Degradation of Polypropylene Membranes Applied in ... - MDPI

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Degradation of Polypropylene Membranes Applied in Membrane Distillation Crystallizer Marek Gryta Institute of Inorganic Technology and Environment Engineering, West Pomeranian University of Technology, Szczecin, ul. Pułaskiego 10, 70-322 Szczecin, Poland; [email protected]; Tel.: +48-91-449-47-30; Fax: +48-91-449-46-86 Academic Editor: Gianluca Di Profio Received: 20 January 2016; Accepted: 23 March 2016; Published: 28 March 2016

Abstract: The studies on the resistance to degradation of capillary polypropylene membranes assembled in a membrane crystallizer were performed. The supersaturation state of salt was achieved by evaporation of water from the NaCl saturated solutions using membrane distillation process. A high feed temperature (363 K) was used in order to enhance the degradation effects and to shorten the test times. Salt crystallization was carried out by the application of batch or fluidized bed crystallizer. A significant membrane scaling was observed regardless of the method of realized crystallization. The SEM-EDS, DSC, and FTIR methods were used for investigations of polypropylene degradation. The salt crystallization onto the membrane surface accelerated polypropylene degradation. Due to a polymer degradation, the presence of carbonyl groups on the membranes’ surface was identified. Besides the changes in the chemical structure a significant mechanical damage of the membranes, mainly caused by the internal scaling, was also found. As a result, the membranes were severely damaged after 150 h of process operation. A high level of salt rejection was maintained despite damage to the external membrane surface. Keywords: membrane distillation; scaling; crystallization; polymer degradation

1. Introduction In the membrane distillation (MD) process, pure water is separated from salt solutions that leads to the supersaturation state of solutes. As a result, a salt crystallizes on the membrane surface resulting in a decrease of process yield and frequently leads to irreversible damage of the membrane modules [1–3]. This problem can be solved by integration of membrane distillation with a crystallization unit—membrane distillation crystallizer (MDC) [4–8]. In this system, not only water can be produced from brines and other solutions, but also the solutes can be recovered as solids in the crystallizer. The application of MDC was proposed in a treatment technology of wastewater generated during an electroplating process [9]. The MDC process can be utilized in the integrated system of water desalination. However, a limited solubility of salts and increasing osmotic pressure results in fresh water obtained in the reverse osmosis (RO) process being constituted of about 40%–50% volume of desalinated seawater and the remaining RO retentate forms a brine, which is subsequently discharged as a secondary waste stream. This problem can be solved by performing the desalination in a RO-MDC system [5,6,10–12]. In this case, the water is produced from RO retentate in the MD stage and solids salts in the crystallizer, which allows realization of a zero discharge concept of the concentrated brine into the environment [11]. The major operational problem in the MDC system is an appropriate control of supersaturation during the process, which eliminates the nucleation and crystals formation on the membrane surface. A crystallizer design and operating effectiveness can influence the scaling intensity that occurrs in the MD module [13–15]. The periodical, cooling crystallizers are often used when temperature has a large Crystals 2016, 6, 33; doi:10.3390/cryst6040033

