ARTICLE pubs.acs.org/IECR
Influence of Triglycerides on Fouling of GlycerolWater with Ultrafiltration Membranes M. A. Indok Nurul Hasyimah, A.W. Mohammad,* and M. Markom Department of Chemical and Process Engineering, Faculty of Engineering & Built Environment, Universiti Kebangsaan Malaysia, Scale-Up and Downstream Processing Research Group, 43600 Bangi, Selangor, Malaysia ABSTRACT: In the present study, the ability of two ultrafiltration (UF) polymeric membranes to clarify synthetic glycerin-rich solutions containing triglycerides (TGs) was evaluated. The membranes were made of poly(ether sulfone) (PES) and poly(vinylidene fluoride) (PVDF) and had molecular-weight cutoff (MWCO) values of 25000 and 30000 Da, respectively. A commercial TG (RBD Palm Olein) was employed, and the effects of membrane surface chemistry and solution pH on the permeation flux and TG retention rate were studied. In glycerolwater mixtures containing TG, the contribution of the solute to fouling was more severe than that of fatty acid (FA), with lower permeation rates being shown. Furthermore, the present study revealed that the nature of the membrane and the pH of the solution have significant effects on the fouling potential and can be used to quantitatively determine the TG rejection rate. Moreover, PVDF membranes were found to provide higher fluxes and lower TG rejection rates (81%) than PES membranes (91%).
1. INTRODUCTION Glycerin, also known as glycerol, is a highly valued chemical in the pharmaceutical, cosmetic, food, tobacco, paint, automotive, leather, and textile industries.1 In general, glycerin is produced as a byproduct of the hydrolysis and saponification of oleochemicals. For instance, glycerin-rich solutions (well-known as sweetwater) are an abundant source of glycerin; however, they contain mixtures of glycerin, water, and impurities such as free fatty acids; unreacted mono-, di-, and triglycerides; inorganic salts; and a variety of nonglycerol organic matter (MONG).2 To produce pure glycerin by hydrolysis, water and impurities must be removed from the crude product.3 The removal of triglycerides from glycerolwater mixtures is of commercial importance because triglycerides can cause severe fouling during membrane clarification. In fact, triglycerides (TGs) reduce the permeate flux over time through concentration polarization and the formation of a cake layer, probably as a result of the deposition of oil droplets on the membrane surface, causing pore blockage and cake layer formation.4 Ultrafiltration is an effective method for the treatment of TGs because additional chemicals are not required to achieve separation.5 In recent years, a considerable amount of attention has focused on the effects of oil droplets on the flux of ultrafiltration processes.68 However, those investigations focused primarily on the ultrafiltration of solutesolvent systems; thus, considerably less attention has been paid to the pretreatment of TGs in aqueous (solutewater) systems. Previously, the abilities of various polymeric membranes to separate oil constituents [free fatty acids (FFAs), diglycerides (DGs), monoglycerides (MGs), and TGs] from organic solvents have been investigated.6 In addition, attempts have been made to separate FAs and TGs in the presence of alcohols, suggesting that membrane materials play an important role in the separation of FAs and TGs.9 In fact, membrane fouling by oilwater mixtures was found to be influenced by the hydrophobicity of the membrane surface and r 2011 American Chemical Society
could be reduced by improving the hydrophilicity behavior.10 Moreover, the performance of dense membranes during the permeation of triglycerides, fatty acids, and TGFA mixtures is dependent on temperature and pressure. In particular, both parameters have significant effects on the permeation rates of TGs and fatty acids.11 Thus, the aim of the present study was to evaluate the use of membranes for the separation of glycerolwater mixtures. In particular, the main objective of the current investigation was to evaluate membrane fouling by palm-oil-based TGs present in glycerolwater mixtures. Moreover, the effects of the membrane surface chemistry and the pH of the solution were investigated in detail. Specifically, a glycerin-rich solution (containing 15% glycerin) was evaluated, and poly(ether sulfone) (PES) membranes with a molecular-weight cutoff (MWCO) of 25 kDa and poly(vinylidene fluoride) (PVDF) membranes with a MWCO of 30 kDa were used. The effects of oil deposition on various ultrafiltration membranes were examined qualitatively and quantitatively by monitoring the flux of the glycerinwater mixture.
