Page 341 - Adsorbents fundamentals and applications

326 SORBENTS FOR APPLICATIONS

New possibilities for developing better sorbents for methane storage are still
available. For example, no reports have appeared on activation of carbon fibers
by using molten KOH, which could yield the optimum micropore distribution and
volume. Single-wall carbon nanotubes with the right sizes, particularly bundled
in the aligned forms, could result in the ideal sorbent for methane storage.

10.5. OLEFIN/PARAFFIN SEPARATIONS
Cryogenic distillation has been used for over 70 years for the recovery of ethy-
lene and propylene from olefin plants, refinery gas streams, and other sources
(Keller et al., 1992). These separations are difficult to accomplish because of the
close relative volatilities. The ethane/ethylene distillation is performed at about
◦
−25 C and 320 psig in a column containing over 100 trays. Propane/propylene
◦
distillation is performed at about −30 C and 30 psig. These are the most energy-
intensive distillations in the chemical and petrochemical industry (Safarik and
Eldridge, 1998).
As discussed in Chapter 10.1, about 20% of the oxygen and nitrogen is now
produced from air by PSA technology. A single PSA unit is capable of producing
more than 200 tons/day of oxygen, and the capacity is steadily increasing as the
technology further improves. Distillation is necessary for larger scales. In the
following discussion, a case will be made that the situation for olefin/paraffin
separation should parallel that of air separation. That is, PSA is capable of small-
scale recoveries of ethylene and propylene, while for large-scale production of
ethylene and propylene in the olefin plants, distillation will clearly remain to be
the process of choice.
A significant amount of the light olefins produced during the refining of crude
oil is used as a fuel or is simply flared. These streams are generally small. In the
production of polypropylene and polyethylene, a significant amount of propy-
lene and ethylene are lost in the purge gas. These gas streams are generally
available at super-atmospheric pressures. For example, the refinery gas streams
are at 100–250 psia pressure range containing 10–35% ethane/ethylene, 2–10
propane/propylene, 8–20% H 2 , 20–45% CH 4 , and trace amounts of higher hydro-
carbons. The modern gas-phase polymerization reactors for both ethylene and
◦
propylene are operated at 20 atm pressure and 85–100 C, and the purge streams
contain about 80–85% olefins and 10–15% paraffins (Rodriguez, 1999). These
ranges of pressure, temperature, and gas composition are ideally suited for PSA
using π-complexation sorbents. Moreover, hydrocarbon emissions are no longer
allowed by federal regulations. Small-scale recovery of olefins is necessary for
both economic and environmental reasons. In fact, PSA using 4A zeolite (under
the name of Petrofin Process) was used for recovery of propylene from the purge
gas streams of polypropylene reactors. As discussed below, 4A zeolite is not a
good sorbent for this separation and hence this process has been discontinued.
10.5.1. Sorbents
Two types of sorbents have been examined for ethane/ethylene and propane/pro-
pylene separations: zeolites/molecular sieves and π-complexation sorbents. The