Mf

MF/UF MF

Product recovered In permeate Product In concentrate

Product recovered In permeate

Species retained by the membrane Is concentrated In retentate. Some losses may occur In permeate. Species Is concentrated In retentate Product In concentrate phase

Species retained by the membrane Is concentrated In the retentate

Permeate sterile air

Oils retained by the membrane are concentrated In retentate

Species retained by the membrane Is concentrated In retentate characteristics, operational aspects and applications will be limited to MF and UF, where the cross-flow mode shows the greatest impact on filtration performance compared with dead end filtration. Figure 1 shows the schematic of cross-flow filtration including the critical issues and operational modes for clarification or concentration using a semipermeable polymeric or inorganic membrane.

Despite the growing use in a broad range of applications, cross-flow filtration still largely remains a semi-empirical science. Mathematical models and correlations are generally unavailable or applicable under very specific and well-defined conditions, owing to the complex combination of hydrodynamic, electrostatic and thermodynamic forces that affect flux and/ or retention. Membrane fouling is not yet fully understood and is perhaps the biggest obstacle to more widespread use of CFF in solid-liquid separations. Membrane cleaning is also not well understood. The success of a membrane-based filtration process depends on its ability to obtain a reproducible performance in conformance with the design specifications over a long period of time with periodic (typically once a day) membrane cleaning.

2.0 CROSS-FLOW vs. DEAD END FILTRATION

The distinction between cross-flow and dead end (also known as through-flow) filtration can be better understood if we first analyze the mechanism of retention. The efficiency of cross-flow filtration is largely dependent on the ability of the membrane to perform an effective surface filtration, especially where suspended or colloidal particles are involved. Table 2 shows the advantages and versatility of cross-flow filtration in meeting a broad range of filtration objectives.Figure 2 illustrates the differences in separation mechanisms of CFF versus dead end filtration.

High recirculation rates ensure higher cross-flow velocities (and hence Reynold's number) past the membrane surface which promotes turbulence and increases the rate of redispersion of retained solids in the bulk feed. This is helpful in controlling the concentration polarization layer. It may be of interest to note that polarization is controlled essentially by cross-flow velocity and not very much by the average transmembrane pressure (ATP). It should also be noted that higher particle or molecular diffusivity under the influence of high shear can enhance the filtration rates. Since diffusivity values of rigid particles (MF) under turbulent conditions are typically much higher than those for colloidal particles or dissolved macromolecules (UF) microfiltration rates tend to be much higher than ultrafiltration rates under otherwise similar conditions.

Clean Product (e.g. antibiotics, bacteria-free or pyrogen-free water)

(clarification/

concentration)

Cross-flow Filtration (polymeric, inorganic)

Critical Issues (fouling, polarization and cleaning)

Concentrated Product (e.g. cells, yeast, enzymes)

Batch, feed & bleed, continuous, diafiltration, multistage Figure 1. Schematic of cross-flow filtration.

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