Chapter 3 Pressure-activated membrane processes M.J. LEWIS, Department of Food Science and Technology, The University of Reading Reading RG6 6AP 3.1 INTRODUCTION Over the last 30 years, a number of membrane processes have evolved, which make use a pressure driving force and a semi-permeable membrane in order to effect a separa tion of components in a solution or colloidal dispersion. The separation is based mainly on molecular size, but to a lesser extent on shape and charge. The three main processes are reverse osmosis(hyperfiltration), ultrafiltration and microfiltration. The dimensions of the components involved in these separations are given in Fig. 3.1, and are typically in the range of less than 1 nm to over 1000 nm. a brief summary of the main differences between them, in terms of the components which are rejected by the membranes, is also illustrated. More recently the process term'nanofiltration' has been introduced, which is somewhere between reverse osmosis(RO) and ultrafiltration, bringing about a separation of low molecular weight components such as monovalent ions and salts from organic SRO Solutions Macromolecules Fat globules Suspensions Fig 3. 1. Size ranges for different membrane processe
Chapter 3 Pressure-activated membrane processes M. J. LEWIS, Department of Food Science and Technology, The University of Reading, Reading RG6 6AP 3.1 INTRODUCTION Over the last 30 years, a number of membrane processes have evolved, which make use of a pressure driving force and a semi-permeable membrane in order to effect a separation of components in a solution or colloidal dispersion. The separation is based mainly on molecular size, but to a lesser extent on shape and charge. The three main processes are reverse osmosis (hyperfiltration), ultrafiltration and microfiltration. The dimensions of the components involved in these separations are given in Fig. 3.1, and are typically in the range of less than 1 nm to over 1000 nm. A brief summary of the main differences between them, in terms of the components which are rejected by the membranes, is also illustrated. More recently the process term ‘nanofiltration’ has been introduced, which is somewhere between reverse osmosis (RO) and ultrafiltration, bringing about a separation of low molecular weight components such as monovalent ions and salts from organic MF --- Solutions Macromolecules Fat globules Ions, sugars Proteins - Suspensions 1 I I I I I I 10-2 1 102 io4 (nm) Fig. 3 1 Size ranges for different membrane processes
6 M.J. Lewis compounds such as sugars. These pressure-activated processes can also be regarded as a continuous spectrum of processes, with no obvious distinct boundaries between ther However, it should be noted that the sizes of the components being separated range over several orders of magnitude, so it is highly likely that the separation mechanisms and hence the operating strategies may change as we move through the spectrum 3.2 TERMINOLOGY The feed material is applied to one side of a membrane, The feed is usually a low viscosity fluid which may sometimes contain suspended matter and which is subjected to a pressure. In most cases the feed flows in a direction parallel to the membrane surface and the term cross-flow filtration is used to describe such applications. Dead-end systems are used, but mainly for laboratory scale separations. The stream which passes through the membrane under the influence of this pressure is termed the permeate(filtrate). After removal of the required amount of permeate, the remaining material is termed the con centrate or retentate. The extent of the concentration is characterised by the concentration factor () which is the ratio of the feed volume to the final concentrate volume(see equation (3.5)) The process can be illustrated simply in Fig. 3. 2(a). From a single membrane process ing stage, two fractions are produced, named the concentrate and permeate. The required extent of concentration may not be achieved in one stage, so the concentrate may be returned to the same module for further concentration or taken to other modules in a cascade, or multistage process. The permeate may also be further treated in a separate process In terms of size considerations alone, one extreme is a membrane with very small pore diameters(tight pores). In this case the permeate will be pure water because even small molecular weight solutes will be rejected by the membrane; high-pressure driving forces are required to overcome frictional resistance and osmotic pressure gradients, If the permeate 1s pi then the process is known as reverse hyperfiltration; it is similar in its effects to evaporation or freeze-concentration. A con centrate will be produced, in which there is virtually no alteration in the proportion of the Feed moss Fig. 3.2. Separation of feed into a concentrate and permeate stream
66 M. J.Lewis compounds such as sugars. These pressure-activated processes can also be regarded as a continuous spectrum of processes, with no obvious distinct boundaries between them. However, it should be noted that the sizes of the components being separated range over several orders of magnitude, so it is highly likely that the separation mechanisms and hence the operating strategies may change as we move through the spectrum. 