Cross-Flow Filtration 281 4.1 Polymeric Microfilters and Ultrafilters Symmetric polymeric membranes possess a uniform pore structure over the entire thickness. These membranes can be porous or dense with a constant permeability from one surface to the other. Asymmetric(also sometimes referred to as anisotropic)membranes, on the other hand, typically show a dense(nonporous )structure with a thin( 0. 1-0.5 um)surface layer ported on a porous substrate. The thin surface layer maximizes the flux and performs the separation. The microporous support structure provides the mechanical streng Polymeric membranes are prepared from a variety of materials using several different production techniques. Table 5 summarizes a partial list of the various polymer materials used in the manufacture of cross-flow filters for both MF and UF applications. For microfiltration applications, typically symmetric membranes are used. Examples include polyethylene, polyvinylidene fluoride(PvdF)and polytetrafluoroethylene(PtFe)mem- brane. These can be produced by stretching, molding and sintering fine- grained and partially crystalline polymers. Polyester and polycarbonate membranes are made using irradiation and etching processes and polymers such as polypropylene, polyamide, cellulose acetate and polysulfone mem- branes are produced by the phase inversion process. j(7t8 Ultrafiltration membranes are usually asymmetric and are also made from a variety of materials but are primarily made by the phase inversion process. In the phase inversion process, a homogeneous liquid phase consisting of a polymer and a solvent is converted into a two-phase system The polymer is precipitated as a solid phase(through a change in temperature, solvent evaporation or addition of a precipitant)and the liquid phase forms the pore system. UF membranes currently on the market are also made from a variety of materials, including polyvinylidene fluoride, polyacrylonitrile, polyethersulfone and polysulfone Microfiltration membranes are characterized by bubble point and pore size distribution whereas the UF membranes are typically described by their molecular weight cutoff (MWCO)value. The bubble point pressure relates to the largest pore opening in the membrane layer. This is measured with the help of a bubble point apparatus. 9 The average pore diameter of a MF membrane is determined by measuring the pressure at which a steady stream of bubbles is observed For MF membranes, bubble point pressures vary depending on the pore diameter and nature of membrane material (e. g hydrophobic or hydrophilic). For example, bubble point values for 0.1 to 0.8 um pore diameter membranes are reported to vary from I bar(equals about
Cross-Flow Filtration 281 4.1 Polymeric Microfilters and Ultrafilters Symmetric polymeric membranes possess a uniform pore structure over the entire thickness. These membranes can be porous or dense with a constant permeability from one surface to the other. Asymmetric (also sometimes referred to as anisotropic) membranes, on the other hand, typically show a dense (nonporous) structure with a thin (0.1-0.5 pm) surface layer supported on a porous substrate. The thin surface layer maximizes the flux and performs the separation. The microporous support structure provides the mechanical strength. Polymeric membranes are prepared from a variety of materials using several different production techniques. Table 5 summarizes a partial list of the various polymer materials used in the manufacture of cross-flow filters for both MF and UF applications. For microfiltration applications, typically symmetric membranes are used. Examples include polyethylene, polyvinylidene fluoride (PVDF) and polytetrafluoroethylene (PTFE) membrane. These can be produced by stretching, molding and sintering finegrained and partially crystalline polymers. Polyester and polycarbonate membranes are made using irradiation and etching processes and polymers such as polypropylene, polyamide, cellulose acetate and polysulfone membranes are produced by the phase inversion proce~s.['1[~1[~] Ultrafiltration membranes are usually asymmetric and are also made from a variety of materials but are primarily made by the phase inversion process. In the phase inversion process, a homogeneous liquid phase consisting of a polymer and a solvent is converted into a two-phase system. The polymer is precipitated as a solid phase (through achange in temperature, solvent evaporation or addition of a precipitant) and the liquid phase forms the pore system. UF membranes currently on the market are also made from a variety of materials, including polyvinylidene fluoride, polyacrylonitrile, polyethersulfone and polysulfone. Microfiltration membranes are characterized by bubble point and pore size distribution whereas the UF membranes are typically described by their molecular weight cutoff (MWCO) value. The bubble point pressure relates to the largest pore opening in the membrane layer. This is measured with the help of a bubble point apparatu~.['l[~I The average pore diameter of a MF membrane is determined by measuring the pressure at which a steady stream of bubbles is observed. For MF membranes, bubble point pressures vary depending on the pore diameter and nature of membrane material (e.g., hydrophobic or hydrophilic). For example, bubble point values for 0.1 to 0.8 pm pore diameter membranes are reported to vary from 1 bar (equals about
