13.12. MICROFILTRATION MEMBRANE PROCESSES

13.12A. Introduction

In microfiltration, pressure-driven flow through a membrane is used to separate micron-size particles from fluids. The size range of particles ranges from 0.02 μm to 10 μm (H2). This microfiltration separates particles from solutions. The particles are usually larger than the solutes in reverse osmosis and ultrafiltration. Hence, osmotic pressure is negligible. At the very low end of the size range, very large soluble macromolecules are retained. Bacteria and other microorganisms (P7) are also retained on these membranes. Other particles in this size range are paint pigment, yeast cells, suspended matter such as cells from fermentation broth, particles in beer pasteurization, and so on. The dividing line between ultrafiltration and microfiltration is not very distinct.

The pore sizes of the membranes and the permeate flux are typically larger than for reverse osmosis and ultrafiltration. Usually the pressure drop used across the membranes varies from 1 psi to 50 psi (H2). Types of membranes are extremely varied and can be ceramics, polymers, and so on.

Many different geometries of membranes are used. These include spiral-wound, plate and frame, hollow fiber, cartridge filters with pleated membranes, and so on. Disposable cartridges are also used.

13.12B. Models for Microfiltration

1. Dead-end microfiltration flow model

In many laboratory batch filtrations, the batch process is run in dead-end flow, with the membrane replacing the conventional filter paper. The particles build up with time as a cake and the clarified permeate is forced through the membrane, as shown in Fig. 13.12-1a. The permeate flux equation is (H2)

Equation 13.12-1

where N_w is the solvent flux in kg/(s · m²), ΔP is the pressure difference in Pa, μ is viscosity of the solvent in Pa · s, R_m is the membrane resistance in m²/kg, and R_c is the cake resistance in m²/kg, which increases with time due to cake buildup.

This Eq. (13.12-1) is similar to Eq. (14.2-8) for ordinary filtration given in Section 14.2 of this text. Solutions to this equation are also given there.

2. Cross-flow microfiltration flow model

In the cross-flow model shown in Fig. 13.12-1b, the operation is similar to that for reverse osmosis and ultrafiltration in that the flow of bulk solution is parallel to the membrane surface and not through it (H2). The permeate flow through the membrane carries particles to the surface, where they form a thin layer. A relatively high flow rate tangential to the surface sweeps the deposited particles toward the filter exit leaving a relatively thin deposited cake layer. This thin cake layer is similar to the gel layer formed in ultrafiltration. This cross-flow is effective in controlling concentration polarization and cake buildup, allowing relatively high fluxes to be maintained.

This concentration-polarization model of convection of particles to the cake layer is balanced by particle diffusion by Brownian diffusion away from the cake surface. This model is similar to the flux Eq. (13.11-3) for ultrafiltration. Other models are also given elsewhere (H2).