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Osmotic distillation with propylene glycol, glycerol and glycerol–salt mixtures
Journal of Membrane Science 229 (2004) 159–170
Osmotic distillation with propylene glycol, glycerol and glycerol–salt mixtures M. Celere, C. Gostoli∗ Dipartimento di Ingegneria Chimica, Mineraria e delle Tecnologie Ambientali, Università degli Studi di Bologna, Viale Risorgimento 2, I-40136 Bologna, Italy Received 11 June 2003; received in revised form 15 October 2003; accepted 17 October 2003
Abstract Aqueous solutions of propylene glycol (PG), glycerol and glycerol–salt mixtures are investigated as alternative to calcium chloride, normally employed to concentrate aqueous mixtures by osmotic distillation (OD). These extractants can overcome the problems of corrosion and scaling associated with the use of brines. The proposed extractants are compared in terms of driving force available for mass transfer, flux achievable, viscosity, penetration pressure through the membrane pores, solvent entrainment. The water activities of the relevant solutions are calculated by predictive methods and, in the case of glycerol–salt mixtures, measured by an isopiestic technique. Propylene glycol and glycerol solutions exhibit similar extractive power; the concentration has to be limited to 70–75 wt.% in order to limit the viscosity, as a consequence these compounds are a bit less effective than highly concentrated CaCl2 solutions. Notwithstanding the good flux achievable, propylene glycol cannot be recommended as extractant for juice concentration, indeed, owing to the not negligible volatility, it diffuses toward the juice in considerable amount, in addition, the penetration pressure through the membrane pores is quite small. The ternary mixture glycerol–salt–water allows to obtain the same flux achievable with glycerol alone, but with a substantially lower viscosity. © 2003 Elsevier B.V. All rights reserved. Keywords: Concentration; Hydrophobic membranes; Osmotic distillation fruit juices
1. Introduction The production of superior quality concentrates, especially fruit juices, has been considered a very important goal and considerable R&D effort has been devoted to develop non thermal concentration techniques. The freeze concentration [1], the FreshNoteTM [2–4] and the OsmotekTM [5] processes are good examples. The FreshNoteTM process is based on reverse osmosis through a combination of low retention-high retention modules [4], the OsmotekTM process is a direct osmosis through hydrophilous membranes with concentrated sugar solutions as extractant. Osmotic distillation (OD), in a sense, is a sort of direct osmosis in that water is removed from the feed by a hypertonic solution, the only difference with respect to the usual direct osmosis relies on the nature of the membrane employed, which is porous and hydrophobic. The advantage is
∗
Corresponding author. Tel.: +39-051-2093144; fax: +39-051-581200. E-mail address: [email protected] (C. Gostoli).
0376-7388/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2003.10.025
that non-volatile solutes are completely retained, thus more effective agents can be used as extractant. OD has been proposed for concentrating juices since 1989 [6], it has been shown that orange juice can be concentrated to 60◦ Brix by using porous membranes made of polytetrafluoroethylene (PTFE) or other hydrophobic material and concentrated brines as extractant. Many papers have been published in the last decade devoted to the physical mechanism involved as well as to applications in the juices production [7–13]. The mechanism of osmotic distillation is quite similar to membrane distillation [14,15]. Both processes are based on the gas membrane idea, which is a thin gas layer entrapped within the pores of a microporous hydrophobic polymer film, typically made of PTFE or polypropylene. The membrane is in contact on both sides with liquid phases at pressure lower than the pressure needed to displace the gas phase in the pores. Volatile compounds can pass across the membrane by diffusion through the gas phase (essentially air) entrapped within the pores; non-volatile solutes such as salts or sugars, on the contrary, are completely retained. The porous
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hydrophobic membrane is only the physical support for the gas membrane, which is the true selective barrier. In both OD and MD we have evaporation of water at the feed side, diffusion of water vapour across the gas membrane and condensation at the downstream side. The driving force for the diffusion is represented by the water vapour pressure gradient through the gas membrane, which in MD is sustained by a temperature difference, whereas in OD is sustained by an activity difference. Concentrated salt solutions, especially calcium chloride, were proposed as extractants [10]. The main weak point of OD is represented by the difficulty to handle concentrated brines. Corrosion and scaling make expensive the regeneration of exhausted brines which hardly can be accomplished by conventional evaporators. Alternative concentration techniques, as solar evaporation or pervaporation [16], have been proposed indeed. In order to overcome the problems associated with the use of brines we looked for organic extractants with good extractive power and easier to handle. The crucial properties to be considered are: high water solubility, high surface tension, negligible volatility, no toxicity. Based on these criteria glycols and glycerol seem possible organic extractants. Extraction with glycols has been already proposed for the ethanol removal from fermentation broth [17,18]. In this work we will explore the possibility of using glycols, glycerol or glycerol–salt mixtures for the concentration of aqueous solutions.
2. Membranes and extractants The description above gives a guideline for the selection of membranes and extractants. A crucial prerequisite for the process is represented by the ability of the membrane to sustain a gas phase within the pores; very important properties are thus the non-wettability of the membrane material, the pore size and the surface tension, which determine the penetration pressure. Pore sizes close to 0.2 m seem to be adequate for the process. Indeed commercial membranes with 0.2 m pore size exhibit a penetration pressure for water of nearly 3 bar [12,14]. Lower pore sizes give of course larger penetration pressure, but also lower permeability due to the Knudsen resistance to mass transfer. On the other hand, since the molecular diffusion rate is independent of the pore size, an increase of pore size above 0.2 m have a minor effect on the mass transfer rate. As regard as the extractant, since the driving force for the water transport through the membrane is given by the difference between the water vapour pressures at the two membrane sides, the “extractive power” of a substance is represented by the lowering of the water vapour pressure in aqueous solutions. A prerequisite is thus the high water solubility. Other crucial properties to be considered are: high surface tension (to get high penetration pressure), negligible
Table 1 Physical properties of propylene glycol and glycerol at 25 ◦ C Property
PG
Glycerol
Molar mass (kg/kmol) Normal boiling point (◦ C) Density (kg/m3 ) Viscosity (mPa s) Vapour pressure (Pa) Surface tension (dyn/cm)
76.096 187.3 1037 23 17.33 36
92.09 290 1261 934 0.023 63
volatility (to avoid counter-diffusion towards the juice and loss during regeneration), no toxicity. Based on the criteria above propylene glycol (PG) and Glycerol were identified as possible extractants. Both compounds are completely miscible with water and fulfil the no toxicity requirement, indeed PG and glycerol are allowed in food as well as in the pharmaceutical industry also as additives. The other relevant properties, reported in Table 1 [19,20], seem acceptable at a first thought: glycerol exhibits a negligible volatility and the surface tension is quite close to that of water. The viscosity of pure glycerol is actually very high, but it quickly lowers when glycerol is diluted with water and for concentrations lesser than 70–75 wt.% it takes acceptable values. Propylene glycol, on the other hand, shows a low, even if non negligible volatility; based on the value of the surface tension, the penetration pressure of PG through the membrane may be estimated to be nearly one half the value for water, larger values are expected for aqueous PG solutions. The extractive power of the extractants considered can be inferred from Fig. 1, which reports the water vapour pressure of various aqueous mixtures versus concentration. The glucose–sucrose mixture with 2:1 weight ratio can be considered as a model solution for several fruit juices. The water activity values were taken from literature data, extensive references and correlations will be given in the next paragraph. Apparently calcium and sodium chloride are equivalent at low concentration, however the driving force available with NaCl is limited because of the low solubility, for example, it is difficult to concentrate the juice to 60◦ Brix by using nearly saturated NaCl solutions as extractant, since the driving force is only 2 mbar. CaCl2 is a very effective extractant, and indeed it has been suggested by many researchers. Propylene glycol and glycerol exhibit similar extractive power; in comparison with CaCl2 these compounds are less effective, if one consider that, in order to limit the viscosity, the concentration should be limited to 70–75 wt.%. However the use of nearly saturated CaCl2 solutions on industrial scale poses serious problems: the solutions are highly corrosive, an accidental temperature drop causes crystallisation with obstruction in the pipes and fittings. All these problems are avoided by using PG or glycerol. The concentration polarisation is expected to play a heavy role in OD with PG or glycerol, due to the high viscosity,
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Fig. 1. Vapour pressure vs. concentration of various aqueous mixtures at 25 ◦ C.
however the phenomenon is probably relevant also with CaCl2 , the viscosity of saturated CaCl2 solutions is indeed nearly 13 mPa s at 25 ◦ C. In addition of the extractants listed above we considered a further possibility, namely ternary mixtures water–glycerol–salts, in order to get the same extractive power available with glycerol alone, but with lower viscosity. Solubility data of NaCl in glycerol aqueous solutions has been given in [21]. In the following we will present several OD experiments with different extractants to discuss the respective advantages and disadvantages. Before this we will present some water vapour pressure (or water activity) data and measurements for the solutions considered, especially for the ternary mixtures for which no literature data are available.
