Acceleration of mass transfer in macroporous catalysts

Gas bubbles entrapped in macroporous catalyst particles that are subjected to hydrostatic pressure oscillations (e.g., during recirculation in a loop reactor), will induce intraparticle liquid flows that enhance mass transfer in excess of diffusion. The permeability and gas holdup of methanogenic gr anules were found to be large enough to enable this ''breathing particle'' mechanism. However, continuous pressure oscillations appeared to disintegra te these biocatalysts within 1 month, although calculations indicate that p ore liquid velocities did not exceed the breakup level. The proposed mechan ism could be demonstrated unequivocally with Pt-loaded sintered glass pelle ts, affecting the gas-producing decomposition of H2O2. At oscillations of 3 bar and 3 Hz, the activity was more than doubled compared with atmospheric conditions. The results were modeled assuming axial dispersion of the inte rnal flow as the accelerating mechanism. The required gas holdup, being the only input variable, corresponded well with the measured value at a consta nt pressure. From the experimental results and model predictions, it is con cluded that the acceleration of mass transfer in gas-producing systems offe rs challenging perspectives for heterogeneous catalysis and biological ferm entations The development of special macroporous carrier materials with a c entral gas-filled cavity could lead to a novel type of reactor where liquid flow through the catalyst is decisive.