Chapter 4 Ultrafiltration M.J. LEWIS, Department of Food Science and Technology, The University of Reading RG6 6AP 4.1 INTRODUCTION Ultrafiltration offers the opportunity to concentrate large molecular weight components without the application of heat or a change of phase. Such components are rejected by the membrane, whereas the permeate produced will contain the low molecular weight components present in the food, at a concentration similar to that in the feed. This results an increase in their concentration both on a wet weight and dry weight basis in the olution. It is a pressure-activated process, with pressures in the range of 1-15 bar; these pressures are considerably lower than those used in reverse osmosis. For many heat labile macromolecules,e.g proteins and starches, concentration by UF at ambient temperature will minimise heat-induced reactions which may adversely influence their functional behaviour in foods. Some important functional properties are solubility, foaming capacity, gelation, emulsification capacity, fat and water binding properties. These are discussed in more detail in section 4.5 In the case of enzymes or pharmaceutical agents, their biological activity needs to be conserved. It also affords the opportunity to separate small molecular weight components from lex mixtures, containing components with a wide range of molecular weights There have also been investigations into using UF for protein fractionation, but this is not straightforward due to the diffuse nature of the membranes and their selectivity UF is also very useful for recovering valuable components from food processing waste treams and fermentation broths, Probably the greatest impetus has come from the dairy industry and dairying applications. However, in all applications, flux decline due oncentration polarisation and fouling are probably the two most important practical
Chapter 4 Ultrafiltration M. J. LEWIS, Department of Food Science and Technology, The University of Reading, RG6 6AP. 4.1 INTRODUCTION Ultrafiltration offers the opportunity to concentrate large molecular weight components without the application of heat or a change of phase. Such components are rejected by the membrane, whereas the permeate produced will contain the low molecular weight components present in the food, at a concentration similar to that in the feed. This results in an increase in their concentration both on a wet weight and dry weight basis in the solution. It is a pressure-activated process, with pressures in the range of 1-15 bar; these pressures are considerably lower than those used in reverse osmosis. For many heat labile macromolecules, e.g. proteins and starches, concentration by UF at ambient temperature will minimise heat-induced reactions which may adversely influence their functional behaviour in foods. Some important functional properties are solubility, foaming capacity, gelation, emulsification capacity, fat and water binding properties. These are discussed in more detail in Section 4.5. In the case of enzymes or pharmaceutical agents, their biological activity needs to be conserved. It also affords the opportunity to separate small molecular weight components from complex mixtures, containing components with a wide range of molecular weights. There have also been investigations into using UF for protein fractionation, but this is not straightforward due to the diffuse nature of the membranes and their selectivity. UF is also very useful for recovering valuable components from food processing waste streams and fermentation broths. Probably the greatest impetus has come from the dairy industry and dairying applications. However, in all applications, flux decline due to concentration polarisation and fouling are probably the two most important practical aspects
98 M.J. Lewis 4.2 PROCESSING CHARACTERISTICS This section will deal with some of the important processing parameters encountered in ultrafiltration. There are various factors which will influence the outcome of the process, such as the concentration factor and rejection. See Section 3.3 The extent of the concentration is defined by the concentration factor (), defined as VE/V(see eq.(3.5). Usually the permeate is the biggest fraction by volume. Milk for heese making is concentrated by UF fivefold, whereas cheese whey is concentrated twentyfold for the production of protein concentrates. Sometimes the resulting permeates are further concentrated by reverse osmosis 4.2.1 Rejection or retention factors The rejection or retention factor(R)of any component is defined as where ce is the concentration of component in the feed and cp is the concentration in the permeate. The rejection is determined experimentally for each component in the feed, by sampling the feed and permeate at the same time and analysing that component. It is very important and will influence the extent(quality)of the separation achievable Rejection values normally range between 0 and 1; sometimes they are expressed as percentages(0 to 100%) when Cp=0; R=l; all the component is retained in the feed when cp = c R=0; the