Chapter 5 Microfiltration A.S. GRANDISON, Department of Food Science and Technology, The University ot Reading, Reading RG6 6AP and T J. A FINNIGAN, Marlow Foods, Middlesbrough 5.1 INTRODUCTION Microfiltration (MF) is the oldest membrane technology, having been used several decades before the first industrial use of reverse osmosis(Glimenius, 1985). However, subsequent development of the technology has been slow. Until recently microfilters were operated in the dead-end mode and were exclusively of the depth-filter type which particles become trapped within the filter structure, but recent developments have led to membrane-type microfilters, with a narrow pore size distribution, which can be operated in the cross-flow mode. This has led to an increase in possible applications, ncluding clarification of fluids in the food and beverage industries, recovery of cells and cell debris in the biotechnology industries, and the treatment of wastes 5.2 THEORY, MATERIALS AND EQUIPMENT Like ultrafiltration (UF), MF is a pressure-driven process employing pressures consider- ably lower than reverse osmosis. In fact the distinction between U MF is Somewhe arbitrary and there is no distinction on purely theoretical grounds. The distinction lies in the size ranges of the materials which are separated. UF is considered to involve the processing of dissolved macromolecules, while MF involves separation of dispersed articles such as colloids, fat globules or cells. MF can be considered to fall between UF and conventional filtration, although there is overlap at both ends of the spectrum. A guide to the pore sizes used for MF could be 0.01-10 um 2. For many years, MF has been applied as a dead-end operation using highly microporous symmetric membranes of the depth-filter type. Such membranes retain particles and consequently result in the build-up of a filter cake. This reduces flow, and when the pressure drop has reached a certain level the membrane must be replaced or removed and regenerated. In addition, the presence of a filter cake radically alters the filtering characteristics, effectively acting as a prefilter which removes particles which
Chapter 5 Microfiltration A. S. GRANDISON, Departrnent of Food Science and Technology, The University of Reading, Reading RG6 6AP and T. J. A. FINNIGAN, Marlow Foods, Middlesbrough. 5.1 INTRODUCTION Microfiltration (MF) is the oldest membrane technology, having been used several decades before the first industrial use of reverse osmosis (Glimenius, 1985). However, subsequent development of the technology has been slow. Until recently microfilters were operated in the dead-end mode and were exclusively of the depth-filter type in which particles become trapped within the filter structure, but recent developments have led to membrane-type microfilters, with a narrow pore size distribution, which can be operated in the cross-flow mode. This has led to an increase in possible applications, including clarification of fluids in the food and beverage industries, recovery of cells and cell debris in the biotechnology industries, and the treatment of wastes. 5.2 THEORY, MATERIALS AND EQUIPMENT Like ultrafiltration (UF), MF is a pressure-driven process employing pressures considerably lower than reverse osmosis. In fact the distinction between UF and MF is somewhat arbitrary and there is no distinction on purely theoretical grounds. The distinction lies in the size ranges of the materials which are separated. UF is considered to involve the processing of dissolved macromolecules, while MF involves separation of dispersed particles such as colloids, fat globules or cells. MF can be considered to fall between UF and conventional filtration, although there is overlap at both ends of the spectrum. A guide to the pore sizes used for MF could be 0.01-10 pm. For many years, MF has been applied as a dead-end operation using highly microporous symmetric membranes of the depth-filter type. Such membranes retain particles and consequently result in the build-up of a filter cake. This reduces flow, and when the pressure drop has reached a certain level the membrane must be replaced or removed and regenerated. In addition, the presence of a filter cake radically alters the filtering characteristics, effectively acting as a prefilter which removes particles which
142 A.S. Grandison and T, J. A. Finnigan could otherwise pass through the membrane itself. On a large scale, therefore, it is only this technique when small amounts of particles are present Cross-flow MF (CMF) is a development which combines the cross-flow technique, as applied to UF and reverse osmosis, with MF CMF can be used to minimise (although it should be emphasised, not completely eradicate) the problems encountered in dead-ene MF, and thus permit the processing of fluids containing quite large amounts of suspended solids on a large scale. The advantages result from the fact that the build-up of filter cake is avoided due to the shearing effect of the feed stream flowing parallel to the membrane (Fig. 5. 1). CMf plants can be operated in the same batch or continuous modes described in Chapter 3 PERMEATE Membrane Feed FIG. 5.1. Principles of (a)'dead-end and (b)cross-tlow'filtration 5.2. 