Practice of Kinetics (Comprehensive Chemical Kinetics, Volume 1)

3 THE S OLI D-L IQ U I D I N TER FA C E 25 IA related method developed by Watanabe et aL3I6 employs the droppingmercury electrode connected to a resonance circuit instead of an ac bridge. Thetotal capacitance of the dropping mercury electrode increases with drop growth andthe resonant ac voltage reaches a maximum at a certain instant in each droppingperiod. The following resonance condition is then satisfied-- - [,+ (I+ 31wc 2where C is the double layer capacitance, R, the solution resistance, L the loadinductance and E the applied ac voltage. A block diagram of the circuit is shown inFig. 40. E, and E are both amplified and fed to the vertical axis of an oscilloscope.The gain ratio of the amplifier required to adjust both traces to the same heightis the value em.,. Known resistance and capacity can then be substituted forthe dropping mercury electrode, to give the same resonance point and Q value.The appropriate values of C and R give the double layer capacitance and solutionresistance directly.3.2 CATALYTIC REACTIONSCatalytic reactions in the liquid phase have been widely studied, in generalusing similar methods, i.e. autoclaves, tubular reactors, to those employed forthe study of gas phase reactions. The stirred tank reactor, in a batch or continuousform, has been widely used. Vigorous agitation is necessary to ensure completemixing and to maintain a uniform distribution of catalyst throughout the solution.Catalyst is usually used in powder form to assist in its dispersion and to reducepore diffusion limitations. The effect of mass transfer to the catalyst surface isusually estimated by varying the rate of agitation. At the point where the degreeof agitation no longer affects the reaction rate the rate 'of mass transfer is consideredto be unimportant. This does not affect the characteristics of pore diffusion,which are still influenced by the fact that mass transfer in liquids is slower than ingases.A particular advantage of liquid phase reactions is the large heat transfercoefficients of liquids. This makes it possible to carry out highly exothermicreactions which would be difficult to control in the gas phase. Solid catalystssuspended in liquids (slurries) have been widely used for hydrogenation3"* 318.This can normally be treated as a liquid phase reaction because the liquid isessentially saturated with hydrogen.References pp. 270-278