Dehydration adsorbent regeneration

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. 

Durable changes of the catalytic properties of supported platinum induced by microwave irradiation have been also recorded [29]. A drastic reduction of the time of activation (from 9 h to 10 min) was observed in the activation of NaY zeolite catalyst by microwave dehydration in comparison with conventional thermal activation [30]. The very efficient activation and regeneration of zeolites by microwave heating can be explained by the direct desorption of water molecules from zeolite by the electromagnetic field this process is independent of the temperature of the solid [31]. Interaction between the adsorbed molecules and the microwave field does not result simply in heating of the system. Desorption is much faster than in the conventional thermal process, because transport of water molecules from the inside of the zeolite pores is much faster than the usual diffusion process. 

The basic adsorption process design. Sub-tasks within that include the adsorbent selection, made in view of aU of the requirements imposed on the dehydration process. The adsorption step time, regeneration and cooHng step times all need to be settled and these in view of mechanical details. The overall vessel configuration, for example, the vessel ID and length, which quantities are typically sized based on pressure drop. Finally we need to make some estimate of the expected service Hfetime for the adsorbent product. 

The use of rotary desiccant wheels for dehydration has been practiced for several decades. Carl Munters [13] created the first rotary adsorbers by impregnating corrugated paper with LiCl and then ran the rotor using direct heat exchange to regenerate the sorbent. More recently Munters, NTI and others have developed improved adsorbents for rotary dehydrators. 

In addition to heterogeneous azeotropic distillation, several alternative methods are available for ethanol dehydration such as extractive distillation, adsorption, and pervapo-ration. A comprehensive review of the subject, including 302 references, has been presented by Vane. A recent paper by Kiss and Paul claims that the heterogeneous azeotropic distillation process is more economical than adsorptive drying because of the large amount of energy required to regenerate the adsorbent. 

As shown by Table 1, the only large-scale application of adsorbent aluminas prior to 1940 was as desiccants. Both air and natural gas dehydration were being routinely performed with waste-heat regeneration of the alumina columns. In later years, dehydration with aluminas was extended to cracked gas and heavier hydrocarbon streams, as well as those containing carbon dioxide and ammonia. 

The principal drawback of the process is the possibility of producing poorly dried gas for a brief period when a hot, regenerated bed is first put on stream for dehydration service. However, operating experience of more than a year with a commercial unit did not show evidence of such occurrences (possibly due to instrumentation limitations). In theory, the heat transfer rone moves through the bed several hundred times faster than the mass transfer zone. As a result, except for a brief period at the beginning of the cycle, the heat transfer zone moves rapidly ahead of the mass transfer zone, providing an adequate supply of cool adsorbent to pick up water contained in the feed gas. 

Any of the solid materials used as desiccants must be regenerated periodically so there must be at least two adsorption towers if dehydration of the gas is to be continuous. A schematic of a typical two-tower adsorption plant is shown in Fig. 16.12. The adsorbent must be heated to high temperature (ca. 400°F, 200 C) to remove the water and regenerate the solid desiccant. This usually is accomplished by heating a slip stream of the gas and circulating the heated gas through the adsorbent bed. 

---