3. Theory 3.1. Mass transfer in OD The basic physical mechanism of the osmotic distillation is well known [7,10,12] and is only briefly recalled here: a porous hydrophobic membrane is in contact with the feed at one side and with the extractant on the other side, the water vapour pressures at the pore mouths are related to the temperature and activities prevailing in the liquids facing the membrane by ∗ Pw1 = Pw1 aw1
(1)
∗ aw2 Pw2 = Pw2
(2)
The resulting transmembrane mass flux, N, is related to the water vapour pressure difference (Pw1 − Pw2 ) by the
relationship: Pw1 − Pw2 N = Km PAlm
(3)
in which PAlm is the logarithmic mean pressure of the air within the pores and Km the membrane permeability depending on the membrane properties, mainly the thickness and porosity. Eq. (3), in which molecular diffusion is assumed to be the prevailing mechanism of mass transfer, showed to be adequate for membrane pores as larger as 0.2 m [12], of course as the pore size decreases, the Knudsen mechanism plays an increasing role. As in all the membrane processes, the flux through the membrane can be limited by the concentration polarisation phenomenon. Since only water passes through the membrane, the solute concentration near the membrane is larger than the bulk value in the feed side, and lower than the bulk value in the extract side. According to the well known film theory model we have ln
x1I N = x1 ρ1 kL1
(4)
ln
N x2 = I ρ x2 2 kL2
(5)
The mass transfer coefficients kL1 and kL2 can be estimated from literature correlations according to the module geometry and flow regime. Making reference to capillary modules, the tube-side mass transfer coefficient is given by the classical Graetz solution, for flow in the shell-side we adopted the “equivalent annulus model” [22], proposed in 1980 to calculate the shell-side mass transfer coefficient in hollow fibre dialysers. It assumes that the capillaries are surrounded by fictitious annular ducts of diameter d2 with overall area equal to the
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free cross-section of the shell, i.e. nd22 = (ds2 − nd2o ), in which ds is the shell diameter, do the outer diameter of the capillaries and n the capillary number. In [22] the ratio α = do /d2 was used as a model parameter, here the shell-side void fraction ε = 1 − α2 is used as a more significant parameter, defined by ε=
ds2 − nd2o ds2
(6)
In terms of the new parameter ε the Leveque solution under the uniform wall flux condition is 1/3 ε2 Sh = 1.302 (7) Gz −2 ln(1 − ε) − ε(2 + ε) in which the Sherwood and the Graetz numbers are defined assuming the outer diameter of the capillaries as characteristic length: kL do Sh = , D
ud2 Gz = o Dl
(8)
Also heat transfer is concerned in OD, indeed the water transport implies evaporation at the feed side and condensation at the extract side. As a consequence a temperature difference is created which reduces the vapour pressure difference across the membrane, i.e. the driving force for the water transport. The thermal effect in OD was analysed in [12].
Fig. 2. Comparison of calculated and experimental values of the water vapour pressure of NaCl and CaCl2 solutions. Experimental data after Robinson and Stokes [25].
the other hand, it will be shown hereafter that predictions based on Eq. (9) are in good agreement with experimental results.
4. Experimental 4.1. Water activity
3.2. Water activity The water activity of aqueous solutions of propylene glycol and glycerol are well represented by the Wilson method [23], the relevant data are reported in Appendix A [24]. Extensive data for electrolyte solutions are reported in [25], the ASOG method [26] showed to be in good agreement with experimental data for both pure salts and salt mixtures. Fig. 2 reports a comparison for the NaCl and CaCl2 solutions. More details on the ASOG method are given in Appendix A. As regard as glycerol–salt mixtures, the simplest prediction method for the water activity comes from the hypothesis that electrolytes and glycerol do not have any effects on each other, so that the non-ideality of the water is the sum of contribution of electrolytes and glycerol. The water activity coefficient in the mixture, γ w , can thus be expressed by Gly
ln γw = ln γwel + ln γw
(9)
in which γwel is the water activity coefficient, calculated by the ASOG method, of the water–electrolyte solution on glycGly erol free basis, and γw the water activity coefficient, calculated by the Wilson method, of the water–glycerol solution on electrolyte free basis. Nasrifar et al. [27] suggested a similar method for water–alcohol–salt mixtures, however the water activities aw were used in Eq. (9) instead of the activity coefficients γ w . In our opinion the “product rule” based on the activity coefficients has more foundations, on
Water activity measurements were performed by the isopiestic method described in the following. The mixture of unknown water activity was put into a glass jar of 50 ml, then a drop of a detector liquid was put on a little boat floating over the sample, the jar was hermetically sealed and maintained in a thermostated room for several days. Finally the concentration of the detector liquid was measured by a refractometer. To test the accuracy of the method and to evaluate the time needed to reach the equilibrium between the liquid in the jar and the liquid in the boat several tests were made with glycerol. To do this five glycerol solutions of concentration between 30 and 80 wt.% were prepared, a set of jars were then filled by each solution, whereas a drop of pure glycerol was putted in the little boats. Every day a jar of each set was taken of the thermostated chamber and the refractive index of the drop was measured and compared to the value of the solution in the jar. Fig. 3 shows the concentration difference between the drops of the detector liquid and the solutions versus time. Apparently the drops approach the concentration of the solutions in all cases, but the time depends of course on the concentration: two days are enough for the jars containing 80 wt.% glycerol, whereas four days are not enough for the jars containing 30 wt.% glycerol. Clearly the time can be minimised using a drop with water activity close
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163
Fig. 3. Glycerol weight percent difference between the drop and the solution vs. time at 25 ◦ C; the different lines correspond to different mass fractions of the solutions in the jars.
to the expected value of the solution, instead of pure glycerol. The method is very simple and accurate, it does not require complex and costly apparatuses, as other methods, but only a refractometer. Accurate measurements will require nearly a week, but the actual working time is very short; several solutions can be analysed in few minutes. The method outlined above was used to measure the water activity of ternary mixtures water–glycerol–NaCl. The procedure was as follows: several glycerol water solutions at different concentrations were prepared, each binary solution was divided in four bottles and different amount of anhydrous salt was added to each bottle. Six jars were then filled from each bottle and put in the thermostated room with the little boats floating over the liquid. Among the six jars of the same concentration, the detector liquid was pure glycerol in three jars whereas in the other three jars the initial glycerol–water solution was used. In this way the drop of the detector liquid approaches the equilibrium staring from a water activity lower than and respectively, larger than the value prevailing in the solution. Refractive index measurements were made after 5–7 days from the preparation; the values were very close each other. 4.2. Osmotic distillation experiments Osmotic distillation experiments were performed by a capillary module (LM2P06) supplied by Microdyn GmbH (Germany) manufactured with Accurel polypropylene capillaries. The relevant characteristics of the module are reported in Table 2. All the properties was given by the producer, but the permeability Km , which was measured in a previous work [12] through OD experiments with NaCl. The experimental set-up is shown schematically in Fig. 4. The feed was circulated in a closed loop connected to the
feed reservoir and to a graduate burette by a gear pump with variable speed (Iwaki MDG-M4T6, Japan). During the runs the valve V was closed, so that the level decrease in the burette was a direct measure of the flux through the membrane. When needed the valve was open to restore the liquid level in the burette. The extractant was circulated co-currently through the module and the reservoir by a gear pump equal to the feed pump. The inlet temperature of the two streams were controlled by two coils immersed in thermostatic baths, inlet and outlet temperatures were recorded. The inlet pressure of the tube-side stream was also measured continuously to be sure that the pressure never exceeded the penetration pressure of the membrane. The volume of the extractant was quite large in order to maintain a nearly constant concentration during the experiments. In all the experiments reported the feed was pure water, the feed flow rate was kept at relatively large value (above 50 l/h), as a consequence the exit temperature was equal to the inlet value in all the runs, the Table 2 The relevant characteristics of the module used in the experiments Module
LM 2P 06
Membrane Pore size (m) Penetration pressure (bar) Void fraction (%)
Accurel PP Q3/2 0.2 3.1 70
Capillary diameter Inner (mm) Outer (mm) Active length (mm) Shell diameter (mm) No. of capillaries Inside area (m2 ) Water vapour permeability (kg/m2 h)
0.6 1.0 250 18 85 0.04 137.4
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Fig. 5. Experimental vs. calculated values of the water vapour pressure of water–glycerol–NaCl mixtures. The digits beside the symbols represent the NaCl wt.% in the mixture.
