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Multi-step membrane processes for the concentration of grape juice
Desalination 191 (2006) 446–453
Multi-step membrane processes for the concentration of grape juice Attila Rektor*, Gyula Vatai, Erika Békássy-Molnár Faculty of Food Science, Department of Food Engineering, Corvinus University of Budapest, Budapest, Hungary Tel. +36 (1) 482-6113; Fax +36 (1) 482-6323; [email protected] Received 15 March 2005; accepted 27 June 2005
Abstract Grape production and viticulture have great traditions in Hungary. Nowadays wine-making is only profitable if the end-product is quality wine. Good-quality wine can be produced from must which has the proper high sugar content. If there is an unfavorable vintage, it is necessary to enrich the must, to increase its sugar content and to eliminate its other defects. Grape juice is not only a sweetener; it can be used for other purposes too because the juice of the grape is a pleasant, fashionable soft drink as well. However, the must storage can cause some difficulties, as fermentation starts easily. The aim of the investigations was to use osmotic distillation and membrane distillation after the microfiltration and reverse osmosis concentration [1] for water removal from grape juice in order to reach a high sugar content, to the extent that makes it possible to preserve the grape juice without cooling. Keywords: Grape juice; Preservation; Membrane distillation; osmotic distillation
1. Introduction One of the most valuable fruits in Hungary is the grape. The conditions of wine growing in the country are very good, and a large volume and good quality of must are produced. The greatest part of the vintage is processed by the viticulture and soft drinks industry. Because the ripening of *Corresponding author.
the grape and the vintage happen once a year, there can be a lack of grapes (or poor quality) or overproduction. The preservation of grape juice can solve the above problems. Viticulture could produce must concentrate from the surplus that could be applied by the upgrading of poor-quality, low-sugarcontent grape juice, or the soft drinks industry could develop a new product for its customers.
Presented at the International Congress on Membranes and Membrane Processes (ICOM), Seoul, Korea, 21–26 August 2005. 0011-9164/06/$– See front matter © 2006 Published by Elsevier B.V.
doi:10.1016/j.desal.2005.06.046
A. Rektor et al. / Desalination 191 (2006) 446–453
The high-quality requirements and expectations of the customers (new foods) do not include chemical preservation, additives and high-temperature applications [2]. Membrane distillation (MD) is a relatively new membrane process in which two aqueous solutions at different temperatures are separated by a microporous hydrophobic membrane. Under these conditions a net pure water flux from the warm side to the cold side occurs. The process takes place at atmospheric pressure and at a temperature that may be much lower than the boiling point of the solutions. The driving force is the vapour pressure difference between the two solution–membrane interfaces due to the existing temperature gradient. The phenomenon can be described as a three-phase sequence: (1) formation of a vapour gap at the warm solution– membrane interface; (2) transport of the vapour phase through the microporous system; (3) its condensation at the cold side membrane–solution interface [3]. Nene et al. [4] made a concentration of 2.5 M and 5 M NaCl solutions by MD. When a 2.5 M NaCl solution was subjected to MD at various values of ∆T using a PP membrane with a nominal pore size of 0.2 µm, an almost linear relation was obtained between flux and ∆T. The temperature difference creates a vapour pressure difference, which leads to water vapour diffusion through the membrane. As the temperature difference increases, so does the vapour pressure difference and thus the MD flux rises. Using NaCl experimental data, the operating temperatures at the following cane sugar concentrations were 75EC on the (hot) sugar side and 25EC on the (cold) water side. Bailey et al. [5] observed that using ultrafiltration (UF) before MD with membranes with pore diameters of 0.1 µm or less resulted in appreciable osmotic distillation flux increase, higher than that observed for juice not subjected to UF. These flux increases have been attributed to a reduction in the viscosity of the concentrated
447
