Molecular sieves as adsorbents

Adsorption processes with molecular sieves as adsorbents are a good tool to separate mixtures of -paraffins, isoparaffins, olefines, and cyclic hydrocarbons according to the PSA mode. 

Dchumidificatlon by other methods Adsorption, e.g., methods using activated silica gel, alumina, or molecular sieves as adsorbents (see Chap. 11), and washing gases with water solutions containing dissolved substances which appreciably lower the partial pressure of the water (see Chap. 8) are other commonly used dehumidification processes, particularly when very dry gases are required. 

Market Data. The largest U.S. manufacturer of molecular sieves for adsorbent and desiccant use is UOP, which has a production capacity of 18—20 million kg/year. W.R. Grace and Zeochem have about 7 and 2 million kg/year capacity, respectively (55). W.R. Grace is the largest producer of siUca gel desiccants. Activated alumina for use as adsorbent and desiccant is produced by LaRoche Chemicals (formerly Kaiser) and by Aluminum Company of America. About one-third of the U.S. supply of activated alumina adsorbent and desiccant is imported by Rhc ne-Poulenc. 

KURASEP [Kuraray Separation] A process for separating nitrogen from air by a variant of the PSA process, using carbon molecular sieve as the adsorbent. Developed by Kuraray Chemical Company. 

Sumitomo-BF A PSA hydrogen purification process using a carbon molecular sieve as the selective adsorbent. Developed by Sumitomo, Japan. 

The following applications include the removal of straight-chain from branched-chain or cyclic molecules. For example, type 5A sieves will adsorb n-butyl alcohol but not its branched-chain isomers. Similarly, it separates n-tetradecane from benzene, or n-heptane from methylcyclohexane. A logical development is the use of molecular sieves as chromatographic columns for particular preparations. 

Finally more nitrogen Is recovered by vacuum desorption The adsorbent used In this process Is apparently the same - zeolite molecular sieves - as that used In oxygen PSA processes ... 

Displacement-Purge and Inert-Purge Cycles By far the most widespread embodiment of these cycles Is the separation of normal and lso-parafflns In a variety of petroleum fractions. These fractions In general contain several carbon numbers and can Include molecules from about C5 to about C g All of these separations use 5A molecular sieve as the adsorbent Its 0.5 nm pore diameter Is such that normal paraffins can enter but lso-parafflns are excluded this constitutes the basis for separation. 

Adsorption Measurement. The capacities of the molecular sieves to adsorb vapor phase 1,3,5-triisopropylbenzene (97 %, Aldrich) and 1,2,4-triisopropylbenzene (99%, Camegie-Mellon University) were measured at 373 K using a McBain-Bakr balance. The adsorption temperature was chosen such that no chemical reactions of the adsorbates were observed. Prior to the adsorption experiment, the NH4+-forms of the solids (except SAPO-37) were dehydrated at 573 K under a vacuum of 10" 2 Torr. The as-made SAPO-37 was calcined at 793 K in an oxygen flow of 6 L/h in-situ in the adsorption system for removal of organic species and dehydration. The vapor pressure at 296 K of 1,3,5- and 1,2,4-triisopropylbenzene is approximately 0.45 Torr. The adsorption experiments were conducted at this pressure. 

Sharp separation consists of obtaining splitting of the mixture into products with a high recovery of target components. The sharpness is defined as the ratio of key component concentrations in products. This should be better than 10. Potential techniques are physical absorption, cryogenic distillation, molecular sieving, as well as equilibrium adsorption when the molar fraction of the adsorbate is less than 0.1. Chemical absorption may also be applicable when the component concentration is low. 

F. Wolf (Martin Luther University, Halle/Salle, Germany, DDR) Tsitsishvilfs results that there are maxima and minima in the curves for the adsorption amounts as functions of the degree of ion exchange were reported at the Molecular Sieves Conference in London, 1967. This seems to be the general behavior of molecular sieves when adsorbing molecules. 

Subsequently, a period of particularly active research into the exploration value of sulphur gases yielded promising results. Hinkle (1978), Hinkle and Harms (1978) and Hinkle and Kantor (1978) used molecular sieves as passive adsorbents of sulphur gases. Lovell (1979), Lovell et al. (1980), Hale and Moon (1982), Hinkle and Dilbert (1984) and Oakes and Hale (1987) used the soils themselves. McCarthy et al. (1986) used soil gases and Hinkle (1986) used both soils and soil gases. The objective of this chapter is to present the rationale and the results of the investigations from this period. 

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