Page 236 - Separation process principles 2

6.3 Graphical Equilibrium-Stage Method for Trayed Towers 201

5. Minimum absorbent (stripping agent) flow rate and absorbent (stripping agent) flow rate exists that corresponds
actual absorbent (stripping agent) flow rate as a multi- to an infinite number of countercurrent equilibrium contacts
ple of the minimum rate needed to make the separation between the gas and liquid phases. In every design problem
involving flow rates of the absorbent (stripping agent) and
6. Number of equilibrium stages and stage efficiency
number of stages, a trade-off exists between the number of
7. Heat effects and need for cooling (heating)
equilibrium stages and the absorbent (stripping agent) flow
8. Type of absorber (stripper) equipment rate at rates greater than the minimum value. Graphical and
9. Height of absorber (stripper) analytical methods for computing the minimum flow rate and
this trade-off are developed in the following sections for a
10. Diameter of absorber (stripper)
mixture that is dilute in the solute(s). For this essentially
The ideal absorbent should (a) have a high solubility for isothermal case, the energy balance can be ignored. As dis-
the solute(s) to minimize the need for absorbent, (b) have a cussed in Chapters 10 and 11, computer-aided methods are
low volatility to reduce the loss of absorbent and facilitate best used for concentrated mixtures, where multicomponent
separation of absorbent from solute(s), (c) be stable to max- phase-equilibrium and mass-transfer effects can become com-
imize absorbent life and reduce absorbent makeup require- plicated and it is necessary to consider the energy balance.
ment, (d) be noncorrosive to permit use of common materi-
als of construction, (e) have a low viscosity to provide low
6.3 GRAPHICAL EQUILIBRIUM-STAGE
pressure drop and high mass- and heat-transfer rates, (f) be
nonfoaming when contacted with the gas so as to make it un- METHOD FOR TRAYED TOWERS
necessary to increase absorber dimensions, (g) be nontoxic Consider the countercurrent-flow, trayed tower for absorp-
and nonflammable to facilitate its safe use, and (h) be avail- tion (or stripping) operating under isobaric, isothermal, con-
able, if possible, within the process, to make it unnecessary tinuous, steady-state flow conditions shown in Figure 6.8.
to provide an absorbent from external sources, or be inex- For convenience, the stages are numbered from top to bot-
pensive. As already indicated at the beginning of this chap- tom for the absorber and from bottom to top for the stripper.
ter, the most widely used absorbents are water, hydrocarbon Phase equilibrium is assumed to be achieved at each of the N
oils, and aqueous solutions of acids and bases. The most trays between the vapor and liquid streams leaving the tray.
common stripping agents are steam, air, inert gases, and That is, each tray is treated as an equilibrium stage. Assume
hydrocarbon gases. that the only component transferred from one phase to the
In general, operating pressure should be high and temper-
ature low for an absorber, to minimize stage requirements
and/or absorbent flow rate and to lower the equipment vol-
ume required to accommodate the gas flow. unfortunately,
both compression and refrigeration of a gas are expensive.
Therefore, most absorbers are operated at feed-gas pressure,
which may be greater than ambient pressure, and ambient
temperature, which can be achieved by cooling the feed gas
and absorbent with cooling water, unless one or both streams
already exist at a subambient temperature. Operating pres-
sure should be low and temperature high for a stripper to
minimize stage requirements or stripping agent flow rate.
However, because maintenance of a vacuum is expensive,
strippers are commonly operated at a pressure just above
ambient. A high temperature can be used, but it should not
be so high as to cause undesirable chemical reactions. Of
course, operating temperature and pressure must be compat-
ible with the necessary phase conditions of the streams being
contacted. For example, an absorber should not be operated
at a pressure and/or temperature that would condense the
feed gas, and a stripper should not be operated at a pressure )/)
and/or temperature that would vaporize the feed liquid. The
possibility of such conditions occurring can be checked
(bottom)
by bubble-point and dew-point calculations, discussed in
Chapter 4.
For given feed-gas (liquid) flow rate, extent of solute
absorption (stripping), operating pressure and temperature, Figure 6.8 Continuous, steady-state operation in a countercurrent
and absorbent (stripping agent) composition, a minimum cascade with equilibrium stages: (a) absorber; (b) stripper.