Source: Kerry M. Dooley and Michael G. Benton, Department of Chemical Engineering, Louisiana State University, Baton Rouge, LA
Liquid-liquid extraction (LLE) is a separation technique used instead of distillation when either: (a) the relative volatilities of the compounds to be separated are very similar; (b) one or more of the mixture components are temperature sensitive even near ambient conditions; (c) the distillation would require a very low pressure or a very high distillate/feed ratio.1The driving force for mass transfer is the difference in solubility of one material (the solute) in two other immiscible or partially miscible streams (the feed and the solvent). The feed and solvent streams are mixed and then separated, allowing the solute to transfer from the feed to the solvent. Normally, this process is repeated in successive stages using counter-current flow. The solute-rich solvent is called the extract as it leaves, and the solute-depleted feed is the raffinate. When there is a reasonable density difference between the feed and solvent streams, extraction can be accomplished using a vertical column, although in other cases a series of mixing and settling tanks may be used.
In this experiment, the operational goal is to extract isopropanol (IPA, ~10 - 15 wt. %, the solute) from a mixture of C8-to-C10 hydrocarbons using pure water as solvent. A York-Scheibel type (vertical mixers and coalescers, one each per physical stage) extraction column is available. Like most extractors, the overall efficiency (number theoretical stages/physical stages) of this column is quite low, especially in comparison to many distillation columns. The low efficiencies arise from both slow mass transfer (two liquid resistances instead of one as in distillation) and often also from maldistribution of the phases. The effect of agitator speed on both the solute recovery in the extract and the overall column efficiency will be evaluated.
Either the a) McCabe-Thiele method, or b) a process simulator (e.g., ASPEN Plus, HYSYS, ChemSep) may be used to estimate the number of equilibrium (theoretical) stages. The McCabe-Thiele method is employed on a solvent-free basis, meaning both the solubilities of the solvent in the extract and of the diluent compound in the raffinate are neglected. A stagewise representation of counter-current liquid-liquid extraction is shown in Figure 1, where F' is the molar flow rate of the feed (approximately constant), S' is the molar flow rate of extract (approximately constant), Xf is the mole fraction of solute in feed, Ys is mole fraction of solute in the solvent, Ye is mole fraction of solute in solvent in extract stream, and Xr is mole fraction of solute in diluent in the raffinate stream.
Figure 1: Stagewise representation of the extraction process.
At steady state, a material balance on the solute between the feed end of the column and any stage, n (dotted outline above) leads to the operating line:
In particular, the equation is satisfied at both ends of the column, so the points (Xf, Ye) and (Xr, Ys) lie on the line.The equilibrium data in the Appendix can be used in conjunction with this equation (either graphically or numerically) to step through the column.
The process simulators can do more rigorous stage-to-stage calculations, but still assuming equilibrium stages. Either the NRTL or UNIQUAC methods (both sets of parameters in the Appendix) can be used to model the equilibrium relationship. Note that the big advantage of the simulators is that they DO tell you how much solvent winds up in the extract and how much diluent winds up in the raffinate. They also can give the exit temperatures for an adiabatic column, or the heat duty needed to keep the column isothermal.
A York-Scheibel apparatus is shown in Figure 2. Feed can be introduced at the bottom (11 stages) or at the middle of the column (6 stages).
Figure 2: York-Scheibel liquid-liquid extraction apparatus.
The extraction unit consists of a 2" I.D. Pyrex column, with 11 extraction stages, each consisting of a one-inch mixing section and a four-inch wire mesh packing (coalescing) section. The column is mechanically agitated by paddlewheel-type (Rushton turbine) agitators. A variable speed motor, with a control knob and digital readout on the control panel, controls the speed of the agitator. Rotameters on the feed and solvent inlets are used to measure those flow rates. Flow rates of the extract and raffinate can be measured with a graduated cylinder and stopwatch.
The following equations relate the rotameter readings to volumetric flow rates (the flows can also be checked with a graduated cylinder):
where Ff is the feed flow rate (~10 wt. % IPA) in mL/min,Rf is the feed rotameter reading,Fs is the solvent flow rate in ml/min, andRs is the solvent rotameter reading.
In this experiment, the properties of n-nonane are a good approximation to those of the hydrocarbon mixture for equilibrium data purposes. The ternary system water/isopropanol/n-nonane exhibits Type I equilibrium behavior (there is some composition range over which phase splitting will not take place) at room temperature. The equilibrium data for this system can be found in the Appendix.
1. Operating the York-Scheibel Column
- Fill the extractor with hydrocarbon mixture /IPA feed (if necessary) and bleed air from the feed line. Turn off the feed flow.
- Start the mixer and keep the agitator speed constant.
- Open the solvent, feed, extract, and raffinate ball valves; and start the flow of solvent (water) into the column.
