optimization of osmotic distillation conditions of

Year 3 (2019) Vol:10 Issued in JULY, 2019 www.ejons.co.uk

OPTIMIZATION OF OSMOTIC DISTILLATION CONDITIONS OF FRUCTOSE SOLUTION USING MICROPOROUS POLYSULFONE MEMBRANES

Gülşen TAŞKIN ÇAKICI

Dr. Öğr. Üyesi, Chemistry and Chemical Process Technology, Yıldızeli Vocational School, Cumhuriyet University, Sivas, Turkey

[email protected]

Oya ŞANLI

Science Faculty, Department of Chemistry, Gazi University, 06500 Teknikokullar, Ankara, Turkey

ABSTRACT In this study, osmotic distillation was applied to concentrate fructose solutions using polysulfone (PSf) membranes. PSf membranes were prepared at relative humidities of 32, 75, 93.0 %, respectively. The effect of various process parameters such as, thickness, pore size, porosity of the membrane, feed, brine flow rates, feed concentration, brine concentration and temperature on the OD fluxes were studied. The highest flux of 0.32 was found optimized conditions, as membrane thickness: 60±5µm, feed flow rate: 200 rpm and brine flow rate: 100 rpm.

Keywords: Osmotic distillation ∙ Polysulfone ∙ Hydrophobic Membrane ∙ Microporous Membrane ∙ Fructose

INTRODUCTION Concentration is an important unit operation in the liquid food industry, especially in fruit juice processing, since it removes a significant amount of water, hence a major reduction in packaging, transport and storage cost with much greater stability of the concentrates, and determines the quality of the final products. The techniques for concentration and their advantages and disadvantages are discussed elsewhere [1-4]. Membrane processes as microfiltration (MF), ultrafiltration (UF) and reverse osmosis (RO) are widely used in the dairy products, food and drinks industry [3]. The potential of developing RO process as a concentration technique to remove water from fruit juices has been of interest to the fruit juice industry for about 30 years. The advantages of RO process over traditional evaporation are in lower thermal damage to product, increase in aroma retention, less energy consumption and lower equipment costs. One of the main disadvantage of RO is its inability to reach the concentration of standard products produced by evaporation because of high osmotic pressure limitation [3,5-8]. Recently, technological advances related to the development of new membranes and improvements in process engineering have been proved to overcome this limitation [3,9-16]. Osmotic distillation (OD) is a new alternative membrane process [17], also known as osmotic evaporation [18], isothermal membrane distillation or gas membrane extraction which is a low energy concentration process. The OD process can be operated at room temperature and atmospheric pressure thus avoiding thermal degradation of the solutions [19]. The membranes used in OD are made of hydrophobic porous polymers, e.g. polytetrafluoroethylene (PTFE), polypropylene (PP), polyvinylidenefluoride (PVDF) membrane, which separates two circulating aqueous solutions at different solute concentrations: a dilute solution and a hypertonic salt solution.

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The main driving force acting on water in osmotic distillation (OD) is vapor-pressure difference. It is therefore particularly adapted to the concentration of heat-sensitive products like fruit juices [18, 20-23] Usually, inorganic salts (NaCl, CaCl2, MgCl2, MgSO4) or organic solvents (glycerol, polyglycerol) can be used as hypertonic salt solutions [24]. Mass transfer Mass flux in osmotic membrane distillation can be estimated by several models, that is, the Knudsen, Poiseuille, ordinary diffusion or combination of the models [25,26]. Practical use of the mentioned models requires the detailed knowledge about the process parameters and membrane properties. The basic equation, which relates the transmembrane flux (J) to the driving force represented by the difference in vapor pressures of the bulk liquids (feed and brine) is given by [27, 28]: J=K ∆P= K (PF –PB) (1) where K is the overall mass transfer coefficient, PF is the water vapor pressure at the bulk feed (F)/ brine (B) solutions. The overall mass transfer coefficient (K) depends on the physicochemical properties of solutions as well as on the hydrodynamic properties of the system and it is defined by the following equation :[25]

