can be neutralized indirectly in a KOH sys- tem to produce the desired CaF2 product. The KOH neutralization system currently used in a UOP-designed unit involves a two-stage process. As HF acid is neutralized by aqueous KOH, soluble potassium fluo- ride (KF) is produced, and the KOH is gradually depleted. Periodically, some of the KF- containing neutralizing solution is withdrawn to the KOH regenerator. In this vessel, KF reacts with a lime slurry to produce insoluble CaF2 and thereby regenerates KF to KOH. 1.46 ALKYLATION AND POLYMERATION FIGURE 1.4.5 UOP HF Alkylation process effluent management. UOP HF ALKYLATION TECHNOLOGY
- 51. The regenerated KOH is then returned to the system, and the solid CaF2 is routed to the neutralizing basin. Effluent Gases. The HF Alkylation unit uses two separate gas vent lines to maintain the separation of acidic gases from nonacidic gases until the acidic gases can be scrubbed free of acid. Acidic Hydrocarbon Gases. Acidic hydrocarbon gases originate from sections of the unit where HF acid is present. These gases may evolve during a unit upset, during a shutdown, or during a maintenance period in which these acidic gases are partially or totally removed from the process vessels or equipment. The gases from the acid vents and from the acid pressure relief valves are piped to a separate closed relief system for the neutralization of the acid contained in the gas. The acid-free gases are then routed from this acid-scrubbing section to the refinery nonacid flare system, where they are disposed of properly by burning. The acidic gases are scrubbed in the acid neutralization and caustic regeneration system, as shown Fig. 1.4.6. This system consists of the relief-gas scrubber, KOH mix tank, liquid- knockout drum, neutralization drum, circulating pumps, and a KOH regeneration tank. Acidic gases, which were either vented or released, first flow to a liquid-knockout drum to remove any entrained liquid. The liquid from this drum is pumped to the neutralization drum. The acidic gases from the liquid-knockout drum then pass from the drum to the scrubbing section of the relief-gas scrubber, where countercurrent contact with a KOH solution removes the HF acid. After neutralization of the HF acid, the nonacidic gases are released into the refinery flare system. The KOH used for the acidic-gas neutralization is recirculated by the circulation pumps. The KOH solution is pumped to the top of the scrubber and flows downward to contact the rising acidic gas stream and then overflows a liquid-seal pan to the reservoir section of the scrubber. In addition, a slipstream of the circulating KOH contacts the acidic gas just prior to its entry to the scrubber. The circulating KOH removes HF through the following reaction: HF ⫹ KOH → KF ⫹ H2O (1.4.19) Maintaining the circulating caustic pH and the correct percentage of KOH and KF requires a system to regenerate the caustic. This regeneration of the KOH solution is per- formed on a batch basis in a vessel separate from the relief-gas scrubber. In this regenera- tion tank, lime and the spent KOH solution are thoroughly mixed. The regenerated caustic solution is pumped back to the scrubber. The CaF2 and any unreacted lime are permitted to settle out and are then directed to the neutralization pit. The regeneration of the spent KOH solution follows the Berthollet rule, by which the insolubility of CaF2 in water per- mits the complete regeneration of the potassium hydroxide according to the following equation: 2KF ⫹ Ca (OH) 2 → 2KOH ⫹ CaF2 (1.4.20) Nonacidic Hydrocarbon Gases. Nonacidic gases originate from sections of the unit in which HF acid is not present. These nonacidic gases from process vents and relief valves are discharged into the refinery nonacid flare system, where they are disposed of by burning. The material that is vented or released to the flare is mainly hydrocarbon in nature. Possibly, small quantities of inert gases are also included. Obnoxious Fumes and Odors. The only area from which these potentially objectionable fumes could originate is the unit’s neutralizing basins. To prevent the discharge of these odorous gases to the surroundings, the neutralizing basins are tightly covered and equipped with a gas scrubber to remove any offensive odors. The UOP HF ALKYLATION TECHNOLOGY 1.47 UOP HF ALKYLATION TECHNOLOGY
- 52. FIGURE 1.4.6 Acid neutralization and caustic regeneration section. 1.48 UOP HF ALKYLATION TECHNOLOGY
- 53. gas scrubber uses either water or activated charcoal as the scrubbing agent. However, in the aforementioned neutralizing system, odors from the basin are essentially nonexistent because the main source of these odors (acid regenerator bottoms) is handled in separate closed vessels. Liquid Effluents. The HF Alkylation unit is equipped with two separate sewer systems to ensure the segregation of the nonacid from the possibly acid-containing water streams. Acidic Waters. Any potential HF containing water streams (rainwater runoff in the acid area and wash water), heavy hydrocarbons, and possibly spent neutralizing media are directed through the acid sewer system to the neutralizing basins for the neutralization of any acidic material. In the basins, lime is used to convert the incoming soluble fluorides to CaF2 . The neutralizing basins consist of two separate chambers (Fig. 1.4.7). One chamber is filled while the other drains. In this parallel neutralizing basin design, one basin has the inlet line open and the outlet line closed. As only a few surface drains are directed to the neutralizing basins, inlet flow normally is small, or nonexistent, except when acid equip- ment is being drained. The operator regularly checks the pH and, if necessary, mixes the lime slurry in the bottom of the basin. After the first basin is full, the inlet line is closed, and the inlet to the second basin is opened; then lime is added to the second basin. The first basin is mixed and checked with pH paper after a period of agitation; if it is acidic, more lime is added from lime storage until the basin is again basic. After settling, the effluent from the first basin is drained. Nonacidic Waters. The nonacid sewers are directed to the refinery water disposal system or to the API separators. UOP HF ALKYLATION TECHNOLOGY 1.49 FIGURE 1.4.7 Neutralizing basin. UOP HF ALKYLATION TECHNOLOGY
