Page 212 - Adsorption Technology & Design, Elsevier (1998)

196 Selected adsorption processes

effluent from the other beds. Union Carbide has employed the four bed PSA
process for plants producing 0.4 x 106 m 3 (measured at STP) of H2 per day.
For producing even greater quantities (ca. 1.4 x 106 m 3 per day) of ultra-
pure hydrogen, as many as nine beds in series are used. For the latter large-
scale process the same steps - repressurization, adsorption, cocurrent
depressurization, countercurrent blowdown and purge - as for the four-bed
process are employed but the sequence and number of pressure equalization
steps differ.

7.3.3 Separation of low molecular weight paraffins
Low molecular weight straight chain paraffins less than the molecular weight
of the homologue cetane (C10 H22) may be recovered at high purity from
naphtha feedstocks by the Iso-Siv process introduced by Union Carbide in
1961 (Symoniak 1980, Cassidy and Holmes 1984). Typical feeds are C5-C9
hydrocarbons containing as much as 50% n-paraffins. A two-bed process is
employed using a 5A zeolite which adsorbs the straight chain hydrocarbons
but excludes branched chain isomers. The sequence of steps for the two-bed
process is illustrated in Figure 7.4. Following the passage of feed at high
pressure through bed 1 when adsorption of n-paraffins occurs, bed 1 is
depressurized cocurrently when the product n-paraffins are desorbed and
collected. Gases remaining in the voids of the bed are removed by
evacuation prior to bed 1 being repressurized. Meanwhile bed 2 goes
through the same sequence of steps, the adsorption step in bed 2 occurring
simultaneously with cocurrent depressurization, vacuum desorption and
repressurization of bed 1.
Separation of hydrocarbons of higher molecular weight than C10 H12 is
accomplished by a different adsorption-separation technique not involving
either a change of pressure or temperature. The C10-C~8 n-paraffins are
strongly adsorbed on a 5A zeolite even at high temperatures. Neither
pressure swing nor thermal swing operations are therefore efficient in
desorbing the adsorbate. Displacement desorption is employed instead,
which involves the displacement of the adsorbate by means of a second
adsorbate gas purge (Chi and Cummings 1978). This technique will be
described in Section 7.6.

7.3.4 Air separation into 02 and N2 employing two different
processes
First we describe the separation of 02 from air using a 5A zeolite adsorbent
in a PSA process. As indicated in Section 7.2.1, a two-bed process described
by Figures 7.1 and 5.15 is used to separate 02 from air. On a 5A zeolite bed