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Long-term performance of membrane distillation process
Journal of Membrane Science 265 (2005) 153–159
Long-term performance of membrane distillation process Marek Gryta ∗ Technical University of Szczecin, Institute of Chemical and Environment Engineering, Department of Water Technology and Environment Engineering, ul. Pułaskiego 10, 70-322 Szczecin, Poland Received 25 January 2005; received in revised form 5 April 2005; accepted 27 April 2005 Available online 11 July 2005
Abstract The results of the over 3 years’ time research on the direct contact membrane distillation applied for production of the demineralised water have been presented. The hydrophobic capillary polypropylene membranes (Accurel PP S6/2) were used in these studies. The inlet temperature of the feed and the distillate was 353 and 293 K, respectively. The membranes were found to be thermally stable, and good separation characteristics was maintained throughout the whole period of the investigations. The SEM observations of the capillary membrane confirmed that their morphology remained unchanged. It was found that water (permeate from reverse osmosis process) did not cause the wetting of the used membranes despite a long-term membrane module exploitation. The precipitation of CaCO3 on the membrane surface was observed when tap water was used directly as a feed. A partial wetting of the membrane was found in this case. The wettability resulted in the increased electrical conductivity of the distillate obtained from 0.9 to 2.5 S/cm. © 2005 Elsevier B.V. All rights reserved. Keywords: Membrane distillation; Hydrophobic membrane; Pilot plant; Membrane wetting
1. Introduction In the process of membrane distillation (MD) the gas phase (non-wetted porous membrane) is in the contact with the liquid phase [1]. During the process only the water vapour and other volatile components present in the feed are transported across the membrane. One of the ways of putting the process into practice is the direct contact MD (DCMD), in which the membrane separates the hot feed from the cold distillate [1–3]. In this case, the vapour pressure gradient, which results from the different temperatures and the solution compositions in the boundary layers adjacent to the membrane, is the driving force of the mass transport across the membrane [1,2,4]. MD process demonstrates the excellent properties to retain the non-volatile solutes, therefore, it is often proposed for the purpose of water desalination and wastewater treatment [1–5]. A number of other applications, such as HCl recovery and the concentration of the salt solutions with their simul∗
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taneous crystallization, are reported [1–6]. The MD process was also successfully applied for the separation of volatile metabolites (e.g. ethanol) in the membrane bioreactors [7]. Membrane distillation has been known for 40 years, however, it is still used on a laboratory scale. The tests, which were conducted in the 1980s with a view to implement this process have resulted in establishing only a few pilot plants [1,2,8]. In recent years, the process has been tested under the industrial conditions for the purpose of fruit juices concentration (osmotic MD) [9]. The difficulties involved are basically associated with a phenomenon of membrane wetting and the formation of deposits on the membrane surface (fouling and scaling). The latter phenomena are responsible not only for a decline of the plant efficiency, but also may cause a damage of the MD module over a short time [4,5,7]. Currently, the membranes for the MD process have not yet been produced, therefore, the hydrophobic membranes manufactured for the microfiltration process are commonly used in the MD experiments [1,2,8]. Moreover, the literature reports on the MD studies usually describe a short-time membrane usage. Only a few papers deal with the studies performed over a period of several weeks [4,5,7,8,10]. Taking
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into consideration the above factors, the question how long the membranes proposed for MD process could be operate still remains unanswered. In many cases the polypropylene Accurel PP capillary membranes proved to be successful in the MD process [2–5,7,8,11,12]. The results of the MD studies with the Accurel PP S6/2 membranes, performed during the 3-year (including the shutdowns periods) module exploitation, were presented in this paper. The MD process was employed to produce the demineralised water.
2. Theory The retention of the gas phase in membrane pores during the MD is an essential condition for the process functioning. This condition is met when the membranes made of highly hydrophobic polymers, such as polyvinylidenefluoride (PVDF), polypropylene (PP) or polytetrafluoroethylene (PTFE) are employed [1–13]. Apart from the hydrophobic character of the membrane material, the liquid surface pressure, pores diameter and the hydraulic pressure influence on the possibility of the liquid penetration into the pores. This relation is described by the Laplace–Young (Kelvin law) equation [1,2,8,14]. P = PF − PD =
−4Bσ cos Θ dp
(1)
where B is the pore geometry coefficient (B = 1 for cylindrical pores), σ is the surface tension of the liquid, Θ is the contact angle, dP is the pore diameter, PF and PD are the hydraulic pressure on the feed and distillate side, respectively. According to Eq. (1), each membrane is characterized by a critical pressure above which the liquid will penetrate the membrane pores. This pressure is known as the liquid entry pressure of the water (LEPW) [1,2,15]. For example, if the contact angle value is 130◦ (PTFE membrane), the water (σ = 72 × 10−3 N/m) flows into the cylindrical pores having the diameter of 1 m at pressure of 185 kPa [1]. Taking into consideration the possibility of membrane wetting, it is recommended that for MD process the maximum diameter of the membrane pores does not exceed 0.5 m [2]. Moreover, the usefulness of Eq. (1) may be limited by the uncertainty of Θ angle value. A number of LEPW values determined experimentally, for the pores diameter within the range 0.2–2 m, were presented in the work [8]. For these presented LEPW results the Θ value calculated from Eq. (1) is equal to 93.5◦ ± 0.2◦ for the polypropylene Accurel PP membranes. At the initial stage of a new MD module working, a small increase of the permeate flux caused by the minor changes in the membrane morphology was observed [10–12]. The hydraulic pressure of the liquid may lead to a slight compaction of membrane, which shortens the vapour diffusion path and as a consequence the permeate flux is enhanced [16]. Moreover, an increase of the process yield may be also
due to a change of the membranes pores size. During the first 72 h of the polypropylene membranes exploitation (Accurel PP, Celgard), an increase of the diameter of the large pores by up to 25% together with a simultaneous collapse of the small ones, was observed [11]. A partial wetting of the membranes, associated with the phenomena proceeding on their surface or resulting from the transformation of the membrane material characteristics, poses problems which are difficult to eliminate [4,10]. The deposit formed on the membrane surface is a factor impeding the possibility of the long-term MD modules exploitation. This deposit causes clogging of the evaporation surfaces which leads to a decline of modules efficiency [8,12,17]. Furthermore, the membrane surface is wetted in the place where the deposit was formed, hence, the liquid may penetrate the adjacent pores [7]. This phenomenon will certainly be accelerated if the formed deposit comprises the salt crystals growing into the pores [4,5,12].
