# Separation Processes Questions and Answers – Stage Efficiency and Column Height for Trayed Columns

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This set of Separation Processes test focuses on “Stage Efficiency and Column Height for Trayed Columns”.

1. When it is not assumed that the vapor and liquid leaving a stage are not at same temperature?
a) When pressure changes from stage to stage
b) When temperature changes from stage to stage
c) When concentration of solute changes from stage to stage
d) When concentration of vapor changes from stage to stage

Explanation: When temperature changes from stage to stage it cannot be assumed that the vapor and liquid leaving a stage are not at same temperature.

2. To determine the actual number of required plates for a trayed column, what must be used?
a) K values
b) Process efficiency
c) Stage efficiency
d) Equilibrium between vapor and liquid

Explanation: To determine the actual number of required plates for a trayed column, stage effieiency or tray efficiency must be used.

3. What is the formula for overall stage efficiency given by lewis?
a) E o = Nt /N a
b) Eo = Nt N a
c) Eo = Nt -N a
d) Eo = Nt +N a

Explanation: The formula for overall stage efficiency given by Lewis is E o = Nt /N a , where Eo is the fractional stage efficiency, N t is the calculated equilibrium stages and N a is the actual number of required plates.

4. The overall stage efficiency is not a complex function of which of the following?
a) Geometry and design of contacting trays
b) Flow rates and flow paths for liquid and vapor phases
c) Compositions and properties of vapor and liquid phases
d) Material of the tray used

Explanation: The overall stage efficiency is not dependent on the material of the plates used.

5. What is the height added above the top tray to remove the entrained liquid?
a) 2ft
b) 4ft
c) 6ft
d) 8ft

Explanation: 4ft is added above the top tray to remove the entrained liquid.

6. What is the height added below the bottom tray for bottom surge capacity?
a) 4ft
b) 6ft
c) 8ft
d) 10ft

Explanation: The height of 10ft is added below the bottom tray for bottom surge capacity.

7. When are the columns in series preferred?
a) If the estimated height is above 100ft
b) If the estimated height Is above 150ft
c) If the estimated height is above 250ft
d) If the number of trays are less

Explanation: If the estimated height is above 250ft, columns in series are preferred.

8. What is the tallest column in the world?
a) 240ft
b) 248ft
c) 300ft
d) 338ft

Explanation: The tallest column in the world located at Shell Company, texas is 338ft.

9. What are the actual numbers of required plates when the overall stage efficiency is 0.8 and the calculated equilibrium stages are 4?
a) 2
b) 3
c) 5
d) 7

Explanation: By E o = Nt /N a , N a = 4/0.8=5.

10. What are the actual numbers of required plates when the overall stage efficiency is 0.9 and the calculated equilibrium stages are 9?
a) 12
b) 11
c) 5
d) 10

Explanation: By E o = Nt /N a , N a = 9/0.9=10.

11. What are the actual numbers of required plates when the overall stage efficiency is 0.7 and the calculated equilibrium stages are 2.8?
a) 2
b) 4
c) 5
d) 7

Explanation: By E o = Nt /N a , N a = 2.8/0.7=4.

12. What are the actual numbers of required plates when the overall stage efficiency is 0.6 and the calculated equilibrium stages are 4.8?
a) 8
b) 3
c) 5
d) 7

Explanation: By E o = Nt /N a , N a= 4.8/0.6=8.

13. What is the overall stage efficiency if the calculated equilibrium stages are 8 and the actual plates required are 12?
a) 0.75
b) 0.6
c) 0.56
d) 0.12

Explanation: By E o = Nt /N a , E o = 8/12 = 0.75.

14. What is the overall stage efficiency if the calculated equilibrium stages are 9 and the actual plates required are 11?
a) 0.75
b) 0.82
c) 0.56
d) 0.12

Explanation: By E o = Nt /N a , E o = 9/1 = 0.82.

15. What is the overall stage efficiency if the calculated equilibrium stages are 7 and the actual plates required are 13?
a) 0.75
b) 0.6
c) 0.54
d) 0.12 