K=( 1𝐾𝐾𝐾𝐾

+ 1𝐾𝐾𝐾𝐾

+ 1𝐾𝐾𝐾𝐾

)-1 (2)

where Km, KF and KB are mass transfer coefficients of membrane, feed boundary layer and brine boundary layer, respectively. The resistivity of the membrane support layer during direct osmosis process was proposed and given by Loeb et al. [29]: km = (1/J) ln(πB/πF) (3) where km is the resistivity of membrane support layer and J is the water flux through the membrane . The osmotic pressure (π) of the concentrated solution is related to water activity of the solution as given by the following equation [30], π = - (RT/V) ln aw =(RT/V) ln( Pw/ P) (4) where R is the gas constant (8.314 J/(K mol)), T is the temperature in K, V is the molar volume (18 ml/mol of water) and aw is the water activity. The boundary layers of concentrated feed and dilutedhypertonic salt solutions present on either side of the membrane significantly affect the resistance to mass transfer. The liquid mass transfer coefficients depend on the properties of the solutions and on hydrodynamic conditions of the systems. These coefficients can be estimated with the help of empirical correlation of dimensionless numbers, namely, Sherwood (Sh), Reynolds (Re) and Schmidt (Sc) numbers. In case of a flat plate membrane cell, the liquid mass transfer coefficient in the feed and brine side boundary layer were estimated using the following correlation [31,32] Sh = 0,66 Re 0,5 Sc0,33 (5)

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Where Sh=ki δDw

, Re =u δ ρμ

, Sc= μρ Dw

, δ the membrane thickness, Dw the water diffusion coefficient, ρ the solution density, μ the solution dynamic viscosity, k the feed or osmotic agent side liquid mass transfer coefficient and u the velocity of the fluid, flowing over the membrane surface [31]. The water diffusion coefficient (Dw) estimated by the following empirical equation [31], Dw = [117,3 x 10 -18 (φ M)0,5 T] / μ v A 0,6 (6) where ϕ is the association factor for water, vA the solute molal volume (m3 kmol−1), T the temperature (K) and M is the molecular weight of the solute. The aim of this paper is to study the effect of operating conditions such as membrane pore size and thickness, feed and hypertonic salt solution flow rates, feed and hypertonic salt solution concentration, bulk temperature on OD flux and transferred water. The fructose solutions are chosen for this study because fructose is the main sugar found in fruit juices.

2.MATERIALS AND METHODS 2.1.Chemicals Polysulfone (PSf) (Mw=35000) was supplied by Aldrich, used as the base polymer in the membrane preparation. N,N- dimethylacetamide (DMAc), sodium chloride (NaCl), calcium chloride dihydrate (CaCl2 2H2O), magnesium chloride (MgCl2), D(-)-Fructose were all purchased from Merck and at they were supplied.

2.2. Preparation of PSf Membranes PSf membranes were prepared using the Vapor Induced Phase Separation Technique (VIPS) at relative humidities of 32, 75, 93 %, by using solution of PSf in DMAc at a concentration of 18% (w/V). A schematic illustration of the VIPS process used in this study was shown in Figure 1.

Figure 1. Schematic illustration of the preparation of the PSf membranes VIPS, (a) water vapour condenses on the surface of polymer solution because of the surface cooling caused by solvent evaporation, (b) porous surface forms after the polymer precipitation and water removal. The relative humidities and temperature were set to 32, 75, 93 %, and 30 oC, respectively, two hours before casting the solutions to ensure that equilibrium would be reached in the whole box by the

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time solutions would be casted. Membrane formation actually depends on (i) how fast water diffusion will overcome the first resistance at the air/matrix interface, that is on the difference of water chemical potential between the gaseous phase and the liquid system, and on (ii) the diffusion rates of water within the system [33]. Predetermined amount of homogeneous polymer solution was cast onto rimmed round glass plates for the complete phase separation, and were exposed to water vapor for 24 hours. Many studies report quite short exposure time to non-solvent vapors ranging from a few seconds [34] to a few minutes [35]. Each of the prepared membranes were washed with water.