Fig. 4. The set-up for osmotic distillation experiments: M, membrane module; F, feed reservoir; E, extractant reservoir; T, thermometers; P, pressure gauge; V, valve.
thermal effect associated to mass transfer is indeed negligible for large flow rates, as shown in [12]. 4.3. Penetration pressure The penetration pressure of PG solutions through the membrane was measured following the method described in [14]. To do this small modules were built sealing two or three capillaries inside a Plexiglas tube, in each run a module was filled by the solution to be tested on both sides, shell-side was connected to a burette, whereas tube-side was connected to a small reservoir containing the same solution. The pressure in the reservoir was then slowly increased by compressed air until gas bubbles appeared shell-side and a constant level increase was observed in the burette. It is
possible to use the same module in several runs, but after each run the membrane has to be washed with ethanol and dried in order to restore the hydrophobicity. 5. Results Fig. 5 shows a comparison between the water activities of glycerol–NaCl–water mixtures measured by the isopiestic method and the values calculated by the product rule, Eq. (9). Apparently there is a very good agreement, i.e. the predictive method is reliable for the system considered. Fig. 6 represents the behaviour of the system Glycerol– water–NaCl in the whole composition range. The water vapour pressure of the mixture at 25 ◦ C is reported as a function of the glycerol mass fraction on a salt free basis, the parameter R represents the mass ratio between NaCl and glycerol, the dashed line represents the saturation. The mixture may be interesting for osmotic distillation, as an example, we can obtain the same water vapour pressure
Fig. 6. Water vapour pressure at 25 ◦ C vs. the glycerol mass fraction on salt free basis. The different lines correspond to different NaCl/glycerol ratio.
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165
Fig. 7. Flux vs. glycol concentration in the extraction of pure water with propylene glycol through capillary membranes at 25 ◦ C. Theoretical values were calculated assuming Km = 137.4 kg/m2 h.
using glycerol 70 wt.%, with a viscosity of 19.3 mPa s or glycerol at 52 wt.% and R = 0.3, with a viscosity of 9.3 mPa s, the advantage is substantial. The results obtained in OD experiments with the various extractant are reported in Figs. 7–10. Fig. 7 reports the results obtained with propylene glycol in the concentration range 35–75 wt.%. The theoretical values were calculated by Eq. (3) assuming Km = 137.4 kg/m2 h [12] and Pw2 equal to the water vapour pressure at the concentration prevailing in the bulk phase, i.e. under the hypothesis of no concentration polarization effects. Apparently the measured fluxes are quite lower than the theoretical values for large glycol concentrations, the
glycol tube-side configuration gives larger fluxes with respect to the shell-side configuration for the same flow rate. This is due to the larger fluid velocity, because of the smaller cross-section. In the case of glycol shell-side the effect of concentration polarisation depends on the flow rate, as expected, even at large flow rate heavy polarisation effects were however observed. The concentration polarisation effects become negligible for glycol concentration lower than 55 wt.% in the shell-side configuration and for glycol concentration lower than 65 wt.% in the tube-side configuration. Fig. 8 reports the results obtained with glycerol in the concentration range 30–70 wt.%. In the glycerol tube-side
Fig. 8. Flux vs. glycerol concentration in the extraction of pure water with glycerol through capillary membranes at 25 ◦ C. Theoretical values were calculated assuming Km = 137.4 kg/m2 h.
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Fig. 9. Flux vs. glycerol concentration on salt free basis in the extraction of pure water with glycerol–NaCl mixtures, R = 0.34 g salt/g glycerol (open symbols), and with glycerol (closed symbols). Extractant shell-side, T = 25 ◦ C.
configuration the concentration was actually limited to 60%, since the pressure drops were too high at larger concentration. The flux values as well as the general behaviour were very close to that observed with PG. Fig. 9 reports the results obtained using as extractant a Glycerol–NaCl–water mixture with NaCl/glycerol ratio R = 0.34 and for comparison, the flux observed with glycerol alone. The experiments started with the highest concentration (50 wt.% glycerol on salt free basis), which, according to Fig. 6, has the same water activity as a glycerol solution 70 wt.%. The flux observed with this ternary mixtures was larger than the flux observed with glycerol 70 wt.%, but the difference was quite small. Apparently the lower viscosity of
Fig. 10. Flux vs. salt concentration in the extraction of pure water with CaCl2 through capillary membranes at 25 ◦ C, extractant shell-side. Theoretical values were calculated assuming Km = 137.4 kg/m2 h.
the ternary mixture reduces the concentration polarisation, but the effect on the flux is not substantial. Nevertheless the use of ternary mixtures is advantageous, the pressure drops were indeed less than one half with respect to a glycerol solution at the same water activity. Fig. 10 reports the results obtained with CaCl2 . As expected this extractant allows flux values quite larger with respect to the other extractants considered, however also in this case the concentration polarisation significantly reduces the flux with respect to the theoretical values. The role of concentration polarisation can be better appreciated reporting the flux versus the driving force, an example is shown in Fig. 11 in which the fluxes observed with various extractants at the same flow rate are reported versus the driving force evaluated at the bulk conditions. Apparently nearly all the data points lay in the same line, which deviates from the straight line, representing the theoretical values, as the driving force increases. The conclusion is drown that, notwithstanding the different values of the viscosity and of the diffusion coefficient, the role of concentration polarisation is nearly the same for the extractants considered. From Fig. 11 it is possible to evaluate the concentration prevailing at the extractant-membrane interface, indeed for any experimental flux value, the effective driving force can be read making reference to the theoretical line, or calculated from Eq. (3). Since the feed was pure water, from the effective driving force one can calculate the water vapour I and finally the composition xI at the interface. pressure Pw2 2 Fig. 12 reports the results obtained in the extraction with glycerol at different flow rates. Finally from Eq. (5) the extractant side mass transfer coefficient kL2 was calculated. Fig. 13 reports the experimental mass transfer coefficients for glycerol 70 wt.% and for CaCl2 40 wt.% versus the flow
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167
Fig. 11. Flux vs. driving force in the extraction of pure water with different extractants through capillary membranes. Extractant shell-side, flow rate 40 l/h, T = 25 ◦ C. Theoretical line corresponds to Km = 137.4 kg/m2 h.
rate. Theoretical values were calculated according to the equivalent annulus model, Eq. (7). For the module used in the experiments the shell-side void fraction is ε = 0.74 and the Sherwood number becomes Sh = 1.223Gz1/3
(10)
According to [22] Eq. (10) is valid for Gz > 40. The diffusion coefficient of glycerol in aqueous solution was calculated from the viscosity data following the procedure 10I reported in [28], for CaCl2 the values D = 1.195 × 10−5 cm2 /s, given by Robinson and Stokes [25] for 38.85 wt.% was used.
Fig. 13. Shell-side mass transfer coefficients vs. the flow rate in the extraction of pure water with glycerol 70 wt.% and with CaCl2 40 wt.%. Extractant shell-side, T = 25 ◦ C. Theoretical values were calculated with the equivalent annuls model [22].
The agreement is very satisfactory for glycerol, whereas for CaCl2 the experimental mass transfer coefficient appears lower than the predicted value. A possible explanation may be the thermal effect, which has been ignored in the analysis. Actually the thermal effect is expected to be relatively small at 25 ◦ C and at the large flow rates used in this work [12], however it may play a role in the extraction with CaCl2 , owing to the large flux.
6. Solvent entrainment Fig. 12. Glycerol mass fraction at the interface vs. the values in the bulk in the extraction of pure water in capillary module, extractant shell-side, T = 25 ◦ C.
The use of organic extractants, as propylene glycol presents some advantages with respect to brine, however
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Table 3 Surface tension and penetration pressure of PG solutions through the capillary membranes wt.%
σ calc (dyn/cm)
σ exp (dyn/cm)
PP (bar)
0 66 71 80 84 91 95 100
48.2 47.7 46.3 46.15 45.9 45.8
71 48.1 45.9 45.2 44.75 42.8 42.2 36
3.1a 1.26 1.15 1.1 1.05 0.98 0.95 0.6
a
Given by the supplier.
in order to accept this extractant for juice concentration any solvent entrainment has to be excluded. Apart from membrane defects, solvent entrainment can occur due to membrane wetting or counter-diffusion through the gas membrane. Membrane wetting occurs if the hydraulic pressure prevailing in the liquid phase exceeds the penetration pressure of the membrane, which is directly related to the surface tension of the liquid. Table 3 reports the surface tension as well as the penetration pressure of the PG solutions as a function of the concentration. σ calc represents the surface tension estimated by the method of Takamura, Kurata and Odani [23] and σ exp the values measured by the apparatus of Fisher-Scientific instruments. Apparently PG solutions exhibit surface tension and penetration pressure through the membrane pores quite lower with respect to water. One can conclude that propylene glycol can be used in osmotic distillation, however a careful pressure control is required to avoid membrane wetting. Counter-diffusion of PG towards the juice through the gas membrane is related to the volatility of PG, which is not negligible (see Table 1). A rough estimation of the ratio between the PG and water fluxes can be made assuming for both compounds independent binary diffusion through the stagnant air. In the case of pure water at one side and a PG solution on the other side and assuming ideal behaviour of PG solutions (Raoult’s law) we have NG DGA MG PG∗ = = 7.77 × 10−3 Nw DwA Mw Pw∗
(11)
The diffusion coefficient of PG vapour in air (DGA ) was estimated by the Fuller equation [23], it results DGA = 0.097 cm2 /s. More accurate value was calculated considering that water and PG vapours together diffuse through the stagnant air and applying the Stefan–Maxwell equation for multicomponent diffusion. The procedure was the same reported in [17], the calculated PG to water flux ratio appeared nearly independent on glycol composition and equal to 7.3 × 10−3 .