- 54. Liquid Process Effluents (Hydrocarbon and Acid). Hydrocarbon and acid effluents originate from some minor undesirable process side reactions and from any feed contaminants that are introduced to the unit. Undesirable by-products formed in this manner are ultimately rejected from the Alkylation unit in the acid regeneration column as a bottoms stream. The regeneration-column bottoms stream consists mainly of two types of mixtures. One is an acid-water phase that is produced when water enters the unit with the feed streams. The other mixture is a small amount of polymeric material that is formed during certain undesirable process side reactions. Figure 1.4.8 represents the HF acid regenera- tion circuit. The first step in the disposal of these materials is to direct the regenerator bottoms to the polymer surge drum, where the two mixtures separate. The acid-water mixture forms an azeotrope, or constant boiling mixture (CBM), which is directed to the neutralizing drum (Fig. 1.4.8) for neutralization of the HF acid. The acid in this CBM ultimately ends up as insoluble CaF2 (as described previously). The polymer that remains in the polymer surge drum is then transferred to the tar neutralizer, where the HF acid is removed. The polymer has excellent fuel oil properties and can then be disposed of by burning as long as applicable regulations allow such. However, by the mid-1980s, technology and special operating techniques such as internal acid regeneration had virtually eliminated this liquid- effluent stream for many units. Solid Effluents Neutralization Basin Solids. The neutralization basin solids consist largely of CaF2 and unreacted lime. As indicated previously, all HF-containing liquids that are directed to the neutralizing basins ultimately have any contained soluble fluorides converted to insoluble CaF2 . The disposal of this solid material is done on a batch basis. A vacuum truck is normally used to remove the fluoride-lime sludge from the 1.50 ALKYLATION AND POLYMERATION FIGURE 1.4.8 HF acid regeneration circuit. UOP HF ALKYLATION TECHNOLOGY
- 55. Another Random Scribd Document with Unrelated Content
- 56. Helga var imponeret og harmfuld. Skønt han med megen selvbeherskelse trådte et skridt tilbage, da han var færdig, så hun nok, at hans hænder rystede. Hun havde heller ikke glemt, at der var en gang, han opførte sig langt mindre værdigt og mandigt end nu. Men sådan friede altså en lærer til en uartig elev, som han ønskede at gifte sig med. Jo tak! Men nu havde Helga fået det lune, at hun vilde se ham ligesom den aften; han skulde næmlig ikke komme nu bagefter og være stor. Hun så hen på ham; han så nok ud til at ha en vilje, som andre måtte bøje sig for, og fortrydelig over ham og halv skamfuld over sig selv udbrød hun: Kan De da ikke forstå, at jeg ikke er voksen endnu, efter den måde, jeg svarer Dem på? Alt det, De siger, er slet ikke noget, man skal sige til et barn. Det skulde være en undskyldning, men han opfattede det anderledes; han kom igen det skridt frem, som han før var gået tilbage, og lidt til, og han så på hende med øjne, der var fulde af tilbedelse og af nyt håb: Jeg tar dig, som du er, Helga, du må være så meget barn, du vil, jeg skal holde endnu mere af dig for det, når blot du — — — Men nu var afstanden mellem dem større end nogen sinde; ham var hun færdig med, han skulde ikke få sagt et ord mere. Den alvor, der kunde ha gjort indtryk på hende, forlod ham, så snart hun sa et ord. Nej, til ham havde hun fra nu af ikke andet end latter. Hun hørte ikke, hvordan det nu blev til, at når der ikke var andet i vejen, så skulde hun med fornøjelse få lov at være barn, så længe hun vilde; det hastede jo heller ikke med at blive gift. Hun bøjede sig ud af vinduet og råbte: Er der ingen til at ringe der nede? Der skal ringes. Drengene så op, hr. Jensen måtte gå bort fra vinduet. — Alle var på plads. Hr. Jensen kom hen til Helga:
- 57. Må jeg låne din bog? De skal altid låne min bog. Kom med den, siger jeg. Helga lukkede den og skubbede den fra sig. Jeg kan altid lugte, når De har haft mine bøger, mumlede hun. Den første, der kom op, kunde ikke noget; Helga skulde komme og hjælpe ham. Jeg er ikke forberedt. Jeg var så træt, da jeg skulde til at læse i Lørdags, af at løbe. Der var nogen efter mig. En anden blev taget op; imens sad Helga og skar ansigt for dem bagved; hun havde særlig ævne til at ligne en gammel kone med mange rynker. Munterheden fik hr. Jensen til at vende sig. Hvad er det? Helga nikkede til Dagmar, og denne sa halvkvalt: Det var Helga, der lavede sådan et morsomt ansigt. Må jeg se det ansigt, Helga? Nej. Så får du en anmærkning. Ja. — Helga ønskede netop en anmærkning. Hvad var mere naturligt, end at en lærer gav en uartig tøs en anmærkning, når hun lige havde erklæret, at hun ikke vilde ha ham? Hun morede sig åbenlyst; der blev røre i klassen; en enkelt dreng hengav sig ubehersket til sin munterhed. Stille! skreg hr. Jensen og foer hen imod ham. Drengen tav og dukkede sig; han kendte hr. Jensens mandagslussinger. Helga tegnede. En dame med en urimelig stor barm sad på en bænk og påhørte med dryppende blikke en kærlighedserklæring fra en
- 58. herre, der lå på knæ foran hende; ved siden af stod hans høje silkehat; neden under skrev hun: Dig æ—ælsker jeg! Dig æ—æ—ælsker jeg!! Billedet sendtes omkring. Eksaminanden oppe ved tavlen, hvis sind var delt mellem forlystelse og frygt for lussinger, så den nye morskab dukke frem, og skønt han ikke kendte dens art, var han lige med eet den, der morede sig bedst og lydeligst. Han knækkede over på midten som en tynd kæp, der er for stærkt belastet, holdt sig ved tavlen og fnyste af grin, så stærk var hans tro, endda han ikke havde set. En lussing på hundrede graders Celsius gjorde ham imidlertid atter til en anstændig elev. Han genindtog retstillingen og begyndte med kridtet at lede efter vinkel AOC. Hvad er det for et papir? Helga fik det straks returneret. Det er en tegning af Helga, oplyste Dagmar. Må jeg se den? Nej. — Helga krøllede den sammen i den hule hånd. Han greb hende om håndleddene, men hun sled sig løs og gemte tegningen i sin ene sko. Hvor blev den af? Den er i min sko. Jeg vil ha den. De vil da ikke ta mig om benene. Der blev stærk uro i klassen. Eksaminanden smilede smærteligt. Han lignede et billede, som et barn har været ved at kolorere, men hvor kun den ene kind er blevet færdig. Han og hr. Jensen tog igen fat på
- 59. at søge efter vinkel AOC; de havde bægge lige svært ved at finde den. Men idet Helga satte sig, kom tegningen som ved et uheld op af hendes sko. Hr. Jensen kastede sig over den. Han var virkelig sådan en fornuftig mand, det kom han til at tænke på, nu da modsætningen mellem hans vanvittige optræden og hans øvrige respektabelhed også blev ham selv påfaldende. Han måtte sætte sig ned, og hele klassen beskuede tavs hans moralske nederlag. Helga blev alvorlig, igen en gang følte hun sig berørt af noget vildt og forfærdeligt; mon det ikke var rent galt at le af det? Bevidstheden af at ha haft en triumf svandt bort, hun stod så ubetydelig og grim over for manden, der sad og stirrede frem for sig med sådan et udtryk i sit ansigt. Resten af timen gik i højtidelig stilhed. Hr. Jensen var en agtet lærer. Du driver det noget vidt, sa han til Helga, da de to var alene. Det er jeg nødt til, svarede hun med bortvendt ansigt, ellers griner de andre ad mig. Må jeg få min tegning igen?
