Resistance analysis for ceramic membrane microfiltration of raw soy sauce

Journal of Membrane Science 299 (2007) 122–129

Resistance analysis for ceramic membrane microfiltration of raw soy sauce Meisheng Li a , Yijiang Zhao a,∗ , Shouyong Zhou a , Weihong Xing b , Fook-Sin Wong c a

Chemistry Department, Huaiyin Teachers College, Key Laboratory for Chemistry of Low-Dimensional Materials of Jiangsu Province, No. 111 West Changjiang Road, Huaian 223300, Jiangsu Province, PR China b Membrane Science and Technology Research Center, Nanjing University of Technology, No. 5 Xinmofan Road, Nanjing 210009, Jiangsu Province, PR China c Institute of Enviromental Science and Engineering, Nanyang Technological University, Innovation Centre Block 2, Unit 237, 18 Nanyang Drive, Singapore 637723, Singapore Received 23 December 2006; received in revised form 8 April 2007; accepted 23 April 2007 Available online 4 May 2007

Abstract Based on resistance-in-series model, influences of membrane microstructure and operational conditions on fouling behavior of ceramic membranes during microfiltration of raw soy sauce were investigated. Results show that total resistance (Rt ) and concentration polarization resistance (Rcp ) increase significantly with increasing nominal pore size while cake resistance (Rc ) and internal fouling resistance (Rif ) decrease slightly. For different materials, the Rt of a ZrO2 membrane is much larger than that of a ␣-Al2 O3 membrane. The fouling resistance of the ZrO2 membrane is Rcp -dominant while Rcp and Rc control the ␣-Al2 O3 membrane separation process. At different operating conditions, the permeability of a 0.2 ␮m ␣-Al2 O3 membrane decreases rapidly due to Rcp , which is the dominant fouling resistance for microfiltration of the high viscosity system. Further flux decline is caused by the growth of a gel-type layer (Rc ) over the membrane surface and Rif , which are the secondary dominant resistances. The different resistances (Rcp , Rc and Rif ) were empirically modeled as a function of the operating conditions (transmembrane pressure, crossflow velocity and solutes concentration) to allow the interpolation for other operational conditions. The main physical chemistry properties of raw soy sauce were evaluated in order to select the suitable membrane pore size and material that provide the highest permeate flux and best clarified soy sauce. © 2007 Elsevier B.V. All rights reserved. Keywords: Ceramic membrane microfiltration; Fouling; Hydraulic resistance; Raw soy sauce

1. Introduction Soy sauce is the most popular liquid condiment used in Chinese cuisine as well as in cuisine of other oriental countries. Produced for thousands of years, soy sauce fermentation is one of the oldest techniques in food preservation as it not only extends the shelf-life but enhances the flavour and nutritional quality of the product [1]. After fermentation, raw soy sauce contains lots of bacteria, and enzymes that would affect the characteristics of the sauce [2]. Therefore, they need to be removed from the raw soy sauce for purification and refinement. A traditional method is pasteurization at 70–85 ◦ C for 30 min, and storage in

∗

Corresponding author. Tel.: +86 517 3525021; fax: +86 517 3526020. E-mail address: [email protected] (Y. Zhao).

0376-7388/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2007.04.033

a tank, in order to stop the microbial and enzymatic reaction and to stabilize the flavor and color, respectively. However, during this treatment, sediment is gradually formed. This sediment is difficult to filter out using traditional method. Its removal from soy sauce after pasteurization entails a loss of approximately 10% of the total volume in the brewery [3]. Moreover, the sediment and heat treatment would still affect the characteristics of the sauce. Recently, crossflow microfiltration has been offered as an attractive and economical alternative for fluid clarification/pasteurization/sterilization in the beverage, brewing, and dairy industries [4]. Ceramic membrane filtration is an advanced method for separating these substances from such fermentation liquid due to their excellent selectivity, permeability and thermal and chemical stability [5]. Wang et al.[6] studied the influences of operating pressure, crossflow velocity and temperature on permeate flux during microfiltration of raw soy sauce using ceramic