juice–membrane boundary layer as the result of protein removal. HPLC measurements showed that the normal concentration of fermentable sugars in standard 68E Brix concentrate can be achieved at a lower Brix value with UF permeate, thereby providing a possible means of reducing the handling of highly viscous streams. UF also resulted in an increase in juice surface tension with a consequent reduction in the tendency for membrane wet-out to occur. Osmotic distillation (OD) can be used to extract selectively the water from aqueous solutions under atmospheric pressure and at room temperature, thus avoiding thermal degradation of the solutions [6]. Comparing reverse osmosis (RO) and the MD process, the OD process has the potential advantage to overcome the drawbacks of RO and MD for concentrating fruit juice because RO suffers from high osmotic pressure of the must components and heat degradation may still occur due to the heat requirement for the feed stream in order to maintain the water vapour pressure gradient for MD. On the other hand, OD does not suffer from any of the problems mentioned above when operated at room temperature [3]. Cassano et al. made similar experiments (kiwi juice concentration with OD) with the permeate of ultrafiltrated kiwi juice. They used UF because after the pressing of kiwi pulp because it was a treatment with pectin breaking enzymes, and this convenient cleaning method was UF [7]. Ali et al. used a pilot OD installation. Its maximal evaporating capacity was 7 kg h!1. It had a polypropylene hollow-fiber membrane that was 800 µm thick, with an average pore diameter of 0.2 µm. The total membrane area installed was 10.2 m2. Their experiments confirmed that with OD applications the organic volatile loss is significantly less than in the case of vacuum evaporation [8]. The aim of this study was the investigation of water removal by MD and OD to reach a high sugar content in the final must product. With
448
A. Rektor et al. / Desalination 191 (2006) 446–453
these treatments the water activity can be reduced to such a low level that the final product (concentrate) can be stored without cooling. 2. Materials and methods Two kinds of grape juice and a model solution of sugar were used during the experiments. The white grape juice was Tokaji Furmint with 18E Brix and the red grape juice was Egri Kékfrankos with 17E Brix. Deionized water was used for condensation (MD) on the cold side of the membrane. The grape juice was sterilized and clarified by microfiltration (MF). MF experiments were carried out on a pilot-plant apparatus, designed at the Department of Food Engineering, and built by Hidrofilt Ltd. The details of the pilot plant are explained elsewhere [1]. In the MD investigations a laboratory-size hydrophobic polypropylene MD membrane (020 CP 2N, Microdyn) was used. The module contained 40 polypropylene capillars with 2.8 mm outer and 1.8 mm inner diameter. The wall thickness of the capillars was 0.5 mm, the average pore size 0.2 µm and the membrane area 0.1 m2. The flow diagram of the laboratory apparatus is shown in Fig. 1.
The liquids from the feed tanks were transported by using peristaltic pumps across a heat exchanger on each side. The warm side (water, sugar solution, grape juice) was heated by a Lauda heating thermostat and the cold side (deionized water) was kept cold by a cooling thermostat. The total solid, i.e., sugar concentration of the sugar solution and must, was measured by a refractometer, taking samples from the retentate side. Each time the sugar content of the permeate was checked, but during all experiments on the permeate side sugar was not detected. The typical OD process (Fig. 2.) involves the use of a concentrated brine at the downstream side of the membrane as the stripping solution. Calcium chloride dihydrate (CaCl2–2H2O) was used as the stripper by the measurements. Because the solubility of the CaCl2 is very good, it can be up to 60 w/w% concentration, which has a very high osmotic pressure (~140 bar). It was chosen because it is not toxic and it is readily available at low cost. Two different capacity peristaltic pumps were used during the experiments. The smaller pump had a 6.28 L/h (Re1 = 43.8) and the larger had a 40.45 L/h (Re2 = 283.2) recirculation flow rate.
Fig. 1. Laboratory membrane distillation apparatus. 1 module, 2 peristaltic pump, 3 must, 4 deionized water, 5 digital scale, 6 heat exchanger, 7 thermostat/heating, 8 thermostat/cooling, 9 personal computer.
A. Rektor et al. / Desalination 191 (2006) 446–453
Fig. 2. OD system configuration with a digital laboratory scale. 1 module, 2 peristaltic pump, 3 must, 4 stripping solution, 5 digital scale.