2. EXPERIMENTAL SECTION Synthetic Mixtures. The experiments were carried out with a feed solution containing 15% (v/v) glycerin and 1% (v/v) triglycerides (TGs). Glycerin (USP, 92.09 g/mol) was purchased from Merck, and commercial triglycerides (RBD Palm Olein, 870 g/mol) were obtained from the local hypermarket. Before being mixed with glycerin, the TGs were initially prepared in a separate beaker and stabilized in ultrapure water at 650 rpm for 40 min to minimize heterogeneity effects. The pH of the feed Received: January 12, 2011 Accepted: May 10, 2011 Revised: May 9, 2011 Published: May 10, 2011 7520
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solution was varied between pH 3 and 10 with a few drops of 0.1 M HCl or 0.1 M NaOH. The pH of the solutions was measured with a pH meter (Mettler Toledo). Materials. Ultrafiltration flat-sheet polymeric membranes made of PES and PVDF were purchased from Sterlitech Corporation. The hydrophilicity or hydrophobicity of the membranes was determined according to the contact angle method. The properties of the membranes are described in the literature.12 Fresh membranes were used in each experiment and were soaked in pure water overnight. The surface charge properties of PES25 and PVDF membranes were analyzed by measuring the streaming potential. The analyses were performed using a ZetaCAD instrument (version 2.01) in the presence of (approximately) 1 L of electrolyte, and all measurements were carried out at room temperature. The electrolyte solution was continuously circulated at high applied pressures. The ZetaCAD instrument was furnished with a software package to measure the voltage (E) across the sample. The samples were pre-equilibrated overnight in the electrolyte solution prior to analysis. The zeta (ζ) potential of the sample was calculated from the slope of the streaming potential against pressure line using the classical HelmholtzSmoluchowski equation ζ¼
ληE εP
ð1Þ
where λ, η, E, ε, and P are the electrical conductivity of the solution, viscosity, streaming potential, permittivity, and differential pressure across the cell, respectively. Apart from the surface charge analysis, contact angle (CA) measurements were carried out to determine the CAs of fresh and fouled membranes. The analysis:: was done using a drop shape analysis system (EasyDrop, KRUSS Gmbh) with drops of 3-μL volume injected using a stainless steel microsyringe needle onto the membrane surface at a rate 30 μL/min. A water drop was captured and recorded for 90 readings. The analyses were repeated three times at different spots, and average values are reported. Filtration Experiments. The experiments were performed at 2.0 bar using a stirred Sterlitech HP4750 dead-end filtration cell with an effective filtration area of 15.2 cm2, as previously reported.13 Homogeneity was achieved by stirring the solution at 450 rpm. To avoid compaction effects and achieve a stable flux, compaction was performed by passing 300 mL of ultrapure (UP) water through the membrane at a pressure of 2.0 bar at least 30 min prior to the filtration experiments. The flux of UP water through the membranes before and after the experiments was determined at a transmembrane pressure (TMP) of 2 bar. The cell and reservoir were filled with the solution, and filtration was initiated. Permeate was collected within 1 h of the start of filtration and used for further characterization. After the filtration period, the cell and membrane were rinsed several times with UP water, and the flux was measured again with UP water as the feed. Analysis of Oil Contents. The oil content in the feed and permeate was extracted with oil extraction solvent S-316 (chlorotrifluoroethylene) and analyzed with an oil content analyzer (Horiba) in units of milligrams per liter. The performance of the filtration process is expressed as the percentage oil removal (R, %) according to the formula R ð%Þ ¼ ð1 Cp =Cf Þ 100
ð2Þ
where Cp and Cf are the concentrations of oil in the permeate and feed, respectively. The particle size distribution of oil droplets
Figure 1. Normalized flux decline of a glycerinwater mixture with TG for PES25 and PVDF membranes.
