Design and simulation of a multiple effect evaporator using vapor bleeding

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Design and simulation of a multiple-effect evaporator using vapor bleeding
A thesis submitted in partial fulfillment of the requirements for the degree of
Bachelor of Technology
In
Chemical Engineering
By
MonalishaNayak
Roll No. 108CH001
Under the guidance of
Dr. ShabinaKhanam
DEPARTMENT OF CHEMICAL ENGINEERING
NATIONAL INSTITUTE OF TECHNOLOGY, ROURKELA
DATE OF SUBMISSION: 07.05.2012

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ACKNOWLEDGEMENT
I express my deep gratitude to my guide Dr. Shabina Khanam, for her ever- lasting patience and inspiration which led me through this project work.
I would also like to acknowledge the contribution of the Department of Chemical Engineering as a whole, and Dr. R.K. Singh, HOD, Chemical Engineering, for having provided us with all the necessary facilities.
MonalishaNayak
Roll No. 108CH001

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CERTIFICATE
This is to certify that the project entitled “Design and simulation of a Multiple effect
evaporator using Vapor bleeding” is an original work done by MonalishaNayak, Roll No.
108CH001, under my guidance and supervision.
DEPARTMENT OF CHEMICAL ENGINEERING,
NATIONAL INSTITUTE OF TECHNOLOGY, ROURKELA
Dr.ShabinaKhanam
Assistant Professor
Department of Chemical Engineering
National Institute of Technology, Rourkela

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LIST OF FIGURES Page no.
Fig.2.1. Schematic diagram of a single-effect evaporator 16
Fig.2.2. Long tube vertical evaporator 19
Fig 2.3.Climbing film evaporator 20
Fig.2.4. Falling film evaporator 21
Fig.2.5. Forced-circulation evaporator 22
Fig.2.6.Plate-type evaporator 23
Fig.3.1. Schematic diagram of triple-effect backward feed evaporator 29
Fig.3.2. Schematic diagram of five-effect forward feed evaporator 30
Fig.4.1. Block diagram of a single effect in a multiple-effect evaporator 31
Fig.4.2. Schematic diagram of a three-effect backward feed evaporator 34
with bleeding

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Fig.4.3. Schematic of the five-effect forward feed evaporator without bleeding 35
Fig.4.4.Schematic of the five effect forward feed evaporator with four stage 36
pre-heating
Fig.4.5. Schematic of the five-effect backward feed flow without bleeding 38
Fig.4.6. Schematic of the five-effect backward flow evaporator with vapor 39
bleeding
Fig 5.1.Schematic of the three-effect evaporator with specified variables 48
Fig. 5.2. Schematic of triple-effect backward feed evaporator with vapor bleeding 50
showing the different variables
Fig.5.3. Schematic of five-effect forward feed system showing the different 53
variables
Fig.5.4. Schematic of the five-effect forward feed evaporator with bleeding 54
Fig.5.5.Schematic of the five-effect backward feed evaporator showing the 56
different variables
Fig.5.6. Schematic of the five-effect backward feed flow system with bleeding 58

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LIST OF TABLES Page no.
Table 3.1. Operating parameters for a three-effect backward feed problem 28
Table 3.2. Operating parameters for the five-effect evaporator 29
Table 5.1. Iterative results for the simple three effect system 47
Table 5.2: Values of different variables for three effect evaporator system 48
Table 5.3. Values of different variables in the triple-effect backward feed 49
evaporator with bleeding
Table 5.4. Values of variables in the five-effect evaporator system 52
Table 5.5 Values of variables in the five-effect forward feed evaporator system 54
with bleeding
Table 5.6.. Values of variables in the five-effect backward feed evaporator system 56
Table 5.7.. Values of variables in the five-effect backward feed evaporator 57
system with bleeding

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NOMENCLATURE
The following nomenclature is used in the development of model equations:
Symbol
Parameter
Unit
F
Feed flow rate
Kg/s
xf
Concentration of feed by weight
-
Tf
Feed temperature
˚C
Ti
Temperature of ith effect
˚C
Li
Liquor flow rate from ith effect
Kg/s
xi
Concentration of liquor coming out of ith effect
-
Cpi
Specific heat capacity of liquor leaving ith effect
KJ/Kg K
ɽi
Boiling point elevation of liquor in ith effect
˚C
hi
Film heat transfer coefficient of liquor in ith effect
KW/m2K
Øi
Heat flux in ith effect
KW/m2
ᴫi
Linear velocity of liquid on tubes in ith effect
Kg/m.s
Vi
Flow rate of vapor coming out of ith effect
Kg/s
V0
Steam flow rate
Kg/s
T0
Steam temperature
˚C
λ0
Latent heat capacity of steam
KJ/Kg
λi
latent heat capacity of vapor leaving ith effect
KJ/Kg
Vbi
Flow rate of bleed stream from Vi
Kg/s
h
Specific enthalpy of liquid phase
KJ/Kg
H
Specific enthalpy of vapor phase
KJ/Kg
Ai
Heat transfer area of ith effect
m2

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ABSTRACT
The objective of this report is to develop a model for a five-effect evaporator system. Vapor bleeding is incorporated in the model as a means to reduce the amount of steam consumed in the evaporator. Since evaporation is the most energy-intensive stage in any industrial operation, measures to reduce energy consumption in the evaporator-house are greatly beneficial towards making an operation cost-effective. Other energy reduction schemes like condensate, feed and product flashing, vapor compression etc. are also available. Vapor bleeding brings about an increase in the steam economy of the process, but at the added cost of the required heat exchangers.
A model for an evaporator used for the concentration of sugar solution is developed using a set of non-linear equations derived from the mass and energy balance relations. These equations are then solved using the Newton-Raphson method by developing a matlab code for the same. For the present system steam requirement without vapor bleeding is computed first. Other variables like effect temperatures and liquor flow rates are also a result of the modeling procedure. The same system is then modeled by incorporating vapor bleeding for which steam requirement is also found. Additionally, the purchase and installation cost of the heat exchangers required are computed.
Vapor bleeding brings an improvement in the steam economy of a process by 26% but at the added cost $1,50,291 of the five heat exchangers. Annual savings of $43,200 in the cost of steam is achieved. The payback period of the modified design of five effect evaporator system is found as 2 years 5 months and 12 days.

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CONTENTS
CHAPTER ONE: INTRODUCTION .............................................................................. 12
CHAPTER TWO: LITERATURE REVIEW .................................................................. 15
2.1. Different types of evaporators .............................................................................................. 15
2.1.1.Horizontal tube evaporators ........................................................................................... 16
2.1.2. Horizontal spray-film evaporators .................................................................................. 16
2.1.3. Short tube vertical evaporators ...................................................................................... 17
2.1.4. Basket type evaporators ................................................................................................. 17
2.1.5.Long tube vertical evaporators ........................................................................................ 18
2.1.7 Climbing film evaporators ............................................................................................... 19
2.1.8. Falling film evaporators .................................................................................................. 19
2.1.9. Rising falling film evaporators ........................................................................................ 20
2.1.10 Forced circulation evaporators ...................................................................................... 21
2.1.11. Plate type evaporators ................................................................................................. 22
2.2. Modeling of a multiple effect evaporator system .................................................................. 22
Equation-based model ............................................................................................................. 22
Cascade algorithm ................................................................................................................... 23
2.3. Energy Reduction Schemes(ERSs) in evaporators. ................................................................. 24
2.3.1. Flash evaporation ........................................................................................................... 24
2.3.2. Vapor compression ........................................................................................................ 24
2.3.3. Vapor bleeding ............................................................................................................... 25
2.4. Sugarcane juice ..................................................................................................................... 26
CHAPTER THREE: PROBLEM STATEMENT ........................................................... 27
3.1. Problem I .............................................................................................................................. 27
3.2. Problem II ............................................................................................................................. 28
CHAPTER FOUR : DEVELOPMENT OF THE MODEL AND SOLUTION PROCEDURE ................................................................................................................... 30
4.1. Model development for a particular effect ............................................................................ 30
4.2. Model development for the three effect backward feed evaporator ..................................... 32
4.3. Model development for the three effect backward feed evaporator with bleeding ............... 33
4.4. Model development for the five- effect forward feed evaporator without bleeding .............. 34
4.5. Model development for the five-effect forward feed evaporator with bleeding .................... 35

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4.6. Model development for the five-effect evaporator operated in backward feed flow without bleeding ...................................................................................................................................... 37
4.7. Model development for the five-effect evaporator operated in backward feed flow with bleeding ...................................................................................................................................... 38
4.8. Solution of the model: Newton Raphson method [2] ............................................................. 40
4.9. Empirical correlations for specific heat, boiling point rise, latent heat and heat transfer coefficient ................................................................................................................................... 40
4.10. Detailed iterative solution procedure of the three-effect backward feed evaporator .......... 41
4.11. Detailed iterative solution procedure of the five-effect backward feed evaporator with bleeding ...................................................................................................................................... 43
CHAPTER FIVE: RESULT AND DISCUSSION ........................................................... 45
5.1. Three effect evaporator operated in backward-feed flow sequence ...................................... 45
5.2. Three-effect evaporator system with vapor bleeding ............................................................ 48
5.2.1 Associated costs .............................................................................................................. 49
5.3. Five-effect forward feed evaporator ..................................................................................... 51
5.4. Five-effect forward feed evaporator with vapor bleeding ...................................................... 52
5.4.1 Costs computation .......................................................................................................... 54
5.5. Five effect evaporator operated in backward feed flow without bleeding ............................. 54
5.6. Five-effect backward feed evaporator with vapor bleeding ................................................... 56
CHAPTER SIX : CONCLUSION .................................................................................... 58
REFERENCES .................................................................................................................. 59
APPENDIX ........................................................................................................................ 61

