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