3.2 TERMINOLOGY The feed material is applied to one side of a membrane. The feed is usually a lowviscosity fluid, which may sometimes contain suspended matter and which is subjected to a pressure. In most cases the feed flows in a direction parallel to the membrane surface and the term cross-flow filtration is used to describe such applications. Dead-end systems are used, but mainly for laboratory scale separations. The stream which passes through the membrane under the influence of this pressure is termed the permeate (filtrate). After removal of the required amount of permeate, the remaining material is termed the concentrate or retentate. The extent of the concentration is characterised by the concentration factor df), which is the ratio of the feed volume to the final concentrate volume (see equation (3.5)). The process can be illustrated simply in Fig. 3.2(a). From a single membrane processing stage, two fractions are produced, named the concentrate and permeate. The required extent of concentration may not be achieved in one stage, so the concentrate may be returned to the same module for further concentration or taken to other modules in a cascade, or multistage process. The permeate may also be further treated in a separate process. In terms of size considerations alone, one extreme is a membrane with very small pore diameters (tight pores). In this case the permeate will be pure water because even small molecular weight solutes will be rejected by the membrane; high-pressure driving forces are required to overcome frictional resistance and osmotic pressure gradients. If the permeate is predominantly water, then the process is known as reverse osmosis or hyperfiltration; it is similar in its effects to evaporation or freeze-concentration. A concentrate will be produced, in which there is virtually no alteration in the proportion of the Concentrate Feed :-+ pi L 1: IR I Permeate + ++- Osmosis Reverse osmosis I (4 (b) Fig. 3.2. Separation of feed into a concentrate and permeate stream
Pressure-activated membrane processes 67 solid constituents. In some applications it is the permeate which is the required material for example the production of 'drinking waterfrom sea-water or 'pure water brackish water. The best processes are those where both the concentrate and the permeate e fully utilised. There have been several comparisons made between evaporation and reverse osmosis in terms of capital costs, energy costs and product quality(Renner, 1991). In general terms Ro is less energy intensive and can improve product quality. Some limitations are the high capital costs, membrane replacement costs and extent of concentration, which is not as high as that obtainable by evaporation If a fluid, for example milk, is separated from water by a semi-permeable membrane (see Fig. 3.2()), there will be a flow of water from the water to the milk, in order equalise the chemical potential of the two fluids; this is termed osmosis. This flow of water can be stopped by applying a pressure to the milk. This pressure that stops the flow termed he osmotic pressure. If a pressure greater than the water will flow from the milk to the water, thereby reversing the natural process of osmosis and achieving a concentration of the milk. Therefore in reverse osmosis, the pressure applied needs to be in excess of the osmotic pressure. Osmotic pressure(r)is a olligative property, the pressure being dependent upon the number of particles and thei molecular weight. In classical terms it is determined from the Gibb's free energy InyX (3.1) whereR= gas constant, T=absolute temperature, y= activity coefficient, X mole frac tion, and Vm= partial molar volume For dilute solutions of non-ionisable materials, the Van t Hoff equation can be used 丌=RT(c/M) (3.2) where c= concentration(kg m")and M= molecular weight For ionisable salts this becomes iRT(c/M) (3.3) where i= the degree of ionisation, e. g. for NaCl, i= 2; for FeCl2, i=3. This equation predicts a linear increase in osmotic pressure with concentration. How ever, this relationship breaks down, even at relatively low concentrations, with the rela- tionship between osmotic pressure and concentration becoming non-linear. For example, the osmotic pressure of a 25% serum albumin solution was 300 t, which is about six times higher than predicted from the Van't Hoff equation. It is also affected by pH This non-linear relationship can be represented by virial type equations 兀=Ac+Bc2+Dc (3.4) where c=concentration and A, B and D are constants. The constants are presented for dextran and whey by Cheryan(1986)
Pressure-activated membrane processes 67 solid constituents. In some applications it is the permeate which is the required material; for example the production of ‘drinking water’ from sea-water or ‘pure water’ from brackish water. The best processes are those where both the concentrate and the permeate are fully utilised. There have been several comparisons made between evaporation and reverse osmosis, in terms of capital costs, energy costs and product quality (Renner, 1991). In general terms RO is less energy intensive and can improve product quality. Some limitations are the high capital costs, membrane replacement costs and extent of concentration, which is not as high as that obtainable by evaporation. If a fluid, for example milk, is separated from water by a semi-permeable membrane (see Fig. 3.2(b)), there will be a flow of water from the water to the milk, in order to equalise the chemical potential of the two fluids; this is termed osmosis. This flow of water can be stopped by applying a pressure to the milk. This pressure that stops the flow is termed the osmotic pressure. If a pressure greater than the osmotic pressure is applied, the water will flow from the milk to the water, thereby reversing the natural process of osmosis and achieving a concentration of the milk. Therefore in reverse osmosis, the pressure applied needs to be in excess of the osmotic pressure. Osmotic pressure (T) is a colligative property, the pressure being dependent upon the number of particles and their molecular weight. In classical terms it is determined from the Gibb’s free energy equation: (3.1) RT ;rl=-1nyX “m where R = gas constant, T = absolute temperature, y= activity coefficient, X = mole fraction, and V, = partial molar volume. For dilute solutions of non-ionisable materials, the Van’t Hoff equation can be used z = RT(C/M) (3.2) where c = concentration (kg m-3) and M = molecular weight. For ionisable salts this becomes ;rl = iRT(c/M) (3.3) where i = the degree of ionisation, e.g. for NaC1, i = 2; for FeC12, i = 3. This equation predicts a linear increase in osmotic pressure with concentration. However, this relationship breaks down, even at relatively low concentrations, with the relationship between osmotic pressure and concentration becoming non-linear. For example, the osmotic pressure of a 25% serum albumin solution was 300 2, which is about six times higher than predicted from the Van’t Hoff equation. It is also affected by pH. This non-linear relationship can be represented by Virial type equations: ;rl = Ac+ Bc2 + Dc3 (3.4) where c = concentration and A, B and D are constants. The constants are presented for dextran and whey by Cheryan (1986)
