Solid bed dehydration systems work on the principle of adsorption. Adsorption involves a form of adhesion between the surface of the solid desiccant and the water vapor in the gas. The water forms an extremely thin film that is held to the desiccant surface by forces of attraction, but there is no chemical reaction. The desiccant is a solid, granulated drying or dehydrating medium with an extremely large effective surface area per unit weight because of a multitude of microscopic pores and capillary openings. A typical desiccant might have as much as 4 million square feet of surface area per pound.
The initial cost for a solid bed dehydration unit generally exceeds that of a glycol unit. However, the dry bed has the advantage of producing very low dew points, which are required for cryogenic gas plants, and is adaptable to very large changes in flow rates. A dry bed can handle high contact temperatures. Disadvantages are that it is a batch process, there is a relatively high pressure drop through the system, and the desiccants are sensitive to poisoning with liquids or other impurities in the gas.
