Cross-Flow Filtration Ramesh. bhave 1.0 INTRODUCTION Cross-flow filtration( CFF)also known as tangential flow filtration is not of recent origin. It began with the development of reverse osmosis(Ro) more than three decades ago. Industrial ro processes include desalting ofsea water and brackish water, and recovery and purification of some fermentation products. The cross-flow membrane filtration technique was next applied to the concentration and fractionation ofmacromolecules commonly recognized as ultrafiltration (UF) in the late 1960s. Major UF applications include electrocoat paint recovery, enzyme and protein recovery and pyrogen re moval(1-(31 In the past ten to fifteen years or so, the applications sphere of cross flow filtration has been extended to include microfiltration(MF) which primarily deals with the filtration of colloidal or particulate suspensions with size ranging from 0.02 to about 10 microns. Microfiltration applications are rapidly developing and range from sterile water production to clarification of beverages and fermentation products and concentration of cell mass, yeast E-coli and other media in biotechnology related applications. 01-14) Table I shows the types of separations achievable with MF, UF and RO membranes when operated in cross-flow configuration. For MF or UF application, the choice of membrane materials includes ceramics, metals or polymers, whereas for RO at the present only polymer membranes are predominantly used. Although cross-flow filtration is practiced in all the above three types of membrane applications, the description of membrane
7 Cross-Flow Filtration Ramesh R. Bhave 1.0 INTRODUCTION Cross-flow filtration (CFF) also known as tangential flow filtration is not of recent origin. It began with the development of reverse osmosis (RO) more than three decades ago. Industrial RO processes include desalting of sea water and brackish water, and recovery and purification of some fermentation products. The cross-flow membrane filtration technique was next applied to the concentration and fractionation ofmacromolecules commonly recognized as ultrafiltration (UF) in the late 1960's. Major UF applications include electrocoat paint recovery, enzyme and protein recovery and pyrogen removal. [11-r31 In the past ten to fifteen years or so, the applications sphere of crossflow filtration has been extended to include microfiltration (MF) which primarily deals with the filtration of colloidal or particulate suspensions with size ranging from 0.02 to about 10 microns. Microfiltration applications are rapidly developing and range from sterile water production to clarification of beverages and fermentation products and concentration of cell mass, yeast, E-coli and other media in biotechnology related applications .[11-[41 Table 1 shows the types of separations achievable with MF, UF and RO membranes when operated in cross-flow configuration. For MF or UF application, the choice of membrane materials includes ceramics, metals or polymers, whereas for RO at the present only polymer membranes are predominantly used. Although cross-flow filtration is practiced in all the above three types of membrane applications, the description of membrane 2 71
Table 1. Separation Spectrum Nominal Size of Examples of Spccics Scparated Process Remarks Spccies 100500 Organic acld acetic acld Dalton itric acld, aImino acids Product recovered in permeate 2002.000 MF/UF Product recovered In permeate peniclllln, cephalosporin 10.000-200000 Proteins/poly saccharides retained by the membrane Is trated in retentate. Some losses 00103 Viruses. Inter concentrated in retentate colloidal silica MF/UF Product In concentrate phase o.1-10t E-coll. Pseudonomus dimInuta Species retained by the membrane mIcroorganisms from af Permeate sterile air ME Oils retained by the membrane are 1-100μ Bacteria cells, yeasts, molds Specles retained by the membrane is concentrated In retentate
Fermentation and Biochemical Engineering Handbook
Cross- Flow Filtration 275 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 I shows the sche matic of cross-flow filtration including the critical issues and operational modes for clarification or concentration using a semipermeable polymeric or 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 yS. 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 Table 2 shows the advantages and versatility of cross-flow filtration in meeting a broad range of filtration objectives. 1-316 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 [5
Cross-Flow Filtration 273 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. Membranefouling 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 membranebased 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 0bjectives.[']-[~1[~] 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 difisivity under the influence of high shear can enhance the filtration rates. Since difisivity 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 condition^.[^]
Clean Product (e.g. antibiotics, bacteria-free or pyrogen-free water) Cross-flow Filtration Feed (polymeric, inorganic) Concentrated Product (clarification/ (e.g. cells, yeast, concentration) Critical Issues (fouling, polarization and cleaning) essss Batch, feed bleed, continuous, diafiltration, multistage Figure 1. Schematic of cross-flow filtration
274 Fermentation and Biochemical Engineering Handbook 1 Y v) Od m m W i - .- - s m iE - Y E Y .- 0 K - I= f 3 0 3 v) a W 75- ou Y .- a 0, W 75 rn m Y V i L 0
Table 2. Cross-Flow Filtration: Key Advantages Process Go Cross-low Fltration Deadend FIltration Ability to handle wide Excellent Generally poor variations in particle size Ability to handle wide Exccllent Poor or unacceptable variations in solids concentration Continuous concentration Excellent Poor or unacceptable witll recycle Waste minimization Superior Can minimize waste If handling low solids feed where cartridge disposal Is infrequent Iligh product purity or yield Excellent: Performance Is generally but may require diafiltration acceptable except in situations to ovcrcome excessive nux involving high solids or loss at higher recovery adsorptive fouling
Cross-Flow Filtration 275 w :: 0 - W c d a 0 0 L 2 3 C m V V 4 5 4 e, 0 C v, 3 a e, L - 0 L 3 r: v) .d 0 E .- I