Molecular sieves are crystalline metal aluminosilicates having a threedimensional interconnecting network of silica and alumina tetrahedra. Natural water of hydration is removed from this network by heating to produce uniform cavities which selectively adsorb molecules of a specific size.
A 4 to 8-mesh sieve is normally used in gasphase applications, while the 8 to 12-mesh type is common in liquidphase applications. The powder forms of the 3A, 4A, 5A and 13X sieves are suitable for specialized applications.
Long known for their drying capacity (even to 90 °C), molecular sieves have recently demonstrated utility in synthetic organic procedures, frequently allowing isolation of desired products from condensation reactions that are governed by generally unfavorable equilibria. These synthetic zeolites have been shown to remove water, alcohols (including methanol and ethanol), and HCl from such systems as ketimine and enamine syntheses, ester condensations, and the conversion of unsaturated aldehydes to polyenals.
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Regeneration in typical cyclic systems constitutes removal of the adsorbate from the molecularsieve bed by heating and purging with a carrier gas. Sufficient heat must be applied to raise the temperature of the adsorbate, the adsorbent and the vessel to vaporize the liquid and offset the heat of wetting the molecular sieve surface. The bed temperature is critical in regeneration. Bed temperatures in the 175-260° range are usually employed for type 3A. This lower range minimizes polymerization of olefins on the molecularsieve surfaces when such materials are present in the gas. Slow heatup is recommended since most olefinic materials will be removed at minimum temperatures; 4A, 5A and 13X sieves require temperatures in the 200-315 °C range.
After regeneration, a cooling period is necessary to reduce the molecular sieve temperature to within 15° of the temperature of the stream to be processed. This is most conveniently done by using the same gas stream as for heating, but with no heat input. For optimum regeneration, gas flow should be countercurrent to adsorption during the heatup cycle, and concurrent (relative to the process stream) during cooling. Alternatively, small quantities of molecular sieves may be dried in the absence of a purge gas by oven heating followed by slow cooling in a closed system, such as a desiccator.
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