ChE 4162 Liquid-Liquid Extraction Operating Manual

Transcription

1 ChE 4162 Liquid-Liquid Extraction Operating Manual Introduction Liquid-liquid extraction is a relatively new unit operation in industry, though it has been used for centuries in laboratories. It is used to separate materials that cannot easily be separated by other methods. The 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 called the raffinate. When there is a reasonable density difference between the feed and solvent streams, this can be accomplished easily using a vertical column. Apparatus The York-Scheibel liquid extraction column shown in Figure 1 can be used to perform a variety of liquid extraction operations. Feed can be introduced at the bottom of the column (11 stages) or at the middle of the column (6 stages). In this experiment, the typical operational goal is to extract isopropanol (5-to-20 wt. %) from ISOPAR E (a mixture of C 8-to-C 10 hydrocarbons with physical and chemical properties nearly those of n- nonane) using pure water as the solvent. The laboratory 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 horizontal thrust agitators mounted on a 3/16" diameter shaft. A variable speed motor, with a control knob and digital readout on the control panel, FIGURE 1 - YORK-SCHEIBEL LIQUID-LIQUID EXTRACTION APPARATUS 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. Edited from Dooley/Hadlock original by HJ Toups and KM Dooley revised Spring

2 Rotameter Equations 1 The following equations relate rotameter readings to volumetric flow rates: Where F f is the feed flow rate (~10 wt. % IPA) in ml/min, R f is the feed rotameter reading, F s is the solvent flow rate in ml/min, and R s is the solvent rotameter reading. F f R f (1) F s Rs (2) These calibrations are useful for planning and trial run purposes. Inasmuch as the feed is variable in composition, the feed rotameter calibration is subject to an additional disclaimer. Equilibrium Data for Water/Isopropanol/n-Nonane In this experiment, we assume the properties of n-nonane are a good approximation of ISOPAR E for equilibrium data purposes. The ternary system water/isopropanol/n-nonane exhibits Type I equilibrium behavior at room temperature. The equilibrium data for this system can be found in an attachment to this operating manual. Equilibrium Data for Water/Isopropanol/n-Nonane/NaCl If the normal ternary system is augmented with NaCl in the incoming aqueous solvent, necessary equilibrium may have to be developed internally to any experimental program. Operating Procedure 1. Bleed a small volume from the raffinate tank and check for water. If water is present, bleed tank until water is removed. This water is added to the extract tank. 2. Pump contents of the raffinate tank back up to the feed tank. 3. Check feed composition either in the feed tank (~ 35L) or at the sample point by GC analysis and adjust the IPA wt. % if it s too low. If too high, do not adjust. N.B.: If the feed composition has been established by your instructor and produced by the Lab Coordinator, it may be not necessary or advisable to adjust it. Please check first. 4. If pure water is being used as solvent, the solvent tank should be full. If not, check with the Lab Coordinator to identify appropriate water solvent source. If NaCl-laden water is being used as solvent, it may be necessary to add salt to the water solvent in an amount suitable to meet program specifications. 5. Fill the extractor with ISOPAR E /IPA feed (if necessary) and bleed air from the feed line. Turn off the feed flow. 1 Ambient temperature information for the calibration is not available. Edited from Dooley/Hadlock original by HJ Toups and KM Dooley revised Spring

3 N.B.: Check with your instructor to determine if the column should be filled with feed. In some studies, it might be advised to first fill it with water and then start feed flow. 6. Start the mixer and keep agitator speed constant. 7. Open the solvent, feed, extract, and raffinate ball valves; and start the flow of solvent (water) into the column. 8. If no interface is present between the solvent entrance and the raffinate exit, let the dispersed phase rise and form the upper interface. 9. When the upper interface forms, (re-)start the feed flow. 10. Control the interface level by adjusting the height of the inverted U on the extract line from the bottom of the tower. 11. Periodically check the raffinate stream for steady state by gas chromatography. 12. Check the composition of the extract stream by hydrometer. This provides a check on attainment of steady state, and on the IPA mass balance. Operating Notes 1. The extract stream (water plus IPA) composition versus specific gravity tables are found in Perry s Handbook. 2 Use these data to interpolate weight percent. (Look closely; there is an error in the Perry s table.) N.B.: If NaCl-laden solvent water is used, it may be necessary to generate composition versus gravity tables internal to the experimental program. 2. The upper interface level adjustment (inverted U) is sensitive. Movements of a fraction of an inch are often sufficient. 3. The specific gravity of the feed or raffinate can (and should) be measured by hydrometer. 4. There appears to be a problem with emulsification in the column at low solvent to feed ratios (ml/min solvent / ml/min feed). Ratios below 1.2 may lead to emulsification and even higher ratios (~1.5) may cause problems at very high agitator speeds (i.e., >1200 rpm). Such conditions should be avoided unless specifically designated as part of an experimental program request. 5. Assume that the properties of n-nonane are a good approximation for those of ISOPAR E, especially for equilibrium data purposes. The ternary water/ipa/n-nonane system exhibits Type I equilibrium behavior at room temperature. Gas Chromatograph (GC) Operation The GC (Agilent 7890B) is used to separate and quantify the components in a sample. Each component will be recorded as a separate peak on the program (all the components of ISOPAR E will be combined by the program to give the "nonane" result). Each peak's area percent reading is related to the wt. % in the sample. Unless instructed otherwise, you must calibrate the GC to obtain this relationship using standard calibration samples, which you will make up yourselves (usually at least 3, e.g., 0.5, 1.5, 2.5 wt%). (Neat ISOPAR E and reagent-grade isopropanol are available) Typical sample injection volume is 0.1 L via a dedicated syringe. The elution order is isopropanol and then Isopar E (which gives several peaks spaced closely). Edited from Dooley/Hadlock original by HJ Toups and KM Dooley revised Spring

