Page 261 - Adsorption Technology & Design, Elsevier (1998)

236 Selected adsorption processes

This fractional vacuum swing process is similar in many respects to the
established vacuum swing adsorption process described in Section 7.3.4 but
utilizes a CaX zeolite as the adsorbent and uses part of the oxygen-enriched
gas produced from the first step of the process.
The phenomenon of surface diffusion, demonstrated by Barrer and
Strachan (1955) may be used to advantage for the separation of some
hydrogen-hydrocarbon mixtures (Sircar, 1993b). Thus, thin microporous
carbon membranes (<5/lm), supported by macroporous sheets of graphite
assembled into a plate and frame membrane module, were able to separate
H2 at 63 % recovery from a refinery waste gas. The feed was passed at high
pressure over the membrane unit which preferentially adsorbed the
hydrocarbons and facilitated surface diffusion of these species through the
membrane to the low pressure side of the module. Adsorption,
accompanied by surface diffusion through a thin microporous membrane,
thus offers possible future applications.
One further development in adsorption technology which holds some
promise, is a cyclic process applicable to the separation of liquids. The
separation method depends on concentration changes and is referred to as
concentration swing adsorption (Rao and Sircar 1992). Separation of a
binary mixture, such as ethanol-water, occurs by the selective liquid phase
adsorption of ethanol (the more strongly adsorbed component) onto the
surface of a porous adsorbent. The cycle consists of four steps:

(1) During the first part of the concentration swing adsorption cycle,
effluent from the column is pure water. This first step is continued
until the whole column is saturated with the ethanol-water feed
mixture.
(2) The second cycle step is then started when the column is rinsed with
pure ethanol. The section of column ahead of the mass transfer zone is
saturated with pure ethanol. The effluent composition during this
second step is close to that of the feed mixture and the step is
continued until the column is saturated with ethanol.
(3) The third step of the cycle is when desorption of ethanol is achieved
by rinsing, in a countercurrent direction, the column with a desorbent
liquid which is adsorbed at least as strongly as ethanol and which does
not form an azeotrope with water. Step 3 is continued until the column
is completely saturated with the desorbent, the effluent ahead of the
mass transfer zone being enriched ethanol, part of which is collected
as product and the remainder being diverted to provide ethanol rinse
for Step 2. If the mass transfer zone for Step 3 is not sharp then the