The role of concentration polarization in ultrafiltration of ovine cheese whey

Journal of Membrane Science 381 (2011) 34–40

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The role of concentration polarization in ultrafiltration of ovine cheese whey Antonia Macedo a,∗ , Elizabeth Duarte b , Maria Pinho c a

Polytechnic Institute of Beja, Escola Superior Agrária, Department of Technology and Applied Sciences, R. Pedro Soares, Ap. 6158, 7801-908 Beja, Portugal Technical University of Lisbon, Instituto Superior de Agronomia, Department of Agricultural and Environmental Chemistry, Tapada da Ajuda, 1349-017 Lisboa, Portugal c Technical University of Lisbon, Instituto Superior Técnico, ICEMS and Department of Chemical and Biological Engineering, Av. Rovisco Pais, 1049-001 Lisboa, Portugal b

a r t i c l e

i n f o

Article history: Received 18 April 2011 Received in revised form 2 July 2011 Accepted 5 July 2011 Available online 12 July 2011 Keywords: Ovine cheese whey Ultrafiltration Concentration polarization Fouling

a b s t r a c t Ultrafiltration of pretreated whey is assessed in terms of permeation patterns for the membranes M1 (ETNA10PP), M2 (GR61PP) and M3 (GR81PP). Permeation experiments were carried out at different transmembrane pressures and cross-flow velocities, for studying the influence of these operating conditions on permeate fluxes. The modeling of ultrafiltration was performed in terms of permeate fluxes adjusting the resistances-in-series model. The results showed that the M1 allows higher permeate fluxes and displays the conventional pattern of linear variation of permeate fluxes with transmembrane pressure for lower pressures. With membranes M2 there is a linear variation of permeate fluxes with transmembrane pressure in all the pressure range and for the three feed circulation velocities studied. The membrane M3 has the lowest permeate fluxes and shows for the two higher feed circulation velocities (0.94 m/s and 1.23 m/s) a linear variation of permeate fluxes with transmembrane pressure, in all the pressure range studied. The modeling of ultrafiltration allowed to conclude that with the membranes M1, the concentration polarization controls the mass transfer in all the pressure range studied, whereas the resistance due to fouling is the main contributor for lowering the permeate fluxes, with the membranes M2 and M3. Once the membranes M1 allow higher permeate fluxes and better selectivity with a negligible contribution of fouling, in the experimental conditions used, they are more suitable for the ultrafiltration of pretreated ovine cheese whey. © 2011 Elsevier B.V. All rights reserved.

1. Introduction Whey is a sub-product from the processing of milk into cheese or into casein. In a dry basis, it contains 70–80% of lactose, 9% of proteins, corresponding to 20% of all milk proteins, and 8–20% of minerals [1]. Other minor components exist, such as peptides of k-casein hydrolysates, lipids and bacteria [1,2]. In contrast with the vast literature on ultrafiltration and nanofiltration of bovine cheese whey [3–10], there is very scarce literature on the processing of ovine cheese whey. The fact that in the ovine whey the ratio total nitrogen/dry matter is much higher and that the content of soluble protein is about the double of the one existing in bovine whey [11], contributed to a growing interest in this product [12–14]. Southern European countries are major producers of ovine cheeses and this production is normally associated with Protected Geographical Indication labels. In some countries, such as Italy, Spain and Portugal, part of the whey from production of cheese is further processed to obtain whey cheeses, designated as Ricotta,

∗ Corresponding author. Tel.: +347 284328603. E-mail addresses: [email protected], [email protected] (A. Macedo), [email protected] (E. Duarte), [email protected] (M. Pinho). 0376-7388/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2011.07.012

Requesón and Requeijão, respectively. However, not all of the whey can be processed due to the high volumes generated and this creates problems in conventional water treatment plants. Moreover, the “deproteinized” whey resulting from the production of Requeijão still leads to the production of a “deproteinized” whey called Sorelho, which contains more than 50% of the original dry matter of the whey [14,15] and when released directly into drains it causes problems in water treatment plants. The ultrafiltration of bovine cheese whey was in the early seventies [16,17] successfully used in the production of whey protein concentrates (WPC) with different degrees of purity (roughly between 35 and 95%), depending on the pretreatment and on the further use of diafiltration [18]. Nevertheless, ultrafiltration of whey is mainly limited by concentration polarization and fouling [19,20], the phenomena responsible for the decrease of permeate fluxes and by changing the selectivity of the process. The main components involved in membrane fouling by dairy fluids are proteins and minerals, especially calcium and phosphorous, in the form of calcium phosphate [20,21]. In the sweet bovine whey, ␣-lactalbumin appears to be the main cause of declining of permeate fluxes in the short term, while ␤-lactoglobulin appears to be responsible for that at the long term [21]. Besides these, other components that can cause fouling in ultrafiltration

