Multiple desiccant beds are used in cyclic operation to dry the gas on a continuous basis. The number and arrangement of the desiccant beds may vary from two towers, adsorbing alternately, to many towers. Three separate functions or cycles must alternately be performed in each dehydrator. They are an adsorbing or gas drying cycle, a heating or regeneration cycle, and a cooling cycle.
Figure 8-21 is a flow diagram for a typical two-tower solid desiccant dehydration unit. The essential components of any solid desiccant dehydration system are:
1. Inlet gas separator.
2. Two or more adsorption towers (contactors) filled with a solid desiccant,
3. A high-temperature heater to provide hot regeneration gas to reactivate the desiccant in the towers.
4. A regeneration gas cooler to condense water from the hot regeneration gas.
5. A regeneration gas separator to remove the condensed water from the regeneration gas.
6. Piping, manifolds, switching valves and controls to direct and control the flow of gases according to the process requirements.
In the drying cycle, the wet inlet gas first passes through an inlet separator where free liquids, entrained mist, and solid particles are removed, This is a very important part of the system because free liquids can damage or destroy the desiccant bed and solids may plug it. If the adsorption unit is downstream from an amine unit, glycol unit or compressors, a filter separator is preferred.
In the adsorption cycle, the wet inlet gas flows downward through the tower. The adsorbable components are adsorbed at rates dependent on their chemical nature, the size of their molecules, and the size of the pores. The water molecules are adsorbed first in the top layers of the desiccant bed. Dry hydrocarbon gases are adsorbed throughout the bed. As the upper layers of desiccant become saturated with water, the water in the wet gas stream begins displacing the previously adsorbed hydrocarbons in the lower desiccant layers. Liquid hydrocarbons will also be absorbed and will fill pore spaces that would otherwise be available for water molecules.