2.3. Equilibrium Water Content (EWC) EWC is related to the porosity of the membrane. Also it indicates the degree of hydropbilicity or hydropbilicity of the membrane [36]. The equilibrium water content (EWC) at room temperature was calculated as follows:

EWC (%) = Ww−WdWw

x 100 (7)

Where ww is weight of wet membranes (g) and wd is weight of dry membranes(g).

2.4.Determination of Porosity of Membranes 2.4.1. Theoretically calculation Asymmetric microporous membranes were characterized by determination of porosity and average pore radius. The membrane porosity, ε, was described as the volume of the pores divided by the total volume of the porous membrane. Porosity (ε) of the membranes was calculated by Eq.(8),

ε = (w1−w2)/ρww1−w2

ρw + w2/ρp (8)

where w1 is weight of the wet membrane, w2 the weight of the dry membrane, ρw the water density and ρp the polymer density [37].

2.4.2.Hg porosimetry Quantachrome Corporation, Poremaster 60 model porosimetry was used for measuring the porosity of the membranes.

2.4.3.Scanning electron microscopy (SEM) Quanta-400F (FEI Company, USA) model electron microscope was used for the SEM micrographs of the membranes. Dry membranes were sputtered with gold in vacuum before viewing under the microscope.

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2.4.4.Osmotic distillation experiments The prepared polysulfone membranes were tested in the osmotic distillation cell to concentrate the fructose solutions. Fructose solutions were prepared with Fructose (C6H12O6) powder of 99.6% purity and distilled water. Hypertonic salt solutions were prepared from NaCl, CaCl2 . 2H2O and MgCl2 salts. The experimental set up of OD used in this work was illustrated in Figure 2. A stirred cell consisting of two identical cylindrical chambers was used. The polysulfone membrane was placed between the chambers, on one side fructose solution (chamber 1), the other side a hypertonic salt solution (chamber 2). The water flux was obtained measuring the variation of the water mass with time in the reservoir connected to chamber 2. In order to check reproducibility, three measurements were performed for each case, and the average values were taken. The temperature was maintained constant by re-circulating water through the thermal jackets. A simulated fructose in an aqueous solution and hypertonic salt solution were circulated using peristaltic pumps.

Figure 2. Schematic diagram of experimental set up for osmotic distillation The water permeation flux (J, kg/m2 h) was calculated by Eq.(9):

J = wA x t

(9)

where w (kg) was the mass permeated in a time t (h), A (m2) was the membrane surface area. [38]

3.RESULTS AND DISCUSSION 3.1.Effect of Morphology of Psf Membrane 3.1.1.Effect of relative humidity

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The hydrophobic polysulfone membranes prepared by Vapor Induced Phase Separation Technique (VIPS) at relative humidities of 32, 75, 93 %, were denoted by as A1, A2 and A3, respectively. Dry thickness (δ) of the prepared membranes was determined as 60±5 µm using a precision micrometer (Aldrich, Germany) by taking the average of at least ten measurements. Morphology of the membranes were studied by scanning electron microscopy (SEM) and characterized in terms of EWC, porosity, …etc. The relative humidity of the air plays an important role in the phase separation dynamics of the polymer film. In the process of VIPS where the initial casting solution contains solvent and polymer, increasing the relative humidity increases the driving force for a net diffusion into and accumulation of water in the film, thereby inducing phase separation. Therefore, relative humidity has a significant influence on the phase inversion kinetics and final membrane morphology in VIPS [39, 40]. SEM micrographs of the prepared polysulfone membranes were shown in Figure 3 a-b-c, for relative humidities of 32, 75, 93 %, respectively. These micrographs provide the information regarding the surface morphology of each membrane. The diameter of the pores was measured and it is observed that with the increase in relative humidity, the pore size of the corresponding membranes decreases. This may be attributed to reducing the relative humidity led strongly reducing the mass transfer driving force for the water intake, whereas the solvent evaporation rate was not directly affected [41]. In the literature similar results were reported concerning the effect of the relative humidity on the membrane morphology [42-47]. Parka et al. [46] noticed an increase of the size of the pores when RH decreased. They showed, using optical microscopy, that the higher the RH, the slower the pore growht and the shorter its duration. In order to elucidate this behavior, they suggested that the coarsening was allowed to proceed for a longer time in the case of low RH values corresponding to a low water concentration in the film-forming system.

