Page 42 - Gas Purification 5E

32 Gas Purification

Perry et al. (1990) defme the following three basic measures of liquid distribution quality:

1. Distribution density (number of liquid streams)
2. Geometric uniformity of distribution points
3. Uniformity of liquid flow in the individual streams

On the basis of available test data they constructed a decision tree to aid in the selection of
the distributor type for various column services and operating conditions. The resulting
guide indicates that trough-type distributors are generally preferred (over orificepan, multi-
pan 2-stage, or spray) for typical absorbers and strippers. The multi-pan 2-stage (MTS) dis-
tributor is a high-perFormance system designed for low liquid rate conditions (less than about
5 gpm per square foot). The development and application of the MTS distributor is described
by Killat and Perry (1991).
For structured packing, Shah (1991) ranks liquid distribution at the top of the list of poten-
tial trouble spots. He points out that orifice distributors are more generally used for relatively
small columns and are not recommended for fouling service or for liquid containing solids.
Notched-trough distributors are particularly well suited for fouling service and are also used
for large diameter columns, but are extremely sensitive to levelness. Generally, 4 to 7 distri-
bution points are used per square foot (45-75/sq. m) of tower cross sectional area. Details of
the design of packed tower intemals are given by Chen (1984). Excellent standardized dis-
tributors for liquid feeding, and packing support plates that provide effective gas distribution
are generally available from major packing suppliers.

Items to be considered in the design of tray columns include

1. Type of tray (e.g., sieve, valve, or bubble-cap)
2. Tray spacing
3. Number and size of openings for gas flow
4. Dimensions of active area
5. Number of passes
6. Size and location of downcomers and weirs
These items are covered in detail in texts on the subject such as Treybal (1980), Van
Winkle (1969), Bolles (1963), Fair (1963), and Perry's Chemical Engineers ' Handbook
(Perry and Green, 1984). Design data and procedures have also been published for specific
tray column types. Sieve trays are reviewed by Chase (1967), Economopoulos (1978), and
Bamicki and Davis (1989); valve trays by Bolles (1976); and slotted sieve trays by Smith
and Delnicki (1975).
A comprehensive algorithm for designing sieve tray towers is presented by Economopou-
10s (1978). More recently, Barnicki and Davis (1989) authored a two-part article about sieve
tray column design, including multipass trays, flow regime effects, and practical cost-effec-
tive standardizations. These authors divide column design into four tasks: (1) determining
the approximate configuration of each tray, (2) selecting a common diameter for the column
and dividing the column into zones of trays with the same number of passes and uniform
active tray areas, (3) assigning hole areas for each tray based on pressure drop, structural,
and flow regime limitations, and (4) checking each tray for excessive entrainment, entrain-
ment flooding, downcomer backup, and weeping. Typical guidelimes for the design of sieve
plate columns as compiled by Bamicki and Davis (1989) are given in Table 1-9.