Page 285 - Pressure Swing Adsorption

262 PRESSURE SWING ADSORPTION PSA PROCESSES 263

Table 6.8. Exergy Analysis of Three PSA Air Separation Processes" kinetic orocess. More significantly, however, the comoanson between the
vacuum swing process (3) and the supra-atmospheric processes ( l and 2)
(1) PSA 0 process (2) PSA N process (3) VSA N 2 process
2 2
shows clearly the thermoctvnam,c advantage of vacuum swing. This advantage
Adsorbem SA zeo\ite CMS CMS stems from the large reduct10n in the energy input so that, eyen when the
Cycle Skarstrom +. P.E. Self-purgmg + P.E. Vac. swmg energy reomred to compress the product 1s allowed for, the net energy
PHIP, (aim) 3.9 /1.0 8.0/1.0 1.0/0.2 reqmre_ment 1s substantially reduced. However, this advantage, which trans-
Recovery(%) 60 33 63
lates directly mto a reduction of the orocess ooeraung cost, must be offset
Energy inpui
(a) Compressor /vac. 41,120 31.500 12.800 against the increased ca01tal costs associated with vacuum swing ooeration
(b) Product compression -2,800 -4,800 +5,700 which reouires both a compressor and a vacuum oumo as well as much iarge;
/expressmn (%) (%) (%) ducts and valves.
Total inpm 38,320 100 26,700 !00 18,500 !00
Product exergy 7,270 5,600 360
Product exergy at I amt 4,470 11.7 800 3.0 4,360 23.5 References
Waste product 6,230 16.3 1,730 6.5 720 3.9
Bed loss 13,430 35.0 12,450 46.5 5,100 27.6 l. C. W. Skarstrom, "Heatless Fract1onat10il of Gases over Solid Adsorbeilts," m Recent
Developments in Separatwn Science, pp. 95-106, Vol. 2, N. N. Li, ed., CRC Press Cleveiand
Compressor/ cooler iosses 14,200 37 9,470 35.5 8,050 43.43 (1972). '
Other losses le.g .. valves. 2,270 8.5 320 1.7
2. D. H. White, 11ie Pressure Swmg Ad.mrptwn Procen, AIChE National Meeting, paper 87h,
e1c.)
New Orleans, LA, March 8 (1988). See also D. 1-1. White and G. Barclay, Chem. Em:. Prog
Process efficiency(%) 11.7 3.0 23.5 85(1), 25 (1989). .
Energy and exergy expressed as J/mole product. Product exergy 1s corrected to I atm. mall cases, 3. D. M. Ruthven, Principles of Adsorpllon and Adsorptwn Procenes, Chap. 7, Wilev. -New
by allowmg for work of expansion or compression. Process efficiency 1s defined bv Eq. 6.15. York (1984).
4. A. Anzelius, Zeit. Angew. Math. Mech. 6, 291-94 (1926).
process with pressure equalization when operated under optimal conditions 5. C. W. Skarsirom, U.S. Patents 2,944,627 (1958) and 3,237,377 (1966) to Esso Research aild
Engineering.
in terms of the power reqmrement.
A similar analysis has also been made for the two-bed nitrogen production 6. G. A Soria!, W. H. Granville, and W. 0. Daley, Chem. Eng. Sci. 38, 1517 (1983).
process including both the self-ourgmg cycle and the vacuum swmg cycle 7. C. G. Coe, G. E. Pams, R. Sdmvasan, and S. R. Auvii,. m hoceeding., oi Smnth
(Figures 3.12 and 3.17). The corresponding Grassman diagrams are shown in Intematwnal _Zeolite Conference, Tokvo, p. 1033, Y. Murakami; A. Liiima, and J. W. Ward.
Figure 6.27. The exerget1c efficiency of the self-purging process 1s about eds., Kodanslla-Elsevier, Tokyo 0986).
17 .6% at an operating pressure of 8 atm, and the corresponding energy· 8. C. G. Coe, Ill Ga.\' Separmion Technoirwy, pp. 149-59, E. F. Yansam a 11 d R. Dewolf<;, etk.
requirement is about 8.7 kWh per kmole of product. For the vacuum swing Elsevier, Amsterdam (1990). ·
cycle the exergetic efficiency is much lower ( ~ 2.8%) since the product is 9. I. Smolarek and M. J. Camphell, m Gas Separation Technoiogy, p. 281, E. F. Vansant and
dclivcn::d at subatmosphenc pressure. However, the energy requirement is R. Dewolfs, eds .. Elsevier, AmsterdJtm (1990).
aiso iower (about 3.6 -kWh per kmolc of product). If the product nitrogen 10. L. B. Batt~, U.S. Patent 3,636,679 lo Union Carbide (1972).
were compressed to 8 atm, the total work requirement would be mcreased to
11. K. Knoblauch, H. Heiilback, and B. Harder, U.S. Patent 4;548,799 (1985), to Bergbau
5.6 kWh/kmole product; but this 1s still substanttally lower than for the
Forschung.
pressure swmg process.
12. H.J. SchrOter and H. Jiintgen, in Adsorption: Sci1:m'.e and Tedmaiom·. p. 269. NATO ASl
A detailed thermoctynam1c comP,arison of the three air separation pro-
158, A. E. Rodrigues, M. D. LeVail, and D. Tondeur. eds., Kluwer, Oordrechl (1989).
cesses, hased on BanerJee's figures, 1s given m Table 6.8. For all three
orocess(::S the majOr sources of inefficiency are the losses in the feed compres~ 13. E. Pilarciyk and_ K. Knoblauch, Sepurat,011 Technology, p. 522, N. Li and H. Strathmann,
eds., Eng. Foundation, NY (1988).
sor or the vacuum, pump and m the adsorbent bed. Companng the two
supra-atmosphenc pressure processes (1 and 2), tile efficiency of the kineti- 14. Nitrotec brochure, Nitrotec Engmeenng Co., Gleil Burnie, MD (1988).
cally controllect mtrogen process 1s substantially lower than that of the 15. Ailon., "Pressure Swing Adsorption Picks Up Steam," Chem. Eng,, 95, s t 26 1988
P.26. ep. • ',
equilihnum-basect oxygen process, reflecting the inherent Irreversibility of the