component is freely permeating In ultrafiltration experiments, some workers have measured negative rejection,i.e Cp>CE, particularly for minerals. It is not immediately obvious why this should have occurred. Possible explanations for this are higher concentrations at the membrane Irface than in the bulk, due to concentration polarisation. However, this is unlikely to be the case for freely permeating species. Another explanation is the basis on which concen- tration is measured (Glover, 1985). This may arise when there is substantial fat in the eed which is rejected by the material. It is suggested that concentrations be expressed in the aqueous portion. A third explanation lies in the Donnan effect; Donnan predicted and later demonstrated that concentration of electrolyte in the solutions on either side of a alysis membrane were unequal when the colloid on one side was electrically charged (see later). For example, at low pH values, where proteins are likely to be positively charged, this could lead to higher concentrations of cations in the permeate membrane for a particular application. Rejection values may also be influenced by operating conditions An idealultrafiltration membrane would have a rejection value of 1.0 for high molecular weight components and zero for low molecular weight components. However, typical values observed for real membranes are between 0.9 and 1.0 for high molecular weights and between 0 and 0. 1 for low molecular weight components. values for
98 M.J.Lewis 4.2 PROCESSING CHARACTERISTICS This section will deal with some of the important processing parameters encountered in ultrafiltration. There are various factors which will influence the outcome of the process, such as the concentration factor and rejection. See Section 3.3. The extent of the concentration is defined by the concentration factor cf), defined as VF/Vc (see eq. (3.5)). Usually the permeate is the biggest fraction by volume. Milk for cheese making is concentrated by UF fivefold, whereas cheese whey is concentrated twentyfold for the production of protein concentrates. Sometimes the resulting permeates are further concentrated by reverse osmosis. 4.2.1 Rejection or retention factors The rejection or retention factor (R) of any component is defined as R = (CF - cp )/cF (4.1) where cF is the concentration of component in the feed and cp is the concentration in the permeate. The rejection is determined experimentally for each component in the feed, by sampling the feed and permeate at the same time and analysing that component. It is very important and will influence the extent (quality) of the separation achievable. Rejection values normally range between 0 and 1; sometimes they are expressed as percentages (0 to 100%). when cp =O; when cp = cF R = 1; all the component is retained in the feed R = 0; the component is freely permeating. In ultrafiltration experiments, some workers have measured negative rejection, Le. cp > cF, particularly for minerals. It is not immediately obvious why this should have occurred. Possible explanations for this are higher concentrations at the membrane surface than in the bulk, due to concentration polarisation. However, this is unlikely to be the case for freely permeating species. Another explanation is the basis on which concentration is measured (Glover, 1985). This may arise when there is substantial fat in the feed which is rejected by the material. It is suggested that concentrations be expressed in the aqueous portion. A third explanation lies in the Donnan effect; Donnan predicted and later demonstrated that concentration of electrolyte in the solutions on either side of a dialysis membrane were unequal when the colloid on one side was electrically charged (see later). For example, at low pH values, where proteins are likely to be positively charged, this could lead to higher concentrations of cations in the permeate. Rejection characteristics can readily be determined for different substances using different membranes. This is one practical way of selecting the most appropriate membrane for a particular application. Rejection values may also be influenced by operating conditions. An ‘ideal’ ultrafiltration membrane would have a rejection value of 1.0 for high molecular weight components and zero for low molecular weight components. However, typical values observed for real membranes are between 0.9 and 1.0 for high molecular weights and between 0 and 0.1 for low molecular weight components. Values for
