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Dr Saad Al-Shahrani

Liquid-Liquid Extraction. CHOICE OF SOLVENT.

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Dr Saad Al-Shahrani

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  1. Liquid-Liquid Extraction CHOICE OF SOLVENT Choosing the best solvent is the most critical aspect of developing a liquid-liquid extraction process. The solvent should have a high selectivity for the extracted solute. The selectivity of a solvent is similar to relative volatility and is given by x1S = weight fraction of component 1 (solute) in the solvent phase x2S = weight fraction of component 2 in the solvent phase x1R = weight fraction of component 1 (solute) in the raffinate phase x2R = weight fraction of component 2 in the raffinate phase ChE 334: Separation Processes Dr Saad Al-Shahrani

  2. Raffinate phase R1, x1R1, x2R1, x3R1 Feed F, z1, z2, z3 Stage solvent phase S1, x1S1, x2S1, x3S1 Fresh solvent So, x1So, x2So, x3So Liquid-Liquid Extraction SINGLE-STAGE CALCULATIONS In one-stage liquid-liquid extractor as shown in the next Figure, Component 1 is the solute, component 2 is the other component in the feed that we are trying to separate from component 1, and component 3 is the solvent. In the normal situation, the process feed contains no solvent, so that z3= 0. We need to specify only two weight fractions in this ternary system. If the solvent is essentially pure, as is often the case, x3So = 1, x1So = 0, x2So = 0 ChE 334: Separation Processes Dr Saad Al-Shahrani

  3. Liquid-Liquid Extraction The total mass balance at this stage is (4) A component balance on the jth component yields (5) Now we define the parameter (6) Where the point M must lie on the straight line joining F and So. It also must lie on a straight line joining S1 and R1 ChE 334: Separation Processes Dr Saad Al-Shahrani

  4. Liquid-Liquid Extraction (7) (8) (9) Since the two liquid phases leaving the system are in phase equilibrium, the points R1 and S1 must be connected by an LLE tie-line. the procedure for determining the compositions and flow rates of the two liquid streams leaving the system is as follows: 1. Calculate M from equation (7). 2. Calculate x1M and x3Mfrom equations (8) and (9). 3. Locate the point M using x1M and x3M. 4. Find the LLE tie-line that passes through the point M. ChE 334: Separation Processes Dr Saad Al-Shahrani

  5. Liquid-Liquid Extraction 5. The points at the two ends of the LLE tie-line give the compositions of the two phases leaving the system: the raffinate-rich phase with composition x1R1 and x3R1and the solvent-rich phase with composition x1S1 and x3S1 6. Calculate the flow rates S1and R1by solving equations (4) and (5) simultaneously: Splitting the mixture of F and So into raffinate-rich phase R1 and solvent-rich phase S1 at ends of a tie-line. ChE 334: Separation Processes Dr Saad Al-Shahrani

  6. Raffinate phase R1, x1R1, x2R1, x3R1 Feed= 10000 z1=0.3 z2= 0.7, z3=0 Stage solvent phase S1, x1S1, x2S1, x3S1 Fresh solvent=5000 x1So,=0 x2So=0, x3So=1.0 Liquid-Liquid Extraction Example:An organic stream, with composition 30 weight percent acetone and 70 weight percent methyl isobutyl ketone and flow rate 10,000 kg/h, is mixed with a pure water solvent with flow rate 5,000 kg/h. What are the compositions and flow rates of the two liquid phases leaving a single-stage liquid-liquid extractor operating at 25oC. Solution: x1So = 0 x2So= 0 x3So= 1.0 z1 = 0.3 z2 =0.7 z3 = 0 ChE 334: Separation Processes Dr Saad Al-Shahrani

  7. Liquid-Liquid Extraction ChE 334: Separation Processes Dr Saad Al-Shahrani

  8. Liquid-Liquid Extraction The point M is thus located at (0.333,0.20), as shown in Figure Note that M lies on the straight line connecting the points F and So ChE 334: Separation Processes Dr Saad Al-Shahrani

  9. Liquid-Liquid Extraction The LLE conjugate line is used to determine the other end of the LLE tie-line on the solvent phase solubility curve. If the tie-line goes through the point M, the compositions of the water solvent phase and organic raffinate phase have been found. We have: ChE 334: Separation Processes Dr Saad Al-Shahrani