3. Experimental The investigations of the DCMD process were carried out in a continuous mode using a fully automated experimental set-up shown in Fig. 1. The MD installation consists of two thermostatic cycles, the feed and the distillate one, which were connected to the membrane module. A filter net (80 mesh) was mounted at the module inlet to prevent clogging of the capillary membranes. The RE4 microprocessor regulators compatible with the PT100 thermometers were employed for to control the operation of the MD installation. The membrane module with the diameter of 0.025 m and the effective length of 0.53 m was installed in a vertical position. The module was equipped with 30 hydrophobic capillary polypropylene membranes (Accurel PP S6/2, Mem-
Fig. 1. The experimental set-up for the DCMD process: 1—MD capillary module, 2—heat exchanger, 3—distillate tank, 4—feed tank, 5—impeller pump, 6—heater, 7—flowmeter, 8—filter, 9—RE4 controller, 10—RP7 current relay, T—thermometer, P—manometer, —conductometer, V1, V2, V3—valves.
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brana GmbH, Germany), with the outside/inside diameter dout /din = 2.6 mm/1.8 mm. The capillary membranes have the pore size with the maximum and nominal diameter of 0.55 and 0.22 m, respectively, and the porosity of 72% (the manufacturer’s data). The membranes were arranged as a parallel bundle of braided capillaries (three membranes in the braid). The total active surface area of membranes (A) for the mass transfer was calculated for the internal capillary diameter and amounted to 0.0889 m2 . The values of the permeate flux were calculated from the equation: Ji =
VD 24 (L/(m2 h)) Ati
(2)
where A = 0.0889 m2 , VD (L) is the permeate volume collected over a period of time ti (h) of the MD process duration. The feed flows inside the capillaries, whereas the distillate flows through the inter-tubular space. The impeller pumps were used to provide the flow of liquids in the MD installation. The volumetric flow of both the permeate and the feed was measured by the flowmeters. The calculated feed flow rate alongside the membrane surface was 1.35 m/s, whereas that of the distillate was 0.25 m/s. The initial hydraulic pressure of both streams (on the feed and distillate side) at the module inlet was equal (regulation by throttling valve V3), and amounted to 52 kPa. Only in the S1 series (the initial 400 h of the experiments), the pressure on the distillate side was lower, reaching 50 kPa. The inlet temperature of the feed and distillate was 353 and 293 K, respectively, and was maintained constant during the MD process. The permeate from the reverse osmosis process (RO), the installation of which was supplied with tap water, was used as a feed in the MD studies. For the RO process the module with the BW3040 element (Film Tec) was applied. Furthermore, at the final stage of the experiments, the MD installation was supplied with the tap water. The MD installation was operated in a continuous mode, in a few measurement series. The installation was shutdown between the measurement series for a period ranging from a few weeks to several months. The scheme of the time-events occurring during the operation of the MD module is presented in Fig. 2. While the installation was shut down, the MD module was filled with the retentate and the distillate obtained from the completed series. Before the next series was started, the installation was emptied and rinsed with the 2% HCl solution, followed by the distilled water. The installation was then subsequently filled with the RO permeate and the MD process was initiated. The studies were conducted in the two modes of the installation operation: Mode I. The MD permeate was collected in a continuous way (valve V1, closed; valve V2, open) and a level of the liquid in the feed tank was kept constant by the continuous dosing of the new portions of the feed water. As a result of the permeation of water vapour through the membranes, a gradual increase of the solute concentration in the recycled
Fig. 2. Time-events during water demineralisation in MD process.
feed was found. This mode was the most frequently used variant of the installation work. Mode II. The obtained permeate was returned to the feed tank (valve V1, open valve; valve V2, closed). In this case, the feed concentration was constant over the entire course of the MD investigations. This mode of work was found its application at weekends. The electrical conductivity and the total dissolved solids (TDS) of the used waters were measured with a 6P Ultrameter (Myron L Company). A Yobin Yvon Ultrace 238 JY inductively coupled plasma atomic emission spectrometer (ICP-AES) was used to determine the content of cations in the samples. The content of the inorganic carbon (IC) and the total organic carbon (TOC), both in the feed and the permeat, was determined using a TOC-Analyzer multi N/C (Analytic Jena). The membrane morphology and the composition of the fouling layer were studied using a Jeol JSM 6100 scanning electron microscopy (SEM) coupled with the energy dispersion spectrometry (EDS). The accelerating voltage used was 20 kV. The samples were sputter coated with gold and palladium.
4. Results and discussion The operation of the presented MD installation (Fig. 1) in a continuous mode allowed the simulation of the conditions of the membrane module work similar to those of the industrial installation. This is of a significant importance in the case of the investigations associated with the evaluation of the fouling and wetting phenomena. In traditional laboratory installations, the membrane temperature decreases during the night shutdown. This may cause the salt precipitation from the feed, which accelerates membranes wetting [4,5,12], and therefore, the results of the laboratory studies may substantially differ from those of the industrial plants. The substances dissolved in the feed water have a significant influence on the membranes wetting. In order to
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remove the solutes, the water treated in the process of the reverse osmosis was used in the MD experiments. The RO pilot installation was supplied with the tap water having the electrical conductivity of 605–650 S/cm. The corresponding solute content (TDS) in the tap water was found to be in the range 409–430 mg/L. The TOC analysis revealed that the tap water contained 29–31 mg/L of inorganic carbon (IC) and 6.8–8.5 mg/L of the total organic carbon (TOC). The average concentrations (in mg/L) of the major ions were as follows: 29.2 Na, 59.2 Ca, 16.6 Mg, 6.1 K and 2.8 Si. The composition of the permeate obtained from the RO process was dependent on the degree of water recovery, which was in the range 60–75%. On an average, the RO permeate contained (mg/L): 0.98 Na, 0.78 Ca, 0.46 Mg, 0.3 K and 0.043 Si. The amount of the compounds containing carbon (TOC analysis) was reduced to a value of 0.6–0.9 mg/L (TOC) and 1.2–2.2 mg/L (IC). The RO permeate has the electrical conductivity of 7.1–11.9 S/cm and the TDS value of 4.5–7.6 mg/L. The MD process, in which the RO permeate was employed as the feed water, was carried out in a few series and the experimental results are shown in Fig. 3. At the initial stage of the investigations, an increase of the module efficiency from 670 to 715 L/(m2 d) was observed. One of the reasons causing a change of the permeate flux may be associated with a change of the membrane morphology, the occurrence of which was indicated in other works [10,11]. The SEM observations performed with the brand-new membrane and the samples of the membranes collected from the module after the MD tests did not reveal any significant differences in the morphology. Thus, it can be concluded that the polymer material employed for membranes production exhibit a good thermal stability. However, even a slight increase of the pore size, difficult to assess visually, can lead to the reduction of the resistances of vapour diffusion across the membrane and to increase the process efficiency [1,2,10]. The observed initial permeate flux increase could also result from the asymmetrical structure of the used membranes. The SEM observation revealed that the Accurel PP membranes display a sponge structure and the pores located
Fig. 3. Variation of the permeat flux and feed concentration (TDS) as a function of operating time of the MD process.
