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 conventionally treated as a quasi homogeneous liquid phase with altered sorption and reaction capacity due to the presence of catalyst particles. This approach may be utterly wrong in any case where phase segregation of the solid takes place. This phenomenon is relatively little studied and it will be demonstrated 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–solid agglomeration, are to be discussed. Taking the example of carbon and alumina supported palladium catalysts employed in the hydrogenation of methyl 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 bubble collision probability and the impact of the size of adhering particles 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 reaction, i.e. the hydrogenation of methyl acrylate to methyl propionate, is studied in a stirred tank reactor for two different catalysts (Pd/C and Pd/Al₂O₃) 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 behaviour is of particular importance as it leads to a substantial increase in the effective particle size resulting in a decreased conversion rate.