Separation of Sucrose and Reducing Sugar in Cane Molasses by Nanofiltration

Transcription

1 Food and Bioprocess Technology (218) 11: ORIGINAL PAPER Separation of and Reducing Sugar in Cane Molasses by Nanofiltration Jianquan Luo 1 & Shiwei Guo 1 & Yuanyuan Wu 1 & Yinhua Wan 1 Received: 3 August 217 /Accepted: 15 January 218 /Published online: 3 January 218 # Springer Science+Business Media, LLC, part of Springer Nature 218 Abstract Recovery of sugars from cane molasses is a promising approach to increase the added value of molasses and reduce its environmental pollution. In this work, for the first time, nanofiltration (NF) was used for the separation of sucrose and reducing sugar in cane molasses by a cascade diafiltration-concentration process. The retention difference between sucrose and reducing sugar by all the tested NF membranes was not distinct at 25 C, while due to the thermal-induced pore size change and enhanced solute diffusivity, the NF retention behavior changed significantly at 6 C, and the DL membrane with a sucrose retention of 96% and a reducing sugar retention 5% was selected for the process optimization and modeling. High temperature (55 6 C), lowpermeateflux(below15lm 2 h 1 ), and high sugar concentration resulted in a low retention of reducing sugar due to the dominant diffusive mass transfer, which was desirable for the molasses separation by NF. Mathematical modeling could well predict the diafiltration and concentration processes if using right sugar retention data. The deviations between prediction lines and experimental data in the cross-flow filtration of real solution were mainly caused by the permeate flux variation rather than membrane fouling. After diafiltration, the ratio of sucrose in total molasses sugar increased from 76.1 to 87.9%, while in the permeate of the second concentration step, the ratio of sucrose was only 2.4%. Thus, the retentate of diafiltration could be directly used for sucrose crystallization to avoid the accumulation of reducing sugar and salts, and the permeate of the second concentration step could be concentrated by NF27 at room temperature to produce syrup drinking. Keywords Diafiltration. Membrane filtration. Sugar recovery. Molasses purification. Modeling Introduction Cane molasses, a thick and brown syrup obtained as a byproduct from the processing of sugarcane into sucrose, consists of fermentable carbohydrates (i.e., sucrose, glucose, fructose) and some non-sugar organic materials (e.g., pigments, amino acids, inorganic salts, phenolic compounds) (Baikow 213). The quantity of cane molasses available in China is about 48, tons/year, and it is mainly used as a supplement for livestock feed and as a source of carbon in fermentation processes (e.g., ethanol production) (Valli et al. 212; * Jianquan Luo Jqluo@ipe.ac.cn * Yinhua Wan yhwan@ipe.ac.cn 1 State Key Laboratory of Biochemical Engineering, Institute of Process Engineering, University of Chinese Academy of Sciences, Chinese Academy of Sciences, Beijing 119, China M. Sharma et al. 216) since the price of cane molasses is lower than $15/ton, and the sugar factory can get little profit from the molasses sale. Moreover, the fermentation with cane molasses as carbon source produces a large amount of highstrength wastewater; thus, the molasses demand in downstream industry is decreasing (Nandy et al. 22). Therefore, recovery of sugars from cane molasses is a promising approach to increase the added value of molasses and reduce its environmental pollution. Considering that pigments, inorganic salts, sucrose, and reducing sugar are the main components in cane molasses, three operations including decolorization, desalination, and sugar fractionation are required to recover the sugars (account for 5% of the total weight) from the molasses. In most patents, cane molasses was decolorized and desalted by chemical treatment (e.g., organic solvent, alkaline, acid) followed by ion exchange (Riffer 1977; Nakasone et al.1985; Ou 1985; Clarke 1995). In addition, an integrated process including softening, filtration, and chromatography (i.e., ion-exclusion) was successfully applied in beet molasses purification

2 914 Food Bioprocess Technol (218) 11: (Lameloise and Lewandowski 1994; Kearney and Kochergin 22); however, it was not economical in cane molasses industry because of its high level of suspended solids and hardness resulting in high cost of the pretreatment (Kearney and Kochergin 22). Recently, membrane technology combined with chemical treatment, activated charcoal adsorption, or ion exchange has also been reported for sugar decolorization (Bernal et al. 216; W. Li et al. 216; Susanto et al. 216; Zhu et al. 216). Regarding the sugar fractionation, Donovan and Hlavacek (22) patented a nanofiltration (NF) process for the separation of sucrose and reducing sugar in cane molasses, and stated that by using NF, reducing sugar content in molasses could be decreased from 23 to 2%, and the ash level reduced from 17 to 7%, while the sucrose