Page 349 - Adsorbents fundamentals and applications

334 SORBENTS FOR APPLICATIONS

10.5.3. Other Sorbents
5A and 13X zeolites have also been considered for olefin/paraffin separations
based on stronger adsorption of olefins. However, additional and substantial
operations for the PSA are needed (e.g., Javelin and Fair, 1993). Rodriguez and
co-workers have made interesting studied on the adsorption properties of these
sorbents (Da Silva and Rodriguez, 1998; Grande et al., 2002). A vacuum swing
cycle was studied for propane/propylene separation by using 13X (Da Silva and
Rodriguez, 2001). Compared with the π-complexation sorbents, however, the
separation results with π-complexation sorbents are significantly better.
Cho and co-workers (Cho et al., 2001; Choudary et al., 2002) have
recently developed AgNO 3 supported on acid-treated clays, named Olesorb-1.
Subatmospheric isotherms have been reported. The diffusion time constants for
the C 2 and C 3 olefins and paraffins at ambient temperature are around 10 −3
1/s. From the isotherms and the diffusion time constants, the sorbent is suitable
for olefin/paraffin separation. The isotherms are shown in Figure 10.38 and
Figure 10.39. The isotherms of AgNO /SiO 2 are compared in these figures. For
3
C 2 separation, the two sorbents appear to be comparable, although the isotherm of
C 2 H 4 on AgNO /SiO 2 is more linear, which helps separation. For C 3 separation,
3
the isotherm of C 3 H 6 on AgNO /SiO 2 is substantially higher as well as being
3
more linear, although AgNO /SiO 2 also adsorbs more C 3 H 8 .Nodataonthe
3
Olesorb-1 were reported for higher pressures. It appears that the olefin capacity of
Olesorb is also limited by its pore volume, since the isotherms seem to be leveling
off at a low pressure. The Olesorb-1 has been evaluated in an experimental four-
bed PSA system, using a VSA cycle similar to the one described above, except
the feed pressure was at ∼1.2 atm and the VSA was run at ambient temperature.
Using a feed containing 83.1% ethylene (16.8% ethane and traces of other gases),
Choudary et al. (2002) obtained a sorbent productivity of 0.04 kg C 2 H 4 /h/kg
sorbent, at 99.8% ethylene product purity and 85% recovery.
From these results, it is seen that olefins can be recovered at high purities
and reasonably high recoveries. The sorbent productivities are higher than that
obtained in air separation by PSA/VSA. As mentioned, for air separation, a
single PSA/VSA can produce over 200 tons/day of oxygen. Thus, it is feasible
to recover olefins at that scale with a single PSA unit.

10.6. NITROGEN/METHANE SEPARATION

The separation of nitrogen from methane is becoming increasingly important for
natural gas recovery and enhanced oil recovery. Natural gases that contain sig-
nificant amounts of nitrogen need to be upgraded in order to meet the pipeline
quality for minimum heating value specifications, typically >90% methane. This
is the case with a large amount of natural gas reserves as well as aging natural
gas wells. This also applies to enhanced oil recovery where nitrogen is injected
into the reservoir. Nitrogen injection increases the level of nitrogen contamina-
tion in the gas fraction recovered from the reservoir, that is, petroleum gases,