Desiccant tower

To get a complete picture of a dryer s dewpoint performance, the dewpoint must be monitored continuously throughout the entire dryer cycle to ensure it remains relatively constant. The dewpoint should be monitored closely when a fresh desiccant tower or cartridge is first brought on-line. If the freshly regenerated desiccant has not been properly cooled, the dewpoint will spike to an unacceptable level and will remain there until the desiccant has cooled to a temperature at which the desiccant can adsorb the moisture from the return air. 

The solid in the tower may be supported on a Witt filter plate, but a plug of glass wool is satisfactory the plug of glass wool above the solid acts as a filter for fine particles of the desiccant. 

In a typical gas oil design, the lighter products overhead from the quench tower/primary fractionator are compressed to 210 psi, and cooled to about 100°F. Some Q plus material is recovered from the compressor knockout drums. The gases are ethanolamine and caustic washed to remove acid gases sulfur compounds and carbon dioxide, and then desiccant dried to remove last traces of water. This is to prevent ice and hydrate formation in the low temperamre section downstream. 

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 dehydraiion unit. The essential components of any solid desiccant dehydration system are ... 

Two or more adsorption towers (contactors) filled with a solid desiccant. 

A high-ieniperature heater to provide hot regeneration gas to reactivate the desiccant in the towers. 

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. 

However, low velocities require towers with large cross-sectional areas to handle a given gas flow, and allow the wet gas to channel through the desiccant bed and not be properly dehydrated. In selecting the design velocity therefore, a compromise must be made between the tower diameter and the maximum use of the desiccant. Figure 8-22 shows a maximum design velocity. Smaller velocities may be required due to pressure drop considerations. 

In its simplest form, an adsorber is normally a cylindrical tower filled with a solid desiccant. The depth of the desiccant may vary from a few feet to 30 ft or more. The vessel diameter may be from a few inches to 10 or 15 ft. A bed height to diameter (L/D) ratio of higher than 2.5 is desirable. Ratios as low as 1 1 are sometimes used however, poor gas dehydration, caused by non-uniform flow, channeling and an inadequate contact time between the wet gas and the desiccant sometimes result. 

Towers are sized for a design pressure drop of about 5 psi through th-j desiccant. The pressure drop can be estimated by ... 

Leaving 6 ft above and below the bed, the total tower length including space to remove the desiccant and refill would be about 28 ft. This yields an L/D of 28/5.5 = 5.0. 

Atmospheric air at the rate of 6.1 million SCFD is compressed to 160 psig in a two-stage compressor JJ-1 that is provided with an intercooler and a knockout drum. Then it proceeds to a packed tower T-l where it is scrubbed with recirculating caustic soda solution. Overhead from T-l is cooled to 14°F in a refrigerated exchanger. After removal of the condensate, this stream proceeds to a dryer system that consists principally of two vessels F-l and F-2 packed with solid desiccant. 

The simplest solid desiccant fixed bed system consists of a minimum of two dryer vessels or towers. One vessel is drying the process... 

Fig. 3. Elution pattern of I-labeled rat prolactin, [ I]rPRL (AFP 1-2), prepared by the glucose oxidase-lactoperoxidase iodination procedure and purified as described in Pig. I for human PRL. The unlabeled hormone was stored as a dry powder, desiccated, and weighed out just prior to iodination this resulted in a maximum yield of peak III material. The peak III monomeric hormone is suitable for use in radioimmunoassay (cf. Fig. 4) (Tower er o/. ).

Figure 4-9. Solid desiccant dehydrator twin tower system. Courtesy Gas Processors Suppliers Association Engineering Data Book, Tenth Edition.

A drying tower, if used, is placed between the bleeding valve and the pump to remove any water vapor from the air during the initial pressure adjustments when the pump is first turned on. Potassium hydroxide was the old-time favorite desiccant, but today, indicating Drierite (CaS04 with CoClj HiO) or Anhydrone [MgfCIQ,) 21- O] is used more often because these desiccants do not liquify when moist. 

---