For each component in the inlet gas stream, there will be a section of bed depth, from top to bottom, where the desiccant is saturated with that component and where the desiccant below is just starting to adsorb that component. The depth of bed from saturation to initial adsorption is known as the mass transfer zone. This is simply a zone or section of the bed where a component is transferring its mass from the gas stream to the surface of the desiccant.
As the flow of gas continues, the mass transfer zones move downward through the bed and water displaces the previously adsorbed gases until finally the entire bed is saturated with water vapor. If the entire bed becomes completely saturated with water vapor, the outlet gas is just as wet as the inlet gas. Obviously, the towers must be switched from the adsorption cycle to the regeneration cycle (heating and cooling) before the desiccant bed is completely saturated with water.
At any given time, at least one of the towers will be adsorbing while the other towers will be in the process of being heated or cooled to regenerate the desiccant. When a tower is switched to the regeneration cycle some wet gas (that is, the inlet gas downstream of the inlet gas separator) is heated to temperatures of 450°F to 600°F in the high-temperature heater and routed to the tower to remove the previously adsorbed water. As the temperature within the tower is increased, the water captured within the pores of the desiccant turns to steam and is absorbed by the natural gas. This gas leaves the top of the tower and is cooled by the regeneration gas cooler. When the gas is cooled the saturation level of water vapor is lowered significantly and water is condensed. The water is separated in the regeneration gas separator and the cool, saturated regeneration gas is recycled to be dehydrated. This can be done by operating the dehydration tower at a lower pressure than the tower being regenerated
or by recompressing the regeneration gas.
Once the bed has been “dried” in this manner, it is necessary to flow cool gas through the tower to return it to normal operating temperatures (about 100°F to 120°F) before placing it back in service to dehydrate gas. The cooling gas could either be wet gas or gas that has already been dehydrated. If wet gas is used, it must be dehydrated after being used as cooling gas. A, hot tower will not sufficiently dehydrate the gas.
The switching of the beds is controlled by a time controller that performs switching operations at specified times in the cycle. The length of the different phases can vary considerably. Longer cycle times will require larger beds, but will increase the bed life. A typical two-bed cycle might have an eight-hour adsorption period with six hours of heating and two hours of cooling for regeneration. Adsorption units with three beds typically have one bed being regenerated, one fresh bed adsorbing, and one bed in the middle of the drying cycle.
Internal or external insulation for the adsorbers may be used. The main purpose of internal insulation is to reduce the total regeneration gas requirements and costs. Internal insulation eliminates the need to heat and cool the steel walls of the adsorber vessel. Normally, a castable re factory lining is used for internal insulation. The refractory must be applied and properly cured to prevent liner cracks. Liner cracks will per mit some of the wet gas to bypass the desiccant bed. Only a small amount of wet, bypassed gas is needed to cause freezeups in cryogenic plants. Ledges installed every few feet along the vessel wall can help eliminate this problem.