Gas phase adsorptive separation

Adsorption as a gas phase separation process fills a space in the spectrum of separations processes that encompasses both purification and bulk separations. The market for gas phase adsorptive separations is of the order of several billion US dollars armuaUy when aU sorbent, equipment and related products are included. 

Tlie growth in both variety and scale of gas-phase adsorption separation processes, particularly since 1970. is due in part to continuing discoveries of new porous, high surface-area adsorbent materials (particularly molecular sieve zeolites and. especially, to improvements in tlie design and modification of adsorbents. These advances have encouraged parallel inventions of new process concepts. Increasingly, the development of new... 

Gas-phase adsorption is widely employed for the large-scale purification or bulk separation of air, natural gas, chemicals, and petrochemicals (Table 1). In these uses it is often a preferred alternative to the older unit operations of distillation and absorption. 

Process Concepts. Hybrid systems involving gas-phase adsorption coupled with catalytic processes and with other separations processes (especially distillation and membrane systems) will be developed to take advantage of the unique features of each. The roles of adsorption systems will be to efficiently achieve very high degrees of purification to lower fouUng contaminant concentrations to very low levels in front of membrane and other separations processes or to provide unique separations of azeotropes, close-boiling isomers, and temperature-sensitive or reactive compounds. 

The great leap forward for chromatography was the seminal work of Martin and Synge (7) who in 1941 replaced countercurrent liquid-liquid extraction by partition chromatography for the analysis of amino acids from wool. Martin also realized that the mobile phase could be a gas rather than a liquid, and with James first developed (8) gas chromatography (GC) in 1951, following the gas-phase adsorption-chromatographic separations of Phillips (9). 

Adsorption is the process whereby molecules of a gas or liquid species adhere to a solid surface. If a certain species A has a greater affinity for the solid surface than another species B in the mixture the preferentially adsorbed species can in principle be separated from the other molecules in the gas or Hquid mixture. If the solid adsorbent is to be reused the adsorbed species must be desorbed from the solid. In gas phase adsorption the adsorbed material is most often removed by changing the temperature and/or the pressure of the system along with a carrier or sweeper gas. For liquid systems a chemical desorbent must be found that preferentially displaces the desired product species from the soHd. The desorbent must itself be easily separated from the product in another separation step, usually distillation. 

Figure 15.23. Hypersorber continuous moving bed gas phase adsorption system (See Mantell, Adsorption, McGraw-Hill, New York, 1951). (a) Schematic pattern of flows of gas and solid adsorbent (Hengstebeck, Petroleum Processing, McGraw-Hill, New York, 1959). (b) Solids flow rate control mechanism, (c) Typical separation performance.

Kovach, J. L. "Gas-Phase Adsorption," In "Handbook of Separation Techniques for Chemical Engineers," Schweitzer, P. A, Ed., McGraw-Hill, New York, 1979 ... 

Isotope separation by means of gas-phase adsorption is quite a new area of study. In the screening test for selective adsorption of CO on zeolite samples from the binary system CO/ CO, it was found that Na-LSX (Si02/Al203 ratio 2.0) adsorbed CO selectively at low temperatures, and fliat its CO separation factor reached 1.1 at 213 K. Compared with cryogenic separation, this separation factor is extremely high. 

Jia Guo and Aik Chong-Lua, Effect of sur ce chemistry on gas -phase adsorption by activated carbon prepared from oil-palm stone with pre-impregnation . Separation and Purfication Technology 18 (2000) 47-55. 

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