Question

The number of bubble caps to be used per tray is determined from

• A. The vapour load
• B. The liquid load
• C. Tray diameter
• D. Allowable gas velocity

Hint:

The number of bubble caps on a tray is calculated using the allowable gas velocity to ensure the vapor flow is handled without flooding or entrainment.

Answer 1

The Correct Option is D

Step 1: Understand bubble cap trays in distillation. 
Bubble cap trays are used in distillation columns to facilitate vapor-liquid contact. Bubble caps are devices on the tray that allow vapor to rise through the tray, bubble through the liquid, and promote mass transfer. The number of bubble caps per tray depends on the design parameters that ensure proper operation without issues like flooding or weeping. 
Step 2: Analyze factors affecting the number of bubble caps. 
Vapor load: The amount of vapor flowing through the column (e.g., vapor flow rate in \( \text{m}^3/\text{s} \) or \( \text{kg}/\text{s} \)). While vapor load influences the overall design, it is not the direct determinant of the number of bubble caps.
Liquid load: The amount of liquid flowing down the tray (e.g., liquid flow rate). This affects tray hydraulics but is not the primary factor for determining the number of bubble caps.
Tray diameter: The diameter of the tray determines the available area for installing bubble caps, but the number of caps is not directly calculated from the diameter alone.
Allowable gas velocity: The maximum vapor velocity through the bubble caps that ensures proper operation (e.g., avoiding entrainment of liquid or excessive pressure drop). The allowable gas velocity (\( u_{\text{allow}} \)) is used to calculate the total vapor flow area required, which determines the number of bubble caps. The vapor flow rate (\( Q_v \)) is related to the velocity and area by: \[ Q_v = u_{\text{allow}} \times A_{\text{total}}, \] where \( A_{\text{total}} \) is the total area of all bubble caps. Each bubble cap has a specific area (\( A_{\text{cap}} \)), so the number of bubble caps (\( N_{\text{caps}} \)) is: \[ N_{\text{caps}} = \frac{A_{\text{total}}}{A_{\text{cap}}} = \frac{Q_v}{u_{\text{allow}} \times A_{\text{cap}}}. \] Thus, the allowable gas velocity directly determines the number of bubble caps needed to handle the vapor flow without operational issues. 
Step 3: Evaluate the options. 
(1) The vapor load: Incorrect, as vapor load (\( Q_v \)) is an input, but the number of bubble caps is determined by dividing this load by the allowable gas velocity and cap area. Incorrect.
(2) The liquid load: Incorrect, as liquid load affects tray hydraulics (e.g., weir height, downcomer design) but not directly the number of bubble caps. Incorrect.
(3) Tray diameter: Incorrect, as tray diameter provides the area for placing bubble caps, but the number of caps depends on the vapor flow and allowable velocity. Incorrect.
(4) Allowable gas velocity: Correct, as the number of bubble caps is calculated based on the allowable gas velocity to ensure proper vapor flow through the caps without flooding or entrainment. Correct. 
Step 4: Select the correct answer. 
The number of bubble caps to be used per tray is determined from the allowable gas velocity, matching option (4).