(The Reynolds numbers were with the physical properties of the distillated water –ρ, η– calculated.) The temperature of both streams was maintained constant at 21EC. During the experiments, the mass change of the cooling water was measured by a digital laboratory balance (PMA 7500, Sartorius) which was connected with a computer where the changes were registered and processed. At the end of every experiment the apparatus and the membrane were washed with water for half an hour and after that rinsed with deionized water. Then the washing procedure was repeated with 0.1% solution of NaOH and again rinsed with water 2–3 times. 3. Results and discussion
449
the must. After preliminary measurements, experiments were carried out with a model solution of sugar (20E Brix), white and red must. The results of the experiments are shown in Figs. 3–6. In Fig. 3 the fluxes of water and the model sugar solution (which was concentrated from 20E Brix to 30E Brix during the experiment) are presented. From the diagram it is obvious that up to a 30E Brix sugar concentration there were no difference in the fluxes. In Fig. 4 the mass of the permeate at different temperatures (15EC and 30EC) for different grape juices is shown. At higher temperatures the distillate mass transfer is faster, but no significant influence of the type of must was observed. At a higher temperature (30EC) at the end of the experiments due to a high total solid/sugar content (30–65E Brix), the mass transfer velocity decreased. The same conclusions are reached in Fig. 5 where the influence of the retentate concentration on the permeate flux is presented. Similar behaviour was observed by Lagana et al. [9] for apple juice concentration, but at higher concentrations (over 64E Brix) the viscosity quickly increased and the retentate side controlled the mass transfer. In Fig. 6 the influence of different temperatures on the concentration changes of different musts is shown. There are no differences between the must at higher temperatures, but at 15EC the concentration rate of the Furmint must was faster; the 30E Brix concentration was reached in 9 h, while for Kékfrankos for the same concentration (30E Brix), a much longer time (~12 h) was necessary.
3.1. Membrane distillation (MD)
3.2. Osmotic distillation (OD)
Preliminary experiments were carried out with deionized water on both sides, using temperatures of 15EC and 30EC. The warm side was less than 50EC in all of these and following experiments to avoid the damage of the valuable components in
In the preliminary experiments similar concentration of model solutions at different recirculation flow rates was studies. The results are presented in the Fig. 7. The aim of these experiments was to find out which resistant is
450
A. Rektor et al. / Desalination 191 (2006) 446–453
Fig. 3. Fluxes of different liquids used at constant temperature (30EC).
Fig. 4. Influence of the driving force (∆T ) on permeated mass.
Fig. 5. Influence of the TS content and temperature on permeate flux.
A. Rektor et al. / Desalination 191 (2006) 446–453
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Fig. 6. Total solid concentration changes during batch processing.
Fig. 7. Concentrations of 10E Brix sugar solutions with combinations of different pumps.
dominant in the mass transfer. The smallest distillate flux was reached in the case of low velocities on both membrane sides (Re1!Re1). If the pumps were used in asymmetric mode (Re1!Re2, Re2!Re1), the distillate flux increased greatly. The flux was slightly higher when the higher capacity pump was on the stripper side. The highest distillate flux was in the symmetric mode of higher capacity pumps (Re2!Re2). The reason for
the 2-h deviation was the lower concentration time in the case of higher flux from the Re2!Re2 symmetric mode. After the measurements with the model solutions, the higher capacity pumps on both membrane sides were used for the concentration of the different EBrix sugar solutions and the 10E Brix diluted must. The effect of the different initial concentrations on the distillate flux and the
452
A. Rektor et al. / Desalination 191 (2006) 446–453
Fig. 8. Comparison of the concentrations of the model solutions and the must.
