For the conditions given in Example 3, what weir height would be necessary to reduce the dimensionless ratio of liquid gradient/pressure-drop head caused by bubble-cap assemblies to the recommended maximum value of 0.4?

Example 3

Determination of pressure drop and liquid height in down comer for bubble-cap plate The following specifications apply to a

bubble-cap plate:

Diameter = 10.0 ft

Tray spacing = 26 in.

Liquid cross flow

Weir length = 6.2 ft

Weir height = 3.0 in.

Skirt clearance = 0.5 in.

Static submergence = 0.5 in.

Rectangular slots Height of slots = 1.5 in.

Width of slots = 0.3 in.

Total riser cross-sectional area = 9 ft2

Caps are bolted to tray (no hold-down bars)

Clearance between bottom of downcomer and plate = 2.5 in.

Number of rows of caps perpendicular to direction of liquid flow = 11

For an average row of caps perpendicular to direction of liquid flow, total free space between risers = 4.4 ft, total free space between caps = 2.7 ft, width of tray = 9 ft

Cross-sectional areas for vapor flow through riser, direction-reversal space, annular cap space, and slots are equal.

Caps are arranged on equilateral-triangle centers staggered perpendicular to direction of liquid flow, and liquid depth above caps is less than 1 in.

This bubble-cap plate is to be used under the following conditions:

Vapor density = 0.15 Ib/ft3

Liquid density = 50 lb/ft3

Superficial vapor velocity based on cross-sectional area of empty tower = 1.8 ft/s

Liquid flow rate = 1 ft3/s Surface tension of liquid is such that Eq. (11) is applicable.

Estimate the gas pressure drop across the tray, the percent of this pressure drop due to liquid head above the top of the bubble-cap slots, and the liquid head in the downcomer.