M. Li et al. / Journal of Membrane Science 299 (2007) 122–129

membranes. They obtained suitable working conditions for the system. However, the permeate flux decreases dramatically during filtration process caused by the adsorption of solutes on outer membrane and inner pore surfaces, blocking of pores by rejected solutes or formation of a thick cake layer of precipitated solutes on top of the membrane [6]. These would reduce productivity, and potentially shorten membrane life. Minimization of membrane fouling is essential to make the membrane process economical. A detailed understanding of causes of flux decline and their relative contribution are essential for minimizing fouling. For example, if adsorption is the main cause of fouling, then improvement of mass transfer by altering hydrodynamics is of little benefit [7]. To understand the mechanisms of flux decline, some researchers proposed the resistance-in-series model and experimental methods to measure each of the fouling resistance separately [8–10]. Based on this model, Psoch et al. [11] evaluated the contribution of fouling resistance, cake resistance and membrane resistance to the overall resistance during filtration of wastewater according to the experimental data. Vladisavljevi’c et al. [12] studied the variation of fouling resistance with filtration time and the effects of operating pressure and crossflow velocity during ultrafiltration of depectinized apple juice. Zhao et al. [13] analyzed the hydraulic resistance in microfiltration of titanium white waste acid through ceramic membranes. They investigated the effects of operational parameters on polarization resistance (Rcp ), cake resistance (Rc ), internal plugging resistance (Rif ) and their relative percentages in the total filtration resistance (Rt ). Nevertheless, the fouling resistance is dependent on operation conditions such as transmembrane pressure and crossflow velocity, as well as conditions of the solution such as concentration and size distribution of colloids, characteristics of membrane such as pore size, chemical nature, etc. [14]. Lack of research in these fouling behaviors limits the wide usage of ceramic microfltation membrane in raw soy sauce sterilization process in China. The main objective of this work is to study the effects of membrane microstructure such as pore size and interface nature, solute concentration and operating conditions such as transmembrane pressure (TMP) and crossflow velocity (CFV) on the permeate flux and fouling resistance during microfiltration of raw soy sauce by ceramic membrane. The experimental results were analyzed in terms of resistance-in-series model. Also the mathematical correlations between each of the resistance (Rc , Rcp and Rif ) and operating conditions were determined. The study aims to contribute fundamentally on the subject of membrane fouling control in the application of MF for clarification of raw soy sauce. 2. Materials and methods 2.1. Membranes and raw soy sauce Four ceramic tubular microfiltration membranes with various nominal pore sizes and different materials (Nanjing Jiusi High-Tech Co. Ltd., Jiangsu, PR China) were selected for this

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Table 1 Characteristics of membrane Characteristics

Membranes for microfiltration

Material Nominal pore size (␮m) Membrane thickness (␮m) Membrane area (m2 ) Inside diameter (ID) (mm) Membrane length (cm)

␣-Al2 O3 ; ZrO2 0.2; 0.5; 0.8 15 0.00328 7 15

experiment. The details of ceramic membrane characteristics are presented in Table 1. Raw soy sauce sampled from a local food plant (Huaian Food Plant, Jiangsu, PR China) was used in this study. Some physical properties of raw soy sauce are given in Table 2. 2.2. Experimental setup and procedure Fig. 1 illustrates the schematic diagram of the crossflow microfiltration set-up. The feed raw soy sauce of 2.5–3 L was recycled between the retentate reservoir and the module by a rotary pump and the feed flow rate was controlled with a laboratory made rotameter. TMP was controlled by the in/outlet valves. The TMP was calculated as the mean value of inlet and outlet pressure. The permeate was collected in a reservoir placed on a digital balance that can be connected to a computer. In total recycle experiments, both permeate and concentrate flows were returned to the feed tank to keep concentrations constant. For all experiments, the system temperature was fixed at 22 ± 3 ◦ C to avoid forming sedimentation and affecting the characteristics of the sauce. The TMPs were 0.05, 0.10, 0.15 and 0.20 MPa. The CFV were 0.29, 0.58 and 0.87 m s−1 . Raw soy sauce was diluted with deionized water into different total solids concentration (0.13, 0.19 and 0.38 kg l−1 ). After each experiment, the membrane was cleaned for the next run. The cleaning process varied according to the feed property. For this feed, the membranes underwent a cleaning process utilizing 2% (w/w) NaOH and 0.15 M HNO3 aqueous solutions at 40 ± 3 ◦ C and finally rinsing off with water till the permeate flux was restored [15]. The membranes should be stored in 1% (w/w) NaOCl solution to prevent bacteria formation. 2.3. Analysis The feed and permeate were analyzed for pH, electric conductivity, viscosity, density, turbidity (NTU), absorbance, total solids and the content of target composition (such as total nitrogen, amino nitrogen, total acid and reducing sugar). The pH values of the solutions were measured using a pH meter (PHS3C, Shanghai, PR China). Viscosity was determined using a rotating viscometer (NDJ-1, Shanghai, PR China) at a constant water bath temperature of 22 ± 3 ◦ C. The density was measured by accurately weighing 10 ml of sample into a 10 ml pycnometer. Turbidity and absorbance were measured with a 2100 Turbidimeter (HACH USA) and a spectrophotometer (722 Shanghai, PR China) at 650 nm, respectively. Total solids con-