concentration time was examined. The differences between the distillate fluxes of the 10 and 20°Brix sugar solutions can be seen in the Fig. 8. Between the must and the model solution concentrations by similar starting °Brix values, the distillate flux of the must was lower and the concentration time higher. With the 60 w/w% stripper 34° Brix concentrate from the 10° Brix sugar content a starting solution was reached. The osmotic pressure of the starting stripper solution was calculated with the van’t Hoff law:
where π is the osmotic pressure of the stripping solution, M is the molarity of the total number of particles, R is the gas constant and T is the operating temperature [10]. The osmotic pressure of the 60 w/w% stripper was 142 bar. With similar w/w % stripper the 20° Brix sugar content starting solution to 54° Brix final value was concentrated. 4. Conclusions Comparing MD and OD with traditional concentration treatments (evaporation), a high sugar
concentration final product can be reached at low temperature using MD and OD. The final concentrations of the two grape juices were over 60 EBrix (MD) at the end of the batch measurements. The successful concentrations of the two traditional Hungarian grape juices are good examples that MD and OD can be applied for other fruit juices. The results of the laboratory experiments are a good basis for the production of high concentration must. The final must concentrate can be used in wine-making technology, brewery and other food industries as sweeteners or additives. References [1] A. Rektor, N. Pap, Z. Kókai, R. Szabó, Gy. Vatai and E. Békássy-Molnár, Application of membrane filtration methods for must processing and preservation, Desalination, 162 (2004) 271–277. [2] I. Kiss, E. Bekassy-Molnar and Gy. Vatai, Must concentrate using membrane technology, Desalination, 162 (2004) 295–300. [3] B. Jiao, A. Cassano and E. Drioli, Recent advances on membrane processes for the concentration of fruit juices, J. Food Eng., 63 (2004) 303–324. [4] S. Nene, S. Kaur, K. Sumod, B. Joshi and K.S.M.S. Raghavaro, Membrane distillation for the concen-
A. Rektor et al. / Desalination 191 (2006) 446–453 tration of raw cane-sugar syrup and membrane clarified sugarcane juice, Desalination, 147 (2002) 157–160. [5] A.F.G. Bailey, A.M. Barbe, P.A. Hogan, R.A. Johnson and J. Sheng, The effect of ultrafiltration on the subsequent concentration of grape juice by osmotic distillation, J. Membr. Sci., 164 (2000) 195– 204. [6] W. Kunz, A. Benhabiles and R. Ben-Aim, Osmotic evaporation through macroporous hydrophobic membranes: a survey of current research and applications, J. Membr. Sci., 121(1) (1996) 25–36.
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[7] A. Cassano, B. Jiao and E. Drioli, Production of concentrated kiwifruit juice by integrated membrane process, Food Res. Internat., 37 (2004) 139–148. [8] F. Ali, M. Dornier, A. Duquenoy and M. Reynes, Evaluating transfers of aroma compounds during the concentration of sucrose solutions by osmotic distillation in a batch-type pilot plant, J. Food Eng., 60 (2003) 1–8. [9] F. Lagana, G. Barbieri and E. Drioli, Direct contact membrane distillation: modelling and concentration experiments, J. Membr. Sci., 166(1) (2000) 1–11. [10] http://www.wwnorton.com/chemistry/concepts/cha pter5/ch5_5.htm.
Multi-step membrane processes for the concentration of grape juice Attila Rektor*, Gyula Vatai, Erika Békássy-Molnár Faculty of Food Science, Department of Food Engineering, Corvinus University of Budapest, Budapest, Hungary Tel. +36 (1) 482-6113; Fax +36 (1) 482-6323; [email protected] Received 15 March 2005; accepted 27 June 2005
Abstract Grape production and viticulture have great traditions in Hungary. Nowadays wine-making is only profitable if the end-product is quality wine. Good-quality wine can be produced from must which has the proper high sugar content. If there is an unfavorable vintage, it is necessary to enrich the must, to increase its sugar content and to eliminate its other defects. Grape juice is not only a sweetener; it can be used for other purposes too because the juice of the grape is a pleasant, fashionable soft drink as well. However, the must storage can cause some difficulties, as fermentation starts easily. The aim of the investigations was to use osmotic distillation and membrane distillation after the microfiltration and reverse osmosis concentration [1] for water removal from grape juice in order to reach a high sugar content, to the extent that makes it possible to preserve the grape juice without cooling. Keywords: Grape juice; Preservation; Membrane distillation; osmotic distillation
1. Introduction One of the most valuable fruits in Hungary is the grape. The conditions of wine growing in the country are very good, and a large volume and good quality of must are produced. The greatest part of the vintage is processed by the viticulture and soft drinks industry. Because the ripening of *Corresponding author.
the grape and the vintage happen once a year, there can be a lack of grapes (or poor quality) or overproduction. The preservation of grape juice can solve the above problems. Viticulture could produce must concentrate from the surplus that could be applied by the upgrading of poor-quality, low-sugarcontent grape juice, or the soft drinks industry could develop a new product for its customers.
