Heat Effect in Adsorption Operation

Adsorption is accompanied by the evolution of heat, and temperature changes affect the adsorption equilibrium relation and, in some cases, adsorption rate. Thus, especially in gas phase adsorption, the effects of heat generation and heat transfer in adsorbent beds must be taken into account. This is essential in the case of thermal regeneration of exhausted adsorbent using steam or hot inert gases, a topic which is discussed in Chapter 9. 

Heat generation also affects measurement of adsorption rate by batch techniques such as the gravimetric method. First, the effect of heat transfer rate on measurement of adsorption rate by a batch method is shown using a simple model as an example of nonisothermal effect. Then fundamental equations for heat transfer in packed beds are shown and simplified models presented. Estimation mehtods for heat transfer rate parameters in packed beds are introduced followed by a discussion of heat transfer in an adsorbent bed in adsorption equilibrium to show the coupling effect of heat and mass dispersion. Finally, the effect of heat transfer on adsorption dynamics in a column is illustrated using simple models. 

Effect of Heat Generation on Adsorption Rate Measurement by a Single Particle Method 

A single particle gravimetric method, for example, is often used to determine adsorption rate. In most cases analysis is based on isothermal adsorption neglecting heat generation due to adsorption. Depending on the system employed and experimental conditions, this assumption may become critical. 

The effect of heat generation can be checked and the critical conditions for negligible heat effect derived (Chihara and Suzuki, 1976) employing a simple model of mass and heat transfer. 

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Heat Effect in Adsorption Operation condition as follows. 

Assumption 6 In the general case, we shall assume that there are no thermal effects and neglect the influence of the heat of adsorption on the band profile. In principle, heat or enthalpy balances for the mobile and stationary phases should be included in the system of equations, as discussed in Section 2.1.4. In practice, however, the temperature excursion associated with the migration of a band seems to be small and no detectable effect has been demonstrated [31]. Accordingly, we ignore the thermal effect in the rest of this book, except in Section 2.1.5. We assiune that the column is operated isothermally. A temperature gradient along the column would require appropriate adjustments in Eq. 2.2 to account for the variations of the isotherm and the kinetic parameters, of u, Di i, and, improbably, F with T, hence, in this case, z. 

The solid support for the liquid phase should be chemically inert and stable at the operating temperatures, and should have a large surface area. The importance of uniform particle-size to facilitate even packing has already been emphasized (see Section 11,1, p. 96). Materials in most common use as inert supports are diatomaceous earth and glass beads, both of which are often pretreated chemically to minimize adsorption effects. Preparations of diatomaceous earth are available commercially, in narrow ranges of particle size, pretreated with acid, alkali, or dichlorodimethyl-silane. A coating of silver has been used for diminishing adsorption on the inert support it probably enhances heat transfer in the columns, as well. 

In evaluating the effect of degassing, one is faced with an uncertainty as to whether the operation may produce changes in the carbon other than removal of preadsorbed gases. Other lines of evidence indicate that qualitative changes in adsorptive power may occur when a carbon is heated above 500° C. More than simple removal of preadsorbed gas may be involved, and a carbon may not have the same intrinsic properties before and after degassing at high temperatures. 

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