Page 18 - Gas Purification 5E

8 Gas Punpcation

inorganic fouling of trays or packing may occur when volatile organic compounds (VOCs)
are steam stripped from water; however, he concludes that neither type of column has an
advantage when fouling occurs. In another paper, Bravo (1993) describes methods to avoid
and contend with the fouling of packing.
Gas purification absorbers often operate with liquids that contain suspended solid parti-
cles. A detailed review of techniques and design issues involved in making a vaporhquid
mass transfer device operate with solid particles in the solvent is given by Stoley and Martin
(1995). They rank mass transfer equipment for such service from most suitable to least suit-
able as

1. grid-structured packing
2. baffle trays (e.g., shed trays, disc-and-donut trays, side-to-side trays)
3. dual-flow trays (e.g., downcomerless perforated trays with large openings)
4. tab trays (e.g., fixed tabs, jet tabs)
5. sieve trays (with downcomers)
6. bubble-cap trays
7. third generation random packings (e.g., Glitsch CMR)
8. second generation random packings (e.g., Glitsch Ballast Plus Rings) or smooth-surface
structured packings
9. aggressive-surface smctured packings
10. first generation random packings (e.g., Raschig rings)
11. complex trays (e.g., film or valve trays)
12. mist eliminator pads or wire packings

As a further guide to the selection of absorbers, the relative costs of six types of tray
columns and ten types of column packings are presented in Table 1-4 (Blecker and Nichols,
1973). Generalized comments on the nature and fields of application for tray, packed, and
spray contactors follow.

Tny Columns

Tray columns (also called plate columns) are particularly well suited for large installa-
tions; clean, noncorrosive, nonfoaming liquids; and low-to-medium liquid flow applications.
Tray columns are also preferred when internal cooling is required in the column. Cooling
coils may be installed directly on individual trays or liquid can readily be removed at one
tray, cooled, and returned to another tray. Perforated trays (also called sieve trays) are widely
used because of their simplicity and low cost.
The formerly popular bubblecap design is now used primarily for columns requiring a
very low liquid flow rate, although structured packing is being used as a replacement for
bubble-cap trays in many such applications. A number of special tray designs have been
developed, including valve, grid, and baffle types to overcome some of the limitations of
simple perforated and bubblecap trays. Valve trays have been particularly popular because
they permit operation over a wider range of flow rates than simple perforated trays without
the high liquid holdup of bubble-cap trays. Examples of proprietary designs are the Koch
Flexitray, Glitsch Ballast Tray, and Nutter Float Valve Tray.
Conventional bubble-cap, perforated, and valve trays operate as crossflow contactors in
which the liquid flows horizontally across the tray and contacts gas flowing vertically