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CHAPTER ONE: INTRODUCTION
Evaporators are used in a process industry to concentrate solutions consisting of a non- volatile solute and volatile solvent, like foods, chemicals, etc. In a majority of the processes, the solvent is water. Dilute solutions contain a large amount of water, and the cost of processing such solutions involves a high equipment cost. The evaporation process proceeds by evaporating a part of the solvent from the solution to increase its concentration. This evaporation is carried out by using steam as the heating medium. Normally, the thick liquor obtained is the valuable product. The material of construction of an evaporator may be any kind of steel. Special materials like copper, stainless steel, nickel, aluminium may be used depending upon the specific properties of the solution to be concentrated.
When a single evaporator is used for concentration, the vapor issuing out of it is condensed and discarded. This type of operation is called single-effect evaporation. When a number of effects are used in series, such that the vapor coming out of one effect is used as a heating medium in the steam chest of the next effect, it is called multiple effect evaporation. Single effect evaporation is simple but fails to utilize the steam effectively, while multiple effect evaporators evaporate more quantity of water per kilogram of steam consumed in the evaporation process. This brings about a saving in the steam cost, but at the same time, the cost of material and installation of the evaporator system increases because of the large number of effects involved. There has to be trade-off between the material cost and the steam cost and the number of effects giving the minimum total cost is the most thermo-economic solution. The maximum number of effects in a multiple effect evaporator usually should not exceed seven, because beyond this, the material and installation cost of the evaporator effects offset the saving achieved in the steam cost.

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The various process industries where evaporation plays a pivotal role are food and pharmaceuticals, pulp and paper, sugar, chlor-alkali, desalination of water etc. The most important application of evaporators is in the food and beverage industry. Evaporation of water from foods and beverages like milk products, fruit juices, various extracts, enables them to last for a longer period of time or helps in maintaining the required consistency, like in case of coffee. Evaporation eliminates excess moisture from pharmaceutical products, thereby improving product stability and enabling easy handling of the product. Preservation of long-term activity and stabilization of enzymes is brought about by evaporating excess moisture. In the pulp and paper industry, sodium hydroxide is recovered in the Kraft process by evaporation.
In an air-conditioning process, evaporation is used to allow the coolant, Freon, to evaporate from liquid to gas while absorbing heat. All industries must dispose off any waste after processing and methods for waste-handling are costly. By removing moisture through vaporization, there is a reduction in the amount of waste to be handled and thus the waste- handling cost incurred by any industry.
Evaporation is a highly energy-intensive process. It consumes a considerable portion of the energy consumed by the whole process. There is a great scope for cutting down the costs involved in an evaporation operation by reducing the live steam requirement in an evaporation operation. Efforts have been made by many researchers such as Khanam et al.(2010), Jorge et al.(2010), to cut down the steam consumption in a multiple-effect evaporator by different operating strategies like feed, condensate and product flashing, vapor compression, vapor bleeding, feed and steam splitting or using an optimal feed flow sequence.

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Harper and Tsao (1972) carried out optimization of multiple effect evaporator and they modified the feed flow pattern. Their work was extended by Nishitani and Kunugita (1979) who considered all possible feed flow sequences to optimize a MEE system. All the mathematical models are solved by developing a set of non-linear equations originating from the corresponding mass and energy balance relations. When the operating strategy of a system is changed, a whole new set of equations results. This problem was addressed by Stewart and Beveridge(1977). They developed a generalized cascade algorithm which could be solved irrespective of the operating strategy involved in the operation.
Many different operating strategies such as flashing, vapor compression, have been studied in literature. In the present work, vapor bleeding as an energy reduction scheme has been elaborated. Vapor bleeding has been applied to three-effect problem first and then to an existing industrial five-effect system both for forward and backward feed flow sequences. Improvement in steam economy, if any, is noted and overall cost computations are made. To meet this target following objectives are to be framed:
 To develop governing equations for the multiple effect evaporator system without vapor bleeding and then for a system with vapor bleeding
 To develop a matlab code for the solution of these equations
 To compute the operating and the capital cost of the original system as well as the modified system.
 To compare the results of different modification to select the best design.

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CHAPTER TWO: LITERATURE REVIEW
Evaporation is a process of concentrating a given solution by heating it to vaporize water. Exposing the solution to a higher surface area or heating it to a higher temperature reduces the time needed to achieve a desired concentration. But increasing the temperature of operation or the residence time in an evaporator might degrade the solution. So, in order to avoid thermal degradation of the solution, the operating temperature, as well as the residence time should be kept as low as possible. This requirement has led to the development of many types of evaporators.
2.1. Different types of evaporators
Evaporators are classified into four categories
1. Evaporators in which the heating medium and the evaporating medium are separated by tubular heating surfaces
2. Evaporators in which the heating medium is confined by coils, jackets, double walls etc.
3. Evaporators in which the heating medium is in direct contact with the evaporating fluid.
4. Evaporators that use solar radiation as the heating medium.
The most commonly used evaporators are the tubular heating-surface evaporators. In these evaporators, the liquid flows past the heating surface either by natural circulation(boiling) or by forced circulation(mechanical methods).

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The different types of evaporators are :
2.1.1.Horizontal tube evaporators
These were the first kind of evaporators that came into use and have the simplest of designs.
It contains a shell and a horizontal tube within it, such that the heating medium is confined
within the tubes and the liquid to be evaporated is in the shell. They have very limited use in
present day applications, they are mostly used for fluids that have low viscosity and are non-scaling.
They have a very low initial investment.
2.1.2. Horizontal spray-film evaporators
They are modified horizontal tube evaporators in which the liquid is distributed by a spray
system and falls down the tubes by the action of gravity.
They give the following advantages:
Fig.2.1. Schematic diagram of a single-effect evaporator[1]

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1. Easy removal of non-condensable vapors
2. Uniform distribution of the liquid
3. Vapor is easily separated from the liquid
4. Convinient operation even with scaling fluids.
2.1.3. Short tube vertical evaporators
They are better known as Calandrias. Liquid circulates past the heating surface by boiling,i.e. natural circulation. The first short tube evaporator was built by Robert. They consist of tubes inside a shell. The tubes may be 2 to 3 inches in diameter and 4 to 10 feet long. A downcomer is present at the centre which enables the flow of liquid from the top to the bottom tube sheet.
2.1.4. Basket type evaporators
The first basket-type evaporator was built in 1877. These evaporators are similar to the short tube vertical evaporators in all respects, the only difference being that the downcomer, instead of being central, is annular. The annular downcomer is installed to allow removal of the evaporator for cleaning and repair purpose. The presence of the downcomer makes the design more economical. Entrainment losses are caused due to violent boiling in the vertical- tube evaporator. A deflector is provided in order to reduce entrainment from spouting.

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2.1.5.Long tube vertical evaporators
They are the most versatile and economical evaporator systems. The tubes herein are 1 to 2
inch in diameter and 12 to 30 feet long. They may be operated as once-through or
recirculating systems. In once-through evaporators, the liquid has a residence time of few
seconds only. Recirculation type evaporators may be batch type or continuos. In a
recirculating evaporator, a particular level in the vapor body has to be maintained, and a
deflector is provided to prevent entrainment in the vapor body. There is a non-uniform
temperature distribution in the tubes and this makes prediction of the tube side temperature
difficult. Due to the appreciable length of the tubes, the effect of hydrostatic pressure head
cannot be ignored.
Fig.2.2.Long tube vertical evaporators ( Source: Unit operations in food engineering,
www.nzifst.org.nz)

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2.1.7 Climbing film evaporators
They are mainly used in industry to concentrate a solution. They are operated under vacuum,
in order to lower the boiling point of the solution and thus increase the temperature difference
driving force. The working principle behind this is the ‘thermo-siphon’ principle. The liquid
rises up in the core of the tube in the form of a thin film, because the liquid flows faster than
the vapor. Such a flow of the liquid is against gravity and thus it is highly turbulent. They
give high heat transfer rates and have a low contact time. They are most ideal for
concentration of heat sensitive materials like juices, pharmaceuticals etc. They provide low
cost operation. They have the least cost per unit capacity available.
2.1.8. Falling film evaporators
In falling film evaporators, the liquid enters the tubes from the top, gets heated and flows
downstream as a film, and leaves from the bottom. The tubes in these type of evaporators are
Fig.2.3. Climbing film evaporators
(Source: GEA Process Engineering Inc.)
A: Product
B: Vapor
C: Concentrate
D: Heating steam
E: Condensate

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about 2 to 10 in. in diameter. The vapor that is evolved from the liquid also moves downward
with the liquid and is removed from the bottom of the unit. They have a liquid-vapor
separator at the bottom to and a distributor for uniform distribution of the feed liquid at the
top. Once-through falling film evaporators have a minimum time of exposure to the heated
surface, and thus can be used for concentration of highly heat sensitive liquids. They are also
effective in handling highly viscous liquids.
Recirculating evaporators distribute the liquid to the tubes by moderate recycling of the liquid
from the bottom to the top of the tubes. Recirculating systems allow larger volume of flow
through the tubes as compared to the once-through evaporators.
2.1.9. Rising falling film evaporators
A high heat transfer rate is obtained by incorporating both the falling film and rising film
evaporators in a single unit. Such type of evaporator is called rising-falling film evaporator.
They have short residence times for the liquid and are hence suitable for heat sensitive
products.
Fig.2.4. Falling-film evaporators
(Source: GEA Process Engineering
Inc.)

