Hydrogen Transfer in Slurries of Carbon Supported Catalyst (HPO Process)
The effect of suspended activated carbon on gas/ liquid mass transfer is studied for the Pd /C-catalyzed hydrogenation of nitrate to hydroxylamine (HPO process). In a bubble column and in a stirred autoclave, physical mass transfer &,a) in the concentrated salt solution is improved by more than a factor of three by the catalyst. Chemical absorption enhancement by the zero-order catalytic reaction is insignificant at the employed catalyst diameter (40 pm), but hydroxylarnine selectivity depends on the absorption regime.
Activated carbon is frequently used as the catalyst support material in catalytic hydrogenation. In slurry reactors, suspended activated carbon can considerably increase the gas/liquid mass transfer rate. This has been observed during physical absorption of propane (Kars et al., 1979), as well as 0, and CO, . In the case of absorption with chemical reaction, higher k,a values along with chemical absorption enhancement were detected, e.g., for the oxidation of sulfide on activated carbon as the catalyst and glucose oxidation on Pt/C catalyst . Physical and chemical absorption enhancement could be observed only if the solid particle size was smaller than the thickness of the liquid side diffusion film. Therefore, the effects were often studied in the stirred cell with plane interface and rather small liquid side mass transfer coefficient k,. Because of the constant interfacial area in this absorber, higher kLu value in the presence of suspended activated carbon is referred to higher k , by adsorption facilitated transport .
In most industrial processes, on the other hand, high turbulence and coarseness of catalyst particles may render these effects negligible. studied the absorption of CO, into carbonate buffer solutions with suspended activated carbon powder in a bubble column. The analysis in the form of the Danckwert's plot indicated 1-43% higher liquid side mass transfer coefficient and up to 19% higher interfacial area at the concentration of 9 g/L activated carbon. Studying oxygen transfer in the same bubble column with suspended catalyst support particles, found that k,u monotonically decreases with increasing concentration of activated carbon in water. The k,a values were well correlated with suspension viscosity in a single relation for adsorptive and non-