- 60. KAPITEL 4 Hvor er hr. Grandlev? blev der skreget. Hvor er frøken Schou? blev der hvisket. Helga løb ned ad perronen og fandt hr. Grandlev. De må skynde Dem, sa hun, ellers blir der ikke plads. Hun tog ham ved det ene ærme og stak resolut i trav med ham. En kupedør fyldtes straks med en forvirret blanding af større hatte og mindre ansigter. Er her plads til os to? spurte Helga. Vilde jahyl lød indbydende derinde fra. La være med at skrige, rådede Helga; ellers får vi frøken Schou herind. Hvor skal hr. Grandlev sidde? spurte hun, mens hun endnu hang på trinet. Her! lød det adskillige steder fra, og flere brøkdele plads blev gjort ryddelige. Der er bedst ved vinduet, afgjorde Helga. Hun satte sig lige overfor. Så gik toget, og realskolen kørte i skoven. Grandlev forstod ikke, hvorfor han netop skulde være inde hos de store piger; han plejede ikke at være særlig meddelsom uden for skoletiden, men det morede ham i al stilhed at høre på dem, og det havde de måske opdaget. Deres samtale var en lydmasse, der
- 61. uafbrudt varierede, og hvis enkelte ord ikke interesserede ham, men i dens stadige vekslen fandtes udtryk for menneskelig-broget mangfoldighed. I dette virvar af lystighed, bedske hentydninger, der sigtede og ramte; gammelklog og fornuftig passiar og ubetænkt vås, der straks fandt sin skarpt bevæbnede kritiker; af blide og hvasse stemmer, af godt hviskende venindeskab og åbenbar hånlig fnysen mod den, der ikke regnedes med — i dette kaos af frisk fornøjelighed og ugidelige suk, af bløde, klare røster, der lod ane om fremtidig moderlighed, og stemmer, der uden nødvendighed sattes op til kattehyl — i alt det var der spirer til de voksne kvindestemmer, som en gang skulde udvikle sig deraf; hver af dem vilde følge sine tonebeslægtede og senere mere og mere harmonere med dem; de blide og de skarpe, de matte og de friske vilde hver gå sine veje, og de vilde ikke senere komme til at lyde sammen som nu. Men i dette stemmemylr var begyndelsen til alt blandet i eet, og det stemte sammen ligesom en sommermorgens mangfoldige lyde. Og her sad Grandlev og hørte på det, som om det var en naturkonsert, han nød på sin morgentur. Som tanke betragtet var det unægtelig primitivt, men som en eller anden mangfoldighed, der bragte ens sind i sammensatte svingninger, var det af værdi. Det var frisk som alt broget, hvor man ikke behøver at gøre sig ulejlighed med at forstå enkelthederne. Han kom til at tænke på den tid, da han begyndte at få ideen til sin doktorafhandling, der havde noget med sammenlignende sprogvidenskab at gøre. Indtrykkene fra de mange sprog havde været så forfriskende for hans sind, der var blevet træt af adskillige års specialstudier, og nu mærkede han en tilsvarende friskhed ved at lytte til et kor af pigestemmer mellem fjorten og atten; der var ingen slags snak, der lignede denne i stemningens omfang, men han havde aldrig i sine rent intellektuelle dage tænkt, at det kunde ha udbytte for ham at høre på den. Måske var det hans stille høren efter, som han særlig havde lagt sig til, siden han var blevet lærer, der havde givet anledning til hans bekendte studier over visse lydlige fænomener ved det danske talesprog. I så fald havde disse bevidstløst passiarende unge pigers megen ordspild dog haft en videnskabelig betydning; han smilede uvilkårlig og så op.
