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# Distillation Design MCQ’s – Minimum Stages

The number of theoretical stages, depends on the

a) Stepping off the tray

b) Reflux ratio

c) Operating line

d) Flow rates

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Answer: b

Explanation: The number of theoretical stages depends on the reflux ratio R = L0/D, while the reflux ratio itself depends upon the operating line.

When R is infinite, the slope of this rectifying becomes

a) 0

b) 1

c) 0.5

d) Can’t be specifiedAnswer: b

Explanation: When R is infinity, the slope of this line becomes 1, while R is zero the slope of this line reaches infinity.

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Answer: b

Explanation: When R is infinity, the slope of this line becomes 1, while R is zero the slope of this line reaches infinity.

If x=0.4 and y =0.6633 and point x_{D} (0.9, 0.9). Then estimate Rmin

a) 0.9

b) 0.4734

c) 0.4

d) 0.66

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Answer: a

Explanation: As R_{min}/R+1 = (0.9-0.6633)/ (0.9/0.4) = 0.4734 then 0.4734/1-0.4734 = 0.9.

If distillate is 95%, and withdrawal is 4 and α=2.5. Estimate total number of tray?

a) 7.7

b) 10

c) 6.7

d) 5.7

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Answer: c

Explanation: Using Fenske’s equation and subsuming values we have 2.66/log 2.5= 6.7.

For non-ideal cases, to calculate number of stages we use

a) Point efficiency

b) Murphree efficiency

c) Tray efficiency

d) Overall efficiency

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Answer: b

Explanation: To calculate the number of stages in a non-ideal case, Murphree efficiency line is used instead of the ideal.

Minimum reflux ratio in a distillation column results in

a) Optimum no. of trays

b) Minimum reboiler size

c) Maximum condenser size

d) Minimum number of trays

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Answer: c

Explanation: Minimum reflux ratio in a distillation column results in maximum condenser size as inversely proportional.

In a tray column, Murphree efficiency

a) Exceed 100%

b) Equal to 100%

c) Can’t be specified

d) Depend upon cases given

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Answer: a

Explanation: In a non-ideal case, Murphree efficiency can exceed 100%, also it depends on the cases given and the feed type.

For value of m, the change in liquid concentration is independent of tray location if

a) L/V = 1

b) V/L =1

c) L/mV=1

d) Vm/L=1

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Answer: c

Explanation: At this condition slope of operating line equals to slope of equilibrium line, i.e. m = 1 this shows the steady state operation.

At minimum reflux, No. stages are

a) Unity

b) Infinite

c) Zero

d) Negative

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Answer: b

Explanation: Minimum reflux and stages are inversely proportional relation, while at maximum reflux the number of stages are zero.

Scaling up from pilot scale to industrial scale only increases

a) Feed rates

b) Reflux ratio

c) No. of stages

d) Composition

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Answer: a

Explanation: There will be no effect on internal property only feed rates are increased after scaling, Only dimensionless properties change like Re, Sc and Sh.