Adiabatic Adsorption in a Column

When gas phase adsorption takes place in a large column, heat generated due to adsorption cannot be removed from the bed wall and accumulated in the bed because of poor beat transfer characteristics in packed beds of particles. A typical model of this situations is an adiabatic adsorption. The fundamental relations for this case are Eqs. (8-22), (8-38), (8-39) and (8-40), which are essentially similar to those employed by Pan and Basmadjian (1970). Thermal equilibrium between particle and fluid is assumed and oidy axial dispersion of heat is taken into account while mass transfer resistance between fluid phase and particle as well as axial dispersion is considered. This situation is identical with the model employed in the previous section. For further simpliHcation, axial dispersion effect may be involved in the overall mass transfer coefficient of the linear driving force model as discussed in Chapter S. In this case, after further justifiable simplifications such as negligible heat capacity and accumulation of adsorbate in void spaces, a set of basic equations to describe heat and mass balances can be ven as follows. 

Fig S8 Typical examples of concentration and temperaure changes of effluent stream from adiabatic adsorption column 

Several works assume that heat transfer between fluid and particle is a major heat transfer parameter (Carter, 1966-1973 Meyer and Weber, 1967-1969 Raghavan and Ruthven, 1983). This situation may become more likely when operation is carried out at high flow rate (/ ep 100). 

Examples of calculation for adiabatic adsorption by means of the above set of equations are given in Fig. 8.8 for adsorption of hydrocarbon gas on activated carbon columns. Effect of heat generation on the shape of adsorption front is clearly shown. By assuming that the equilibrium constant varies with keeping the other parameters constant, changes in thermal waves are also illustrated, i.e. when velocity of adsorption front is slower than that of the thermal wave which is generated at the adsorption front, the thermal wave proceeds in front of the adsorption front while in the opposite case the formation of adsorption front is greatly affected by the temperature increase in the bed. 

Analogous to Eq. (3-36), propagation speed of heat wave in packed beds, Fh, is given from Eq. (8-54) as 

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