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Crystals 2016, 6, 33

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influence on the solubility. Therefore, this method is not useful for NaCl, because temperature has little effect on the solubility in this case [4,5], and evaporative crystallizers are used for crystallization of this salt. A continuous NaCl crystallization can be carried-out using Oslo fluidized-bed crystallizer [16]. In units of this type, a bed of crystals is suspended in the saturated solution. The salt removal by fluidized-bed method was also applied in water desalination processes. The salt removal by seeding in a fluidized-bed crystallizer was found to be a feasible method for the reduction of scaling tendency of seawater [17]. This method is based on introducing seed (salt crystals) of the same type as the scaling materials to the metastable salt solution in order to reduce its supersaturation by heterogeneous crystallization. The fluidized-bed reactor was applied for scaling elimination during water desalination by RO process [18]. In the MD process, hydrophobic membranes are used, the pores of which can be only filled by the gas phase. The formation of deposits on the membrane surface (fouling and scaling) cause the wettability of the surface pores by separated solutions [2–5]. The water evaporation from the crystal surface facilitates the secondary nucleation; therefore, the surface scaling can also develop into internal membrane scaling. The growth of salt crystals in the pores interior may cause the blockage of pores, enhancement of surface roughness, and even mechanical damage of the membrane walls [2,5,13]. With regard to these phenomena, an elimination of crystallization in the MD module is a basic condition for an efficient operation of MDC. A fulfillment of the above mentioned condition is possible when the applied parameters of MDC operation prevent the formation of a supersaturation state at the membrane surface. This can be achieved by selecting a temperature and the flow rate of feed and distillate through the MD module [5,11]. It is also essential that the residence time of solution in the crystallizer avoids the supersaturation state caused by rapid water evaporation in the MD process [14]. With regard to this, it is advantageous to limit a magnitude of the driving force of mass transfer, which is proposed by using low feed temperature [5,6,8]. Such effect can be also obtained by elevation of distillate temperature, which additionally limits the heat loss by conduction [15]. However, the effectiveness of these solutions requires the selection of operation parameters for a given MDC design. For this reason, it is more advantageous to use a multistage MDC with a continuous operation, in which unsaturated solution is obtained in the final stage (before MD unit) by heating the feed and by adding a fresh portion of unsaturated salt solution into the feed [7,11]. The crystallization of salt on the surface of hydrophobic membranes caused their wettability due to contact of the feed with salt crystals surface. The membrane wettability in the MD process can also be as a result of chemical degradation of polymer, which causes the formation of hydrophilic groups on the membrane surfaces [19]. In the MD process, the membranes from polypropylene are frequently used [7,9]. These membranes exhibit a good thermal and chemical resistance, which allowed to maintain their non-wettability during long-term studies of water desalination [20]. During the studies with concentrated NaCl solutions, it was found that a four-year contact of PP film with the saturated salt solution caused only a slight degradation of polymer [21]. It can be assumed that the conditions existing in the MDC (e.g., elevated temperature and salt crystallization) can cause a larger degradation of PP. In the presented work, the studies were performed on the resistance to polymer degradation and the wettability of PP membranes used for the separation of supersaturated NaCl solutions. A scaling intensity in the MDC installation can be reduced by using at least a two-stage system: crystallizer-MD installation. However, the salt crystallization may proceed in the MD module even in a multi-stage MDC. In order to accelerate the degradation process, a high feed temperature was used and a single-stage MDC, which facilitates the salt crystallization on the membrane surface [2,6]. 2. Experimental Section The membrane degradation usually proceeds very slowly and negative effects of this process are observed after e.g., 1–2 years of MD module exploitation [19–22]. Therefore, a single-stage MDC


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installation schematically presented in Figure 1 was used for accelerate the polypropylene degradation installation presented in Figure 1 was used for accelerate the polypropylene Crystals 2016, 6, schematically 33 3 of 14 phenomenon during a short period laboratory examination. degradation phenomenon during aofshort period of laboratory examination. installation schematically presented in Figure 1 was used for accelerate the polypropylene degradation phenomenon during TaF short period of laboratory examination. 3

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Figure 1. Scheme of experimental set-up. 1—MD module; 2—crystallizer; 3—Nüga temperature

Figure 1. Scheme of experimental set-up. 1—MD module; 2—crystallizer; 3—Nüga temperature regulator; 4—magnetic stirrer; 5—peristaltic pump; 6—thermostat; 7—distillate tank; 8—balance. regulator; 4—magnetic stirrer; 5—peristaltic pump; 6—thermostat; 7—distillate tank; 8—balance. Figure 1. Scheme of experimental set-up. 1—MD module; 2—crystallizer; 3—Nüga temperature

A submerged MD membrane module (without housing) was assembledtank; inside the upper part regulator; 4—magnetic stirrer; 5—peristaltic pump; 6—thermostat; 7—distillate 8—balance. A MD module (without inside the upper part of of submerged crystallizer in themembrane applied experimental set-up housing) (Figure 1).was Theassembled studies were performed using A submerged MD membrane module (without housing) was assembled inside the upper part crystallizer in the applied experimental (Figure 1).and Thethe studies were performed using alternately alternately one of the modules havingset-up a similar design external surface of respectively 0.01 2PP 2 (MD2). of2the crystallizer inhaving themapplied experimental set-up (Figure 1). The studies were performed using m (MD1) and 0.0096 In each module, ten polypropylene membranes Accurel one of modules a similar design and thecapillary external surface of respectively 0.01 m (MD1) 2 alternately ofGmbH, theIn modules havingGermany) aten similar design and the external surface of respectively 0.01 S6/2 (Membrana Wuppertal, were assembled (Figure membranes 2). The capillary diameters and 0.0096 m one (MD2). each module, capillary polypropylene Accurel PP S6/2 m2 (MD1) andmm, 0.0096 m2 (MD2). In maximum each module, ten capillary polypropylene Accurel PPwere were 1.8/2.6 nominal and diameters of the pores2).were 0.2 µm and 0.6 µm, (Membrana GmbH, Wuppertal, Germany) were assembled (Figure Themembranes capillary diameters S6/2 (Membrana Germany) were assembled (Figure 2). The capillary diameters respectively, and GmbH, the openWuppertal, porosity was 73% (manufacturer’s data). 1.8/2.6 mm, nominal and maximum diameters of the pores were 0.2 µm and 0.6 µm, respectively, and were 1.8/2.6 mm, nominal and maximum diameters of the pores were 0.2 µm and 0.6 µm, the open porosity was 73% (manufacturer’s data). respectively, and the open porosity was 73% (manufacturer’s data).

