Page 228 - Adsorption Technology & Design, Elsevier (1998)

208 Selected adsorption processes

size of water is 0.28 nm the sieve can adsorb water molecules but exclude the
larger ethanol (0.44 nm) molecules.

Drying of vaporized ethanol-water mixtures
The drying of gases and vapours usually implies the removal of low
concentrations of water and is accomplished as briefly described in Section
7.5.1. Because of inefficiencies caused by the release of large quantities of
heat, such a process is not suitable for the removal of large quantities of
water from gaseous streams such as would be present in a vaporized
azeotropic mixture of ethanol and water emerging from a distillation
column and which has a composition of 10.6 mole % H20 at 1 bar
pressure. Union Carbide, however, operate an adsorptive heat recovery
(AHR) drying system (Garg and Ausikaitis 1983, Garg and Yon 1986)
which enables much of the heat released on adsorption to be retained by
the adsorbent bed thus enabling the stored heat to be utilized for the
regeneration step of the cycle.
The principle upon which the design of an AHR system depends is to
ensure that the temperature wave front which traverses the adsorption bed
travels at a velocity such that it is retained within the mass transfer zone.
Both Ruthven (1984) and Yang (1987) have given analytical expressions
which describe the conditions for which the concentration and temperature
waves travel at identical velocities. Furthermore, Yang (1987) deduced
that the condition for coincident temperature and concentration wave
fronts is
(Cs/Cpg) > (q*/y) (7.1)
where Cpg is the heat capacity of the adsorbate and other carrier gas, cs is the
heat capacity of the solid adsorbent and (q*/y) is (at the bed inlet) the ratio of
the equilibrium mass of adsorbate adsorbed per unit mass of adsorbent to
the mole fraction of adsorbate in the gas phase. This inequality turns out to
be similar to the empirical cross-over ratio R (Garg and Ausikaitis 1983)
described previously in Chapter 6 (equation 6.18). The parameter R is a
measure of the non-isothermality of a system. When R > I heat is removed
from the mass transfer zone with comparative ease and the system remains
approximately isothermal. This is because the temperature wave is far ahead
of the concentration front so that heat is convected from the bed before the
mass transfer zone has traversed the adsorbent bed. When R approaches, or
is equal to, unity the temperature wave is located within the mass transfer
zone and heat is retained in the bed during the whole time that the
concentration front moves through the bed. Finally, when R < 1 the
temperature wave lags behind the mass transfer zone. Clearly then, if the