Practice of Kinetics (Comprehensive Chemical Kinetics, Volume 1)

234 EXPERIMENTAL METHODS FOR HETEROGENEOUS REACTIONStended to confirm the view that the evaluation of kinetic data and scale up offluidised beds is not easy. The two main disadvantages are firstly, that gas bubblesappear, giving a variable bed density, and secondly, that the low gas velocity andturbulent motion of the catalyst particles permit back mixing of the products.The catalyst particles are generally of a small size and catalyst attrition due toabrasion and impact with the walls is quite high.A third type of reactor system has been developed by Emmett er al.215*216known as the microreactor. This apparatus has the advantage of speed and simplicityand is particularly useful for evaluating the product distribution from complexreactions. Fig. 29 illustrates the principle of this type of reactor. It consists of asmall catalytic reactor placed directly above a gas chromatograph. An injectionof reactant is made through the top of the reactor into a stream of carrier gas,usually hydrogen, helium or nitrogen. The slug of reactant passes through thereactor, the products and reactants are separated by the chromatographic columnand subsequently recorded by the detector. Systems for working at superatmosphericpressure have been devised2”, but most of the work has been of a qualitativenature. The problem of obtaining good kinetic data from a microreactor system isreceiving attention and Schwab and Watson218 have shown that kinetic data canbe as reliable as that from a continuous flow system. Hall er al.219 have recentlymeasured the kinetics of butenes isomerisation in a microcatalytic reactor” ’.The advantages of the pulse technique are that only small quantities of reactantare required and the results are obtained rapidly. A major difficulty in obtainingkinetic measurements is the absence of a steady state, causing the pulse to changeshape as the products are formed and as it passes through the reactor. SomeCeramic tube (1”LD.)Nichrome windingGlass woolCatatyst badUKnife blade heaterStainless steelchromatographic column(y~.aR,O.l80‘1.~..~3srFig. 29. Micro-catalytic reactor used in conjunction with a gas chromatograph. From P. H.Emmett, New Approaches to the Study of Catalysis, Pennsylvania University Press, 1962, p. 149.