A Basic Primer on Liquid-Liquid Extraction
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The document provides an overview of liquid-liquid extraction (LLE) as a separation technique in chemical processing, highlighting its significance when distillation is not economically viable. It discusses the basic principles of LLE, types of columns, and the factors influencing their design. Additionally, it addresses various applications of LLE, including the recovery of non-volatile components and the removal of high-boiling substances from dilute solutions.
A Basic Primer on Liquid-Liquid Extraction
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- 2. 32 Processing | OCTOBER 2020 hemical engineers are often faced with the task to design challenging separation processes for product recovery or puri- fication. This article looks at the basics of one powerful and yet overlooked separation tech- nique: liquid-liquid extraction. here are other unit operations used to separate compounds, such as distillation, which is taught extensively in chemical engineering curriculums. If a separation is feasible by distillation and is economical, there is no reason to consider liquid-liquid extraction (LLE). However, dis- tillation may not be a feasible solution for a number of reasons, such as: • If it requires a complex process sequence (several distillation columns) • If the components to be removed or recovered have low volatility • If it results in high CapEx and OpEx • In these instances, LLE may be the best separa- tion technology to pursue. The basics of liquid-liquid extraction While distillation drives the separation of chemicals based upon diferences in relative volatility, LLE is a separation technology that exploits the diferences in the relative solubilities of compounds in two immis- cible liquids. Typically, one liquid is aqueous, and the other liquid is an organic compound. Used in multiple industries including chemical, pharmaceutical, petrochemical, biobased chemicals and lavor and fragrances, this approach takes careful process design by experienced chemical engineers and scientists. In many cases, LLE is the best choice as a separation technology and well worth searching for a qualified team to assist in its development and design. Figure 1 can be used to gain an understanding of exactly what LLE is. A feed consisting of mostly com- ponent A plus component C (solute) is well mixed with solvent B. The goal is to achieve mass transfer of solute C from feed A into solvent B. After the maxi- mum amount of mass transfer is achieved, the phases C chemical processing liquid-liquid extraction A basic primer on An introduction to LLE and agitated LLE columns | By Don Glatz and Brendan Cross, Koch Modular 32 Processing | OCTOBER 2020
- 3. OCTOBER 2020 | www.processingmagazine.com 33 are separated into an extract phase (solvent C plus the extracted solute B) and a rainate phase (feed A plus the residual solute C) and possible the soluble amount of solvent B. Assuming that equilibrium was achieved during the mixing phase, this process represents a single theoretical stage of extraction. As shown in Figure 1, for a single theoretical stage, there are a number of parameters identiied at equilibrium, including frac- tion unextracted, distribution coefficient and the extraction factor. he key for LLE is to select a solvent with as high a distribution coeicient as possible for the solute to be extracted and as low as possible of all other components present in the feed. For most LLE processes, a single stage of extrac- tion such as this will not provide a suicient amount of solute removal from the feed stream. In order to achieve higher levels of extraction, there are two low patterns in which LLE can be operated: cross flow extraction and countercurrent extraction. Cross low extraction consists of a series of mixing and decants using fresh solvent each time. hus, after the phase have separated, fresh solvent would be added to the rainate and once again they would be mixed and allowed to decant. his procedure would be repeated as many times as necessary to get from the feed concentration of solute C to the required raf- inate concentration, shown as R4 in Figure 2. In countercurrent extraction, the feed lows in one direction from stage to stage while the solvent lows in the opposite direction relative to the feed. Both liquid phases are lowing continuously through the extractor. his type of process is shown for a column type contactor in Figure 3. In Figure 3, the feed is the heavy phase so it is added to the top of the column. It is also the continuous phase, so it essentially ills the column. he solvent is the light phase so it is added to the bottom of the column. It lows as droplets up through the continuous phase and thus it is the dis- persed phase in the column. Rules of thumb for which phase should be continuous and which phase should be dispersed are provided later. An important step in understanding an LLE pro- cess and eventually designing LLE columns is deter- mining the liquid-liquid equilibrium data for the solvent of choice. he equilibrium data provides the equilibrium curve shown in Figure 4. he operating line in Figure 4 is determined by the actual inlet and outlet concentrations of solute during steady state column operation. hese two lines allow for a graphi- cal determination of the number of theoretical stages. he total number of theoretical stages required can be stepped of from the operating line to the equilibri- um line, as shown in Figure 4, and is analogous to the method used for absorption and stripping columns. As an example of this procedure, Figure 5 Figure 1 Figure 2
