Page 220 - Adsorption Technology & Design, Elsevier (1998)

202 Selected adsorption processes

During the period when bed 1 is adsorbing, bed 2 is being desorbed which
includes the times required for heating and cooling. The two-bed thermal
swing process requires that the time taken for desorbing gases from the first
bed matches the time allowed for adsorption in the second bed, otherwise
flow of the product gas would be discontinuous. Similarly, the time allocated
for adsorption in bed 2 must equal the time required for desorption in bed 1.
Total cycle times for thermal swing processes are of the order of hours
(rather than minutes as for pressure swing adsorption cycles) because of the
thermal inertia of the packed beds.
In a two-bed thermal swing process the limitation imposed by equal
times for adsorption and desorption engenders inflexibility of operation
and reduces the effective loading capacity of the beds. If the more
strongly adsorbed component of the mixture to be separated has an equil-
ibrium isotherm convex to the axis representing quantity adsorbed
(favourable type of isotherm), the desorption part of the cycle becomes
the limiting factor of the overall cycle time. For a favourable type of
adsorption isotherm the concentration wave travelling through the bed
tends toward a constant pattern, but during desorption the wave becomes
dispersed thus broadening the mass transfer zone (see Chapter 6). If,
therefore, both flow rate and temperature were maintained at the same
values for both adsorption and desorption, a longer period would be
required for desorption than adsorption. The requirement for equal
adsorption and desorption times in a two-bed temperature swing cycle,
therefore, means that only a fraction of the adsorbate present in the feed
can be removed during the desorption step of the cycle. Bed capacity is
consequently not fully utilized.
Because of the long cycle times required for thermal swing separations
this mode of operation is used almost exclusively for the removal of low
concentrations of adsorbable gases from feed streams. Furthermore, sub-
stantial amounts of energy can be used in supplying sensible heat and the
heat of desorption unless the concentration of the component to be
removed is small. As mentioned in Section 7.1, a combination of inert gas
purge stripping and thermal swing operations may be used for the desorp-
tion of strongly adsorbed components.

7.4.2 Three-bed systems
When the length of the mass transfer zone (MTZ) is long relative to the
length of the bed in which adsorbate is in equilibrium with adsorbent the
extent of bed utilization is small (see Chapter 6). An improvement in total
bed capacity usage, however, may be achieved by the use of three
adsorbent beds. The operating sequence in such cases is illustrated in