content increased to 75%, which could be fed to crystallization equipment for sucrose recovery. In our previous study, an integrated membrane process including a loose ultrafiltration (UF) for clarification, a tight UF for decolorization, and a NF for sucrose concentrating and glucose/fructose/salt removing was proposed for sugarcane juice refining (Luo et al. 216). These indicated that NF process was promising for sugar fractionation in cane molasses purification. However, to the best of our knowledge, there has been no systematic work on the separation of sucrose and reducing sugar in cane molasses. NF, as a membrane technology based on both charge effect and size exclusion, is a powerful tool for separation of small molecules and salts, which has attracted growing attention in many fields such as wastewater treatment, water purification, desalination, food processing, and bio-separation (Luo and Wan 213; Wang et al. 22; Altmann et al. 216; Meyer et al. 217). Since the molecular weight difference between sucrose and glucose/fructose is only 162 Da, the separation of sucrose and reducing sugar is challenging. Goulas et al. (22) found that using DL membrane at 2.7 bar, the sucrose rejection was 99 and 97% at 25 and 6 C, respectively, while fructose rejection was 77 and 53%, respectively, and the lower sugar rejection at higher temperature was caused not only by the enhanced diffusivity of solutes but also by the larger effective pore size (i.e., thinner layer of adsorbed water molecules on the pore wall at higher temperature). Kuhn et al. (21) reported that the rejection of glucose/fructose by six NF membranes was below 3% at 25 C, while the sucrose rejection was up to 93%. Zhao et al. (213) showedthatat 3 C, the rejection difference between sucrose and glucose was decreasing with increase of permeate flux, for example, their rejections were 42 and 96% at 5 Lm 2 h 1,respectively, but increased to 8 and 99.8% at 2 Lm 2 h 1,respectively. Bandini and Morelli (217) recently claimed that the rejections of dextrose and fructose reduced at higher temperature, while such reduction became smaller at higher permeate flux. Therefore, it could be concluded that the separation efficiency of disaccharide and monosaccharide by NF was mainly governed by their diffusivity difference, and higher temperature and lower permeate flux are preferred thanks to the more significant diffusive mass transfer. Accordingly, effective separation of sucrose and reducing sugar in cane molasses can be achieved by NF at high temperature, especially at low permeate flux. However, most studies only focused on the purification of oligosaccharides (e.g., fructo-oligosaccharides, galacto-oligosaccharides) by NF (Goulas et al. 22; Kuhn et al. 21; Pruksasri et al. 215; Córdova et al. 216). The present work was undertaken to select a suitable NF membrane for separation of sucrose and reducing sugar (i.e., glucose and fructose) in cane molasses, investigate the effect of process parameters on the separation performance, and clarify the mechanisms behind various phenomena induced by temperature and permeate flux. Moreover, a diafiltration followed by multistage concentration processes was proposed, aiming at improving the NF separation efficiency to sucrose and reducing sugar. We also attempted to establish mathematical models for diafiltration/concentration process both under bench-scale dead-end mode and pilot-scale cross-flow mode. Finally, a real solution (cane molasses after partial decolorization by a tight UF) was treated by the cross-flow NF module to validate the proposed process and models. This study not only elucidated the separation mechanisms for small neutral solutes by NF, but also offered an alternative to recover sugars from cane molasses. Materials and Methods Cane Molasses, Chemicals, and Membranes Cane molasses was provided by Yaoming Sugar Industry Co., Ltd. (Guangdong, China), which contained ~ 8 wt.% soluble solids (~ 47.4 wt.% sucrose and ~ 16.5 wt.% reducing sugar). About 1.5 kg cane molasses was diluted with 3 L deionized water, and the solution was adjusted to ph 7 by adding solid sodium hydroxide. After ph adjustment, this diluted molasses was centrifuged at 5g and 25 C for 15 min by a refrigerated centrifuge (4k-15, Sigma, Germany), which has a viscosity of Pa s at 55 6 C ( Pa s at 2 C). The supernatant was decolorized by 2 kda spiral-wound UF membrane (Sepro Membranes, USA) under concentration mode, and the resulting permeate (i.e., light-colored molasses) for NF diafiltration contained g L 1 sucrose, g L 1 reducing sugar, and 5 ms cm 1 conductivity., glucose, and fructose were of analytical grade and purchased from Xilong Scientific Co., Ltd. (Guangdong, China). For the model solution with mixed sugars, the total sugar concentration was 19 g L 1, and the concentration ratio of sucrose, glucose, and fructose was 14:25:25 except elsewhere stated. Nine commercial NF membranes were tested in this work, and their main properties are summarized in Table 1.