68 M.J. Lewis Osmotic pressures are highest for low molecular weight solutes, so the highest osmotic pressures arise for salt and sugar solutions. Concentration of such solutions results in a large increase in their osmotic pressure. On the other hand, proteins and other macromol ecules do not produce high osmotic pressures. There will only be small increases during their concentration as well as small differences in osmotic pressure between the feed and permeate in ultrafiltration. Values for osmotic pressures are not easy to find in the literature and a selection of values is given in Table 3. 1. A further complication wi foods and other biological systems is their complexity, with not just one but many components. In reverse osmosis the applied pressure must exceed the osmotic pressure, and the driving force term in reverse osmosis is normally the difference between the applied pressure and the osmotic pressure. It could be that osmotic re is one of the factors that limits the extent of concentration. One suggested experimental method for measuring osmotic pressure is to determine the pressure that would give zero flux, by extrapolation. In ultrafiltration and microfiltration, there is little osmotic pressure differ ence over the membrane as the low molecular weight components are almost freely permeating(see equation(3. 8) Table 3. 1. Osmotic pressures of some solutions Solution Osmotic pressure Sugar beet 20°Brix 34.1 Tomato paste 33°Brix 69.0 15°Brix Citrus juice 10°BriX 34°Brix 690 Sucrose Br Coffee extract 28%TS 34.0 Sea-wate 3.5%o salt 15.0%o sal 138.0 Milk Lactose 1%w/v 3.7 Sor piled from data in Cheryan(1986)and Lewis(1982) Some equations for osmotic pressure are given by Cheryan(1986) As the membrane pore size increases, the membrane becomes permeable to low molecular weight solutes in the feed; even the transport mechanisms are likely to change Lower pressure driving forces are required as osmotic pressure differences between the feed and permeate are reduced. However, molecules of a larger molecular weight are still rejected by the membrane. Therefore some separation of the solids present in the feed takes place; the permeate contains low molecular weight components at approximately the same concentration as they are in the feed, and the concentrate contains large
68 M. J.Lewis Osmotic pressures are highest for low molecular weight solutes, so the highest osmotic pressures arise for salt and sugar solutions. Concentration of such solutions results in a large increase in their osmotic pressure. On the other hand, proteins and other macromolecules do not produce high osmotic pressures. There will only be small increases during their concentration as well as small differences in osmotic pressure between the feed and permeate in ultrafiltration. Values for osmotic pressures are not easy to find in the literature and a selection of values is given in Table 3.1. A further complication with foods and other biological systems is their complexity, with not just one but many components. In reverse osmosis the applied pressure must exceed the osmotic pressure, and the driving-force term in reverse osmosis is normally the difference between the applied pressure and the osmotic pressure. It could be that osmotic pressure is one of the factors that limits the extent of concentration. One suggested experimental method for measuring osmotic pressure is to determine the pressure that would give zero flux, by extrapolation. In ultrafiltration and microfiltration, there is little osmotic pressure difference over the membrane as the low molecular weight components are almost freely permeating (see equation (3.8)). Table 3.1. Osmotic pressures of some solutions So I LI ti on Osmotic pressure (bar) Sugar beet 20" Brix 34.1 Tomato paste 33" Brix 69.0 Apple juice 15" Brix 20.4 Citrus juice 10" Brix 14.8 34" Brix 69.0 Sucrose 44" Brix 69.0 Coffee extract 28% TS 34.0 Sea-water 3.5% salt 23.2 15.0% salt 138.0 Milk 6.9 Whey 6.9 Lactose 1% w/v 3.7 Compiled from data in Cheryan (1986) and Lewis (1982). Some equations for osmotic pressure are given by Cheryan (1986). As the membrane pore size increases, the membrane becomes permeable to low molecular weight solutes in the feed; even the transport mechanisms are likely to change. Lower pressure driving forces are required as osmotic pressure differences between the feed and permeate are reduced. However, molecules of a larger molecular weight are still rejected by the membrane. Therefore some separation of the solids present in the feed takes place; the permeate contains low molecular weight components at approximately the same concentration as they are in the feed, and the concentrate contains large