4 First, be sure that the helium, air and H 2 cylinders are open and showing pressure on their regulators. If not, see the Lab Coordinator or your Instructor. The control program should be open on the computer (che-uolab-gc5.lsu.edu). If not, click on ChE-7890B-01 online. A popup will appear, select Upload from Instrument. Load method START.M. Do NOT save any changes to this method (or any other), unless instructed. Wait at least 10 min before injecting anything, and be sure the GC comes to its Ready (green button) state in the program. There are 3 main icons on the right of the top menu. The 1 st one will tell you if you are ready, and if there is a problem inform you what it is. The 2 nd icon will give the chromatogram plot. The Instructor or lab coordinator will demonstrate the correct syringe injection technique (0.1 L). You inject to the REAR injector. After injection, hit the START button on the GC. When the analysis finishes, get the area% s by clicking the Data Analysis button on the left side of the program display. Assuming one injection will give perfect results is erroneous. You need repetition to establish decent precision, and some samples will give totally bogus results that should be discarded completely. You should be able to recognize these. At the end of each day, do NOT turn off the GC NOR close the supply gas cylinders! Load the Standby method, however. Some Safety Considerations 1. Return all unneeded feed and raffinate sample material to the raffinate storage tank, place the empty bottles on top of the LLE GC, and put the bottle caps in the drawer. If there are feed or raffinate samples that you must save till a later time to analyze by GC, cover the cap of these bottles with Parafilm, place them in a beaker in the laboratory refrigerator. Make sure the beaker is labeled with your name(s) and the chemical nature of the samples. 2. Return all extract sample material to the extract storage tank. 3. Clean up any spills. 4. Be careful with the hydrometers - they are fragile. Shutdown Procedure When leaving for the day, turn off the agitator (unless instructed otherwise) and main power switch. Close the feed and solvent ball valves. Leave the raffinate and extract ball valves open. Analysis of Solvent Extraction Several methods are available to analyze liquid-liquid extraction data. Some of these are described here: a) McCabe-Thiele method, b) the Kremser equation, and c) Aspen or HYSYS simulation software. McCabe-Thiele Method (Solute-free Basis) 3 A stagewise representation of counter-current liquid-liquid extraction is shown in Figure 2. Edited from Dooley/Hadlock original by HJ Toups and KM Dooley revised Spring FIGURE 2 - STAGEWISE REPRESENATION OF THE EXTRACTION PROCESS

5 F = moles/sec of feed diluent, n-nonane, flowing in the column; approximately constant through the column. S = moles/sec of extraction solvent, water, flowing in the column; approximately constant through the column. X f = moles IPA per mole n-nonane in the feed. Y s = moles IPA per mole water in entering solvent = 0. Y e = moles IPA per mole water in extract stream leaving the column. X r = moles IPA per mole n-nonane in the raffinate stream leaving the column. At steady state, a material balance on IPA between the feed end of the column and any stage, n (see dotted outline above) is: Solving for Y n+1 produces the operating line: F X f SYn 1 SYe F Xn (3) Y n1 F X S n Y e F X S f (4) Since F and S are approximately constant through the column, this will be a straight line with slope F/S. It relates the compositions Y n+1 and X n of the two immiscible streams passing one another at any point in the column. In particular, the equation is satisfied at both ends of the column, so the points (X f, Y e) and (X r, Y s) lie on the line. Hence, if these two points are determined experimentally, the operating line can be plotted as a straight line on the X-Y diagram. You can use the data in the handout to generate an equation for the equilibrium curve, and plot it on the same diagram. Then the number of theoretical stages required for the given separation can be stepped off in the usual McCabe-Thiele procedure. Or you can do this numerically using Excel or MATLAB. The Kremser Equation If the equilibrium line, as well as the operating line, is straight, then the Kremser equation (see your Unit Operations textbook or Perry's for more detailed information on Solvent Extraction) can be used to solve for the number of theoretical stages. The equilibrium line must be of the following form: Where the slope, m, is constant. Y = mx (5) If the equilibrium line is not straight but the slope does not change much, the geometric mean of m at the two ends of the column can be used in the Kremser equation to given an approximate answer. The ASPEN and HYSYS programs Edited from Dooley/Hadlock original by HJ Toups and KM Dooley revised Spring