3 Food Bioprocess Technol (218) 11: Table 1 Main properties of the NF membranes examined Membrane Manufacturer Surface material MgSO 4 rejection (%) a Permeability (L h 1 m 2 bar 1 ) b NF27 Dow-Filmtec Poly(piperazinamide) ±.3 XN45 Trisep Poly(piperazinamide) ±.2 DK GE-osmonics Polyamide ±.5 DL GE-osmonics Polyamide ± 1.1 NT12 Microdyn-Nadir Poly(piperazinamide) ±.3 NT13 Microdyn-Nadir Polyamide ±.6 NF7 Development Center Poly(piperazinamide) ±.8 NF4-I of Water Treatment Poly(piperazinamide) ±.2 Technology, Hangzhou NF4-II Poly(piperazinamide) ± 1.4 a MgSO 4 rejection determination conditions: 2 ppm, bar, 25 C b Pure water permeability was measured at a constant flux of Lm 2 h 1,25 C Experimental Setup and Procedure Dead-end Filtration at Constant Flux The dead-end filtration experiments were conducted in a laboratory-constructed magnetically stirred cell at constant flux mode (Fig. 1a). This device could be fitted with a membrane disc with an effective membrane surface area of m 2, and the working volume of the cell was 13 ml. Feed solution was pre-filled into an injection column (Superloop 5 ml, Pharmacia, Sweden) and then pumped into cell for the concentration mode, while for the diafiltration mode, deionized water was pumped into cell directly. More detailed information of this setup could be found in our previous work (Luo et al. 29). A new membrane was used for each series of experiments, and each membrane disc was firstly dipped in 5% ethanol solution for about 5 s to remove manufacturing residues from the membrane and then soaked in deionized water for at least 12 h prior to use. Before the water permeability measurement at different fluxes, all the virgin membranes were compacted at 35 bar for 3 min. For the membrane selection and process optimization, 26 ml model solution was concentrated to 13 ml at Lm 2 h 1 and 12 rpm, and two temperature of 25 and 6 C was employed except elsewhere stated. During the concentration process, the first 1 ml permeate was discarded to eliminate the interference effect of the residual water in the pipeline, and the rest 12 ml permeate and the corresponding retentate were collected for analysis. The average transmembrane pressure (TMP) during the concentration process was recorded. For the cascade NF processes, the operating procedure was described as follows. Fourteen milliliters of model solution was injected into the cell, and after discarding the first 1 ml permeate, 39 ml deionized water was pumped into the cell at Lm 2 h 1, 12 rpm, and 6 C. The permeate was collected every 5 ml in vials for analysis (the average TMP for each sample was also recorded). The permeate from diafiltration operation was further concentrated by NF at 39.8 Lm 2 h 1 and 6 C, where 13 ml model solution (sucrose 1.4 g L 1,reducingsugar14.7gL 1 )was Fig. 1 Schematic diagrams of a dead-end filtration under constant flux mode and b cross-flow filtration under constant pressure mode

4 916 Food Bioprocess Technol (218) 11: concentrated to 13 ml, and the permeate sample was collected every 15 ml for analysis. The permeate from the first concentration operation was further concentrated by NF at Lm 2 h 1 and 6 C, where 13 ml model solution (sucrose 1.5 g L 1, reducing sugar 12. g L 1 ) was concentrated to 13 ml, and the permeate sample was collected every 15 ml. Membrane permeability was examined again after the filtration to monitor the irreversible fouling. Cross-flow Filtration at Constant Pressure The cross-flow filtration was performed in a pilot-scale device (Fig. 1b) consisting of a 5-L feed tank with an insulating jacket, a feed pump, a spiral-wound membrane module (effective membrane area =.34 m 2 ), and a water bath tank equipped with a circulating pump. The feed temperature could be controlled by the circulating water bath. The diafiltration was carried out at 25 bar, 55 C, and a cross-flow velocity of 5Lmin 1, where 3 L real solution (light-colored cane molasses) was diafiltrated by 9 L deionized water at constant volume diafiltration mode. Then, the first concentration operation was conducted at 1 bar, 55 C, and 5 L min 1, where 8 L permeate from the diafiltration process was concentrated to.8 L. Finally, the second concentration operation was carried out at 5 bar, 55 C, and 5 L min 1, where 6.5 L permeate from the first concentration process was concentrated to.65 L. After each test, NF membrane module was rinsed using hot deionized water with the same temperature as feed temperature. Membrane permeability was examined before and after the filtration to monitor the irreversible fouling. Analytical Methods content was measured by Roe colorimetric method (J. Li 28), and reducing sugar concentration was determined by the method of Miller with dinitro salicylic acid reagent (Miller 1959). Calculation and Modeling The observed retention (R obs ) of sucrose and reducing sugar is defined as follows: R obs ¼ 1 C p 1; ð1þ C r;av where C p is the sugar concentration in the permeate and C r,av is the average concentration in the retentate for a specific operation