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2.1.10 Forced circulation evaporators
Natural circulation evaporators give uneconomically low heat transfer coefficients with
viscous liquids since the velocity of liquid entering the tubes is low, only about 0.3 to 1.2
m/s.
Forced-circulation evaporators provide for a centrifugal pump which forces the liquid
through the tubes at a higher velocity, about 2 to 5.5 m/s. Sufficient static head is provided to
the tubes to ensure that the liquid does not boil over the heating surface, in order to avoid the
fouling characteristics of the liquid. It has a shell and tube heat exchanger, with vertical or
horizontal tubes. There is a significant reduction in the cost of evaporation of viscous liquids
by using forced circulation evaporators, despite the added cost of pumping. Because of the
high liquid velocities, they take very short residence times and are good for moderately heat
sensitive liquids. Forced circulation evaporators are also used for evaporating salting liquors,
or those that have foaming tendency.
Fig.2.5. Forced-Circulation Evaporators [1]

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2.1.11. Plate type evaporators
Plate evaporators consist of corrugated and framed plates that are suitable for scaling liquids,
since the scales can be easily flaked off the plates. They provide relatively larger surface
areas than other type of evaporators. The liquid is pumped between the thin plates, and the
heating medium is provided between the mating surfaces. They have a single pass operation
and thus a short contact time with the heating surface, making them suitable for heat-sensitive
liquids. The product quality is better than other evaporators. They have a low liquid hold-up
and produce minimal waste. They can be easily scaled up, and need low installation cost due
to their compact size and light weight.
2.2. Modeling of a multiple effect evaporator system
There are two different ways of modeling a multiple effect evaporator:
Equation-based model
In the equation based model, the various operating parameters of the different effects are
related by the mass and enthalpy balance relations, the heat transfer rates and the phase
equilibrium relationships for the liquid and vapor (Radovic et.al,1979). This results in a set
of non-linear equations. The number of variables and equations depends on the operating
Fig.2.6. Plate type evaporators
(Source: GEA Process Engineering
Inc.)

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strategy of the evaporator system. This system of non-linear equations is then solved, iteratively or by developing a code for the Newton-raphson method. Pre-developed solvers are also available for obtaining the solution of non-linear systems.
This model is disadvantageous when the operating strategy of a system changes, like when the feed flow sequence is altered, or when feed or product flashing is incorporated because the original equations now fail to hold true and a completely new set of equations has to be developed again.
Equation based models for the design of chemical reactors and mass transfer separation equipment have been developed in literature by Itahara and Steil (1966) using programming.
A model based on boiling point rise and enthalpy relationships developed by curve fitting procedures was presented by Lambert et al. (1987).
Similar equation based models are previously published in literature by Holland (1975), Mathur (1992), Bremford and Muller-Steinhagen (1994), Bhargava (2004).
Cascade algorithm
In cascade algorithm, generalized equations are written for a model which are independent of the operating strategy. This model can handle any feed flow sequence, condensate, feed or product flashing, and vapor bleeding. The liquor flow rate entering each effect is taken as the total of the fraction of feed entering that effect and the fraction of feed entering that effect from the previous effect. On changing the operating condition in any way, the feed flow sequence in the algorithm does not need to be changed, and the same program can be used.(Bhargava et al., 2010)
Other generalized models on similar lines have also been developed by Stewart and Beveridge (1977) and Ayangbile et al. (1984).

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2.3. Energy Reduction Schemes(ERSs) in evaporators.
2.3.1. Flash evaporation
Flash evaporation is the process in which a part of the heated liquid instantly vaporizes when its pressure is suddenly reduced. The reduction in pressure is brought about by passing the liquid through a throttling device. The vessel within which flash evaporation occurs is called as a flash drum. A single component liquid flashes into vapor but a in the case of a multicomponent liquid, the flash vapor contains a greater amount of the more volatile component than the residual liquid.
A flashing liquid utilizes the energy it possesses in excess of the evaporation enthalpy, thus flashing helps in reduction of energy consumption. In a normal evaporation operation, either the feed, the evaporation product, or the condensate can be flashed.
2.3.2. Vapor compression
In vapor compression, the pressure of the vapor leaving the evaporator system is increased by using a blower or compressor. The purpose of compression is to make the vapor suitable for heating in the steam chest of the same effect in place of steam. The vapor issuing out of an effect in an evaporator has the same temperature as the effect, and thus is unsuitable for use as a heating medium in the same effect since the temperature difference driving force in this case would be zero. However, if the pressure of this vapor is reduced, the boiling point of the vapor increases and it stays at a higher temperature than the effect temperature, and there is

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some driving force for heat transfer. But the latent heat of this vapor has been lowered after compression.
2.3.3. Vapor bleeding
Vapor bleeding is a scheme in which a part of the vapor coming out of one effect is withdrawn and utilized to preheat the liquor entering any other effect. I a backward feed evaporator, the liquor coming out of any effect is at a temperature lower than the next subsequent effect it is going to enter. Therefore, a part of the heat provided by the heating medium in that effect is first utilized to raise the temperature of the liquor to that of the effect, and the remaining heat is devoted towards evaporating water from the liquor. In an evaporation operation, the goal is to obtain maximum concentration, by evaporating as much water as possible. Therefore, if the heat utilized to achieve the temperature rise is met from somewhere else, the entire amount of heat in the effect could be utilized for evaporation of water only, and we would thus have a higher evaporator economy.
To fulfill this purpose, vapor is bled out of a particular effect and additional heat exchangers are installed to raise the temperature of the liquor entering any effect, using this bled vapor as the heating medium.
There is additional cost incurred in this process, like the capital cost of the heat exchangers, but great savings are brought about in the cost of steam.

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2.4. Sugarcane juice
Sugarcane is a low-cost agricultural resource, produced mainly in tropical and sub-tropical regions. Sugarcane juice is the juice extracted from pressed sugarcane. It is consumed as a beverage worldwide. Sugarcane is commercially grown in South east Asia, South Asia and Latin America. One of the most important applications of sugarcane juice is in the production of ethanol biofuel. Sugarcane juice is used as a raw material in sugar industry. After some pre-treatment steps, the juice is concentrated in large evaporators to produce sugar. This evaporation removes about 98% of the water in the juice and sugar crystals are formed.
The evaporator house of a sugarcane industry is the most energy-intensive part. In the present study, vapor bleeding as a method to reduce the steam consumption, and thus the cost of operation of the evaporation operation, is studied in various patterns of triple-effect and five- effect evaporators.
Sugarcane juice is composed of combinations of sugars, including glucose and fructose. Some soluble solids like sugars and organic acids are also present. It is alkaline in nature. The rheology and fluid dynamics properties of sugarcane juice were studied by Zailer Astolfi- Filho et al. in February 2011 [15].

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CHAPTER THREE: PROBLEM STATEMENT
3.1. Problem I
A triple-effect evaporator is considered which is used for concentrating sugarcane juice. The base-case operating parameters are listed in Table 3.1. The schematic diagram of the system in shown in Fig.3.1. The feed flow sequence is backward, that is the feed is fed to the 3rd effect, from there the liquor moves to the second effect, and from the second to the first effect. Live steam is fed to the 1st effect only.
Table 3.1. Operating parameters for a three-effect backward feed problem
S.No.
Parameter(s)
Value(s)
1
Total no. of effects
3
2
Number of effects supplied with live steam
1
3
Live steam temperature, To
121.1˚C
4
Feed flow rate, F
22680 kg/hr
5
Feed temperature, Tf
22˚C
6
Feed concentration, xf
10% by wt.
7
Area of each effect, A
100m2
8
Temperature of last effect
52˚C
9
Feed flow sequence
Backward
10
Length of evaporator tubes
4 m

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Fig.3.1. Schematic diagram of triple-effect evaporator system
3.2. Problem II
In this problem, a five-effect evaporator is considered which is operated in forward feed sequence. The operating parameters and schematic diagram of the system are shown in Table 3.2 and Fig. 3.2, respectively.

