Separation of metal oxides from supercritical water by crossflow microfiltration

Low-shear crossflow microfilters rated for 34.5 MPa and 455 degrees C were evaluated for their ability to remove alpha-alumina particles from supercritical water. The crossflow microfilters were tested in conjunction with a 2.5 L/h bench-scale apparatus and a 150 L/h pilot plant. The filter elements were made of sintered Stainless Steel 316L tubes with a nominal pore size of 0.5 mu m Process variables included volumetric feed concentration, feed flow rate, temperature, and fluid density and viscosity. The mean particle diameter was about 1.6 mu m. Filtration characteristics were similar at supercritical water conditions to those at ambient conditions: Increased shear rates and decreased fluid viscosity resulted in increased filtrate fluxes. Filtrate flow deterioration over time and the required transfilter pressure drop were about 40% less at supercritical water conditions as compared to the performance obtainable at ambient conditions. Higher shear rates delayed the establishment of steady state operating conditions. Filtrate flux could be augmented by pressure swings or periodic increases in the shear rate. Particle separation efficiencies typically exceeded 99.9%. A modified concentration polarization model for turbulent flow yielded steady state filtrate fluxes that were within a factor of two of the experimental results. Back diffusion was modeled as a process-in-series of molecular diffusion and eddy diffusion. The proposed model was consistent with findings of numerical diffusion studies and the theory of concentration polarization as recently presented in the literature. To save on energy costs it might be possible to achieve the benefits of filtration at supercritical conditions at near-critical subcritical conditions: viscosity and mass diffusion coefficients are similar but the density of subcritical water is 3 to 4 times higher than supercritical water thus increasing the filtrate mass flow rate by a factor of 3 to 4 for a given filter.