282 Fermentation and Biochemical Engineering Handbook 14.5 psi)to 15 bar. I However, due to the limited mechanical resistance of some membrane geometries(e.g, tubular and to some extent hollow fiber) such measurements cannot be performed for smaller pore diameter MF and UF membranes. The bubble point apparatus can also be used to determine the pore size distribution of the membrane Table 5. Polymeric Microfilters and Ultrafilters teri MIcrofilter Ultrafilter Configuration Acrylic polymer X X W HFF Nylon based polyester HFF, PS, FS Polybenzimidazole X Polycarbonate Polyethersulfone Polyethylene Poly propylene HFF. FS, T Polysulfone FIFF SW,T FS polyletralluorocthylene X PolyvInylidene fluoride x Ps- Pleated Sheet T-Tubular (including wide channel) s· Flat sheet HFF. Hollow fine fiber Since the majority of UF membranes have dense surface layers, it is difficult to characterize them with a true pore size distribution. Therefore, polymeric UF membranes are described by their ability to retain or allow passage of certain solutes. The MwCO values for UF membranes can range from as low as 1000 dalton(tight uf) to as high as 200, 000 Dalton(loose F). This roughly corresponds to an"equivalent pore diameter range from about 1 nanometer(nm)to 100 nm( 0. 1 um)as described in Ref. 10
282 Fermentation and Biochemical Engineering Handbook 14.5 psi) to 15 bar.[’] However, due to the limited mechanical resistance of some membrane geometries (e.g., tubular and to some extent hollow fiber) such measurements cannot be performed for smaller pore diameter MF and UF membranes. The bubble point apparatus can also be used to determine the pore size distribution of the membrane. Table 5. Polymeric Microfilters and Ultrafilters Material Microfilter Ultrafilter Connvuration Acrylic polymer X X HFF Cellulosic polymer X X FS. PS, SW, HFF Nylon based polyester X X HFF, PS. Fs Polyamide X HFF, FS Polybenzamidazole X Fs. sw Polycarbonate X Fs Polyelhersulfone X SW, T Polye Lliylene X Fs Poly p ro py le ne X HFF, FS. T I’olysulTone X IIFF, SW, T, FS I~olylelrafluorocthylcne X X FS. T Polyvinylidene fluorlde X X SW, T, FS. PF PF - Plate and Frame PS - Pleated Sheet FS - Flat Sheet SW - Spiral Wound T - Tubular (including wide channel) HFF - Hollow Fine Fiber Since the majority of UF membranes have dense surface layers, it is difficult to characterize them with a true pore size distribution. Therefore, polymeric UF membranes are described by their ability to retain or allow passage of certain solutes. The MWCO values for UF membranes can range from as low as 1000 dalton (tight UF) to as high as 200,000 Dalton (loose UF). This roughly corresponds to an “equivalent” pore diameter range from about 1 nanometer (nm) to 100 nm (0.1 pm) as described in Ref. 10
Cross-Flow Filtration 283 Different membrane materials with similar or identical mwco value may show different solute retention properties under otherwise simila operating conditions. If adsorption effects are negligible, such a result can be attributed primarily to the differences in their pore size distributions. This is illustrated in Fig. 3. It can be seen that, although the two membranes are rated by the same MwCO value, their retention characteristics are distinctly different(sharp versus diffuse Polymeric cross-flow filters are available in many geometries. These are listed in Table 6. It is obvious that no single geometry can provide the versatility to meet the broad range of operating conditions and wide variations in properties. Some cross-flow filters such as cartridge filters have low initial capital cost but high replacement costs and tubular filters may show longer service life but higher operating costs. The optimization of CFF for a specific application may depend on economic and/or environmental factors and is almost impossible to generalize 1.0 Ideal Cutoff fuse Cutoff 10,000 Molecular Weight Figure 3. Rejection coefficient as a function of molecular weight cutoff of an ultrafiltration membrane
Cross-Flow Filtration 283 Different membrane materials with similar or identical MWCO value may show different solute retention properties under otherwise similar operating conditions. If adsorption effects are negligible, such a result can be attributed primarily to the differences in their pore size distributions. This is illustrated in Fig. 3. It can be seen that, although the two membranes are rated by the same MWCO value, their retention characteristics are distinctly different (sharp versus diffuse). Polymeric cross-flow filters are available in many geometries. These are listed in Table 6. It is obvious that no single geometry can provide the versatility to meet the broad range ofoperating conditions and wide variations in properties. Some cross-flow filters such as cartridge filters have low initial capital cost but high replacement costs and tubular filters may show longer service life but higher operating costs. The optimization of CFF for a specific application may depend on economic and/or environmental factors and is almost impossible to generalize. 1 .o Y C P) U .- g 0 U C 0 U a, aJ a: .- 4.1 '-7 0 ideal Culolf - 1,000 /*- / 1 / / / / /" Dilluse Cutoff '/ / / / 10,000 100,000 Molecular Weight Figure 3. Rejection coefficient as a function ofmolecular weight cutoff of an ultrafiltration membrane
Table 6. Polymeric Cross-flow Filters: Module Geometries Module geomet Special Features/Remarks Flat she Typical spacing between sheets is 0.25 to 2.5 mm and are used for laboratory evaluations (small surface area modules) Hollow Fine fiber The internal diameter generally ranges form 0. 25 to I mm This type of module geometry cannot handle large amounts of suspended solids or fibrous materials. Plate and Frame Trame devices to handle larger processing volumes e Flat sheet membrane elements are assembled in plate and Pleated Sheet Typical spacing between sheets is 0.25 to 2.5 mm. The sheels are enclosed in cylindrical cartridge Not suitable to handle high solids Spiral wound Typlcal spacing between the membrane sheets is 0.25 mm Not sultable to handle high solids. Tubular The internal dlaineter can range from 2 to 6 mm. Sultable for handling higher solids loading Tubular (wide Channel) The Internal diameter is typically greater than 6 mm and can be as high as 25 mm. The advantage Is lower pressure drop and abilily to handle high solids/ brous materials at the expense of higher energy cost