- 62. Hr. Grandlev sidder og kritiserer, sa Berta, og der blev med et stille. Hvad tænker De egentlig ved at høre på al vort vås? Å, jeg ved ikke; jeg tænker mig nærmest, at sådan må det ha lydt i Babelstårnet, efter at sprogforvirringen var godt og vel kommet i gang. Nå! Ja, men så må De hellere sige os noget andet: Hvad hører der til for at være voksen? Helga tror ikke, hun er det, og Dagmar mener nok, hun er det. Hvad, hvornår er man så voksen? Han mente, det var et relativt begreb, og søgte at forklare det, men det relative interesserede ikke. Jeg tror, Dagmar ved det, sa Berta. Nej, erklærede Dagmar og rødmede. Jo, sa Helga, man kan se det på hende. Dagmar mener, forklarede Berta, at man er voksen, når man kan blive forelsket. Dagmar forsøgte ikke at benægte det; hun rødmede og så ulykkelig ud og vendte blikket mod himlen, hvorved hun igen kom i ligevægt. Det var Dagmars sidste udvej, når hun ikke vidste, hvad hun skulde sige eller gøre: at se mildt til stjærnerne. Jeg synes ikke, sa Helga, at det at være forelsket er noget tegn på, at man er voksen; tværtimod. Ja, hvad ler De af, hr. Grandlev? Jeg lo ikke, jeg så blot i ånden den myndige institutbestyrerinde, som De en gang har i sinde at udvikle Dem til. Er jeg myndig? spurte Helga med en så rolig og kold forundring, at al samtale stansede i nogle minutter. Ja vist er hun myndig, tænkte Grandlev, og da han så udtrykket i hendes ansigt, var han nød til at beskæftige sig med hende resten af kørslen. Det var ham ikke muligt at finde tilbage til sin forrige
- 63. tankegang. Det var ganske vist også dumt udtrykt, gik han videre, idet han forestillede sig, hvordan hun havde opfattet hans bemærkning; der var skrappe lærerinder på skolen, som det måtte være fornærmende at blive sammenlignet med. Han havde blot villet udtrykke sin anerkendelse af hendes væsens sikre og sunde ligevægt, men det måtte jo ventes, at han slap dårligt fra det, da den art overvejelser lå så langt uden for hans speciale. Han vilde gærne glatte på det, men opdagede i det samme, at hun indgød ham en vis respekt, ti hvorfor blev han ellers ilde til mode ved tanken om at skulle gøre hende en slags undskyldning? Et mærkeligt faktum, værd at anholde. Derved kom han til at huske på noget, så at sige noget udestående, han havde med hende. Et par øjne — et simpelt individ vilde sige: et par øjne, hun havde lånt ham, men det var jo slet ikke ham, hun havde set på den gang; henover hans hoved — som om han aldeles intet var — havde hun slynget et stort og fuldt blik langt ud i verden; han blev ganske lille under det; og skønt det ikke vedkom ham det fjærneste, vilde det ikke la sig glemme igen. Han havde tænkt over det og over hende med, hans interesse havde været ham selv påfaldende; han havde forklaret sig sin mærkelige sindsbevægelse over det nye og enestående, der var i at møde sådan et par øjne; der måtte tages billigt hensyn til hans ringe erfaring på kvindeøjnenes område. Det havde været ham overraskende, at hun beskæftigede ham så meget, og han havde omhyggeligt udredet sig alle grundene dertil; han havde skarpsindigt fået meget ud af lidt, hvad der var en af hans videnskabelige specialiteter. Ja. Og hver gang han havde facit — og det var rigtigt —, så var der et par øjne for meget, de kunde ikke skaffes bort. Han så dem til uventede, endog ubelejlige tider. De var så rige på — hvad? De så efter, ja de havde allerede set så meget, som han ikke kendte. De drev ham ud at spasere, når han havde andet at gøre; de fik ham til, i ren videnskabelig utilfredshed, at lægge bøgerne og gå fra dem; nu havde han opdaget, at der var noget, de ikke vidste. Men ret beset var det ikke nogen overmåde intelligent opdagelse. Han tænkte sig resolut alle muligheder; han plejede ikke at gå af vejen for sandheder, ikke en gang for sandsynligheder. Men her
- 64. fandt han ingen grund at bygge nogen hypotese på; han var ikke forelsket, han kunde se på hende med den største koldblodighed. Hendes øjne var udmærkede, men det udtryk havde han ikke set siden, og det var jo det, alting var bygget på. Nå, hvor kunde man så bygge på noget, der måske viste sig en gang om året? Hvis det var hans form for den berømte kærlighed, så kunde den passe sig selv, han skulde ikke tænke på at træffe nogen som helst foranstaltninger i anledning af den. Der var et spørsmål, som han aldrig kunde blive klar over: Gjorde han alt for meget ud af dette attenårige pigebarn, eller var hun mere værd, end han havde nogen anelse om? Det stod ham ikke klart, om det var latterligt, at han var så optaget af en skolepige, eller om hun muligvis var den rigeste af de to og kunde le ad hans lærde fattigdom. Een vished havde han erhvervet sig den dag: Hans øjne var matte og savnede udtryk; de havde set og læst meget, men øjne kunde også bruges til andet, som ikke skulde foragtes; ti var det et sømmeligt resultat af det sete og læste, at han slog sine øjne ned for hendes? Han kunde ikke lide denne tanke, den gjorde ham, der var stærk i kundskab, til den ringere, og dog er kundskab såre god; han havde altid følt sig som den, der i visse henseender er bedre og stærkere. Men ringere var han, og det i en komisk grad. Sad han ikke her og anstillede disse lange betragtninger i anledning af hende, mens hun for længe siden var færdig med ham og nu talte ivrigt om ting, der intet havde med ham at gøre. Men det kunde også blot bevise, at han var den, der tænkte dybest over foreteelserne, og deri var ikke noget mærkeligt. — Da de gik fra stasjonen ad skoven til, kom Helga til at gå ved siden af ham. Nu ved jeg altså, hvad De mener om mig, sa hun lidt besværligt. Han blev behageligt overrasket ved, at hun ikke havde glemt det endnu. Hvad er det egentlig, De sigter til? spurte han. Det, De sa om mig som institutbestyrerinde.
- 65. Nå ja. — Nu var han ikke længere så ivrig efter at forklare hende sin mening med de uheldige ord. Hun var et barn, at hun hængte sig i sådan noget. — Det var ikke ord, jeg la nogen som helst betydning i, Helga. Tænk ikke mere på det. Hun kom til at fryse over hele kroppen. Med sådan et par ord smed han hende langt væk som noget ligegyldigt uden nogen som helst betydning. Å, jeg havde bare tænkt mig, De mente noget med det. Ellers vilde jeg ha sagt Dem, at — —. Men det er jo slemt nok. Hvad gør vi ved det? De ved det vel ikke. Nej. Farvel da. Helga er nok i dårligt humør, sa Berta, da de store piger efter frokosten lå oppe på udsigtsbakken og snakkede og røg cigaretter. Hvem har bildt dig det ind? spurte Helga. Du siger slet ingenting, svarede Berta. Det er fordi, jeg tænker på noget, jeg har oplevet i den skole, hvor jeg gik, før jeg kom hertil. Å, lad os høre det, bad de allesammen. For mig gærne. Bestyrerinden i den skole kaldte vi Sule-Rikke, fordi hun var så fed naturligvis, og så var der også en fedtet, langhåret, bleg teologisk kandidat, som hed Amerikansk Olje. Så en dag skulde vi ha Oljen til historie, og vi blev enige om, at jeg skulde stoppe en sko ind i kakkelovnen, så det lugtede, så der ikke var til at være, for så kunde vi blive fri for ham og få gymnastik i stedet for. Men om forladelse, Sule-Rikke fik det hoved på, at hun vilde ha os i første time, og hun var ellers ikke til at gøre sjov med. Men jeg havde jo været der med skoen, og hun sad på katedret og gloede og snusede og gjorde ved, og så sa hun, hvad det var for en lugt, men det var der jo ingen, der vidste.
