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1.4 Adsorbent Regeneration Regeneration involves removing or desorbing the adsorbed molecules from the adsorbent and can be achieved by changing either the temperature or pressure. If the waste stream flow rate and composition are predictable, regeneration can occur at a preset time. However, if flow rate or the VOC concentration vary unpredictably, a sensor can be used to control when the regeneration cycle occurs. [18, 19] Three types of regeneration systems are available: thermal swing regeneration, vacuum regeneration and pressure swing adsorption. Thermal Swing Regeneration: In systems using thermal swing regeneration, the temperature is increased (swung) from the ambient conditions under which VOC is adsorbed to higher temperatures for the regeneration cycle. Increasing the temperature, increases the kinetic energy of the VOC molecules. The molecules are desorbed when the kinetic energy of the VOC molecules increases sufficiently to overcome the forces holding molecules to the adsorbent. The temperature required depends on the VOC present, but is typically between 250 to 350oF. Once the VOC is desorbed, cooled air is passed through the adsorber vessel to cool and dry the adsorbent before returning the vessel back to the adsorption cycle. Thermal regeneration has historically used steam, which provides the energy to desorb the adsorbate and also carries the desorbed compounds out of the adsorber vessel. The steam is then condensed and the VOC can either be recovered from the wastewater or the wastewater can be sent to wastewater treatment. Hence, steam-based regeneration systems will have additional equipment and operating costs associated with recovering the VOC and/or disposing of the wastewater generated. [19] In addition to steam, other heat sources have been developed, including using microwaves, embedded heaters, and heated nitrogen. These alternative heat sources make VOC recovery easier and avoid the wastewater treatment costs associated with the steam-based systems. [19] The physically adsorbed species in the smallest pores of the solid and the chemisorbed species may require rather high temperatures to be removed, and for all practical purposes cannot be desorbed during regeneration. Approximately 3 to 5 percent of organics adsorbed on virgin activated carbon is either chemisorbed or very strongly physically adsorbed and is difficult to desorb during regeneration. [1] Vacuum Regeneration: In a vacuum regeneration system, a vacuum pump is used to lower the pressure in the adsorber to below the vapor pressure of the adsorbed VOC. This causes the VOC to boil off from the adsorbent. Some systems use a small flow of purge gas (such as dry nitrogen) to transport of the VOC vapor to the recovery vessel. [19] 1-10PDF Image | Carbon Adsorbers
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