Regeneration desorption rates during

Figure 5. Desorption rates from Z200H during regeneration (Q) water, flZD benzene...

However, we have to reflect on one of our model assumptions (Table 5.1). It is certainly not justified to assume a completely uniform oxide surface. The dissolution is favored at a few localized (active) sites where the reactions have lower activation energy. The overall reaction rate is the sum of the rates of the various types of sites. The reactions occurring at differently active sites are parallel reaction steps occurring at different rates (Table 5.1). In parallel reactions the fast reaction is rate determining. We can assume that the ratio (mol fraction, %a) of active sites to total (active plus less active) sites remains constant during the dissolution that is the active sites are continuously regenerated after AI(III) detachment and thus steady state conditions are maintained, i.e., a mean field rate law can generalize the dissolution rate. The reaction constant k in Eq. (5.9) includes %a, which is a function of the particular material used (see remark 4 in Table 5.1). In the activated complex theory the surface complex is the precursor of the activated complex (Fig. 5.4) and is in local equilibrium with it. The detachment corresponds to the desorption of the activated surface complex. 

Another factor that is particularly important in the regeneration of molecular sieve driers is the rate at which the temperature is raised during regeneration. If this is too rapid relative to the rate of moisture removal, one may get rapid desorption of moisture from the initial section of the bed, which is in contact with the hot desorbent gas, followed by condensation of liquid water in the cooler regions some distance from the inlet, with serious consequences for adsorbent life. 

The comparison between the extract mass during the extraction (10 %) and the desorption (1.25 %) shows us that other effects than solubility are more crucial in the desorption process. The butylacetate regeneration is better than xylene regeneration zeolithe is saturated by a mixture composed with butyl acetate (50 %) and xylenes isomeres (30 %), the extracts composition is butyl acetate (60-65 %) and xylenes isomers (35-40 %). The equilibrium thermodynamic and adsorption data could help us to explain these results. To increase the C02 flow rate (Figure 3) contribute to decrease the desorption time but the lowest flow rate does not permit to desorbe completely zeolithe this is suggestive of a film transfer resistance at lower flow rates. 

Figure 5 shows reactor profiles for the adiabatic case during the regeneration. In the case shown, the desorption flux of A exceeds the rate of conversion of A near reference temperatures. Near the feed end (xwO.7), a significant portion of desorbed A is not con-... 

Figure 5. Profiles in the PSR during regeneration in the isothermal (top) and ar diabatic (bottom) case. Legend A dimensionless temperature (T/Tref), O desorption flux of A (Sa), reaction rate 1 (ri), 0 convective and diffusive axial flow of A (Fa). Parameters Series iv from figure 4 (V (purge)=50), time IXTads into the purge step.

A proper kinetic description of a catalytic reaction must not only follow the formation and conversion of individual intermediates, but should also include the fimdamental steps that control the regeneration of the catalyst after each catalytic turnover. Both the catalyst sites and the surface intermediates are part of the catalytic cycle which must turn over in order for the reaction to remain catalytic. The competition between the kinetics for surface reaction and desorption steps leads to the Sabatier principle which indicates that the overall catalytic reaction rate is maximized for an optimal interaction between the substrate molecule and the catalyst surface. At an atomic level, this implies that bonds within the substrate molecule are broken whereas bonds between the substrate and the catalyst are made during the course of reaction. Similarly, as the bonds between the substrate and the surface are broken, bonds within the substrate are formed. The catalyst system regenerates itself through the desorption of products, and the self repair and reorganization of the active site and its environment after each catalytic cycle. 

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