Some experiments were also performed with the rig used in OD experiments (Fig. 4). The two circuits were filled with known volumes (nearly 1 l) of pure water and PG solution, respectively, the system was then taken in operation for several hours keeping the water pressure a bit larger than the glycol pressure. At the end the overall water flux was evaluated as the volume change of the two liquids as well as from the glycol concentration. The glycol counter-flux was evaluated measuring the PG content in the water by HPLC using ethanol as internal standard. Several runs were performed with PG initial concentration ranging from 65 to 78 wt.%. The observed PG to water flux ratios seemed independent of the initial glycol concentration and ranged from 6.4 to 9.2 × 10−3 with average value 7.6 × 10−3 . The results above clearly show that PG cannot be recommended as extractant for juice concentration. Let us, for example, consider the case in which a juice is concentrated from 12 to 60◦ Brix with PG and then reconstructed with water to 12◦ Brix: the final juice will contain nearly 0.6 wt.% of PG. Even if PG is a not toxic compound, this value cannot be accepted. In similar experiments performed with glycerol solutions the glycerol concentration in the water after the runs was not detectable with the analytical method used, the theoretical calculations gave a glycerol to water flux ratio of nearly 10−6 , which gives a glycerol content in the final juice (after concentration to 60◦ Brix and reconstruction) of only 8 mg/kg (8 ppm).
7. Conclusions Several extractants have been compared. Among these CaCl2 showed to be the most effective, however organic extractants as PG and Glycerol allow to obtain comparable fluxes with advantages related to the absence of corrosion and scaling. PG and Glycerol are equivalent as regard as the extractive power, however PG showed a low penetration pressure through the membrane pores, in addition, it diffused towards the juice in considerable amount, owing to the not negligible volatility at room temperature. The weak point of glycerol is the high viscosity, nevertheless the operation is possible also with narrow capillaries, as those used in this study, limiting the concentration to nearly 70–75 wt.%. Interesting possibilities are offered by the ternary mixtures water–glycerol–NaCl, or possibly other salts. Indeed the same flux achievable with glycerol alone can be obtained, but with a substantially lower viscosity. As the driving force, and thus the flux, increase, large concentration polarisation is observed, the effect is nearly the same for all the extractants tested. The shell-side mass transfer rate is adequately described by the equivalent annulus model.
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Nomenclature
ln(Λ21 ) = A21 +
aw do ds D Gz kL Km l M n N Pw R Sh x
with:
Greek ε γ ρ σ
water activity outer diameter shell diameter diffusion coefficient Graetz number mass transfer coefficient membrane permeability module length molecular weight number of capillaries mass flux water vapour pressure NaCl/glycerol mass ratio Sherwood number mole fraction
Glycerol PG
169
B21 T
(A.4)
A12
B12
A21
1.59191 6.99834
−199.24 −1969.27
0.88587 −17.630
B21 −640.96 5128.72
A.2. The modified ASOG method It is a group contribution method in which the activity coefficient γ i of a component in a mixture consist of a configurational contribution (γiC ), due to differences in molecular size, a group interaction contribution (γiR ), due to differences in intermolecular forces, and a Debye–Hückel (D–H) contribution (γwD–H ) which takes into account the effect of electrostatic interactions. ln γ = ln γ C + ln γ R + ln γ D–H (A.5)
letters shell void fraction activity coefficient density surface tension
i
i
i
w
where ln γiC = ln
υiFH j
υjFH xj
+1−
υiFH
j
υjFH xj
,
j = 1, 2, . . . , no. comp.
Subscripts 1 feed side 2 extract side A air G glycol lm logarithmic mean w water
ln γiR =
k
(A.6) (i)
υki [ln Γk − ln Γk ],
k = 1, 2, . . . , no. groups ln Γk = −ln
Superscripts I interface * pure water
Xn akn + 1 −
n
(A.7)
k, m, n = 1, 2, . . . , no. groups Xn =
j xj υjn
j xj
Appendix A
k
υkj
,
n
Xn ank , m Xm anm
j = 1, 2, . . . , no. comp.; k, n = 1, 2, . . . , no. groups
A.1. The Wilson method
ln akn = mkn +
The water vapour pressure is given by Pw = Pw∗ xw γw
(A.1)
in which Pw∗ is the pure water vapour pressure, xw the water mole fraction and γw the water activity coefficient given by ln(γw ) = −ln(xw + Λ12 xg ) Λ12 Λ21 + xg − xw + Λ12 xg Λ21 xw + xg
(A.2)
in which xg = 1 − xw is the solute mole fraction. The temperature dependence of the parameters Λ12 and Λ21 is given by ln(Λ12 ) = A12 +
B12 T
(A.3)
nkn T
(A.8)
(A.9) (A.10)
where υiFH is the number of non-hydrogen atoms in molecule i, υki is the number of non-hydrogen atoms in group k of (i) molecule i, xj is the mole fraction of component j, γ k and γk are the group activity coefficients of group k in the system and at standard state (pure component), respectively, akn is the group interaction parameter characteristic of groups k and n (akn = ank ), mkn and nkn are the group pair parameters characteristic of group k and n (temperature independent), and T is the temperature in K. The last term in Eq. (A.5) is √ √ 2AM 1 D–H ln γw = 3 1+b I − √ − 2 ln(1 + b I) b 1+b I (A.11)
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where A=
1.327757(105 )ρ1/2 (εT)3/2
(A.12)
b=
6.359696ρ1/2 (εT)1/2
(A.13)
and M is the molar mass of water (kg/mol), I the ionic strength (I = (1/2) mi z2i ), ρ the density of pure water (kg/m3 ), ε the dielectric constant of pure water, T the temperature (K), mi the molality of ion i and zi is the charge number of ion i. For the electrolyte solutions of our interest the parameters are (W stands for water, CH for the hydrated cation):
υiFH υki
W
CH
Cl−
1.0 1.6
1 + hc 1.6hc
1.0 1.0
hc is hydration number of cations (1 for Na+ and 3.1 for Ca2+ ). The ASOG pair parameters mkn and nkn are: W
CH −6.584 1925.5
W CH
22.79 2116
Cl−
29.45 −3851
2.116 3871
Cl− 1.9896 −341.1
m n
−69.07 20353
m n m n
References [1] W.H.J.M. van Pelt, Die Wirtschaftlichkeitmehrstufiger Gefrierkonzentrierungsverfahren (Economics of Multistage Freeze Concentration Processes), Confructa Studien Mai/Juni III/84, pp. 225–239. [2] J.B. Walker, Membrane process for production of superior quality fruit juice concentrates, in: Proceedings of the International Congress on Membrane, vol. 1, Chicago, 20–24 August 1990, pp. 283–285. [3] M. Cheryan, Membranes in food processing, in: M.K. Turner (Ed.), Effective Industrial Membrane Processes: Benefits and Opportunities, Elsevier, Amsterdam, 1991. [4] C. Gostoli, S. Bandini, R. Di Francesca, G. Zardi, Analysis of a reverse osmosis process for concentrating solutions of high osmotic pressure: the low retention method, in: Proceedings of the Trans IChemE on Food and Bioproducts Processing, vol. 74, Part C, June 1996, pp. 101–109. [5] E.G. Beaudry, J.R. Herron, C.E. Jochums, L.E. Medina, Apparatus for direct osmotic concentration of fruit juices, Osmotek Inc. (OSMO), US Patent 5281430 (25 January 1994).