A. Macedo et al. / Journal of Membrane Science 381 (2011) 34–40

membranes are residues from previous processing, as particulate matter (casein fines), residual fat (phospholipids), products resulting from proteolysis of k-casein (casein macropeptides), enzymes (rennet or chymosin), microorganisms resulting from the initial culture and soluble caseins [22]. Several authors have described the physical–chemical aspects of membrane fouling by dairy fluids, as well as methods to minimize this phenomenon. Among this are: pretreatment of the feed [22], optimization of hydrodynamics in the flow channels of the feed and adjustment of process conditions [23], surface modification of membranes/appropriate selection of membrane material [24] and selection of the most suitable filtration system [19,25]. The present work investigates the ultrafiltration permeation performance of ovine whey through: (1) assessment of membrane permeation characteristics; (2) modeling of ultrafiltration permeation; (3) determination of optimal operating conditions of transmembrane pressure and feed circulation velocity. 2. Experimental 2.1. Ovine whey Ovine whey was collected at “Ovelheira, Casa Agrícola de la Féria, Lda”, a Portuguese cheese and “Requeijão” factory, located at the Protected Geographical Region of Serpa cheese. Immediately after reception, ovine whey was filtered and afterwards skimmed by means of a Westfalia separator. Samples of the initial cheese whey and pretreated cheese whey were object of the following determinations: pH (Metrohm pH meter); specific conductivity (Methrom AG CH-9100 Herisau); viscosity (Viscotester VT 550); total solids, according to the gravimetric AOAC procedure [26]; total suspended solids [27]; protein by the Kjeldahl method; lactose, according to Munson and Walker process; fat content in the whey was determined using Gerber’s butyrometric determination and in the pretreated whey by the Soxhlet extraction method [27], sodium and potassium by flame photometry (Corning photometer 410), calcium and magnesium by atomic absorption spectrometry (Thermo Jarrell Ash); chloride by Charpentier–Volhard method and phosphate, by vanadomolibdophosphoric acid method [27]. Table 2 presents the values of these parameters for cheese whey and pretreated cheese whey. 2.2. Membranes and filtration unit Three commercial ultrafiltration membranes M1 (ETNA10PP), M2 (GR61PP) and M3 (GR81PP) supplied by Alfa Laval Denmark were tested. In order to eliminate fluctuations and assure reproducibility in the permeation essays, the membranes were subjected to compaction through permeation of pure water (<1 ␮S/cm) at 7.3E + 5 Pa for 3 h. The pure water permeability of the three membranes was determined. The permeation flux of pure water through a membrane, Jw , is given by the Eq. (1): Jw =

Lp P P =  Rm

(1)

where Lp (m) is the hydraulic permeability of the membrane, which only depends on its morphological characteristics;  (Pa s) is the viscosity of pure water; P (Pa) is the applied transmembrane pressure and Rm (m−1 ), the membrane resistance. The hydraulic permeability of the membranes is the slope of the straight line yielding the variation of the permeate fluxes at various transmembrane pressures in the range 1.0 × 105 Pa to 6.0 × 105 Pa, at a feed circulation velocity of 0.94 m/s and a temperature of 25 ◦ C. The membrane resistance is the inverse of the hydraulic permeability.

35

Table 1 Membrane cleaning and disinfection. Operation

Pressure (bar) Cleaning Temperature pHa NaOH (% w/v) Na-EDTA (% w/v) HNO3 (% w/v) Citric acid (% w/v) Disinfection H2 O2 (mg/L), a 25 ◦ C

Membrane

Time (min)

M3;M2

M1

1

1

50 1–13 0.40 0.50 0.25 0.50

50 1–11.5 0.05 0.20 0.25 0.50

15 15 15 15

1000

1000

30

a

Since pH-limits dominate the concentrations were adjusted to the right pH value.

