Interaction forces and suspension characteristics in an oscillatory membrane photocatalytic reactor

Abstract

The effects of the interaction between the hydrodynamic and colloidal forces on the suspension and performance characteristics of a slurry photocatalytic reactor equipped with oscillatory membrane and lateral turbulence promoters are investigated. Aggregation of catalyst particles is influenced by the magnitude of colloidal forces between the particles in relation to the hydrodynamic stresses acting on the aggregate. The interaction of the axial oscillatory shear with the transverse motion of the turbulence promoters (TP) results in generation of vortices and eddies that extended further away from the membrane surface leading to reducing particle aggregation. For the range of oscillatory conditions investigated, the aggregate size remained in the viscous subrange and decreased with increasing the oscillation intensity where an almost three folds decrease in aggregate size was achieved at high oscillation intensities. This resulted in reducing particles settling and deposition on the membrane surface which increased the catalyst specific area and volume fraction in suspension and enhanced both membrane flux and reaction rates. Improvement of the latter is also achieved by the microscale eddies attenuation of the concentration boundary layer surrounding the catalyst particles which results in better mass transfer. Using methylene blue dye (MB) degradation with ZnO photocatalyst under UV illumination as a model photocatalytic reaction, the investigation showed that increasing the oscillation intensity resulted in higher membrane flux and reaction rates that asymptotically approached maximum corresponding to near maximum catalyst suspension. This was estimated based on the magnitude of the eddy forces in the suspension in relation to the sedimentation and permeate drag forces acting on the catalyst particles. Comparison between the results of the present investigation and data from the literature showed satisfactory agreement.