- 4. 34 Processing | OCTOBER 2020 represents the extraction of a carboxylic acid from a fermentation broth using an organic solvent. he red line represents the equilibrium data generated using actual feed and solvent determined via laboratory testing. he operating line is drawn by determining the material balance required to achieve 95% recov- ery. In order to achieve 95% recovery of the carboxylic acid, three theoretical stages are required. Another way to estimate the number of theoreti- cal stages required for the LLE system is using the Kremser Equation. his equation considers the dis- tribution coeicient, extraction factor and concentra- tion of the solute in the feed, rainate and solvent, all on a solute-free basis. A graphical representation of the Kremser equation using diferent extraction factors is shown in Figure 6. When is it recommended to use LLE? • LLE is often used for the removal of high boil- ing components from dilute aqueous streams such as wastewater or fermentation broths. Two great examples of this would are phenol extrac- tion using MIBK and acetic acid extraction using ethyl acetate. Distillation being used as the sole separation technology for these applications would be cost-prohibitive because of the high amount of energy required. For example, if an aqueous feed contains < 30% acetic acid, liquid- liquid extraction is usually a more economical solution than distillation for removal and recov- ery of acetic acid because the operating costs associated with distilling away the water far out- weigh the capital costs of the liquid-liquid extrac- tion process equipment. • LLE is also suited for the removal of hydro- philic particles from organic streams. For these applications, water is the solvent. A couple of examples of this type of application would be water extracting DMSO from a polymer solution and neutralizing & water extracting acids from organic streams. • When recovery of non-volatile components is BUILT TO FILL. BUILT FOR LIFE. Just built beter... We also provide installaion, parts and ield service 833-467-3432 www.specialtyequipment.com READY TO SHIP Mobile Pail Filler This mobile pail iller was created for batch illing operaions at muliple locaions. It is top and sub- surface pail iller that moves around on casters. This iller will handle 4 to 7 gallon containers and only requires 25 lbs. of ineria to move it. » Improved Eiciencies & Accuracy » Increase Producion & Maximize Upime » Heavy-Duty & Long Lasing » User Friendly Beneits This soluion has a variety of product opions available to meet your needs. To learn more, contact us by calling 833-467-3432. Figure 3 Figure 4
- 5. OCTOBER 2020 | www.processingmagazine.com 35 required, LLE is an excellent choice, particularly as compared against distillation. Regular distil- lation is inefficient because of the high energy consumption that is required to boil water away from the non-volatiles. LLE is a better choice from an operating cost perspective. Extraction and recovery of metals are classically done via LLE processes. • If the compound of interest is heat-sensitive and decomposes with temperature/time, then distil- lation may not be a good choice. LLE can often be performed at low temperatures at atmospheric pressure and thus prevent such decomposition. • When the feed contains azeotropic mixtures, a complicated distillation process (such as a 2-col- umn, pressure swing operation) will be required. LLE can be considered as a cost-efective alter- nate if the proper solvent can be identiied. LLE column technology As Figure 7 shows, there are three major types of extraction equipment: mixer settlers, column contac- tors, and centrifugal extractors. his article will spe- ciically focus on LLE columns. here are two major types of liquid-liquid extrac- tion columns: static and agitated. here are three types of static columns: spray tow- ers, packed columns and trayed columns. Spray type columns are rarely used because they typically only provide up to one theoretical stage. Packed columns are used in reining and petrochemicals applications and use random or structured packing such as SMVP BUILT TO FILL. BUILT FOR LIFE. Just built beter... We also provide installaion, parts and ield service 833-467-3432 www.specialtyequipment.com READY TO SHIP Palleized Drum Filler & IBC / Tote Filler The most versaile and widely accepted illing machine is this palleized drum and IBC/tote iller. It is both a top and sub-surface liquid illing machine for illing a pallet of drums or a single IBC/ tote ranging from 5 to 550 gallons. Standard and explosion proof opion available. » Improved Eiciencies & Accuracy » Increase Producion & Maximize Upime » Heavy-Duty & Long Lasing » User Friendly Beneits This soluion has a variety of product opions available to meet your needs. To learn more, contact us by calling 833-467-3432. Figure 5 Figure 6 “The benefits of LLE position it as a powerful separation technol- ogy able to achieve product purification and recovery, particularly where distillation is not an economically viable option.”
