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Lectrodryer is a registered trademark for equipment to remove water and other similar trace constituents from a fluid. One common use is to remove water vapor from compressed air, gases and atmospheric air. Other uses include removing traces of ammonia, carbon dioxide and impurities from gases, and removing dissolved water from liquid hydrocarbons. Actually, the name is somewhat misleading since we design and manufacture a “reactivator.” A dryer is easily built. It is as simple as purchasing a container of desiccant from any desiccant manufacturer and putting an inlet and outlet connection on it. The fluid to be dried can then be passed through the container and water will be removed. The trick, however, is to use it a second time and this requires returning the desiccant to its original state of condition or as nearly to it as possible. This is reactivation.
Lectrodryers utilize a material known either as desiccant or adsorbent such as molecular sieve, activated alumina, and silica gel. All of these materials have tremendous surface areas available in small volumes due to their porosity. This porosity provides parking spaces for molecules of materials which are to be removed from a fluid. For example, it has been calculated that one cubic inch of silica gel desiccant contains pores having a surface area of about 90,000 square feet. In effect, the adsorbent is similar to a sponge, allowing the molecules of the adsorbate (an adsorbed substance) to condense on the surfaces. In the process of doing so, there is a release of energy known as the “heat of adsorption.”
Depending upon the temperature and the partial pressure of the adsorbate, a condition is finally reached when the sponge is full and has to be wrung out before it can be used again.
The most common type of adsorption system employs a dual-tower, fixed bed, cyclic operation. While one tower is adsorbing for a period of time, the other tower is reactivating. Since adsorbing is typically a continuous operation, the beds are continuously switched back and forth between the adsorption and reactivation modes of operation. The entire gas or liquid stream flows through the particular bed in the adsorption position. Simultaneously, the bed in the reactivation position is being reactivated by flowing gases from various sources. In many applications, the adsorbate in the fluid is water and the unit itself is referred to as a “dryer.” As the wet process stream enters the fresh desiccant bed, the water vapor is adsorbed in a finite length of bed called a mass transfer zone (MTZ). This MTZ is defined to be the bed length through which the concentration of the adsorbate is reduced from essentially inlet to outlet conditions. As the wet gas continues to flow, the bed may be divided into three distinct zones: the dynamic equilibrium zone, the MTZ, and the remaining active zone. The dynamic equilibrium zone is where the adsorbate is in equilibrium with the adsorbent and no mass transfer occurs. The final zone in the bed is the yet unused part or the “active zone.” The bed is exhausted when the leading edge of the MTZ reaches the outlet end of the bed.
1. Adsorbent in dynamic equilibrium with inlet gas
2. Gas at this point contains 95% of inlet adsorbate concentration
3. Gas at this point has 5% of inlet adsorbate concentration
4. Active adsorbent
After the bed is exhausted, the towers are switched. The tower that was adsorbing is now put on reactivation (also known as regeneration) and the fresh reactivated tower is put on adsorption. This reactivation period is the period of time during which the adsorbate is removed and the adsorbent is returned to its original condition (known as desorption). Two basic types of regenerative systems are used on Lectrodryer units, thermal and pressure swing. Thermal reactivation of the adsorbent involves a transfer of energy to drive the molecule from the surface and a simultaneous purging (by a regeneration gas) to sweep it out of the environment so that it will not be re-adsorbed. Most reactivations are performed with heat, raising the temperature of the total environment to whatever level is necessary to accomplish desorption. Reactivation heating continues until all of the adsorbate is driven from the active surfaces. The adsorbent then has to be cooled back down to ambient temperatures before it can be used again. The total reactivation cycle includes both the heating and cooling phases.
Since thermal regeneration is used on most of the Lectrodryer units, this will be discussed first. There are several ways of heating the desiccant beds, the two most popular are those that employ heaters embedded in the desiccant itself, or a heater external to the bed which depends upon a convection flow of some fluid to raise it to the desired temperatures.
The embedded type of reactivation is the most efficient since it puts the heat where it is needed the most. Electric heaters of the nichrome wire wound type can be distributed through the bed, or sheath-type heating elements can also be used. Lectrodryer units with internal heaters are the Type-T, AIR, GAS-B, and the GAS-CC units (up to size 210. GAS-CC-350 and above use an external heater).
Thermal regeneration is used on all Lectrodryer units except the Type-R heatless. This unit uses pressure swing regeneration. Desorption is carried out at a pressure lower than that of adsorption and occurs primarily through the stripping action of a purge gas. The adsorbent releases the adsorbate at this lower pressure because of the adsorbent’s natural tendency to come to equilibrium with its surroundings. The desiccant beds, themselves, are switched from the adsorption step to the desorption step before the thermal front associated with adsorption reaches the effluent end of the onstream bed. In this manner, the beds retain sufficient heat to compensate for the endothermic heat of desorption. Because of this, cycling between beds is typically three to five minutes.
