Effect of foam stirrer design on the catalytic performance of rotating foam stirrer reactions

The liquid–solid mass transfer rate in a rotating foam stirrer reactor and in a slurry reactor is studied using the hydrogenation of styrene as a model reaction. The rotating foam stirrer reactor is a novel type of multi-phase reactor where highly open-celled materials, solid foams, are used as a catalyst support and as a stirrer. The design of the foam stirrer has a strong influence on the liquid–solid mass transfer rate. Using a donut-shaped foam block configuration, the liquid solid mass transfer coefficient, kLS, is five times lower than in a blade configuration. The reduced liquid circulation through the foam block structure is explained by a higher frictional pressure drop. The pore Reynolds number indicates that the flow is in the laminar regime for the foam block stirrer while it is turbulent for the blade stirrer. Local measurements of kLS indicate a mass transfer profile along the height of the foam block structure, allowing a pertinent choice of the catalyst location within the foam block and/or suggesting changes of the foam block design to avoid the bottom effects, for instance, using a conical foam block shape. However, the foam block stirrer offers higher liquid–solid interfacial area than a blade stirrer, resulting in higher kLSaLS. At a power input above and using a 20 ppi foam block with the top part catalytically active, the reaction rate is not liquid–solid mass transfer limited. Compared to a Rushton stirrer, the liquid–solid mass transfer rate is enhanced because of a high liquid–solid interfacial area and a fast refreshment of the catalyst surface. kLSaLS values of 0.5 s-1 are obtained compared to 0.04 s-1 for the slurry system. The same production rate as in the slurry reactor is achieved using 62% less catalyst. As the catalyst is immobilized on the stirrer, an additional advantage of the rotating foam stirrer reactor is the absence of a catalyst separation step. Attrition and agglomeration of the catalyst do not occur and the foam catalyst can be reused