(a) (b) (c)

Figure 3. SEM micrographs of the prepared polysulfone membranes [(a ) RH: 32%, (b) RH: 75 % , (c) RH: 93%]

3.1.2.Porosity and EWC EWC and the porosity values of prepared membranes were calculated from Eq (7) and (8), which values are summerized Table 1. As it is seen from the table, A3 membranes have smaller pore size and higher porosity than the A1 and A2 membranes. Although, A3 membranes have greater porosity than the A1 membranes, which EWC values is less than A1 membranes. Thus, for larger pore size, due to enhanced segmented gap between polymer chains, the water content is higher [47,48].

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Table 1. The porosity and EWC values of prepared membranes

Relative Humidity (RH)

93 % 75 % 32 %

Porosity (ε) % 75.9 70.7 63.4 (Theoretical)

Porosity (ε) % 81.9 69.6 61.7 (Hg Porosimetry)

EWC (%) 67.8 65.7 68.9

3.1.3. Effect of the membrane thickness on the OD The influence of membrane thickness (δ) was studied using the PSf membranes of the thickness on the performance of the OD. The overall thickness of these membranes were 90±5, 60±5 and 30 ±5 µm. The osmotic medium was NaCl hypertonic salt solution (20% (w/w)) in all cases and water was used as feed. The experimental conditions for the three respective experiments were: water and hypertonic salt solution temperature, 30 ±1 0C, feed and hypertonic salt solution flow rate, 50 rpm. The results were presented in Figure 4.

Figure 4. Effect of the membrane thickness on flux (Brine: 20% (w/w) NaCl Feed: Water T=30±1°C, Brine flow rate and Feed flow rate: 50 rpm)

As it can be seen from the figure, the highest flux was obtained from the 60µm thickness membrane. Similar results were observed in the literature [49]. Beaudry and Lampi [50] reported that the osmotic flux would change to be proportional to the membrane thickness. Herron et al. [51] found that the membrane thickness appreciably affected the osmotic flux. Petrotos et al. [52]

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concluded that there is an increasing trend in flux due to the decrease in membrane thickness. The mass transfer through membrane pores is inversely proportional to the membrane thickness [53,54].

3.1.4.Effect of the pore size of the prepared membranes Prepared membranes using the VIPS and the simulation are denoted by cases A1, A2, A3 were tested on the OD. The osmotic medium was NaCl brine (20% (w/w)) in all cases and water was used as feed. The experimental conditions for the three respective experiments were: water and brine temperature, 30 ±1 0C, feed and brine flow rate, 50 mL min-1. The results were shown in Figure 5.

Figure 5. Effect of the pore size of the prepared membranes on flux (Brine: 20% (w/w) NaCl, Feed: 10 %(w/w) Fructose T=30±1°C, Brine flow rate and Feed flow rate: 50 rpm, δ: 60±5µm) On the SEM micrographs (Fig. 3 a-b-c), A1 have larger pores than A2 and A3. Despite having greater pores having significantly greater pores the other two membranes, A1 membrane has less permeate water. As it can be seen from the figures, A2 membrane has much mass transfer of water and water flux. This result may be attributed that the water flux (or transmembrane flux) is independent of pore size of the membrane. Since the mechanism of mass transfer in both the cases is in the transient region between Knudsen and molecular diffusion, no significant dependency of transmembrane flux on pore size was observed [30,31,50]. Mengual et al. [55] has also reported no marked change in the flux between the membranes having the pore radius 0.2 and 0.45 µm, perhaps due to insignificant change in mechanism of diffusion. However, for the membrane pore size of 1.00 µm, therewas a significant increase in flux, probably due to increased contribution of molecular diffusion as compared to Knudsen diffusion [30].