was analyzed with a particle analyzer (Mastersizer 2000 Malvern) with a Hydro 2000MU dispersing unit and detected by means of laser diffractometry (within approximately 30 s). The following parameters were set: refractive index (RI) values for water and basis solution (15% v/v glycerolwater solution, 20 °C) are 1.33 and 1.35106, respectively and the measurement cycle is 10 s. The analysis was repeated twice, and the average readings were obtained by Mastersizer2000 software. The output of the measurements is depicted in a graph of volume (%) against particle size (μm) within the range from 0.01 to 2000 μm. Fouling Resistance Measurements. Membrane fouling is a major factor limiting membrane processes and is attributed to the accumulation and deposition of solutes in the feed on the membrane surface or within the membrane pores. Generally, membrane fouling is classified into two stages: pore blocking and cake layer formation. To describe membrane fouling behavior due to the deposition of TGs, the cake blocking model was applied by assuming that the retention of TGs was high. The behavior of the flux was interpreted according to the equation14 t 1 ¼ KCF V þ V JVO
ð3Þ
where t is the time (s); V is the volume of permeate per unit surface area of membrane (m); JVO is initial flux (m/s); and KCF is the cake layer kinetic constant (s/m2), which can be obtained from the slope of the linear relationship shown in eq 3.
3. RESULTS AND DISCUSSION Effects of Membrane Characteristics on Fouling Behavior. Figure 1 depicts the normalized flux (J/J0) of a glycerinwater mixture containing TGs as a function of time. The data refer to samples obtained after 60 min of operating time. As shown in Figure 1, the flux declined rapidly during the first 15 min of filtration, and constant values were achieved at longer operating times for the PES25 membrane. However, for the PVDF membrane, the flux decayed continuously but slowly, moving toward constant values after 40 min. Membrane fouling might have occurred at the beginning of the permeation process because of the relatively high flux at the beginning of the run, which resulted in a higher rate of fluid concentration and a rapid increase in the thickness of the fouled layer.13 These results are in 7521
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Table 1. Contact Angles, Works of Adhesion, and Capillary Pressures for PES25 and PVDF Membranes after Fouling contact angle, θ
Figure 2. Cross-sectional structures of (a) PVDF and (b) PES25 membranes.
agreement with those of a previous study in which a sharp decrease in flux during the early stages of the process was attributed to lipid agglomeration on the membrane surface and small pore sizes.15 Moreover, the stabilization of the flux at long operating times implies that a gel layer formed on the membrane surface during the final stages of filtration (1560 min)16 because of the presence of wax, which can increase the thickness of the deposited layer on the membrane surface and occlude internal pores. It is inferred that, as more particles deposit with time, the flux decreases continuously, but at ever lower rates.17 According to Figure 1, it is noticeable that the relative fluxes for the two membranes became slower after 10 min of filtration and almost reached a steady state for the PES25 membrane. This might be due to the deposition of solute particles not only on the surface, but also in multiple overlapping layers in the deposit. However, throughout the entire process, the relative flux through the PVDF membrane was higher than that through the PES25 membrane because the solute particles can easily pass through the PVDF, probably because of the surface chemistry of the membrane itself and the larger cutoff value than for the PES25 membrane. Figure 2a shows the cross-sectional structure of the PVDF membrane, magnified by a factor of 5000. It can be clearly observed that the PVDF membrane has a thinner skin layer and a larger number of small pores at the pore wall compared to the PES25 membrane (as shown in Figure 2b). Hence, the pore volume of the PVDF membrane was higher than that of the PES25 membrane and allowed the solute molecules to permeate through the membrane, leading to a higher relative flux. Additionally, the pore size of the membrane also plays an important role in membrane fouling.18 Therefore, membranes with large pore sizes might not retain the oil phase and might be less susceptible to fouling. Moreover, the larger decline in the normalized flux observed for the hydrophobic UF membrane (PES25) was partly due to strong hydrophobichydrophobic interactions between the oil droplets and the membrane surface. Hence, the droplets tend to adsorb directly on the hydrophobic membrane surface, resulting in serious membrane fouling. On the other hand, because the PES25 membranes had greater hydrophobicity than the PVDF membrane, its water contact angle was 74.10°, which was higher than that of the PVDF membrane (as reported in Table 1). However, after the deposition of TGs, the contact angle of hydrophobic surfaces tend to be in the range 0 < θo/w < 90°.19 Thus, the PES25 membrane was easily wetted and fouled with oil droplets, leading to low surface tension and enhanced adhesion between the oil and the membrane material. The aforementioned hypothesis was confirmed by determining the contact angle after fouling and the work of adhesion (Wa), as reported in Table 1. In particular, the liquid in a wetted pore requires less energy to pass through the pores and reach the other side of the membrane. In fact, oil droplets are