- 66. Så blir I her en time i eftermiddag og lige sådan i morgen, til jeg får at vide, hvad det er, og hvem der har lavet det. Og så op med vinduerne, og dær sad vi. Sule-Rikke hun hentede sig et uldent sjal, så var hun klaret. Vi gumlede noget på dette her, og der kom sedler hen til mig, at jeg skulde sige, hvordan det hang sammen. Ja, ja, tænkte jeg, når I ikke holder bedre sammen, så skal jeg nok huske jer til en anden gang, og så rejser jeg mig og siger, at det er mig, der har puttet en sko i kakkelovnen. Vil du så ta en ildtang og ta den ud med? sa Rikke. Jeg fiskede skoen op og bar den ud, så røgen stod omkring. Hun gabede på mig hele tiden, men sa ingenting. Hun skældte aldrig ud. Var der flere med til det? spurte hun så. Det er mig alene, der har fundet på det, sa jeg. Det vil jeg gærne tro, sa Rikke og så hel venlig ud af arrigskab. Så kan Helga sætte sig ned på sin plads igen. Den er god, tænkte jeg, og så gik vi videre i fransken. Jeg blev hørt, men jeg kunde det så godt, at Sule-Rikke blev violet af ærgrelse. Men da frikvarteret kom, sa hun til mig: Det er jo kedeligt, at du ikke kan blive flyttet op i næste klasse, men det er din egen skyld. Bagefter kom de andre og skulde være medlidende, men det skulde jeg naturligvis ikke ha noget af. I timen efter fik vi så Amerikansk Olje, og Rikke gik ude på gangen og lurede, for der var tit spetakler i hans timer, men den gang var der nu ingen. Det var heldigvis koldt den dag, så hun måtte trave godt til for at holde varmen, og lidt før timen var forbi, kom hun ind og spurte, om der havde nogen været uartig, og skævede hen til mig, men der havde nok ingen været, og om nogen ikke havde kunnet sin historie, men Oljen mente ikke, der var nogen. Og Rikke kom hen og skulde høre mig om igen, men det
- 67. fik hun ikke noget ud af, så hun måtte nøjes med at glo på mig. Siden den dag var jeg ikke mere med til at lave kunster, de andre måtte gøre det alene, og på den måde gik det efterhånden i stå. Det var såmænd ikke morsomt. Til sidst en dag holdt Sule-Rikke en lovtale over mig, hvordan jeg havde forbedret mig, det var da hendes værk, og det var hun stolt over, for i grunden holdt hun så meget af mig, især nu da jeg var sådan, som en sød, ung pige skal være. Men det var endnu værre, at jeg var kommet i kridthuset hos Amerikansk Olje, det var siden han kunde styre mig, det var han naturligvis forfærdelig taknæmlig for. Han smilede altid så fedtet til mig og vilde gærne røre ved mig med sine oljefingre. Han var alligevel fælere end Rikke. Hvorfor det? spurte Berta. Å, han var da, eller skulde ha været et mandfolk. Han friede til mig, det vil sige, han var ved at begynde på det. Jeg skulde ha været Fru Amerikansk Olje. Jeg skulde komme til ære og værdighed. Uha, jeg føler mig hel fedtet endnu, når bare jeg tænker på ham. Dagmar gik alene inde i skoven med sænket hoved og de foldede hænder hængende ned foran sig; hun vidste ikke, hvor hun var, og hun vilde heller ikke sætte pris på, om nogen kom og fortalte hende det, medmindre det var en bestemt, som hun nok vidste. Egenlig legede hun røvere og soldater og var selv røver, men det havde hun glemt for længe siden. Hun kom til at tænke på det, da hun så en skikkelse rejse sig bag et træ, og hun følte, at hun ikke gjorde sin pligt. Da hun fik løftet sine vemodige øjne op fra skovbunden, opdagede hun, at skikkelsen var Helga. Ser jeg forbrølet ud? spurte hun. Har du grædt?
- 68. Nå, du kan altså ikke se det. Er jeg grøn om munden? Jeg tror nok, jeg har tygget græs. Lad os gå hen til en grøft og vaske os. Hvordan er det, du ser ud, Helga? Jeg er vel ikke køn i dag? Nej, jeg vil gærne tro det. Hvad har du bestilt? Ja, nu leger vi jo. Ellers har jeg talt med hr. Grandlev. Om kærlighed? Sa du, at du er forelsket i ham? Nej, svarede Dagmar smigret og flov. Det er jeg da heller ikke. Du skulde bare ha sagt ham det. Så skulde du se — — Hvad så? Å, Helga, sådan noget snak. Så havde han vredet halsen om på dig, for han kan ikke lide dig. Men Helga, hvad er der i vejen? Uha, lad være. Jeg er ikke oplagt til at være kælen. Det er også noget vrøvl; han elsker dig næmlig, han har selv sagt mig det, for mig fortæller han alting. Det er ikke sandt. Skal vi gå hen i grøften og drukne dig af ulykkelig kærlighed? For du tror ikke, det var løgn, hvad jeg sa, og det er det desværre. Jeg kan holde dig i benene så længe. Kom så. Hodet først. Dagmar lo gennem tårer, og det klædte hende. I dag er du ikke rar. — Hun smilte undskyldende i anledning af det stærke udtryk, hun brugte. Ja, du blir hel grim i dit pæne ansigt af at se på mig, og jeg blir endnu værre af at se på dig. Når jeg en aften møder dig og Grandlev nede ved fjorden, skal jeg smide jer i vandet, og når I så er blevet begravede i een grav, så sætter jeg mig oven på som en kulsort ugle