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Table 3.2. Operating parameters for the five-effect evaporator
S.No.
Parameter(s)
Value(s)
1
Total no. of effects
5
2
Number of effects supplied with live steam
1
3
Live steam temperature, To
120˚C
4
Feed flow rate, F
37.8 kg/s
5
Feed temperature, Tf
108˚C
6
Feed concentration, xf
13% by wt.
7
Area of first effect, A1
1800
8
Area of second effect, A2
1400
9
Area of third effect, A3
600
10
Area of fourth effect, A4
300
11
Area of fifth effect, A5
300
12
Temperature of last effect
62˚C
13
Length of evaporator tubes
4 m

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CHAPTER FOUR : DEVELOPMENT OF THE MODEL AND SOLUTION PROCEDURE
4.1. Model development for a particular effect
The block diagram of a single effect is shown in Fig.4.1. The effect belongs to an evaporator being operated in forward feed flow sequence. Ti is the temperature of the ith effect. Li-1 is the flow rate of liquor leaving effect i-1 entering effect i and Li is the flow rate of liquor leaving effect i. Vi-1 is the vapor flow rate leaving effect i-1, which is used as a heating medium in effect i. Vi is the vapor flow rate leaving effect i, which leaves at the temperature of the effect, i.e., Ti.
Fig.3.2. Schematic diagram of the five-effect forward feed system with one stage pre-heating

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The equation based model is developed by writing the mass and energy balance relations for each effect, as well as the equations for heat-transfer rates[2].
The overall mass balance over ith effect becomes
Li-1 = Li + Vi (4.1)
Component mass balance is written as
Li-1 xi-1 = Li xi = Lf xf (4.2)
Overall energy balance
Li-1 hi-1 + Qi -ViHi-Lihi= 0 (4.3)
Hi = λi+ hi (4.4)
Thus, equation 4.3 becomes
Li -1 hi-1 – Li hi + Qi - Vi λi = 0 (4.5)
The enthalpy balance on the steam chest is given by
Fig.4.1. Block diagram of a single effect in a multiple effect evaporator operating in forward feed flow

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Qi = Vi-1λi-1 (4.6)
And the rate of heat transfer is given by
Qi = Ui Ai (Ti-1 - Ti) (4.7)
Substituting the relation for Q1 , we finally get two equations for any ith effect
Enthalpy balance: f1 = Li -1 hi-1 – Li hi + Vi-1λi-1 - Vi λi
Heat transfer rate : f2= Ui Ai (Ti-1 - Ti) - Vi-1λi-1
Similar relations are written for all the effects, depending on the operating strategy. The number of equations obtained is equal to the number of variables to be solved for. The set of non-linear equations is solved using Newton-Raphson method to get converged values for all the variables.
4.2. Model development for the three effect backward feed evaporator
The following six equations are obtained from the mass and enthalpy balance, and heat transfer rate equations for the three effects. A total of six non-linear equations is to be solved to obtain the variables Vo, T1, T2, L1, L2, L3.
f1 = L2Cp2T2’ + Voλo – (L2 - L1)H1 - L1Cp1 (T1+ ɽ1) (4.8)
f2 = U1A(To – T1) - Voλo (4.9)
f3 = L3 Cp3 (T3 +ɽ3) + (L2 - L1)λ1 – (L3 -L2) H2 - L2Cp2(T2+ ɽ2) (4.10)
f4 = U2A(T1-T2) - (L2 - L1)λ1 (4.11)
f5= FCpFTF + (L3 -L2) λ2 – (F-L3) H3 - L3Cp3(T3 +ɽ3) (4.12)

33
f6= U3A(T2-T3) - (L3 -L2) λ2 (4.13)
4.3. Model development for the three effect backward feed evaporator with bleeding
The schematic of the three effect backward feed system with bleeding is shown in Fig 4.2. Here, two bleed streams are withdrawn from the vapor leaving effects 1 and 2 to pre heat the liquor entering effects 2 and 3 respectively. Therefore, we have two additional equations obtained as a result of the enthalpy balance over the bleed streams.
Fig.4.2. Schematic diagram of a three effect backward feed system with bleeding
f1 = L2Cp2T2’ + Voλo – (L2 - L1)H1 - L1Cp1 (T1+ ɽ1) (4.14)
f2 = U1A(To – T1) - Voλo (4.15)
f3 = L3 Cp3 (T3 +ɽ3) + (L2 - L1-Vb1)λ1 – (L3 -L2) H2 - L2Cp2(T2+ ɽ2) (4.16)
f4 = U2A(T1-T2) - (L2 - L1-Vb1)λ1 (4.17)

34
f5= FCpFTF + (L3 -L2-Vb2) λ2 – (F-L3) H3 - L3Cp3(T3 +ɽ3) (4.18)
f6= U3A(T2-T3) - (L3 -L2-Vb2) λ2 (4.19)
f7 = Vb1 λ1 – L2 Cp2 (T1-T2) (4.20)
f8 = Vb2 λ2 – L3 Cp3 (T2-T3) (4.21)
4.4. Model development for the five- effect forward feed evaporator without bleeding
A five-effect system is modeled using the mass and energy balance relations. A total of 10 equations are obtained for the variables V0, T1, T2, T3, T4, L1, L2, L3, L4 and L5. In this system, the inlet temperature of the feed is at 108˚C. Therefore, an additional quantity of steam is consumed in preheating the feed from its normal temperature to the required feed inlet temperature.
f1 = FCpfTf + Voλo – (F - L1)H1 - L1Cp1 (T1+ ɽ1) (4.22)
f2 = U1A1(To – T1) - Voλo (4.23)
f3 = L1 Cp1 (T1 +ɽ1) + (F- L1)λ1 – (L1 -L2) H2 - L2Cp2(T2+ ɽ2) (4.24)
Fig.4.3. Schematic of the five-effect forward feed evaporator without bleeding

35
f4 = U2A2(T1-T2) - (F - L1)λ1 (4.25)
f5= L2 Cp2 (T2 +ɽ2) + (L1 -L2) λ2 – (L2-L3) H3 - L3Cp3(T3 +ɽ3) (4.26)
f6= U3A3(T2-T3) - (L1 -L2) λ2 (4.27)
f7 = L3 Cp3 (T3 +ɽ3) + (L2 –L3) λ3 – (L3-L4) H4 – L4Cp4(T4 +ɽ4) (4.28)
f8 = U4A4(T3-T4) - (L2 –L3) λ3 (4.29)
f9 = L4 Cp4 (T4 +ɽ4) + (L3 –L4) λ4 – (L4-L5) H5 – L5Cp5(T5 +ɽ5) (4.30)
f10 = U5A5(T4-T5) - (L3 –L4) λ4 (4.31)
4.5. Model development for the five-effect forward feed evaporator with bleeding
In this case, the temperature of the feed is 108˚C, which is far higher than the normal temperature of 30˚C. Therefore, the feed has to be pre-heated to a temperature of 108˚C from 30˚C. To carry out this pre-heating, vapor is bled from the evaporator effects.
In the forward feed case, the preheating of the feed can be done either by employing additional steam to raise the feed temperature from 30˚C to 108˚C directly, or it may be done in four steps, from 30˚C to 50˚C, 50˚C to 70˚C, 70˚C to 88˚C and finally from 88˚C to 108˚C. The first step, i.e. from 30˚C to 50˚C is met by bleeding vapor coming out of effect 4. The heating from 50˚C to 70˚C is done by bleeding vapor from effect 3. The temperature of the feed is then raised from 70˚C to 88˚C by bleeding a part of the vapor coming out of the 2nd effect. Further preheating from 88˚C to 108˚C is carried out by employing additional steam.
Fig.4.4. shows the schematic of the five-effect forward feed evaporator employing vapor bleeding for four-stage preheating

36
f1 = FCpfTf + Voλo – (F - L1)H1 - L1Cp1 (T1+ ɽ1) (4.32)
f2 = U1A1(To – T1) - Voλo (4.33)
f3 = L1 Cp1 (T1 +ɽ1) + (F- L1)λ1 – (L1 -L2) H2 - L2Cp2(T2+ ɽ2) (4.34)
f4 = U2A2(T1-T2) - (F - L1)λ1 (4.35)
f5= L2 Cp2 (T2 +ɽ2) + (L1 -L2-Vb2) λ2 – (L2-L3) H3 - L3Cp3(T3 +ɽ3) (4.36)
f6= U3A3(T2-T3) - (L1 -L2-Vb2) λ2 (4.37)
f7 = L3 Cp3 (T3 +ɽ3) + (L2 –L3-Vb3) λ3 – (L3-L4) H4 – L4Cp4(T4 +ɽ4) (4.38)
f8 = U4A4(T3-T4) - (L2 –L3-Vb3) λ3 (4.39)
f9 = L4 Cp4 (T4 +ɽ4) + (L3 –L4-Vb4) λ4 – (L4-L5) H5 – L5Cp5(T5 +ɽ5) (4.40)
f10 = U5A5(T4-T5) - (L3 –L4-Vb4) λ4 (4.41)
Fig.4.4. Schematic of the five effect forward feed evaporator with four stage pre-heating

37
f11 = Vb2 λ2 – FCpf(88-70) (4.42)
f12 = Vb3 λ3 – FCpf(70-50) (4.43)
f13 = Vb4 λ4 – FCpf(50-30) (4.44)
f14= V0’λ0 – FCpf(108-88) (4.45)
4.6. Model development for the five-effect evaporator operated in backward feed flow without bleeding
The same five-effect evaporator is now operated in backward-flow, with and without bleeding. In backward flow, the feed temperature can be raised to a maximum of 52˚C in order to maintain a 10˚C temperature approach. This is done by employing additional steam (in the case without bleeding).Fig.4.5. shows the schematic of the five-effect backward flow evaporator without bleeding.
f1 = L2Cp2 ( T2+ɽ2)+ Voλo – (L2 - L1)H1 - L1Cp1 (T1+ ɽ1) (4.46)
Fig.4.5. Schematic of the five-effect backward feed flow without bleeding

38
f2 = U1A1(To – T1) - Voλo (4.47)
f4 = U2A2(T1-T2) - (L2 - L1)λ1 (4.49)
f5= L4 Cp4 (T4 +ɽ4) + (L3 -L2) λ2 – (L4 - L3) H3 - L3Cp3(T3 +ɽ3) (4.50)
f6= U3A3(T2-T3) - (L3 -L2) λ2 (4.51)
f7 = L5 Cp5 (T5 +ɽ5) + (L4 –L3) λ3 – (L5 - L4) H4 – L4Cp4(T4 +ɽ4) (4.52)
f8 = U4A4(T3-T4) - (L4 –L3) λ3 (4.53)
f9 = F Cpf Tf + (L5 –L4) λ4 – (F - L5) H5 – L5Cp5(T5 +ɽ5) (4.54)
f10 = U5A5(T4-T5) - (L5 –L4) λ4 (4.55)
4.7. Model development for the five-effect evaporator operated in backward feed flow with bleeding
With vapor bleeding, the feed temperature is raised by bleeding a part of the vapor leaving the 5th effect. Then a part of the vapor leaving the 4th effect is used to preheat the liquor entering effect 4. Similarly, five bleed streams are taken and the additional cost of five heat exchangers has to be considered Fig.4.6. shows the schematic of the same system with vapor bleeding.

