Adsorption - Principles of Chemical Separations with Environmental Applications - page 185

q

T a

q a

TSA

PSA

T d

q d

P d P a P

Figure 7.1 Isotherm diagram illustrating pressure-swing (PSA) and temperature-
swing (TSA) processes [5]. Reproduced with permission of the American Institute
of Chemical Engineers. Copyright c

1988, AIChE. All rights reserved.

relationship). The isotherm should span the concentration range required. If there is a
temperature change, then this information also needs to be available over the temperature
range of interest. These data are needed at both the adsorption and desorption (sorbent
regeneration) conditions. It is usually necessary to change the operating conditions of
the adsorption process during regeneration to make desorption more thermodynamically
favorable. This can involve an increase in temperature and/or a reduction of partial pressure,
and data at these new conditions can be helpful in the analysis.
Figure 7.1 illustrates the data required for the various sorption options. The sorption
isotherm is plotted for two temperatures. For a pressure-swing process (PSA), the adsorp-
tion step is performed at P a and the desorption at P d . The maximum amount of solute
removed and recovered per mass of sorbent is q a − q d . The temperature-swing process

(TSA) is also illustrated. A combination of temperature and pressure swings can be used,
although this is rarely done in practice. Note that the absolute values of the pressure and
temperature as well as the change (swing) affect the productivity of the process. In addi-
tion, pressure changes can be accomplished very rapidly while temperature changes occur
much more slowly since the entire bed must be heated or cooled. Typical cycle times for
PSA are 1-5 minutes while it is 2 hours or longer for TSA. Once the sorption isotherm
information is obtained, then other factors need to be considered in adsorption design [5]:
1 Sorbent capacity (i.e., how much material is adsorbed per unit quantity of sorbent). The
surface area per unit volume of the sorbent has an important influence on this value. For
this reason, sorbents are usually highly porous materials. Obviously, one would want
the capacity to be as high as possible.
2 The purity requirement of the fluid phase (i.e., how much material needs to be removed).
3 Sorbent selectivity. This can be accomplished by three mechanisms: (a) selective
binding to the sorbent surface (equilibrium); (b) excluding certain components based