Dynamic adsorption in adsorber beds

Adsorption Dynamics. An outline of approaches that have been taken to model mass-transfer rates in adsorbents has been given (see Adsorption). Detailed reviews of the extensive Hterature on the interrelated topics of modeling of mass-transfer rate processes in fixed-bed adsorbers, bed concentration profiles, and breakthrough curves include references 16 and 26. The related simple design concepts of WES, WUB, and LUB for constant-pattern adsorption are discussed later. 

Adsorption for gas purification comes under the category of dynamic adsorption. Where a high separation efficiency is required, the adsorption would be stopped when the breakthrough point is reached. The relationship between adsorbate concentration in the gas stream and the solid may be determined experimentally and plotted in the form of isotherms. These are usually determined under static equilibrium conditions but dynamic adsorption conditions operating in gas purification bear little relationship to these results. Isotherms indicate the affinity of the adsorbent for the adsorbate but do not relate the contact time or the amount of adsorbent required to reduce the adsorbate from one concentration to another. Factors which influence the service time of an adsorbent bed include the grain size of the adsorbent depth of adsorbent bed gas velocity temperature of gas and adsorbent pressure of the gas stream concentration of the adsorbates concentration of other gas constituents which may be adsorbed at the same time moisture content of the gas and adsorbent concentration of substances which may polymerize or react with the adsorbent adsorptive capacity of the adsorbent for the adsorbate over the concentration range applicable over the filter or carbon bed efficiency of adsorbate removal required. 

It is a mass transfer between a mobile, solid, or liquid phase, and the adsorption bed packed in a reactor. To carry out adsorption, a reactor, where a dynamic adsorption process will occur, is packed with an adsorbent [2], The adsorbents normally used for these applications are active carbons, zeolites and related materials, silica, mesoporous molecular sieves, alumina, titanium dioxide, magnesium oxide, clays, and pillared clays. 

Finally, the capacity of activated carbons to adsorb surfactants in dynamic conditions has been evaluated. Dynamic adsorption has been carried out in filter columns (diameter 29 nun) in conditions proposed in France [39] bed depth - 6 cm, graining of carbon 0,6—1,2 mm, filtration rate — 6 m/h. Colunuis have been filled with 3 fractions of carbon grain in equal volumes (the total volume of carbon - about 39.6 cm ). Carbons have been flooded with water and then de—aerated in vacuo during Ih. Adsorption has been carried out by passing solution of sodium laurylosulphate SLS (1 mg/dm ) through the bed. 

On the other hand, in dynamic conditions of adsorption on filters column effect occurs, that is the most polluted water contacts with carbon just partically used and then reaches layers of bed adsorbing better. Process of dynamic adsorption may be compared to adsorption on a series consisting of portions of powdered carbons con-tinously used in static conditions. 

The dynamic adsorption capacity of the bed ranged from 23.2 mg SO2 per gram of adsorbent, at 890 mm Hg, 180°C and 7 liters/minute, to 101.6 mg SO2 per gram of adsorbent, at 1610 mm Hg, 100°C and 5 liters per minute. It increased with the SO2 feed concentration, or partial pressure, and decreasing flow rate, as can be seen in Figures 3 to 5. The dynamic adsorption capacity decreased markedly with increasing temperature as can be seen in Figure 6. 

Since it is difficult to obtain point by poim concentration data in adsoiption beds, most dynamic adsorption data are obtained in the form of Fig. 12.7-10, In Fig- 12.7-la, the area iebai represents ihe uptake of the adsorbate, as does the stoichiometric area Idea, and can therefore be used to calculate (he equilibrium loading. If a stable mass transfer zone has developed. Ihe breakthrough time 0 and the "stoichiometric time 9., can be used to calculate LUB by the equation... 

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