39
f1 = L2Cp2 ( T2+ɽ2)+ Voλo – (L2 - L1)H1 - L1Cp1 (T1+ ɽ1) (4.56)
f2 = U1A1(To – T1) - Voλo (4.57)
f4 = U2A2(T1-T2) - (L2 - L1)λ1 (4.59)
f5= L4 Cp4 (T4 +ɽ4) + (L3 -L2) λ2 – (L4 - L3) H3 - L3Cp3(T3 + ɽ3) (4.60)
f6= U3A3(T2-T3) - (L3 -L2) λ2 (4.61)
f7 = L5 Cp5 (T5 +ɽ5) + (L4 –L3) λ3 – (L5 - L4) H4 – L4Cp4(T4 + ɽ4) (4.62)
f8 = U4A4(T3-T4) - (L4 –L3) λ3 (4.63)
f9 = F Cpf Tf + (L5 –L4) λ4 – (F - L5) H5 – L5Cp5(T5 + ɽ5) (4.64)
f10 = U5A5(T4-T5) - (L5 –L4) λ4 (4.65)
f11 = Vb5 λ5 – FCpf(T5-10-30) (4.66)
f12 = Vb4 λ4 – L5Cp5(T4-5-( T5 + ɽ5)) (4.67)
Fig.4.6. Schematic of the five-effect backward flow evaporator with vapor bleeding

40
f13= Vb3 λ3 – L4Cp4 (T3-5-( T4 + ɽ4)) (4.68)
f14 = Vb2 λ2 – L3Cp3 (T2-5-( T3 + ɽ3)) (4.69)
f15 = Vb1 λ1 – L2Cp2 (T1-5-( T2 + ɽ2)) (4.70)
4.8. Solution of the model: Newton Raphson method [2]
The obtained equations for the model are written in a compact form as
JkΔXk = -fk
Where Jkis the jacobian matrix
And ΔXk= Xk+1 – Xk = [ΔV0 ΔT1ΔT2 ΔL1 ΔL2 ΔL3]’ (for the case of 4.2)
Xk+1 and Xk are respectively the values of the variables obtained after (k+1)thand kthiterations respectively.
An assumed set of values for all the variables are provided. The initial guess values for temperature are provided by assuming that there is equal driving force in each effect, and the initial values for the liquor flow rates are provided assuming equal vaporization in each effect.
If the functional values f1, f2….f6 and their partial derivatives occurring in the jacobian matrix are continuos, and the determinant of Jkis not equal to zero, the Newton Raphson method will provide a converged solution with a limited number of iterations.
4.9. Empirical correlations for specific heat, boiling point rise, latent heat and heat transfer coefficient

41
The values of specific heat capacity, boiling point elevation of the liquor and latent heat capacity of the vapor are calculated from empirical correlations. Empirical correlations are also used for calculating the film heat transfer coefficient of the liquor.
The correlations given in equations 4.8 to 4.13 are used to calculate the specific heat capacity, boiling point elevation[5] and film heat transfer coefficient [10] respectively for the liquor and equation 4.11 gives the latent heat for the vapor[5] coming out of a particular effect.
Cp = 4.19-2.35x, (4.8)
ɽ = 1.78x+6.22(x^2) (4.9)
hi = 218 + 24ø -3700x + 1090Г + 32(T+ɽ) (4.10)
λ = 2823.2-4.9783T (4.11)
where x is the concentration of the liquor by wt.
Cp is the specific heat capacity in KJ/Kg K.
ɽ is the boiling point elevation in the liquor in ˚C
ø is the heat flux in the effect, KW/m2
Г is the linear velocity of the liquid on the tubes, kg/m.s
T is the temperature of the effect from where the vapor is leaving,˚C
λ is the latent heat of the vapor in KJ/Kg.
4.10. Detailed iterative solution procedure of the three-effect backward feed evaporator
The model equations are shown in equations 4.8 to 4.13

42
The jacobian matrix is
λo
-L1Cp1-(L2- L1)Cp1
L2Cp2
-Cp1(T1+ɽ1)+ Cp1T1+λ1
Cp2(T2+ɽ2)- (Cp1T1+λ1)
0
- λo
-U1A
0
0
0
0
0
0
-L2Cp2- Cp2(L3-L2)
- λ1
λ1-Cp2(T2+ɽ2)+ (Cp2T2+ λ2)
Cp3(T3+ɽ3)- (Cp2T2+λ2)
0
U2A
- U2A
λ1
- λ1
0
0
0
0
0
- λ2
λ2-Cp3(T3+ɽ3)+ Cp3T3+ λ3
0
0
U3A
0
λ2
- λ2
Given, T0=121.1˚C and T3=51.6 ˚C
Assuming equal temperature drop in each effect,
ΔT1 = ΔT2 = ΔT3 = = = 23.17 ˚C
This gives T1 = 97.93 ˚C and T2 = 74.76 ˚C

43
Let us assume the final concentration is 10% ( x1=0.1) and there is equal vaporization in each effect,
L1 = = 4536 kg/hr
ΔV1 = ΔV2 = ΔV3 = = 6048 kg/hr
This gives L2= 10584 kg/hr, x2 = 0.214
and L3= 16632 kg/hr, x3 = 0.136
This problem may be solved iteratively and the results are given in Table 5.1.
4.11. Detailed iterative solution procedure of the five-effect backward feed evaporator with bleeding
The model equations for this case are shown through Eqs 4.56 to 4.70
Variables: V0, T1, T2, T3, T4, L1, L2, L3, L4 , L5, Vb1, Vb2 ,Vb3, Vb4 ,Vb5
The guess values of the variables to be provided for the 1st iteration are calculated in a similar way, i.e. by assuming equal driving force and equal vaporization in each effect. The guess values for the bleed streams are also calculated from the guess values for temperature and liquor flow rates.
This gives the assumed values to be :
V00 = 5.7 kg/s
T10 = 108.4 ˚C
T20 = 96.8 ˚C

44
T30 = 85.2 ˚C
T40 = 73.6 ˚C
L10 = 9.828 kg/s
L20 = 15.418 kg/s
L30 = 21.008 kg/s
L40=26.598 kg/s
L50 = 32.188 kg/s
Vb10 = 0.096 kg/s
Vb20 = 0.19 kg/s
Vb30 = 0.25 kg/s
Vb40 = 0.31 kg/s
Vb50 = 1.46 kg/s
A matlab code is developed for solving the system and the converged solution is listed in Table 5.7.

45
CHAPTER FIVE: RESULT AND DISCUSSION
The present Chapter shows the results obtained from the theoretical investigation carried out in the present work. The MEE systems considered in this work are three effect and five effect evaporator systems which are utilized for concentrating sugar solution as described in Chapter 3. For this system different models are developed considering vapor bleeding. These models consists of set of non-linear equations is developed in Chapter 4. The results obtained are discussed in the subsequent paragraphs:
5.1. Three effect evaporator operated in backward-feed flow sequence
The operating parameters as mentioned in Table 3.1.are used to solve the model developed in Section 4.10.using matlab code. This model is for three-effect evaporator without the use of vapor-bleeding. To solve this model, empirical correlations are used for computing boiling- point elevation and specific heat of liquor and the latent heat capacity of vapor as described through Eqs.4.8-11. Empirical correlations are also used to calculate the film-heat transfer coefficient for the liquor which have been mentioned in equation 4.10 . As temperature and

46
concentration dependent properties are accounted in the model an iterative approach is used to get the final results. The results of all iteration with different parameters are shown in Table 5.1. For this computation, To, λo, ho and hd are considered as 121.1˚C, 2220 kJ/kg, 9000 kW/m2 K and 566 kW/m2 K, respectively[3].
Table 5.1. Iterative results for the simple three effect system
Itr. no.
Effect no.
T(˚C)
L(kg/h)
x
Cp(kJ/kgK)
ɽ(˚C)
λ(KJ/ kgm)
ᴫ(kg/m/s)
ø(kW/ m2)
hi(kW/
m2 K)
U(kW/ m2 K)
1
1
97.93
4536
0.5
3.015
2.445
2336
0.315
22.82
2471
438.1
2
74.76
10584
0.214
3.686
0.667
2451
0.735
39.24
3581.7
463.59
3
51.59
16632
0.136
3.869
0.358
2566
1.155
41.18
3623
464.27
2
1
98.66
4291
0.528
2.947
2.679
2332
0.298
39.33
2773.8
446.75
2
74.76
11120
0.204
3.711
0.622
2451
0.772
44.24
3779.1
466.74
3
51.59
17441
0.13
3.884
0.337
2566
1.211
43.04
3751.6
466.32
3
1
99.16
4023
0.564
2.865
2.980
2330
0.279
39.21
2646.1
443.3
2
74.99
10988
0.206
3.705
0.632
2450
0.763
45.07
3787.6
466.87
3
51.59
17398
0.130
3.884
0.338
2566
1.208
43.62
3761.3
466.47
4
1
99.36
3917
0.579
2.829
3.116
2329
0.272
38.55
2576.6
441.3