[6] R.A. Johnson, R.H. Valks, M.S. Lefevre, Osmotic distillation-a low temperature concentration technique, Aust. J. Biotechnol. 3 (3) (1989) 206–217. [7] W. Kunz, A. Benhabiles, R. Ben-Aim, Osmotic evaporation through macroporous hydrophobic membranes: a survey of current research and applications, J. Membr. Sci. 121 (1996) 25–36. [8] P.A. Hogan, R.P. Canning, P.A. Peterson, R.A. Johnson, A.S. Michaels, A new option: osmotic distillation, Chem. Eng. Prog. (July 1998) 49–61. [9] J.I. Mengual, Z. Ortiz De, L. Pena, A. Velasquez, Osmotic distillation through porous hydrophobic membranes, J. Membr. Sci. 82 (1993) 129–140. [10] M. Courel, M. Dornier, G.M. Rios, M. Reynes, Modelling of water transport in osmotic distillation using asymmetric membrane, J. Membr. Sci. 173 (2000) 107–122. [11] F. Vaillant, E. Jeanton, M. Dornier, O’Brien,M. Reynes, M. Decloux, Concentration of passion fruit on industrial pilot scale using osmotic evaporation, J. Food Eng. 47 (2001) 195–202. [12] C. Gostoli, Thermal effects in osmotic distillation, J. Membr. Sci. 163 (1) (1999) 75–91. [13] J. Romero, M.G. Rios, J. Sanchez, S. Bocquet, A. Savedra, Modeling heat and mass transfer in osmotic evaporation process, Am. Inst. Chem. J. 49 (2) (2003) 300–308. [14] G.C. Sarti, C. Gostoli, S. Matulli, Low energy cost desalination processes using hydrophobic membranes, Desalination 56 (1985) 277–286. [15] K.W. Lawson, D.R. Lloyd, Membrane distillation, J. Membr. Sci. 124 (1997) 1–25. [16] J. Sheng, Pilot scale brine concentration process using membrane pervaporation, J. Membr. Sci. 87 (1994) 131–137. [17] S. Bandini, C. Gostoli, Ethanol removal from fermentation broth by gas membrane extraction, J. Membr. Sci. 70 (1992) 119–127. [18] C. Gostoli, S. Bandini, Gas membrane extraction of ethanol by glycols: experiments and modelling, J. Membr. Sci. 98 (1995) 1– 12. [19] R.H. Perry, D.W. Green, Perry’s Chemical Engineers’ Handbook, McGraw-Hill, New York, 1997. [20] R.C. Weast, Handbook of Chemistry and Physics, CRC Press, Boca Raton, 1974. [21] D.H.T. Chen, A.R. Thompson, Isobaric vapour–liquid equilibria for the systems glycerol–water and glycerol–water saturated with sodium chloride, J. Chem. Eng. Data 15 (4) (1970) 471–474. [22] C. Gostoli, A. Gatta, Mass transfer in a hollow fiber dialyzer, J. Membr. Sci. 6 (1980) 133–148. [23] R.C. Reid, J.M. Prausnitz, B.E. Poling, The Properties of Gases and Liquids, McGraw-Hill, New York, 1987. [24] J. Gmehling, U. Onken, Vapour Liquid Equilibrium Data Collection, vol. 1, Part 1, Chemistry Data Series, Dechema, 1981. [25] A. Robinson, R.H. Stokes, Electrolyte Solutions, Butterworths, London, 1959. [26] A. Correa, J.F. Comesana, J.M. Correa, A.M. Sereno, Measurement and prediction of water activity in electrolyte solutions by modified ASOG group contribution method, Fluid Phase Equilib. 129 (1997) 267–283. [27] Kh. Nasrifar, M. Moshfeghian, R.N. Maddox, Prediction of equilibrium conditions for gas hydrate formation in the mixtures of both electrolytes and alcohol, Fluid Phase Equilib. 146 (1998) 1– 13. [28] R.P. Danner, T.E. Daubert, Manual for predicting chemical process design data: data prediction manual, Am. Inst. Chem. Eng., 1983.
Osmotic distillation with propylene glycol, glycerol and glycerol–salt mixtures M. Celere, C. Gostoli∗ Dipartimento di Ingegneria Chimica, Mineraria e delle Tecnologie Ambientali, Università degli Studi di Bologna, Viale Risorgimento 2, I-40136 Bologna, Italy Received 11 June 2003; received in revised form 15 October 2003; accepted 17 October 2003
Abstract Aqueous solutions of propylene glycol (PG), glycerol and glycerol–salt mixtures are investigated as alternative to calcium chloride, normally employed to concentrate aqueous mixtures by osmotic distillation (OD). These extractants can overcome the problems of corrosion and scaling associated with the use of brines. The proposed extractants are compared in terms of driving force available for mass transfer, flux achievable, viscosity, penetration pressure through the membrane pores, solvent entrainment. The water activities of the relevant solutions are calculated by predictive methods and, in the case of glycerol–salt mixtures, measured by an isopiestic technique. Propylene glycol and glycerol solutions exhibit similar extractive power; the concentration has to be limited to 70–75 wt.% in order to limit the viscosity, as a consequence these compounds are a bit less effective than highly concentrated CaCl2 solutions. Notwithstanding the good flux achievable, propylene glycol cannot be recommended as extractant for juice concentration, indeed, owing to the not negligible volatility, it diffuses toward the juice in considerable amount, in addition, the penetration pressure through the membrane pores is quite small. The ternary mixture glycerol–salt–water allows to obtain the same flux achievable with glycerol alone, but with a substantially lower viscosity. © 2003 Elsevier B.V. All rights reserved. Keywords: Concentration; Hydrophobic membranes; Osmotic distillation fruit juices
1. Introduction The production of superior quality concentrates, especially fruit juices, has been considered a very important goal and considerable R&D effort has been devoted to develop non thermal concentration techniques. The freeze concentration [1], the FreshNoteTM [2–4] and the OsmotekTM [5] processes are good examples. The FreshNoteTM process is based on reverse osmosis through a combination of low retention-high retention modules [4], the OsmotekTM process is a direct osmosis through hydrophilous membranes with concentrated sugar solutions as extractant. Osmotic distillation (OD), in a sense, is a sort of direct osmosis in that water is removed from the feed by a hypertonic solution, the only difference with respect to the usual direct osmosis relies on the nature of the membrane employed, which is porous and hydrophobic. The advantage is
∗
Corresponding author. Tel.: +39-051-2093144; fax: +39-051-581200. E-mail address: [email protected] (C. Gostoli).
0376-7388/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2003.10.025
that non-volatile solutes are completely retained, thus more effective agents can be used as extractant. OD has been proposed for concentrating juices since 1989 [6], it has been shown that orange juice can be concentrated to 60◦ Brix by using porous membranes made of polytetrafluoroethylene (PTFE) or other hydrophobic material and concentrated brines as extractant. Many papers have been published in the last decade devoted to the physical mechanism involved as well as to applications in the juices production [7–13]. The mechanism of osmotic distillation is quite similar to membrane distillation [14,15]. Both processes are based on the gas membrane idea, which is a thin gas layer entrapped within the pores of a microporous hydrophobic polymer film, typically made of PTFE or polypropylene. The membrane is in contact on both sides with liquid phases at pressure lower than the pressure needed to displace the gas phase in the pores. Volatile compounds can pass across the membrane by diffusion through the gas phase (essentially air) entrapped within the pores; non-volatile solutes such as salts or sugars, on the contrary, are completely retained. The porous
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hydrophobic membrane is only the physical support for the gas membrane, which is the true selective barrier. In both OD and MD we have evaporation of water at the feed side, diffusion of water vapour across the gas membrane and condensation at the downstream side. The driving force for the diffusion is represented by the water vapour pressure gradient through the gas membrane, which in MD is sustained by a temperature difference, whereas in OD is sustained by an activity difference. Concentrated salt solutions, especially calcium chloride, were proposed as extractants [10]. The main weak point of OD is represented by the difficulty to handle concentrated brines. Corrosion and scaling make expensive the regeneration of exhausted brines which hardly can be accomplished by conventional evaporators. Alternative concentration techniques, as solar evaporation or pervaporation [16], have been proposed indeed. In order to overcome the problems associated with the use of brines we looked for organic extractants with good extractive power and easier to handle. The crucial properties to be considered are: high water solubility, high surface tension, negligible volatility, no toxicity. Based on these criteria glycols and glycerol seem possible organic extractants. Extraction with glycols has been already proposed for the ethanol removal from fermentation broth [17,18]. In this work we will explore the possibility of using glycols, glycerol or glycerol–salt mixtures for the concentration of aqueous solutions.
2. Membranes and extractants The description above gives a guideline for the selection of membranes and extractants. A crucial prerequisite for the process is represented by the ability of the membrane to sustain a gas phase within the pores; very important properties are thus the non-wettability of the membrane material, the pore size and the surface tension, which determine the penetration pressure. Pore sizes close to 0.2 m seem to be adequate for the process. Indeed commercial membranes with 0.2 m pore size exhibit a penetration pressure for water of nearly 3 bar [12,14]. Lower pore sizes give of course larger penetration pressure, but also lower permeability due to the Knudsen resistance to mass transfer. On the other hand, since the molecular diffusion rate is independent of the pore size, an increase of pore size above 0.2 m have a minor effect on the mass transfer rate. As regard as the extractant, since the driving force for the water transport through the membrane is given by the difference between the water vapour pressures at the two membrane sides, the “extractive power” of a substance is represented by the lowering of the water vapour pressure in aqueous solutions. A prerequisite is thus the high water solubility. Other crucial properties to be considered are: high surface tension (to get high penetration pressure), negligible
Table 1 Physical properties of propylene glycol and glycerol at 25 ◦ C Property
PG
Glycerol
Molar mass (kg/kmol) Normal boiling point (◦ C) Density (kg/m3 ) Viscosity (mPa s) Vapour pressure (Pa) Surface tension (dyn/cm)
76.096 187.3 1037 23 17.33 36
92.09 290 1261 934 0.023 63
volatility (to avoid counter-diffusion towards the juice and loss during regeneration), no toxicity. Based on the criteria above propylene glycol (PG) and Glycerol were identified as possible extractants. Both compounds are completely miscible with water and fulfil the no toxicity requirement, indeed PG and glycerol are allowed in food as well as in the pharmaceutical industry also as additives. The other relevant properties, reported in Table 1 [19,20], seem acceptable at a first thought: glycerol exhibits a negligible volatility and the surface tension is quite close to that of water. The viscosity of pure glycerol is actually very high, but it quickly lowers when glycerol is diluted with water and for concentrations lesser than 70–75 wt.% it takes acceptable values. Propylene glycol, on the other hand, shows a low, even if non negligible volatility; based on the value of the surface tension, the penetration pressure of PG through the membrane may be estimated to be nearly one half the value for water, larger values are expected for aqueous PG solutions. The extractive power of the extractants considered can be inferred from Fig. 1, which reports the water vapour pressure of various aqueous mixtures versus concentration. The glucose–sucrose mixture with 2:1 weight ratio can be considered as a model solution for several fruit juices. The water activity values were taken from literature data, extensive references and correlations will be given in the next paragraph. Apparently calcium and sodium chloride are equivalent at low concentration, however the driving force available with NaCl is limited because of the low solubility, for example, it is difficult to concentrate the juice to 60◦ Brix by using nearly saturated NaCl solutions as extractant, since the driving force is only 2 mbar. CaCl2 is a very effective extractant, and indeed it has been suggested by many researchers. Propylene glycol and glycerol exhibit similar extractive power; in comparison with CaCl2 these compounds are less effective, if one consider that, in order to limit the viscosity, the concentration should be limited to 70–75 wt.%. However the use of nearly saturated CaCl2 solutions on industrial scale poses serious problems: the solutions are highly corrosive, an accidental temperature drop causes crystallisation with obstruction in the pipes and fittings. All these problems are avoided by using PG or glycerol. The concentration polarisation is expected to play a heavy role in OD with PG or glycerol, due to the high viscosity,
M. Celere, C. Gostoli / Journal of Membrane Science 229 (2004) 159–170
161
Fig. 1. Vapour pressure vs. concentration of various aqueous mixtures at 25 ◦ C.