Fig. 4. SEM micrograph of the inner surface of the Accurel PP S6/2 membrane.
on the membrane surface are larger. The existing differences occur in the layer adjacent to the surface up to the depth of about 20–40 m. The pore size distribution is more uniform in the deeper layers of the membranes wall [12]. Some of the pores located on the membrane surface are very large, sometimes having the diameter of 5–10 m (Fig. 4). It was found, that the pore size on the outer surface (distillate side) was significantly larger than that on the surface inside the capillaries (feed side). According to Eq. (1) for the Θ angle value equal 93.5◦ , the pores with the diameter equal 5 m are filling with water at the pressure of about 35 kPa. Such a small LEPW value means that the surface large pores could be filled with water (especially on the distillate side) at the initial stage of the MD tests, which caused a decrease of thickness of the gas layer entrapped in the membrane. This shortens the vapour diffusion path and consequently the MD process efficiency was increased. Unfortunately, wetting of the large surface pores initiates the same process for the smaller pores adjacent to them. Finally, the liquid filled the pores throughout the wall and the MD process stops in a given part of the membrane surface, which results in the decline of the membrane module efficiency. After the initial changes, the course of the MD process was stabilized and its efficiency amounted to about 700 L/(m2 d) over the next 200 h (Fig. 3). However, at the end of the S1 series (period 230–400 h), the module efficiency decreased to about 640 L/(m2 d). The quality of the obtained distillate was not deteriorate and the electrical conductivity remained stable at a level of 1.6 S/cm. This indicated that the observed decline of the module efficiency was not the result of the membrane wettability. In the case of the membrane wetting, the solutes diffuse through the pores filled with the liquid, which resulted in a significant increase of the permeate electrical conductivity [4,14]. Moreover, in the S1 series, the hydraulic pressure on the feed side was slightly higher than that of the distillate side, which additionally enhanced the possibility of leakage and consequently the distillate contamination.
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Fig. 6. The result of SEM-EDS analysis of deposit formed on the filter net (Fig. 5). Fig. 5. SEM micrograph of the filter net with the deposit formed during the MD process.
It was found that the major reason of the observed permeate flux decline was the formation of the deposits layer in the filter mounted at the inlet of the module (Fig. 1). The SEM image of the filter surface covered by deposit after completing the S1 series is shown in Fig. 5. The formation of deposit was also observed in other measurement series. The deposit caused the clogging of the flow channels through the filter net, which led to an increase of the flow resistance, thus the feed hydraulic pressure increased from 50 to 54 kPa. The feed was pumped with the use of the impeller pump, therefore an increase of the feed flow resistance caused a decline of the flow rate. As a consequence, the feed flow rate in the module decreased from 1.35 to 1.23 m/s. A decrease of the flow rate caused an unfavourable increase of the temperature polarization and comprises a possible reason of the observed reduction of the MD process efficiency [1,2,4,8]. Furthermore, the trace amounts of deposit were accumulated at the entrances of some capillary membranes and it caused a disturbance of the feed flow inside the capillaries. The mechanisms of this phenomenon was presented in work [18]. The removal of the deposit both from the filter and the membrane inlets enables the recovery of the initial process efficiency each time (Fig. 3). This result confirmed the previous results, that the formation of deposits at the inlet of the capillary membranes comprises one of the major problems of the MD process operation over an extended period [18]. The SEM-EDS analysis demonstrated that the deposit accumulated on the filter net contained a large amounts of Ca and Si (Fig. 6). During the performed MD tests the installation worked mostly according to Mode I. Hence, a constant dosing of the feed water to the feed tank led to a gradual increase of the solutes in the feed (Fig. 3, TDS). However, the obtained concentration of the solutes was relatively low, thereby, one can hardly explain the formation of deposit at this concentration. This problem will be the subject of the further works. The experimental results presented in Fig. 3 were obtained during almost the 2-year studies of the MD process performance. In these investigations the RO permeate was
used exclusively as a feed water. Several shutdown periods occurred between the measurement series. The module was filled with the solutions from the accomplished measurement series over these periods. The S2 series was started after a standstill lasting 1 year and the permeate flux about 690 L/(m2 d) was obtained. That efficiency was similar to that obtained for a new module. Thus, it can be concluded that 1year storage of the module with the channels filled with the processing water will not result in the membranes wettability. However, the formation of biofilm on the walls of the feed and the distillate tanks was observed during the installation shutdown. The SEM image of the deposit collected from the tank walls and subsequently separated with the use of the MILLEX-HV filter is presented in Fig. 7. The microbial tests, demonstrated in previous works [17], indicated that nonfermenting gram-negative rods were present in the collected deposit. Therefore, taking into consideration the possibility of biofouling occurrence it is recommended to drain the liquids from the MD installation before a prolonged standstill. A period of time of the standstill between the measurement series was presented in Fig. 2. Irrespective of their duration, the MD installation efficiency was found to be similar to the initial permeate flux (Fig. 8). The stability of the electrical conductivity of obtained distillate confirms that the membranes wettability was not observed in the course of feeding
Fig. 7. SEM micrograph of the micro-organisms in the deposit collected from the distillate tank walls.
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Fig. 8. The influence of the operating time of the MD process and the nature of the feed water on the changes of the permeate flux and the electrical conductivity of distillate.
the MD installation with the RO permeate. The higher values of the electrical conductivity (Fig. 8, at the beginning of the measurement series), resulted from the quality of the distilled water, which was used for filling the distillate tank at the start of the installation. A rapid decline of the electrical conductivity of the water in the distillate tank, observed after consecutive hours of process operation, confirmed that the obtained permeate was pure water. The average value of the distillate electrical conductivity amounted to 1 S/cm. However, a lower value of the electrical conductivity equal to 0.4 S/cm was achieved in other MD investigations with the same membranes, as reported in work [8]. In the experimental set-up (Fig. 1), the container lids were not airtight and the ambient air could penetrate the distillate. The installation was located in the chemical laboratory so the effect of the gaseous emissions on the quality of the produced distillate was noticeable. Gases (such as NH3 or CO2 ) while dissolving in the distillate, increased the value of the electrical conductivity. The TOC analysis (Fig. 9) was found to be a more appropriate method to evaluate the performance of membrane separation over the discussed period. Despite the increasing value of the feed concentration, the IC content in the distil-
Fig. 9. The changes of TOC and IC concentration in the feed and the distillate as a function of the operating time of the MD, feed: RO permeate.