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Figure 2. The design of used submerged MD module. 1—Accurel PP S6/2 membrane; 2—module head; 3—membrane potting. Figure 2. The design of used submerged MD module. 1—Accurel PP S6/2 membrane; 2—module

Figure 2. The design ofpotting. used inside submerged MD module. PP ±S6/2 2—module A solution temperature the crystallizer was1—Accurel fixed at 363 1 K membrane; by Nűga temperature head; 3—membrane head; 3—membrane potting. regulator (Germany). The distillate (323 K) flowed inside the capillaries (lumen side), from the A solution temperature the crystallizer was pump fixed at 363 ± 1toKobtain by Nűga temperature bottom to the upper part of theinside MD module. A peristaltic was used the volume flow regulator (Germany). The distillate (323 K) flowed inside the capillaries (lumen side), from velocity of distillate stream equal to 6 ± 0.2 mL/s. At the beginning of each measuremental series, the A solution temperature inside the crystallizer was fixed at 363 ˘ 1 K by Nuga ˝ temperature bottom to the upper part of the MD module. A peristaltic pump was used to obtain the volume flow distillate loop was refilled by 0.5 L distillate water (2–3 µS/cm). The permeate flux was calculated regulator (Germany). The distillate (323 K) flowed inside the capillaries (lumen side), from the bottom velocity of distillate equal to 6 ±volume 0.2 mL/s. At the beginning the basis in the distillate over studied time.measuremental series, the to theon upper partofofchanges the stream MD module. A peristaltic pump wasperiod usedofof toeach obtain the volume flow velocity distillate loop NaCl was refilled by 0.5 L distillate water (2–3solution µS/cm).(10 The permeate flux calculated A diluted (ChemPur, Piekary Śląskie, Poland) g/L) was used forwas determination of distillate stream equal to 6 ˘ 0.2 mL/s. At the beginning of each measuremental series, the distillate onmaximal the basispermeate of changes in the distillate volume over studied period of time. of flux for both modules MD1 and MD2. A solution of 300 g NaCl/L was utilized loop to was refilled byof0.5 L decline distillate water (2–3saturated µS/cm). The permeate flux was on the basis A diluted (ChemPur, Piekary Śląskie, Poland) solution (10the g/L) was usedcalculated for determination study a levelNaCl flux when almost solution fed MD module. of changes in the distillate volume over studied period of time. of maximal permeate flux for both modules MD1 and ions, MD2.and A solution 300 gmay NaCl/L was utilized Seawater contains, besides NaCl, several other some ofofthem form sparingly ´ askie, A NaClof (ChemPur, Sl ˛ saturated Poland) solution was for determination to diluted studysolutes. a level flux decline when almost solution fed(10 the MD module. soluble Therefore, thePiekary crystallization studies were carried outg/L) using theused saturated brine (at of maximal permeate flux for bothprepared modules MD1 andcrystal MD2.and A solution 300 NaCl/L wasThis utilized contains, besides NaCl, several other ions, some of of them form sparingly roomSeawater temperature) which was with NaCl salts purchased in may aglocal market. 2−, K+,the 2+, Si4+, and 2+ soluble solutes. Therefore, the crystallization studies were carried out using saturated brine (at salt beside NaCl (98.5%) contained several other ions, such as SO 4 Ca Mg to study a level of flux decline when almost saturated solution fed the MD module. room temperature) which was prepared with NaCl crystal salts purchased in a local market. This (manufacturer’s data, Kłodzko Salt Mine, Wieliczka, Poland). The content of crystallizer was Seawater contains, besides NaCl, several other ions, and some of them may form sparingly soluble 2+ salt Therefore, beside NaCl (98.5%) contained several other carried ions, such SO42−the , K+,saturated Ca2+, Si4+, brine and Mg agitated using a magnetic stirrer (1000studies rpm). were solutes. the crystallization outasusing (at room (manufacturer’s data, Kłodzko Salt Mine, Wieliczka, Poland). The content of crystallizer was temperature) which was prepared with NaCl crystal salts purchased in a local market. This salt beside agitated using a magnetic stirrer (1000 rpm). 2´ + 2+ 4+ 2+

NaCl (98.5%) contained several other ions, such as SO4 , K , Ca , Si , and Mg (manufacturer’s data, Kłodzko Salt Mine, Wieliczka, Poland). The content of crystallizer was agitated using a magnetic stirrer (1000 rpm).


