Viscosity, hydrogen transfer

A major factor to consider in pitch carbonizations is the facility for hydrogen transfer reactions. These stabilize otherwise reactive radical species and permit the growth of mesophase in a fluid of low viscosity. Hydroaromaticity and methylene linkages are able to transfer hydrogen without creating radicals. 

Viscosity of the medium can also play a role in the kinetics due to the importance of diffusion in the observed rate constants. In the bulk radical polymerization of 2-phenoxyethyl methacrylate, thiol chain-transfer reagents operate at rates close to those observed for MMA while the rate of CCT catalyzed by 9a is an order of magnitude slower (2 x 103 at 60 °C) than that of MMA.5 The thiol reactions involve a chemically controlled hydrogen transfer event, whereas the reaction of methacrylate radicals with cobalt are diffusion controlled. The higher bulk viscosity of the 2-phenoxyethyl methacrylate has a significant influence on the transfer rate. 

It may be seen that the mobilities of the abnormally conducting ions in BuSO and D2S04 are quite comparable with those of the corresponding ions in HgO and B20 despite the fact that the viscosity of sulphuric acid is 27 5 times that of water at 25°. The macroscopic viscosity evidently has little effect on the rate of the proton-transfer conduction process. It is interesting to note that in both water and sulphuric acid, the mobilities of the abnormal ions are decreased by substituting deuteri urn for hydrogen, and that in each case the mobilities of the anions are less than those of the corresponding cations. 

The size of the bubbles produced in the reactor and the gas volume fraction will depend on the agitation conditions, and the rate at which fresh hydrogen is fed to the impeller, as shown in Fig. 4.20. (Some hydrogenation reactors use gas-inducing impellers to recirculate gas from the head-space above the liquid while others use an external compressor.) For the purposes of the present example, the typical values, db = 0.8 mm and eg m 0.20, will be taken. Also dependent to some extent on the agitation conditions are the values of the transfer coefficients kL and k, although these will depend mainly on the physical properties of the system such as the viscosity of the liquid and the diffusivity of the dissolved gas. The values taken here will be kL = 1.23 x 10 5 m/s and k, = 0.54x 10"3 m/s. 

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