period of time for sampling, that is, the average of the solute concentration in the retentate at the beginning and that at the end of collecting a given volume of permeate sample. For a batch filtration, the average sugar concentration in the retentate is calculated as follows: C r;av ¼ 1 2 ð C f þ C r Þ; ð2þ where C f and C r are the sugar concentrations in the feed and retentate. Modeling for Dead-end Filtration Under Constant Flux Mode (Fig. 1a) (a) Diafiltration process During the constant volume diafiltration, the loss of solute in the dead-end cell was equal to the mass in the permeate, that is, V dc ¼ C p dv; ð3þ where V and V are the cell volume and diafiltration solvent volume, respectively, and V isthesameasv r (retentate volume). Substituting Eq. (1) intoeq.(3), and then integrating it at V =,C = C f gives, C r ¼ C f exp (b) ðr obs 1Þ V V Concentration process : ð4þ During the concentration process, the solute mass in the cell was equal to the feed mass minus the mass in the permeate, V dc ¼ C f d C p dv p ; ð5þ where and V p are the feed and permeate volumes, respectively, /V should be larger than 1, and V p = 13. Substituting Eq. (1) intoeq.(5) and then integrating it at = V, C = C f, following equation can be obtained: R obs exp ðr obs 1Þ 1 1 V C r ¼ C f : ð6þ R obs 1 Modeling for Cross-flow Filtration Under Constant Pressure Mode (Fig. 1b) (a) Diafiltration process The diafiltration process in cross-flow filtration was similar to that in dead-end filtration, which could be modeled by Eq. (4).

5 Food Bioprocess Technol (218) 11: (b) Concentration process In dead-end filtration, fresh feed was continuously pumped into the cell, while in cross-flow filtration, all the feed was poured into the feed tank in the beginning and then recirculated in the system. The solute mass in the retentate is equal to the feed mass minus the mass in the permeate: C r V r ¼ C f C p V p ; ð7þ where V r is the retentate volume. Substituting Eq. (1) into Eq. (7), the following equation can be obtained: ð1 þ R obs ÞC f þ C f ð1 R obs Þ V C r ¼ r 1 þ V f þ R obs 1 V : ð8þ f V r V r Results and Discussion Effect of Membrane Type on Separation of and Reducing Sugar As listed in Table 2 (Ribeiro et al. 26), the molecular weight difference between sucrose and glucose/fructose is only 162 Da, and their Stokes radius difference is around.1 nm; thus, it is difficult to separate them only based on size exclusion at room temperature. Regarding the solute diffusivity, when the temperature increased from 25 to 6 C, their diffusivity difference raised from to m 2 s 1, implying that the separation of sucrose and reducing sugar by NF could be enhanced at higher temperature. It was reported that with increasing temperature, solute rejection by NF would decrease due to not only the increase in the solute diffusivity but also the enlargement of the membrane pore size (Dang et al. 214). Many studies confirmed the increase in effective pore size of NF membranes at higher temperature by both modeling and physical characterization (R. R. Sharma and Chellam 25; Ben Amar et al. 27; Saidani et al. 21; Mänttäri et al. 22). Such thermal-induced pore size change was caused by two possible reasons: one is the thinner hydration layer on the pore wall owing to higher diffusivity of water molecules (Luo and Wan 213); another is the structural changes in network pores due to the thermal expansion of polymer (Saidani et al. 21). First, the separation of sucrose and reducing sugar by different NF membranes was evaluated at 25 C, and as shown in Fig. 2a, except for XN45, NT12, and NF4-II, the retention of sucrose and reducing sugar by each NF membrane is similar, indicating that these NF membranes cannot be used for this purpose at room temperature. For XN45 and NF4-II, the retention of sucrose was higher than 95%, while their reducing sugar retention was around 7 and 5%, respectively, implying that their separation efficiency was not high and economical. Surprisingly, for NT12, reducing sugar had a much higher retention than sucrose (8 vs. 5%), and this might be related to its special pore structure or material affinity. Actually, Koschuh et al. also found similar results with polyethersulphone NF membranes (R glucose > R sucrose ) (Koschuh et al. 25). When temperature became 6 C, the sugar retention behavior greatly changed for all the NF membranes as expected. Comparing the results in Fig. 2a, b, it was found that for NF27 and XN45, the retention of both sucrose and reducing sugar distinctly declined at 6 C, especially for reducing sugar. This was reasonable considering the increase in the solute diffusivity and the enlargement of the membrane pore size at higher temperature (Goulas et al. 22; Dang et al. 214). For NT13 and NF7, the retention of reducing sugar decreased as expected, while their sucrose retention Table 2 Main properties of the sugars in cane molasses (Ribeiro et al. 26) Index Glucose Fructose Structure Molecular weight (Da) Stokes radius (nm) Diffusivity (1-9 m 2 s -1 ) at 25 o C Diffusivity (1-9 m 2 s -1 ) at 6 o C