- 69. og tuder og gnistrer med øjnene, og hver nat går der et menneske fra forstanden af skræk ved at komme forbi og se mig. Skal vi ikke gå tilbage til de andre, Helga? Nej, jeg vil blive her og øve mig i at tude. Når I blir gift, skal I ha et gæsteværelse stående, dær skal tante Helga bo, når hun kommer og besøger jer. Gå så med dig. Må jeg ikke blive lidt hos dig, Helga? Jeg trænger sådan til at tale med dig, hvis du bare vil være fornuftig ligesom et andet menneske. Nej. Jeg skal også hen og brøle noget mer, så jeg har ingen brug for dig. Men hun græd ikke, da Dagmar var borte. Hun satte sig på et gærde i kanten af skoven og så ud over en mark og tænkte: Hvad så? Langs en bivej gennem marken stod en række gamle, stævnede popler, hvis forunderlige hæslighed pludselig blev hende indlysende. Der strømmede noget igennem hende, noget velgørende, der spændte og trykkede; begyndelsen til et smil åbnede hendes læber. Et ujævnt geled af lemlæstede kroppe, fulde af knuder, der gærne kunde være ar efter sygdomme, der havde ædt lemmerne bort. En sørgelig allé af spedalske, der rokkede fremad mod et sted, hvor der kunde tænkes at være en almisse at få. De krummede sig allesammen i en eller anden retning, som om de lyttede efter noget, men deres stakkels hjælpeløse udtryk fortalte, at de intet opfattede. De var berøvede alt, indtil det blotte skrog; nogle kummerlige duske med grimme, gule blade strittede til værs, det var deres føletråde, det eneste, de havde tilbage, hvormed de kunde skønne sig til, hvad der foregik omkring dem. De viftede ikke med deres grene som andre træer, de ytrede ikke noget med deres blade, og hvis de raslede et øjeblik, så lød det ganske skeletagtigt, ikke som en livsytring. Helga var betaget af dette billede på stum og blind elendighed; hun følte, at tilværelsen rummede meget, hun endnu ikke anede, men
- 70. som hun burde kende, som en gang vilde åbenbare sig, og medens hun sad dær ubevægelig i en tung og rig stemning, havde hun en stærk foruddannelse om kommende rigdom. — — Råb inde fra skoven vakte hende; hun sprang op og så sig undrende om. Hun gik lidt og kunde ikke rigtig orientere sig i sine egne stemninger. Nu da hun gik tilbage ad den samme sti, forekom det hende næmlig, at hun egenlig var i dårligt humør, men ved hjælp af lidt eftertanke viste det sig, at hun bare havde båret sig temmelig tosset ad over for Dagmar, hvad der så var gået af hende, men det var ellers ligegyldigt. Hun lod sine fødder slæbe gennem løvet, og på hendes ansigt var et stillestående smil, mærket af et lykkeligt øjeblik, der er blevet koldt og endnu holder sindet fast i en erindrende stivnen. Hun bar sit legeme fremad, som om det var en uvedkommende byrde, der skulde med, og som for resten heller ikke vejede noget. Inden hun nåede det sted, hvor de skulde samles, kom hr. Jensen hende i møde. Hvor har du været, Helga? Dær henne ad; hun gjorde et kast med hodet, der meget ubestemt angav retningen. Jeg kan ikke forstå, du er rendt fra os andre. Er det virkelig for at undgå mig, at du gør det? Hvad har du taget dig til? Men ikke for alverden kunde det falde Helga ind at fortælle, at hun havde set på gamle poppeltræer. Måske var der eet menneske — ja måske; men at han her kom netop nu og muligvis vilde fri om igen, det var sandelig groft. Hun fik en anelse om, hvor tosset det ene undertiden kan følge efter det andet her i verden. Derfor vendte hun sig og gik rask ind ad mødestedet til. Men så hændte det samme om igen i form af frøken Schou. Tillader du, jeg følges med dig?
- 71. Helga så forbavset på hende. Du spaserer meget med hr. Jensen, sa frøknen, rolig i bevidstheden om, at hun her kunde møde med moderlig overlegenhed og fortjente at blive påhørt med taknæmlighed. — Jeg vil bare gøre dig opmærksom på det; du har vel som sædvanlig ikke tænkt over det, men det er lidt påfaldende, folk taler om det. Helgas tidligere stemning mødtes her med fornæmmelsen af stor plathed, men resultatet af blandingen var ikke færdigt; hun tav. Du er vel ikke fornærmet? Nå, det har du da heller ingen grund til; men hvis du for en gangs skyld skulde ha lyst til at overveje, hvad jeg siger, så vil du indse, hvorfor denne spaseren ikke er heldig. Kampen i Helgas sind endte med lystighed; hun lo uden at lægge bånd på sig, indtil hun fik kvalme ved at se frøknens ansigt, hvor et samtidigt udtryk af overlegenhed og underlegenhed frembragte en besynderlig og flov virkning. Også hende løb Helga fra. Hun var egenlig både vred og afskyligt til mode, da hun nåede pavillonen; det var slet ikke morsomt alligevel, selv om hun havde let. Hun hadede menneskene, sådan som de kom den ene efter den anden — —. Grandlev rejste sig fra et af de små borde. Men hvad er der dog i vejen med vor Helga? sa han. De ser jo helt ulykkelig ud. Helga blev varm. Det var så velgørende, at han straks så, at der var noget, der trykkede hende. Og med det samme var det væk. Der er ingenting, sa hun og så op på ham. Nå nej; jeg tror virkelig heller ikke. De er nok rask til at skifte ansigt. Ja, heldigvis, svarede hun overgiven. Men skal vi ikke danse, hr. Grandlev?
- 72. Jo, vi skal danse. Kom De bare med mig.