membrane
fresh
fouled
Wa 102 (N/m)
Pc (bar)
PES25
74.10
47.60
12.05
1.44
PVDF
72.60
88.30
7.41
0.06
more likely to enter and adhere to the pore walls as well as the surface of the membrane, which reduces the effective diameter of the pore and causes blockages.19 As a result, deposited TGs form a layer on the surface of the membrane and resist the transport of water. Conversely, for the PVDF membranes, the contact angle after fouling with TGs increased from 72.60° to 88.30°, leading to slow and incomplete wetting. Thus, because of high surface tension, liquid on the nonwetted material does not spontaneously enter the pores, and adhesive forces between the liquid and membrane are relatively weak.20 In fact, the adhesive forces between oil and hydrophobic surfaces are much stronger than those between oil and less hydrophobic surfaces.21 Thus, oil easily adheres to the surface of a hydrophobic membrane, which leads to significant fouling. The works of adhesion (Wa) of the membrane materials are presented in Table 1 and were was determined by applying the equation Wa ¼ γL ðcos θ þ 1Þ
ð4Þ
where Wa, γL, and θ are the work of adhesion (N/m); the liquid surface tension (N/m); and the contact angle at the solidliquid interface, respectively. For oil droplets, the work of adhesion of the hydrophobic membrane was approximately 12.05 102 N/ m and was higher than that of the less hydrophobic membrane because of the greater adhesion between the oil droplets and the membrane surface, which led to a severe reduction in the flux during the clarification of glycerolwater mixtures. In addition, the effect of capillary pressure, Pc, on membrane fouling during the removal of TGs was calculated according to the equation Pc ¼
2γo=w cos θo=w rp
ð5Þ
where Pc is the capillary pressure (N/m2), rp is the radius of the membrane pore (m), γo/w is the interfacial tension of oil and water (N/m), and θo/w is the contact angle of an oil droplet on the membrane surface in the presence of water. As shown in Table 1, the capillary pressures of the two membranes were positive, and values of 1.44 and 0.06 bar were obtained for PES25 and PVDF membranes, respectively, after fouling with TGs. Consequently, the operating pressure exceeded Pc, and therefore, the droplets could easily pass through the membrane pores at higher operating pressures21 and block membrane pores, leading to severe membrane fouling. Nevertheless, it is noted that the capillary pressure of the PVDF membrane was lower than that of the PES25 membrane, implying that the adhesion of the oil droplets to the pore wall was comparatively weak. The tendency of the oil droplets to enter the pores and adhere to the nonwetted membrane surface was low, probably because of high surface tension, resulting in less fouling. The rejections of TGs by the two membrane materials are listed in Table 2 (in original pH) and were calculated in terms of R values (eq 2). Because the decline in flux during the early stages 7522
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Table 2. Measured Contact Angles and Oil Rejection of PES25 and PVDF Membranes after Filtration for Different Feed Characteristics rejection (%) contact angle membrane PES 25
PVDF
pH
after fouling (θ) before 15 min after 15 min
3.12
47.10
91.60
94.70
4.69 (original)
47.60
79.91
88.83
10.07
62.00
77.89
78.36
3.12 4.82 (original)
79.40 88.30
87.03 67.40
80.31 84.67
10.07
81.20
58.00
66.13
of the process (15 min) was more pronounced than that at longer operating times (after 15 min), the rejection of TGs was determined before and after 15 min of filtration. As shown in Table 2, the PES25 membrane exhibited the highest rejection of TGs; the TG rejection rate of the PES25 membrane was equal to 80% during the first 15 min of operation and was greater than 85% after 15 min of filtration. Conversely, the rejection rate of the PVDF membrane was slightly lower than that of the PES25 membrane, and ranged between 67% and 85% before and after 15 min of filtration. This shows that the PVDF membrane inefficiently retained the oil droplets within the first 15 min of filtration compared to the PES25 membrane, mainly because of its larger MWCO than for PES25, and gradually improved as the oil droplets were constantly deposited on the surface. Effects of Feed Characteristics on Fouling Behavior. Figures 3 and 4 show the normalized fluxes of PES and PVDF membranes over time, as well as the particle size distributions of oil droplets in acidic and alkaline solutions, respectively. The results indicate that the normalized flux and size of the oil droplets were strongly affected by the pH of the feed solution. Specifically, the fluxes through PES and PVDF membranes significantly increased as the pH increased from 3 to 10. Thus, the permeate flux was strongly dependent on the interactions between the membrane material and the oil droplets in solution. Figure 4 shows the effect of pH on the particle size distribution of oil droplets. In acidic solution, the size distribution of the droplets was broad, and particle sizes between 1500 μm were observed. The average size of oil droplets in acidic solution was 79.621 μm, whereas the average particle size under neutral conditions (without the addition of acid or alkali) was 44.774 μm. However, in alkaline solution, the distribution of oil droplets was significantly different