Info

source: From H. Z. Kister, Distillation Operation, Copyright © 1990 McGraw-Hill, Inc. Reprinted by permission.

source: From H. Z. Kister, Distillation Operation, Copyright © 1990 McGraw-Hill, Inc. Reprinted by permission.

The apparent residence time is the ratio of the downcomer volume to the clear liquid flow in the downcomer. The downcomer volume is the tray spacing times the average downcomer cross-section area. The true residence time is the ratio of froth volume in the downcomer to the frothy liquid flow in the downcomer. The true residence time can alternatively be expressed as the ratio of the clear liquid volume in the downcomer to the clear liquid flow. The definition adopted here is that of the apparent downcomer residence time. The author found this definition to be better and to be consistent with :>ublished criteria for downcomer residence times (1).

Kister (1) reviewed various published criteria available for downcomer residence times, and recommended those by Bolles (47) and Erbar and Maddox (49). Both sets of guidelines are similar, and express downcomer residence time as a function of the system's tendency to foam (Table 6.6).

Minimum downcomer width and minimum downcomer area. As the downcomer becomes smaller, its width decreases faster than its length, turning the downcomer into a long and narrow slot. This geometry increases the resistance to liquid downflow and to upflow of disengaging vapor. Small downcomers are also extremely sensitive to foaming, fouling, construction tolerances, and introduction of debris. Smaller weirs associated with small downcomers distort the liquid flow pattern as it approaches the weir ("weir constriction effect"), which in-

table 6.6 Recommended Minimum Residence Time In the Downcomer

Foaming tendency

Example

Residence Time, s

Low

Low -molecular-weight hydrocarbons,* alcohols

Was this article helpful?

0 0

Post a comment