Presented at the International Congress on Membranes and Membrane Processes (ICOM), Seoul, Korea, 21–26 August 2005. 0011-9164/06/$– See front matter © 2006 Published by Elsevier B.V.
doi:10.1016/j.desal.2005.06.046
A. Rektor et al. / Desalination 191 (2006) 446–453
The high-quality requirements and expectations of the customers (new foods) do not include chemical preservation, additives and high-temperature applications [2]. Membrane distillation (MD) is a relatively new membrane process in which two aqueous solutions at different temperatures are separated by a microporous hydrophobic membrane. Under these conditions a net pure water flux from the warm side to the cold side occurs. The process takes place at atmospheric pressure and at a temperature that may be much lower than the boiling point of the solutions. The driving force is the vapour pressure difference between the two solution–membrane interfaces due to the existing temperature gradient. The phenomenon can be described as a three-phase sequence: (1) formation of a vapour gap at the warm solution– membrane interface; (2) transport of the vapour phase through the microporous system; (3) its condensation at the cold side membrane–solution interface [3]. Nene et al. [4] made a concentration of 2.5 M and 5 M NaCl solutions by MD. When a 2.5 M NaCl solution was subjected to MD at various values of ∆T using a PP membrane with a nominal pore size of 0.2 µm, an almost linear relation was obtained between flux and ∆T. The temperature difference creates a vapour pressure difference, which leads to water vapour diffusion through the membrane. As the temperature difference increases, so does the vapour pressure difference and thus the MD flux rises. Using NaCl experimental data, the operating temperatures at the following cane sugar concentrations were 75EC on the (hot) sugar side and 25EC on the (cold) water side. Bailey et al. [5] observed that using ultrafiltration (UF) before MD with membranes with pore diameters of 0.1 µm or less resulted in appreciable osmotic distillation flux increase, higher than that observed for juice not subjected to UF. These flux increases have been attributed to a reduction in the viscosity of the concentrated
447
juice–membrane boundary layer as the result of protein removal. HPLC measurements showed that the normal concentration of fermentable sugars in standard 68E Brix concentrate can be achieved at a lower Brix value with UF permeate, thereby providing a possible means of reducing the handling of highly viscous streams. UF also resulted in an increase in juice surface tension with a consequent reduction in the tendency for membrane wet-out to occur. Osmotic distillation (OD) can be used to extract selectively the water from aqueous solutions under atmospheric pressure and at room temperature, thus avoiding thermal degradation of the solutions [6]. Comparing reverse osmosis (RO) and the MD process, the OD process has the potential advantage to overcome the drawbacks of RO and MD for concentrating fruit juice because RO suffers from high osmotic pressure of the must components and heat degradation may still occur due to the heat requirement for the feed stream in order to maintain the water vapour pressure gradient for MD. On the other hand, OD does not suffer from any of the problems mentioned above when operated at room temperature [3]. Cassano et al. made similar experiments (kiwi juice concentration with OD) with the permeate of ultrafiltrated kiwi juice. They used UF because after the pressing of kiwi pulp because it was a treatment with pectin breaking enzymes, and this convenient cleaning method was UF [7]. Ali et al. used a pilot OD installation. Its maximal evaporating capacity was 7 kg h!1. It had a polypropylene hollow-fiber membrane that was 800 µm thick, with an average pore diameter of 0.2 µm. The total membrane area installed was 10.2 m2. Their experiments confirmed that with OD applications the organic volatile loss is significantly less than in the case of vacuum evaporation [8]. The aim of this study was the investigation of water removal by MD and OD to reach a high sugar content in the final must product. With
448
A. Rektor et al. / Desalination 191 (2006) 446–453
these treatments the water activity can be reduced to such a low level that the final product (concentrate) can be stored without cooling. 2. Materials and methods Two kinds of grape juice and a model solution of sugar were used during the experiments. The white grape juice was Tokaji Furmint with 18E Brix and the red grape juice was Egri Kékfrankos with 17E Brix. Deionized water was used for condensation (MD) on the cold side of the membrane. The grape juice was sterilized and clarified by microfiltration (MF). MF experiments were carried out on a pilot-plant apparatus, designed at the Department of Food Engineering, and built by Hidrofilt Ltd. The details of the pilot plant are explained elsewhere [1]. In the MD investigations a laboratory-size hydrophobic polypropylene MD membrane (020 CP 2N, Microdyn) was used. The module contained 40 polypropylene capillars with 2.8 mm outer and 1.8 mm inner diameter. The wall thickness of the capillars was 0.5 mm, the average pore size 0.2 µm and the membrane area 0.1 m2. The flow diagram of the laboratory apparatus is shown in Fig. 1.