3.1.5.Effect of the flow rate Experiments were performed for fructose solution by varying brine and feed flow rate from 50 to 300 ml min−1, and maintaining the concentrations of brine at 20 % (w/w) (sodium chloride) and feed solution at 10 %(w/w) (fructose). The variation of OD flux with brine and feed side flow rate were reported in Figure 6.

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Figure 6.Effect of flow rate on flux (Brine: 20% (w/w) NaCl Feed: 10 %(w/w) Fructose, T=30±1°C, δ: 60±5µm) The increase in flux with an increase in velocity is because of reduction in concentration polarization effect due to the reduction in hydrodynamic boundary layer thickness [30,55]. Similar results were observed in the literature [3, 37, 56].

3.1.6. Effect of flow rate of the hypertonic salt solution The brine flow rate was varied from 0 to 200 ml.min-1 and maintaining the concentration of brine at 20 % (w/w) (sodium chloride), while flow rate (200 ml.min−1) and concentration of fructose (10% (w/w)) solution was maintained. The effect of brineflow rate on flux were shown in Figure 7 .

Figure 7. Effect of flow rate brine on flux (Brine: 20% (w/w) NaCl Feed: 10 %(w/w) Fructose, T=30±1°C, and Feed flow rate: 200 rpm, δ: 60±5µm)

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As it is seen from the figure increased OD flux when the brine is circulated at 100 rpm. This OD flux increase can be attributed to the increasing flow rate of the hpertonic salt solution which results in stronger shear stress along the condensation side of the membrane. [23, 24, 38, 49]. M. Courel et al. [57] reported that controlling the hydrodynamic circulation conditions on the hypertonic salt solution side can lead to significant flux improvement.

3.1.6. Effect of flow rate of the feed solution Experiments were performed for concentration of fructose solution (10 %(w/w) by varying feed flow rate from 0 ml min−1 to 200 ml min−1, while flow rate (100 ml min−1) and concentration of brine 20 % (w/w) (sodium chloride) was maintained. The effect of feed flow rate on flux were shown in Figure 8 .

Figure 8. Effect of flow rate feed solution on flux (Brine: 20% (w/w) NaCl Feed: 10 %(w/w) Fructose, T=30±1°C, Brine flow rate: 100 rpm, δ: 60±5µm) As it is clearly seen from the figure, as the OD flux increases, the feed flow rate increases. The similar the results was found in literatures [24, 56, 58]. B.R. Babu et al. [31] have determined that at low feed concentrations, the effect of feed flow rate on flux was not significant. However, the effect of feed flow rate was found more important at higher concentration in case of sweet-lime juice. The variations of feed flow rate and the calculated reynold numbers values are all presented in Table 2. Increasing the feed flow rate improves the water flux (Table 2). This may be attributed to the reduction in the hydrodynamic boundary layer thickness, which results in a larger Reynolds number (flow rate), affects the mass transfer resistance on the membrane surface and consequently increases the transmembrane flux [4]. Dova et al. [59] stated that the resistance of the membrane in the water path is proportional the feed flow rate. Moreover, hydrodynamic shear forces may increase with increasing feed flow rates, which leads to less cake formation of flocculation of foulants (mainly pectin) on the membrane surface. This phenomenon results in less fouling and consequently a better process performance [4, 60].

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Table 2. Values of reynold numbers

Flow rate 0 50 100 200

(rpm)

Re 0 1.82 3.65 7.30

3.1.7.Effect of feed concentration The effect of feed concentration on OD flux for fructose solution was presented in Figure 9. The feed concentration was varied from 10 to 40 % (w/w) and feed flow rate was adjusted 200rpm.