47
2
75.11
10916
0.208
3.702
0.638
2449
0.758
45.27
3785.8
466.85
3
51.59
17363
0.131
3.883
0.339
2566
1.206
43.87
3763.5
466.5
5
1
99.43
3878
0.585
2.816
3.168
2328
0.269
38.25
2548.8
440.48
2
75.15
10888
0.208
3.701
0.641
2449
0.756
45.33
3784.7
466.83
3
51.59
17350
0.131
3.883
0.339
2566
1.205
43.96
3764.4
466.52
The final converged values for all the variables are presented in Table 5.2 and Fig 5.1 which shows steam economy of simple three effect evaporator system is 1.45.
Table 5.2: Values of different variables for three effect evaporator system
Effect no.
Liquor flow rate
L(kg/hr)
Temperatureof effect
T(˚C)
Steam consumption
Vo(kg/hr)
Steam economy
1
4291
98.6580
3147.7
2.3
2
11120
74.7649
3
17441
51.5900

48
Fig 5.1.Schematic of the three-effect evaporator with specified variables
According to Al-Sahali et al.(1997), the cost of steam is $0.3 per m3. The steam requirement for this operation is 3147.7 kg/hr. Considering density of steam from the steam tables as 1.157 kg/m3 the cost of this operation per year becomes $816.
5.2. Three-effect evaporator system with vapor bleeding
The model of the three effect evaporator system considered in Section 5.1 is now solved by incorporating vapor bleeding. Two preheaters are used for vapor bleeding. Similar correlations for all the parameters are chosen as in the previous case. The results are iterated to obtain the final converged values as shown in Table 5.3. The schematic diagram of this system is shown in Fig. 5.2.
Table 5.3. Values of different variables in the triple-effect backward feed evaporator with bleeding

49
Effect no.
Liquor flow rate
L(kg/hr)
Temperature of effect
T(˚C)
Bleed stream of vapor (kg/hr)
Steam consumption
Vo(kg/hr)
Steam economy
1
15213
87.1413
678.5
3082
2.38
2
21337
69.8225
1072
3
22549
51.5900
5.2.1 Associated costs
Fig. 5.2. Schematic of triple-effect backward feed evaporator with vapor bleeding showing the different variables

50
Cost of steam for this operation comes out to be $799.Two heat exchangers are to be installed to carry out the pre-heating operation.These exchangers are shell and tube exchangers. The installation cost of these exchangers is to be taken into account.
Area of each heat exchanger handling a particular bleed-stream
= Heat duty/(Overall heat transfer coefficient*temperature driving-force)
The overall heat-transfer coefficient for an exchanger handling steam and organic liquid is 1000 W/m2K [3].This gives the areas of the two exchangers to be 163.77 cm2 and 240 cm2 respectively. Considering stainless-steel to be the material of construction for the shell and tube heat exchanger, capital cost of heat exchangers is found as $60139 using following expression [4]where A is the area of heat-transfer surface in m2:
Cost ($) = 30000+1650A0.81
Comparison of results of three effect evaporator with and without vapor bleeding shows that the steam economy of the system increases with bleeding. The increase in steam economy of the evaporator system is 0.08, i.e. per kg of steam consumed, 0.08 kg more liquid is evaporated with bleeding. This is as vapor bleeding is done to preheat the liquor near to the temperature of the effect before it is entering into the effect so that the liquor can quickly attain the boiling temperature inside the effect. Thus, evaporation rate increases in the system. The steam consumption of the system falls from 3147.7 kg/hr to 3082 kg/hr when bleeding is incorporated. The cost of steam for the system thus decreases. The savings brought about in the cost of steam come out to be $1,22,400 per year.
Payback period = [16]

51
Assuming the salvage value of both the heat exchangers to be $ 10000 and the service life to be 10 years, yearly depreciation = =$5014
Total depreciable cost=60139-10000=$50139
This gives the payback period to be = 4 months 22 days.
5.3. Five-effect forward feed evaporator
A model is developed for a five-effect evaporator system with forward-feed and solved using the Newton-Raphson method. Operating parameters have been mentioned in Table 3.2. Here, the feed temperature is 108 ˚C, i.e.-far above normal temperature. So, additional amount of steam is used up to pre-heat the feed from its normal temperature, i.e. 30˚C to 108˚C. The empirical correlations for specific heat, latent heat, boiling point elevation and film coefficient used here are the same as those used in the three-effect system. A matlab code is developed to solve the system and the converged results are presented in Table 5.4. The results for the same are also shown schematically in Fig.5.3.
Table 5.4. Values of variables in the five-effect evaporator system
Effect no.
Liquor flow rate
L(kg/s)
T(˚C)
Steam consumption
Vo(kg/s)
Steam economy
1
37.1875
117.0435
1.8853
3.92
2
36.4428
115.1293
3
35.3516
109.6693
4
33.3294
93.4221
5
29.7991
62.0000

52
Fig.5.3. Schematic of five-effect forward feed system showing the different variables
Additional steam consumed to raise the feed temperature from 30˚C to 108˚C is found as 5.14 kg/s thus, total steam consumption for this system is 7.025 kg/s. Total cost of steam consumed per hour, taking the cost of steam to be $ 0.3 per m3 is $6552.
5.4. Five-effect forward feed evaporator with vapor bleeding
The five-effect forward feed system is modified using vapor bleeding to pre-heat the feed liquor form normal temperature(30˚C) to the feed inlet temperature, i.e. 108˚C in three stages, from 30˚C to 50˚C, 50˚C to 70˚C and 70˚C to 88˚C. The additional sensible heat requirement to raise the temperature from 88˚C to 108˚C is provided by the steam. The additional costs incurred are the costs of the three heat exchangers, and that of the additional steam required. The results of this model and schematic diagram are shown in Table 5.5 and Fig.5.4, respectively.

53
Table 5.5 Values of variables in the five-effect forward feed evaporator system with bleeding
Effect no.
Liquor flow rate
L(kg/s)
Temperature
T(˚C)
Bleed stream
Vb(kg.s)
Steam consumption
Vo(kg/s)
Steam economy
1
35.6191
113.7696
1.8820
4.07
2
33.0146
106.8789
1.1536
3
30.9778
95.9885
1.2521
4
29.5999
83.8982
1.2208
5
27.9590
62.0000
Fig.5.4. Schematic of the five-effect forward feed evaporator with bleeding

54
Additional amount of steam required to carry out the preheating of the feed required from 88˚C to 108˚C is 1.32 kg/s.Therefore, the total steam requirement is found as 2.715 kg/s.
5.4.1 Costs computation
Cost of steam per hour(at the rate of $0.3 per m3) is found as $6540. Total capital cost of the three exchangers is obtained as $180108 considering areas of the three heat-exchangers as98.31 m2, 83.8 m2 and 68 m2, respectively.
The steam consumption of the five effect system falls from 1.8853 kg/s to 1.8820 kg/s when bleeding is incorporated. There is an improvement in steam economy, from 3.92 to 4.02, from the system without bleeding to the system with bleeding. The annual savings in the cost of steam come out to be $86400.
Assuming the salvage value of the equipments to be $10000 and service life to be 10 years, annual depreciation = $17010
Total depreciable amount = $170108
This gives the payback period to be 1 year, 7 months and 23 days.
5.5. Five effect evaporator operated in backward feed flow without bleeding
The five-effect evaporator with the same operating parameters is operated in backward feed-flow sequence. Additional steam is employed to pre-heat the feed. The values of the variables calculated as a result of the matlab code are presented in Table 5.6 and shown schematically in Fig.5.5.

55
Table 5.6.. Values of variables in the five-effect backward feed evaporator system
Effect no.
Liquor flow rate
L(kg/s)
T(˚C)
Steam consumption
Vo(kg/s)
Steam economy
1
28.0716
111.8301
1.5783
4.12
2
30.9464
102.6847
3
32.9625
87.4288
4
34.0202
70.8826
5
34.5686
62.0000
Fig.5.5.Schematic of the five-effect backward feed evaporator showing the different variables
Steam required to pre-heat the feed from 30 ˚C to 108˚C is 5.14 kg/s. Therefore the total cost of steam consumed per hour becomes $1873. The cost of additional heat exchanger of area 296 m2 is $30000.

56
5.6. Five-effect backward feed evaporator with vapor bleeding
Vapor bled out of the stream leaving the 5th effect is used to pre-heat the feed. The maximum temperature to which the feed can be raised should be 5˚C less than that of the 5th effect, in order to maintain a temperature approach of 5˚C. Similarly, vapor bled out of the streams leaving 1st, 2nd, 3rd and 4th effects are used to pre-heat the liquor entering the same effect, allowing the liquor to rise to a temperature 5˚C less than the temperature of the effect which it is going to enter.
Table 5.7 lists in a tabular manner the values of the various variables obtained as a result of the matlab code. Fig.5.6 shows these variables in a schematic diagram of the evaporator system. The steam economy increases from 4.12 to 5.19 with vapor bleeding.
Table 5.7. Values of different variables in the five-effect backward feed flow system with bleeding
Effect no.
Liquor flow rate
L(kg/s)
T(˚C)
Bleed stream
Vb(kg.s)
Steam consumption
Vo(kg/s)
Steam economy
1
26.4812
109.1736
0.2977
1.5833
5.19
2
30.3150
97.8452
0.5649
3
32.9499
82.0112
0.4381
4
34.2353
68.6073
0.0664
5
34.7078
62.0000
1.4598

57
Fig.5.6. Schematic of the five-effect backward feed flow system with bleeding
Cost of steam at the rate of $0.3 per m3 is $1879. Areas of the heat exchangers that need to be installed come out to be 201.66 m2, 30.14 m2, 124.8 m2, 348.76 m2 and 85.48 m2 respectively, from right to left. This gives the capital cost of the exchangers to be $1,50,291. The annual savings in the cost of steam with bleeding are $43,200. Assuming the salvage value of the heat transfer equipment to be $10000 and its service life to be 10 years, the payback period comes out to be 2 years 5 months and 12 days .

