Page 207 - Adsorption Technology & Design, Elsevier (1998)

Selected adsorption processes 193

been adopted for many drying plants especially when the air or gas pressure
supply is available at a moderately high pressure. In a typical two-bed
adsorption unit, each bed is subjected to pressurization and adsorption steps
followed by countercurrent blowdown and purge. Cycle times vary from I to
10 minutes - very much shorter than the corresponding thermal swing
process (compare with Section 7.4.1). Short cycle times help to conserve the
heat of adsorption and experimental evidence for this indicates that
temperatures in each bed vary by less than 10~ (Chihara and Suzuki 1983).
To obtain a high purity product with a low dewpoint the purge to feed flow
ratios should be between 1.1 and 2.0 and the ratio of high pressure (when
adsorbing) to low pressure (during regeneration) should be greater than the
reciprocal of the mole fraction of the product contained in the feed.

7.3.2 Hydrogen purification (Stewart and Heck 1969, Cassidy and
Holmes 1984)
Pressure swing adsorption units are used extensively for the purification of
hydrogen streams containing small amounts of low molecular weight
hydrocarbons. For most adsorbents, hydrogen is hardly adsorbed. Thus the
consequence is that ultra-high purity hydrogen may be recovered using
almost any adsorbent. Losses from blowdown and purge, although rela-
tively large, do not militate against a PSA process as recoveries of hydrogen
over 85% are possible and the feed gases wasted during blowdown and
purge steps are of little economic value.
Figure 7.2 is a sketch of a typical four-bed commercial hydrogen
purification unit together with the pipe layout and bed interconnections and
valves while Figure 7.3 indicates the sequence of steps for each of the four-
beds. In bed I while adsorption (i.e. when feed flows through the bed at high
pressure) is occurring, countercurrent depressurization takes place in bed 2
followed by a purge step and pressure equalization with bed 3. The
operations, all of which are occurring simultaneously, are designed (a) to
force any adsorbed hydrocarbons to the entrance (bottom) of bed 2 leaving
the majority of the adsorbent free of adsorbate in readiness for repressuriza-
tion with feed and (b) to reduce the consumption of mechanical energy.
Pressure is then equalized between beds I and 2. Following these two latter
steps, bed I is depressurized cocurrently and its pressure then equalized with
bed 3. Meanwhile, bed 2 is being repressurized countercurrently. Counter-
current depressurization and purge then occur in bed I followed by pressure
equalization with bed 4 as adsorption occurs in bed 2. Subsequently pressure
in bed 1 is equalized with bed 2 followed by the repressurization of bed 1. As
bed 1 is repressurized, bed 2 is depressurized and then its pressure shared
with bed 4. Purge and pressurization in each bed is achieved by means of the