Ultrafiltration 99 minerals often are usually in the region of 0.1, but may be as high as 0.5, if the mineral binds to macromolecules. It is important to appreciate that any component with a rejection value greater than 0 will increase in concentration during the course of an ultrafiltration process. Rejection values can be used to check the integrity and performance of a membrane. Some values for components in dairy processing are given in Table 4. 1. Note the relatively high values for minerals, which suggests some binding to the proteins, particularly for calcium and magnesium. Membrane manufacturers some times present performance data in terms of rejection values of a range of components of different molecular weights(see Table 4.2). This will give some guidelines in terms of selection. However, very rarely are those components selected that one is interested in An alternative form of representation widely used is the molecular weight cut-off value. Table 4. 1. Rejection characteristics obtained during ultra filtration of dairy products Product Protein Lactose Ash Sweet whey 0.85-1.0 00.2 0-0.5 Ac 0.85-1.0 00.2 Skim milk 0.965-1.0 Whole milk 0.965-0.999 00.03 00.1 a Based on Kjeldahl nitrogen x 6.38 Taken from Lewis(1982) Table 4. 2. Some cited rejection characteristics for different components MW 3000 1000030000 100000 Insulin 600 >0.98 Cytochrome C 12400 045 >0.98 095 0.75 0.20 67000>0.980.98 0.95 Adapted from data from Amicon(1992) The molecular weight cut-off values for UF membranes range betv and 300 000. At values of about 2000, it overlaps with nanofiltration or loose reverse osmosis, whereas at 30 000 it overlaps with microfiltration. Generally the applied pressure required will decrease with increasing cut-off value and pressures in the range 1-15 bar are used It is implied that a membrane with a molecular weight cut-off of 5000, would reject all omponents with that molecular weight value or higher (R= 1)and allow components below that molecular weight to permeate freely. Often dextrins have been used for esti mating molecular weight cut-off, but these are linear molecules. However, due to the
Ultrafiltration 99 minerals often are usually in the region of 0.1, but may be as high as 0.5, if the mineral binds to macromolecules. It is important to appreciate that any component with a rejection value greater than 0 will increase in concentration during the course of an ultrafiltration process. Rejection values can be used to check the integrity and performance of a membrane. Some values for components in dairy processing are given in Table 4.1. Note the relatively high values for minerals, which suggests some binding to the proteins, particularly for calcium and magnesium. Membrane manufacturers sometimes present performance data in terms of rejection values of a range of components of different molecular weights (see Table 4.2). This will give some guidelines in terms of selection. However, very rarely are those components selected that one is interested in. An alternative form of representation widely used is the molecular weight cut-off value. Table 4.1. Rejection characteristics obtained during ultrafiltration of dairy products Product Proteina Lactose Ash Sweet whey 0.85-1.0 0-0. 2 0-0.5 Acid whey 0.85-1.0 0-0.2 0-0.5 Skim milk 0.965-1 .O 0-0.2 0-0.5 Whole milk 0.965-0.999 0-0.03 0-0.1 a Taken from Lewis (1982). Based on Kjeldahl nitrogen x 6.38. Table 4.2. Some cited rejection characteristics for different components MW 3000 10 000 30 000 100 000 Insulin 6 000 >0.98 - - - Cytochrome C 12 400 >0.98 0.85 0.45 - a-Ch ymotrypsinogen 24 500 >0.98 0.95 0.75 0.20 Albumin 67 000 >0.98 >0.98 0.95 0.30 Adapted from data from Amicon (1992). The molecular weight cut-off values for UF membranes range between about 2000 and 300 000. At values of about 2000, it overlaps with nanofiltration or ‘loose reverse osmosis’, whereas at 30 000 it overlaps with microfiltration. Generally the applied pressure required will decrease with increasing cut-off value and pressures in the range 1-15 bar are used. It is implied that a membrane with a molecular weight cut-off of 5000, would reject all components with that molecular weight value or higher (R = 1) and allow components below that molecular weight to permeate freely. Often dextrins have been used for estimating molecular weight cut-off, but these are linear molecules. However, due to the
100 M.J. Lewis diffuse nature of the membrane, this is not so. This approach ignores molecular shape face and within the membrane itself, A nponents in the feed, and at the membrane sur- vertheless it is useful for a preliminary(initial) selection of a suitable membrane. However, it tells you nothing about the rejection value of a component below the molecular weight cut-off, say 500 or 1000. In fact, it rather implies that such components would be freely permeating. In reality, this is not the case as most membranes are diffuse in their separation ability. The concept of a sharp and diffuse membrane is useful in this respect(see data from Table 4.2) Figure 4. I shows the rejection characteristics of two such membranes. The sharp membrane is an ideal situation, offering the perfect separation. Real membranes offe quite diffuse rejection characteristics, requiring a molecular weight difference of about tenfold to provide an effective separation. Therefore they would give a poor separation of components with slight differences in molecular weights, even components with differences up to two times would not necessarily be well separated. For example it ould not be easy to fractionate the proteins in cheese whey or to separate mono saccharides from disaccharides. McGregor (1986)has undertaken some interesting xperiments, using electrophoresis to examine the sharpness of separations performed on mixtures of protein of different molecular weights. His results showed considerable differences in the sharpness of the separation between different membranes with the same nominal molecular weight cut-off value. Gekas et al.