Equilibrium Conversion Heterogeneous Systems

We will concentrate here on the most important type of these systems, those involving gas-solid reactions, such as combustion of solid fuels, decomposition of solids, and reduction of metal oxides. For the rather rare case of gas-liquid systems, see - among others - Sandler (1977) or Smith and Van Ness (1987). 

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Chemical reactions obey the rules of chemical kinetics (see Chapter 2) and chemical thermodynamics, if they occur slowly and do not exhibit a significant heat of reaction in the homogeneous system (microkinetics). Thermodynamics, as reviewed in Chapter 3, has an essential role in the scale-up of reactors. It shows the form that rate equations must take in the limiting case where a reaction has attained equilibrium. Consistency is required thermodynamically before a rate equation achieves success over tlie entire range of conversion. Generally, chemical reactions do not depend on the theory of similarity rules. However, most industrial reactions occur under heterogeneous systems (e.g., liquid/solid, gas/solid, liquid/gas, and liquid/liquid), thereby generating enormous heat of reaction. Therefore, mass and heat transfer processes (macrokinetics) that are scale-dependent often accompany the chemical reaction. The path of such chemical reactions will be... 

The high molar conversion obtainable in esterifications catalyzed by dry mycelia encouraged us to investigate the partition of water in these heterogeneous systems further. The objechve was to verify whether mycelia were able to affect the thermodynamic equilibrium of the reactions by modifying the parhtioning of the water formed during the esterificahon. The synthesis of two esters by lyophilized mycelia of R. oryzae CBS 112.07 was studied and the results were compared with those obtained with a commercial immobilized CALB (Novozym 435). 

H-Mordenite catalyzes the smooth conversion of simple aldehydes and alcohols to form acetals at 30° in the liquid phase. From the examples in Table XXVII, it is apparent that in these heterogeneous catalytic systems, acetal formation is dependent on the structures of both the aldehyde and the alcohol involved. Thus, for a given aldehyde, yields of acetal decreased in the order primary > secondary > tertiary that is, branching at the a-carbon of the alcohol reduced the equilibrium conversion to acetal. In the isobutyraldehyde reactions, an extremely sharp drop in conversion was observed upon changing from isopropanol to fert-butanol as reactant. This observation suggests that, in addition to the increased steric interactions between organic reactants encountered in the tert alcohol system, molecular sieving-type interactions within the narrow mordenite pore system are operative. 

Further removal of the dispersion medium results in a conversion of gel into a solid macroscopic phase, i.e. into the soap crystal. Based on the results of the X-ray diffraction analysis, soap crystals were shown to have a lamellar structure. The surfactant - water system can thus undergo transitions into various states, depending on the content of components from a homogeneous system (surfactant molecular solution) to lyophilic colloidal state and further to macroscopic heterogeneous system (soap crystals in water). Different states of the system can be described by a particular thermodynamic equilibrium, i.e. ... 

Calculation of equilibrium conversions for heterogeneous systems and for multiple reactions is then outlined mostly through some typical examples. 

The mechanism given above places no restrictions on the source of the reversible poison. Consequently, the poisoning can be due not to an adsorption competition between the reactant and a diluent but to an adsorption competition between the reactant and one or more of the reaction products. When this occurs the products will determine the kinetics in the flow type and static systems where appreciable conversion is allowed. Under these conditions the kinetics may be expressed by equations similar to equation (6), and the order will be determined by the magnitude of constants similar to H which depend upon the various velocity constants and adsorption equilibrium constants of the heterogeneous reaction. 

Surface conversion due to reactions of the dissolved species with the mineral surface can be predicted using thermodynamic stability diagrams for heterogeneous mineral systems based on relevant mineral dissociation equilibria. This is illustrated in Fig. 3.10 for the calcite/apatite/dolomite system. The activities of Ca + species in equilibrium with various solid phases show that the singular point for calcite and apatite is 9.3. Above this pH, apatite is less stable than calcite and hence conversion of apatite surface to that of calcite can be expected in calcite-apatite system. Similarly, the calcite-dolomite and apatite-dolomite singular points occur at pH 8.2 and 8.8, respectively. 

Lastly, Chapter 8 examines the matter of membrane reactors, which pertains to chemically reacting systems, either homogeneous or heterogeneous (catalyzed). The selective removal of one or another of the reaction products by membrane permeation shifts the reaction conversion equilibrium to the right. ... 

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