6 a dtmp (Bar) 918 Food Bioprocess Technol (218) 11: Fig. 2 Retention of sucrose and reducing sugar by different NF membranes at a 25 C and b 6 C; TMP during separation of sucrose and reducing sugar by different NF membranes at c 25 C and d 6 C. Permeate flux = Lm 2 h T=25 o C b T=6 o C DK XN45 NF27 DL DK XN45 NF27 DL c 4 32 T=25 o C NF4-II NF4-I NF7 NT13 NT12 : 7 gl -1 : 25 gl T=6 o C NF4-II NF4-I NF7 NT13 NT12 : 14 gl -1 : 5 gl -1 TMP (Bar) DK XN45 NF27 DL NF4-II NF4-I NF7 NT13 NT12 DK XN45 NF27 DL NF4-II NF4-I NF7 NT13 NT12 unexpectedly increased to 94% at 6 C, which was possibly caused by the positive change in polymer physical state (pore deformation) for these two membranes (Saidani et al. 21). This phenomenon became more obvious for NT12, where the retention of both sucrose and reducing sugar increased to 9% at 6 C (their retention was only 5 and 8% at 25 C, respectively). However, NF4-I and NF4-II could not sustain the high temperature, and the sugar retention greatly dropped at 6 C. Regarding DK and DL with good reputations in the applications under extreme conditions, their sucrose retention could be kept at 96% at 6 C, while the reducing sugar retention greatly declined to 15 and 5%, respectively, meaning that their slightly increased pore size was perfect for the separation of sucrose and reducing sugar. Compared to the results in literature, the obtained separation factor for sucrose and reducing sugar was the highest in this study (Table 3), which was attributed to the suitable NF membrane, operating temperature, and permeate flux. Moreover, as shown in Fig. 2c, d, the operating TMP at constant flux for different membranes is corresponding to their sugar retention, that is, larger sugar retention resulted in higher TMP, implying that the permeate flux in this case is dominated by osmotic pressure difference across the membrane, and lower reducing sugar retention was not only good for separation efficiency but also helpful to save energy. Thus, the DL membrane was selected for the further study. Effect of Operating Parameters on Separation of and Reducing Sugar Separation of sucrose and reducing sugar by DL was conducted at 45, 55, and 6 C, respectively, and when temperature ascended from 45 to 55 C, the sucrose retention decreased from 96 to 94%, and the reducing sugar retention declined from 42 to 14% (Fig. 3a). However, the average TMP during the filtration increased from 29 to 3 bar. Theoretically, the operating TMP was supposed to descend due to lower sugar retention and smaller solvent viscosity at higher temperature. Thus, such unexpected TMP increase was caused by increasing osmotic pressure at higher temperature (osmotic pressure calculation: π =nrt, T is temperature) (Luo et al. 216). When temperature increased to 6 C, the sugar retention did not further decline but the TMP decreased from 3 to 27 bar, which was caused by the dominant viscosity reduction (Luo and Wan 213). AsshowninFig.3b, the rotating speed from 8 to 16 rpm in the dead-end cell has negligible effect on the sugar retention and TMP, implying that the back diffusion of these small solutes is intensive at higher temperature and the concentration polarization layer is quite thin. As seen in Fig. 3c, when the permeate flux increases from 6.64 to Lm 2 h 1, the TMP went up linearly, indicating that these experiments were operated below Bthreshold flux^

7 Food Bioprocess Technol (218) 11: Table 3 Comparison of sucrose/ reducing sugar separation by NF membranes with the results in literature Membrane used (temperature) retention (%) retention (%) Separation factor (R suc /R red ) Reference DL (6 C) Goulas et al. (22) NF45 (32 C) Wang et al. (22) DL (25 C) Kuhn et al. (21) NF-3D (3 C) Zhao et al. (213) DL (6 C) Present study (Luo and Wan 213). Moreover, the effect of permeate flux had negligible effect on the sucrose retention, while the reducing sugar retention enhanced obviously when the permeate flux increased from to Lm 2 h 1, implying that a rpm Lm -2 h b o C Lm -2 h TMP TMP Temperature ( o C) Rotating speed (rpm) 16 c o C 12 rpm 4 3 d 1 8 : reducing sugar (g/g) =14:5 6 o C 12 rpm Lm -2 h TMP TMP Permeate flux (Lm -2 h -1 ) Total concentration (gl -1 ) e 1 8 : 14 gl -1 6 o C 12 rpm Lm -2 h TMP :1 14:3 14:5 /reducing sugar (g/g) Fig. 3 Effect of temperature (a), rotating speed (b), permeate flux (c), total sugar concentration (d), and sucrose/reducing sugar ratio (e) on the retention of sucrose and reducing sugar as well as TMP by DL membrane

