Semicontinuous adsorption

Fixed-bed adsorbers may be operated in either intermittent or semicon-tinuous mode. A typical removal system is a semicontinuously operated dnal-bed system one bed is in adsorption mode while the other is being re generated (Fig. 13.23). " The adsorption performance of the bed can he monitored by analyzing the outlet gas. Once organic vapors are detected in the gas stream, the incoming gas stream is routed to the parallel adsorber, and the exhausted bed is regenerated. The adsorption and desorption cycles can also be fixed. 

Maas R, Chaudhari S (2005) Adsorption and biological decolourization of azo dye Reactive Red 2 in semicontinuous anaerobic reactors. Process Biochem 40 699-705... 

Pad Processes. In a continuous or semicontinuous processes, the fabric is first padded with the dye solution. The substantive adsorption of dyes during padding is avoided by employing minimum amounts of pad liquor at ambient temperatures. For even penetration of the dye, the material is then steamed without intermediate drying for 1-3 min in saturated steam (pad steam process ). Alternatively, it is heated to 80-85 °C and rolled up in a closed chamber where it is slowly rotated for 2-8 h (pad roll process). 

From the experimental point of view there is one more experiment that can be done to obtain adsorption rate parameters and this is the use of a semicontinuous approach. Here the adsorbate is fed at a mass flow rate that is equal to the rate of adsorption, and a very sensitive pressure transducer is slaved to a mass flow controller for the adsorbate. As this gas is adsorbed, and were there no flow into the system, the pressure would drop. This would lead to the complexities we have just analyzed. If, however, the mass flow controller is slaved in such a way that it opens whenever there is a slight 6P of pressure drop below the fixed experimental pressure set point, then the pressure can be maintained as a constant. The mass flow rate into the system is the same as the mass rate of adsorption "out" of the gas phase and "onto" the adsorbent phase II. 

For commercial applications, an adsorbent must be chosen carefully to give the required selectivity, capacity, stability, strength, and regenerability. The most commonly used adsorbents are activated carbon, molecular-sieve carbon, molecular-sieve zeolites, silica gel, and activated alumina. Of particular importance in the selection process is the adsorption isotherm for competing solutes when using a particular adsorbent. Most adsorption operations are conducted in a semicontinuous cyclic mode that includes a regeneration step. Batch slurry systems are favored for small-scale separations, whereas fixed-bed operations are preferred for large-scale separations. Quite elaborate cycles have been developed for the latter. 

The complexities of moving and handling solids add to the cost of the operation, and so a conimon technique in the field of adsorption is the use of a fixed bed of solids. The process becomes unsteady-state and semicontinuous. The solids remain in place as the fluid passes through. Occasionally, operation of the bed must stop to allow removal or regeneration of the solids. 

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