Gas-solid adhesion and solid-solid agglomeration of carbon supported catalysts in three phase slurry reactors

In the study of three phase slurry reactors the slurry phase is conventiona lly treated as a quasi homogeneous liquid phase with altered sorption and r eaction capacity due to the presence of catalyst particles. This approach m ay be utterly wrong in any case where phase segregation of the solid takes place. This phenomenon is relatively Little studied and it will be demonstr ated that it may have a considerable impact on the operation of three phase reactors. Two examples of segregation, i.e. gas-solid adhesion and solid-s olid agglomeration, are to be discussed. Taking the example of carbon and a lumina supported palladium catalysts employed in the hydrogenation of methy l acrylate towards methyl propionate, the segregation of the catalyst phase by adhesion to gas bubbles is studied. This adhesion may take place up to complete coverage of the gas bubbles but it may also be entirely absent. A quantitative model is developed based on the film theory, the particle to b ubble collision probability and the impact of the size of adhering particle s on the effective film thickness. This model is used to describe adhesion under non-stagnant conditions and the impact it has on the overall G-L mass transfer rates. The conversion rate of a mass transfer limited model react ion, i.e. the hydrogenation of methyl acrylate to methyl propionate, is stu died in a stirred tank reactor for two different catalysts (Pd/C and Pd/Al2 O3) in order to verify the model. It is quantitatively demonstrated that G- L mass transfer rates may be increased considerably as a result of adhesion . The second, closely related, phenomenon studied is the segregation of the solid and liquid phase by agglomeration of the catalyst particles. This be haviour is of particular importance as it leads to a substantial increase i n the effective particle size resulting in a decreased conversion rate. (C) 1999 Elsevier Science B.V. All rights reserved.