58
CHAPTER SIX : CONCLUSION
In the present study, the simulation of a three-effect and a five-effect forward as well as backward feed evaporator is done with and without bleeding. The aim is to find the most thermo-economically viable solution for the system. Model is developed and solved using the Newton-Raphson method by developing a matlab code.
Taking the results into account, the following inference is drawn from the present study:
1. The steam economy of an evaporator when operated in backward feed is higher than when it is operated in forward feed flow.
2. Vapor bleeding increases the steam economy of all the sytems but at the added cost of the heat exchangers that need to be installed to carry out the bleeding operation
3. Vapor bleeding when applied to a five-effect backward feed problem shows better results than when applied to a forward feed problem for the same system.
4. The payback period of the heat exchangers is the highest in the five-effect backward feed problem, since here the maximum number of exchangers is required, as well as the head load on the exchangers is maximum.

59
REFERENCES
[1] Warren McCabe, Julian Smith, Peter Harriott, Unit Operations of Chemical Engineering, 7thed.
[2] Charles Donald Holland, Fundamentals and Modeling of Separation Processes: Absorption, Distillation, Evaporation and Extraction, 1st ed., p 25-29, 1975
[3] Kern Donald Q. process Heat Transfer. Tata McGraw- Hill, 106, 845
[4] Shenoy, Uday V., Heat Exchanger network-synthesis: Optimiztion process by energy and resource analysis, 1st ed.
[5] Geankoplis, C.J., Transport processes and Separation Process Principles, 4th edition.
[6] Peters Max.S., Timmerhaus Klaus D., Plant Design and Economics for Chemical Engineers, 4th edition.
[7] Radovic, L. R., Tasic, A.Z., Grozanic, D.K., Djordjevic, B.D., &Valent, V.J., Computer design and analysis if operation of a multiple effect evaporation system in the sugar industry, Industrial and Engineering Chemistry Process Design and Development, 1979
[8] Khanam S, Mohanty B.Energy reduction schemes for multiple effect evaporator systems. Applied Energy 87 (2010), 1102–1111.
[9] Jorge L.M.M., Righetto A.R., Polli P.A., Santos O.A.A., FilhoMaciel R. Simulation and analysis of a sugarcane juice evaporation system. Journal of Food Engineering 99 (2010) 351–359

60
[10] AdibTarif Ali, Heyd Bertrand, Vasseur Jean. Experimental results and modeling of boiling heat transfer coefficients in falling film evaporator usable for evaporator design. Chemical Engineering and Processing 48 (2009) 961–968
[11] Prost J.S., Gonza´lez M.T., Urbicain M.J. Determination and correlation of heat transfer coefficients in a falling film evaporator.Journal of Food Engineering 73 (2006) 320–326
[12] Mohammad Al Sahali, HishamEttouney, Developments in thermal desalination processes, Design, energy and costing aspects, 2007.
[13] R. Bhargava , S. Khanam, B. Mohanty, A.K. Ray, " Selection of optimal feed flow sequence for a multiple effect evaporator system”, Computers and Chemical Engineering 32 2203–2216, 2008
[14] Khanam S., Mohanty B. Development of a new model for a multiple effect evaporator(2010). Computers and Chemical Engineering
[15] ZailerAstolfi-Filho, Vânia Regina Nicoletti Telis, Eduardo Basilio de Oliveira, Jane Sélia dos Reis Coimbra, Javier Telis-Romero. Rheology and fluid dynamics properties of sugarcane juice. Biochemical Engineering Journal 53(February 2011), 260-265
[16] peterstimmerhaus, Plant design and economics for chemical engineers

61
APPENDIX
In the appendix, the matlab code developed for the solution of model developed under Section 4.7 is given:
T0=120;
lat0=2225.804;
Tf=108;
F=37.8;
xf=0.13;
cpf=4.19-2.35*xf;
V0=5.7;
T=rand(5,1);
x=rand(5,1);
L=rand(5,1);
V=rand(5,1);
V1=rand(5,1);
A=rand(5,1);
cp=rand(5,1);
bpe=rand(5,1);
lat=rand(5,1);
gamma=rand(5,1);
phi=rand(5,1);
A(1)=1800;
A(2)=1400;
A(3)=600;

62
A(4)=300;
A(5)=300;
T(1)=108.4;
T(2)=96.8;
T(3)=85.2;
T(4)=73.6;
T(5)=62;
L(5)=32.188;
L(4)=26.598;
L(3)=21.008;
L(2)=15.418;
L(1)=9.828;
V1(1)=0.096;
V1(2)=0.19;
V1(3)=0.25;
V1(4)=0.31;
V1(5)=1.46;
ho=9000;
hd=566;
hi=rand(5,1);
U=rand(5,1);
fori=1:5
V(i)=5.6;
x(i)=37.8*0.13/L(i);

63
cp(i)=4.19-2.35*x(i);
bpe(i)=1.78*x(i)+6.22*(x(i)^2);
lat(i)=2823.2-4.9783*T(i);
gamma(i)=L(i)/4;
end
phi(1)=V0*lat0/A(1);
phi(2)=[V(1)*lat(1)]/A(2);
phi(3)=[V(2)*lat(2)]/A(3);
phi(4)=[V(3)*lat(3)]/A(4);
phi(5)=[V(4)*lat(4)]/A(5);
fori=1:5
hi(i)=218+24*phi(i)-37*100*x(i)+1090*gamma(i)+32*[T(i)+bpe(i)];
U(i)=[(1/hi(i))+(1/ho)+1/hd]^-1;
end
f=rand(15,1);
j=rand(15,15);
z=rand(15,1);
z(1:15,1)=1;
count=0;
fori=1:50
f(1)=(V0*lat0+L(2)*cp(2)*(T(2)+bpe(2))-(L(2)-L(1))*(lat(1)+cp(1)*T(1))- L(1)*cp(1)*(T(1)+bpe(1)))*1000;

64
f(2)=U(1)*A(1)*(T0-T(1))-V0*lat0*1000;
f(3)=((L(2)-L(1)-V1(1))*lat(1)+L(3)*cp(3)*(T(3)+bpe(3))-(L(3)-L(2))*(lat(2)+cp(2)*T(2))- L(2)*cp(2)*(T(2)+bpe(2)))*1000;
f(4)=U(2)*A(2)*(T(1)-T(2))-(L(2)-L(1)-V1(1))*lat(1)*1000;
f(6)=U(3)*A(3)*(T(2)-T(3))-(L(3)-L(2)-V1(2))*lat(2)*1000;
f(8)=U(4)*A(4)*(T(3)-T(4))-(L(4)-L(3)-V1(3))*lat(3)*1000;
f(9)=((L(5)-L(4)-V1(4))*lat(4)+F*cpf*Tf-(F-L(5))*(cp(5)*T(5)+lat(5))- L(5)*cp(5)*(T(5)+bpe(5)))*1000;
f(10)=U(5)*A(5)*(T(4)-T(5))-(L(5)-L(4)-V1(4))*lat(4)*1000;
f(11)=(V1(5)*lat(5)-3670.85)*1000;
f(12)=(V1(4)*lat(4)-L(5)*cp(5)*(T(4)-5-(T(5)+bpe(5))))*1000;
f(13)=(V1(3)*lat(3)-L(4)*cp(4)*(T(3)-5-(T(4)+bpe(4))))*1000;
f(14)=(V1(2)*lat(2)-L(3)*cp(3)*(T(2)-5-(T(3)+bpe(3))))*1000;
f(15)=(V1(1)*lat(1)-L(2)*cp(2)*(T(1)-5-(T(2)+bpe(2))))*1000;
j(1,1)=lat0*1000; j(1,2)=(-(L(2)-L(1))*cp(1)-L(1)*cp(1))*1000; j(1,3)=L(2)*cp(2)*1000; j(1,4)=0; j(1,5)=0;
j(1,6)=((lat(1)+cp(1)*T(1))-cp(1)*(T(1)+bpe(1)))*1000; j(1,7)=(cp(2)*(T(2)+bpe(2))- (lat(1)+cp(1)*T(1)))*1000; j(1,8)=0; j(1,9)=0; j(1,10)=0;