Fig. 10. SEM micrograph of CaCO3 deposit on the membrane surface, feed: tap water.
late was similar to the analytic zero. This confirms a fact that regardless of the time of the process duration, membranes demonstrated a high retention of inorganic solutes. Only a trace amount of TOC, probably associated with the volatile compounds, was detected in the distillate. It is characteristic feature of the MD process that volatile organic compounds, similarly to water vapour, are not retained [1,2]. In the final stage of the MD studies, tap water was applied as the feed water, which led to a rapid decline of the process efficiency. As a result of heating the feed, the HCO3 − ions present in the water undergo the decomposition and a considerable amount of CaCO3 precipitates on the membrane surface (Fig. 10), which reduces the surface of water vapourization (pores) [5]. The formed deposit was removed every 40–80 h by rinsing the module with a 2–5 wt.% HCl solution which enabled the recovery of the initial process efficiency. However, the results demonstrated in Fig. 8 show that a multiple repetition of this operation resulted in a gradual decline of the maximum flux of the permeate (feed—distilled water). At the same time, an increase of the electrical conductivity of produced distillate was observed. This confirms that the observed decrease of the maximum permeate flux was caused by a partial wetting of the membranes. This conclusion can be confirmed by the fact that the module efficiency increased from 555 to 650 L/(m2 d) as a result of the membranes drying (Fig. 8, 2800 h). Unfortunately, the permeate flux was found to decline after restarting the MD process with tap water. After 3300 h of running the MD process investigations, the installation was finally rinsed with a HCl solution. Subsequently, the MD process was operated for almost 200 h, feeding the installation with the distilled water. The permeat flux, which was obtained at that time (period 3300–3500 h), was found to be stable. That confirms that deposits formed on the membranes surface (Fig. 10) were responsible for the decline of the efficiency in the previous stage of the module operation (feed—tap water). As it has been demonstrated in previous works, the formation of deposits on the membrane surface has accelerated wetting of the membrane pores [4,12].
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5. Conclusions The experimental results have demonstrated that the wettability of polypropylene Accurel PP S6/2 membranes by pure water does not occur in the MD process. These enable a long-term operation of the MD modules made from these membranes for the separation of water solutions without the suspended solids. Therefore, the Accurel PP membranes can be recommended for the application in the membrane distillation process. It was found that the MD modules filled with processing water could be storage for a few months without a deterioration of its efficiency. However, it is recommended that to drain the installation before standstill, due to the possibility of occurrence of biofouling phenomenon. In order to prevent a clogging of the capillary inlets by the suspensions present in the feed, it is important to locate the filtering systems close to the MD modules. The deposits formed on the hydrophobic surface of membrane cause that the pores adjacent to the deposit will be filled with liquid. As a result of the partial wetting of the membranes, the MD process efficiency decreases from 700 to 550 L/(m2 d). Moreover, the electrical conductivity of the obtained distillate was increased from 0.9 to 2.5 S/cm. The removal of deposits from the membranes surfaces (e.g. CaCO3 by washing them with a HCl solution) and rinsing the membranes with pure water, followed by drying, facilitates the recovery of the initial efficiency of the membrane module.
Acknowledgements This work was supported by a grant from Polish State Committee for Scientific Research.
References [1] K.W. Lawson, D.R. Lloyd, Membrane distillation, J. Membr. Sci. 124 (1997) 1.
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[2] A. Burgoyne, M.M. Vahdati, Direct contact membrane distillation, Sep. Sci. Technol. 35 (2000) 1257. [3] M. Tomaszewska, Membrane distillation – examples of applications in technology and environmental protection, Pol. J. Environ. Stud. 9 (2000) 27. [4] M. Gryta, Concentration of NaCl solution by membrane distillation integrated with crystallization, Sep. Sci. Technol. 37 (15) (2002) 3535. [5] K. Karakulski, M. Gryta, A. Morawski, Membrane processes used for potable water quality improvement, Desalination 145 (2002) 315. [6] V. Calabro, B.L. Jiao, E. Drioli, Theoretical and experimental study on membrane distillation in the concentration of orange juice, Ind. Eng. Chem. Res. 33 (1994) 1803. [7] M. Gryta, The fermentation process integrated with membrane distillation, Sep. Purif. Technol. 24 (2001) 283. [8] K. Schneider, W. H¨olz, R. Wollbeck, Membranes and modules for ransmembrane distillation, J. Membrane Sci. 39 (1988) 25. [9] F. Vaillant, E. Jeanton, M. Dornier, G.M. O’Brien, M. Reynes, M. Decloux, Concentration of passion fruit juice on an industrial pilot scale using osmotic evaporation, J. Food Eng. 47 (2001) 195. [10] F.A. Banat, J. Simandl, Theoretical and experimental study in membrane distillation, Desalination 95 (1994) 39. [11] A.M. Barbe, P.A. Hogan, R.A. Johnson, Surface morphology changes during initial usage of hydrophobic, microporous polypropylene membranes, J. Membr. Sci. 172 (2000) 149. [12] M. Gryta, Direct contact membrane distillation with crystallization applied to NaCl solutions, Chem. Pap. 56 (2002) 14. [13] M. Tomaszewska, Preparation and properties of flat-sheet membranes from poly(vinylidene fluoride) for membrane distillation, Desalination 104 (1996) 1. [14] H. Udriot, U. van Stockar, New method for the determination of wettability in a membrane distillation process, in: Advances in separation processes, Hemisphere Publishing Corporation, London, 1990, pp. 135–145. [15] K. Byoung-Sik, P. Harriott, Critical entry pressure for liquids in hydrophobic membranes, J. Colloid Interface Sci. 115 (1987) 1. [16] K.W. Lawson, M.S. Hall, D.R. Lloyd, Compaction of microporous membranes used in membrane distillation. I. Effect on gas permeability, J. Membr. Sci. 101 (1995) 99. [17] M. Gryta, The assessment of microorganism growth in the membrane distillation system, Desalination 142 (2002) 79. [18] K. Karakulski, M. Gryta, Water demineralisation by NF/MD integrated processes, Desalination 177 (2005) 109.
