Page 231 - Separation process principles 2

196 Chapter 6 Absorption and Stripping of Dilute Mixtures

6.1 EQUIPMENT

Absorption and stripping are conducted mainly in trayed
towers (plate columns) and packed columns, and less often
in spray towers, bubble columns, and centrifugal contactors,
as shown schematically in Figure 6.2. A trayed tower is a
vertical, cylindrical pressure vessel in which vapor and liq-
uid, which flow countercunrently, are contacted on a series of
trays or plates, an example of which is shown in Figure 6.3.
Liquid flows across each tray, over an outlet weir, and into a
downcomer, which takes the liquid by gravity to the tray
below. Gas flows upward through openings in each tray,
bubbling through the liquid on the tray. When the openings
are holes, any of the five two-phase-flow regimes shown in

~,-d
b-~ray diameter,
Liquid Liquid
in
Figure 6.3 Details of a contacting tray in a trayed tower.
[Adapted from B.F. Smith, Design of Equilibrium Stage Processes,
McGraw-Hill, New York (1963).]

Gas in Gas in Figure 6.4, and considered in detail by Lockett [2], may
occur. The most common and favored regime is the froth
Liquid out Liquid out regime, in which the liquid phase is continuous and the gas
passes through in the form of jets or a series of bubbles. The
(a) (b) spray regime, in which the gas phase is continuous, occurs
for low weir heights (low liquid depths) at high gas rates. For
1 Gas out low gas rates, the bubble regime can occur, in which the
h liquid is fairly quiescent and bubbles rise in swarms. At
11111111 high liquid rates, small gas bubbles may be undesirably
emulsified. If bubble coalescence is hindered, an undesirable
Liquid -
in foam forms. Ideally, the liquid carries no vapor bubbles
Liquid (occlusion) to the tray below, the vapor carries no liquid
in Gas-liquid
droplets (entrainment) to the tray above, and there is no
weeping of liquid through the holes in the tray. With good
contacting, equilibrium between the exiting vapor and liquid
phases is approached on each tray.
Gas in
Liquid out
Liquid
JL out t- as in
(c) (d)

Liquid
Vapor

Figure 6.4 Possible vapor-liquid flow regimes for a contacting
(e) tray: (a) spray; (b) froth; (c) emulsion; (d) bubble; (e) cellular
Figure 6.2 Industrial equipment for absorption and stripping: foam.
(a) trayed tower; (b) packed column; (c) spray tower; (d) bubble [Reproduced by permission from M.J. Lockett, Distillation Tray
column; (e) centrifugal contactor. Fundamentals, Cambridge University Press, London (1986).]