- If no interface is present between the solvent entrance and the raffinate exit, let the dispersed phase rise and form the upper interface.
- When the upper interface forms, (re-)start the feed flow.
- Control the interface level by adjusting the height of the inverted U on the extract line from the bottom of the tower.The upper interface level adjustment (inverted U) is sensitive. Movements of a fraction of an inch are often sufficient.
- Periodically check the raffinate stream for steady state using gas chromatography. The gas chromotagraph will separate and quantify the components in the sample.
- Use a hydrometer to measure the specific gravity of the extract stream and determine the composition. (This also can help confirm that steady state has been reached.) The extract stream composition vs. specific gravity tables can be found in Perry's Handbook.3 This data can be used to interpolate for weight percent of IPA.
- Use a hydrometer to measure the specific gravity of the feed and raffinate for use in subsequent calculations.
2. Shutdown Procedure
- Once the experiments are complete, turn off the agitator and main power switch.
- Close the feed and solvent ball valves, leaving the raffinate and extract ball valves open.
Liquid-liquid extraction, or LLE, is a technique used to separate liquids that can not be separated with distillation due to temperature-sensitive components or similar solvent boiling points. Mass transfer in LLE is driven by the solubility difference of the solute in the immiscible or partially miscible feed and solvent streams. The feed stream containing the solute is mixed with the solvent stream, often using an agitator, allowing the solute to transfer from the feed to the solvent. The depleted feed, known as the raffinate, is separated from the extract, which is the solute-rich solvent phase. This video will illustrate an extraction of isopropanol from n-nonane, using pure water and study how operating variables affect the overall column efficiency.
LLE is typically performed in continuous stages using co-current or counter-current flow. Counter-current systems are generally preferred, as they tend to be more efficient. Usually, the stages are housed within a single unit. Counter-current extraction columns can be set up two ways. When the solvent is heavier than the feed liquid, or diluent, the solvent is introduced at the top of the column and the solute then exits at the bottom. When the solvent is lighter than the diluent, the solvent is introduced at the bottom of the column, and the solute will exit the column at the top. At steady-state, the material balance of the solute between the feed-end of the process and any stage denoted by N is as shown. Where X is the mole fraction of solute in the diluent, Y is the mole fraction of solute to the solvent. F is the molar flow-rate of feed diluent and S is the molar flow-rate of solvent. The analysis of theoretical plates is used to evaluate the efficiency of the separation process. These plates are hypothetical stages where two phases are in equilibrium with each other. If the two liquids are in equilibrium at a stage, meaning that there would be no change in concentration of either given longer mixing time, then the stage is considered to be a theoretical plate. The higher the number of theoretical plates, the more efficient the process. Operating variables such as temperature, pressure, flow rates and agitator speed affect efficiency and therefore theoretical plate analysis. The following experiment will examine an LLE process to extract the solute isopropanol from n-nonane using water as the solvent. This system, containing three liquids, is called a ternary system. Often, all three liquid components are miscible to some degree. Equilibrium behavior for these and other solvents can be found in the literature. Now that the basics of LLE and its operation have been explained, let's take a look at a separation process.
A York-Scheibel column will be used for this experiment. It is an agitated column with internal paddle-wheel impellers, connected to one vertical drive. Each level contains wire-mesh packing to enable phase-separation, and is separated by partitions to provide individual stages. First use the control knob and digital readout on the control panel to control the speed of the agitator. Use the rotameters on the feed and solvent inlets to measure the flow rate of feed and solvent. Use graduated cylinders and a watch to measure the flow rates of the extract and raffinate. Now start the mixer and keep agitator speed constant at 300 rpm. Open the ball valves for the solvent, feed, extract and raffinate. Start the flow of water into the column to obtain the desired solvent-to-feed molar ratio at a rate of 200 milliliters per minute. Observe whether an interface between solvent entrance and raffinate exit is present, and if not, let the dispersed phase rise to form the upper interface. Start the feed flow when the upper interface forms. Carefully adjust the height of the inverted U on the extract line from the bottom of the tower, to control the level of the upper interface between the two phases. This assures that the raffinate phase does not flow into the extract tank if adjusted too low, or that the extract is not flowing into the raffinate tank if interface is set too high.
Collect samples every 10 minutes at the raffinate sample point in four milliliter bottles. Use gas chromatography, or GC, to quantify the components and confirm that a steady-state has been reached. Next, using a clean graduated cylinder, collect 250 milliliters of the extract at the sample point, then measure the specific gravity or relative density of the sample to water, using a hydrometer. Interpolate the weight per cent of the isopropanol in the sample using the provided table, which displays extract stream composition versus specific gravity. Repeat the procedure for two other lower feed-rates. Make sure to keep both the solvent-to-feed ratio and the agitator speed constant. When finished, turn off the agitator and main power switch and close the feed and solvent ball valves, leaving the raffinate and extract ball valves open. Now let's evaluate the results.