however the phenomenon is probably relevant also with CaCl2 , the viscosity of saturated CaCl2 solutions is indeed nearly 13 mPa s at 25 ◦ C. In addition of the extractants listed above we considered a further possibility, namely ternary mixtures water–glycerol–salts, in order to get the same extractive power available with glycerol alone, but with lower viscosity. Solubility data of NaCl in glycerol aqueous solutions has been given in [21]. In the following we will present several OD experiments with different extractants to discuss the respective advantages and disadvantages. Before this we will present some water vapour pressure (or water activity) data and measurements for the solutions considered, especially for the ternary mixtures for which no literature data are available.
3. Theory 3.1. Mass transfer in OD The basic physical mechanism of the osmotic distillation is well known [7,10,12] and is only briefly recalled here: a porous hydrophobic membrane is in contact with the feed at one side and with the extractant on the other side, the water vapour pressures at the pore mouths are related to the temperature and activities prevailing in the liquids facing the membrane by ∗ Pw1 = Pw1 aw1
(1)
∗ aw2 Pw2 = Pw2
(2)
The resulting transmembrane mass flux, N, is related to the water vapour pressure difference (Pw1 − Pw2 ) by the
relationship: Pw1 − Pw2 N = Km PAlm
(3)
in which PAlm is the logarithmic mean pressure of the air within the pores and Km the membrane permeability depending on the membrane properties, mainly the thickness and porosity. Eq. (3), in which molecular diffusion is assumed to be the prevailing mechanism of mass transfer, showed to be adequate for membrane pores as larger as 0.2 m [12], of course as the pore size decreases, the Knudsen mechanism plays an increasing role. As in all the membrane processes, the flux through the membrane can be limited by the concentration polarisation phenomenon. Since only water passes through the membrane, the solute concentration near the membrane is larger than the bulk value in the feed side, and lower than the bulk value in the extract side. According to the well known film theory model we have ln
x1I N = x1 ρ1 kL1
(4)
ln
N x2 = I ρ x2 2 kL2
(5)
The mass transfer coefficients kL1 and kL2 can be estimated from literature correlations according to the module geometry and flow regime. Making reference to capillary modules, the tube-side mass transfer coefficient is given by the classical Graetz solution, for flow in the shell-side we adopted the “equivalent annulus model” [22], proposed in 1980 to calculate the shell-side mass transfer coefficient in hollow fibre dialysers. It assumes that the capillaries are surrounded by fictitious annular ducts of diameter d2 with overall area equal to the
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free cross-section of the shell, i.e. nd22 = (ds2 − nd2o ), in which ds is the shell diameter, do the outer diameter of the capillaries and n the capillary number. In [22] the ratio α = do /d2 was used as a model parameter, here the shell-side void fraction ε = 1 − α2 is used as a more significant parameter, defined by ε=
ds2 − nd2o ds2
(6)
In terms of the new parameter ε the Leveque solution under the uniform wall flux condition is 1/3 ε2 Sh = 1.302 (7) Gz −2 ln(1 − ε) − ε(2 + ε) in which the Sherwood and the Graetz numbers are defined assuming the outer diameter of the capillaries as characteristic length: kL do Sh = , D
ud2 Gz = o Dl
(8)
Also heat transfer is concerned in OD, indeed the water transport implies evaporation at the feed side and condensation at the extract side. As a consequence a temperature difference is created which reduces the vapour pressure difference across the membrane, i.e. the driving force for the water transport. The thermal effect in OD was analysed in [12].
Fig. 2. Comparison of calculated and experimental values of the water vapour pressure of NaCl and CaCl2 solutions. Experimental data after Robinson and Stokes [25].
the other hand, it will be shown hereafter that predictions based on Eq. (9) are in good agreement with experimental results.
4. Experimental 4.1. Water activity
3.2. Water activity The water activity of aqueous solutions of propylene glycol and glycerol are well represented by the Wilson method [23], the relevant data are reported in Appendix A [24]. Extensive data for electrolyte solutions are reported in [25], the ASOG method [26] showed to be in good agreement with experimental data for both pure salts and salt mixtures. Fig. 2 reports a comparison for the NaCl and CaCl2 solutions. More details on the ASOG method are given in Appendix A. As regard as glycerol–salt mixtures, the simplest prediction method for the water activity comes from the hypothesis that electrolytes and glycerol do not have any effects on each other, so that the non-ideality of the water is the sum of contribution of electrolytes and glycerol. The water activity coefficient in the mixture, γ w , can thus be expressed by Gly
ln γw = ln γwel + ln γw
(9)
in which γwel is the water activity coefficient, calculated by the ASOG method, of the water–electrolyte solution on glycGly erol free basis, and γw the water activity coefficient, calculated by the Wilson method, of the water–glycerol solution on electrolyte free basis. Nasrifar et al. [27] suggested a similar method for water–alcohol–salt mixtures, however the water activities aw were used in Eq. (9) instead of the activity coefficients γ w . In our opinion the “product rule” based on the activity coefficients has more foundations, on
Water activity measurements were performed by the isopiestic method described in the following. The mixture of unknown water activity was put into a glass jar of 50 ml, then a drop of a detector liquid was put on a little boat floating over the sample, the jar was hermetically sealed and maintained in a thermostated room for several days. Finally the concentration of the detector liquid was measured by a refractometer. To test the accuracy of the method and to evaluate the time needed to reach the equilibrium between the liquid in the jar and the liquid in the boat several tests were made with glycerol. To do this five glycerol solutions of concentration between 30 and 80 wt.% were prepared, a set of jars were then filled by each solution, whereas a drop of pure glycerol was putted in the little boats. Every day a jar of each set was taken of the thermostated chamber and the refractive index of the drop was measured and compared to the value of the solution in the jar. Fig. 3 shows the concentration difference between the drops of the detector liquid and the solutions versus time. Apparently the drops approach the concentration of the solutions in all cases, but the time depends of course on the concentration: two days are enough for the jars containing 80 wt.% glycerol, whereas four days are not enough for the jars containing 30 wt.% glycerol. Clearly the time can be minimised using a drop with water activity close
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163
Fig. 3. Glycerol weight percent difference between the drop and the solution vs. time at 25 ◦ C; the different lines correspond to different mass fractions of the solutions in the jars.