Pressure-activated membrane processes 69 molecular weight components at an increased concentration, compared to the feed. Note that some of the low molecular weight components will be retained in the concentrate. It this fractionation and concentration process that makes the ultrafiltration process more interesting than reverse osmosis, although, as mentioned earlier, there is no sharp demarcation between the processes. More porous membranes still allow not only sugars and salts, but also macromolecules, to pass through, but retain particular matter and fat r than 100 nm(see Fig 3. 1); this is termed microfiltration. Because of their increased potential for separating components in mixed feeds, ultrafiltration and microfiltration are covered in more detail in Chapters 4 and 5. However, much of the discussion, particularly that on membrane performance and rejection, will also be pertinent to all three pressure-activated processes. Major points of difference are discussed later in this chapter. 3.3 CONCENTRATION FACTOR AND REJECTION Two important processing parameters for all pressure activated processes are the concell- tration factor ()and the membrane rejection characteristics, The concentration factor is defined as follows Concentration factor (f)=VE/Vc (3.5) where VF=feed volume and Vc=final concentrate volume The term volume reduction factor (VRF) is sometimes used VRF=100V-V)/V=1001-1/f) Thus a process with a concentration factor of 10 would have a volume reduction factor of The permeate volume(Vp) equals the feed volume minus the concentrate volume (assuming no losses) As soon as the concentration factor exceeds l the volume of permeate will exceed that of the concentrate. Concentration factors may range from as low as 1.5 for some visco materials, to up to 50 for dilute protein solutions, e.g. chhana whey (Jindal and Grandison 1992). Generally higher concentration factors are used for ultrafiltration than for reverse osmosis, e.g. up to 25-30 for UF of cheese-whey, compared to 5 for RO of cheese-whey A mass balance for the process can be applied and is useful for estimating the distribution of components between the permeate and concentrate, or for estimating the losses that are incurred in practical situations T n or retention factor(R)of any component is defined where cp is the concentration of component in the feed and cp is the concentration in the
Pressure-activated membrane processes 69 molecular weight components at an increased concentration, compared to the feed. Note that some of the low molecular weight components will be retained in the concentrate. It is this fractionation and concentration process that makes the ultrafiltration process more interesting than reverse osmosis, although, as mentioned earlier, there is no sharp demarcation between the processes. More porous membranes still allow not only sugars and salts, but also macromolecules, to pass through, but retain particular matter and fat globules, i.e. greater than 100 nm (see Fig. 3.1); this is termed microfiltration. Because of their increased potential for separating components in mixed feeds, ultrafiltration and microfiltration are covered in more detail in Chapters 4 and 5. However, much of the discussion, particularly that on membrane performance and rejection, will also be pertinent to all three pressure-activated processes. Major points of difference are discussed later in this chapter. 3.3 CONCENTRATION FACTOR AND REJECTION Two important processing parameters for all pressure activated processes are the conceirtration factor cf) and the membrane rejection characteristics. The concentration factor is defined as follows: Concentration factor (f) = VF 1 V, (3.5) where VF = feed volume and V, = final concentrate volume. The term volume reduction factor (VRF) is sometimes used: VRF = 1oo(VF - V,)/~F = 100(1- 1/f) (3.6) Thus a process with a concentration factor of 10 would have a volume reduction factor of 90%. The permeate volume (V,) equals the feed volume minus the concentrate volume (assuming no losses) (3.7) As soon as the concentration factor exceeds 1, the volume of permeate will exceed that of the concentrate. Concentration factors may range from as low as 1.5 for some viscous materials, to up to 50 for dilute protein solutions, e.g. chhana whey (Jindal and Grandison, 1992). Generally higher concentration factors are used for ultrafiltration than for reverse osmosis, e.g. up to 25-30 for UF of cheese-whey, compared to 5 for RO of cheese-whey. A mass balance for the process can be applied and is useful for estimating the distribution of components between the permeate and concentrate, or for estimating the losses that are incurred in practical situations. vp = v, - v, The rejection or retention factor (R) of any component is defined as R = CF - cp/cF (3.8) where CF is the concentration of component in the feed and cp is the concentration in the permeate
M.J Lewis It can be determined experimentally for each and every component in the feed, by sampling the feed and permeate at the same time and analysing the component in ques tion. It is a very important property of a membrane, as it will influence the extent (quality )of the separation that can be achieved Rejection values normally range between 0 and 1; sometimes they are expressed as percentages(0-100%) when c E 0: nt is retained in the feed when c =c R=0; the component is freely permeating An ideal RO membrane would give a rejection value for all components of 1, whil an ideal UF membrane, being used to concentrate a high molecular weight component or remove a low molecular weight component would give respective rejection values of 1 and 0. If the concentration factor and rejection value are known, the yield of any component, which is defined as the fraction of that component present in the feed, which is recovered in the concentrate, can be estimated. Obviously for reverse osmosis, the yield for an ideal membrane is 1.0. Rejection data for membranes and their effects on eld and separation performance will be discussed in greater detail in Chapter 4 3.4 MEMBRANE CHARACTERISTICS The membrane itself is crucial to the process. The first commercial membranes were made of cellulose acetate and these are