  10. Liquid-Liquid Extraction MULTIPLE STAGES WITH CROSSFLOW OF SOLVENT If the process liquid stream from the first stages fed into a second extractor and mixed with more fresh solvent, as shown in the Figure, we have what is called cross-flow extraction. The process can be described the same as for a single stage .it is simply repeated again for each stage, using the raffinate phase from the upstream stage as the feed to each stage. ChE 334: Separation Processes Dr Saad Al-Shahrani

  11. Liquid-Liquid Extraction Mix points for cross-flow extractor. ChE 334: Separation Processes Dr Saad Al-Shahrani

  12. Liquid-Liquid Extraction MULTISTAGE COUNTERCURRENT EXTRACTION Multistage countercurrent extraction is the most commonly encountered liquid-liquid extraction process. The raffinate and solvent streams travel countercurrent to each other through N stages. The flow rate of the raffinate leaving the last stage (tray N) is R. Mass and component balances around the entire cascade give ChE 334: Separation Processes Dr Saad Al-Shahrani

  13. Liquid-Liquid Extraction We next define a pseudo flow rate M and pseudo compositions x1M and x3M: ChE 334: Separation Processes Dr Saad Al-Shahrani

  14. Liquid-Liquid Extraction If the fresh solvent flow rate Soand composition are given along with the feed flow rate F and composition, we can locate the point M on the straight line connecting the points F and So. From equations (10) through (12), it is clear that M must also lie on the straight line connecting S1and RN, as shown in the figure. ChE 334: Separation Processes Dr Saad Al-Shahrani

  15. Liquid-Liquid Extraction In the typical design problem, you will be given: The point RNwill be given, i.e., the concentration of the raffinate phase leaving the final stage will be specified so as to recover the desired amount of the solute from the feed. Note: in any real system we cannot recover all of the solute from the feed Since the points RN and M are known, a straight line can be drawn to the solubility curve to determine the composition of the x1S1 and x3S1 of the S1 stream. Equations (10) and (11) can be used to solve for the flow rates RNand S1. ChE 334: Separation Processes Dr Saad Al-Shahrani

  16. Liquid-Liquid Extraction Since S1and R1are in phase equilibrium, we can use an LLE tie-line to determine the point R1on the solubility curve. ChE 334: Separation Processes Dr Saad Al-Shahrani

  17. Liquid-Liquid Extraction Component and mass balances around the first stage can then be used to calculate the flow rate S2and compositions x1S2 and x3S2 of the solvent entering stage 1 from stage 2: Once S2is known, R2can be found at the other end of the LLE tie-line. This computational procedure can be repeated from stage to stage until enough stages have been used to produce a process stream that meets or exceeds the specifications on RN(the final raffinate phase leaving the unit). ChE 334: Separation Processes Dr Saad Al-Shahrani

  18. Liquid-Liquid Extraction Graphical procedure First, define another pseudo flow rate Δ and pseudo compositions x1Δ and x3Δ This fictitious Δstream serves the same purpose as the operating lines did in binary distillation. We can calculate the compositions of "passing" streams in the column. It provides a graphical way to solve the three mass balances that describe this ternary system simultaneously: ChE 334: Separation Processes Dr Saad Al-Shahrani

  19. Liquid-Liquid Extraction The pseudo composition xjΔis entirely fictitious and, therefore, can be less than zero or greater than unity. Using equations (10) and (19), we see that Therefore, the Δ point must lie on two straight lines, one through the points RN and S0 and the other through the points F and S1. ChE 334: Separation Processes Dr Saad Al-Shahrani

  20. Liquid-Liquid Extraction Δ can lie either to the left or to the right of the phase diagram ChE 334: Separation Processes Dr Saad Al-Shahrani

  21. Liquid-Liquid Equilibrium The mass balances for the first stage (equations (16) through (18), the definition of Δ (equations (19) through (21), and equation(22) can be combined to give The two streams R1and S2 that pass each other between the first and second stages are related to each other by the Δ point. Hence, if we know R1we can determine S2by using the straight line that connects R1and Δ. ChE 334: Separation Processes Dr Saad Al-Shahrani

  22. Liquid-Liquid Extraction ChE 334: Separation Processes Dr Saad Al-Shahrani

  23. Liquid-Liquid Extraction Example: A liquid-liquid ternary phase diagram for isopropyl alcohol (IPA), toluene, and water at 25oC. Feed flow rate is 100 kg/hr, and feed compositions 40 weight percent IPA and 60 weight percent toluene. Fresh solvent is pure water at a flow rate of 100 kghr. Determine the number of equilibrium stages required to produce a raffinate stream that contains 3 weight percent IPA. ChE 334: Separation Processes Dr Saad Al-Shahrani

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