from that in acidic and neutral solutions, and a peak at 39.905 μm was observed. The peak at 39.905 μm in the alkaline solution was smaller than that in the acidic solution and reduced about 50% from the size because the addition of NaOH allowed the oil droplets to break up into smaller particles and induce the saponification, which results in the formation of liquid soap. As a result, both membranes were less susceptible to membrane fouling at higher pH. However, at low pH values, the addition of HCl caused the droplets to coalesce and form large oil droplets. Thus, large oil droplets could deposit on the membrane, forming a hydrophobic film or cake layer, which would cause the pores to clog and lead to severe membrane fouling.22 To further elucidate the effects of pH on the fouling of PES and PVDF membranes, the wettability of the oil on the membrane was used to assess the electrochemical interactions
Figure 3. Normalized flux declines of PES25 and PVDF membranes in solutions with different pH values.
Figure 4. Oil droplet distributions in solutions with different pH values.
between the membrane surface and the charged oil droplets, which are strongly influenced by the solution pH. As reported in the literature, if the two interfaces (membrane and oil droplets) have like charges, then a repulsion force will occur, which is likely to produce water-wetness.23 Conversely, if the interfaces have different charges, an attractive force occurs, which enhances the oil-wetting of the membrane surface. In fact, the ζ potential of oil droplets has been estimated by certain studies,24,25 confirming that the droplets are negatively charged, whereas the ζ potentials for PES25 and PVDF membranes are clearly depicted in Figure 5. The isoelectric points (IEPs) for the two membranes were found to be at pH 5.5; thus, the PES25 and PVDF membranes had positive charges at pH values below the IEP and vice versa. Therefore, at low pH, the interfaces (the membrane and oil droplets) have opposite charges, leading to an attractive force between the surface and the droplets, which promotes oil adsorption on the surface as well as in the pores. In contrast, at higher pH values, the PES25 membrane and the oil droplets became negatively charged, which enhanced the repulsion force and prevented oil adsorption on the surface. Therefore, alkaline conditions diminish the oil wettability of the membranes, leading to an improved permeate flux and lower fouling. Nevertheless, under acidic and alkaline conditions, the PVDF membrane 7523
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Figure 5. pH dependence of the ζ potential for PES25 and PVDF membranes.
presented a larger volume of permeate than the PES25 membrane because of the less hydrophobicity of PVDF, which causes strong repulsive forces between the membrane material and the oil droplets. Therefore, the nature of the PVDF membrane might limit the deposition of oil droplets on the pore walls and reduce pore blockages, which results in a higher permeate flux. As shown in Table 2, the contact angle values for PES membranes were lower at low pH and increased significantly at high pH. This implies that there was a decrease in the wettability of the oil on the membrane in basic solutions compared to acidic conditions. However, the contact angles for the fouled PVDF membrane were higher than those for the fresh PVDF membrane in both solutions, which indicated that the surface was incompletely wetted by the oil droplets. The aforementioned hypothesis was supported by the finding that the TG rejection rates of both membranes were high at a low pH (shown in Table 2). According to Table 2, after 15 min of permeation, the PES25 membrane exhibited the highest rejection for TGs, and values greater than 95% were obtained. However, the rejection rate of the PVDF membrane was slightly lower than that of the PES25 membrane and varied between 66% and 87%. Fouling Resistance Analysis. During membrane fouling, the permeate flux declines dramatically as a result of pore blocking and the formation of a cake layer on the surface of the membrane. The effect of cake layer formation during the clarification of glycerinrich solutions containing TGs is important. Thus, this phenomenon has been studied during the filtration of oilwater mixtures.26 Figures 6 and 7 show the correlations of the experimental data to the cake filtration model (eq 2) for PES25 and PVDF membranes, respectively, under different feed characteristics. The values of KCF and JVO were obtained by linear regression analyses and are reported in Table 3. The results clearly indicate that the best agreement with the experimental data occurred with the PVDF membrane, for which a correlation coefficient of R2 > 0.9 was obtained. However, the correlation coefficient for the PES25 membrane was lower than that for the PVDF membrane and fell in the range 0.7 < R2 < 0.8. The fit of the cake blocking model to the results for the PES25 membrane might have been poor because of the occurrence of pore blocking throughout the entire filtration process. However, for the PVDF membrane, a switch in the fouling mechanism from standard pore blocking to cake layer formation was observed at approximately 2 min (as indicated by a narrow curve around 2 min on the X axis), which suggested that oil droplets were deposited on the
Figure 6. Cake filtration model of PES25 membranes at different pH values: (a) pH 3.12, (b) pH 4.69, and (c) pH 10.07.