termed first-generation membranes. For food processing applications, they had some limitations, with temperatures below 30C and PH range of 3-6. These were followed in the mid-1970s by other polymeric membranes cond-generation membranes), with polyamides and, in particular, polysulphones being widely used for foods. The resulting improvements in cleaning and hygiene are covered in Section 3.8. It is estimated that over 150 organic polymers have now been investigated for membrane applications, Inorganic membranes based on sintered and ceramic materials are also now available. The physical structure of these membranes is complex and as most of them are used for microfiltration. their structure is described in more detail in Chapter 5 The main terms used to describe membranes are microporous or asymmetric. Microporous membranes have a uniform porous structure throughout, although the pore size may not be uniform across the thickness of the membrane. They are usually charac terised by a nominal pore size and no particle larger than this will pass through the nembrane. In contrast to this. most membranes used for ultrafiltration are of a type, having a dense active layer or skin of 0.5-1 um in thickness, and a further support layer which is much more porous and of greater thickness(Fig. 3.3). Overall the porosity of these membranes is high, although the surface porosity may be low, with quoted alues in the range 0.3-15%(Fane and Fell, 1987). Often the porous path may be quite tortuous, the distance covered by the solvent or solute being much greater than the thickness of the membrane; the term tortuosity has been used as a measure of this property. The pores are not of a uniform size, as can be seen when viewed under the electron microscope, and are best characterised by a pore size distribution. This
70 M. J.Lewis It can be determined experimentally for each and every component in the feed, by sampling the feed and permeate at the same time and analysing the component in question. It is a very important property of a membrane, as it will influence the extent (quality) of the separation that can be achieved. Rejection values normally range between 0 and 1; sometimes they are expressed as percentages (0-1 00%). when cp = 0; when cp = CF R = 1; all the component is retained in the feed R = 0; the component is freely permeating. An ideal RO membrane would give a rejection value for all components of 1, whilst an ideal UF membrane, being used to concentrate a high molecular weight component or remove a low molecular weight component would give respective rejection values of 1 and 0. If the concentration factor and rejection value are known, the yield of any component, which is defined as the fraction of that component present in the feed, which is recovered in the concentrate, can be estimated. Obviously for reverse osmosis, the yield for an ideal membrane is 1.0. Rejection data for membranes and their effects on yield and separation performance will be discussed in greater detail in Chapter 4. 3.4 MEMBRANE CHARACTERISTICS The membrane itself is crucial to the process. The first commercial membranes were made of cellulose acetate and these are termed first-generation membranes. For foodprocessing applications, they had some limitations, with temperatures below 30°C and a pH range of 3-6. These were followed in the mid-1970s by other polymeric membranes (second-generation membranes), with polyamides and, in particular, polysulphones being widely used for foods. The resulting improvements in cleaning and hygiene are covered in Section 3.8. It is estimated that over 150 organic polymers have now been investigated for membrane applications. Inorganic membranes based on sintered and ceramic materials are also now available. The physical structure of these membranes is complex, and as most of them are used for microfiltration, their structure is described in more detail in Chapter 5. The main terms used to describe membranes are microporous or asymmetric. Microporous membranes have a uniform porous structure throughout, although the pore size may not be uniform across the thickness of the membrane. They are usually characterised by a nominal pore size and no particle larger than this will pass through the membrane. In contrast to this, most membranes used for ultrafiltration are of asymmetric type, having a dense active layer or skin of 0.5-1 pm in thickness, and a further support layer which is much more porous and of greater thickness (Fig. 3.3). Overall the porosity of these membranes is high, although the surface porosity may be low, with quoted values in the range 0.3-15% (Fane and Fell, 1987). Often the porous path may be quite tortuous, the distance covered by the solvent or solute being much greater than the thickness of the membrane; the term tortuosity has been used as a measure of this property. The pores are not of a uniform size, as can be seen when viewed under the electron microscope, and are best characterised by a pore size distribution. This