The liquids from the feed tanks were transported by using peristaltic pumps across a heat exchanger on each side. The warm side (water, sugar solution, grape juice) was heated by a Lauda heating thermostat and the cold side (deionized water) was kept cold by a cooling thermostat. The total solid, i.e., sugar concentration of the sugar solution and must, was measured by a refractometer, taking samples from the retentate side. Each time the sugar content of the permeate was checked, but during all experiments on the permeate side sugar was not detected. The typical OD process (Fig. 2.) involves the use of a concentrated brine at the downstream side of the membrane as the stripping solution. Calcium chloride dihydrate (CaCl2–2H2O) was used as the stripper by the measurements. Because the solubility of the CaCl2 is very good, it can be up to 60 w/w% concentration, which has a very high osmotic pressure (~140 bar). It was chosen because it is not toxic and it is readily available at low cost. Two different capacity peristaltic pumps were used during the experiments. The smaller pump had a 6.28 L/h (Re1 = 43.8) and the larger had a 40.45 L/h (Re2 = 283.2) recirculation flow rate.
Fig. 1. Laboratory membrane distillation apparatus. 1 module, 2 peristaltic pump, 3 must, 4 deionized water, 5 digital scale, 6 heat exchanger, 7 thermostat/heating, 8 thermostat/cooling, 9 personal computer.
A. Rektor et al. / Desalination 191 (2006) 446–453
Fig. 2. OD system configuration with a digital laboratory scale. 1 module, 2 peristaltic pump, 3 must, 4 stripping solution, 5 digital scale.
(The Reynolds numbers were with the physical properties of the distillated water –ρ, η– calculated.) The temperature of both streams was maintained constant at 21EC. During the experiments, the mass change of the cooling water was measured by a digital laboratory balance (PMA 7500, Sartorius) which was connected with a computer where the changes were registered and processed. At the end of every experiment the apparatus and the membrane were washed with water for half an hour and after that rinsed with deionized water. Then the washing procedure was repeated with 0.1% solution of NaOH and again rinsed with water 2–3 times. 3. Results and discussion
449
the must. After preliminary measurements, experiments were carried out with a model solution of sugar (20E Brix), white and red must. The results of the experiments are shown in Figs. 3–6. In Fig. 3 the fluxes of water and the model sugar solution (which was concentrated from 20E Brix to 30E Brix during the experiment) are presented. From the diagram it is obvious that up to a 30E Brix sugar concentration there were no difference in the fluxes. In Fig. 4 the mass of the permeate at different temperatures (15EC and 30EC) for different grape juices is shown. At higher temperatures the distillate mass transfer is faster, but no significant influence of the type of must was observed. At a higher temperature (30EC) at the end of the experiments due to a high total solid/sugar content (30–65E Brix), the mass transfer velocity decreased. The same conclusions are reached in Fig. 5 where the influence of the retentate concentration on the permeate flux is presented. Similar behaviour was observed by Lagana et al. [9] for apple juice concentration, but at higher concentrations (over 64E Brix) the viscosity quickly increased and the retentate side controlled the mass transfer. In Fig. 6 the influence of different temperatures on the concentration changes of different musts is shown. There are no differences between the must at higher temperatures, but at 15EC the concentration rate of the Furmint must was faster; the 30E Brix concentration was reached in 9 h, while for Kékfrankos for the same concentration (30E Brix), a much longer time (~12 h) was necessary.
3.1. Membrane distillation (MD)
3.2. Osmotic distillation (OD)
Preliminary experiments were carried out with deionized water on both sides, using temperatures of 15EC and 30EC. The warm side was less than 50EC in all of these and following experiments to avoid the damage of the valuable components in
In the preliminary experiments similar concentration of model solutions at different recirculation flow rates was studies. The results are presented in the Fig. 7. The aim of these experiments was to find out which resistant is
450
A. Rektor et al. / Desalination 191 (2006) 446–453
Fig. 3. Fluxes of different liquids used at constant temperature (30EC).
Fig. 4. Influence of the driving force (∆T ) on permeated mass.
Fig. 5. Influence of the TS content and temperature on permeate flux.
A. Rektor et al. / Desalination 191 (2006) 446–453
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Fig. 6. Total solid concentration changes during batch processing.