First, let's take a look at the per cent recovery of isopropanol with varied feed-flow rate and varied agitation rate. With increased feed-flow rate, per cent recovery increases and levels off. This is typical of a system which is not near flooding. Increased agitation rate also increased per cent recovery. Stage efficiency, calculated using theoretical plate analysis via computer simulation is also affected by these parameters. As expected, both stage efficiency and per cent recovery increase with higher flow-rate and agitation. This is due to improved mixing, which results in smaller droplets and improved dispersion, thereby improving mass transfer, however, both relationships plateau at higher feed rates. Efficiency and per cent recovery level off and eventually decrease due to emulsification and flooding. The formation of an emulsion negatively impacts recovery and efficiency because the phases can no longer separate cleanly in order to move up or down to the next stage. This can be a problem in systems like the York-Scheibel unit, making mixer and settler vessels in series an appealing alternative.
Liquid-liquid extraction is a separation technique used in a wide range of separations and can be used in a variety of setups. In the case where the emulsification of phases is a challenge, mixer settler tanks in series can be used. This simple setup utilizes a tank where the two phases are mixed by an agitator. The two phases then coalesce in the settler tank, where the heavy phase eventually settles to the bottom and is removed through an outlet on the bottom of the tank. The light phase settles to the top and is removed via another outlet. Another separation technique that harnesses the solubility properties of a solute is solid-liquid extraction. In solid-liquid extraction, the solute present in a solid matrix is extracted into a liquid through vigorous mixing. This technique is used on a large scale for many applications, such as the removal of toxins like the herbicide Atrazine from soil.
You've just watched Jove's introduction to liquid-liquid extraction. You should now understand the basics of an LLE extraction using a York-Scheibel column and how process variables such as agitator and flow-rate can affect the solute recovery and efficiency of the column. Thanks for watching.
Figures 3 and 4 show results when both the agitation and feed flow rates were varied over a wide range. The overall efficiency and recovery increase before becoming asymptotic, which is fairly typical of liquid-liquid extractors that are not at or near flooding. At near flooding conditions, the overall efficiency and recovery are expected to sharply decrease. Note that, unlike distillation, flooding can take place in liquid-liquid extraction at either high solvent or high feed rates (or ratios).1 In this experiment, the lighter organic phase is also the dispersed (droplet) phase, so at high feed rates it is expected that the droplets coalescence prior to flooding, leading to lower rates of mass transfer and, therefore, lower recoveries and efficiencies. At high solvent rates the droplets should remain small, so it is expected that the recovery and efficiency remain high until very near the flooding point.
Figure 3: Percent recovery of IPA from hydrocarbon mixture into water, for a York-Scheibel column, 11 stages, 16 - 18 mol% IPA in Isopar E (feed), S/F (molar) = 1.5.
Figure 4: Percent overall stage efficiency for IPA extraction using a York-Scheibel column, 11 stages, 16 - 18 mol% IPA in hydrocarbon mixture (feed), S/F (molar) = 1.5.
As seen from Figures 3 and 4, increasing the agitation rate increases both the recovery and overall efficiency. This is because with greater power input the droplets of the dispersed phase are smaller - the observed dependence is roughly inverse with respect to agitator speed.4 The "a" parameter (interfacial area/total volume) that appears in mass transfer correlations and fundamental mass flux equations can be written as follows for uniform-size spherical droplets:
a = 6 ε/d (4)
where ε is the volume fraction of the dispersed phase. While ε can increase with an increase in either phase's superficial velocity, its changes are usually less marked than the change of diameter with respect to speed. So (usually) the more speed, the more interfacial area, leading to faster mass transfer.
The exception to the above discussion is at very high speeds, which were not reached in Figs. 3 and 4, where the two phases are so well-mixed that if the interfacial tension between them is low, emulsification will take place. The formation of an emulsion negatively impacts recovery and efficiency because the phases can no longer separate cleanly to move up or down to the next physical stage.Emulsification is a problem in many liquid-liquid extractions and where it cannot be limited a series of mixer and settler vessels in series is often preferred to column-type designs such as sieve trays or York-Scheibel units.
Applications and Summary
Liquid-liquid extraction (LLE) is an alternative to distillation which relies upon solvent-feed immiscibility (or slight miscibility) and favorable solute partition coefficients to attain high solute recoveries in a solvent phase at as low a solvent/feed ratio as practical. Although the range of flows (the "turndown") over which LLE will be effective is often limited, and while stage efficiencies are low such that phase equilibrium is not attained, certain mixtures just cannot be separated using other methods in a continuous countercurrent process. Mathematical analysis of the equilibrium operation of such extractors follows a familiar McCabe-Thiele-type procedure (although reflux is often lacking, so only one operating line). The non-equilibrium ("rate-based") analysis of LLEs is complex and depends strongly on the relative velocity between the two phases (the "slip velocity"), bubble size, and dispersed phase fraction, all of which can be observed but are difficult to predict.