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Table 2 The physical chemistry properties of raw soy sauce in its feed and permeate clarified by ceramic membrane with various pore sizes and materials (P = 0.10 MPa, T = 22 ± 3 ◦ C and V = 0.58 m s−1 ) Analysis

Feed

pH Total solids (kg L−1 ) Density 22 ◦ C (g mL−1 ) Turbidity (NTU) Viscosity (mPa s) Absorbance at 650 nm Total nitrogen (g/100 mL) Amino nitrogen (g/100 mL) Reducer sugar (g/100 mL) Total bacterial count Percentage of removing bacteria (%) Sediment (yes or no) Steady flux (L m−2 h−1 )

3.81 0.38 1.197 18.7 6.32 0.742 1.74 0.870 2.70 3200 Yes

Permeate 0.2 ␮m (␣-Al2 O3 )

0.2 ␮m (ZrO2 )

0.5 ␮m (␣-Al2 O3 )

0.8 ␮m (␣-Al2 O3 )

3.80 0.27 1.123 1.03 4.85 0.587 1.65 0.853 2.10 30 99.1 No 12

3.82 0.26 1.121 0.813 4.86 0.597 1.64 0.853 2.13 35 98.9 No 9.5

3.81 0.29 1.124 1.55 4.87 0.582 1.67 0.854 2.21 80 97.5 No 8.0

3.80 0.31 1.124 1.61 5.12 0.551 1.70 0.856 2.34 200 93.8 No 3.5

centration of all the samples were measured by taking a known volume of sample in a petri dish and keeping it in an oven maintained at 105 ± 2 ◦ C, till complete drying of the sample [16]. Total nitrogen, amino nitrogen, reducer sugar and total bacterial count were examined by Huaian Food Plant according to National Standard of the People’s Repubulic of China (GB2717-81 and GB2719-81). 2.4. Experimental analysis of various resistances According to the resistance-in-series model [8], the overall filtration resistance of a microfiltration can be given by summing all sources of filtration resistances. Thus, the basic filtration equation can be written in the following form: JRSS =

P p = μRt μRSS (Rm + Rcp + Rc + Rif )

(1)

where JRSS is the filtration rate, P the filtration pressure, μRSS the viscosity of the permeate (RSS) at 22 ± 3 ◦ C, and Rt , Rcp ,

Rc , Rif , and Rm are the overall filtration resistance, the cake resistance, the concentration polarization, layer resistance, the membrane internal fouling resistance and the clean membrane resistance, respectively. The values of Rt can be calculated from the filtration data in Eq. (1) while Rm from the pure water flux before filtration. Rcp , resistance of polarization layer results from the concentration polarization, can be removed by rinsing with water at very low flow rate. Rc , resistance of cake layer results from the deposition of particles and other solute on the membrane surface, can be removed by brushing the membrane surface. Rif , resistance of internal fouling results from the plugging of the macromolecules and fine particles in membrane pores, can be removed with the method of chemical cleaning combined with back flushing. Experimentally [13], the resistances defined can be determined from the value of flux in four different periods. First, pure water flux (Jw1 ) was determined using a clean membrane, and then the steady flux (JRSS ) of filtration of raw soy sauce was measured. After the filtration of raw soy sauce, the membrane was rinsed with pure water to eliminate all traces of the solution. This rinsing was to remove the polarization layer and the water flux measured afterwards was Jw2 . The next step was cleaning of the membrane with a suitable brush, follow by rinsing with pure water. This cleaning was to remove the deposited cake and the water flux measured afterwards was Jw3 . Then each resistance could be calculated using the experimental flux data. In the experiment, the filtration of the raw soy sauce was continued unless the flux tending to a steady value. This meant that the fouling resistance was totally established. 3. Results and discussion 3.1. Influence of ceramic membrane microstructure on resistances

Fig. 1. Schematic diagram of the laboratory set-up.