Fig. 7. Concentrations of 10E Brix sugar solutions with combinations of different pumps.
dominant in the mass transfer. The smallest distillate flux was reached in the case of low velocities on both membrane sides (Re1!Re1). If the pumps were used in asymmetric mode (Re1!Re2, Re2!Re1), the distillate flux increased greatly. The flux was slightly higher when the higher capacity pump was on the stripper side. The highest distillate flux was in the symmetric mode of higher capacity pumps (Re2!Re2). The reason for
the 2-h deviation was the lower concentration time in the case of higher flux from the Re2!Re2 symmetric mode. After the measurements with the model solutions, the higher capacity pumps on both membrane sides were used for the concentration of the different EBrix sugar solutions and the 10E Brix diluted must. The effect of the different initial concentrations on the distillate flux and the
452
A. Rektor et al. / Desalination 191 (2006) 446–453
Fig. 8. Comparison of the concentrations of the model solutions and the must.
concentration time was examined. The differences between the distillate fluxes of the 10 and 20°Brix sugar solutions can be seen in the Fig. 8. Between the must and the model solution concentrations by similar starting °Brix values, the distillate flux of the must was lower and the concentration time higher. With the 60 w/w% stripper 34° Brix concentrate from the 10° Brix sugar content a starting solution was reached. The osmotic pressure of the starting stripper solution was calculated with the van’t Hoff law:
where π is the osmotic pressure of the stripping solution, M is the molarity of the total number of particles, R is the gas constant and T is the operating temperature [10]. The osmotic pressure of the 60 w/w% stripper was 142 bar. With similar w/w % stripper the 20° Brix sugar content starting solution to 54° Brix final value was concentrated. 4. Conclusions Comparing MD and OD with traditional concentration treatments (evaporation), a high sugar
concentration final product can be reached at low temperature using MD and OD. The final concentrations of the two grape juices were over 60 EBrix (MD) at the end of the batch measurements. The successful concentrations of the two traditional Hungarian grape juices are good examples that MD and OD can be applied for other fruit juices. The results of the laboratory experiments are a good basis for the production of high concentration must. The final must concentrate can be used in wine-making technology, brewery and other food industries as sweeteners or additives. References [1] A. Rektor, N. Pap, Z. Kókai, R. Szabó, Gy. Vatai and E. Békássy-Molnár, Application of membrane filtration methods for must processing and preservation, Desalination, 162 (2004) 271–277. [2] I. Kiss, E. Bekassy-Molnar and Gy. Vatai, Must concentrate using membrane technology, Desalination, 162 (2004) 295–300. [3] B. Jiao, A. Cassano and E. Drioli, Recent advances on membrane processes for the concentration of fruit juices, J. Food Eng., 63 (2004) 303–324. [4] S. Nene, S. Kaur, K. Sumod, B. Joshi and K.S.M.S. Raghavaro, Membrane distillation for the concen-
A. Rektor et al. / Desalination 191 (2006) 446–453 tration of raw cane-sugar syrup and membrane clarified sugarcane juice, Desalination, 147 (2002) 157–160. [5] A.F.G. Bailey, A.M. Barbe, P.A. Hogan, R.A. Johnson and J. Sheng, The effect of ultrafiltration on the subsequent concentration of grape juice by osmotic distillation, J. Membr. Sci., 164 (2000) 195– 204. [6] W. Kunz, A. Benhabiles and R. Ben-Aim, Osmotic evaporation through macroporous hydrophobic membranes: a survey of current research and applications, J. Membr. Sci., 121(1) (1996) 25–36.
453
[7] A. Cassano, B. Jiao and E. Drioli, Production of concentrated kiwifruit juice by integrated membrane process, Food Res. Internat., 37 (2004) 139–148. [8] F. Ali, M. Dornier, A. Duquenoy and M. Reynes, Evaluating transfers of aroma compounds during the concentration of sucrose solutions by osmotic distillation in a batch-type pilot plant, J. Food Eng., 60 (2003) 1–8. [9] F. Lagana, G. Barbieri and E. Drioli, Direct contact membrane distillation: modelling and concentration experiments, J. Membr. Sci., 166(1) (2000) 1–11. [10] http://www.wwnorton.com/chemistry/concepts/cha pter5/ch5_5.htm.