SEPARATION PROCESSES
For numbers 1 to 2:
1. Determine the relative volatility of n-butane/n-pentane mixture at 300C and 1 atm abs
a. 2.55
c. 3.46
b. 3.04
d. 4.28
2. One hundred mols of 15% by mole n-butane/n-pentane mixture is distilled until the residual liquid contains 45.5 mols. Using the
value of the relative volatility obtained in problem no.1, find the average concentration of the distillate in mol% butane.
a. 20.6
c. 32.4
b. 24.7
d. 48.5
For numbers 3 to 5:
The distribution equilibrium for A between an extract solvent S and a raffinate solvent B is given Y=2X where Y=mass of A per unit
mass of S, X=mass of A per unit mass of B. The extract and raffinate solvents are immiscible with each other at all concentrations
of A. From these data, calculate the amount of extract solvent needed per 100 kg solution containing 30% A in B in 95% of A is to
be removed for each of the following arrangements:
3. Single stage contact
a. 665 kg
b. 565 kg
c. 180 kg
d. 70 kg
4. Three-stage batch contact, one third of the solvent will be used in each contact
a. 60 kg
c. 210 kg
b. 665 kg
d. 180 kg
5. Three-stage counter-current operation
a. 70 kg
b. 79 kg
c. 280 kg
d. 160 kg
6. Powdered limestone (CaCO3) containing 10000 ppm NaOH is to be washed in a two-step continuous counter current washing
system to remove most of the NaOH. Two cells and 100000 L of water per 10 MT/day of limestone feed are used. The slurry
discharged and removed from the underflow contains 0.091 MT of water per MT CaCO3. Assuming complete mixing and washing,
calculate the NaOH content of the washed and dried limestone in ppm.
a. 0.77 ppm
c. 0.82 ppm
b. 0.65 ppm
d. 0.97 ppm
7. Ten kg of a solid containing 45% of soluble material were treated with 15 kg of a solvent containing the same solute at 2%
concentration in a vessel under constant agitation. After a long time, the solution and the solid were separated by pressing. The
solid analyzed 0.8 kg of solvent per kg of inert solid. The extract quantity obtained in kg was therefore
a. 14
c. 28
b. 22
d. 19
For numbers 8 to 10:
An aqueous solution containing 1.5 kmol X / m3 is fed at 36 ml/s to the top of the column of height 1.60 m and cross sectional area
of 0.0045 m2 and it leaves at the bottom with 1.4 kmol / m3. An organic solvent containing 0.006 kmol X / m 3 flows counter to the
aqueous phase at 9 ml/s. The equilibrium relationship is given by: CX organic = 0.3 CX aqueous. Calculate:
8. The log mean concentration difference for the transfer.
a. 0.162
c. 0.233
b. 0.098
d. 0.345
9. The overall volumetric transfer coefficient based on the organic phase.
a. 1.15x10-3 /s
c. 4.23x10-3 /s
b. 3.08x10-3 /s
d. 2.60x10-3 /s
10. The height of transfer unit.
a. 0.88 m
b. 0.71 m
c. 0.54 m
d. 0.65 m
11. It is desired to produce an overhead product containing 80 mol% benzene from a feed mixture of 68 mol% benzene and 32
mol% toluene. If the mixture is subjected to equilibrium distillation at 1 atm, what is the fraction of the liquid that is vaporized?
a. 1/3
c. 1/2
b. 1/4
d. 2/3
12. A vessel contains a liquid mixture of 50% benzene and 50% toluene by weight at 100 0C. Calculate the average molecular
weight of the vapor in contact with the solution.
a. 85 g/mol
c. 84 g/mol
b. 82 g/mol
d. 86 g/mol
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For numbers 13 to 15:
A 537 kg solution of 50% wt octane and 50% wt heptane is batch-distilled until only 4.74 lbmols remain in the still. Assuming α=2.0,
calculate the following:
13 The total moles of distillate
a. 6.36 lbmol
b. 5.09 lbmol
c. 8.34 lbmol
d. 7.75 lbmol
14. The percentage by mol of heptane in the distillate
a. 40%
c. 30%
b. 50%
d. 60%
15. The octane concentration of the residue by weight
a. 40%
c. 30%
b. 50%
d. 60%
16. The vapor pressures of n-heptane and toluene at 373 K are 106 and 73.7kN/m2 respectively. What is the mole fraction of nheptane in the vapor phase at 373 K if the total pressure is 101.3kN/m2?
a. 0.856
c. 0.896
b. 0.942
d. 0.964
17. It is desired to reduce the concentration of pyridine in 500 kg of aqueous solution from 20 weight percent to 5 wt percent in a
single batch extraction using chlorobenzene as solvent. Equilibrium compositions (end points of the tie line) in terms of weight
percent of pyridine-water-chlorobenzene are (5, 95, 0) and (11, 0, 89). The amount of pure solvent required in kg for the operation
is
a. 607
c. 639
b. 931
d. 1501
18. The vapor phase composition and relative volatilities (with respect to n-propane) on an ideal tray of a distillation column are:
Component
Mole fraction in vapor
Relative volatility
Methane
0.12
10
Ethane
0.28
4
n-propane
0.6
1
The mole fraction of n-propane in the liquid phase is _____.
a. 0.90
c. 0.83
b. 0.77
d. 0.65
For numbers 19 to 20:
One hundred kg of a solid containing 40% solute A and 60% inert B is treated with solvent C in a single stage single contact
extraction. The extracted solids are then screw pressed. The pressed solids contain 1.2 kg solution per kg of inert. Entrainment of
solids in the extract may be neglected.
