SCHOOL OF ENGINEERING & PHYSICAL SCIENCES

Chemical Engineering

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B49CA1

SEPARATION PROCESSES A

Semester 1 – 2013/14

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THURSDAY 12TH DECEMBER 2013

17:00 – 19:00

Edinburgh Campus

Instructions to candidates:

Answer TWO questions from Section A and TWO questions from Section B.

Candidates are expected to make reasonable assumptions where necessary

Where a distribution of marks within a question is shown, this should not be

taken to be definitive but is for guidance only

(A breakdown of marks is provided as a guide within each question)

Diagrams included with paper:

x-y diagram for cyclohexane-cyclohexanone at 1 bar (2 x)

T-x-y diagram for cyclohexane-cyclohexanone at 1 bar

triangular diagram for di-isobutyl ketone-water-acetic acid at 24°C (2 x)

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SECTION A

1.

The single feed to a continuous distillation column is a two-phase vapour-liquid mixture

consisting of 75 mole % vapour. The feed composition is 50% cyclohexane and 50%

cyclohexanone on a molar basis and feed flow rate is 120 kmol h-1. Two product

streams leave the column. The upper product (distillate) stream is 75 mole %

cyclohexane, while the bottom product stream is 5 mole % cyclohexane. Column

pressure may be assumed constant at 1 bar, while reflux ratio is 0.5 kmol kmol-1. The

column has a total condenser, which produces a saturated liquid condensate, and a

partial reboiler.

The x-y and T-x-y diagrams provided will be needed to answer this question.

a) Find the number of theoretical plates required and identify the optimal feed plate,

explaining your reasoning clearly.

(13 marks)

b) Find the minimum reflux ratio.

(4 marks)

c) Find the temperatures of the vapour entering the condenser, the distillate product

and the two-phase feed.

(6 marks)

d) Find the compositions of the saturated liquid and vapour phases present in the

feed.

(2 marks)

(Any diagrams used should be attached securely to your answer book and handed in to

show working. Write your matriculation number but not your name on them)

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2.

Water will be used as an extracting solvent in a liquid-liquid extraction process to

recover acetone from a solution of acetone dissolved in tetrachloro-methane. It may be

assumed that water and tetrachloro-methane are completely immiscible. The

equilibrium distribution of acetone between water and tetrachloro-methane under

process conditions is given by:

(kg A/kg W) = 4.2(kg A/kg TCM)

Where A, W and TCM are acetone, water and tetrachloro-methane respectively.

a) At first, a continuous single-stage extraction is tried. The feed is a 12.5 mass %

solution of acetone in tetrachloro-methane, which enters the process at 4,000 kg

min-1. Find the flow rate of pure water needed to recover 95% of the acetone.

(6 marks)

b) In an attempt to improve performance, a two-stage counter-current system is

tried, as illustrated below. The feed is the same as that used in part ‘a’, while the

flow rate of pure water extracting solvent is 5,000 kg min-1.

Find the percentage recovery of acetone and comment on the effect of using two

equilibrium stages rather than one.

(17 marks)

c) Find the minimum flow rate of water that would be needed to achieve 95%

recovery of acetone in this system if an infinite number of counter-current

equilibrium stages were available.

(2 marks)

5,000 kg min-1

Water

4,000 kg min-1

feed

2 1

Final

raffinate

Final extract

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3. Answer ALL PARTS of this question.

a) 100 kg of a liquid mixture consisting of 50% acetic acid and 50% di-

isobutylketone (DIBK) by mass is mixed with 200 kg water in a process carried

out at 24°C.

i. Using the triangular diagram supplied, find the quantities and

compositions of the two phases that form, and the percentage recovery of

acetic acid in the aqueous extract.

(12 marks)

ii. Find the quantity of pure acetic acid that must be added to the mixture to

form a single liquid phase.

(5 marks)

b) Discuss the vapour phase pressure drop that occurs as vapour passes upwards

through a distillation column sieve plate, identifying the main mechanisms

involved. Describe the adverse consequences of excessive pressure drop and

explain how columns can be designed to avoid these.

(8 marks)

(Any diagrams used should be attached securely to your answer book and handed

in to show working. Write your matriculation number but not your name on them)

END OF SECTION A

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SECTION B

4.

Winkelmann’s method is used to determine the diffusivity of the vapour of a liquid A (molar mass 119.5 kg kmol-1) in a gas B.

a) Draw the experimental system, indicating its significant features and the hypothesis under which the method works

(4 marks) b) Explain how the expression:

*

0

*

0

0ln

2

2

ln2

AT

T

L

T

AT

T

L

T

cc

cMDc

L

cc

cMDc

LL

LL

t

Equation 1.

is derived and indicate the meaning of the various terms in Equation 1. (9 marks)

c) Experiments at 1 bar and 321 K give the following results:

Time

(min)

Liquid level

(cm)

0 0.0

26.7 0.25

456.7 2.32

2810.0 6.70

4821.7 9.03

6385.0 10.48

Use Equation (1) to calculate the diffusivity of A if its vapour pressure at 321K is

37 kN m-2 and the density of liquid A is 1540 kg m-3.

(12 marks)

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5.

Consider the diffusion of gas A in gas B.

a) Give a practical example which can be described by equimolar counter-diffusion

and one example where diffusion of A through stagnant B happens instead.

(3 marks)

b) Write the equations that regulate equimolar counter-diffusion and diffusion of A

through stagnant B.

(3 marks)

c) Describe the hypothesis under which the Whitman 2-film theory applies.

(5 marks)

d) Derive an expression relating the overall mass transfer coefficient KG to the film

mass transfer coefficients, hL and hG, using a driving force diagram where the

equilibrium relationship is a straight line. Make use of the relevant graph to help

the derivation.

(14 marks)

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6.

Liquid A of molar mass 74 kg kmol-1 is removed from aqueous solution by liquid/liquid

extraction with organic phase B. At a particular point in the process, the concentration of

A in the aqueous phase is 5% by mass while that in the organic phase is 0.8% by mass.

The transfer rate of A is 1.2 x 10-6 kmol m-2 s-1.

The process is operated at 20°C and the densities of the aqueous and organic phases

at this temperature are 998 kg m-3 and 879 kg m-3 respectively. It may be assumed that

the resistance to mass transfer lies solely in a pair of liquid films at the liquid/liquid

interface and that 60% of this resistance lies in the aqueous phase. For dilute solutions,

the equilibrium distribution of A between the two phases is given by:

cA1 = 5 cA2

where cA1 and cA2 are the concentrations (kmol m-3) of A in the aqueous and organic

phases respectively.

a) Find the overall mass transfer coefficients K1 and K2, based on the aqueous and organic phases respectively

(10 marks)

b) Find the film mass transfer coefficients, hD1 and hD2 (8 marks)

c) Find the interfacial concentration of A in each phase

(7 marks)

END OF PAPER

Page 1 of 2 B49CA – chart (2 copies)

Page 2 of 2 B49CA – chart (2 copies)

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Vapour-Liquid Equilibrium Diagram for Cyclohexane-Cyclohexanone at 1 bar

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Mole Fraction of Cyclohexane (x or y)

T-x-y Diagram for Cyclohexane-Cyclohexanone at 1 bar