Page 56 - Adsorbents - fundamentals and applications

SIMPLE CRITERIA FOR SORBENT SELECTION 41

selectivity of the sorbents as defined by Gaffney et al. (1993). An adiabatic sepa-
ration factor, computed by taking the ratio of working capacities determined under
non-isothermal, multicomponent conditions, was proposed by Ackley (1998). By
taking the product of working capacity and adiabatic separation factor, Ackley
et al. (2000) compared the relative merits of different sorbents for a particular
gas separation application. Lastly, an example of a sorbent parameter appears in
a patent by Notaro et al. (1998), which mainly concerns itself with the separa-
tion of N 2 from air. This parameter, referred to as “Adsorption Figure of Merit
(AFM),” was defined as follows:

α 2
AFM = N 2 · ads (3.57)
α des

where N 2 is the delta N 2 loading (or the difference between N 2 adsorption
amounts at high pressure and low pressure), and α ads and α des are the adsorption
selectivities (for N 2 /O 2 ) under adsorption and desorption conditions. The AFM
was used to assign the zones in which different adsorbents would be used in
a layered bed configuration. However, the reasoning behind the AFM was not
explained by Notaro et al. (1998), and it is obvious that this parameter is an
empirical rule of thumb. Also, it should be noted that this parameter is not
dimensionless and thus restricts its generalization.
Building on the various methods described above, Rege and Yang (2001a and
b) proposed a simple parameter when comparing two adsorbents for a particular
binary gas separation on the basis of their equilibrium adsorption capacities. The
parameter is composed of two factors. One is related to the equilibrium selectivity
of the sorbent, which is an inherent property of the sorbent. The other factor is
related to the working capacity of the sorbent, which is defined as the difference
in the adsorbed amount at the adsorption and the desorption pressures. Thus,
the latter factor is related to the particular PSA cycle used for performing the
separation. The applicability of the parameter has been demonstrated by using
two examples from air separation.

Theoretical Basis for Simple Parameter: Sorbent Selection Parame-
ter (S). A parameter for the selection of an adsorbent should preferably have
the following attributes: It should be readily estimable without complicated cal-
culations, and it should incorporate the essential nature of the isotherms under
the PSA operating conditions, which will dictate the selectivity and hence the
performance of the adsorbents. The PSA parameter discussed here applies only
to sorbents with negligible mass transfer limitations or at most, having close
diffusivity values for both components. It is further assumed that the adsorp-
tion isotherms follow Langmuir behavior, although this parameter can be easily
extended to other isotherm models as well. By Langmuir behavior it is meant that
the sorbents exhibit a limiting coverage as the saturation pressure is approached
and that they have an energetically homogeneous surface. The Langmuir equation,