# Chemical Reaction Engineering Questions and Answers – Microbial Fermentation – 2

This set of Chemical Reaction Engineering Interview Questions and Answers for Experienced people focuses on “Microbial Fermentation – 2”.

1. State true or false.
For product limiting microbial fermentation in CSTR with CC0=0, CR0=0 and high CA0, washout occurs at kτ =1.
a) True
b) False

Explanation: Maximum production of cells occurs at kτ =2. The concentration at which maximum production rate occurs is CR = $$\frac{C_R^*}{2}.$$

2. The plot representing the variation of CC with the residence time in a PFR for substrate limiting microbial fermentation reaction is ____
a)
b)
c)
d)

Explanation: Initially, the curve is very close to exponential decay. Deviation occurs when CA and CC change during the run. With a further increase in time, the microbial cells run out of food.

3. The slope of the curve of $$\frac{C_C}{r_C}$$ and $$\frac{1}{C_A}$$ for the substrate limiting microbial reaction in a batch reactor is ____
a) $$\frac{C_M}{k}$$
b) $$\frac{1}{k}$$
c) –$$\frac{C_M}{k}$$
d) –$$\frac{1}{k}$$

Explanation: The equation representing the kinetics is, $$\frac{C_C}{r_C} = \frac{1}{k} + \frac{C_M}{k}\frac{1}{C_A}.$$ The slope is $$\frac{C_M}{k}$$ and the line intercept is $$\frac{1}{k}.$$

4. The plot of $$\frac{1}{C_A}$$ and τ for mixed flow substrate limiting fermentation reaction in fermenters, assuming no cells in feed stream is ___
b) Linear
c) Logarithmic
d) Exponential

Explanation: The relationship between CA and τ for mixed flow fermenter with CC0=0 is $$\frac{1}{C_A} = \frac{k}{C_M}$$ τ – $$\frac{1}{C_M}.$$ The plot is linear with a slope $$\frac{k}{C_M}.$$

5. Optimum operations for CSTR carrying out substrate limiting fermentation reaction occurs at ___
a) τ = $$\frac{N}{N-1}$$
b) kτ = $$\frac{N-1}{N}$$
c) kτ = $$\frac{N}{N-1}$$
d) kτ = $$\frac{N^2}{N-1}$$

Explanation: The maximum operation for a single mixed flow fermenter occurs at $$\frac{C_A}{C_{A0}} = \frac{1}{N+1}.$$ kτ = $$\frac{N}{N-1}.$$

6. The plot representing the variation of rate, rC with CA for Monod type microbial fermentation occurring in a PFR is ____
a)
b)
c)
d)

Explanation: The rate is lower at the beginning. There is a leap in rate as reaction proceeds and then the rate decreases. The area under the curve gives the residence time.

7. Given the value of τopt for product limiting microbial fermentation is 2 hrs, then the value of τwashout (in hour) is ____
a) 1
b) 2
c) 3
d) 4

Explanation: τopt = 2τwashout.
τwashout = 1 hour.

8. If CA0 = 8 and CM = 1, the value of N is ____
a) 4
b) 1
c) 2
d) 3

Explanation: N = (1+$$\frac{C_{A0}}{C_M}$$)0.5 = (1+$$\frac{8}{1}$$)0.5 = 3.

9. The value of CR,opt for product limiting microbial fermentation (in kg/m3), given the value of CR*=60 kg/m3 is ____
a) 30
b) 20
c) 15
d) 10

Explanation: CR,opt = $$\frac{C_R*}{2} = \frac{60}{2}$$ = 30 kg/m3.

10. The value of τm,washout given N = 4 and k=1 is____
a) 1.06
b) 1.34
c) 2.45
d) 3.86

Explanation: τm,washout = $$\frac{N^2}{(N^2-1)k} = \frac{16}{15}$$ = 1.06

Sanfoundry Global Education & Learning Series – Chemical Reaction Engineering.

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