Page 41 - Gas Purification 5E

Introduction 31

A, = Active area, the area on the tray actively involved in gadliquid contact, typically the
column cross section less two downcomers, sq. ft
Ah = Total slot, perforated, or open valve area on plate, sq. ft
A, = Net area for vapor flow, typically the column cross section less one downcomer
(used for calculating U), sq. ft

Fair (1987) points out that when the ratio of &/A, is smaller than 0.1, jetting occurs
because of the high velocity of gas through the tray openings. Fair suggests that the calculat-
ed allowable velocity be corrected as follows:

WAa ufl
0.10 1 .oo
0.08 0.90
0.06 0.80

where U is the allowable velocity as calculated by the above equations, and U, is the comt-
ed allowable velocity.
A more accurate (and somewhat more complicated) correlation for predicting entrainment
flooding on sieve and valve trays is proposed by Kister and Haas (1990). Their correlation is
also derived from the original work of Souders and Brown, but provides a modified
approach for determining K,. They introduce, as an important parameter in the correlation,
the clear liquid height at the froth-to-spray-regime transition, and suggest that it be calculat-
ed by an equation attributed to Jeronimo and Sawistowski 0973).

General Design Considerations

Packed Columns

practical guidelines for random-packed tower design are given by Coker (1991) as follows:

1. The ratio of the diameter of the column to the packing should be at least 15: 1.
2. Because of deformability, plastic packing is limited to an unsupported height of 10-15
feet, and metal to 2&25 feet.
3. Liquid redistributors are required every 5-10 tower diameters for rings, and at least every
20 feet for all types of dumped packing.
4. The number of liquid streams provided by the feed distributor should be 3-5 per square
foot in towers larger than 3 feet in diameter.

Efficient liquid and gas distribution is a key requirement for high performance packed
columns. The packed bed will normally cause the liquid flow to spread throughout the col-
umn as it flows downward, however, this results in a loss of effective column height.
A detailed study of liquid and gas distribution in commercial packed columns is described
by Moore and Rukovena (1986). They conclude that the importance of high performance
distributors increases as the stage count per bed increases. Liquid and gas flow rates as well
as packing type or size have little or no effect. Practical guidelines for selecting, designing,
and installing packed column distributors are given by Bonilla (1993).