Long-term performance of membrane distillation process Marek Gryta ∗ Technical University of Szczecin, Institute of Chemical and Environment Engineering, Department of Water Technology and Environment Engineering, ul. Pułaskiego 10, 70-322 Szczecin, Poland Received 25 January 2005; received in revised form 5 April 2005; accepted 27 April 2005 Available online 11 July 2005
Abstract The results of the over 3 years’ time research on the direct contact membrane distillation applied for production of the demineralised water have been presented. The hydrophobic capillary polypropylene membranes (Accurel PP S6/2) were used in these studies. The inlet temperature of the feed and the distillate was 353 and 293 K, respectively. The membranes were found to be thermally stable, and good separation characteristics was maintained throughout the whole period of the investigations. The SEM observations of the capillary membrane confirmed that their morphology remained unchanged. It was found that water (permeate from reverse osmosis process) did not cause the wetting of the used membranes despite a long-term membrane module exploitation. The precipitation of CaCO3 on the membrane surface was observed when tap water was used directly as a feed. A partial wetting of the membrane was found in this case. The wettability resulted in the increased electrical conductivity of the distillate obtained from 0.9 to 2.5 S/cm. © 2005 Elsevier B.V. All rights reserved. Keywords: Membrane distillation; Hydrophobic membrane; Pilot plant; Membrane wetting
1. Introduction In the process of membrane distillation (MD) the gas phase (non-wetted porous membrane) is in the contact with the liquid phase [1]. During the process only the water vapour and other volatile components present in the feed are transported across the membrane. One of the ways of putting the process into practice is the direct contact MD (DCMD), in which the membrane separates the hot feed from the cold distillate [1–3]. In this case, the vapour pressure gradient, which results from the different temperatures and the solution compositions in the boundary layers adjacent to the membrane, is the driving force of the mass transport across the membrane [1,2,4]. MD process demonstrates the excellent properties to retain the non-volatile solutes, therefore, it is often proposed for the purpose of water desalination and wastewater treatment [1–5]. A number of other applications, such as HCl recovery and the concentration of the salt solutions with their simul∗
Fax: +48 91 449 46 86. E-mail address: [email protected].
0376-7388/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2005.04.049
taneous crystallization, are reported [1–6]. The MD process was also successfully applied for the separation of volatile metabolites (e.g. ethanol) in the membrane bioreactors [7]. Membrane distillation has been known for 40 years, however, it is still used on a laboratory scale. The tests, which were conducted in the 1980s with a view to implement this process have resulted in establishing only a few pilot plants [1,2,8]. In recent years, the process has been tested under the industrial conditions for the purpose of fruit juices concentration (osmotic MD) [9]. The difficulties involved are basically associated with a phenomenon of membrane wetting and the formation of deposits on the membrane surface (fouling and scaling). The latter phenomena are responsible not only for a decline of the plant efficiency, but also may cause a damage of the MD module over a short time [4,5,7]. Currently, the membranes for the MD process have not yet been produced, therefore, the hydrophobic membranes manufactured for the microfiltration process are commonly used in the MD experiments [1,2,8]. Moreover, the literature reports on the MD studies usually describe a short-time membrane usage. Only a few papers deal with the studies performed over a period of several weeks [4,5,7,8,10]. Taking
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into consideration the above factors, the question how long the membranes proposed for MD process could be operate still remains unanswered. In many cases the polypropylene Accurel PP capillary membranes proved to be successful in the MD process [2–5,7,8,11,12]. The results of the MD studies with the Accurel PP S6/2 membranes, performed during the 3-year (including the shutdowns periods) module exploitation, were presented in this paper. The MD process was employed to produce the demineralised water.
2. Theory The retention of the gas phase in membrane pores during the MD is an essential condition for the process functioning. This condition is met when the membranes made of highly hydrophobic polymers, such as polyvinylidenefluoride (PVDF), polypropylene (PP) or polytetrafluoroethylene (PTFE) are employed [1–13]. Apart from the hydrophobic character of the membrane material, the liquid surface pressure, pores diameter and the hydraulic pressure influence on the possibility of the liquid penetration into the pores. This relation is described by the Laplace–Young (Kelvin law) equation [1,2,8,14]. P = PF − PD =
−4Bσ cos Θ dp
(1)
where B is the pore geometry coefficient (B = 1 for cylindrical pores), σ is the surface tension of the liquid, Θ is the contact angle, dP is the pore diameter, PF and PD are the hydraulic pressure on the feed and distillate side, respectively. According to Eq. (1), each membrane is characterized by a critical pressure above which the liquid will penetrate the membrane pores. This pressure is known as the liquid entry pressure of the water (LEPW) [1,2,15]. For example, if the contact angle value is 130◦ (PTFE membrane), the water (σ = 72 × 10−3 N/m) flows into the cylindrical pores having the diameter of 1 m at pressure of 185 kPa [1]. Taking into consideration the possibility of membrane wetting, it is recommended that for MD process the maximum diameter of the membrane pores does not exceed 0.5 m [2]. Moreover, the usefulness of Eq. (1) may be limited by the uncertainty of Θ angle value. A number of LEPW values determined experimentally, for the pores diameter within the range 0.2–2 m, were presented in the work [8]. For these presented LEPW results the Θ value calculated from Eq. (1) is equal to 93.5◦ ± 0.2◦ for the polypropylene Accurel PP membranes. At the initial stage of a new MD module working, a small increase of the permeate flux caused by the minor changes in the membrane morphology was observed [10–12]. The hydraulic pressure of the liquid may lead to a slight compaction of membrane, which shortens the vapour diffusion path and as a consequence the permeate flux is enhanced [16]. Moreover, an increase of the process yield may be also
due to a change of the membranes pores size. During the first 72 h of the polypropylene membranes exploitation (Accurel PP, Celgard), an increase of the diameter of the large pores by up to 25% together with a simultaneous collapse of the small ones, was observed [11]. A partial wetting of the membranes, associated with the phenomena proceeding on their surface or resulting from the transformation of the membrane material characteristics, poses problems which are difficult to eliminate [4,10]. The deposit formed on the membrane surface is a factor impeding the possibility of the long-term MD modules exploitation. This deposit causes clogging of the evaporation surfaces which leads to a decline of modules efficiency [8,12,17]. Furthermore, the membrane surface is wetted in the place where the deposit was formed, hence, the liquid may penetrate the adjacent pores [7]. This phenomenon will certainly be accelerated if the formed deposit comprises the salt crystals growing into the pores [4,5,12].