Pressure-activated membrane processes 71 ctive layer Microporous Fig. 3.3. Closer examination of the membrane structure distribution can be measured by electron microscope techniques, or by combined bubble size and solvent permeability methods( Munari et al., 1985). It is claimed that thi method is capable of measuring the pore-size distribution in the thin skin. Another technique mentioned by Fane and Fell is the capillary condensation/permeability method Pore sizes may range from 1 to 100 nm. This distribution of pore size is one of the main factors preventing a sharp separation of components of almost similar size, e.g. mono and di-saccharides(see Rejection). An indirect measurement of pore size can be made by measuring the permeability of solutes, such as dextrans with a range of molecular It can be seen that in physical terms alone, there are a number of membrane structures available. The membrane also has a chemical nature, and many materials have been evaluated. It may be hydrophilic or hydrophobic in nature. Fane and Fell(1987) stated that the hydrophobic nature can be characterised by measuring its contact angle( e). The higher the contact angle the more hydrophobic is the surface. Polysulphones are generally much more hydrophobic than cellulosic membranes. There was shown to be a good correlation between flux decline and hydrophobicity, with the least hydrophobic mem brane showing the least flux loss over a period of 150 min. The surface may also be charged. All these factors will give rise to interactions between components in the feed and influence the components passing through the membrane, as well as the fouling of the membrane The physical chemistry of membranes has been described in more detail by Cheryan (1986), Gutman(1987)and Tsujita(1992) 3.5 PERMEATE RATE Two other important processing parameters are the flux or permeate rate and the power The flux is usually expressed in terms of volume per unit time per unit area (I m"). Expressed in this way it permits a ready comparison of different membrane configurations of different surface areas. It can also be expressed as a permeate velocity If energy is to be taken into account, it may be relevant to measure and maximise the flux to energy consumption ratio. Flux values may range from higher than 500 1 m h less than 51 m-2 h te: Imperial units are still sometimes used, where l gal ft-2d-=2.036- Factors affecting the flux rate are the applied pressure: the flow rate and viscosity, both of which affect turbulence, and the processing temperature. Increasing th
72 M.. Lewis temperature and inducing more turbulence increases the flux. However, the flux is only ffected by the applied pressure in the pressure-dependent region. These factors are discussed in more detail in Chapter 4 The power utilisation(W) is related to the pressure(head) developed and the mass flow rate as follows: where m= mass flow rate(kg s"), h=head developed(m)and g= acceleration du (981ms-2) ture rise. Cooling may be necessary if a constant processing temperature is required oera- This energy is largely dissipated within the fluid as heat and will result in a tempera- The membrane offers a resistance to the transfer of bo th solvent (normally water) and The permeate flux is a measure of the flow rate of solvent through the membrane, whereas the rejection describes the amount of solute which passes through(see eq. (3. 8)). From a process engineering standpoint, it is highly desirable to be able to predict the flux and rejection from the physical properties of the solution, the membrane characteristics and the hydrodynamics of the flow situation, in order to optimise the performance of the system. Membrane operations have been subject to a number of modelling processes, in order to achieve these objectives. However, before these models are discussed in more detail, it is important to consider the phenomena of concentration polarisation and uling 3.6 TRANSPORT PHENOMENA AND CONCENTRATION POLARISATION A very important consideration for pressure-driven membrane processes is that the sepa ration takes place not in the bulk of solution, but in a very small region close to the membrane, known as the boundary layer, as well as over the membrane itself. This gives rise to the phenomenon of concentration polarisation over the boundary layer. (Note that streamline flow the whole of the fluid will behave as a boundary layer. ) It is mani fested by a quick and significant reduction(2-10 fold)in flux when water is replaced by the feed solution, for example in a dynamic start oncentration polarisation occurs whenever a component is rejected by the membrane. As a result, there is an increase in the concentration of that component at the membran rface oncentration gradient over the boundary layer. Eventually dynamic equilibrium is established, where the convective flow of the component to the membrane surface equals the flow of material away from the surface, either in the permeate or back into the bulk of the solution by diffusion, due to the concentration gradient established. This increase in concentration, especially of large molecular weight components, offers a very significant additional resistance. It may also give rise to the formation of a gelled or fouling layer on the surface of the membrane(see Fig. 3. 4) Whether this occurs will depend upon the initial concentration of the component and the physical properties of the solution; it could be very important as it may affect the subsequent separation performance. Concentration polarisation itself is a reversible phenomenon; thus if the solution is then replaced by water, the original water flux should be restored. However, this rarely occurs in practice due to the occurrence of fouling
72 M.J.Lewis temperature and inducing more turbulence increases the flux. However, the flux is only affected by the applied pressure in the pressure-dependent region. These factors are discussed in more detail in Chapter 4. The power utilisation (W) is related to the pressure (head) developed and the mass flow rate as follows: W = m'hg (3.9) where m' = mass flow rate (kg s-'), h = head developed (nz) and g = acceleration due to gravity (9.81 m s-*). This energy is largely dissipated within the fluid as heat and will result in a temperature rise. Cooling may be necessary if a constant processing temperature is required. The membrane offers a resistance to the transfer of both solvent (normally water) and solute. The permeate flux is a measure of the flow rate of solvent through the membrane, whereas the rejection describes the amount of solute which passes through (see eq. (3.8)). From a process engineering standpoint, it is highly desirable to be able to predict the flux and rejection from the physical properties of the solution, the membrane characteristics and the hydrodynamics of the flow situation, in order to optimise the performance of the system. Membrane operations have been subject to a number of modelling processes, in order to achieve these objectives. However, before these models are discussed in more detail, it is important to consider the phenomena of concentration polarisation and fouling. 