The resistances and their relative percentages in microfiltration of raw soy sauce through the ␣-Al2 O3 membrane as a function of nominal pore sizes are shown in Figs. 2 and 3.

M. Li et al. / Journal of Membrane Science 299 (2007) 122–129

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Table 3 Hydraulic resistances and relative percentages as a function of ceramic membrane materials (0.2 ␮m membrane, P = 0.10 MPa, T = 22 ± 3 ◦ C, V = 0.58 m s−1 and Cb = 0.38 kg L−1 )

Fig. 2. Effect of membrane pore size on hydraulic resistances (␣-Al2 O3 membrane, P = 0.10 MPa, T = 22 ± 3 ◦ C, V = 0.58 m s−1 and Cb = 0.38 kg L−1 ).

The following operating conditions were used in the experiments: transmembrane pressure, 0.1 MPa; tangential velocity, 0.58 m s−1 ; temperature, 20 ± 3 ◦ C; total solids concentration, 0.38 kg L−1 . It could be observed that Rcp was the main component in the fouling resistances and Rm was negligible compared with the fouling resistances, which was only 1–5% of Rt . Rt and Rcp increased significantly with increasing nominal pore size while Rc and Rif decreased slightly. Rif could be neglected when the nominal pore sizes were greater than 0.5 ␮m, which was only 2.2% of Rt . The percentage of Rc decreased significantly with increasing membrane pore size. For the large-pore-size (0.8 ␮m) membrane the Rc represented only 1.5%. The variation of Rcp , Rc and Rif with increasing membrane pore size was close to that measured by Ousman et al. [8]. But the dominant fouling resistance in this work was Rcp , while Rc was dominant in Ousman’s work. Also, for the large pore size membranes, Rcp was

Fig. 3. Hydraulic resistance percentages as a function of membrane pore size (␣-Al2 O3 membrane, P = 0.10 MPa, T = 22 ± 3 ◦ C, V = 0.58 m s−1 and Cb = 0.38 kg L−1 ).

Materials

Rm

Rcp

Rc

Rif

Rt

␣-Al2 O3 ×1011 m−1 Percentage

2.321 4.71

21.84 44.3

18.10 36.7

7.027 14.3

49.30 100

ZrO2 ×1011 m−1 Percentage

2.400 3.27

67.01 91.4

0.959 1.31

73.33 100

2.966 4.04

higher than the smaller pore membranes, due to very high initial flux (at constant TMP, larger pore membrane gave higher flux at initial filtration). Since concentration polarization is a function of flux, so Rcp was high. Rc and Rif was low for larger pore membrane most likely the problematic foulants are smaller than 0.8 ␮m therefore not retained by the membrane. This was also supported by the turbidity measurement in Table 2, where the permeate was more turbid for 0.8 ␮m membrane. However, the 0.2 ␮m membrane could remove these particles (lower permeate turbidity) and therefore the foulants accumulated on the surface. Table 3 summarizes the respective resistance values and their relative percentages for the 0.2 ␮m membranes made from different materials (ZrO2 , ␣-Al2 O3 ). During crossflow filtration, the Rt of the ZrO2 membrane was much higher than that of the ␣-Al2 O3 membrane. This could be explained by the charge behavior of membrane surface in solution with different pH values. As shown in Fig. 4, the pH value of raw soy sauce was 3.8, While the IEP (isoelectric point) of raw soy sauce solution is at the pH of 4.7–4.9[17], so the ζ-potential is positive. The zeta potential of ZrO2 and ␣-Al2 O3 membrane at pH 3.8 was +3.65 mV and +46.1 mV, respectively (Fig. 4). The repulsion between solutes and surface of ␣-Al2 O3 membrane is much larger than that of ZrO2 membrane. Then the adsorption of solutes on ZrO2 membrane surface was easier. The adsorption resistance of ZrO2 membrane would be larger.

