Page 309 - Pressure Swing Adsorption

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286 PRESSURE SWING ADSORPTION EXTENSIONS OF THE PSA CONCEPT 287
----- small scales, at which heat dissmat1on 1s rapid relative to the rate of heat
100.0 ----------- crnlization. The "molecular gate" can operate isothermally only at relatively
generatmn by adsorotion. ln order to operate a Iarge-scaie version of this
orocess. it would be necessary to mtroduce heat exChanged at both ends of
the adsorbent bed, thus effectively converting the rnoiccular gate to a TCPSA
80.0
unit.
0 The orospects for commercialization of the TCPSA process are difficuit to
assess. The mdicatwns are that m terms of energy efficiency and adsorbent
productivity the TCPSA process is, for many applications, significantly more
60.0 economic than a traditional PSA orocess. The prospects for developing an
I o =ta1 1morovect adsorbent contactor appear orom1smg, and this would further
j - ll..E + axial disr,cmon enhance the competitive position of TCPSA. The issue of capital cost 1s more
-- LDF + axial dispemoo difficult to assess, since no large TCPSA umt has yet been built. However, 1t
40.0 - - ' ILE {no dispers100) I seems Iikely that the remainmg economic barriers to the commercialization

--· l.DF(nodispemoo) of TCPSA system will be overcome eventually, and· a raoid spread of this
technology to a range of commercially important :separations may occur
within a few years.
· 1:.1-.__,_ __ 2...loo-__,_ __ 4....1.oo _ __._ __ 6_._oo _ _._ _ __.soo
20 0 The single column RPSA system is, m ormcioie, ooerable at large scale.
0 I However, as the scale of the process mcreases, the issue of pressure drop
parocle stze (microns) (and therefore power consumot1on) becomes mcreasmgly important. As a
·I result of the high-pressure drop, the power consumption m a RPSA system
Figure 7.15 Effect of particic size on purity of the oxygen product from a RPSA I will always be higher than that of a well-designed two-column process
process showing comoanson of experimental results with the profiles calculated from I operated under comoarable conditions. The economic viability of this type of
13
process 1s therefore limited to small-scale applications.
vanous theoretical ffiodeis. (From Al pay et ai. 1 with permission.)
Of the three systems described in this chapter oti.ly the TCPSA process
appears to offer reasonable orospects for large-scale ooerat10ns. However,
product. However, shorter feed time leads to mcreased oxygen recovery. the economic operation of such a process at high throughputs cteoends on the
Energy consumotion will therefore depend on the desired product ounty. develooment of a new and more efficient mass transfer device to avoid the
Recovery increases with both increasmg delay and exhaust time. The product inherent limitations of a packed bed contactor.
oxygen concentration shows a broad optimum with increasing delay but
decreases monotonically with decreasmg exhaust time.
Jn a more recent study Aloay, Kenney, and Scott 13 investigated the effect
of particle size on a RPSA air separation umt using 5A zeolite. The results
References
shown in Figure 7.15 show clearly the existence of an optimum particle size
for product:ennchment. A s1mulation including both pressure droo and mass
tra~sfer limitations shows that the product enrichment is limited for small 1. N. H. Sweed and R. H. Wilhelm, Ind. Eng. Chem. Fund. 8, 221 (1968). See also N. H.
Sweed, A!ChE Symp. Ser. 80(233), 44 (1984).
oart1cle sizes by axial dispersion and the pressure dynamics of the system,
and for larger particles by intraparucte diffusional resistance. 2. R. L. Pigford, B. Baker, and D. E. Blum, Ind. Eng. Chem. Fund., 8,848 (1969) .
., G. E. Keller and C.H. A. Kuo, U.S. Patent 4,354,854 (1982), to Union Carbide Corp.
4. B. G. Keefer, U.S. Patents. 4,702,903, 4,816.121, 4,801,308, 4,968,329, 5,096,469, and
7.4 Future Prospects 5,082,473; Canadian Patent 1256088; European Patent 0143537.
5. P.H. Turnock and R.H. Kadlec, A/Ch£ J. 17,335 (1971).
None of the systems described in this chapter has yet been developed beyond
the laboratory scale; so the ouestion anses as to the prospects for commer- 6. D. E. Kowler and R. H. Kadlec, A/Ch£ J. 18, 1207 (I 972).