pore wall and reduced the cross section of the membrane pores, causing a dramatic decline in the flux during the early stages of the separation process. On the other hand, pore fouling became insignificant at longer operating times because the thickness of the cake layer increased, 7524
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Table 3. Cake Model Parameters for PES25 and PVDF Membranes JVO 105 (m/s) KCF 105 membrane PES25
PVDF
pH
(s/m2)
predicted experimental
R2
3.12
1 103
1.21
1.97
0.7391
4.69 (original) 10.07
40.00 20.00
4.14 5.83
2.96 4.27
0.7990 0.9853
3.12
5.10
8.56
7.24
0.9947
4.82 (original)
2.91
5.82
6.14
0.9797
10.07
0.69
6.50
6.36
0.9674
for the PVDF membrane at all pH values, which suggests that cake formation was dominant for the hydrophobic material, probably because of the chemical nature of the PES25 membrane, which is more oleophilic than the PVDF membrane and exhibits a stronger net attractive force to TGs. In addition, oil droplet deformation might increase the packing density of the cake layer, which would increase the cake resistance and cause a dramatic decline in the flux.27 The JVO values for both membranes after the first layer of cake formed on the membrane surface were estimated from the intercept of eq 2. It is noticeable that the estimated values are larger than the initial experimental flux for PES25 membranes under all conditions, suggesting that hydrophobic membranes are prone to pore blocking and cake formation during the entire filtration time. However, the difference between the predicted and experimental values was insignificant for PVDF, showing that the cake formation model accurately fits the experimental data for the PVDF membrane.
Figure 7. Cake filtration model of PVDF membranes at different pH values: (a) pH 3.12, (b) pH 4.82, and (c) pH 10.07.
which improved the fouling resistance. The results obtained for both membranes revealed that the membrane material had a significant effect on the mechanism of fouling. The KCF value (Table 3) for the PES25 membrane was much greater than that
4. CONCLUSIONS In this study, the removal of TGs by UF polymeric membranes was studied during the clarification of a glycerin-rich solution. Based on the results of the present study, the following conclusions were made: The surface chemistry of the membranes and the feed characteristics have significant effects on the permeate flux. The hydrophobicity of the membrane and the pH of the solution affect the wetting process and induce adhesive forces between the oil droplets and the membrane surface. Moreover, the cake formation model was used to predict the initial flux and to determine the fouling resistance over time. The results suggest that hydrophilic membranes present a lower kinetic constant, which indicates that such membranes resist fouling and maintain high fluxes in the presence of oil. The linear relationship regression (R2 > 0.9) for PVDF membranes suggests that the cake model can be used to predict the behavior of the fouling and membrane resistance. Nevertheless, the cake model did not fit the experimental data for the hydrophobic membrane and could not be used to predict the flux over the entire range of filtration times, as pore blocking was dominant at the early stage whereas cake layer formation occurred at longer operating times. ’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected]. Tel.: þ(603)-89216410. Fax: þ(603)-89216148. 7525
dx.doi.org/10.1021/ie2000727 |Ind. Eng. Chem. Res. 2011, 50, 7520–7526