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The salt crystallization was carried out using a batch or fluidized-bed crystallizer. In the first case (MD1 module), a crystallizer was filled with 3 L of brine, then a solution was preheated to 363 K and the MD process was further carried out for about 5 h. After completing the process, the MD1 module was taken out from crystallizer and was then stored in distilled water overnight (Mode I). In the second case (fluidized-bed crystallizer) the MD2 module was used. The process was performed in the following way: after preheating 3 L of brine to 363 K, about 30–45 g of NaCl crystals (initial 80 h of studies) or 90 g of NaCl (from 80 h of studies) was added to the crystallizer, and finally saturated solution (363 K) with suspension of salt crystals was obtained. An intensive mixed (1000 rpm) suspension of salt crystals created the fluidized bed. Two methods were applied (Mode I and Mode II) for a periodical dissolution of salt crystals from membrane surfaces in MD2 module. Mode I was similar as in the case of MD1 module—after 5 h of process the MD2 module was taken out from crystallizer and was then rinsed with distilled water. In the second variant of process operation (Mode II), the membranes were additionally rinsed with distilled water for about 1 min at every 30 min, which allowed removal of the NaCl crystals from the membrane surface. The morphology of membrane and the deposit formed during the process was studied using a Jeol JSM 6100 scanning electron microscopy (SEM-EDS). The samples for cross-section observations were prepared by fracturing the capillary membranes in liquid nitrogen. All the samples were sputter coated with gold and palladium. The electrical conductivity of solutions was measured with a 6P Ultrameter (Myron L Company, Carlsbad, CA, USA). In order to identify the functional groups on the polypropylene surface, the attenuated total reflection Fourier transform infrared (ATR-FTIR) analyses were performed using a Nicolet 380 FT-IR spectrophotometer connected with Smart Orbit diamond ATR accessory (Thermo Electron Corp., Waltham, MA, USA). The measurements of phase transition temperatures of polymers can be useful indicator for evaluation of the extent of polymer degradation. The thermal properties of used polypropylene membranes were evaluated by differential scanning calorimetry (DSC). The measurements were performed with the use of Q1000 TA Instruments apparatus, at heating-cooling rate of 10 deg/min. 3. Results and Discussion 3.1. Batch Crystallizer A maximum yield of applied design of submerged MD modules was determined using a dilute NaCl solution (10 g NaCl/L) as a feed. The permeate flux at a level of 580–600 L/m2 ¨ d was achieved for MD1 module (feed temperature 363 K, distillate temperature 323 K). However, the permeate flux decreased to 540 L/m2 ¨ d during 15 h of module operation (Figure 3). From the previous studies [22] it is a known fact that a fraction of the pores located on the surface of Accurel PP membranes underwent the wettability at the initial period of MD process, resulting in a decline of the permeate flux. A low value of obtained electrical conductivity of distillate in the range of 4–6 µS/cm (feed 10 g NaCl/L), confirms that the membranes underwent only the surface wettability during the first 15 h of studies. In the second stage of studies, an NaCl solution with initial concentration of 300 g/L was used as a feed to determine of MD efficiency. The presence of salt decreased the driving force (partial pressure difference), and as a result, the permeate flux for almost saturated NaCl solution was decreased from 540 to 150 L/m2 ¨ d, whereas a value of the electrical conductivity increased to 70 µS/cm (Figure 3). This value of conductivity was increased to 90 µS/cm, and the permeate flux decreased to 110 L/m2 ¨ d after 40 h of MD1 module exploitation. When brine was exchanged into a dilute NaCl solution, the maximal permeate flux was also significantly decreased during the separation process, from 540 to 480 L/m2 ¨ d (Figure 3, 44 h). These results indicate that not only the pores on the membrane surface, but also some pores inside the wall underwent the wettability. The wetting of pores may be caused by the crystallization of salt on the membrane surface, which is frequently observed in MDC studies [2,5].


