Page 344 - Adsorbents fundamentals and applications

OLEFIN/PARAFFIN SEPARATIONS 329

at ambient temperature, AlPO 4 -14 did not appear to be a suitable sorbent for
propane/propylene separation. Padin et al. (2000) suggested that at higher tem-
◦
peratures (e.g., 120 C) the propylene isotherm became linear while maintaining
adequate capacity such that AlPO 4 -14 was an effective sorbent for the separation.
A direct comparison of PSA separation using the two types of sorbents
(AgNO /SiO 2 vs. AlPO 4 -14) was made by Rege and Yang (2002). A four-step
3
PSA cycle was used in the simulations. The steps involved in each cycle were as
follows: (1) pressurization with the feed gas mixture; (2) high-pressure adsorption
with feed gas (i.e., feed step); (3) high-pressure co-current purge with part of the
compressed C 3 H 6 -rich product obtained in step 4; (4) countercurrent blowdown
to a low pressure. All the steps were of equal time duration.
Figure 10.34 shows the schematic of the PSA cycle used. As can be seen,
a portion of the high-purity C 3 H 6 from the countercurrent blowdown step was
compressed to the feed pressure and used for rinsing the bed co-currently in
step (3). It has been shown earlier that purging with the strongly adsorbed com-
ponent results in a significant increase in the purity of that component in the
product stream (Tamura, 1974; Cen and Yang, 1986; Yang, 1987). In the present
work, the product of the high-pressure rinse step is recycled and mixed volu-
metrically with the feed gas supplied to step (2). The concentration of the feed
gas to step (2) was averaged volumetrically while its temperature was kept the
same as the feed gas temperature. The model and the numerical technique used
in this study are described in detail in previous literature (Rege et al., 1998).
The three important PSA performance parameters are product purity, recovery,

Adsorption
product

Pressurization I High-pressure feed II Co-current high-pressure purge III Low-pressure countercurrent blowdown IV

Olefin-paraffin Feed High-purity olefin
product
Figure 10.34. Schematic of the four-step PSA cycle used for C 3 H 6 /C 3 H 8 separation (Rege and
Yang, 2002, with permission).