All the permeation experiments were carried out in a plate & frame filtration unit, Lab Unit M20 from Alfa Laval (Denmark, Nakskov), with a total membrane surface area of 0.072 m2 , described in [28]. A schematic illustration of the experimental apparatus is displayed in Fig. 1. 2.3. Permeation experiments in total recirculation mode The ultrafiltration experiments of the ovine whey were performed in total recirculation mode for the M1, M2 and M3 membranes with transmembrane pressure ranging from 0.6 × 105 Pa to 6 × 105 Pa and the cross-flow velocities of 0.47, 0.94 and 1.23 m/s. The Reynolds numbers corresponding to these feed circulation velocities are 341, 682 and 893, respectively, and they were computed through the definition of the Reynolds number (Re = (vdh )/) where  is the specific mass of the pretreated whey, v is the tangential mean feed velocity, dh is the hydraulic diameter of the feed channel, calculated as four times the ratio of the straight section and wetted perimeter [29] and  is the dynamic viscosity of the pretreated whey. The temperature was kept at 25 ◦ C. During the experiments, samples of concentrates and permeates were taken for analysis of total protein and lactose and determination of selectivity in terms of apparent rejections. The apparent rejection coefficient, R, is defined as R = (Cb − Cp )/Cb , where Cb and Cp are the concentrations of solute in the bulk concentrate and permeate, respectively. After the permeation experiments with the pretreated cheese whey the membranes were subjected to a process of cleaning and disinfection (CIP) to restore the initial permeation water fluxes, which was conducted according to the manufacturer’s instructions. In Table 1, the cleaning and disinfection procedure is shown. 3. Theory Pressure-driven membrane processes and namely ultrafiltration are governed by mass transfer mechanisms that are very distinct whether one deals with the membrane phase or the fluid phase adjacent to the membrane. To overcome the complexity of this mechanistic approach, the resistance-in-series model is used. The variation of the ultrafiltration permeation flux, J, as a function of the transmembrane pressure is then given by: J=

P (Rm + Rcp + Rf )

(2)

where  is the dynamic viscosity of the permeate; Rm is the membrane resistance given by Eq. (1), Rcp is the resistance due to concentration polarization and Rf is the fouling resistance. Concentration polarization resistance arises due to solute retention and formation of a layer at the membrane interface with a

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A. Macedo et al. / Journal of Membrane Science 381 (2011) 34–40

Fig. 1. Schematic illustration of Lab-unit M20 [28]. (1) Tank, (2) valve, (3) filter, (4) cross-flow pump, (5) safety valve, (6) heat exchanger, (7) dampener, (8) release valve, (9) hydraulic oil pump, (10) cross-flow pump control, (11) module, (12) pressure gauge and (13) pressure control valve.

high concentration of solute. When the feed solute concentration is kept constant (in total recirculation mode experiments) and for a feed circulation velocity and temperature fixed, Rcp depends on the applied transmembrane pressure. Membrane fouling is a complex phenomenon that depends on the membrane material properties, solute properties and operating parameters. It can occur on the top surface of the membrane (external fouling) due to adsorption or accumulation of solutes that do not enter the pores or inside them (internal fouling). Since in cheese whey the molecular weights of the main whey proteins, ␤-lactoglobulin and ␣-lactalbumin are 36.6 kDa (dimer form) and 14.2 kDa respectively, and the molecular weight cut-off of the membranes used in this study are 10 kDa and 20 kDa it is expected that there is both external and internal fouling. If the following assumptions are taken: (i) Rcp varies linearly with pressure, Rcp = P, in the region where the permeate fluxes vary linearly with the transmembrane pressure; (ii) Rf is constant, which is valid when the blockage is caused by specific membrane–solute interactions, that are little affected by operating parameters [19]; Then, Eq. (2) can be rewritten as: J=

P (Rm + P + Rf )

(3)

This equation explicit an asymptotic behavior of permeate flux as a function of pressure and can be linearized as follows: 1 1 = (Rm + Rf ) +  J P

(4)