19.The mass fraction of the solute in the extract if 90% of the solute in the feed is to be recovered in a single stage contact is
a. 0.03
c. 0.07
b. 0.06
d. 0.05
20. The percent recovery of the solute if the extraction is carried out in two crosscurrent stages if 50% of the solvent amount in
problem no. 9 is used in each stage is
a. 95%
c. 97%
b. 90%
d. 94%
For numbers 21 to 22:
Cod liver oil is obtained from crushed livers by extraction with an organic solvent. A sample that contains 0.335 kg of oil per each
kg of exhausted liver is fed into a multistage extractor that operates under countercurrent, where pure organic solvent is employed.
It is desired to recover 90% of the oil in the final overflow, with a composition 60% in weight of oil. If retention of the solution is 2
kg per each 3.2 kg of insoluble solids, calculate the amount of solvent required and the number of ideal stages required to carry
out the desired extraction. Calculations are made assuming that 100 kg of food is introduced into the extractor.
21. Amount of solvent required
a. 40 kg
b. 60 kg
c. 50 kg
d. 70 kg
22. Number of ideal stages
a. 6
b. 4
c. 5
d. 3
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23. A countercurrent extraction battery is used to extract NaOH from theoretical feed of NaOH and CaCO3 produced by the
causticization of lime, but with 4 lb H2O per lb CaCO3. The final extract solution is to contain 10% NaOH on recovery of 98% of the
NaOH. How many theoretical stages will be required? The underflow may be assumed constant at 2 lb solution per lb CaCO3.
a. 2
c. 4
b. 3
d. 5
24. If the operation of the extraction units could be improved so that the underflow contained only 1 lb solution per lb CaCO3, how
many stages would be required?
a. 2
c. 4
b. 3
d. 5
25. A multiple contact extraction is to treat 1.25 tons per hour of dry black ash containing 40% Na 2CO3 and 60% insoluble matter
with 30 gpm of water. If 5% of the Na2CO3 remains unextracted, calculate the number of stages. The mass ratio of insoluble matter
to solvent in the underflow from the stage is 1:2.
a. 2
c. 3
b. 1
d. 4
26. Two ideal cross-current stages operate to extract P from a feed containing P and Q. The mass flowrates of P and Q fed to
stage 1 are 1000 kg/h and 10000 kg/h, respectively. Pure solvent (S) is injected at mass flowrates of 5000 kg/h and 15000 kg/h to
stages 1 and 2, respectively. The equilibrium relation is given by Y* = 1.5X, where X is the mass of P per unit mass of S in the
extract, which is in equilibrium with the raffinate. The mass flowrate of P (in kg/hr) in the raffinate from stage 2 is _____
a. 188 kg/hr
c. 195 kg/hr
b. 176 kg/hr
d. 150 kg/hr
27. A fractionator recovers 95 mol% n-propane as the distillate from an equimolar mixture of n-propane and n-butane. The
condensate is saturated liquid at 550C. The Antoine equation is of the form, ln (Psat in bar) = A – B / (T in K + C), and the constants
are provided below:
A
B
C
n-propane
9.1058
1872.46
-25.16
n-butane
9.0580
2154.90
-34.42
Assuming Raoults’ law, the condenser pressure (in bar) is _____.
a. 15 bar
c. 12 bar
b. 18 bar
d. 10 bar
28. 100 kg of a feed containing 50 wt% of a solute C is contacted with 80 kg of a solvent containing 0.5 wt% C in a mixer-settler
unit. From this operation, the resultant extract and raffinate phases contain 40 wt% and 20 wt% of C, respectively. If E and R
denote the mass of the extract and raffinate phases, respectively, the ratio E/R is
a. 0.55
c. 0.42
b. 0.70
d. 0.67
29. To plan a liquid–liquid extraction we need to know the solute’s distribution ratio between the two phases. One approach is to
carry out the extraction on a solution containing a known amount of solute. After extracting the solution, we isolate the organic
phase and allow it to evaporate, leaving behind the solute. In one such experiment, 1.235 g of a solute with a molar mass of 117.3
g/mol is dissolved in 10.00 mL of water. After extracting with 5.00 mL of toluene, 0.889 g of the solute is recovered in the organic
phase. How many extractions will we need to recover 99.9% of the solute?
a. 5
c. 8
b. 6
d. 4
30. Streptomycin is used as an antibiotic to fight bacterial diseases, and is produced by the fermentation of a bacterium in a
biological reactor with a nutrient of glucose and amino acids. After the fermentation process, Streptomycin is recovered by
contacting the fermentation broth with an organic solvent in an extraction process. The extraction process is able to recover the
Streptomycin because Streptomycin has a greater affinity for dissolving in the organic solution than in the aqueous solution.
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Determine the mass fraction of Streptomycin in the exit organic solvent assuming that no water exits with the solvent exits
with the aqueous solution. Assume that the density of the aqueous solution is 1 g/ml and the density of the organic solvent is
0.6 g/ml.
a. 0.25
c. 0.34
b. 0.19
d. 0.48
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