3. Experimental The investigations of the DCMD process were carried out in a continuous mode using a fully automated experimental set-up shown in Fig. 1. The MD installation consists of two thermostatic cycles, the feed and the distillate one, which were connected to the membrane module. A filter net (80 mesh) was mounted at the module inlet to prevent clogging of the capillary membranes. The RE4 microprocessor regulators compatible with the PT100 thermometers were employed for to control the operation of the MD installation. The membrane module with the diameter of 0.025 m and the effective length of 0.53 m was installed in a vertical position. The module was equipped with 30 hydrophobic capillary polypropylene membranes (Accurel PP S6/2, Mem-
Fig. 1. The experimental set-up for the DCMD process: 1—MD capillary module, 2—heat exchanger, 3—distillate tank, 4—feed tank, 5—impeller pump, 6—heater, 7—flowmeter, 8—filter, 9—RE4 controller, 10—RP7 current relay, T—thermometer, P—manometer, —conductometer, V1, V2, V3—valves.
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brana GmbH, Germany), with the outside/inside diameter dout /din = 2.6 mm/1.8 mm. The capillary membranes have the pore size with the maximum and nominal diameter of 0.55 and 0.22 m, respectively, and the porosity of 72% (the manufacturer’s data). The membranes were arranged as a parallel bundle of braided capillaries (three membranes in the braid). The total active surface area of membranes (A) for the mass transfer was calculated for the internal capillary diameter and amounted to 0.0889 m2 . The values of the permeate flux were calculated from the equation: Ji =
VD 24 (L/(m2 h)) Ati
(2)
where A = 0.0889 m2 , VD (L) is the permeate volume collected over a period of time ti (h) of the MD process duration. The feed flows inside the capillaries, whereas the distillate flows through the inter-tubular space. The impeller pumps were used to provide the flow of liquids in the MD installation. The volumetric flow of both the permeate and the feed was measured by the flowmeters. The calculated feed flow rate alongside the membrane surface was 1.35 m/s, whereas that of the distillate was 0.25 m/s. The initial hydraulic pressure of both streams (on the feed and distillate side) at the module inlet was equal (regulation by throttling valve V3), and amounted to 52 kPa. Only in the S1 series (the initial 400 h of the experiments), the pressure on the distillate side was lower, reaching 50 kPa. The inlet temperature of the feed and distillate was 353 and 293 K, respectively, and was maintained constant during the MD process. The permeate from the reverse osmosis process (RO), the installation of which was supplied with tap water, was used as a feed in the MD studies. For the RO process the module with the BW3040 element (Film Tec) was applied. Furthermore, at the final stage of the experiments, the MD installation was supplied with the tap water. The MD installation was operated in a continuous mode, in a few measurement series. The installation was shutdown between the measurement series for a period ranging from a few weeks to several months. The scheme of the time-events occurring during the operation of the MD module is presented in Fig. 2. While the installation was shut down, the MD module was filled with the retentate and the distillate obtained from the completed series. Before the next series was started, the installation was emptied and rinsed with the 2% HCl solution, followed by the distilled water. The installation was then subsequently filled with the RO permeate and the MD process was initiated. The studies were conducted in the two modes of the installation operation: Mode I. The MD permeate was collected in a continuous way (valve V1, closed; valve V2, open) and a level of the liquid in the feed tank was kept constant by the continuous dosing of the new portions of the feed water. As a result of the permeation of water vapour through the membranes, a gradual increase of the solute concentration in the recycled
Fig. 2. Time-events during water demineralisation in MD process.
feed was found. This mode was the most frequently used variant of the installation work. Mode II. The obtained permeate was returned to the feed tank (valve V1, open valve; valve V2, closed). In this case, the feed concentration was constant over the entire course of the MD investigations. This mode of work was found its application at weekends. The electrical conductivity and the total dissolved solids (TDS) of the used waters were measured with a 6P Ultrameter (Myron L Company). A Yobin Yvon Ultrace 238 JY inductively coupled plasma atomic emission spectrometer (ICP-AES) was used to determine the content of cations in the samples. The content of the inorganic carbon (IC) and the total organic carbon (TOC), both in the feed and the permeat, was determined using a TOC-Analyzer multi N/C (Analytic Jena). The membrane morphology and the composition of the fouling layer were studied using a Jeol JSM 6100 scanning electron microscopy (SEM) coupled with the energy dispersion spectrometry (EDS). The accelerating voltage used was 20 kV. The samples were sputter coated with gold and palladium.
4. Results and discussion The operation of the presented MD installation (Fig. 1) in a continuous mode allowed the simulation of the conditions of the membrane module work similar to those of the industrial installation. This is of a significant importance in the case of the investigations associated with the evaluation of the fouling and wetting phenomena. In traditional laboratory installations, the membrane temperature decreases during the night shutdown. This may cause the salt precipitation from the feed, which accelerates membranes wetting [4,5,12], and therefore, the results of the laboratory studies may substantially differ from those of the industrial plants. The substances dissolved in the feed water have a significant influence on the membranes wetting. In order to
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remove the solutes, the water treated in the process of the reverse osmosis was used in the MD experiments. The RO pilot installation was supplied with the tap water having the electrical conductivity of 605–650 S/cm. The corresponding solute content (TDS) in the tap water was found to be in the range 409–430 mg/L. The TOC analysis revealed that the tap water contained 29–31 mg/L of inorganic carbon (IC) and 6.8–8.5 mg/L of the total organic carbon (TOC). The average concentrations (in mg/L) of the major ions were as follows: 29.2 Na, 59.2 Ca, 16.6 Mg, 6.1 K and 2.8 Si. The composition of the permeate obtained from the RO process was dependent on the degree of water recovery, which was in the range 60–75%. On an average, the RO permeate contained (mg/L): 0.98 Na, 0.78 Ca, 0.46 Mg, 0.3 K and 0.043 Si. The amount of the compounds containing carbon (TOC analysis) was reduced to a value of 0.6–0.9 mg/L (TOC) and 1.2–2.2 mg/L (IC). The RO permeate has the electrical conductivity of 7.1–11.9 S/cm and the TDS value of 4.5–7.6 mg/L. The MD process, in which the RO permeate was employed as the feed water, was carried out in a few series and the experimental results are shown in Fig. 3. At the initial stage of the investigations, an increase of the module efficiency from 670 to 715 L/(m2 d) was observed. One of the reasons causing a change of the permeate flux may be associated with a change of the membrane morphology, the occurrence of which was indicated in other works [10,11]. The SEM observations performed with the brand-new membrane and the samples of the membranes collected from the module after the MD tests did not reveal any significant differences in the morphology. Thus, it can be concluded that the polymer material employed for membranes production exhibit a good thermal stability. However, even a slight increase of the pore size, difficult to assess visually, can lead to the reduction of the resistances of vapour diffusion across the membrane and to increase the process efficiency [1,2,10]. The observed initial permeate flux increase could also result from the asymmetrical structure of the used membranes. The SEM observation revealed that the Accurel PP membranes display a sponge structure and the pores located
Fig. 3. Variation of the permeat flux and feed concentration (TDS) as a function of operating time of the MD process.
