Practice of Kinetics (Comprehensive Chemical Kinetics, Volume 1)

2 THE SOLID-GAS INTERFACE 23 1area is much less than unity’93. True first order reactions which are pore diffusionlimited, still show first order kinetics if Knudsen diffusion is operative, but showan apparent order between 1 and 4 if bulk diffusion is rate controlling. If thereaction is not first order and diffusion effects are present, changes in gas pressurewhich may result during reaction will cause a change in effectiveness factor andhence the apparent order.Further possible disadvantages with static systems are that impurities maypoison the catalyst, changing the activity with time, or the reaction productsmay act as inhibitors.(b) Flow systemsThe design and construction of flow systems is more complex than similarbatch systems’78*’00 and requires larger volumes of reactant. It is often desirableto have automatic control of pressure, temperature and flow rates to increase theaccuracy of measurement as well as the safety of operation.Kinetic measurements in flow systems are usually made with the catalyst packedin a tubular reactor. Three methods of operation are possible: (i) as an integralreactor; (ii) as a differential reactor, and (iii) as a recirculation or recycle reactor.The integral reactor operates at a large degree of reactant conversion, contacttime being varied by a change in flow rate or catalyst volume. Graphical differentiationof the results gives the rate of reaction which allows the construction of akinetic equation. However, the procedure is not very accurate, especially forcomplex reactions. Since the heat of reaction is released or adsorbed, it is difficultto maintain isothermal conditions in the converter. In a differential converter,the contact time is small so that the gas composition is approximately constantthroughout the catalyst bed. The rate coefficient can be measured directly, butthe main difficulty lies in obtaining sufficient analytical accuracy to measure thedifference between inlet and exit gas compositions. An advantage of the differentialreactor is that mass transfer to the catalyst surface can be neglected, because of thelack of concentration gradients.The analytical problems associated with differential reactors can be overcomeby the use of the recirculation reactor. A simplified form, called a “Schwab”reactor, is described by Weisz and Prater” ’. Boreskov.and other Russian workershave described a number of other modifications2 05. The recirculation reactoris equivalent kinetically to the well-stirred continuous reactor’ O6 or backmixreactor1”, which is widely used for homogeneous liquid phase reactions. Fig. 28illustrates the principle of this system. The reactor consists of a loop containinga volume of catalyst V and a circulating pump which can recycle gas at a muchhigher rate, G, than the constant feed and withdrawal rates F.The conversion rate across the catalyst bed isR = (G+F)(X,-XJ/VReferences pp. 270-278