Figure 9. Effect of feed concentration on flux (Brine: 20% (w/w) NaCl, T=30±1°C, Brine flow rate: 100 rpm, Feed flow rate: 200 rpm, δ: 60±5µm) As shown in the figure, the flux decreased as the increase of the concentration of the feed solution from 10 to 40 % (w/w). Since the water vapour pressure is the driving force of OD process and it relates to the water activity, therefore, the increase of feed concentration reduced the water activity and flux. Similar results were also given in the literature [24, 31, 38]. The overall mass transfer coefficient (K) was calculated using Eq (1) and results are given Table 3.

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Table 3.Calculation of the overall mass transfer coefficient (K) at various feed concentrations

Feed concentration Overall mass transfer coefficient (wt%) K (10-3 kg m-2 h-1 Pa-1)

10 0.35

20 0.28

30 0.15

40 0.13

The increase in the feed concentration increases the resistance of the feed boundary layer, which leads to decrease in the overall mass transfer coefficient [61].

3.1.8. Effect of hypertonic salt solution concentration The concentration of brine was varied over 15-25 % (w/w) (sodium chloride). During the experiments, brine flow rate was maintained constant (100 rpm) at 30 °C. The values of flux observed at different concentrations of brine was shown in Figure 10.

Figure 10. Effect of hypertonic salt solution concentration on flux (Feed: 10 %(w/w) Fructose, T=30±1°C, Brine flow rate: 100 rpm, Feed flow rate: 200 rpm, δ: 60±5µm) As shown in the figure, the concentration of hypertonic salt solution as approaches saturation, the flux has not changed much.

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3.1.9.Effect of temperature The effect of temperature on OD flux was studied in the range of 30-50 0C and were demonstrated in Figure 11.

Figure 11. Effect of temperature on flux (Feed solution: 10% (w/w) Fructose, Brine : 20% (w/w) NaCl, T=30±1°C, Brine flow rate: 100 rpm, Feed flow rate: 200 rpm, δ: 60±5µm, ♦ 30 ±1 ºC, ● 40±1 ºC, ▲ 50 ±1 ºC) Increasing the process temperature positively affects the osmotic flux [62]. Increased the temperature decreases the viscosity and increases the diffusion coefficient, thus the OD flux is increasing. So, high temperatures give more kinetic energy to water vapour to transport through the membrane [ 24]. Babu et al. [30] verified that increasing the temperature from 25 to 45 0C enhances water flux by 78% when processing pineapple juice. Similar results were obtained by Nayak and Rastogi [63] , who showed that increasing the temperature from 25 to 40 0C resulted in an increase of the concentration of anthocyanin extract from kokum. The transfer of the molecules through a polymeric membrane follows on Arrhenius type of relation [64,65] J=A exp(Ea/RT) (10) Where J is the mass transferred per unit area in unit time (kg/m2 h), A is the preexponential factor and Ea is the overall activation energy in kj/mol. The overall activation energy was found to be as 12.61 kj/mol. The calculated values of the diffusion coefficient of water (Eq.6) were presented in Table 4. As, the diffusion coefficient of water is proportional to the absolute temperature divided by the viscosity of the solvent (Eq.6). The viscosity decreases with the increase in temperature, the diffusion coefficients also increases [32].

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Table 4. Diffusion coefficients of water at various temperatures

Temperature (0C) 30 40 50

Diffusion coefficient (m 2 s -1) 4.21x 10 -11 5.32 x 6.40 x 10-11

3.1.9.Effect of type of hypertonic salt solution The hypertonic salt solution is the source of the driving force in the OD process, and its selection for the concentration of liquid foods. The hypertonic salt solution should have a higher osmotic pressure than the feed solution and be recoverable, non-toxic and inexpensive. Usually, inorganic salts (NaCl, CaCl2, MgCl2, MgSO4) or organic solvents (glycerol, polyglycerol) can be used as hypertonic salt solutions. In this study, NaCl, CaCl2 2H2O and MgCl2 salts were used to prepare hypertonic salt solution. NaCl Hypertonic salt solution NaCl was studied at part effect of hypertonic salt solution concentration. The results were given in Figure 10. MgCl2

Experiments were performed for hypertonic salt type of MgCl2 solution by varying concentration from 15 to 30 %(w/w). The results were presented in Figure 12.