Eq. (4) expresses a linear relationship between the reciprocals of permeate fluxes and the reciprocals of pressure. The slope of the straight lines is (Rm + Rf ) and the intercept . In the range of pressure where exists a linear relationship like this, we can determine Rf

from the slope and  from the intercept. The viscosity of the permeate is determined by the Einstein equation for dilute solution with volume fractions of solids less than 0.40 [32]. This volume fraction was calculated in relation to the concentration of lactose, the main solute of the permeate. 4. Results and discussion 4.1. Characterization of the ovine cheese whey and membranes The filtration pretreatment of the ovine cheese whey yields the results shown in Table 2. As can be seen in Table 2, the pretreatment allowed the removal of approximately 99% of the fat and 87% of the total suspended solids. Table 2 Cheese whey and pretreated cheese whey characterization. Parameter

pH Specific conductivity (mS/cm) Viscosity (mPa s) Total solids (kg/m3 ) Total suspended solids (kg/m3 ) Lactose (kg/m3 ) Protein (kg/m3 ) Fat (kg/m3 ) Sodium (kg/m3 ) Potassium (kg/m3 ) Calcium (kg/m3 ) Magnesium (kg/m3 ) Chloride (kg/m3 ) Phosphate (kg/m3 )

Value Cheese whey

Pretreated cheese whey

5.62 20.9 2.0 108.3 30.2 52.0 17.7 20.8 7.1 1.0 0.5 0.1 7.4 1.4

5.58 21.0 1.4 87.3 4.0 52.1 17.1 0.2 7.1 1.0 0.5 0.1 7.5 1.5

The ultrafiltration experiments were carried out with membranes characterized through the hydraulic permeability, as shown in Table 3. The material of the membranes and their molecular weight cut-off, provided by the manufacturer, are also presented.

A. Macedo et al. / Journal of Membrane Science 381 (2011) 34–40

37

Table 3 Characteristics of M1, M2 and M3 membranes. Membrane

M1

M2

M3

Material Molecular weight cut-off (kDa) Pure water hydraulic permeability, Lp (m)

Composite fluoro polymer 10 8.73 × 10−14

Polysulphone 20 7.88 × 10−14

Polyethersulphone 10 4.01 × 10−14

Fig. 2. Variation of permeate fluxes with transmembrane pressure for the membranes M1, at three feed velocities 0.47 m/s (Re = 341), 0.94 m/s (Re = 682) and 1.23 m/s (Re = 893) and T = 25 ◦ C.

The results in Table 3, show that the pure water permeability of the membranes increases along the sequence Lp (M3)
Fig. 3. Variation of permeate fluxes with transmembrane pressure for the membranes M2, at three feed velocities 0.47 m/s (Re = 341), 0.94 m/s (Re = 682) and 1.23 m/s (Re = 893) and T = 25 ◦ C.

Fig. 2 displays for the M1 membrane and for the three feed circulation velocities a typical variation of the UF permeate fluxes versus the transmembrane pressure difference. It shows a linear variation for the low pressure range, below 0.14–0.16 MPa, a transition region and then a constant limiting flux for the higher pressure range, above 0.20–0.30 MPa. These limiting fluxes increase with increasing feed circulation velocity. On the other hand, in the region of linear variation of the permeate fluxes, low pressure region, they are practically independent of the feed circulation velocity and they are close to the pure water permeate fluxes (Jw ). In Fig. 3, relative to membranes M2, a linear behavior of permeate fluxes versus pressure is observed for the three feed circulation velocities. The increase in feed velocity has little influence on the permeate fluxes. At the feed circulation velocities of 0.94 m/s and 1.23 m/s they are even fitted by the same straight line. Permeate flows are similar and so they were jointly represented in Fig. 3. The permeate fluxes obtained with the cheese whey are always far lower than those of pure water fluxes. These results suggest that there was initially a rapid fouling of the membrane that caused the lowering of permeate fluxes, which can be later confirmed by the high value of resistance to fouling (Table 5). Fig. 4 shows the variation of permeate fluxes with pressure for membranes M3. For the lower feed circulation velocity, 0.47 m/s, a linear variation occurs up to 0.30 MPa, then there is a transition region and a constant limiting flux of 14.32 L/h m2 is reached for the higher pressures of 0.50–0.60 MPa. For the two higher feed circulation velocities a linear variation exists in all the pressure range studied. For these membranes, the permeate fluxes are always lower than the corresponding pure water fluxes, which suggests that a rapid fouling of the membrane has occurred initially. This behavior is in agreement with the high values obtained for Rf , as shown in Table 6. The highest permeate fluxes were obtained with the membranes M1 and the lowest ones for the membranes M3, throughout the studied range of pressures and for the three velocities. For all the

Fig. 4. Variation of permeate fluxes with transmembrane pressure for the membranes M3, at the three feed circulation velocities 0.47 m/s (Re = 341), 0.94 m/s (Re = 682) and 1.23 m/s (Re = 893) and T = 25 ◦ C.

