Practice of Kinetics (Comprehensive Chemical Kinetics, Volume 1)

2 THE SOLID-GAS INTERFACE 233is liquifiable, an alternative solution is to cool and recycle the liquid. However,a relatively large preheater is required to vaporise the reactants again.The gas flow through tubular reactors is of particular importance because thecomposition at any point is influenced by the linear velocity of the gas, the sue ofthe reactor and the size of the catalyst particles. When gas flows through a pipeat low linear velocity (low Reynolds number), the radial velocity is not uniform.As the linear velocity increases, turbulence increases and the velocity profileapproaches what is called “plug” flow. However, in a packed bed, plug flow cannever be completely attained because of the high voidage near the reactor wall.Certain criteria have been calculated which allow mixing effects to be minimised.These can be summarised as follows:(1) Inert packing equivalent to a length of six particle diameters shall be presentin front of the catalyst bed2’ O. This allows transition to steady flow conditionscharacteristic of the packed bed.(2) To eliminate wall effects, the ratio of converter diameter to pellet diametershould be greater than 30. The velocity profile then deviates from uniformityby about 20p4. For a differential reactor, ratios greater than 5 are probablyacceptable.(3) An aspect ratio (bed length to pellet diameter) greater than 20 and preferablygreater than 100 should be used to minimise axial mixing’”.The criteria listed above are more difficult to attain in a laboratory reactorthan on the full scale. In particular, the opposing requirements of uniform flowand good heat transfer make (2) difficult to achieve.(c) Special types of reactorOne type of reactor which can be useful for kinetic measurements is the continuousstirred tank reactor (CSTR). The kinetic model is identical with that forthe recirculation reactor’06*212, and the designs are based on the reactors used forhomogeneous reactions. Carberry et a1.206,214 have described a CSTR whichcontains catalyst inside the blades of the impellor. Ford and Perlmutter’’3 coatedthe catalyst on the walls of the tank. The disadvantages of the CSTR comparedto the recirculation reactor are that the free volume in the reactor is greater thanthat of a packed bed, only a limited range of particle sizes can be accommodatedand the extent of gas-solid contacting is less well defined. However, a circulationpump is not required.A fluidised bed reactor is often used on the large scale in exothermic reactions,because of its very good heat transfer properties. The catalyst bed is said to befluidised when the particle weight is just equal to the viscous drag of the flowinggas and the particle remains suspended in the gas stream. The bed then resemblesa dense fluid. Because of their industrial importance, chemical engineers have donea good deal of work aimed at characterising their . Ho wever, thebehaviour of fluidised beds is not yet well understood and recent studies haveRrfmencrs pp. 270-278