CATALYTIC REACTORS AND MASS TRANSFER

Most real reactors are not homogeneous but use catalysts (1) to make reaction occur at temperatures lower than would be required for homogeneous reaction and (2) to attain a higher selectivity to a particular product than would be attained homogeneously. One may then ask whether any of the previous material on homogeneous reactions has any relevance to these situations. The answer fortunately is yes, because the same equations are used. However, catalytic reaction rate expressions have a quite different meaning than rate expressions for homogeneous reactions. 

Catalysts are substances added to a chemical process that do not enter into the stoichiometry of the reaction but that cause the reaction to proceed faster or make one reaction proceed faster than others. 

By far the most efficient catalysts are enzymes, which regulate most biological reactions. Biological catalysts are without question the most important catalysts (to us) because without them life would be impossible. Enzymes are proteins that may be either isolated molecules in solution (homogeneous) or molecules bound to large macromolecules or to a cell wall (heterogeneous). We have not yet learned how to create catalysts with nearly the efficiency and selectivity of nature s enzyme catalysts. We will consider biological reactors at the end of this chapter as the example of the most efficient chemical reactor possible. 

We should be clear as to what a catalyst can and cannot do in a reaction. Most important, no catalyst can alter the equilibrium composition in a reactor because that would violate the Second Law of Thermodynamics, which says that equilibrium in a reaction is uniquely defined for any system However, a catalyst can increase the rate of a reaction or increase the rate of one reaction more than another. One can never use a catalyst to take a reaction from one side of equilibrium to another. The goal in reaction engineering is typically to find a catalyst that will accelerate the rate of a desired reaction so that, for the residence time allowed in the reactor, this reaction approaches equihbrium while other undesired reactions do not. Attempts to violate the laws of thermodynamics always lead to failure, but maity engineeis still try. 

The first comphcation with catalytic processes is that we need to maintain the catalyst in the reactor. With a homogeneous catalyst (catalyst and reactants in the same phase), the 

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We have frnished our discussions of the fundamentals of catalytic reactions, catalytic reactors, and mass transfer effects. While we noted that most catalytic reactions can be made to exhibit complicated kinetics, we have confined our considerations to the almost trivial reaction r" = k"CA- We did this because the algebra was messy enough with the simplest kinetics. 

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