Fig. 4. The ζ-potential of ZrO2 and ␣-Al2 O3 powder suspensions as a function of pH.

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Simultaneously, the majority of adsorption solutes were longchain macromolecules. This caused the increase of the Rcp . So the fouling resistance of ZrO2 membrane was Rcp -dominant. The Rm , Rc and Rif values of ZrO2 membrane remained negligible in all cases as compared with fouling and reached a maximum of 1.3–4.1% of the Rt . But for ␣-Al2 O3 membrane, the dominant fouling resistances are Rc and Rcp . Rif, which was 14.3% of Rt , was secondary dominant resistance. Also, Rm of ␣-Al2 O3 membranes could be neglected, which accounted for only 4.71% of the Rt . The main physical chemistry properties of raw soy sauce in its feed and permeate clarified by ceramic membranes with various pore sizes and materials are reported in Table 2. According to these results, we could select the suitable membrane pore size and material. As show in Table 2, the turbidities of raw soy sauce permeate treated by different ceramic membranes were lower than 1.70 NTU. More than 90% insoluble fine particles (Turbidity) were removed. It indicated that the filtration had good clarification efficiency for raw soy sauce. The total nitrogen and amino nitrogen recovered more than 95 and 98%, respectively, in all permeate sauces. However, the bacteria retention ratio varied from 93.8 to 99.1% for different membranes, and the biggest one was 99.1% for 0.2 ␮m ␣-Al2 O3 membrane. Also the steady flux did not increase with increasing nominal pore size. The 0.2 ␮m ␣-Al2 O3 membrane had a better flux than others. It is well known that particle-to-pore size relation is key in determining permeability. When the nominal pore size increases, small particles can travel through the cake layer, initially plug the pores by bridging, and thus create internal pore restrictions to fluid flow. Continued buildup with time may also lead to an increase of concentration polarization and an external cake. These may cause that the steady flux decrease with increasing nominal pore size. Based on above results, it could be concluded that the suitable membrane, which provided the clearest soy sauce permeate, highest retention ratio of bacteria and highest permeate flux, was the 0.2 ␮m ␣-Al2 O3 membrane. Thus, the 0.2 ␮m ␣-Al2 O3 membrane was selected for analyzing the influence of operating conditions on resistances.

Fig. 5. Effect of TMP on the filtration resistances due to different sources (0.2 ␮m ␣-Al2 O3 membrane, T = 22 ± 3 ◦ C, V = 0.58 m s−1 and Cb = 0.38 kg L−1 ).

particles in cake layer may be pressured to travel through the cake layer gradually, initially plug the pores by bridging or transporting into pores, and thus create internal pore restrictions to fluid flow. These reasons may cause the slight rise of Rif and the slight decrease of Rc with increasing TMP when the TMP was higher than 0.10 MPa. As shown in Fig. 6, Rcp (43–51% of Rt ) and Rc (25–39% of Rt ) were the main components of the fouling resistances at low operating pressure range, while Rcp (66–70% of Rt ) became the dominant fouling resistance and Rc (11–13% of Rt ) became the secondary dominant resistance at high operating pressure range. Rif, which was 14–18% of Rt , was secondary dominant resistance continuously. Rm was negligible compared with the fouling resistances, which was only 2–7% of Rt .

3.2. Influence of operating conditions on resistances 3.2.1. Transmembrane pressure (TMP) Figs. 5 and 6 show the hydraulic resistances and their relative percentages as a function of TMP for 0.2 ␮m ␣-Al2 O3 membrane. It was observed that the Rt and Rcp increased rapidly during lower operating pressure range and gradually when TMP was higher than 0.15 MPa. Increase of Rt was mainly caused by the variation of Rcp . The variation tendency of Rcp was close to the results of previous studies [18,19] that Rcp was proportional to the TMP. Rc grew gradually with increasing TMP from 0.05 to 0.10 MPa, but decreased slightly with increasing TMP. An increase of TMP could enhance the convective flow of colloid particle towards the membrane surface, which subsequently enhanced the polarization and deposition of particle. This explained the increase of Rcp and Rc with pressure. However, when the TMP is higher than 0.10 MPa, some fine colloid

Fig. 6. Hydraulic resistance percentages as a function of TMP (0.2 ␮m ␣-Al2 O3 membrane, T = 22 ± 3 ◦ C, V = 0.58 m s−1 and Cb = 0.38 kg L−1 ).