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’ ACKNOWLEDGMENT This work was funded by the Malaysian Ministry of Science and Technology through the Science Fund (02-01-02-SF0529) and an FRGS grant (UKM-KK-03-FRGS0115-2010). The authors also thank the Laboratory of Environmental Technology Department (UniKL MICET) for analyzing oil contents. ’ REFERENCES (1) Burshe, M. C.; Sawant, S. B.; Pangarkar, V. G. Dehydration of glycerinwater mixtures by pervaporation. J. Am. Oil Chem. Soc. 1999, 76, 209–214. (2) Khairnar, D. B.; Pangarkar, V. G. Dehydration of glycerin/water mixtures by pervaporation using homo and copolymer membranes. J. Am. Oil Chem. Soc. 2004, 81, 505–510. (3) Jeromin, L.; Johannisbauer, W.; Blum, S.; Sedelies, R.; Moormann, H.; Holfoth, B.; Plachenka, J. Process for the purification of glycerol water. U.S. Patent 5,527,974, 1996. (4) Cheryan, M.; Rajagopalan, N. Membrane processing of oily streams. Wastewater treatment and waste reduction. J. Membr. Sci. 1998, 151, 13–28. (5) Pagliero, C.; Ochoa, N.; Marchese, J.; Mattea, M. Degumming of crude soybean oil by ultrafiltration using polymeric membranes. J. Am. Oil Chem. Soc. 2001, 78, 793–796. (6) Koike, S.; Subramanian, R.; Nabetani, H.; Nakajima, M. Separation of oil constituents in organic solvents using polymeric membranes. J. Am. Oil Chem. Soc. 2002, 79, 937–942. (7) Subramanian, R.; Nakajima, M.; Raghavarao, K. S. M. S.; Kimura, T. Processing vegetable oils using nonporous denser polymeric composite membranes (Review). J. Am. Oil Chem. Soc. 2004, 81, 313–322. (8) Garcia, A.; Alvarez, S.; Riera, F.; Alvarez, R.; Coca, J. Sunflower oil miscella degumming with polyethersulfone membranes: Effect of process conditions and MWCO on fluxes and rejections. J. Food Eng. 2006, 74, 516–522. (9) Krishna Kumar, N. S.; Bhowmick, D. N. Separation of fatty acids/ triacylglycerol by membranes. J. Am. Oil Chem. Soc. 1996, 73, 399–401. (10) Yan, L.; Hong, S.; Meng, L. L.; Yu, S. L. Application of the Al2O3PVDF nanocomposite tubular ultrafiltration (UF) membrane for oily wastewater treatment and its antifouling research. Sep. Purif. Technol. 2009, 66, 347–352. (11) Subramanian, R.; Raghavarao, K. S. M. S.; Nakajima, M.; Nabetani, H.; Yamaguchi, T.; Kimura, T. Application of dense membrane theory for differential permeation of vegetable oil constituents. J. Food Eng. 2003, 60, 249–256. (12) Amin, I. N. H. M.; Mohammad, A. W.; Markom, M.; Leo, C. P. Effects of palm oil-based fatty acids on fouling of ultrafiltration membranes during the clarification of glycerin-rich solution. J. Food Eng. 2010, 101, 264–272. (13) Amin, I. N. H. M.; Mohammad, A. W.; Markom, M.; Leo, C. P.; Hilal, N. Flux decline study in ultrafiltration of glycerin-rich fatty acid solutions. J. Membr. Sci. 2010, 351, 75–86. (14) Ochoa, N. A.; Masuelli, M.; Marchese, J. Effect of hydrophilicity on fouling of an emulsified oil wastewater with PVDF/PMMA membranes. J. Membr. Sci. 2003, 226, 203–211. (15) de Souza, M. P.; Petrus, J. C. C.; Goncalves, L. A. G.; Viotto, L. A. Degumming of corn oil/hexane miscella using a ceramic membrane. J. Food Eng. 2008, 86, 557–564. (16) Ochoa, N.; Pagliero, C.; Marcheses, J.; Mattea, M. Ultrafiltration of vegetables oils degumming by polymeric membranes. Sep. Purif. Technol. 2001, 2223, 417–422. (17) Zhao, Y.; Tan, Y.; Wong, F.-S.; Fane, A. G.; Nanping, Xu. Formation of dynamic membranes for oily water separation by crossflow filtration. Sep. Purif. Technol. 2005, 44, 212–220. (18) Keurentjes, J. T. F.; Doornbusch, G. I.; van’t Riet, K. The removal of fatty acids from edible oil. Removal of the dispersed phase of a water-in-oil dispersion by a hydrophilic membrane. Sep. Sci. Technol. 1991, 26, 409–423.
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