Obtaining Low Pressures with Cryosorption Pumps
Certain solid materials, such as molecular sieves, charcoals, and silica gels, have the ability to sorb large quantities of gases. A practical application of this phenomena is the removal of gases from the gas phase of a chamber to produce low pressures in vacuum systems. The process involved is one of physical adsorption and should be differentiated from chemisorption. Fundamentally the difference between these two processes is a function of the forces between sorbate and sorbent. The forces involved in physical absorption are similar to the forces active in condensation or liquefaction, viz., van der Waal’s forces (5 to 10 kcal/mole of gas). In chemisorption, the forces are similar to those in chemical species (10 to 100 kcal/mole of gas): however, chemisorption should not be confused with chemical reaction in which the sorbent and sorbate lose their identity. Physical adsorption is rapid in contrast to chemisorption, being primarily diffusion controlled. In chemisorption the rate depends on the resistance to surface reaction and involves an activation energy.
KeywordsMolecular Sieve Physical Adsorption Staging Operation Union Carbide Corporation Ultimate Pressure
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