1 Membrane configurations and characteristics The geometric designs of MF membranes are the same as for UF as described in Chapter 3. Hence the module housings and ancillary equipment are also similar. Also the mem brane types are the same as for UF, i.e. cellulose and synthetic polymers(described in Chapter 3)or inorganic. It is notable that the development of inorganic membranes has been towards applications in MF rather than UF and reverse osmosis. In fact some type of inorganic membrane are only available with pore sizes in the MF range Various inorganic materials have been employed for membrane manufacture including glass, metals and compounds of aluminium, zirconium and titanium, and the geometries can vary radically from conventional membrane design The structures and methods of manufacture of inorganic membranes are described in greater detail by Rios et aL. (1989). Inorganic membranes consist of two parts-a macro porous support and the active membrane coated onto the surface. The supporting
142 could otherwise pass through the membrane itself. On a large scale, therefore, it is only practicable to use this technique when small amounts of particles are present. Cross-flow MF (CMF) is a development which combines the cross-flow technique, as applied to UF and reverse osmosis, with MF. CMF can be used to minimise (although it should be emphasised, not completely eradicate) the problems encountered in dead-end MF, and thus permit the processing of fluids containing quite large amounts of suspended solids on a large scale. The advantages result from the fact that the build-up of filter cake is avoided due to the shearing effect of the feed stream flowing parallel to the membrane (Fig, 5.1). CMF plants can be operated in the same batch or continuous modes as described in Chapter 3. A, S. Grandison and T. J. A. Finnigan FEED Filter cake 0 OOOc$ O O O AP{ lo+:0 **;," ' ' ' ' L +A Membrane PERMEATE (a) 0 000 0 - 0 O O+ 0 oo -0 Ooo0 0 C+SCC++++ Concentrate 0- Feed 0 00 Ap-- - __---_ - Membrane Permeate (b) FIG. 5. I. Principles of (a) 'dead-end' and (b) 'cross-tlow' filtration. 5.2.1 Membrane configurations and characteristics The geometric designs of MF membranes are the same as for UF as described in Chapter 3. Hence the module housings and ancillary equipment are also similar. Also the membrane types are the same as for UF, i.e. cellulose and synthetic polymers (described in Chapter 3) or inorganic. It is notable that the development of inorganic membranes has been towards applications in MF rather than UF and reverse osmosis. In fact some types of inorganic membrane are only available with pore sizes in the MF range. Various inorganic materials have been employed for membrane manufacture including glass, metals and compounds of aluminium, zirconium and titanium, and the geometries can vary radically from conventional membrane design. The structures and methods of manufacture of inorganic membranes are described in greater detail by Rios et al. (1989). Inorganic membranes consist of two parts-a macroporous support and the active membrane coated onto the surface. The supporting
Microfiltration 143 materials must drain away the permeate without any hydrodynamic resistance, and thus have a pore diameter of about 10 um or more. They are produced from sintered fine owdered materials including alumina, carbon, stainless steel and nickel. The tubular or multichannel geometries of modules(e.g. Fig. 5.2(a-c) are produced by extrusion of the Filtrate flows in conduits to downstream d of module Plugged passageways which isolate filtrate conduits from feed flow channels Opposite end has exactly ame appearance Passageways coated FEED
Microfiltration 143 materials must drain away the permeate without any hydrodynamic resistance, and thus have a pore diameter of about 1Opm or more. They are produced from sintered fine powdered materials including alumina, carbon, stainless steel and nickel. The tubular or multichannel geometries of modules (e.g. Fig. 5.2 (a-c)) are produced by extrusion of the Filtrate flows in conduits to downstream end of module
144 A. S. Grandison and T.J. A. finnigan Surface view Zirconia mesh support membranes sy of SFEC):(b) different designs of Ceraver alumina membranes (re n of Membralox):(c)alumina membrane module (reproduced by permission of CeraMem) ;(d)ceramic/metal-mesh composite membrane (reproduced by permission of Nww Acumem Ltd powder and binder in aqueous media. The membrane layer may be coated directly onto the macroporous support where pore size is quite large, but for UF and the smaller pore size MF membranes an intermediate sintered, ceramic layer is necessary due to the surface rugosity of the support. The membrane layer (usually composed of alumina, titania or zirconia) is formed by coating the support with a colloidal suspension and firing at a lower temperature than the firing temperature of the support. To prevent rapid flux decline, the membrane thickness must not exceed a few microns- titanium and zirco- nium membranes of thickness 3-5 Am have been achieved. Accurate control of the col lodal particle size allows the possibility of producing membranes with an extremely narrow pore size distribution compared to conventional membranes. The final pore size is also related to the sintering temperature. An example of the structure of a sintered ce- ramic membrane is shown in Fig 5.3
