Cyclohexene mass transfer rate

Interfacial areas per unit volume in falling-film microreactors have been reported to be as high as 25,000 m m, as compared with the values of 1-200 m m typical in bubble columns. This effect is particularly important in gas-liquid reactions because the rate of mass transfer from the gas to the liquid limits the reaction rate. For the hydrogenation of cyclohexene to cyclohexane in this type of reactor, the mass transfer rate constant Kid) was found to be in the range 3-7 s which is two orders of magnitude higher than that for conventional reactors. 

Precise experimental measurements of mass transfer rates in such systems are still rare. Losey et al [50] microfabricated a packed-bed structure (Figure 11.8) in which thqr estimated the mass transfer rate by performing the fast cyclohexene hydrogenation... 

Tphe rate-limiting processes in catalytic reaction over zeolites remain A largely undefined, mainly because of the lack of information on counterdiffusion rates at reaction conditions. Thomas and Barmby (7), Chen et al. (2, 3), and Nace (4) speculate on possible diffusional limitations in catalytic cracking over zeolites, and Katzer (5) has shown that intracrystalline diffusional limitations do not exist in liquid-phase benzene alkylation with propene. Tan and Fuller (6) propose internal mass transfer limitations and rapid fouling in benzene alkylation with cyclohexene over Y zeolite, based on the occurrence of a maximum in the reaction rate at about 100 min in flow reaction studies. Venuto et al (7, 8, 9) report similar rate maxima for vapor- and liquid-phase alkylation of benzene and dehydro-... 

Alkene hydrogenation was also suggested to test for mass transfer effects during liquid-phase hydrogenations236,237. The method is based on the linear poisoning of hydrogen addition to alkenes (cyclohexene and apopinene) by CS2. When the active sites of Pd or Pt catalysts are titrated with CS2 the decrease in rate is linear unless mass transfer limitations occur. 

A study has been made of the batch-wise partial hydrogenation of benzene and of toluene to cyclohexene and the methyl-cyclohexenes, respectively, over ruthenium as a catalyst in the presence of an aqueous zinc sulphate solution. The reaction has been performed in a stirred autoclave at 423 K and at a total pressure of 5.0 MPa. Both the hydrogenation rates of benzene and of toluene are governed by diffusion processes, which can be shown by applying the Carberry and Wheeler-Weisz criteria. The high selectivities and yields of cyclohexene and methylcyclohexenes are mainly explained by mass transfer limitation of hydrogen and cyclic alkenes towards the active catalyst surface. The lower toluene hydrogenation rate can be explained from the lower solubility of toluene in water as compared to benzene. 

In the presence of an aqueous zinc solution both rates of hydrogenation of benzene and of toluene are governed by mass transfer limitations. The high selectivities and yields of cyclohexene and of the methyl-substituted cyclohexenes are mainly due to the mass-transfer limitation of the re-adsorption of this intermediates. The Carberry and Wheeler-Weisz criteria support these conclusions. 

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