Question

Liquid remaining in unit volume of bed after bed is drained is called as

• A. Final holdup
• B. Total holdup
• C. Static holdup
• D. Dynamic holdup

Hint:

Static holdup in a packed bed is the liquid retained after draining, due to capillary forces, while dynamic holdup is the liquid flowing during operation.

Answer 1

The Correct Option is C

Step 1: Understand liquid holdup in packed beds. 
In a packed bed (e.g., in a packed column or reactor), liquid holdup refers to the amount of liquid retained in the bed. This is relevant in processes like absorption, distillation, or catalytic reactions where liquid flows through a bed of solid packing material. Holdup is categorized based on the state of the bed:
Total holdup: The total amount of liquid in the bed during operation, including both liquid that flows (dynamic) and liquid that remains trapped (static).
Dynamic holdup: The portion of liquid that is actively flowing through the bed during operation.
Static holdup: The liquid that remains in the bed after the bed is drained (i.e., after stopping the liquid flow and allowing the bed to drain under gravity). This liquid is held by capillary forces, surface tension, or adhesion to the packing material.
Final holdup: Not a standard term in this context, but could be misinterpreted as the remaining liquid after some process; static holdup is the correct term.
Step 2: Analyze the definition in the question. 
The question specifies the “liquid remaining in unit volume of bed after bed is drained.” This refers to the liquid that stays in the bed due to capillary and surface forces after gravity drainage, which is the definition of static holdup. Static holdup is typically expressed as a fraction of the bed volume (e.g., volume of liquid per unit volume of bed). 
Step 3: Evaluate the options. 
(1) Final holdup: Incorrect, as “final holdup” is not a standard term; the correct term is static holdup. Incorrect.
(2) Total holdup: Incorrect, as total holdup includes both dynamic and static holdup during operation, not just after draining. Incorrect.
(3) Static holdup: Correct, as static holdup is the liquid remaining in the bed after it has been drained, held by capillary forces. Correct.
(4) Dynamic holdup: Incorrect, as dynamic holdup refers to the liquid flowing through the bed during operation, not after draining. Incorrect. 
Step 4: Select the correct answer. 
The liquid remaining in unit volume of bed after the bed is drained is called static holdup, matching option (3).