38

A. Macedo et al. / Journal of Membrane Science 381 (2011) 34–40

Fig. 5. Variation of the inverse of the permeate fluxes with reciprocals of transmembrane pressure obtained with membranes M1: experimental results (symbols) and regression lines.

membranes, the higher cross-flow velocities of 0.94 and 1.23 m/s allowed an increase of the permeate fluxes. This is certainly due to the reduction of the boundary layers and thicknesses. The experimental data relative to membrane M1 (Fig. 2) are represented as 1/J versus 1/P in Fig. 5 and fitted by straight lines (Eq. (4)). This yields the values of Rf and  and therefore Rcp that are shown in Table 4. Table 4 shows that in the linear zone the fouling resistance is negligible at the three feed circulation velocities tested. The resistance due to concentration polarization can be calculated from Rcp = P and it decreases with the increasing feed velocity, due to the thinning of the boundary layer near the membrane, in accordance to film theory. For the same reason the limiting flux increases with the increase of the feed circulation velocity. So it appears that, in the case of membranes M1, the phenomenon of concentration polarization plays an important role as far as the resistance to mass transfer goes. The straight lines, 1/J versus 1/P, relative to membranes M2 are shown in Fig. 6.

Fig. 6. Variation of the inverse of the permeate fluxes with reciprocals of transmembrane pressure obtained with membranes M2: experimental results (symbols) and regression lines.

Fig. 7. Variation of the inverse of the permeate fluxes with reciprocals of transmembrane pressure obtained with membranes M3: experimental results (symbols) and regression lines.

Table 5 shows that the fouling resistance is the main resistance to mass transfer and it is always higher than the intrinsic membrane resistance Rm . This may be due to a narrowing of the pores and/or internal pore fouling caused by adsorption of the whey proteins since these membranes have a cut-off of 20 kDa. Meireles and others observed a narrowing of the membrane pores due to the adsorption of ␤-lactoglobulin and ␣-lactalbumin in permeation tests carried out with polysulphone membranes of 20 kDa, as the membranes M2 [24]. Fouling resistance decreases with increasing feed circulation velocity due to the reduction of the hydraulic resistance of the fouling layer. The concentration polarization resistance is lower than Rm and has a minor contribution to the lowering of permeate fluxes. The straight lines, 1/J versus 1/P, relative to membranes M3 are shown in Fig. 7. Table 6 shows that for membranes M3, the resistance due to fouling is the main resistance to mass transfer and this is always higher than the intrinsic resistance of the membrane. The resistance is halved with the increase of feed velocity and this plays a major role in the control of membrane fouling. The resistance due to concentration polarization is smaller than Rm and it is more important for the combination of high velocities and pressures due to the higher permeate fluxes and solutes accumulation in the boundary layer. These results show that, under the experimental conditions used, M1 membranes are less susceptible to fouling. It is likely that this different behavior may be related to the membrane material, since M1 membranes have a more hydrophilic character, because they are chemically modified by bonding a polysaccharide derivative to a microporous substrate to reduce protein adsorption [30]. There are two asymptotic behaviors for the low and high pressure regions: (i) a linear variation of flux with pressure as the pressure tends to zero; (ii) a region of constant flux, that is reached for the higher pressures. Churchill and Usagi [31] proposed a methodology to yield a simple expression in the all pressure range. These are shown in Table 7 and they are represented graphically in Fig. 2, where the lines present the curve fitting obtained according to the method proposed in [31]. With regard to membranes M3 and at the lowest feed circulation velocity, 0.47 m/s, it can be observed also a trend towards a similar asymptotic behavior, although much less pronounced, in the range of pressures studied. In this case, the use of the same methodol-