- 73. KAPITEL 5 Eksamen nærmede sig. I fjerde klasse fik den engelske grammatik en sidste overhaling. Eleverne havde nu opgivet alle slags løjer; hvis der forefaldt noget morsomt, var det kun i anledning af et dumt svar; ellers gik det ganske fagmæssigt til; enhver var optaget af at få med, hvad der ikke tidligere var blevet forstået, eller fastslå det kendte for sidste gang. Kun Helga var udeltagende; hun var blevet så underlig i den senere tid; hun vilde ikke gå aftenture med Dagmar, ikke repetere sammen med Berta, og i timerne lukkede hun ikke sin mund op. Grandlev undrede sig også over, at hun ikke svarede sammen med de andre. Forstår De det ikke, Helga? spurte han. Hun stivede sig af mod rygstødet og så op med panden fuld af rynker. Det gør jeg nok, sa hun. Hvad vil det sige? Det ved jeg ikke. Hm, De har tid til at være uhøflig. Lad os håbe, det morer Dem. Hun følte den trods, der steg op i hende, som noget velgørende. Bare han vilde håne hende rigtigt; hun trængte sådan til noget, der var bittert, og som sved. Vær så venlig at sige, om De forstår det, jeg gennemgår. Hvis det anstrænger Dem at benytte en høflig tone — hvad det vistnok gør
- 74. for tiden —, så svar, som det er Dem bekvemmest. Det ved jeg ikke; det har jeg slet ikke tænkt over. Han stod foran hende med den engelske grammatik i hånden. De forbavser mig unægtelig, skønt jeg har dårlig tid til at være forbavset over Dem. Er det sådan, De for fremtiden skal være? Er det med det sind, De skal gå ud i verden og — nå et eller andet mål? For nogen øjeblikkelig grund til Deres opførsel gives der jo ikke. Helga følte alvoren i hans stemme; men hun vidste ikke, hvor langt hans spørsmål egenlig rakte; der kunde ligge så meget i det. Havde hun kun turdet se på ham. Hun kunde blive god, hvornår det skulde være, dersom hun blot vidste — — men måske ønskede han bare at ydmyge hende. Nå, sa han; det var ellers ikke Helgas fremtid, der interesserede os; vi vil hellere gå over til de ubestemte stedord. Dermed var hun afskåret fra at sige mere, og hun var dog endnu ikke mættet i trods, hun var endnu ikke tilfredsstillet i ydmygelse. Hvordan skulde han forstå det? Han måtte jo tro, hun bar sig sådan ad for at være uartig mod ham, og så var det af en hel anden grund. Hvor var det ikke dumt, at hun var så gammel, at han ikke kunde rykke hende i håret. Nu måtte hun se op på ham, mens han var optaget af grammatikken — og så mødte hun hans blik. Det var ikke vredt eller bebrejdende, men søgende, uundgåeligt; hun tog ikke sine øjne til sig, det var ikke hendes sædvane, men dette blik ramte hende som en fysisk smærte; ingen havde nogen sinde set på hende med den magt i sine øjne, det trængte så voldsomt ned i hende, og da hun straks efter lukkede øjnene, blev det ligesom siddende der nede, det tog en uhyre plads op, hun var ikke længere sin egen herre og kendte ikke rigtig sig selv igen. Hun blev så træt over hele kroppen, og hun frøs. Dersom hun nu havde været hjemme, kunde det ha været dejligt at ligge på en sofa og blive ved at ha det som nu. Imidlertid gjorde hun
- 75. sig det bekvemt dær, hvor hun sad, og hun blev helt ked af det, da det ringede, og hun måtte af sted. Grandlev blev tilbage; måske vilde han tale med hende, irettesætte hende, eller hvad det nu var; hun skyndte sig i rystende hast med at samle bøgerne, det kunde ikke gå an, at han talte til hende; hun havde ikke spor af herredømme over sig selv. Hvad kunde der ske? Men hun tabte bøgerne; den ene af eleverne gik efter den anden, de blev alligevel ene. Hun havde aldrig været så bange. Hvis der er noget, De ikke forstår, Helga, sa han tæt ved siden af hende, så vil jeg naturligvis gærne hjælpe Dem. Jeg forstår godt — — —. Hun forsøgte at rynke panden, men den glattedes straks igen ud af sig selv, så træt var hun. Nu var hun bunden af hans stemme, kunde ingen steder komme. Hun anede ikke, hvad det kunde falde hende ind at gøre om et øjeblik, hun var umyndig over sit eget sind. Men så opgav han hende heldigvis og gik. Det kan da vel ikke være meningen, tænkte han, mens han gik, at jeg skulde være forelsket i dette halvvoksne pigebarn, hvis kultur er så yderst primitiv. Helga løb hjem, hele vejen, det varede fem minutter. Hun vilde ligge på sofaen med ansigtet nedad og lukke øjnene og blive borte. Men som hun løb, fik hun smag for hurtig bevægelse; den passede alligevel bedre; hun smed bøgerne ind ad døren og gik en lang tur. I begyndelsen var det dog tungt at gå. Det var ligesom egnen om hendes hjærte var fyldt med vådt sand, men det blev lettere under gangen; hun mærkede, hvordan byrden svandt ind i omfang og tyngde, til sidst var der i dens sted noget, der bar oppe; nu kunde hun slet ikke blive træt. Det vilde gøre godt at ha mange grøfter at springe over lige i træk. Det skulde blot ha været i hendes barndom nu, på den store eng hjemme ved gården.
- 76. Det var så godt og frisk, sommeren var så ny endnu. Vinden puslede varmt i hendes nakkehår, som vilde den lege, hun kom til at smile ved det. Der var i hendes sind en smærtelig højtidelighed, der gjorde, at solskinnet så helt anderledes ud, end det plejede; men det undrede hende slet ikke, at verden således havde skiftet udseende. Hun havde det, som den der har oplevet noget stort og afgørende, og som derefter ser på tingene med andre øjne. Men det vigtigste var, at der i dag var sket noget, der bestemte hendes fremtid, lige meget hvad der ellers fulgte på, lykke eller skuffelse. Nu bar hun noget i sin sjæl, som en anden havde lagt deri, måske kunde ingen af dem gøre for det, men det var dog sket, som ikke kunde være anderledes. Hun bøjede hodet med stolt og hemmelig undseelse: Hun var ikke længere sin egen; men til gengæld havde hun del i en anden, som muligvis slet intet anede om det hele. Det første hun sansede, da hun kom hjem, var, at hun var forfærdelig sulten, og da der var rådet bod på det, følte hun sig egenlig meget ulykkelig; løftelsen var borte. Men nu skulde der læses leksjer, så kunde der væres ulykkelig bag efter. Hun samlede de bøger sammen, der skulde bruges, og ordnede dem metodisk i en bunke, de sværeste øverst, de andre aftagende i vanskelighed ned efter, og så tog hun fat, læste undertiden højt, hvis det kneb med opmærksomheden, og tiden gik. Den gik måske ikke let, men moeren kunde ingenting mærke på hende. Ved hendes højre albu steg bøgerne til værs; det blev en hel bunke, den kunde hun. Aftensmaden tog ti minutter, hun repeterede indvendig, mens hun spiste, og gik videre, som om der intet ophold havde været. Klokken slog otte. Otte. Så gik han tur. Helga blev varm og god, hun stansede midt i et uregelmæssigt verbum og besluttede, at hun vilde altid være rar mod ham, hun vilde begynde straks, nu i aften og derefter blive ved. Hun måtte ud og træffe ham.
