Page 359 - Adsorbents fundamentals and applications

344 SORBENTS FOR APPLICATIONS

This comparison is a direct reflection of the relative pore volumes. The tendency
of the N 2 isotherm on Sr-ETS-4 to form a plauteau at above 5 atm is also
undesirable for PSA separation, particularly when a high feed pressure of natural
gas is available (which is usually the case).

10.6.3. PSA Simulation: Comparison of Sorbents
From the N 2 /CH 4 isotherms and diffusivity data, the most promising sorbents
appeared to be Sr-ETS-4, Mg-clinoptilolite, and purified clinoptilolite. Thus, these
three sorbents were compared for N 2 /CH 4 separation (Jayaraman et al., 2002)
by using a proven numerical PSA model (Rege et al., 1998). The overall diffu-
2
sion time constants (D/R ) at 295 K for N 2 were (in 1/s) 1.1 × 10 −3 (purified
clinoptilolite); 1.8 × 10 −2 (Mg-clinoptilolite); 3.1 × 10 −3 (Sr-ETS-4). The cor-
responding values for CH 4 were 2.0 × 10 −5 (purified clinoptilolite); 6.0 × 10 −5
(Mg-clinoptilolite); 1.2 × 10 −3 (Sr-ETS-4) (Jayaraman et al., 2002). Heats of
adsorption and other input data are available elsewhere (Jayaraman et al., 2002).
The standard five-step PSA cycle was employed, which consisted of: pressur-
ization, high-pressure feed, co-current depressurization (or blowdown), counter-
current blowdown and evacuation, and low-pressure purge. The cycle conditions
were optimized for each run. The results are summarized in Table 10.11.
With a feed mixture containing 85% CH 4 and 15% N 2 , the results show that
over 90% methane product purity at high recoveries and high throughputs are
possible with both purified clinoptilolite and Sr-ETS-4. From the comparison, it
is seen that for a feed pressure of 7 atm, the PSA results are comparable for
purified clinoptilolite and Sr-ETS-4. For Mg-clinoptilolite, both methane purity
and recovery were low. The sorbent productivities for both purified clinoptilolite
and Sr-ETS-4 were higher than those obtained for air separation with zeolites. At
feed pressures higher than 7 atm, the separation will further improve for clinop-
tilolites because the N 2 adsorption amount will continue to rise with pressure,
although the improvement will be less for Sr-ETS-4 as the N 2 capacity is already
nearly saturated at 7 atm. PSA upgrading of natural gas by Sr-ETS-4 has already
been commercialized (Engelhard, 2001).

10.7. DESULFURIZATION OF TRANSPORTATION FUELS
Due to worldwide environmental mandates, refiners are facing the challenge of
producing increasingly cleaner fuels (Avidan et al., 2001). The primary focus of
the new regulations is the reduction of sulfur in gasoline and diesel. Other rules
stipulate reduced levels of aromatics, especially benzene, benzene derivatives,
olefins, and oxygenates. Sorbents for removal of aromatics will be discussed in
Section 10.8 of this chapter.
In 1998, the European Union first mandated new sulfur specifications for
drastically reduced levels that started to be phased in from the year 2000 (Knud-
sen et al., 1999). Similar regulations were legislated in the United States and
elsewhere soon after. The EPA Tier II regulations require reductions of sulfur in