REGENERATION OF SPENT ADSORBENT

The simplest and the most common way of regenerating an adsorbent in industrial applications is by heating. The vapour pressure exerted by the adsorbed phase increases with 

The relatively poor conductivity of a packed bed makes it difficult to get the heat of regeneration into the bed, either from a jacket or from coils embedded in the packing. This is more easily achieved by preheating the purge stream. Even in the best conditions, it takes time for the temperature of the bed to rise to the required level. Thermal regeneration is normally associated with long cycle times, measured in hours. Such cycles require large beds and, since the adsorption wave occupies only a small part of the bed on-line, the utilisation of the total adsorbent in the unit is low. 

It is good operating practice to regenerate a bed in the reverse direction to that followed during adsorption. This ensures that the adsorbent at the end of the bed, which controls the quality of the treated stream, is that which is most thoroughly regenerated. Carter(44) has quantified the effect and showed that regeneration is achieved in a shorter time in this way. 

Better utilisation of adsorbent would be achieved if a unit could be designed in which adsorbent were removed for regeneration as soon as it became saturated, and even better 

The mixture to be separated is fed to the centre of the column down which activated carbon moves slowly. Immediately above the feed, the rising gas is stripped of ethylene and heavier components leaving hydrogen, methane and any non-adsorbed gases to be discharged as a top-product. The adsorbent with its adsorbate continues down the column into an enriching section where it meets an upwards stream of recycled top-product. The 

Adsorption is an unsteady process and regeneration or reactivation of the adsorbent is needed for recyclic use. The primaiy objective of regeneration is to restore the adsorption capacity of exhausted adsorbent while the secondary objective is to recover valuable components present in the adsorbed phase, if any. 

Since adsorption operations are a cyclic process composed of adsorption step and regeneration step, efficiency and cost of regeneration play important roles in the overall feasibility of an adsorption process. 

There are several alternative processes available for the regeneration of spent adsorbents I) desorption by inert stream or low pressure stream, 2) desorption at high temperature where adsorption isotherm is considerably advantageous for desorption, 3) desorption by changing affinity between adsorbate and adsorbent by chemical reagent, 4) desorption by extracting adsorbates by strong solvents, and 5) removal of adsorbates by thermal decomposition or biochemical decomposition. 

Methods I and 2 are commonly used for regeneration of adsorbents used for gaseous phase adsorption. Naturally, method 2 can be applied for liquid phase adsorption if the equilibrium relation allows in specific cases. Fig. 9.1 shows these schemes of desorption. Desorption using an inert stream free of adsorbent is essentially the same operation as adsorption, which can be analyzed by the same basic equation with different initial, and boundary conditions. The same is true of desorption at high temperature (thermal desorption) except that the equilibrium relation is very different. Also, in the actual operation of thermal desorption, nonisothermal treatment becomes important in most cases. The combination of desorption at low pressure and adsorption at high pressure is the principle of pressure swing operation (PSA), which is discussed in Chapter II. 

Methods 3 and 4 are specific to liquid phase adsorption and especially effective when recovery of adsorbate is desirable. Desorption by alkaline solution is often used for recovery of organic acids adsorbed on 

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The adsoiptive capacity of carbons is obviously finite. Consequently, it is gradually decreased and finally exhausted after the adsorption of the maximum possible amount of a substance. The exhausted carbon is then characterized as spent and has to be regenerated, reactivated, or properly disposed off. The regeneration of spent adsorbents is the most difficult and expensive part of adsorption technology. It accounts for about 75% of total operating and maintenance cost for a fixed-bed GAC operation. 

The target compounds of ozonation are often found in the water phase, although there are applications in which they are found in the solvent, e. g. highly lipophilic PAHs in dispersed oil droplets, or adsorbed on the solid, e. g. in the regeneration of spent adsorbents or treatment of contaminated soils. Generally, the tendency of a solute to partition between water and solvent or water and solid has to be considered carefully in three-phase systems. 

Mourand J T, Crittenden J C, Hand D W, Perram D L, Notthakun S (1995) Regeneration of spent adsorbents using homogeneous advanced oxidation, Water Environmental Research 67 355-363. 

Adsorptive materials have been used for many years for the purification of chemicals. These materials include carbons, clays, and synthetic aluminosilicates. The disposal or regeneration of spent adsorbent is usually expensive, and generally viewed as a costly inconvenience. 

Chapter 9 is devoted to methods for the regeneration of spent adsorbents. Since adsorption separation is a transient technique, regeneration of adsorbents after the period of adsorption is an important part of an adsorption purification or separation system. Recovery of valuable adsorbates vrill also become increasingly important. 

This book comprises 12 chapters covering an introduction, porous adsorbents, adsorption equilibrium, diffusion in porous particles, kinetics of adsorption in a vessel, kinetics of adsorption in a column (chromatographic analysis and breakthrough curves), heat effects, regeneration of spent adsorbent, chromatographic separation, pressure swing adsorption and adsorption for energy transport. The text comprises a blend of mathematical analysis and descriptions of plant and processes. Each chapter is fully referenced. 

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