to the expected value of the solution, instead of pure glycerol. The method is very simple and accurate, it does not require complex and costly apparatuses, as other methods, but only a refractometer. Accurate measurements will require nearly a week, but the actual working time is very short; several solutions can be analysed in few minutes. The method outlined above was used to measure the water activity of ternary mixtures water–glycerol–NaCl. The procedure was as follows: several glycerol water solutions at different concentrations were prepared, each binary solution was divided in four bottles and different amount of anhydrous salt was added to each bottle. Six jars were then filled from each bottle and put in the thermostated room with the little boats floating over the liquid. Among the six jars of the same concentration, the detector liquid was pure glycerol in three jars whereas in the other three jars the initial glycerol–water solution was used. In this way the drop of the detector liquid approaches the equilibrium staring from a water activity lower than and respectively, larger than the value prevailing in the solution. Refractive index measurements were made after 5–7 days from the preparation; the values were very close each other. 4.2. Osmotic distillation experiments Osmotic distillation experiments were performed by a capillary module (LM2P06) supplied by Microdyn GmbH (Germany) manufactured with Accurel polypropylene capillaries. The relevant characteristics of the module are reported in Table 2. All the properties was given by the producer, but the permeability Km , which was measured in a previous work [12] through OD experiments with NaCl. The experimental set-up is shown schematically in Fig. 4. The feed was circulated in a closed loop connected to the
feed reservoir and to a graduate burette by a gear pump with variable speed (Iwaki MDG-M4T6, Japan). During the runs the valve V was closed, so that the level decrease in the burette was a direct measure of the flux through the membrane. When needed the valve was open to restore the liquid level in the burette. The extractant was circulated co-currently through the module and the reservoir by a gear pump equal to the feed pump. The inlet temperature of the two streams were controlled by two coils immersed in thermostatic baths, inlet and outlet temperatures were recorded. The inlet pressure of the tube-side stream was also measured continuously to be sure that the pressure never exceeded the penetration pressure of the membrane. The volume of the extractant was quite large in order to maintain a nearly constant concentration during the experiments. In all the experiments reported the feed was pure water, the feed flow rate was kept at relatively large value (above 50 l/h), as a consequence the exit temperature was equal to the inlet value in all the runs, the Table 2 The relevant characteristics of the module used in the experiments Module
LM 2P 06
Membrane Pore size (m) Penetration pressure (bar) Void fraction (%)
Accurel PP Q3/2 0.2 3.1 70
Capillary diameter Inner (mm) Outer (mm) Active length (mm) Shell diameter (mm) No. of capillaries Inside area (m2 ) Water vapour permeability (kg/m2 h)
0.6 1.0 250 18 85 0.04 137.4
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Fig. 5. Experimental vs. calculated values of the water vapour pressure of water–glycerol–NaCl mixtures. The digits beside the symbols represent the NaCl wt.% in the mixture.
Fig. 4. The set-up for osmotic distillation experiments: M, membrane module; F, feed reservoir; E, extractant reservoir; T, thermometers; P, pressure gauge; V, valve.
thermal effect associated to mass transfer is indeed negligible for large flow rates, as shown in [12]. 4.3. Penetration pressure The penetration pressure of PG solutions through the membrane was measured following the method described in [14]. To do this small modules were built sealing two or three capillaries inside a Plexiglas tube, in each run a module was filled by the solution to be tested on both sides, shell-side was connected to a burette, whereas tube-side was connected to a small reservoir containing the same solution. The pressure in the reservoir was then slowly increased by compressed air until gas bubbles appeared shell-side and a constant level increase was observed in the burette. It is
possible to use the same module in several runs, but after each run the membrane has to be washed with ethanol and dried in order to restore the hydrophobicity. 5. Results Fig. 5 shows a comparison between the water activities of glycerol–NaCl–water mixtures measured by the isopiestic method and the values calculated by the product rule, Eq. (9). Apparently there is a very good agreement, i.e. the predictive method is reliable for the system considered. Fig. 6 represents the behaviour of the system Glycerol– water–NaCl in the whole composition range. The water vapour pressure of the mixture at 25 ◦ C is reported as a function of the glycerol mass fraction on a salt free basis, the parameter R represents the mass ratio between NaCl and glycerol, the dashed line represents the saturation. The mixture may be interesting for osmotic distillation, as an example, we can obtain the same water vapour pressure
Fig. 6. Water vapour pressure at 25 ◦ C vs. the glycerol mass fraction on salt free basis. The different lines correspond to different NaCl/glycerol ratio.
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165
Fig. 7. Flux vs. glycol concentration in the extraction of pure water with propylene glycol through capillary membranes at 25 ◦ C. Theoretical values were calculated assuming Km = 137.4 kg/m2 h.
using glycerol 70 wt.%, with a viscosity of 19.3 mPa s or glycerol at 52 wt.% and R = 0.3, with a viscosity of 9.3 mPa s, the advantage is substantial. The results obtained in OD experiments with the various extractant are reported in Figs. 7–10. Fig. 7 reports the results obtained with propylene glycol in the concentration range 35–75 wt.%. The theoretical values were calculated by Eq. (3) assuming Km = 137.4 kg/m2 h [12] and Pw2 equal to the water vapour pressure at the concentration prevailing in the bulk phase, i.e. under the hypothesis of no concentration polarization effects. Apparently the measured fluxes are quite lower than the theoretical values for large glycol concentrations, the
glycol tube-side configuration gives larger fluxes with respect to the shell-side configuration for the same flow rate. This is due to the larger fluid velocity, because of the smaller cross-section. In the case of glycol shell-side the effect of concentration polarisation depends on the flow rate, as expected, even at large flow rate heavy polarisation effects were however observed. The concentration polarisation effects become negligible for glycol concentration lower than 55 wt.% in the shell-side configuration and for glycol concentration lower than 65 wt.% in the tube-side configuration. Fig. 8 reports the results obtained with glycerol in the concentration range 30–70 wt.%. In the glycerol tube-side
Fig. 8. Flux vs. glycerol concentration in the extraction of pure water with glycerol through capillary membranes at 25 ◦ C. Theoretical values were calculated assuming Km = 137.4 kg/m2 h.
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Fig. 9. Flux vs. glycerol concentration on salt free basis in the extraction of pure water with glycerol–NaCl mixtures, R = 0.34 g salt/g glycerol (open symbols), and with glycerol (closed symbols). Extractant shell-side, T = 25 ◦ C.
configuration the concentration was actually limited to 60%, since the pressure drops were too high at larger concentration. The flux values as well as the general behaviour were very close to that observed with PG. Fig. 9 reports the results obtained using as extractant a Glycerol–NaCl–water mixture with NaCl/glycerol ratio R = 0.34 and for comparison, the flux observed with glycerol alone. The experiments started with the highest concentration (50 wt.% glycerol on salt free basis), which, according to Fig. 6, has the same water activity as a glycerol solution 70 wt.%. The flux observed with this ternary mixtures was larger than the flux observed with glycerol 70 wt.%, but the difference was quite small. Apparently the lower viscosity of
Fig. 10. Flux vs. salt concentration in the extraction of pure water with CaCl2 through capillary membranes at 25 ◦ C, extractant shell-side. Theoretical values were calculated assuming Km = 137.4 kg/m2 h.
the ternary mixture reduces the concentration polarisation, but the effect on the flux is not substantial. Nevertheless the use of ternary mixtures is advantageous, the pressure drops were indeed less than one half with respect to a glycerol solution at the same water activity. Fig. 10 reports the results obtained with CaCl2 . As expected this extractant allows flux values quite larger with respect to the other extractants considered, however also in this case the concentration polarisation significantly reduces the flux with respect to the theoretical values. The role of concentration polarisation can be better appreciated reporting the flux versus the driving force, an example is shown in Fig. 11 in which the fluxes observed with various extractants at the same flow rate are reported versus the driving force evaluated at the bulk conditions. Apparently nearly all the data points lay in the same line, which deviates from the straight line, representing the theoretical values, as the driving force increases. The conclusion is drown that, notwithstanding the different values of the viscosity and of the diffusion coefficient, the role of concentration polarisation is nearly the same for the extractants considered. From Fig. 11 it is possible to evaluate the concentration prevailing at the extractant-membrane interface, indeed for any experimental flux value, the effective driving force can be read making reference to the theoretical line, or calculated from Eq. (3). Since the feed was pure water, from the effective driving force one can calculate the water vapour I and finally the composition xI at the interface. pressure Pw2 2 Fig. 12 reports the results obtained in the extraction with glycerol at different flow rates. Finally from Eq. (5) the extractant side mass transfer coefficient kL2 was calculated. Fig. 13 reports the experimental mass transfer coefficients for glycerol 70 wt.% and for CaCl2 40 wt.% versus the flow
M. Celere, C. Gostoli / Journal of Membrane Science 229 (2004) 159–170
167
Fig. 11. Flux vs. driving force in the extraction of pure water with different extractants through capillary membranes. Extractant shell-side, flow rate 40 l/h, T = 25 ◦ C. Theoretical line corresponds to Km = 137.4 kg/m2 h.
rate. Theoretical values were calculated according to the equivalent annulus model, Eq. (7). For the module used in the experiments the shell-side void fraction is ε = 0.74 and the Sherwood number becomes Sh = 1.223Gz1/3
(10)
According to [22] Eq. (10) is valid for Gz > 40. The diffusion coefficient of glycerol in aqueous solution was calculated from the viscosity data following the procedure 10I reported in [28], for CaCl2 the values D = 1.195 × 10−5 cm2 /s, given by Robinson and Stokes [25] for 38.85 wt.% was used.
Fig. 13. Shell-side mass transfer coefficients vs. the flow rate in the extraction of pure water with glycerol 70 wt.% and with CaCl2 40 wt.%. Extractant shell-side, T = 25 ◦ C. Theoretical values were calculated with the equivalent annuls model [22].
