Practice of Kinetics (Comprehensive Chemical Kinetics, Volume 1)

230 EXPERIMENT A L METHOD s FOR HET ER o GENE ous RE ACT I o NSoperative. For the non isothermal case, effectiveness factors greater than unity arepossible. Weisz and have considered this problem in some detail andconstructed a number of graphs for various heats of reaction and activationenergies. When a reaction is limited by pore diffusion, the reaction rate is proportionalto 46. If the temperature effects can be expressed as a simple Arrheniusrelationship k, = A exp(-E/RT), then the measured activation energy E willbe about half the true activation energy. Very low values of the activation energy,i.e. 1-2 kcal . mole-' are only observed when mass transfer to the external catalystsurface is limiting the rate.Determination of the effects of pore diffusion, where the catalyst is being poisoned,have been discussed by Wheeler" 3i Complex kinetics involving parallelor consecutive reactions have also been studied by Wheeler" 3v and more recentlyby J~ttrand'~~by Roberts and Satte~field'~'. The effects of volume change andheat transfer have been considered by Weekmant9' and by Wurzbachertg9.2.2.3 Reactor designThe principles of correct reactor design for heterogeneous reactions are thesame as those for homogeneous reactions, but with additional restrictions causedby the presence of a separate phase which constitutes the catalyst. The kineticinformation obtained should accurately represent the steady state chemical activityand selectivity of the catalyst. This is true whether the information is required todetermine the mechanism of the process, to evaluate different catalysts,or to providedesign information for a full scale chemical process. As in the case of homogeneousreactions, the advantages and differences of static and flow systems must be considered.The subject of reactor design has been reviewed by Hougenand Watson"'and by Levenspiel's'.(a) Static systemsReactions at subatmospheric pressure can be carried out with the aid of highvacuum techniques in glass apparatus, although metal components to fabricatevacuum systems are now supplied by a number of manufacturers. The techniquesand precautions necessary are similar to those required for the study of adsorptionkinetics (Section 2.1). For batch reactions at superatmospheric pressure autoclavesare usually used. Because of the safety aspects, apparatus and particularlyreactors must be designed to the correct engineering specifications. The design,construction and details of commercially available apparatus for the study ofbatch reactions have been discussed at length by Komarewsky et al.178 and byMelville and Gowenlock200.It is important to ensure that gaseous diffusion is not influencing the kineticmeasurements. This is generally true if the rate coefficient per unit geometrical