65
j(1,11)=0;j(1,12)=0;j(1,13)=0;j(1,14)=0;j(1,15)=0;
j(2,1)=-lat0*1000; j(2,2)=-U(1)*A(1); j(2,3)=0; j(2,4)=0; j(2,5)=0; j(2,6)=0; j(2,7)=0; j(2,8)=0; j(2,9)=0; j(2,10)=0;j(2,11)=0;j(2,12)=0;j(2,13)=0;
j(2,14)=0;j(2,15)=0;
j(3,1)=0; j(3,2)=0; j(3,3)=(-(L(3)-L(2))*cp(2)-L(2)*cp(2))*1000; j(3,4)=L(3)*cp(3)*1000; j(3,5)=0; j(3,6)=-lat(1)*1000;
j(3,7)=(lat(1)+lat(2)+cp(2)*T(2)-cp(2)*(T(2)+bpe(2)))*1000; j(3,8)=(cp(3)*(T(3)+bpe(3))- (lat(2)+cp(2)*T(2)))*1000; j(3,9)=0; j(3,10)=0;
j(3,11)=-lat(1)*1000;j(3,12:15)=0;
j(4,1)=0; j(4,2)=U(2)*A(2); j(4,3)=-U(2)*A(2); j(4,4)=0; j(4,5)=0; j(4,6)=lat(1)*1000; j(4,7)=- lat(1)*1000; j(4,8)=0; j(4,9)=0; j(4,10)=0;
j(4,11)=lat(1)*1000;j(4,12:15)=0;
j(5,1)=0; j(5,2)=0;j(5,3)=0; j(5,4)=(-(L(4)-L(3))*cp(3)-L(3)*cp(3))*1000; j(5,5)=L(4)*cp(4)*1000; j(5,6)=0;
j(5,7)=-lat(2)*1000; j(5,8)=(lat(2)+lat(3)+cp(3)*T(3)-cp(3)*(T(3)+bpe(3)))*1000; j(5,9)=(cp(4)*(T(4)+bpe(4))-(lat(3)+cp(3)*T(3)))*1000; j(5,10)=0;
j(5,11)=0;j(5,12)=-lat(2)*1000; j(5,13:15)=0;

66
j(6,1)=0; j(6,2)=0; j(6,3)=U(3)*A(3); j(6,4)=-U(3)*A(3); j(6,5)=0; j(6,6)=0; j(6,7)=lat(2)*1000; j(6,8)=-lat(2)*1000; j(6,9)=0; j(6,10)=0;
j(6,11)=0; j(6,12)=lat(2)*1000;j(6,13:15)=0;
j(7,1)=0; j(7,2)=0; j(7,3)=0; j(7,4)=0; j(7,5)=(-(L(5)-L(4))*cp(4)-L(4)*cp(4))*1000; j(7,6)= 0; j(7,7)=0;
j(7,8)=-lat(3)*1000;j(7,9)=(lat(3)+lat(4)+cp(4)*T(4)- cp(4)*(T(4)+bpe(4)))*1000;j(7,10)=(cp(5)*(T(5)+bpe(5))-(lat(4)+cp(4)*T(4)))*1000;j(7,11)=0;
j(7,12)=0; j(7,13)=-lat(3)*1000; j(7,14)=0; j(7,15)=0;
j(8,1)=0; j(8,2)=0; j(8,3)=0; j(8,4)=U(4)*A(4); j(8,5)=-U(4)*A(4); j(8,6)=0; j(8,7)=0; j(8,8)=lat(3)*1000; j(8,9)=-lat(3)*1000; j(8,10)=0;j(8,11)=0;
j(8,12)=0;j(8,13)=lat(3)*1000;j(8,14)=0;j(8,15)=0;
j(9,1)=0; j(9,2)=0; j(9,3)=0; j(9,4)=0; j(9,5)=0; j(9,6)=0; j(9,7)=0; j(9,8)=0;
j(9,9)=-lat(4)*1000; j(9,10)=(lat(4)+cp(5)*T(5)+lat(5)- cp(5)*(T(5)+bpe(5)))*1000;j(9,11:13)=0; j(9,14)=-lat(4)*1000;j(9,15)=0;
j(10,1)=0; j(10,2)=0; j(10,3)=0; j(10,4)=0; j(10,5)=U(5)*A(5); j(10,6)=0; j(10,7)=0; j(10,8)=0; j(10,9)=lat(4)*1000; j(10,10)=-lat(4)*1000;
j(10,11:13)=0;j(10,14)=lat(4)*1000;j(10,15)=0;
j(11,1:14)=0;j(11,15)=lat(5)*1000;

67
j(12,1:4)=0; j(12,5)=-L(5)*cp(5)*1000;j(12,6:9)=0; j(12,10)=-cp(5)*(T(4)-5- (T(5)+bpe(5)))*1000;j(12,11:13)=0; j(12,14)=lat(4)*1000;j(12,15)=0;
j(13,1:3)=0; j(13,4)=-L(4)*cp(4)*1000;j(13,5)=L(4)*cp(4)*1000;j(13,6:8)=0; j(13,9)=- cp(4)*(T(3)-5-(T(4)+bpe(4)))*1000;j(13,10)=0;j(13,11:12)=0;
j(13,13)=lat(3)*1000;j(13,14:15)=0;
j(14,1:2)=0; j(14,3)=-L(3)*cp(3)*1000; j(14,4)=L(3)*cp(3)*1000; j(14,5:7)=0; j(14,8)=- cp(3)*(T(2)-5-(T(3)+bpe(3)))*1000; j(14,9:11)=0;j(14,12)=lat(2)*1000;
j(14,13:15)=0;
j(15,1)=0; j(15,2)=-L(2)*cp(2)*1000; j(15,3)=L(2)*cp(2)*1000;j(15,4:6)=0; j(15,7)=- cp(2)*(T(1)-5-(T(2)+bpe(2)))*1000;j(15,8:10)=0;
j(15,11)=lat(1)*1000;j(15,12:15)=0;
z=-inv(j)*f;
V0=0.1*V0+0.5*z(1);
T(1)=T(1)+z(2);
T(2)=T(2)+z(3);
T(3)=T(3)+z(4);
T(4)=T(4)+z(5);

68
L(1)=L(1)+z(6);
L(2)=L(2)+z(7);
L(3)=L(3)+z(8);
L(4)=L(4)+z(9);
L(5)=L(5)+z(10);
V1(1)=V1(1)+z(11);
V1(2)=V1(2)+z(12);
V1(3)=V1(3)+z(13);
V1(4)=V1(4)+z(14);
V1(5)=V1(5)+z(15);
V(1)=L(2)-L(1);
V(2)=L(3)-L(2);
V(3)=L(4)-L(3);
V(4)=L(5)-L(4);
V(5)=F-L(5);
fori=1:5
x(i)=37.8*0.13/L(i);
cp(i)=4.19-2.35*x(i);
bpe(i)=1.78*x(i)+6.22*(x(i)^2);
lat(i)=2823.2-4.9783*T(i);
gamma(i)=L(i)/4;
end

69
phi(1)=V0*lat0/A(1);
phi(2)=[V(1)*lat(1)]/A(2);
phi(3)=[V(2)*lat(2)]/A(3);
phi(4)=[V(3)*lat(3)]/A(4);
phi(5)=[V(4)*lat(4)]/A(5);
fori=1:5
hi(i)=218+24*phi(i)-37*100*x(i)+1090*gamma(i)+32*[T(i)+bpe(i)];
U(i)=[(1/hi(i))+(1/ho)+1/hd]^-1;
end
end

70
RESULTS
For 1st iteration:
The jacobian matrix obtained is as follows:
From columns 1 to 7
1*106 *
2.2258
-0.0465
0.0531
0
0
2.2762
-2.2732
-2.2258
-0.8621
0
0
0
0
0
0
0
-0.0723
0.0765
0
-2.2836
4.6207
0
0.6898
-0.6898
0
0
2.2836
-2.2836
0
0
0
-0.0968
0.0999
0
-2.3413
0
0
0.3004
-0.3004
0
0
2.3413
0
0
0
0
-0.1209
0
0
0
0
0
0.1518
-0.1518
0
0
0
0
0
0
0
0
0
0
0
0
0
0.1527
0
0
0
0
0
0
0
0
0
0
0
0
0
-0.1233
0
0
0
0
0
-0.0999
0.0999
0
0
0
0
-0.0765
0.0765
0
0
0
0
-0.0531
0.0531
0
0
0
-0.0186

71
From columns 8 to 15
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
-2.3615
0
0
-2.2836
0
0
0
0
0
0
0
2.2836
0
0
0
0
4.7376
-2.4307
0
0
-2.3413
0
0
0
-2.3413
0
0
0
2.3413
0
0
0
-2.3990
4.8538
-2.4941
0
0
-2.3990
0
0
2.3990
-2.3990
0
0
0
2.3990
0
0
0
-2.4568
4.9697
0
0
0
-2.4568
0
0
2.4568
-2.4568
0
0
0
2.4568
0
0
0
0
0
0
0
0
2.5145
0
0
-0.0237
0
0
0
2.4568
0
0
-0.0228
0
0
0
2.3990
0
0
-0.0213
0
0
0
2.3413
0
0
0
0
0
0
2.2836
0
0
0
0
ΔXk = -2.3575
2.9704
4.2089
-0.6546
-4.1744
16.8817
14.6375
11.6148
7.451
2.4473

72
0.1198
0.2653
0.2195
-0.1856
-0.0002
Modified values after 1st iteration:
V0 = -0.6087
T1 = 111.3704
T2 = 101.0089
T3 = 84.5454
T4 = 69.4256
L1 = 26.7097
L2 = 30.0555
L3 = 32.6228
L4 = 34.0493
L5 = 34.6353
The final converged solution comes out to be :
Effect no.
Liquor flow rate
L(kg/s)
T(˚C)
Bleed stream
Vb(kg.s)
Steam consumption
Vo(kg/s)
Steam economy
1
26.4812
109.1736
0.2977
1.5833
5.19
2
30.3150
97.8452
0.5649
3
32.9499
82.0112
0.4381
4
34.2353
68.6073
0.0664
5
34.7078
62.0000
1.4598

Design and simulation of a multiple effect evaporator using vapor bleeding

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