Nonisothermal Sorption

Because the heat effects associated with adsorption are comparatively large, the assumption of isothermal behavior is a valid approximation only when uptake rates are relativeily slow. The problem of nonisothermal sorption under conditions typical of a gravimetric uptake rate experiment has been analyzed by Lee and Ruthven. ) Two distinct situations were considered nonisothermal sorption in a single particle under conditions such that intraparticle diffusional resistance is the dominant resistance to mass transfer and sorption in a bed of particles under conditions such that the mass transfer rate is controlled by diffusion into the particle bed rather than by intraparticle diffusion. 

In a nonisothermal system there are two effects the temperature dependence of the equilibrium adsorbed phase concentration at the adsorbent surface and the temperature dependence of the diffusivity. The latter effect may be eliminated by reducing the size of the concentration step over which the uptake curve is measured, but the former effect is independent of step size. 

Heat conduction through an adsorbent particle or through an assemblage of adsobent particles is generally much faster than heat transfer at the external surface so it is usually a good approximation to consider the particle as essentially isothermal with all heat transfer resistance concentrated in the external film. This is essentially the same situation as in a nonisothermal catalyst particle.  

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The proposed form of data presentation became highly popular and opportune in gas chromatography. Up to the present, some thousand references to the Kovat s work [1] have been known. The RI values are proportional to the free energies of sorption this is their thermodynamic interpretation. Further development of the RI concept was aimed at its application to nonisothermal conditions of gas chromatographic (GC) analysis. For linear temperature programming regimes (which are characterized by two variables initial temperature, Tq, and rate of its increase, r, deg X min ), the linear relationship (3) does not hold. In some partial cases, other linear dependence seems more precise for the retention time approximation ... 

Several theoretical studies of natural gas storage on activated carbons have been undertaken. A molecular simulation of CH4 adsorption predicts that the maximum storage capacity by a palletized and a monolith activated carbon will be 146 and 209m /m, respectively [43]. Nonisothermal fiU-discharge and dynamics of CFI4 ad(de)sorption in ANG systems have also been evaluated [44]. 

The apparent simplicity of this approach is, however, deceptive. For measurement of intracrystalline diffusion the method works well when diffusion is relatively slow (large crystals and/or low diffusivity), but when sorption rates are rapid the uptake rate may be controlled by extracrystalline diffusion (through the interstices of the adsorbent bed) and/or by heat transfer. The intrusion of such effects is not always obvious from the shape of the uptake curve, but it may generally be detected by changing the sample quantity and/or the sample configuration. It is in principle possible to allow for such effects in the mathematical model used to interpret the uptake curves (Fig. 2), and indeed the modeling of nonisothermal systems has been studied in considerable detail [8-12]. However, any such intrusion will obviously diminish the accuracy and confidence with which the intracrystalline diffusivities can be determined. 

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