(1990)found that experimental flux and rejection data correlated better with porosimetric data(pore size and pore size distri bution as measured by bubble pressure and solvent permeabilities)than molecular weight cut-off value These types of observation illustrate that although some physicochemical measure- ments might be useful, the selection of the best membrane is best done experimentally, by measuring the rejection characteristics of the components to be separated at the selected There is also evidence that the rejection value for most components increases during the course of an ultrafiltration process. Some of the experimental work on rejection measurement and practical problems involved are described in Section 4.2.4 Fig. 4. 1. Characteristics of a sharp and diffuse membrane: Ij, ideal, 10 000 molecular weight cut- off: 12. ideal, 100 000 molecular weight cut-off: S, sharp membrane: D, diffuse mem
1.0 c 0 8 0.5 '6 a: .- c 0 - - -4' I
Ultrafiltration 101 4.2.2 Yield Ultrafiltration is now being used to concentrate and recover some very valuable com- pounds. The yield or recovery of a component is a very important variable, as it will strongly influence the economics of the process. present in the feed, which is retained in the concentrate. For recovery of compollco. a The yield of a component is defined as the fraction of that component, origina mportant to have a high yield. However, when washing out components, such as toxins, the yield should be lov For a batch process, it can be shown that the yield of any component depends upon the concentration factor and rejection Concentration factor and the yield (n is given by Y= mass component in final concentrate=vacc (4.3) mass substance in feed where Vc and Ve are the volumes of feed and concentrate and cc and cF are the concen trations in the concentrate and feed If we now consider a batch concentration process depicted by Fig. 4.2, where permeate is removed and the retentate is recycled At any instance let the volume of the concentrate V and the concentration of the component of interest =c Permeate Fig. 4.2. Batch concentration process for yield calculation Let the removal of a small volume of permeate(dv), result in a change of concentration A mass balance on the component will give the following equation (feed)(concentrate) rmeate Rejection (R)=(c-Cp)
Ultrafiltration 101 4.2.2 Yield Ultrafiltration is now being used to concentrate and recover some very valuable compounds. The yield or recovery of a component is a very important variable, as it will strongly influence the economics of the process. The yield of a component is defined as the fraction of that component, originally present in the feed, which is retained in the concentrate. For recovery of components it is important to have a high yield. However, when washing out components, such as toxins, the yield should be low. For a batch process, it can be shown that the yield of any component depends upon the concentration factor and rejection. Concentration factor cf) = vF/vC (4.2) and the yield (Y) is given by mass component in final concentrate - VCCC Y= -- mass substance in feed VFCF (4.3) where V, and VF are the volumes of feed and concentrate and cc and CF are the concentrations in the concentrate and feed. If we now consider a batch concentration process depicted by Fig. 4.2, where permeate is removed and the retentate is recycled: At any instance let the volume of the concentrate = V and the concentration of the component of interest = c in" f" Permeate CP Fig. 4.2. Batch concentration process for yield calculation. Let the removal of a small volume of permeate (dV), result in a change of concentration (dc) * A mass balance on the component will give the following equation: VC = (V-dV)(c-dc) + cpdV (feed) (concentrate) (permeate) Rejection (R) = (c - cp )/c
102 M.J. Lewis Thus Vc=(V-dv)(c-dc)+c(l-r)dv (Note: d vdc is assumed to be negligible.) Integration between the final and initial conditions gives In(VFVc)=l/R In(cc/cF) If In(Va/Vc) is plotted against In(cc/ce), the gradient is 1/R vE/vc=f=(cc/cF) (4.5) From eqs.(4.2)and(4.3), it can be shown that Substitution into eq (4.5)gives VE/Vc =(f 1/R Therefore =(y/R. This simplifies to Y=fR-I. Therefore the yield r=f <s However, this equation applies only if the rejection remains constant. Nevertheless,it extremely useful, as it gives an insight into the features of the separation process, Let us consider the two extreme values of rejection IfR= l, then yield 1; all the material is recovered in the concentrate i.e. it is not possible to remove all of a component from a feed by ultrafiltration alone Diafiltration may be more useful in helping to achieve this objective(see Section 4.4). However, for most components being concentrated, the rejection values are close to 1.0, typically 0.9-1.0, whereas for those being removed the values would be between 0 Table 4.3 shows a range of yield values for some different concentration factors. One interesting point is that losses can be quite high, even though the rejection value appears good; e.g. for R=0.95 and a concentration factor of 20, the yield is 0.86. Therefore 14%0