3.6 TRANSPORT PHENOMENA AND CONCENTRATION POLARISATION A very important consideration for pressure-driven membrane processes is that the separation takes place not in the bulk of solution, but in a very small region close to the membrane, known as the boundary layer, as well as over the membrane itself. This gives rise to the phenomenon of concentration polarisation over the boundary layer. (Note that in streamline flow the whole of the fluid will behave as a boundary layer.) It is manifested by a quick and significant reduction (2-10 fold) in flux when water is replaced by the feed solution, for example in a dynamic start. Concentration polarisation occurs whenever a component is rejected by the membrane. As a result, there is an increase in the concentration of that component at the membrane surface, together with a concentration gradient over the boundary layer. Eventually a dynamic equilibrium is established, where the convective flow of the component to the membrane surface equals the flow of material away from the surface, either in the permeate or back into the bulk of the solution by diffusion, due to the concentration gradient established. This increase in concentration, especially of large molecular weight components, offers a very significant additional resistance. It may also give rise to the formation of a gelled or fouling layer on the surface of the membrane (see Fig. 3.4). Whether this occurs will depend upon the initial concentration of the component and the physical properties of the solution; it could be very important as it may affect the subsequent separation performance. Concentration polarisation itself is a reversible phenomenon; thus if the solution is then replaced by water, the original water flux should be restored. However, this rarely occurs in practice due to the occurrence of fouling
Pressure-activated membrane processes 73 C Boundary Bulk Fig 3.4.(a)Concentra etected by a decline of flux rate at constant composition. Fouling is caused by the deposition of material on the surface of the membrane or within the pores of the membrane. Fouling is irreversible and the flux needs to be restored by cleaning. There- fore, during a concentration process, flux declines due to a combination of these two phenomena A number of mechanisms have been proposed to explain the transport of solvent and and Nichols(1992) The simplest mechanism to visualise conceptually is a simple sieving mechanism, based on size. However, for reverse osmosis, this does not explain the high rejection of salt and the permeation of water, as the molecules are about the same size. Other physice hemical factors concerned with the structure of the membrane and interaction of solvent and solutes with the membrane influence the performance. It is believed that for reverse osmosis, the phenomena are more complex than those occurring with ultrafiltration which is generally regarded as a sieving process. Therefore the models proposed for reverse osmosis and ultrafiltration are different in nature One of the most common models used for reverse osmosis is the solute diffusion model, in which the solvent flux (Jw) is influenced in the main by the pressure driving force and the solute flux (s) by diffusion. All the resistance to mass transfer takes place in the active layer, which is between 0.5 and 1.0 um in thickness. In this case the pressure driving force is the difference between the applied pressure and the osmotic pressure. The transport of solvent and solute are not connected. It is assumed that the solute dissolves the skin instantaneously and then passes through the pores of membrane by diffusion, whereas the flow of solvent(which also dissolves) is influenced by the pressure differential. The relationship between concentration dissolved in the membrane at its surface and that in the solution depends upon the partition coefficient for that component
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74 M.J. Lewis Thus the diffusivity and membrane thickness and its partition coefficient all influence the transport of solute( Convective flow of solute is ignored at high solute rejections. According to this model, increasing the pressure will have a preferential effect on solvent flow, thereby reducing solute concentration in the permeate and rejection. It also predicts that increasing the solute concentration in the feed preferentially increases solute transport and increases the concentration in the permeate, thereby decreasing rejection. Increasing the temperature increases both solvent flux and solute flux by about the same amounts, thereby leaving the rejection unchanged. A 1C change in temperature changes the flux(solute and solvent) by approximately 3% Although many of these trends are observed with simple solutions, the theory does not account for all the observed facts for multi-component solutions, whereby the presence of one component increases the permeability of other components. Glover(1985) pointed out that this theory does not describe the behaviour of complex systems precisely, but it gives a background of understanding. In multicomponent systems, differences in the partition coefficients could explain the difference in permeabilities between different A second proposed model is the preferential adsorption, capillary flow model, which predicts that the component concentrated preferentially in the permeate will be that component which is adsorbed most