The agreement is very satisfactory for glycerol, whereas for CaCl2 the experimental mass transfer coefficient appears lower than the predicted value. A possible explanation may be the thermal effect, which has been ignored in the analysis. Actually the thermal effect is expected to be relatively small at 25 ◦ C and at the large flow rates used in this work [12], however it may play a role in the extraction with CaCl2 , owing to the large flux.
6. Solvent entrainment Fig. 12. Glycerol mass fraction at the interface vs. the values in the bulk in the extraction of pure water in capillary module, extractant shell-side, T = 25 ◦ C.
The use of organic extractants, as propylene glycol presents some advantages with respect to brine, however
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Table 3 Surface tension and penetration pressure of PG solutions through the capillary membranes wt.%
σ calc (dyn/cm)
σ exp (dyn/cm)
PP (bar)
0 66 71 80 84 91 95 100
48.2 47.7 46.3 46.15 45.9 45.8
71 48.1 45.9 45.2 44.75 42.8 42.2 36
3.1a 1.26 1.15 1.1 1.05 0.98 0.95 0.6
a
Given by the supplier.
in order to accept this extractant for juice concentration any solvent entrainment has to be excluded. Apart from membrane defects, solvent entrainment can occur due to membrane wetting or counter-diffusion through the gas membrane. Membrane wetting occurs if the hydraulic pressure prevailing in the liquid phase exceeds the penetration pressure of the membrane, which is directly related to the surface tension of the liquid. Table 3 reports the surface tension as well as the penetration pressure of the PG solutions as a function of the concentration. σ calc represents the surface tension estimated by the method of Takamura, Kurata and Odani [23] and σ exp the values measured by the apparatus of Fisher-Scientific instruments. Apparently PG solutions exhibit surface tension and penetration pressure through the membrane pores quite lower with respect to water. One can conclude that propylene glycol can be used in osmotic distillation, however a careful pressure control is required to avoid membrane wetting. Counter-diffusion of PG towards the juice through the gas membrane is related to the volatility of PG, which is not negligible (see Table 1). A rough estimation of the ratio between the PG and water fluxes can be made assuming for both compounds independent binary diffusion through the stagnant air. In the case of pure water at one side and a PG solution on the other side and assuming ideal behaviour of PG solutions (Raoult’s law) we have NG DGA MG PG∗ = = 7.77 × 10−3 Nw DwA Mw Pw∗
(11)
The diffusion coefficient of PG vapour in air (DGA ) was estimated by the Fuller equation [23], it results DGA = 0.097 cm2 /s. More accurate value was calculated considering that water and PG vapours together diffuse through the stagnant air and applying the Stefan–Maxwell equation for multicomponent diffusion. The procedure was the same reported in [17], the calculated PG to water flux ratio appeared nearly independent on glycol composition and equal to 7.3 × 10−3 .
Some experiments were also performed with the rig used in OD experiments (Fig. 4). The two circuits were filled with known volumes (nearly 1 l) of pure water and PG solution, respectively, the system was then taken in operation for several hours keeping the water pressure a bit larger than the glycol pressure. At the end the overall water flux was evaluated as the volume change of the two liquids as well as from the glycol concentration. The glycol counter-flux was evaluated measuring the PG content in the water by HPLC using ethanol as internal standard. Several runs were performed with PG initial concentration ranging from 65 to 78 wt.%. The observed PG to water flux ratios seemed independent of the initial glycol concentration and ranged from 6.4 to 9.2 × 10−3 with average value 7.6 × 10−3 . The results above clearly show that PG cannot be recommended as extractant for juice concentration. Let us, for example, consider the case in which a juice is concentrated from 12 to 60◦ Brix with PG and then reconstructed with water to 12◦ Brix: the final juice will contain nearly 0.6 wt.% of PG. Even if PG is a not toxic compound, this value cannot be accepted. In similar experiments performed with glycerol solutions the glycerol concentration in the water after the runs was not detectable with the analytical method used, the theoretical calculations gave a glycerol to water flux ratio of nearly 10−6 , which gives a glycerol content in the final juice (after concentration to 60◦ Brix and reconstruction) of only 8 mg/kg (8 ppm).
7. Conclusions Several extractants have been compared. Among these CaCl2 showed to be the most effective, however organic extractants as PG and Glycerol allow to obtain comparable fluxes with advantages related to the absence of corrosion and scaling. PG and Glycerol are equivalent as regard as the extractive power, however PG showed a low penetration pressure through the membrane pores, in addition, it diffused towards the juice in considerable amount, owing to the not negligible volatility at room temperature. The weak point of glycerol is the high viscosity, nevertheless the operation is possible also with narrow capillaries, as those used in this study, limiting the concentration to nearly 70–75 wt.%. Interesting possibilities are offered by the ternary mixtures water–glycerol–NaCl, or possibly other salts. Indeed the same flux achievable with glycerol alone can be obtained, but with a substantially lower viscosity. As the driving force, and thus the flux, increase, large concentration polarisation is observed, the effect is nearly the same for all the extractants tested. The shell-side mass transfer rate is adequately described by the equivalent annulus model.
M. Celere, C. Gostoli / Journal of Membrane Science 229 (2004) 159–170
Nomenclature
ln(Λ21 ) = A21 +
aw do ds D Gz kL Km l M n N Pw R Sh x
with:
Greek ε γ ρ σ
water activity outer diameter shell diameter diffusion coefficient Graetz number mass transfer coefficient membrane permeability module length molecular weight number of capillaries mass flux water vapour pressure NaCl/glycerol mass ratio Sherwood number mole fraction
Glycerol PG
169
B21 T
(A.4)
A12
B12
A21
1.59191 6.99834
−199.24 −1969.27
0.88587 −17.630
B21 −640.96 5128.72
A.2. The modified ASOG method It is a group contribution method in which the activity coefficient γ i of a component in a mixture consist of a configurational contribution (γiC ), due to differences in molecular size, a group interaction contribution (γiR ), due to differences in intermolecular forces, and a Debye–Hückel (D–H) contribution (γwD–H ) which takes into account the effect of electrostatic interactions. ln γ = ln γ C + ln γ R + ln γ D–H (A.5)
letters shell void fraction activity coefficient density surface tension
i
i
i
w
where ln γiC = ln
υiFH j
υjFH xj
+1−
υiFH
j
υjFH xj
,
j = 1, 2, . . . , no. comp.
Subscripts 1 feed side 2 extract side A air G glycol lm logarithmic mean w water
ln γiR =
k
(A.6) (i)
υki [ln Γk − ln Γk ],
k = 1, 2, . . . , no. groups ln Γk = −ln
Superscripts I interface * pure water
Xn akn + 1 −
n
(A.7)
k, m, n = 1, 2, . . . , no. groups Xn =
j xj υjn
j xj
Appendix A
k
υkj
,
n
Xn ank , m Xm anm
j = 1, 2, . . . , no. comp.; k, n = 1, 2, . . . , no. groups
A.1. The Wilson method
ln akn = mkn +
The water vapour pressure is given by Pw = Pw∗ xw γw
(A.1)
in which Pw∗ is the pure water vapour pressure, xw the water mole fraction and γw the water activity coefficient given by ln(γw ) = −ln(xw + Λ12 xg ) Λ12 Λ21 + xg − xw + Λ12 xg Λ21 xw + xg
(A.2)
in which xg = 1 − xw is the solute mole fraction. The temperature dependence of the parameters Λ12 and Λ21 is given by ln(Λ12 ) = A12 +
B12 T
(A.3)
nkn T
(A.8)
(A.9) (A.10)
where υiFH is the number of non-hydrogen atoms in molecule i, υki is the number of non-hydrogen atoms in group k of (i) molecule i, xj is the mole fraction of component j, γ k and γk are the group activity coefficients of group k in the system and at standard state (pure component), respectively, akn is the group interaction parameter characteristic of groups k and n (akn = ank ), mkn and nkn are the group pair parameters characteristic of group k and n (temperature independent), and T is the temperature in K. The last term in Eq. (A.5) is √ √ 2AM 1 D–H ln γw = 3 1+b I − √ − 2 ln(1 + b I) b 1+b I (A.11)
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M. Celere, C. Gostoli / Journal of Membrane Science 229 (2004) 159–170
where A=
1.327757(105 )ρ1/2 (εT)3/2
(A.12)
b=
6.359696ρ1/2 (εT)1/2
(A.13)
and M is the molar mass of water (kg/mol), I the ionic strength (I = (1/2) mi z2i ), ρ the density of pure water (kg/m3 ), ε the dielectric constant of pure water, T the temperature (K), mi the molality of ion i and zi is the charge number of ion i. For the electrolyte solutions of our interest the parameters are (W stands for water, CH for the hydrated cation):
υiFH υki
W
CH
Cl−
1.0 1.6
1 + hc 1.6hc
1.0 1.0
hc is hydration number of cations (1 for Na+ and 3.1 for Ca2+ ). The ASOG pair parameters mkn and nkn are: W
CH −6.584 1925.5
W CH
22.79 2116
Cl−
29.45 −3851
2.116 3871
Cl− 1.9896 −341.1
m n
−69.07 20353
m n m n
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