- 6. 36 Processing | OCTOBER 2020 What if you had an experienced engineer on call? Join EASA and access tools you need to stay competitive and effectively repair service and sell rotating equipment. l Leverage technical publications including ANSI/EASA AR100: EASA Recommended Practice for the Repair of Rotating Electrical Apparatus and the EASA Technical Manual. l Stay sharp with relevant training for all skill levels. l Connect with your peers and suppliers worldwide. 1331 Baur Blvd. • St. Louis, MO 63132 USA +1 314 993 2220 • easainfo@easa.com • www.easa.com EASA’s team of experts is ready to answer your questions regarding mechanical application procedures, failure analysis, AC and DC, pumps, vibration analysis and more. Get the edge on the competition and join now! Tom Bishop, P.E. Matthew Conville, P.E. Mike Howell Carlos Ramirez Gene Vogel Chuck Yung Join EASA and discover new levels of success. Visit easa.com/join packing. Sieve tray columns are also used in the rein- ing and petrochemicals industry. These columns consist of a series of sieve trays with either upcom- ers or downcomers, depending on which phase is continuous. Agitated extraction columns offer a number of advantages over static columns. he primary advan- tage is that agitated columns ofer many more theo- retical stages than static column, thus the amount of solvent required is signiicantly reduced. he primary operating cost for an LLE process is often the down- stream recovery of the solvent, usually by distillation. Thus, the operating cost for LLE systems using an agitated extraction column will result is a large reduc- tion in the operating cost as compared to using a stat- ic column. For agitated columns there are three main types: pulsed, rotary and reciprocating. Pulsed columns are often used in the nuclear and inorganic chemicals/metals industries. Examples of the internals use for pulsed columns include packing, sieve trays and disc and donut. For these columns, the internals are stationary, and agitation is applied by pulsing the contents of the column with a certain amplitude and frequency. Columns with rotating internals include RDCs (rotating disc contactors) and SCHEIBEL columns (turbine type impellers). Based upon data published in the literature, the SCHEIBEL column has the high- est eiciency. he KARR column is unique in that it utilizes a plate stack that reciprocates. This type of mixing action has been proven to be optimal for sys- tems with low density diference between the phases and/or low interfacial tension — i.e. systems that tend to emulsify easily such as fermentation broths. RDC, SCHEIBEL and KARR columns are all used in the chemical, biochemical, petrochemical, refining, mining/metals and pharmaceutical industries. Factors influencing the design of LLE columns There are many factors that must be considered when looking to design an extraction column, including the following: • Tendency to emulsify • Efects of impurities or solids • Coalescing/wetting characteristics Figure 7 Figure 8
- 7. OCTOBER 2020 | www.processingmagazine.com 37 • Phase separation time • Mass transfer rate • Rag layer and/or emulsion band formation • Phase ratio variability and density gradients • Axial and radial mixing • Interfacial and drop turbulence efects Trying to factor all of these variables into a theo- retical design has proven to be very difficult. Since there are so many variables in play, the best method of design for liquid-liquid extraction columns is via a well-deined pilot test, followed by proven empiri- cal scale-up. The purpose of pilot plant testing for LLE is to determine the optimal throughput capacity (which sets the diameter) and the HETS (which sets the height of the column). During pilot testing there are a number of variables that must be considered in order to provide the optimal column design. A brief description of these variables follows: Selecting the continuous vs.the dispersed phase Designing a column with the best choice of continu- ous and dispersed phases is critical for an extraction process. In the end, the pilot plant testing will deter- mine the best mode of operation. However, some rules of thumb are as follows: • Phase flowrate ratio — For static columns, the most efficient mode of operation is for the low flowing phase to be the continuous phase. However, for agitated columns the high lowing phase should be the continuous phase. • Viscosity — If either the feed or solvent stream is very viscous, then this almost always is selected as the disperse phase in the column. • Surface Wetting — It is important that the con- tinuous phase preferentially “wets” the internals surfaces of the column. If the disperse phase wets the internals, then the droplets will coalesce resulting in loss of efficiency and throughput. As a rule of thumb, if the aqueous phase is the continuous phase, then the internals should be metal. If the organic phase is the continuous phase, then the internals should be PTFE or an analogous material. • Marangoni Efect — he result from this phenom- enon is that mass transfer of the solute from the con- tinuous to the dispersed phase will be more eicient thantransferfromdispersetocontinuousphase. Other LLE phenomena Interface behavior (rag layer, emulsion band) — Very often in LLE processes, either a rag layer (solids) or an emulsion band (2-phase region) forms at the inter- face. Pilot plant testing with actual feed and solvent can identify either one of these and the degree to which they are problematic, so that acceptable solu- tions can be identiied. Entrainment — Entrainment is not about soluble material, but rather when discrete, non-soluble drop- lets of one phase exit the column with the other phase. he degree of entrainment can be easily measured dur- ing pilot test operating by using a standard laboratory centrifuge. If the entrainment is too large, there are a number of corrective actions such as expanding the diameter of the disengaging chamber and putting a coalescing element in the disengaging chamber. Flooding — When the dispersed phase stops lowing countercurrently to the continuous phase, this is con- sidered looding in an extraction column. It generally occurs when the continuous phase velocity gets too large (capacity limit) or in agitated columns when the agitation speed is too high, thus creating dispersed Hayward is a registered trademark of Hayward Industries, Inc. © 2020 Hayward Industries, Inc. Thermoplastic Valves | Actuation & Controls | Strainers | Filters | Bulkhead Fittings & Tank Accessories | Pumps At Hayward Flow Control, we hear you. We’re in this with you – and work to tackle your biggest challenges with made-in-America products. From design to engineering to assembly – we do it all under one roof in North Carolina so you can get what you need without delay. You’ll see this commitment to performance in big ways in our TBH Series True Union Ball Valves. They handle the widest range of flow control needs – with options up to 6-inches in diameter. With a direct-mount ISO flange, integrated standard lock plate and bolt eyelet mounting foot pad, whatever your demands: TBH delivers rugged performance and exceptional safety and operation. Ready to learn more? Visit haywardflowcontrol.com to view our complete lineup of American-made products. STAY AHEAD OF THE PACK WHEN TIMES GET TOUGH, HOW DO YOU PERSEVERE? TRUE GRIT AND DETERMINATION. “In many cases, LLE is the best choice as a separation technology and well worth searching for a qualified team to assist in its development and design.”
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