Transport Limitations in Experimental Catalytic Reactors

Hence we need respective criteria for the design and operation of a laboratory reactor to ensure negligible deviations from the ideal. Subsequently, we repeat these criteria, which were already derived in Sections 4.7, 4.10.6.5, and 4.10.7.2, and specify them for laboratory-scale experiments. In the next subsection, the criteria for ideal plug flow behavior (exclusion of an influence of axial and radial dispersion of mass and heat are covered), and in the subsequent subsection, the criteria for gradientless deal particle behavior (exclusion of an influence of interphase and intraparticle transport of mass and heat) are outlined. 

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Mears, D. E., Tests for transport limitations in experimental catalytic reactors, lEC Process Des. Dev, 10, 541-547 (1971). 

Mears, D.E. (1971). Tests for Transport Limitations in Experimental Catalytic Reactors. 

Advanced learners should then also study Section 4.11.4 (on transport limitations in experimental catalytic reactors) and Sections 4.11.5.2-4.11.5.4, where some more complex examples are given (heterogeneously catalyzed gas-phase reaction, catalytic multiphase reaction, and non-isothermal oxidation of carbon). 

HI H. Hofmann Progress in modeling of catalytic fixed-bed reactors, Germ.Chem.Eng. ( 1979)258-267 /8/ D.E. Hears Diagnostic Criteria for heat transport limitation in fixed-bed reactors, J.Catal.20(1971) 127-131 /9/ D.E. Hears Tests for transport limitations in experimental catalytic reactors, Ind.Eng.Chem.Proc.Des.Dev.10(1971)541-547... 

Heterogeneously catalyzed reactions. Macroscopic fluid models are combined with microscopic transport models in the catalyst particles to describe how concentration changes with time and position in a catalytic reactor. Special considerations must be given to the selection of experimental temperature and catalyst particle size to minimize (and hopefully eliminate) internal transport limitations on the catalytic reaction rate. The next requirement is that the flow pattern in the reactor Is accurately represented by the well-mixed or plug-flow assumption. The subsequent discussion applies to gas-phase reactants. 

While the above criteria are useful for diagnosing the effects of transport limitations on reaction rates of heterogeneous catalytic reactions, they require knowledge of many physical characteristics of the reacting system. Experimental properties like effective diffusivity in catalyst pores, heat and mass transfer coefficients at the fluid-particle interface, and the thermal conductivity of the catalyst are needed to utilize Equations (6.5.1) through (6.5.5). However, it is difficult to obtain accurate values of those critical parameters. For example, the diffusional characteristics of a catalyst may vary throughout a pellet because of the compression procedures used to form the final catalyst pellets. The accuracy of the heat transfer coefficient obtained from known correlations is also questionable because of the low flow rates and small particle sizes typically used in laboratory packed bed reactors. 

Reactive depletion of CO from catalyst sites leads to much higher rates of secondary olefin hydrogenation reactions as pellets and reactors become limited by the rate of arrival of fresh reactants at catalytic sites. We have simulated intrapellet CO depletion experimentally by continuously decreasing the space velocity of mixtures with a H2/CO ratio (3 1) higher than the stoichiometric consumption value (—2.1 1). This reactant ratio was chosen because it corresponds to the relative rates of H2 and CO transport in stoichiometric mixtures through FT liquids at 473 K. As a result, the resulting axial gradients that occur in the catalyst bed as H2 and CO reactants are consumed resemble those that develop within transport-limited catalyst pellets. 

The catalytic behavior of enzymes in immobilized form may dramatically differ from that of soluble homogeneous enzymes. In particular, mass transport effects (the transport of a substrate to the catalyst and diffusion of reaction products away from the catalyst matrix) may result in the reduction of the overall activity. Mass transport effects are usually divided into two categories - external and internal. External effects stem from the fact that substrates must be transported from the bulk solution to the surface of an immobilized enzyme. Internal diffusional limitations occur when a substrate penetrates inside the immobilized enzyme particle, such as porous carriers, polymeric microspheres, membranes, etc. The classical treatment of mass transfer in heterogeneous catalysis has been successfully applied to immobilized enzymes I27l There are several simple experimental criteria or tests that allow one to determine whether a reaction is limited by external diffusion. For example, if a reaction is completely limited by external diffusion, the rate of the process should not depend on pH or enzyme concentration. At the same time the rate of reaction will depend on the stirring in the batch reactor or on the flow rate of a substrate in the column reactor. 

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