Fig. 4. SEM micrograph of the inner surface of the Accurel PP S6/2 membrane.
on the membrane surface are larger. The existing differences occur in the layer adjacent to the surface up to the depth of about 20–40 m. The pore size distribution is more uniform in the deeper layers of the membranes wall [12]. Some of the pores located on the membrane surface are very large, sometimes having the diameter of 5–10 m (Fig. 4). It was found, that the pore size on the outer surface (distillate side) was significantly larger than that on the surface inside the capillaries (feed side). According to Eq. (1) for the Θ angle value equal 93.5◦ , the pores with the diameter equal 5 m are filling with water at the pressure of about 35 kPa. Such a small LEPW value means that the surface large pores could be filled with water (especially on the distillate side) at the initial stage of the MD tests, which caused a decrease of thickness of the gas layer entrapped in the membrane. This shortens the vapour diffusion path and consequently the MD process efficiency was increased. Unfortunately, wetting of the large surface pores initiates the same process for the smaller pores adjacent to them. Finally, the liquid filled the pores throughout the wall and the MD process stops in a given part of the membrane surface, which results in the decline of the membrane module efficiency. After the initial changes, the course of the MD process was stabilized and its efficiency amounted to about 700 L/(m2 d) over the next 200 h (Fig. 3). However, at the end of the S1 series (period 230–400 h), the module efficiency decreased to about 640 L/(m2 d). The quality of the obtained distillate was not deteriorate and the electrical conductivity remained stable at a level of 1.6 S/cm. This indicated that the observed decline of the module efficiency was not the result of the membrane wettability. In the case of the membrane wetting, the solutes diffuse through the pores filled with the liquid, which resulted in a significant increase of the permeate electrical conductivity [4,14]. Moreover, in the S1 series, the hydraulic pressure on the feed side was slightly higher than that of the distillate side, which additionally enhanced the possibility of leakage and consequently the distillate contamination.
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Fig. 6. The result of SEM-EDS analysis of deposit formed on the filter net (Fig. 5). Fig. 5. SEM micrograph of the filter net with the deposit formed during the MD process.
It was found that the major reason of the observed permeate flux decline was the formation of the deposits layer in the filter mounted at the inlet of the module (Fig. 1). The SEM image of the filter surface covered by deposit after completing the S1 series is shown in Fig. 5. The formation of deposit was also observed in other measurement series. The deposit caused the clogging of the flow channels through the filter net, which led to an increase of the flow resistance, thus the feed hydraulic pressure increased from 50 to 54 kPa. The feed was pumped with the use of the impeller pump, therefore an increase of the feed flow resistance caused a decline of the flow rate. As a consequence, the feed flow rate in the module decreased from 1.35 to 1.23 m/s. A decrease of the flow rate caused an unfavourable increase of the temperature polarization and comprises a possible reason of the observed reduction of the MD process efficiency [1,2,4,8]. Furthermore, the trace amounts of deposit were accumulated at the entrances of some capillary membranes and it caused a disturbance of the feed flow inside the capillaries. The mechanisms of this phenomenon was presented in work [18]. The removal of the deposit both from the filter and the membrane inlets enables the recovery of the initial process efficiency each time (Fig. 3). This result confirmed the previous results, that the formation of deposits at the inlet of the capillary membranes comprises one of the major problems of the MD process operation over an extended period [18]. The SEM-EDS analysis demonstrated that the deposit accumulated on the filter net contained a large amounts of Ca and Si (Fig. 6). During the performed MD tests the installation worked mostly according to Mode I. Hence, a constant dosing of the feed water to the feed tank led to a gradual increase of the solutes in the feed (Fig. 3, TDS). However, the obtained concentration of the solutes was relatively low, thereby, one can hardly explain the formation of deposit at this concentration. This problem will be the subject of the further works. The experimental results presented in Fig. 3 were obtained during almost the 2-year studies of the MD process performance. In these investigations the RO permeate was
used exclusively as a feed water. Several shutdown periods occurred between the measurement series. The module was filled with the solutions from the accomplished measurement series over these periods. The S2 series was started after a standstill lasting 1 year and the permeate flux about 690 L/(m2 d) was obtained. That efficiency was similar to that obtained for a new module. Thus, it can be concluded that 1year storage of the module with the channels filled with the processing water will not result in the membranes wettability. However, the formation of biofilm on the walls of the feed and the distillate tanks was observed during the installation shutdown. The SEM image of the deposit collected from the tank walls and subsequently separated with the use of the MILLEX-HV filter is presented in Fig. 7. The microbial tests, demonstrated in previous works [17], indicated that nonfermenting gram-negative rods were present in the collected deposit. Therefore, taking into consideration the possibility of biofouling occurrence it is recommended to drain the liquids from the MD installation before a prolonged standstill. A period of time of the standstill between the measurement series was presented in Fig. 2. Irrespective of their duration, the MD installation efficiency was found to be similar to the initial permeate flux (Fig. 8). The stability of the electrical conductivity of obtained distillate confirms that the membranes wettability was not observed in the course of feeding
Fig. 7. SEM micrograph of the micro-organisms in the deposit collected from the distillate tank walls.
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Fig. 8. The influence of the operating time of the MD process and the nature of the feed water on the changes of the permeate flux and the electrical conductivity of distillate.
the MD installation with the RO permeate. The higher values of the electrical conductivity (Fig. 8, at the beginning of the measurement series), resulted from the quality of the distilled water, which was used for filling the distillate tank at the start of the installation. A rapid decline of the electrical conductivity of the water in the distillate tank, observed after consecutive hours of process operation, confirmed that the obtained permeate was pure water. The average value of the distillate electrical conductivity amounted to 1 S/cm. However, a lower value of the electrical conductivity equal to 0.4 S/cm was achieved in other MD investigations with the same membranes, as reported in work [8]. In the experimental set-up (Fig. 1), the container lids were not airtight and the ambient air could penetrate the distillate. The installation was located in the chemical laboratory so the effect of the gaseous emissions on the quality of the produced distillate was noticeable. Gases (such as NH3 or CO2 ) while dissolving in the distillate, increased the value of the electrical conductivity. The TOC analysis (Fig. 9) was found to be a more appropriate method to evaluate the performance of membrane separation over the discussed period. Despite the increasing value of the feed concentration, the IC content in the distil-
Fig. 9. The changes of TOC and IC concentration in the feed and the distillate as a function of the operating time of the MD, feed: RO permeate.