8 92 Food Bioprocess Technol (218) 11: a lower permeate flux with dominant diffusive mass transfer was preferred for separation of sucrose and reducing sugar. Effect of Sugar Concentration and Ratio on Separation of and Reducing Sugar Cane molasses normally contains 5 65% sugars and cannot be directly treated by NF because of its high osmotic pressure and high viscosity. Thus, a dilution operation is required, and the effect of total sugar concentration on the sugar retention was investigated. As seen in Fig. 3d, with increase of sugar concentration from 95 to 228 g L 1, the TMP raises remarkably due to the higher osmotic pressure and larger solution viscosity, while the sugar retention decreases slightly because the concentration polarization becomes more serious and the concentration gradient-driven mass transfer across the membrane is enhanced. A lower retention of reducing sugar is desirable while a higher sucrose retention is preferred. Therefore, a suitable dilution factor is important to achieve both high separation efficiency and high sucrose recovery. During the NF of diluted molasses, the concentration ratio of sucrose and reducing sugar was changing attributed to their different retention, and thus, the effect of concentration ratio on the sugar retention was examined. As seen in Fig. 3e, with increase of reducing sugar content (sucrose concentration is constant), both sucrose and reducing sugar retention slightly decreases because of the increasing concentration polarization, indicating that the sugar concentration ratio has negligible effect on the separation efficiency while the viscosityinduced mass transfer determines the sugar retention (at constant operating conditions). Separation of and Reducing Sugar in Cane Molasses by Cascade NF Process In order to improve the separation efficiency and sugar recovery, an integrated diafiltration-concentration process (cascade NF process) was proposed as illustrated in Fig. 4. First, constant volume diafiltration was carried out to remove most of reducing sugar by adding deionized water, and the permeate from diafiltration was further concentrated by the same NF membrane (DL), where the retentate rich in sucrose was recycled to the diafiltration step, and the permeate was concentrated by the second concentration step to further recover the residual sucrose. Finally, a relatively pure reducing sugar could be obtained in the permeate of the second concentration step. This cascade NF process was conducted by both deadend and cross-flow filtrations (using model solution and lightcolored cane molasses, respectively) at 55 6 C, and the experimental data and modeling results were compared to gain deeper understanding on this separation process. NF Diafiltration As shown in Fig. 5a, the sucrose concentration in the retentate is decreasing, which is in agreement with the prediction line (R sucrose = 94%), while the retention of reducing sugar (3 4%) in the prediction line is much higher than that obtained in Fig. 3a (< 1%). This was explained as follows: the sugar concentration in the retentate was decreasing during diafiltration, while for the results in Fig. 3a, the concentration mode was used and the sugar concentration in the retentate was increasing; since their initial feed concentrations were the same, the average concentration in the retentate for the diafiltration experiment should be much lower than that for the concentration experiment; Fig. 3d, e showed that with decreasing sugar concentration, the retention of reducing sugar increased significantly; thus, during the diafiltration, the prediction line with a higher reducing sugar retention of 3 4% could match the experimental data. Moreover, as shown in Fig. 5b, the TMP was declining during diafiltration because of less osmotic pressure resulted from sugar molecules as well as lower solution viscosity (smaller filtration resistance). While the light-colored molasses was used as feed and a cross-flow membrane system was employed (Fig. 5c), the sucrose concentration in the retentate followed the prediction line with a sucrose retention of 94% at the beginning of diafiltration (V/V < 1.5), but when V/V was larger than 2, the sucrose retention in the experiment was found to be much higher than 94%. For reducing sugar, the prediction line with a retention of 4% greatly deviated from the experimental data, even when predicting the reducing sugar concentration in the retentate by a retention of 6%, a deviation still occurred as V/ V > 2.5. Such increase in sugar retention was mainly caused by the augment of permeate flux during diafiltration under constant TMP mode (see Fig. 5d), which was verified by the results in Fig. 3c. Generally, more serious fouling formation (as additional selective layer or narrowing membrane pores) Fig. 4 Schematic diagram of integrated diafiltrationconcentration process for the separation of sucrose and reducing sugar in light-colored molasses

