No desiccant is perfect or best for all applications. In some applications the desiccant choice is determined primarily by economics. Sometimes the process conditions control the desiccant choice. Many times the desiccants are interchangeable and the equipment designed for one desiccant can often be operated effectively with another product. Table 8-4 illustrates the most common desiccants used for gas dehydration and some conservative parameters to use for initial design. Desiccant suppliers’ information should be consulted for detail design.
All desiccants exhibit a decrease in capacity (design loading) with increase in temperature. Molecular sieves tend to be the less severely affected and aluminas the most affected by temperature.
Aluminas and molecular sieves act as a catalyst with H2S to form COS. When the bed is regenerated, sulfur remains and plugs up the spaces. Liquid hydrocarbons also present a plugging problem to all desiccants, but molecular sieves are less susceptible to contamination with liquid hydrocarbons.
Silica gels will shatter in the presence of free water and are chemically attacked by many corrosion inhibitors. The chemical attack permanently destroys the silica gels. The other desiccants are not as sensitive to free water and are not chemically attacked by most corrosion inhibitors. However, unless the regeneration temperature is high enough to desorb the inhibitor, the inhibitor may adhere to the desiccants and possibly cause coking.
The alumina gels, activated aluminas, and molecular sieves are all chemically attacked by strong mineral acids and their adsorptive capacity will quickly decline. Special acid resistant molecular sieve desiccants are available.