To perfectly describe the hydraulics and mass transfer of a typical LLE is beyond the capability of even the most sophisticated process simulators, at present. Therefore, design of industrial units still relies on scale-up from pilot-plant-type units, such as that which was tested in this experiment. Normally the engineer attempts to duplicate key descriptors such as the "a" parameter, solvent/feed ratio, total agitator power input/volume, feed location and number of physical stages to keep the stage efficiency and recovery constant during scale-up. Even so, scale-up is an inexact science, and impurities, which alter the interfacial tension, can greatly impact the performance of real systems. The more factors that are held constant, the more likely the scale-up will be successful.
There are many different LLE contactors: a series of mixers - settler vessels, structured packings similar to those used in absorbers and distillation columns, sieve tray columns, rotating disk contactors (similar to the York-Scheibel, but with baffles instead of mesh), Kuhni contactors (a combination of rotating disk and sieve trays), and Podbielniak contactors ("Pods"), where the flow is radial and centrifugal force is used to enhance liquid phase separation.5
A classic example of industrial LLE is the separation of acetic acid from water using ethyl ether or ethyl acetate;6 it is preferred over distillation at lower acetic acid concentrations. Possibly the biggest volume LLE process is that of propane deasphalting, which is used to refine lubricating oils in refineries at near-supercritical conditions.1 However, most applications are found in the production of specialty chemicals and in pharmaceutical industries, ranging from citric acid extraction from fermentation broth to purification of antibiotics and protein purifications.1In these cases, a wide variety of oxygenated organic solvents or two-phase aqueous systems (with one phase being mostly water and the other aqueous dissolved salts and polymers) are utilized. For the latter, a typical polymer system is poly(ethylene glycol)/dextran with NaCl and Na2SO4 as salts. Applications include red blood cell separation and extraction of the phophofructokinase enzyme from S. cerevisiae.7
Appendix – Equilibrium Data8
Experimental Tie Lines in Mole Percent at 25 °C
Specific Model Parameters in Kelvin
|UNIQUAC||NRTL (a = 0.2)|
R1 = 0.92 R2 = 2.7792 R3 = 6.523
Q1 = 1.4 Q2 = 2.508 Q3 = 5.476
|Mean Deviation between Calculate and Experimental Concentrations in Mol. %|
|UNIQUAC (specific parameters)||1.4|
|NRTL (specific parameters)||0.54|
|UNIQUAC (default parameters)||1.68|
- T.C. Frank, L. Dahuron, B.S. Holden, W.D. Prince, A.F. Seibert and L.C. Wilson, Ch. 15 of “Chemical Engineers Handbook, 8th Edition”, R.H. Perry and D.W. Green, Eds., McGraw-Hill, New York, 2008.
- W.L. McCabe, J.C. Smith, and P. Harriott, “Unit Operations of Chemical Engineering”, 7th Ed., McGraw-Hill, New York, 2005, Ch. 23; C.J. Geankoplis, “Transport Processes and Unit Operations”, 3rd Ed., Prentice-Hall, Englewood Cliffs, 1993, Ch. 12; R.K. Sinnott, “Coulson and Richardson’s Chemical Engineering Vol. 6 – Chemical Engineering Design (4th ed.): http://app.knovel.com/hotlink/toc/id:kpCRCEVCE2/coulson-richardsons-chemical/coulson-richardsons-chemical
- B.E. Poling, G.H. Thomson, D.G. Friend, R.L. Rowley and W.V. Wilding, Ch.2 of “Chemical Engineers Handbook, 8th Edition”, R.H. Perry and D.W. Green, Eds., McGraw-Hill, New York, 2008.
- J.C. Godfrey, R. Reeve and F.I.N. Obi, Chem. Eng. Prog. Dec. 1989. pp. 61-69; I. Alatiqi, G. Aly, F. Mjalli and C.J. Mumford, Canad. J. Chem. Eng., 73, 523-533 (1995).
- http://www.pharmaceuticalonline.com/doc/podbielniak-contactor-a-unique-liquid-liquid-0003 (accessed 12/19/16).
- C.J. King, Ch. 18.5 of “Handbook of Solvent Extraction”, T.C. Lo, M.H.I Baird and C. Hanson, Eds., Wiley, New York, 1983.
- “Methods in Enzymology, Vol. 228, Aqueous Two-Phase Systems,” H. Walter and G. Johannson, Eds., Academic, San Diego, 1994.
- A.I. Vorobeva and M.Kh. Karapetyants, Zh. Fiz. Khim., 41, 1984 (1967). Fits to data from: J. Gmehling, and U. Onken, "Vapor-liquid equilibrium data collection", Dechema, 1977.