102 M. J. Lewis Thus cp=c(l -R) Eliminating cp gives VC= (V-dV) (c-dc)+c(l-R)dV - Vdc = cR dV (Note: dVdc is assumed to be negligible.) dV cc dc 1 - -- I v -IcF -zi Integration between the final and initial conditions gives: In (vF/vC)=l/R In (cC/cF) (4.4) vF/Vc = f = (cC/cF)l'R If In (VF/Vc) is plotted against In (cc/cF), the gradient is 1/R. (4.5) From eqs. (4.2) and (4.3), it can be shown that CC/CF = yf Substitution into eq. (4.5) gives vF/vC =(fY)'lR Therefore f = (fY)'IR. This simplifies to Y = f '-'. Therefore the yield y= fR-1 (4.6) However, this equation applies only if the rejection remains constant. Nevertheless, it is extremely useful, as it gives an insight into the features of the separation process. Let us consider the two extreme values of rejection: If R = 1, then yield = 1; all the material is recovered in the concentrate. If R = 0, then yield = l/J in this case the yield is determined by the concentration factor. As the concentration factor is finite (typically 2-20), the yield can never be zero; i.e. it is not possible to remove all of a component from a feed by ultrafiltration alone. Diafiltration may be more useful in helping to achieve this objective (see Section 4.4). However, for most components being concentrated, the rejection values are close to 1.0, typically 0.9-1.0, whereas for those being removed the values would be between 0 and 0.1, Table 4.3 shows a range of yield values for some different concentration factors. One interesting point is that losses can be quite high, even though the rejection value appears good; e.g. for R = 0.95 and a concentration factor of 20, the yield is 0.86. Therefore 14%
Ultrafiltration 103 Table 4.3. Yield values for different concentration factors and rejections Concentration factor Re ection 0 0.1 0.2 0.5 090951.00 0.500.540.570.710930.971.0 0200.240.28 45 850921.0 0.100.1 0.160.32 0.050.0 0.090.22 861.0 0.020.030.040.140.680.821.0 of the component is lost in the permeate. Yield values are also sometimes quoted as percentage However, this equation gives the maximum yield, which would be for a batch process The yield is likely to be lower for a continuous single or multistage process, simply because steady state is achieved at higher levels of concentration. For such a process th ield is given by ∫-R(f-1) The concentration of a component in the final resulting concentrate(cc)can be calculated from the following equation Cc=cFF However, there is some evidence that rejection does not remain constant During a batch ultrafiltration experiment the rejection of most components rises, as has been observed on many occasions(see Fig 4.3) 4.2.3 Average rejectic situations where the rejection does change significantly, an alternative evaluation procedure is to measure the yield for the process, and then to work backwards to calculate the rejection value, which would have given rise to that yield. This rejection value is termed the average rejection value(Rav C CC ce f If this expression for yield is equated with that from eq (4.6)
Ultrafiltration 103 Table 4.3. Yield values for different concentration factors and rejections Concentration factor Rejection 0 0.1 0.2 0.5 0.9 0.95 1.00 2 0.50 0.54 0.57 0.71 0.93 0.97 1.0 5 0.20 0.24 0.28 0.45 0.85 0.92 1.0 10 0.10 0.13 0.16 0.32 0.79 0.89 1.0 20 0.05 0.07 0.09 0.22 0.74 0.86 1.0 50 0.02 0.03 0.04 0.14 0.68 0.82 1.0 of the component is lost in the permeate. Yield values are also sometimes quoted as percentages. However, this equation gives the maximum yield, which would be for a batch process. The yield is likely to be lower for a continuous single or multistage process, simply because steady state is achieved at higher levels of concentration. For such a process the yield is given by 1 Y= f - R(f -1) The concentration of a component in the final resulting concentrate (CC) can be calculated from the following equation: CC = CF Yf (4.7) cc=cF fR (4.8) or However, there is some evidence that rejection does not remain constant. During a batch ultrafiltration experiment the rejection of most components rises, as has been observed on many occasions (see Fig. 4.3). 4.2.3 Average rejection In situations where the rejection does change significantly, an alternative evaluation procedure is to measure the yield for the process, and then to work backwards to calculate the rejection value, which would have given rise to that yield. This rejection value is termed the average rejection value (Rav) (4.9) 1 y - vccc - cc VFCF CF f If this expression for yield is equated with that from eq. (4.6):