284 Fermentation and Biochemical Engineering Handbook d 0 L v) - Y W d rn - rn 0 c'! m E E 00 cdn c 0 i
Cross-Flow Filtration 285 4.2 Inorganic Microfilters and ultrafilters Cross-flow membrane filters made from inorganic materials, primarily ceramics and metals, utilize entirely different manufacturing processes compared with their polymeric counterparts. 3I Although carbon membranes do not qualify under the inorganic definition, they will be included here due to the similarities with inorganic membranes with regard to their material properties such as thermal, mechanical and chemical resistance as well as similarity in production techniques. Table 7 lists the various commonly used materials and membrane geometries in MF and UF modules Commercial ceramic membranes are made by the slip-casting process This consists of two steps and begins with the preparation of a dispersion of fine particles(referred to as slip) followed by the deposition of the particles A majority of commonly used inorganic membranes are composites consisting of a thin separation barrier on porous support(e. g, Membralox& zirconia and alumina membrane products). Inorganic MF and UF mem branes are characterized by their narrow pore size distributions. This allows the description of their separative performance in terms of their true pore diameter rather than MwCO value which can vary with operating conditions This can be advantageous in comparing the relative separation performance of two different membranes independent of the operating conditions. ME membranes, in addition, can be characterized by their bubble point pressures Due to their superior mechanical resistance bubble point measurements can be extended to smaller diameter MF membranes(0. 1 or 0. 2 um)which may have bubble point pressure in excess of 10 bar with water. 191 Typical pore size distributions of inorganic MF and UF membranes are shown in Fig. 4. The narrow pore size distribution of these membrane layers is evident and is primarily responsible for their superior separation capabili ties. The manufacturing processes for inorganic membranes have advanced to the point of delivering consistent high quality filters which are essentially defect free. Inorganic MF and UF membranes also display high flux values (see Table 8)which they owe to their composite/asymmetric nature combined with the ability to operate at high temperatures, pressures and shear rates Two kinds of membrane geometries are predominantly used, the tubular multi-lumen and the multichannel monoliths with circular, hexagonal or honeycomb structures. The number of channels can vary from I to 60
Cross-Flow Filtration 285 4.2 Inorganic Microfilters and Ultrafilters Cross-flow membrane filters made from inorganic materials, primarily ceramics and metals, utilize entirely different manufacturing processes compared with their polymeric counterparts, [31 Although carbon membranes do not qualifjl under the inorganic definition, they will be included here due to the similarities with inorganic membranes with regard to their material properties such as thermal, mechanical and chemical resistance as well as similarity in production techniques. Table 7 lists the various commonly used materials and membrane geometries in MF and UF modules. Commercial ceramic membranes are made by the slip-casting process. This consists of two steps and begins with the preparation of a dispersion of fine particles (referred to as slip) followed by the deposition of the particles on a porous A majority of commonly used inorganic membranes are composites consisting of a thin separation barrier on porous support (e.g., MembraloxB zirconia and alumina membrane products). Inorganic MF and UF membranes are characterized by their narrow pore size distributions. This allows the description of their separative performance in terms of their true pore diameter rather than MWCO value which can vary with operating conditions. This can be advantageous in comparing the relative separation performance of two different membranes independent of the operating conditions. MF membranes, in addition, can be characterized by their bubble point pressures. Due to their superior mechanical resistance bubble point measurements can be extended to smaller diameter MF membranes (0.1 or 0.2 pm) which may have bubble point pressure in excess of 10 bar with water.L9] Typical pore size distributions of inorganic MF and UF membranes are shown in Fig. 4. The narrow pore size distribution of these membrane layers is evident and is primarily responsible for their superior separation capabilities. The manufacturing processes for inorganic membranes have advanced to the point of delivering consistent high quality filters which are essentially defect free. Inorganic MF and UF membranes also display high flux values (see Table 8) which they owe to their composite/asymmetric nature combined with the ability to operate at high temperatures, pressures and shear rates. Two kinds of membrane geometries are predominantly used, the tubular multi-lumen and the multichannel monoliths with circular, hexagonal or honeycomb structures. The number of channels can vary from 1 to 60