Fig. 10. SEM micrograph of CaCO3 deposit on the membrane surface, feed: tap water.
late was similar to the analytic zero. This confirms a fact that regardless of the time of the process duration, membranes demonstrated a high retention of inorganic solutes. Only a trace amount of TOC, probably associated with the volatile compounds, was detected in the distillate. It is characteristic feature of the MD process that volatile organic compounds, similarly to water vapour, are not retained [1,2]. In the final stage of the MD studies, tap water was applied as the feed water, which led to a rapid decline of the process efficiency. As a result of heating the feed, the HCO3 − ions present in the water undergo the decomposition and a considerable amount of CaCO3 precipitates on the membrane surface (Fig. 10), which reduces the surface of water vapourization (pores) [5]. The formed deposit was removed every 40–80 h by rinsing the module with a 2–5 wt.% HCl solution which enabled the recovery of the initial process efficiency. However, the results demonstrated in Fig. 8 show that a multiple repetition of this operation resulted in a gradual decline of the maximum flux of the permeate (feed—distilled water). At the same time, an increase of the electrical conductivity of produced distillate was observed. This confirms that the observed decrease of the maximum permeate flux was caused by a partial wetting of the membranes. This conclusion can be confirmed by the fact that the module efficiency increased from 555 to 650 L/(m2 d) as a result of the membranes drying (Fig. 8, 2800 h). Unfortunately, the permeate flux was found to decline after restarting the MD process with tap water. After 3300 h of running the MD process investigations, the installation was finally rinsed with a HCl solution. Subsequently, the MD process was operated for almost 200 h, feeding the installation with the distilled water. The permeat flux, which was obtained at that time (period 3300–3500 h), was found to be stable. That confirms that deposits formed on the membranes surface (Fig. 10) were responsible for the decline of the efficiency in the previous stage of the module operation (feed—tap water). As it has been demonstrated in previous works, the formation of deposits on the membrane surface has accelerated wetting of the membrane pores [4,12].
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5. Conclusions The experimental results have demonstrated that the wettability of polypropylene Accurel PP S6/2 membranes by pure water does not occur in the MD process. These enable a long-term operation of the MD modules made from these membranes for the separation of water solutions without the suspended solids. Therefore, the Accurel PP membranes can be recommended for the application in the membrane distillation process. It was found that the MD modules filled with processing water could be storage for a few months without a deterioration of its efficiency. However, it is recommended that to drain the installation before standstill, due to the possibility of occurrence of biofouling phenomenon. In order to prevent a clogging of the capillary inlets by the suspensions present in the feed, it is important to locate the filtering systems close to the MD modules. The deposits formed on the hydrophobic surface of membrane cause that the pores adjacent to the deposit will be filled with liquid. As a result of the partial wetting of the membranes, the MD process efficiency decreases from 700 to 550 L/(m2 d). Moreover, the electrical conductivity of the obtained distillate was increased from 0.9 to 2.5 S/cm. The removal of deposits from the membranes surfaces (e.g. CaCO3 by washing them with a HCl solution) and rinsing the membranes with pure water, followed by drying, facilitates the recovery of the initial efficiency of the membrane module.
Acknowledgements This work was supported by a grant from Polish State Committee for Scientific Research.
References [1] K.W. Lawson, D.R. Lloyd, Membrane distillation, J. Membr. Sci. 124 (1997) 1.
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[2] A. Burgoyne, M.M. Vahdati, Direct contact membrane distillation, Sep. Sci. Technol. 35 (2000) 1257. [3] M. Tomaszewska, Membrane distillation – examples of applications in technology and environmental protection, Pol. J. Environ. Stud. 9 (2000) 27. [4] M. Gryta, Concentration of NaCl solution by membrane distillation integrated with crystallization, Sep. Sci. Technol. 37 (15) (2002) 3535. [5] K. Karakulski, M. Gryta, A. Morawski, Membrane processes used for potable water quality improvement, Desalination 145 (2002) 315. [6] V. Calabro, B.L. Jiao, E. Drioli, Theoretical and experimental study on membrane distillation in the concentration of orange juice, Ind. Eng. Chem. Res. 33 (1994) 1803. [7] M. Gryta, The fermentation process integrated with membrane distillation, Sep. Purif. Technol. 24 (2001) 283. [8] K. Schneider, W. H¨olz, R. Wollbeck, Membranes and modules for ransmembrane distillation, J. Membrane Sci. 39 (1988) 25. [9] F. Vaillant, E. Jeanton, M. Dornier, G.M. O’Brien, M. Reynes, M. Decloux, Concentration of passion fruit juice on an industrial pilot scale using osmotic evaporation, J. Food Eng. 47 (2001) 195. [10] F.A. Banat, J. Simandl, Theoretical and experimental study in membrane distillation, Desalination 95 (1994) 39. [11] A.M. Barbe, P.A. Hogan, R.A. Johnson, Surface morphology changes during initial usage of hydrophobic, microporous polypropylene membranes, J. Membr. Sci. 172 (2000) 149. [12] M. Gryta, Direct contact membrane distillation with crystallization applied to NaCl solutions, Chem. Pap. 56 (2002) 14. [13] M. Tomaszewska, Preparation and properties of flat-sheet membranes from poly(vinylidene fluoride) for membrane distillation, Desalination 104 (1996) 1. [14] H. Udriot, U. van Stockar, New method for the determination of wettability in a membrane distillation process, in: Advances in separation processes, Hemisphere Publishing Corporation, London, 1990, pp. 135–145. [15] K. Byoung-Sik, P. Harriott, Critical entry pressure for liquids in hydrophobic membranes, J. Colloid Interface Sci. 115 (1987) 1. [16] K.W. Lawson, M.S. Hall, D.R. Lloyd, Compaction of microporous membranes used in membrane distillation. I. Effect on gas permeability, J. Membr. Sci. 101 (1995) 99. [17] M. Gryta, The assessment of microorganism growth in the membrane distillation system, Desalination 142 (2002) 79. [18] K. Karakulski, M. Gryta, Water demineralisation by NF/MD integrated processes, Desalination 177 (2005) 109.