M. Li et al. / Journal of Membrane Science 299 (2007) 122–129

Fig. 7. Influence of CFV on hydraulic resistance (0.2 ␮m ␣-Al2 O3 membrane, T = 22 ± 3 ◦ C, P = 0.10 MPa and Cb = 0.38 kg L−1 ).

3.2.2. Crossflow velocity (CFV) Figs. 7 and 8 show various sources of filtration resistances and their relative percentages in crossflow microfiltration under various CFV for 0.2 ␮m ␣-Al2 O3 membrane. It was found that Rt and Rcp decreased rapidly with increasing CFV from 0.29 to 0.58 m s−1 and gradually with increasing CFV. Variation of Rt was also mainly caused by the variation of Rcp. The decrease of Rcp was due to the thinner diffusion layer caused by the increase of CFV. This influence would be due to its role in the shearing stress. Among the Rt , Rc increased slightly with increasing CFV from 0.29 to 0.58 m s−1 but decreased with increasing CFV continuously. It is well known that the deposit layer will become thinner with increasing CFV due to its role in the shearing stress. This is the positive influence of velocity on the filtration. On the other hand, increase of CFV will cause the long-chain macromolecules and colloid particles of cake formed becoming finer due to the selective deposition caused

Fig. 8. Hydraulic resistance percentages as a function of CFV (0.2 ␮m ␣-Al2 O3 membrane, T = 22 ± 3 ◦ C, P = 0.10 MPa and Cb = 0.38 kg L−1 ).

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Fig. 9. Influence of total solids concentration on hydraulic resistance (0.2 ␮m ␣-Al2 O3 membrane, P = 0.10 MPa, T = 22 ± 3 ◦ C and V = 0.58 m s−1 ).

by crossflow effect and produce a negative influence on the filtration [13,20]. The two competing factors may contribute to the change of Rc with increase of velocity. During the low CVF range (from 0.29 to 0.58 m s−1 ) the contribution of the latter to cake was more than the former, thus Rc increased with increasing CFV. With increasing CFV continuously, the contribution of the latter to cake became less than the former, thus Rc decreased with increasing CFV. Rif decreased with CFV within the low velocity range of less than 0.58 m s−1 and then grew slightly at the high range. The decrease of Rif with CFV could be explained by the effect of increasing of gel layer. The fine colloid particles were more difficult to transport into membrane pores. The increase of Rif with CFV may also be explained by the effect of shear stress. When the particle size and the thickness of the cake decreased with the CFV, the penetration of the finer particles in the membrane pores was enhanced and the Rif increased a little.

Fig. 10. Hydraulic resistance percentages as a function of total solids concentration (0.2 ␮m ␣-Al2 O3 membrane, P = 0.10 MPa, T = 22 ± 3 ◦ C and V = 0.58 m s−1 ).

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Table 4 Values of α, β, γ and δ for Eqs. (2)–(4) Parameter