A. Macedo et al. / Journal of Membrane Science 381 (2011) 34–40

39

Table 4 Equations of 1/J versus 1/P in the linear region (with correlation coefficients  and the level of significance p for a confidence interval of 95%) and plateau region, values of Rf and  at three feed velocities, for membranes M1 (Rm = 1.15 × 1013 m−1 ). Rf (m−1 )

 (m−1 MPa−1 )

Plateau region

0.47

1 1 = 3.25 × 10−3 × + 4.60 × 10−3 J P

Negligible

–

J = 35.42

0.94

 = 0.974; p = 9.16 × 10−6 P ≤ 0.14 MPa 1 1 = 1.78 × 10−3 × + 1.09 × 10−2 J P

Negligible

3.77 × 1013

J = 55.89

1.23

 = 0.975; p = 7.11 × 10−8 P ≤ 0.20 MPa 1 1 = 1.99 × 10−3 × + 9.07 × 10−3 J P

Negligible

3.14 × 1013

J = 58.86

Feed circulation velocity v, m/s

Equations of the linear regression 1/J versus 1/P

 = 0.981; p = 1.82 × 10−8 P ≤ 0.20 MPa

Table 5 Equations of 1/J versus 1/P, values of Rf and  at three velocities for membranes M2 (Rm = 1.27 × 1013 m−1 ). Feed circulation velocity v, m/s

Equations of the linear regression 1/J versus 1/P over the full range of transmembrane pressures

0.47

1 1 + 7.26 × 10−3 = 1.21 × 10−2 × J P

0.94; 1.23

 = 0.999; p < 2.2 × 10−16 1 1 = 1.04 × 10−2 × + 5.23 × 10−3 J P  = 0.999; p < 2.2 × 10−16

Rf (m−1 )

 (m−1 MPa−1 )

2.92 × 1013

2.51 × 1013

2.34 × 1013

1.81 × 1013

Table 6 Equations of 1/J versus 1/P in the linear region and horizontal lines in the plateau region, values of Rf and  at three feed velocities, for membranes M3 (Rm = 2.49 × 1013 m−1 ). Feed circulation velocity v, m/s

Linear region equations of the linear regression 1/J versus 1/P 1 1 = 2.51 × 10−2 × + 7.40 × 10−3 J P

0.47

 = 0.994; p = 4.82 × 10−8 P ≤ 0.30 MPa 1 1 + 2.05 × 10−2 = 1.53x10−2 × J P

0.94; 1.23

 = 0.993; p < 2.2 × 10−16 (full range of transmembrane pressures)

Rf (m−1 )

 (m−1 MPa−1)

Plateau region

6.18 × 1013

2.56 × 1013

J = 14.32 P > 0.40 MPa

2.89 × 1013

7.10 × 1013

–

Table 7 Equations of the asymptotes for the low and high pressures ranges and equations in the all pressure range [31], for three feed circulation velocities, with the membranes M1. v (m/s)

Equations of the asymptotes

Equations for the all pressure range

P 3.25×10−3 +4.60×10−3 P

0.47

J=

0.94

P ≤ 0.10 MPa J = 35.42 P ≥ 0.20MPa P J= −3 −2

1.23

P ≤ 0.20 MPa J = 55.89 P ≥ 0.3 MPa P J= −3 −3

1.78×10

1.99×10

+1.09×10

+9.07×10

J=

J=

P

J=

P

P



(2.51 × 10−2 + 7.40 × 10−3 P) × 1 + (P/(0.359 + 0.106P))

1+

P 

(1.99×10−3 +9.07×10−3 P)×

ogy [31] allows to obtain Eq. (5) for the all pressure range, that is displayed in Fig. 4.

11

1/11

(5)

For membranes M3 at the two highest feed circulation velocities, v = 0.94 and v = 1.23 m/s, a linear behavior of fluxes versus pressure is observed: J = 41.20 × P;  = 0.997; p < 2.2 × 10−16 . The same is true for membranes M2, at the three feed circulation velocities: v = 0.47 m/s (J = 63.98 × P,  = 0.994, p < 2.2 × 10−16 ); v = 0.94

1+

 P

(1.78×10−3 +1.09×10−2 P)×

P ≤ 0.20 MPa J = 58.86 P ≥ 0.3 MPa

J=

 

