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MODULE TITLE : MASS TRANSFER OPERATIONS
TOPIC TITLE : EXTRACTION
TUTOR MARKED ASSIGNMENT 4
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MTO - 4 - TMA (v1)
© Teesside University 2011
Published by Teesside University Open Learning (Engineering)
School of Science & Engineering
Teesside University
Tees Valley, UK
TS1 3BA
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IMPORTANT
Before you start please read the following instructions carefully.
1. This assignment forms part of the formal assessment for this module. If you fail to reach the required standard for the assignment then you will be allowed to resubmit but a resubmission will only be eligible for a Pass grade, not a Merit or Distinction.
You should therefore not submit the assignment until you are reasonably sure that you have completed it successfully. Seek your tutor's advice if unsure.
2. Ensure that you indicate the number of the question you are answering.
3. Make a copy of your answers before submitting the assignment.
4. Complete all details on the front page of this TMA and return it with the completed assignment including supporting calculations where appropriate. The preferred submission is via your TUOL(E) Blackboard account:
https://eat.tees.ac.uk
5. Your tutor’s comments on the assignment will be posted on Blackboard.
1. (a) Define the following terms used in extraction, quoting an appropriate industrial example in each case:
(i) solute
(ii) solvent
(iii) extract
(iv) raffinate.
(b) Draw block diagrams to illustrate the following methods of extraction:
(i) simple multiple contact
(ii) countercurrent multiple contact.
2. (a) State three factors which influence the effectiveness of:
(i) solid-liquid extraction
(ii) liquid-liquid extraction.
(b) In each case, explain how your chosen factors influence theeffectiveness of extraction.
3. (a) 670 kg h–1 of a slurry containing 120 kg solute and 50 kg solvent is to be extracted. The maximum permitted amount of solute in the final raffinate is 5 kg h–1. When a simple mixer-settling unit is used to separate extract and raffinate, the amount of solvent retained by the solid is 50 kg. Assuming perfect mixing and a constant ratio of solvent in extract and raffinate, determine the number of stages and the strength of the total extract for each of the following conditions:
(i) Simple multiple contact is used for the extraction with a solvent addition of 100 kg h–1 per stage
(ii) The same total amount of solvent as in (i) is provided in countercurrent operation
(iii) Half the total amount of solvent in (i) is used in countercurrentoperation.
(b) Which of these methods would you recommend for the extraction?Give three reasons for your choice.
(c) Suggest two alternative ways of carrying out the extraction and the circumstances under which they might be used.
4. 40 kg s–1 of heptane is to be used to extract sunflower oil from sunflower seeds in a counter-current process which uses a centrifuge to separate extract and raffinate. 100 kg s–1 of sunflower seeds which contain 40% oil are to be extracted until the final raffinate contains less than 2% by mass of oil. The ratio of solution to insoluble solids in the raffinate is 1:4 by mass and no insoluble solids are present in the extract. There is sufficient solvent to ensure all the oil is dissolved.
Determine:
(a) the composition and amount of the final extract and raffinate (b) the number of stages required.
A blank triangular diagram is provided in FIGURE 1.

FIG. 1
5. 1000 kg h–1 of a 25% dioxane in water solution are to be extracted using benzene as the solvent (at concentrations quoted in this example, benzene is totally immiscible with water). The extraction can either be carried out:
(i) using 500 kg h–1 benzene at each stage of simple multiple contact
or (ii) using 1500 kg h–1 benzene in countercurrent multiple contact.
Determine and specify the number of stages, in each case, required to extract 95% of the original dioxane. The equilibrium data is given as follows:
x (g/kg water) 48 100 168 191 257 312 350 y (g/kg benzene) 45 91 178 214 315 394 451
6. The table overleaf shows experimentally determined extract and raffinate concentrations of ethanoic acid (Solute A) in water (original solvent S) and an extracting solvent, L, called methyl isobutylketone at 20°C. (Results are rounded off to whole percentages.)
Note: methyl isobutylketone is abbreviated to MIBK and ethanoic acid to HAc
EXTRACT WEIGHT % Mixture RAFFINATE WEIGHT %
HAc A
MIBK L
Water S
HAc A
MIBK L
Water S
0 98 2 1 0 2 98
2 95 3 2 3 2 95
9 86 5 3 12 2 86
17 74 9 4 20 4 76
25 61 14 5 26 6 68
31 47 22 6 33 12 55
34 35 31 7 35 22 43
(a) Plot the ternary equilibrium diagram for this system on FIGURE 2 atthe end of this TMA – you should take several photocopies of this for use in this question.

(b) 50 kg of feed solution of 50% A in S (HAc in water) is to be

extracted such that the raffinate is less than 5% A (HAc) . The extraction can be performed in one of two ways:
(i) 50 kg of L (MIBK solvent) added at each stage of simple multiple contact
OR (ii) 65 kg of L (MIBK solvent) in counter current multiple contact.
Use your equilibrium diagram to obtain and specify the number of stages in each case.

FIG. 2

Question No.
Question No.