This set of tricky Mass Transfer Questions & Answers focuses on “Time period Calculation in Drying”.

1. Find the time taken to dry 50% to 40% of moisture (dry basis) if critical moisture content is 30%. If the weight of the wet solid is 5 kg and the product of area and the constant rate is 0.5 kg/min.

a) 20 sec

b) 40 sec

c) 60 sec

d) 80 sec

View Answer

Explanation: Time for drying = ( wet solid weight * change in moisture )/ Area * constant rate

= (5 * 0.1/0.5) min = 1 min = 60 sec.

2. Find the period if the initial and the final moisture content are 60% and 40%. Also the critical moisture content is 30%.

a) Constant rate period

b) Falling rate period

c) Unsaturated period

d) None of the mentioned

View Answer

Explanation: Here the final moisture content is greater than critical moisture content so it must be a constant rate period.

3. Find the period if the initial and the final moisture content are 50% and 20%. Also the critical moisture content is 30%.

a) Constant rate period

b) Falling rate period

c) saturated period

d) None of the mentioned

View Answer

Explanation: Here the final moisture content is less than critical moisture content so it must be a falling rate period or unsaturated drying.

4. At the constant drying period of a batch process, the rate of drying varies accordingly.

a) True

b) False

View Answer

Explanation: For the constant drying period the rate of drying remains constant by analysing drying curve.

5. Find the change in moisture content from the given data

Given data:

Time for drying= 6 sec

Area * Constant rate = 8 kg per sec

Weight of wet solid = 100 kg

a) 0.21

b) 0.12

c) 0.48

d) 0.84

View Answer

Explanation: Difference in moisture = (time *area* rate )/Weight of the solid = 0.48.

6. Find the initial moisture content for the batch process if the final moisture content is 25% (dry basis) and the constant moisture content is 40%. (Includes constant and falling rate period)

a) 35

b) 30

c) 60

d) None of the mentioned

View Answer

Explanation: Here the initial moisture content must be greater than critical moisture content.

7. Find the final moisture content for the batch process if the initial moisture content is 75% (dry basis) and the constant moisture content is 40%. (Includes constant and falling rate period)

a) 45

b) 50

c) 60

d) None of the mentioned

View Answer

Explanation: Here the final moisture content must be less than critical moisture content.

8. Falling rate period of a batch process starts at

a) Saturated drying surface

b) Unsaturated drying surface

c) Both unsaturated and saturated region

d) None of the mentioned

View Answer

Explanation: The bound moistures gets evaporated only at unsaturated surface which is a falling rate period.

9. If the time taken for the moisture removal is obtained from the formula

Time = Ss(Xc – Xe)/ Nc * A ln ((X1-Xe)/(X2-Xe)) then the falling curve will be curves symmetrically

Where, Ss= Wet solid weight

Xc,Xe- constant & equilibrium moisture content

X1,X2 – initial and final moisture content

a) True

b) False

View Answer

Explanation: This relation gives the linear relation so the falling rate curve will be linear.

10. There will be a change in the rate of drying of a batch process after the constant drying process.

a) True

b) False

View Answer

Explanation: After the constant drying curve, the unsaturated drying starts then the rate of drying decreases due to moisture removal.

11. Find the Constant rate representation

a) X

b) Y

c) Z

d) A

View Answer

Explanation: Constant drying period has constant rate.

12. Find the representation of critical moisture content value

a) A

b) B

c) Z

d) Y

View Answer

Explanation: At the end of constant rate period we get critical moisture content value.

13. Find the equilibrium content value

a) A

b) B

c) C

d) Y

View Answer

Explanation: After the rate has stopped, the remaining moisture is known as equilibrium moisture content.

14. Falling rate period of the batch drying curve = _______ + ___________

a) Internal movement, unsaturated surface drying

b) Internal movement, saturated surface drying

c) External movement, unsaturated surface drying

d) External movement, saturated surface drying

View Answer

Explanation: From the drying curve we know the falling rate period represents the bound moisture removal so it includes internal movement and unsaturated surface drying.

15. Find the region if the final and the initial moisture content are 40% and 30% (Dry basis) and the critical moisture content is 45%.

a) Constant rate

b) Falling rate

c) Internal movement

d) None of the mentioned

View Answer

Explanation: Here both initial and final moisture content are less than critical moisture content so the rate is falling rate period.

**Sanfoundry Global Education & Learning Series – Mass Transfer.**

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