- 77. Hun blæste de udpillede øjenbryn af bogen, fik tøjet på og løb ned ad trappen. Ved det næstnederste trin stansede hun og kom i tanke om noget: Nu gjorde hun jo netop det, hun var blevet enig med sig selv om, at hun ikke vilde gøre. Den bestemmelse havde hun altså glemt, endskønt den ikke var ret mange timer gammel. Hun kunde næmlig ikke være bekendt at nærme sig ham efter den opførsel, hun havde vist. Hun rystede på hodet, idet hun gik, hun syntes absolut ikke om sig selv, og hun var ikke rigtig glad mere. Da hun gik igennem gaden, så hun sky og uvillig på folk, hun mødte; mon de allerede vidste noget? Nu var hun da hjælpeløst forelsket; hun så op og smilede, det var i grunden så sørgeligt. Uden for byen var der desværre tre veje, som der kunde være tale om. Hendes øjne blev tørre, mens hun spekulerede over, hvilken af dem hun helst skulde prøve. Den i midten havde måske mest sandsynlighed for sig, fordi den var en slags middelvej, men hun kunde ikke bestemme sig, hun fik hjærtebanken af angst, mens hun tøvede, og samtidig med, at hun bag ved højtideligheden fandt sig selv latterlig, forekom det hende, at der efter dette valg fulgte noget, der ikke kunde gøres om. Hun valgte den vej til højre og gik til med lange, hurtige skridt. Men mens hun derved fjærnede sig fra de to andre veje, de to andre muligheder, og fik lyst til at vende om og prøve en af dem, indså hun, at det var unyttigt altsammen; hun kunde derfor lige så godt blive på den vej, hvor hun var. Hvad var det for en forventning om store ting, der havde drevet hende hjemme fra? Nu viste der sig intet andet end den slappeste kedsommelighed, og det blev heller ikke helt godt, fordi hun opdagede ham langt borte, ti da den første hjærtebanken var gået over, forstod hun jo nok, at der kunde ikke ske andet, end at han gik forbi hende og tog hatten af. Hvorfor havde hun ikke kunnet tænke sig det, da hun sad hjemme? Men hun var nu en gang komisk den aften, og siden bevidstheden derom imod sædvane ikke gjorde hende mere fornuftig, så kunde hun gærne blive ved. Hun vendte derfor om og gik så langsomt, at han var nød til at indhente hende.
- 78. Endnu kunde hun af og til se sig om efter ham; han var ikke langsynet, men hun måtte dog være forsigtig, og der forløb en uhyre tid med mange skiftende stemninger, fra det øjeblik hun sidste gang vendte sig om, indtil hun hørte hans trin bag ved sig. Godaften, Helga, sa han alvorligt og venligt, idet han gik forbi. Så kunde hun gå hjem og i seng og spekulere over det; hun glædede sig til det, men det var ikke så rart, som hun havde tænkt sig, for nu kunde hun hverken løbe fra det eller læse sig fra det, hun var hjælpeløs midt i det. Nu kom også de øjne igen; de var ikke ret strænge eller hårde, men de var så stærke, de havde sådan en magt over hende. Ingen havde sådan regeret over hende som de øjne. Hun vred sig i sengen, så hendes underlagen blev som et tov, men hun slap ikke fra de øjne. Hun måtte hellere se at forlige sig med dem. Hun tænkte på den sidste skovtur; de var inde at danse Lanciers sammen. Han stod med hende i hånden, når det ikke var deres tur; hver gang de havde været ude, beholdt han hendes hånd. I de sidste ture rystede den forfærdeligt, og hun vidste ikke, hvad hun skulde gøre; hun turde ikke trække den til sig. Hun prøvede at gøre den stiv, hun strittede med fingrene, men det kunde ikke gå an; desuden blev den straks ustyrlig igen. Men han opdagede ingenting; det kunde hun skele sig til. Hvor de havde haft det godt med ham i den første tid, Dagmar, Berta og hun. Han snakkede så tit med dem i frikvartererne, og når han var i godt humør, kunde han drille meget morsomt. Men det var forbi. Når eksamen var overstået, skulde han forloves med Dagmar. Helga brød han sig ikke om, vilde ikke en gang skænde på hende. Han kunde vel ikke se andet, end at hun var uartig; det var alt, hvad Helga var. Det var rigtigt af ham, at han foretrak Dagmar, for hun var den bedste af de to, men han burde blive ved med at bryde sig lidt om hende ligesom før. Nu da der slet ingenting var, syntes hun, at lidt
- 79. var så uhyre meget. Måske kunde det lidt være blevet til mere, hvis hun havde gjort noget for det. Flere andre mænd, som hun bare havde ladet være at støde bort, var blevet forelskede i hende — — — Hvad var det? Hun var så træt i benene; hun trak dem op under sig og mærkede, hvordan blodet strømmede ned igennem dem; hun havde nok som sædvanlig stemmet dem mod fodenden af sengen. Og mens hun nu lå, så hun med eet hans ansigt for sig, så tydeligt som om han virkelig stod ved hendes seng. Hun forstod, at dette var en slags gave, hvor den så kom fra; hun lå ganske stille og nød det og var helt glad. Han viste sig i profil; hun kendte godt den flip, han havde på. Hver eneste småting var der, også den lille brune plet ud for øret. Udtrykket var det, han havde, når han stod ved vinduet og fulgte med i bogen; øjnene var sænkede. Hvad mon det var for en dag, hun sådan havde stirret på ham? Hun vidste, at det ansigt havde hun fra en bestemt lejlighed. Det blev stående, det forsvandt ikke straks igen, men det kunde jo forsvinde, inden hun anede noget. Hun sprang ud af sengen, tændte lampe, hentede karton og blyant og satte sig op i sengen og tegnede med sit atlas over knæene. Panden og håret kom straks, næsen lykkedes, skægget var også let nok, hagen tegnede hun uden at tænke over det. Men øjnene — —. Hun lukkede sine egne; jo hun havde dem så nøjagtigt for sig. Et par raske streger — — nu var det allerede ham. Hun trykkede hænderne ind mod sit bryst, hun blev så beklemt og overvældet — —. Øjet blev færdigt, det så eftertænksomt nedad, rynkerne strålede i et fint knippe ud fra øjenkrogen. Hun kunde kæle for det, men det skulde først være færdigt. Hun lavede resten af hodet, håret gik ikke ret langt ned på halsen, det var så pænt. Og øret var fint; han havde ikke disse gule, magre læderøren, som så mange mænd har, de var fyldige og sad passende tæt ind til hodet. Hun pressede papiret ind til sig, men tog det straks ud igen og så efter, om det var knækket. En lille fold på kinden gik hun efter med
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