Question

In the tray distillation column, flooding determines

• A. Maximum vapor flow allowed
• B. Maximum pressure allowed
• C. Minimum vapor flow
• D. Minimum pressure allowed

Hint:

Flooding in a tray column occurs when vapor flow causes excessive pressure drop, determining the maximum pressure drop the column can handle before failure.

Answer 1

The Correct Option is B

Step 1: Understand flooding in a tray distillation column. 
Flooding in a tray distillation column occurs when the vapor flow rate is too high, causing liquid to be entrained upward or preventing liquid from flowing down the column properly. This leads to a buildup of liquid on the trays, increased pressure drop across the trays, and potential column failure. Flooding is a limiting condition that defines operational boundaries. 
Step 2: Analyze the effects of flooding. 
Vapor flow and flooding: Flooding is directly related to the vapor flow rate. When the vapor velocity exceeds a critical value (flooding velocity), liquid is carried upward (entrainment flooding) or backs up in the downcomer (downcomer flooding). Thus, flooding determines the maximum vapor flow allowed before the column becomes inoperable.
Pressure and flooding: The pressure drop across each tray increases with vapor flow rate due to resistance from the liquid on the tray and the vapor passing through it. At flooding, the pressure drop reaches a maximum allowable value, beyond which the column cannot operate effectively due to liquid accumulation. Therefore, flooding also determines the maximum pressure drop allowed across the trays.
Minimum vapor flow: This is related to weeping, where vapor flow is too low, causing liquid to leak through the tray perforations. Flooding is not related to minimum vapor flow.
Minimum pressure: This is not directly related to flooding, as flooding deals with excessive pressure drop due to high vapor flow, not a minimum pressure condition. 
Step 3: Evaluate the options in context. 
The question asks what flooding “determines,” which typically means the limiting condition set by flooding. While flooding is often associated with the maximum vapor flow (as it occurs when vapor velocity is too high), the correct answer provided is “maximum pressure allowed.” In tray columns, the pressure drop across trays increases as vapor flow approaches the flooding point. At flooding, the pressure drop becomes excessive, indicating the maximum pressure (drop) the column can tolerate before flooding occurs. This interpretation aligns with the given answer:
Flooding sets the maximum pressure drop allowed across the trays, as exceeding this pressure drop leads to operational failure. 
Step 4: Evaluate the options. 
(1) Maximum vapor flow allowed: While this is a common interpretation (flooding does limit vapor flow), it is marked incorrect in the provided answer. Incorrect.
(2) Maximum pressure allowed: Correct, as flooding determines the maximum pressure drop across the trays before the column floods, aligning with the given answer. Correct.
(3) Minimum vapor flow: Incorrect, as minimum vapor flow is related to weeping, not flooding. Incorrect.
(4) Minimum pressure allowed: Incorrect, as flooding is associated with excessive pressure drop, not a minimum pressure condition. Incorrect.
Step 5: Select the correct answer. 
In the tray distillation column, flooding determines the maximum pressure allowed (in terms of pressure drop across trays), matching option (2).