6 These can do rigorous stage-to-stage calculations assuming equilibrium stages. It is available on the computers in the lab and in If it is to be used in analysis of program results, the instructor will either show you how to use it or will supply you a template that employs either the NRTL or UNIQUAC methods to model the equilibrium relationship. Droplet Diameter and Holdup in Liquid/Liquid Extraction What follows here is some mass transfer theory related to liquid-liquid extraction. It may or may not be relevant to individual experimental program requirements. The mass transfer coefficient(s) and the interfacial area for mass transfer together make up the resistance or lack thereof to movement of solute from one phase to another. Key experimental variables can affect either or both of these items for the better or the worse. In order to determine the effects of the key variables in this experiment on the interfacial area between the phases, two approaches are possible. One is to find a specific correlation for interfacial area for a stirred contactor of this type (4-blade paddles). The other is to use separate correlations for the "holdup" (volume dispersed phase/total volume) and the Sauter mean drop size, d vs, which is the average drop size based on a volume/surface area calculation. The interfacial area in a stage can be related to these two quantities (derive the relationship). For holdup,, many conflicting correlations exist. 3-4 However, a working model is: ε AE m U n d exp(b U c ) (6) Where A and B are constants which depend on almost all of the standard liquid physical properties of both phases, U c is the superficial continuous, and U d the superficial dispersed phase axial velocities, m ~0.75 and n~1. The quantity E is a fundamental one in all two-phase flow processes, called the energy dissipation/unit mass. An increase in E always leads to smaller droplet or bubble sizes, and usually (but not always, especially at low U d) to a higher value of holdup,. E is given for the case of a two-phase liquid/liquid agitated dispersion by: 3 4N pn D πd H E 2 c 5 R (7) Where N is the rotor speed, D R is the rotor diameter, D c is the column diameter, H is the stage or compartment height, and N p is the power number, a characteristic function for a given type of agitator. For turbulent flow, N p is roughly a constant. For laminar flow, N p is inversely proportional to N. For droplet size, almost all correlations, for any agitator type, are of the form: 3-4 Edited from Dooley/Hadlock original by HJ Toups and KM Dooley revised Spring

7 d D vs R K We 0.6 (8) K is (again) a physical property-dependent constant, and We is the characteristic Weber number, which for this process is: We N 2 ρ d σ D 3 R (9) Where = interfacial tension between phases, and d is the density of the dispersed phase. The Weber number, We, represents the ratio of inertial (kinetic) forces to forces arising from surface tension; analogous to the Reynolds number, Re (the ratio of inertial to viscous forces). From the above equations and the mass transfer coefficient correlations, the effects of such variables as N, U d and U c on the mass transfer flux can be derived. The relevant mass transfer correlations for both phases can be found in many sources. 3-7 References 1. Taylor, J. A., Uncertainty Analysis, (Last accessed 01/13/06). 2. B.E. Poling, G.H. Thomson, D.G. Friend, R.L. Rowley and W.V. Wilding, Ch.2 of Chemical Engineers Handbook, 8 th Edition, R.H. Perry and D.W. Green, Eds., McGraw-Hill, New York, Another good reference other than your book: T.C. Frank, L. Dahuron, B.S. Holden, W.D. Prince, A.F. Seibert and L.C. Wilson, Ch. 15 of Chemical Engineers Handbook, 8 th Edition, R.H. Perry and D.W. Green, Eds., McGraw-Hill, New York, J.C. Godfrey, R. Reeve and F.I.N. Obi, Chem. Eng. Prog. Dec pp A. Kumar and S. Hartland. Ind. Eng. Chem. Res. 34, (1995). 6. McCabe, W. L., J. C. Smith and P. Harriott. Unit Operations of Chemical Engineering, 7 th Ed. New York: McGraw-Hill, Alatiqi, I, Aly, G., Mjalli, F. and Mumford, C.J., Canad. J. Chem. Eng. 73, 523 (1995). Edited from Dooley/Hadlock original by HJ Toups and KM Dooley revised Spring