9 Food Bioprocess Technol (218) 11: Fig. 5 Experimental/predicted solutes concentration in the retentate (a) and TMP (b) during diafiltration step using dead-end filtration cell under constant flux mode. Model solution: sucrose 143gL 1, reducing sugar 51 g L 1, permeate flux = Lm 2 h 1 ;experimental/ predicted solutes concentration in the retentate (c) and permeate flux (b) during diafiltration step using cross-flow module under constant pressure mode. Light-colored molasses: sucrose g L 1, reducing sugar g L 1, TMP = 25 bar a C oncentration in retentate (gl -1 ) c C oncentration in retentate (gl -1 ) Model solution =94% =3% =4% V/V o Light-colored molasses =94% =4% =6% b d P ermeate flux (Lm -2 h -1 ) Model solution V/V o Light-colored molasses V/V V/V for the real solution would increase the solute retention (Luo et al. 213); however, in this work, the permeability loss after experiment was only 3.95% (Table 4), indicating that the different permeate flux (13.27 vs Lm 2 h 1 ) and feed concentrations (194 vs g L 1 ) were mainly responsible for the sugar retention variation, and membrane fouling would not be a bottleneck for this application in industry. NF Concentration In order to improve sucrose recovery, a concentration operation under a higher permeate flux of 39.8 Lm 2 h 1 was carried out to treat the permeate from the diafiltration step. The sugar retention was first measured at such permeate flux, which was 97 and 7% for sucrose and reducing sugar, respectively. This remarkable increment in reducing sugar retention at higher permeate flux indicated that the separation of sucrose and reducing sugar by NF could not be operated at high permeate flux, where the diffusive mass transfer was not dominant. As shown in Fig. 6a, the prediction line well matches the experimental data for sucrose, while for reducing sugar, as /V > 5, the prediction line with a retention of 7% deviates from the experimental data, and the experimental retention reduces to 65% because of the increasing sugar content in the retentate during the concentration operation. This increment in sugar concentration not only reduced the retention of reducing sugar but also elevated the TMP (Fig. 6b). When concentrating the permeate from the diafiltration of real solution, the sugar concentration in the retentate of cross-flow membrane module was increasing with /V as the same as those for model solution, because its average permeate flux was similar as that in dead-end filtration (Fig. 6c, d). In the second concentration step, a high permeate flux was used, and the sucrose retention raised to 98% and the reducing sugar retention increased to 76 78%. As seen in Fig. 7a, the reducing sugar concentration becomes much higher than the sucrose concentration in the retentate, implying that a further concentration operation is meaningless. The TMP during the Table 4 Water permeability of DL membranes before and after filtration during different steps Water permeability (Lm 2 h 1 bar 1 ) Diafiltration step 1st concentration step 2nd concentration step Before After Before After Before After Model solution with dead-end filtration 8.1 ± ± ± ± ± ±.4 Light-colored molasses with cross-flow filtration

10 922 Food Bioprocess Technol (218) 11: Fig. 6 Experimental/predicted solutes concentration in the retentate (a) and TMP (b) during the first concentration step using dead-end filtration cell under constant flux mode. Model solution: sucrose 1.5 g L 1, reducing sugar 14.6 g L 1, permeate flux = 39.8 Lm 2 h 1 ; experimental/predicted solutes concentration in the retentate (c) and permeate flux (b) during the first concentration step using cross-flow module under constant pressure mode. NF permeate of light-colored molasses: sucrose 5.77 g L 1, reducing sugar 7.9 g L 1,TMP=1bar a C oncentration in retentate (gl -1 ) c C oncentration in retentate (gl -1 ) Model solution =97% =65% =7% /V Light-colored molasses =97% =65% b TMP(bar) d P ermeate flux (Lm -2 h -1 ) Model solution /V o Light-colored molasses /V r /V r Fig. 7 Experimental/predicted solutes concentration in the retentate (a) and TMP (b) during the second concentration step using dead-end filtration cell under constant flux mode. Model solution: sucrose 1.53 g L 1, reducing sugar 12. g L 1, permeate flux = Lm 2 h 1 ; experimental/predicted solutes concentration in the retentate (c) and permeate flux (b) during the second concentration step using cross-flow module under constant pressure mode. NF permeate of light-colored molasses: sucrose.68 g L 1, reducing sugar 6.34 g L 1, TMP = 5 bar a C oncentration in retentate (gl -1 ) c C oncentration in retentate (gl -1 ) Model solution =98% =76% =78% /V o Light-colored molasses =97% =78% =55% b d P ermeate flux (Lm -2 h -1 ) Model solution /V o Light-colored molasses /V r /V r

11 Food Bioprocess Technol (218) 11: Fig. 8 Aproposedcascade nanofiltration process for separation of sucrose and reducing sugar based on the experimental data and mathematical modeling concentration process continuously ascended as expected (Fig. 7b). Regarding the real solution, as shown in Fig. 7c, the sucrose concentration in the retentate was well predicted by the model with a retention of 97%, while the prediction line with a reducing sugar retention of 78% was far higher than the experimental data. This was caused by a much lower permeate flux in cross-flow filtration (3 4 vs Lm 2 h 1,see Fig. 7d), and its retention of reducing sugar reduced to 55%. It can be concluded that the reducing sugar retention was greatly affected by the permeate flux, and the separation of sucrose and reducing sugar by NF should be operated below 15 Lm 2 h 1 at 55 6 C. Moreover, as listed in Table 