104 M.J. Lewis 04 叶2456}。。 Concentration factor Fig. 4. 3. Change in rejection during UF:(a)glucosinolates; ( b)total solids; (c)protein f= Rlogf=log(cc/cF R= log(cc/cF)/logf This expression for the rejection is effectively an average rejection(Ray) for the process Therefore Rav =log(cc/cF (4.11) Estimation of average rejection is based upon knowing the initial and final concentrations and the concentration factor It is interesting to note that, in this case, the membrane rejection can be determined without sampling the permeate herefore the average rejection is defined as the rejection value which would provide the same yield which was actually found in the process, even though the instantaneou rejection may have been changing throughout 4.2.4 Practical rejection data Although some of the proposed models predict how rejection will be influenced by operating conditions and pH, there is often little agreement between theory and practice
0.4 0.2 0 - - IIIIIIIII
Ultrafiltration 105 for most food systems. Therefore it is very important to measure rejection data under the prevailing operating conditions Lewis(1982) has compiled rejection data for different systems. It was not al ways clear some confusion between the terms rejection and yield in some of the earlier repord R whether rejection data for proteins was based upon crude protein or true protein. U filtration could be useful for removing non-protein nitrogen. There also appeared Table 4. 1 shows some rejection data for some dairy products, reported by the Interna tional Dairy Federation (1979) Figure 4.3 shows rejection data taken during the batch ultrafiltration process during concentration of rapeseed meal, for crude protein, total solids and glucosinolates. For all components, there is an increase in rejection as concentration proceeds, with the increase being most marked between concentration factors of 1 and 2. Many investigators have reported similar increases in rejection as concentration proceeds Table 4.4 shows some data for the average rejection data for proteins and glucosinolate, extracted at different pH values, determined by the method above, Yield alues are also presented. In such complex systems the performance is also strongly affected by pH(see Sections 4.3.2 and 4.5.2) Table 4.4. Average rejection(Ray) and yield values for glucosinolates and crude protein during batch ultra filtration processes at different pH values Glucosinolate Crude protein 2.5 0.5000.45) 0.970.95) 0.39(0.38) 0.93(0.89) 7.0 0.28(0.31) 081(0.74) 9.0 0.36(0.36) 0.95(0.92) 11.0 0.44(041) 0.85(0.92) Yield values in brackets Glucosinolates are expressed as isothiocyanates Therefore on values are very important as they influence the nature of the separation obtained, as well as the yield (or loss)of components. These aspects assume greater importance as the value of the product increases. Changes in rejection during process could also be indicative of some important changes taking place at the surface of the membrane. The effects of pressure and temperature on rejection, as predicted by some f the models, are discussed in Chapter 3. Some practical problems associated with UF of proteins, such as adsorption and pH effects, are described by Sirkar and Prasad(1987) 4.3 PERFORMANCE OF ULTRAFILTRATION SYSTEMS Permeate flux In UF process applications, the two most important parameters are the membrane
Ultrafiltration 105 for most food systems. Therefore it is very important to measure rejection data under the prevailing operating conditions. Lewis (1982) has compiled rejection data for different systems. It was not always clear whether rejection data for proteins was based upon crude protein or true protein. Ultrafiltration could be useful for removing non-protein nitrogen. There also appeared to be some confusion between the terms rejection and yield in some of the earlier reports. Table 4.1 shows some rejection data for some dairy products, reported by the International Dairy Federation (1979). Figure 4.3 shows rejection data taken during the batch ultrafiltration process during concentration of rapeseed meal, for crude protein, total solids and glucosinolates. For all components, there is an increase in rejection as concentration proceeds, with the increase being most marked between concentration factors of 1 and 2. Many investigators have reported similar increases in rejection as concentration proceeds. Table 4.4 shows some data for the average rejection data for proteins and glucosinolate, extracted at different pH values, determined by the method above. Yield values are also presented. In such complex systems the performance is also strongly affected by pH (see Sections 4.3.2 and 4.5.2). Table 4.4. Average rejection (Rav) and yield values for glucosinolates and crude protein during batch ultrafiltration processes at different pH values. PH Glucosinolate Crude protein 2.5 0.50 (0.45) 0.97 (0.95) 3.5 0.39 (0.38) 0.93 (0.89) 7.0 0.28 (0.31) 0.81 (0.74) 9.0 0.36 (0.36) 0.95 (0.92) 11.0 0.44 (0.41) 0.85 (0.92) Yield values in brackets. Glucosinolates are expressed as isothiocyanates. Therefore, rejection values are very impofiant as they influence the nature of the separation obtained, as well as the yield (or loss) of components. These aspects assume greater importance as the value of the product increases. Changes in rejection during a process could also be indicative of some important changes taking place at the surface of the membrane. The effects of pressure and temperature on rejection, as predicted by some of the models, are discussed in Chapter 3. Some practical problems associated with UF of proteins, such as adsorption and pH effects, are described by Sirkar and Prasad (1987). 4.3 PEKFORMANCE OF ULTRAFILTRATION SYSTEMS Permeate flux In UF process applications, the two most important parameters are the membrane