α

β

Rcp Rc Rif

−1.935 × 1013

3.694 × 108

1.570 × 1012 1.527 × 1012

γ 9.222 × 1012

2.500 × 106 7.884 × 106

−4.852 × 109 1.859 × 109

As shown in Fig. 8, the main component of the fouling resistances was still Rcp (58–65% of Rt ) at different velocities. Rc (11–36% of Rt ) and Rif (14–27%) were the secondary dominant resistances. Rm could still be negligible compared with the fouling resistances, which was only 2–5% of Rt . 3.2.3. Total solids concentration (TSC) In order to understand the role of viscosity on different fouling behavior, raw soy sauce was diluted with deionized water into different total solids concentration (TSC). The viscosity of raw soy sauce for different TSC (0.38, 0.19 and 0.13 kg L−1 ) are 6.32, 3.65 and 2.54 mPa s at 22 ± 3 ◦ C, respectively. The resistance and their relative percentages as a function of TSC are shown in Figs. 9 and 10. It is well known that increase of concentration will enhance the concentration polarization and adsorption of colloid macromolecule, which result in the increase of Rc and Rcp . And these cause the growth of gel-layer, the fine colloid particles and macromolecule are difficult to transport into membrane pores. So Rif and its relative percentage decreased linearly with TSC. However, different from the particle suspension system [13], Rt decreased slightly with increasing TSC in this colloid system due to the change of viscosity with TSC. As shown in Fig. 10, the main component of the fouling resistances was Rif (42–53% of Rt ) at low TSC (from 0.13 to 0.19 kg L−1 ). Rc (24–36% of Rt ) was the secondary dominant resistance. Rcp could be neglected when TSC was at 0.13 kg L−1 , which was only 6%–8% of Rt . Nevertheless, with the increase of TSC, the relative percentage of it grew rapidly until becoming the dominant resistance at high TSC (0.38 kg L−1 ). Rm was negligible, which was only 1–5% of Rt . 3.2.4. Correlations between the resistances and operating conditions According to the results of previous studies [18,21], the Rcp was proportional to the P. Rif was proportional to the TSC [22,23]. Rc could be expressed as a function of the TSC [24]. However, it was observed that the experimental results of Rcp , Rc and Rif were transmembrane pressure (P), crossflow velocity (ν) and total solids concentration (C) dependent. Then, an empirical polynomial correlation of Rcp , Rc and Rif was determined, using the program STATISTICA® , and the empirical models presented the form: Rcp = α + β × P + γ × e−v + δ × ln c

(2)

Rc = α + β × P + γ × v−2 + δ × c

(3)

Rif = α + β × P + γ × e−v + δ × c

(4)

δ

R 2.327 × 1012

−1.979 × 109 −5.891 × 109

0.9662 0.5197 0.8833

α, β, γ and δ values are presented in Table 4. According to these fitting parameters, the polynomial estimated relation of Rcp , Rc and Rif , as a function of P, ν and C, would allow the interpolation for other operational conditions in microfiltration of raw soy sauce system by ceramic membrane. However, the correlation coefficient (R) was too low for Rc . This may be due to the instability of cake layer during microfiltration of the complex raw soy sauce feed. Therefore, it would be of interest for further study. 4. Conclusions Ceramic tubular MF membranes with different mean pore sizes and materials were used to clarify raw soy sauce. The results showed that the suitable membranes, which provided the best permeate quality and highest permeate flux, was the 0.2 ␮m ␣-Al2 O3 .membrane. More than 99% of bacteria could be removed from raw soy sauce. The ceramic membrane filtration was a reliable substitute to traditional methods of clarifying raw soy sauce. During the microfiltration of raw soy sauce, the total filtration resistance (Rt ) and the concentration polarization resistance (Rcp ) increased significantly with increasing nominal pore size while the cake resistance (Rc ) and the internal fouling resistance (Rif ) decreased slightly. Rif could be neglected when the nominal pore sizes were greater than 0.5 ␮m, which was only 2.2% of Rt . For different materials, the Rt of the ZrO2 membrane was much larger than that of the ␣-Al2 O3 membrane. The fouling resistance of ZrO2 membrane was Rcp -dominant. The Rm (the membrane’s own resistance), Rc and Rif values of the ZrO2 membrane remaind negligible, which reached a maximum of 1.3–4.1% of the Rt . As for the ␣-Al2 O3 membrane, the dominant fouling resistances were Rc and Rcp . Under different operating conditions used in the experiments, the permeability of the 0.2 ␮m ␣-Al2 O3 membrane decreased rapidly due to concentration polarization resistance (Rcp ) and Rcp was the dominant fouling resistance for microfiltration of such high viscosity system. Further flux decline was caused by the growth of a gel-type layer (cake resistance, Rc ) over the membrane surface and internal fouling resistance (Rif ), which were the secondary dominant resistance. The different resistances (Rcp , Rc and Rif ) were empirically modeled as a function of the operating conditions, such as transmembrane pressure, crossflow velocity and total solutes concentration, to allow the interpolation for other operational conditions. The results will provide theoretical foundations for carrying out necessary means to reduce fouling in the application of MF for clarification of raw soy sauce, which is a important aspect of membrane usage.

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