144 A. S. Grandison and T. J. A. Finnigan Surface view Cross-section I 1 I Nickellbased Zirconia meshsupport membranes Fig. 5.2. Some designs of inorganic membranes: (a) CARBOSEP membrane composed of zirconia on a carbon support (courtesy of SFEC); (b) different designs of Ceraver alumina membranes (reproduced by permission of Membralox@); (c) alumina membrane module (reproduced by permission of CeraMem); (d) ceramic/metal-mesh composite membrane (reproduced by permission of NWW Acumem Ltd). powder and binder in aqueous media. The membrane layer may be coated directly onto the macroporous support where pore size is quite large, but for UF and the smaller poresize MF membranes an intermediate sintered, ceramic layer is necessary due to the surface rugosity of the support. The membrane layer (usually composed of alumina, titania or zirconia) is formed by coating the support with a colloidal suspension and firing at a lower temperature than the firing temperature of the support. To prevent rapid flux decline, the membrane thickness must not exceed a few microns - titanium and zirconium membranes of thickness 3-5 pn have been achieved. Accurate control of the colloidal particle size allows the possibility of producing membranes with an extremely narrow pore size distribution compared to conventional membranes. The final pore size is also related to the sintering temperature. An example of the structure of a sintered ceramic membrane is shown in Fig. 5.3
Microfiltration 145 Microporous→ membrane Macroporous→一 Fig. 5.3. Electron micrograph showing structure of Ceraver sintered embrane with 0. 2 Am pore size(reproduced by permission of Mem Early work on inorganic membranes used glass, but the first reliable cross-flow system was the CARBOSEP membrane made of a microporous layer of zirconia coated onto a macroporous carbon support(Fig. 5. 2(a). New products have subsequently appeared including several designs of alumina membrane (e. g. Ceraver and CeraMem designs Figs. 5.2(b)and 5. 2(c). A novel design is the composite membrane produced by Acumen omposed of a zirconia ceramic membrane with nickel-based superalloy mesh support (Fig.5.2(d) The potential advantages of inorganic membranes result from their greater structural strength and resistance to abrasive degradation, as well as improved chemical and temperature properties. Their rigidity and strength allow the processing of feed materials, such as cheese curd(Mahaut et al., 1982)or particulate materials which would not be possible using conventional designs. Another possibility is that they could be used in conjunction with fluidised turbulence promoters to increase permeate flux, which would be out of the question with the more fragile surfaces of organic membranes. The wide ph ranges of inorganic membranes (e.g. alumina membranes are resistant to pHs ranging from 0.5 to 13.5, although phosphoric and hydrofluoric acids should be avoided) are a great advantage during cleaning and sterilisation. In-place cleaning regimes using high concentrations of caustic soda (3%), nitric acid (2%) and sodium hypochlorite are possible. The modules can withstand temperatures of several hundred oC, which is far beyond the temperatures used for food processing. However, this is an advantage during eaning cycles and the modules can be sterilised by steam. In practice, operating temperatures are limited by other components such as gaskets, but it could be feasible to
Microfiltration 145 Microporous - membrane Macroporous - support Fig. 5.3. Electron micrograph showing structure of Ceraver sintered alumina membrane with 0.2 pm pore size (reproduced by permission of Membralox@). Early work on inorganic membranes used glass, but the first reliable cross-flow system was the CARBOSEP membrane made of a microporous layer of zirconia coated onto a macroporous carbon support (Fig. 5.2(a)). New products have subsequently appeared including several designs of alumina membrane (e.g. Ceraver and CeraMem designs - Figs. 5.2(b) and 5.2(c)). A novel design is the composite membrane produced by Acumem composed of a zirconia ceramic membrane with nickel-based superalloy mesh support (Fig. 5.2(d)). The potential advantages of inorganic membranes result from their greater structural strength and resistance to abrasive degradation, as well as improved chemical and temperature properties. Their rigidity and strength allow the processing of feed materials, such as cheese curd (Mahaut et al., 1982) or particulate materials which would not be possible using conventional designs. Another possibility is that they could be used in conjunction with fluidised turbulence promoters to increase permeate flux, which would be out of the question with the more fragile surfaces of organic membranes. The wide pH ranges of inorganic membranes (e.g. alumina membranes are resistant to pHs ranging from 0.5 to 13.5, although phosphoric and hydrofluoric acids should be avoided) are a great advantage during cleaning and sterilisation. In-place cleaning regimes using high concentrations of caustic soda (3%), nitric acid (2%) and sodium hypochlorite are possible. The modules can withstand temperatures of several hundred "C, which is far beyond the temperatures used for food processing. However, this is an advantage during cleaning cycles and the modules can be sterilised by steam. In practice, operating temperatures are limited by other components such as gaskets, but it could be feasible to