The vacuum use of molecular sieves and other desiccants

Abstract

The use of molecular sieve as a desiccant in vacuum systems has been investigated. A comparison of the water adsorption efficiency of the sieve, under static equilibrium conditions, with two other regenerable desiccants (silica gel and activated alumina) shows the sieve to be far superior. It is also shown that the non-regenerable desiccant phosphorus pentoxide, Has a greater adsorption capacity than the molecular sieve. A comparison of the adsorption capacity of two column lengths of sieve under dynamic conditions is made, and the pressure differentials across the columns are investigated. The increase in the adsorption efficiency when the system is backed by an air ballast rotary pump is shown. Data are also given for moisture breakthrough, heat of adsorption, and regeneration times. As the sieve readily adsorbs air at atmospheric pressure, the time required for initial degassing is shown. An appendix describes a subsidiary water absorption test, made on a sample of phosphorus pentoxide, to show the progressive effect of surface deliquescence and relative vapour pressures.