P (3.25×10−3 +4.60×10−3 P)×

1+

P 0.115+0.163P

9 1/9

P 9.95×10−2 +0.609P

P 0.117+0.534P

11 1/11

11 1/11

and v = 1.23 m/s (J = 76.25 × P,  = 0.998, p < 2.2 × 10−16 ) in the all pressure range. These are shown in Figs. 3 and 4. In Figs. 2–4 we can see that there is a good agreement between the equations proposed with the experimental results. Then, the model proposed in 3 is adequate to explain these results. The apparent rejections for total protein are similar and higher than 0.90 for all the membranes and lactose rejections increase linearly with increasing pressure, standing in a smaller range for membranes M1 (between 3 and 21%). These data show that membranes M1 have higher permeate fluxes and lower lactose rejections, keeping high rejections of protein.

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A. Macedo et al. / Journal of Membrane Science 381 (2011) 34–40

5. Conclusions The ultrafiltration of pretreated ovine cheese whey with the M1, M2 and M3 membranes yields very different permeation patterns. With membranes M1, a typical variation of the UF permeate fluxes versus the transmembrane pressure difference is shown, through a linear variation for the low pressure range, a transition region and then a constant limiting flux for the higher pressure range. These limiting flux increase with the increase of the feed circulation velocity. In the region of linear variation of the permeate fluxes, they are practically independent of the circulation velocity and they are close to the pure water fluxes. For the lower feed velocity (0.47 m/s), membranes M3 also show a trend towards a similar asymptotic behavior, although much less pronounced than with membranes M1, in the range of pressures studied. For both membranes M3 and M2, the permeate fluxes vary linearly with the transmembrane pressure in the all pressure range. The permeate fluxes measured with membranes M3 and M2 are always lower than the corresponding pure water fluxes, in all the range of pressures. The highest permeate fluxes were obtained with the membranes M1 and the lowest ones with the membranes M3, throughout the studied range of pressures and for the three velocities. For all the membranes, the higher cross-flow velocities of 0.94 and 1.23 m/s resulted in reduction of the boundary layers and thicknesses. The ultrafiltration modeling allowed to conclude that with the membranes M1, the concentration polarization controls the mass transfer in all the pressure range studied, whereas with the membranes M2 and M3 is the resistance due to fouling the main contributor for lowering the permeate fluxes. Then, membranes M1 are less susceptible to fouling compared to membranes M3 and M2 and this is likely related to the membrane material. Once the membranes M1 allow higher permeate fluxes and better selectivity, with a negligible contribution of fouling, in the experimental conditions used, they are more suitable for the ultrafiltration of pretreated ovine cheese whey. Acknowledgements The authors would like to thank “Ovelheira-Casa Agrícola de La Féria Lda.” for their cooperation. A. Macedo would like to thank to project Agro n◦ 327 – Medida 8 Acc¸ão 8.1. and FCT – Fundac¸ão para a Ciência e Tecnologia for financial support (SFRH/BD/50227/2009). References [1] G. Daufin, F. René, P. Aimar, Les Separations par Membrane Dans les Procédés de Lˇıindustrie Alimentaire , Collection Sciences et Techniques Agroalimentaires, Paris, 1998. [2] P. Walstra, T.J. Geurts, A. Noomen, A. Jellema, M.A.J.S. van Boekel, Ciencia de la Leche y Tecnologia de los Productos Lácteos, Editorial Acribia, S.A., Zaragoza, 2001. [3] J.-L. Maubois, A. Pierre, J. Fauquant, M. Piot, Industrial fractionation of main whey proteins , Int. Dairy Fed. Bull. 212 (1987) 154–159. [4] G. Daufin, F. Michel, J.-P. Labbé, A. Quémerais, A. Grangeon, Ultrafiltration of defatted whey: improving performance by limiting membrane fouling , J. Dairy Res. 60 (1993) 79–88. [5] G. Daufin, J.-P. Labbé, A. Quémerais, F. Michel, U. Merin, Optimizing clarified whey ultrafiltration: influence of pH , J. Dairy Res. 61 (3) (1994) 355–363. [6] A.L. Zydney, Protein separations using membrane filtration: new opportunities for whey fractionation , Int. Dairy J. 8 (1998) 243–250. [7] C.V. Morr, L. Barrantes, Lactose-hydrolysed cottage cheese whey nanofiltration retentate in ice cream , Milchwissenschaft-Milk Sci. Int. 53 (10) (1998) 568–572.

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