P. Maarten Biesheuvel

Design of ceramic membrane supports: permeability, tensile strength and stress

A membrane support provides mechanical strength to a membrane top layer to withstand the stress induced by the pressure difference applied over the entire membrane and must simultaneously have a low resistance to the filtrate flow. In this paper an experimental and a theoretical approach toward the design of a ceramic membrane support are combined. In the experimental part, the influence of the particle size, sintering time and sintering temperature on the permeability and strength of supports made by colloidal processing of submicron-sized alumina powders is investigated and compared with dry-pressed samples. In the theoretical part, a condensed expert system is set up that comprises the main relations necessary to describe the maximum filtrate flow of an incompressible fluid through a multilayered tubular inorganic membrane. The model can be adapted to describe other geometries and fluids. From calculations it becomes clear that optimum values exist for the dimensions and material properties of the support. Hence, support design is not straightforward and needs a comprehensive approach incorporating simultaneously all relevant design characteristics.

Graded membrane supports produced by centrifugal casting of a slightly polydisperse suspension

Tubular structures of a continuous particle size gradient are formed if a hollow cylindrical mold filled with a suspension of dispersed powder with a size distribution is centrifuged around its center axis. The mean particle size in the final structure increases gradually with increasing radial coordinate. Because the bulk properties can be optimized simultaneously with the surface composition, this process has advantages for the production of porous tubular ceramic membrane supports in case subsequent membrane layers are coated on the inner surface of the support. Particle velocities and concentrations in the suspension, as well as the compact profile, are numerically analyzed for completely filled molds. Using the analysis the composition at each location in the compact can be predicted, which can be used to calculate the permeance (flux per unit pressure difference), as well as the particle composition of the inner and outer surfaces.

A novel interconnected fluidised bed for the combined flash pyrolysis of biomass and combustion of char.

A novel system of two adjacent fluidised beds operating in different gas atmospheres and exchanging solids was developed for the combined flash pyrolysis of biomass and combustion of the produced char. Fluidised sand particles (200 μm < dp < 400 μm) are transported from the pyrolysis reactor to the combustor through an orifice and recycled by a standpipe, riser and cyclone. Advantages of the new design are its compactness and the high level of heat integration. The solids circulation rate and holdup distribution between the two compartments could be controlled adequately in experiments at room temperature and atmospheric pressure. A model, developed to predict the solids and gas exchange between the two reactor compartments, was validated with experiments in which the three relevant gas flows, the orifice diameter and the particle diameter were varied.

Particle segregation during pressure filtration for cast formation

When pressure filtration is used to produce composite materials from a mixed suspension, segregation of the different particle types is apt to occur due to different sedimentation velocities. This results in an inhomogeneous, thus inferior product. To understand and reduce segregation, a model is proposed to describe segregation due to gravitational settling during pressure filtration from a dispersed suspension consisting of two types of particles with different sizes and/or densities. The model combines the consolidation mechanisms of sedimentation and filtration and predicts the composition profile in the formed material. Experimentally determined cast profiles are in good agreement with model predictions.

Influence of suspension concentration on cast formation time in pressure filtration

Expressions for the formation rate of an incompressible cast in batchwise pressure filtration are formally derived from Darcys law and Kynch-theory for pure filtration and filtration with sedimentation and compared with experimental results from the filtration of submicron α-alumina dispersed in water. The influence of suspension concentration on the cast formation time is investigated numerically. A maximum cast formation time is found at a certain suspension concentration φs for pure filtration and a constant filling height. The value of the maximum cast formation time depends solely on the cast concentration if the filter resistance is negligible. For a constant final cast thickness, the cast formation time always decreases with increasing φs. The cast formation time in a filtration set-up decreases when particle sedimentation is important, especially when sedimentation and filtration have the same direction.