Figure 12. Effect of hypertonic salt solution concentration on flux (Feed: 10 %(w/w) Fructose, T=30±1°C, Brine: MgCl2, Brine flow rate: 100 rpm, Feed flow rate: 200 rpm, δ: 60±5µm) CaCl2.2H2O Experiments were performed for hypertonic salt type of CaCl2.2H2O solution by varying concentration from 20 to 40 %(w/w). The results were presented in Figure 13.

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Figure 13. Effect of hypertonic salt solution concentration on flux (Feed: 10 %(w/w) Fructose, T=30±1°C, Brine: CaCl2.2H2O, Brine flow rate: 100 rpm, Feed flow rate: 200 rpm, δ: 60±5µm) The results of compared hypertonic salt solutions were given in Figure 14. As it is seen from the figures, MgCl2 was more productive and contributed higher flux than NaCl and CaCl2. Similar result were given in the literature [32, 66].

Figure 14. Effect of type of hypertonic salt solution on flux (Feed: 10 %(w/w) Fructose, T=30±1°C, Brine flow rate: 100 rpm, Feed flow rate: 200 rpm, δ: 60±5µm) Experiments were performed with OD systems of 20 % (w/w) NaCl/fructose solution, 20 % (w/w) CaCl2 2H2O/ fructose solution and 20% (w/w) MgCl2 /fructose solution. It may be stated that equivalent weights of salts increase in the order of NaCl > CaCl2 2H2O > MgCl2. Therefore, the osmotic activity (ratio of solubility to its equivalent weight) of MgCl2 and CaCl2 2H2O are the better hypertonic salt solution than NaCl [67]. The overall mass transfer coefficient was calculated using Eq (1). (Table 5)

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Table 5. Calculation of the overall mass transfer coefficient (K) of hypertonic salt solutions

Formulation Concentration (wt%) Overall mass transfer coefficient K (10 -3 kg m-2 h-1 Pa-1)

NaCl 20 0.35

MgCl2 20 0.78

CaCl2.2H2O 20 0.71

Table 5 shows the OD performance data for the brine. A finding was that NaCl, the brine that gave the lowest flux, had the lowest overall mass transfer coefficient than others. The lower flux was a consequence of the relatively low water-solubility of this salt.

4. CONCLUSIONS In this study has been investigated a general characterization of the concentration of fructose solution by means of osmotic distillation process distillation in a polysulfone flat membrane contactor. The influence of various process parameters such as type, pore size and thickness of the membrane, concentration and flow rate of the hypertonic salt solution and feed, temperature concerning transmembrane flux were studied for OD systems.

• Increase in the thickness of the membranes decreased OD flux.

• Increase in the porosity of the membranes, flow rate of solutions and temperature increased of OD flux.

• The highest flux of 0.32 was found at optimized conditions, as porosity of membranes: 69.6 %, membrane thickness: 60±5µm , feed flow rate: 200 rpm and brine flow rate: 100 rpm, feed concentration: 10% (w/w) Fructose, hypertonic salt solution concentration: 20% (w/w) NaCl.

• OD hypertonic salt solutions are concerned, MgCl2 was more efficient and provided higher water fluxes than NaCl and CaCl2.2H2O .

ACKNOWLEDGEMENTS We are grateful to Gazi University Research Fund for the support this study.

NOMENCLATURE J flux, kg m -2 h-1 K overall mass transfer coefficient, kg m-2 h-1 Pa-1 k partial mass transfer coefficient

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V molar volume of water, 18 mL mol-1 Re Reynolds number Sc Schmidt number Sh Sherwood number w weight, g A area, m2 t time, h D diffusion coefficient, m 2 s -1 M molecular weight u velocity, m s-1 P water vapor pressure Greek symbols µ viscosity, Pas ε porosity π osmotic pressure φ association factor ρ density, kgm-3 Subscript F feed B brine w water m membrane Abbreviations RH Relative humidity EWC equilibrium water content

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