106 M.J. Lewis ejection(see also Chapter 3)and the flow rate of permeate or permeate flux, hereafter abbreviated to'flux'. The flux will probably be measured in gallons/ min or litres/ hour, but it is usually presented in terms of volume per unit time per unit area(I m h -) Expressed this way it allows a ready comparison of the performance of different mem brane configurations with different surface areas. Flux values may be as low as 5 or as high as 450 I m-h. The flux is one of the major factors influencing the viability of many processe UF processes have been subject to a number of modelling processes, in an attempt to predict flux rates and rejection values from the physical properties of the solution, the membrane characteristics and the hydrodynamics of the flow situation, in order to opti mise the performance of the process 4.3.1 Transport phenomena and concentration polarisation Ultrafiltration is usually regarded as a sieving process and in this sense the mechanisms are simpler than for RO. However, it is important to remember that for pressure-driven membrane processes, 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 oundary layer) 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, and a concentration gradient over the boundary layer. This increase in concentra tion offers a very significant additional resistance, and for macromolecules may also give se to the formation of a gelled or fouling layer on the surface of the membrane(see Fig 3.4). It is interesting to note that the boundary layer does not establish itself immediately at the point where the fluid first contacts the membrane. Rather it takes some distance fo it to be fully established. This distance taken for it to be fully established has been defined as the entry length, and the process of establishment is illustrated for a tubular membrane in Fig. 4.4. Howell et al.( 1990)have analysed flux conditions over the entry length and have concluded that the flux and wall concentrations change quite consider ably over the developing boundary layer, although changes were less marked for a fouled membrane. There would also be less likelihood of operating in the pressure-independent Membrane d Retentate Boundary layer Membrane Permeate Fig. 4.4. Development of the concentration polarisation or boundary layer
106 M. J. Lewis rejection (see also Chapter 3) and the flow rate of permeate or permeate flux, hereafter abbreviated to 'flux'. The flux will probably be measured in gallons/min or litres/hour, but it is usually presented in terms of volume per unit time per unit area (1 m-2 h-l). Expressed this way it allows a ready comparison of the performance of different membrane configurations with different surface areas. Flux values may be as low as 5 or as high as 450 1 m-* h-'. The flux is one of the major factors influencing the viability of many processes. UF processes have been subject to a number of modelling processes, in an attempt to predict flux rates and rejection values 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 process. 4.3.1 Transport phenomena and concentration polarisation Ultrafiltration is usually regarded as a sieving process and in this sense the mechanisms are simpler than for RO. However, it is important to remember that for pressure-driven membrane processes, 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.) 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, and a concentration gradient over the boundary layer. This increase in concentration offers a very significant additional resistance, and for macromolecules may also give rise to the formation of a gelled or fouling layer on the surface of the membrane (see Fig. 3.4). It is interesting to note that the boundary layer does not establish itself immediately at the point where the fluid first contacts the membrane. Rather it takes some distance for it to be fully established. This distance taken for it to be fully established has been defined as the entry length, and the process of establishment is illustrated for a tubular membrane in Fig. 4.4. Howell et al. (1990) have analysed flux conditions over the entry length and have concluded that the flux and wall concentrations change quite considerably over the developing boundary layer, although changes were less marked for a fouled membrane. There would also be less likelihood of operating in the pressure-independent Permeate Membrane Feed inlet --+- Retentate Boundary layer Membrane Permeate Fig. 4 4. Development of the concentration polansation or boundary layer