strongly on the membrane surface. The component that is preferentially adsorbed onto the membrane surface provides a thin layer of tha component adjacent to the surface. This thin layer then moves through the pores of the membrane by capillary flow, under the influence of a pressure gradient, and in this way thus preferentially transported through the membrane, For membranes of a hydrophilic nature, the component preferentially absorbed and transported is water. Thus it is postulated that there is a thin film of water adjacent to the membrane surface. This theory also explains the low rejections and sometimes negative rejections for highly polar organic solutes, found with cellulose acetate membranes, due to their preferential adsorption on the surface. Other situations, where rejection of organic solutes decreases with increasing pressure, are explained by their adsorption onto the more hydrophobic regions of the membrane A third model is based upon the wetted surface mechanism, whereby water adsorbs onto the surface of the membrane by hydrogen bonding. It is postulated that these clusters f water prevent solute entering the pores and that the water passes through the mem- brane from one adsorbed site to the next. The energy requirements for water migration are much less than salt migration, thereby promoting separation of the salt and water. All these models are based on knowing the transport mechanisms involved. The physical hemistry of a wide variety of membrane materials, including permeability data, diffusion data and sorption data which are required for the models described earlier, been reviewed by Tsujita(1992). Also reviewed are transport properties related equilibrium thermodynamics with uncharged and charged membranes Other models are based upon irreversible thermodynamics, where the driving force for transport olvent and solute is expressed in terms of differences in their chemica potential over the membrane. However, the fluxes for solvent and solute are coupled and the flux for each component is influenced by the chemical potential difference for both components. With such models, the exact mechanisms are not known, but the
74 M.J.Lewis Thus the diffusivity and membrane thickness and its partition coefficient all influence the transport of solute. (Convective flow of solute is ignored at high solute rejections.) According to this model, increasing the pressure will have a preferential effect on solvent flow, thereby reducing solute concentration in the permeate and increasing rejection. It also predicts that increasing the solute concentration in the feed preferentially increases solute transport and increases the concentration in the permeate, thereby decreasing rejection. Increasing the temperature increases both solvent flux and solute flux by about the same amounts, thereby leaving the rejection unchanged. A 1°C change in temperature changes the flux (solute and solvent) by approximately 3%. Although many of these trends are observed with simple solutions, the theory does not account for all the observed facts for multi-component solutions, whereby the presence of one component increases the permeability of other components. Glover (1985) pointed out that this theory does not describe the behaviour of complex systems precisely, but it gives a background of understanding. In multicomponent systems, differences in the partition coefficients could explain the difference in permeabilities between different components. A second proposed model is the preferential adsorption, capillary flow model, which predicts that the component concentrated preferentially in the permeate will be that component which is adsorbed most strongly on the membrane surface. The component that is preferentially adsorbed onto the membrane surface provides a thin layer of that component adjacent to the surface. This thin layer then moves through the pores of the membrane by capillary flow, under the influence of a pressure gradient, and in this way is thus preferentially transported through the membrane. For membranes of a hydrophilic nature, the component preferentially absorbed and transported is water. Thus it is postulated that there is a thin film of water adjacent to the membrane surface. This theory also explains the low rejections and sometimes negative rejections for highly polar organic solutes, found with cellulose acetate membranes, due to their preferential adsorption on the surface. Other situations, where rejection of organic solutes decreases with increasing pressure, are explained by their adsorption onto the more hydrophobic regions of the membrane. A third model is based upon the wetted surface mechanism, whereby water adsorbs onto the surface of the membrane by hydrogen bonding. It is postulated that these clusters of water prevent solute entering the pores and that the water passes through the membrane from one adsorbed site to the next. The energy requirements for water migration are much less than salt migration, thereby promoting separation of the salt and water. All these models are based on knowing the transport mechanisms involved. The physical chemistry of a wide variety of membrane materials, including permeability data, diffusion data and sorption data which are required for the models described earlier, have been reviewed by Tsujita (1992). Also reviewed are transport properties related to nonequilibrium thermodynamics with uncharged and charged membranes. Other models are based upon irreversible thermodynamics, where the driving force for transport of solvent and solute is expressed in terms of differences in their chemical potential over the membrane. However, the fluxes for solvent and solute are coupled and the flux for each component is influenced by the chemical potential difference for both components. With such models, the exact mechanisms are not known, but the
