Journal of Membrane Science 278 (2006) 239–250 Optimization of the membrane and pore design for micro-machined membranes G. Brans a , R.G.M. van der Sman a , C.G.P.H. Schro¨ en a,∗ , A. van der Padt b , R.M. Boom a a Food and Bioprocess Engineering Group, Wageningen University, P.O. Box 8129, 6700 EV Wageningen, The Netherlands b Corporate Research Friesland Foods BV, P.O. Box 87, 7400 AB Deventer, The Netherlands Received 28 July 2005; received in revised form 1 November 2005; accepted 4 November 2005 Available online 15 December 2005 Abstract For micro-machined membranes, it is possible to choose pore size, pore geometry and membrane porosity, within certain limits. Different pore geometries (circular, square, slit shaped and triangular pores), particle size to pore size ratios, pore edges and membrane porosities were evaluated with lattice-Boltzmann computer simulations and torque balance considerations for various modes of operation. We focused on hydrodynamic interactions and assumed uncharged neutral surfaces of the particle and the pore. However, the model can easily be extended with additional relations for such interactions in practical systems with defined properties. It was concluded that pore geometry can have a large effect on the flux (up to 60%). Further, the effect of shielding could be quantified. Above a surface coverage of 0.05, the particles effectively shield each other from the flow field, therewith necessitating either a higher cross flow velocity or a lower transmembrane pressure for particle removal. Based on the simulations, an extended criterion for the critical flux was developed, which includes the effects of pore geometry, particle to pore size ratio and membrane porosity. Different optimal membrane choices follow for processes aimed at retention of all particles, and for processes aimed at fractionation of particles into different fractions. © 2005 Elsevier B.V. All rights reserved. Keywords: Microsieve; Membrane; Pore-blocking; CFD; Lattice-Boltzmann 1. Introduction New types of microfiltration membranes, such as silicon or silicon nitride microsieves [1] and metal microfilters [2] have become available. These are made by photolithographic treatment of a silicon wafer and subsequent etching, or electro- chemical metal deposition on a skeleton in an electrolysis bath, respectively. Compared to conventional ceramic and polymer membranes, these membranes have exemplary properties, such as a smooth and flat surface, a very low membrane resistance and narrow pore size distribution. The porosity and pore shape can be chosen almost freely, and therewith these membranes open areas for membrane filtration that have not been possible before. The pores are usually placed in a regular pattern and the poros- ity can be much higher compared to conventional membranes, up to 0.8 for rectangular shapes [3], Fig. 1. By realizing separa- ∗ Corresponding author. Tel.: +31 317 482231; fax: +31 317 482237. E-mail address: karin.schroen@wur.nl (C.G.P.H. Schro¨ en). tions that were not possible before, these membranes could truly create a revolution in microfiltration. Although the membranes themselves have almost ideal prop- erties, common membrane filtration phenomena, such as con- centration polarization, pore blocking, and cake formation still affect membrane performance [4]. In fact, due to the very high permeabilities of these membranes, these phenomena will be even more important than with conventional membranes. Depending on the degree of “fouling”, three filtration regimes can be identified [5]: (I) In the sub-critical flux regime, the membrane surface is still free of particles. Due to the removal of fluid at the membrane, concentration polarization takes place, as in any filtration regime. In this regime, the back-transport of particles away from the membrane can easily keep up with the convective transport of particles towards the mem- brane. Thus, concentration polarization hardly influences the flux (linear relation between transmembrane pressure 0376-7388/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2005.11.007