4, the membrane permeability decline after filtrations is not significant for both model and real solution (less than 1%), especially for cross-flow filtrations with real solution (less than 5%), meaning that the residual pigments in light-colored molasses would not cause obvious fouling to polyamide NF membrane and the cross-flow operation can further reduce membrane fouling. Anyway, the separation performance of dead-end and cross-flow filtrations was similar under the same operating conditions, and the results obtained with model solution as feed could be used to guide industrial process. Figure 8 shows a proposed cascade NF process for separationofsucroseandreducingsugarbasedontheexperimental data and mathematical modeling. By using three times water for diafiltration of light-colored molasses, the ratio of sucrose in total sugar increased from 76.1 to 87.9%, and the impurities (e.g., reducing sugar, monovalent salts, and small pigments) could be partly removed from the molasses. In the NF process patented by Donovan and Hlavacek (22), under a permeate flux of 14 Lm 2 h 1 at 65 C, the ratio of sucrose in total sugar increased from 72.1 to 96.3% when three times water was consumed, while this ratio was 62.% in the permeate (in the present study, it was 41.9%). This comparison indicated that the NF membrane used in Donovan and Hlavacek s work had lower rejection of both sucrose and reducing sugar than that in our study, and their process would obtain higher purity of sucrose but much lower sucrose recovery. Then, the treated diluted molasses could be used for sucrose crystallization, and the diafiltration permeate was further concentrated by 3 times (the resulting concentrate has similar sucrose content as the diluted molasses) and then recycled to the diafiltration step. In the permeate of Table 5 Estimated economic analysis for sugar recovery from cane molasses by membrane process Cost ($/ton molasses) Cane molasses Membrane and filtration process Evaporation/crystallization Income ($/ton molasses) White sugar Fructose/glucose syrup The prices of molasses, white sugar, and syrup in Chinese domestic market are used for calculation; the membrane modules are supposed to be used for at least 1 year, supposing that 5% of sucrose in molasses can be crystallized and 8% of fructose/glucose can be extracted as liquid

12 924 Food Bioprocess Technol (218) 11: the second concentration step, the ratio of sucrose was only 2.4%, and this stream could be concentrated by NF27 at room temperature to produce syrup drinking. A rough cost estimation for the separation of sucrose and reducing sugar from cane molasses by membrane processes is shown in Table 5. Supposing that 5% of sucrose in molasses can be crystallized and 8% of fructose/glucose can be extracted as liquid, this novel technology is economically feasible if the membrane modules are able to maintain their performance at 6 C for at least 1 year. Conclusion This work demonstrated that a cascade nanofiltration (NF) process consisting of one diafiltration and two concentration steps could accomplish the separation of sucrose and reducing sugar (i.e., glucose and fructose) in cane molasses at high temperature. All the tested NF membranes could not separate sucrose and reducing sugar at 25 C, while they showed totally different retention behavior at 6 C. NT12, NT13, NF7, NF4-I, and NF4-II were not thermal stable at 6 C, and the sucrose retention of NT12, NT13, and NF7 unexpectedly increased at higher temperature. The retention of both sucrose and reducing sugar for NF27 and XN45 obviously declined at 6 C as expected, displaying low separation efficiency and sucrose recovery. For DK and DL, the sucrose retention was kept above 95% at 6 C, while the reducing sugar retention greatly declined to 15 and 5%, respectively, which was suitable for this purpose. High temperature (55 6 C), low permeate flux (< 15 Lm 2 h 1 ) and high sugar concentration resulted in a low retention of reducing sugar due to the dominant diffusive mass transfer, which was desirable for the molasses separation by NF. Diafiltration/concentration of the model solution and light-colored molasses by the DL membrane could be well predicted by the mathematical modeling if using right sugar retention data. The deviations between prediction lines and experimental data in the cross-flow filtration of real solution were mainly caused by the permeate flux variation rather than membrane fouling. Moreover, the separation performance of the dead-end and cross-flow filtrations was similar under the same operating conditions, and the results obtained by deadend filtration with model solution could be used to guide industrial process. Acknowledgements The authors thank the Development Center of Water Treatment Technology, Hangzhou, for kindly providing membrane samples used in this study. Funding Information The authors thank the Key Research Program of Chinese Academy of Sciences (No. KFZD-SW-211-3) for the financial supports and. 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