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Flux: - 10 g/L - 300 g/L - conductivity

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Time [h] 3. Changes of the permeate fluxand anddistillate distillate electrical electrical conductivity during MDMD of NaCl FigureFigure 3. Changes of the permeate flux conductivity during of NaCl solutions. initial concentration: g/L,300 300g/L g/L and 293 K) K) solution. Module MD1.MD1. solutions. FeedFeed initial concentration: 1010 g/L, andsaturated saturated(at(at 293 solution. Module

The above phenomenon was confirmed by the results of consecutive measurements of MDC

The above phenomenon wasa confirmed thetemperature) results of consecutive measurements of MDC (Figure 3, from 50 h), in which saturated (atby room NaCl solution was used as a feed. (Figure 3, from h), in whichaasuspension saturatedof(at temperature) solution used in asthe a feed. After 2–3 h50 of MD process, anroom increasing number ofNaCl salt crystals waswas observed After feed. 2–3 hAfter of MD process,measurements, a suspension of increasing salt crystals was in the completing thean presence of saltnumber crystals of was also found on theobserved membrane surfaces. The MD1 module was stored overnight in of thesalt distilled water, resulting in theon dissolution of feed. After completing measurements, the presence crystals was also found the membrane salt crystals from membrane However,in thethe periodical of salt crystals deposited of surfaces. The MD1 module was surfaces. stored overnight distilleddissolution water, resulting in the dissolution on the membrane surfaces (probably also inside the pores) caused the electrical conductivity of on salt crystals from membrane surfaces. However, the periodical dissolution of salt crystals deposited distillate to systematically increase in the consecutive days, reaching a value of 450 µS/cm after the membrane surfaces (probably also inside the pores) caused the electrical conductivity of distillate additional 35 h of crystallizer operation (Figure 3, 83 h). Such a value of electrical conductivity to systematically increase in the consecutive days, reaching a value of 450 µS/cm after additional corresponds to a concentration of about 0.2 g NaCl/L, thus a 99.9% of salt retention was maintained. 35 h of crystallizer operation (Figure 3, 83 h). Such a value of electrical conductivity corresponds However, taking into account the need of long-term exploitation the MDC system, the obtained to a concentration of about 0.2 g NaCl/L, thusworks, a 99.9% of salt was maintained. However, results confirmed a conclusion from other that one retention of basic conditions of appropriate takingoperation into account the need of long-term exploitation the MDC system, the obtained results confirmed of MDC is elimination of salt crystallization on the membrane surface [5]. The scaling a conclusion works,was thatstudied one ofusing basic conditions of appropriate of MDC is intensity from in the other MD1 module SEM methods, and the results ofoperation these observations were presented in Section 3.3. on the membrane surface [5]. The scaling intensity in the MD1 elimination of salt crystallization module was studied using SEM methods, and the results of these observations were presented in 3.2.3.3. Fluidized-Bed Crystallizer Section A heterogeneous crystallization on the surface of seeding salt crystals is proposed as a method

3.2. Fluidized-Bed Crystallizer of scaling elimination in the installations for water desalination [17,18]. The effectiveness of such solution was also tested in this work by location of the MD2 module directly inside a fluidized-bed

A heterogeneous crystallization on the surface of seeding salt crystals is proposed as a method crystallizer. The saturated brine with NaCl crystals was rigorously mixed in order to create the bed of scaling elimination in the installations for water desalination [17,18]. The effectiveness of such of suspended crystals around the membranes in the MD2 module assembled in the upper part of solution was also tested1).inThe this work by location of the MD2 module directly inside4 and a fluidized-bed crystallizer (Figure obtained results of these studies were presented in Figures 5. crystallizer. The saturated brine with NaCl crystals was rigorously mixed in order to create the bed Although the values of maximal permeate flux of the modules MD1 and MD2 were similar of suspended around theflux membranes in MD2 the MD2 module inwas theatupper (Figures 3crystals and 6), the permeate obtained for module duringassembled MDC studies a levelpart of of 2·d (Figure crystallizer (Figure 1). The obtained results of these studies were presented in for Figures 4 and 5.3). 3–4 L/m 4), which was significantly smaller than the values obtained MD1 (Figure It should remembered, that each point on theof figures representsMD1 the average permeate Although the be values of maximal permeate flux the modules and MD2 wereflux similar obtained during 5 h of MD process operation. In the case of results presented in Figure 3 (module (Figures 3 and 6), the permeate flux obtained for MD2 module during MDC studies was at a level of MD1), saturated solution room temperature) as a feed. Due to preheating of the 3). 2 ¨ da(Figure 3–4 L/m 4),salt which was (at significantly smaller was thanused the values obtained for MD1 (Figure feed to 363 K, this solution became unsaturated at the beginning of measurement and the It should be remembered, that each point on the figures represents the average permeate flux supersaturation state was achieved after 2–3 h of MD process. A growth of NaCl crystals in the feed obtained during 5 h of MD process operation. In the case of results presented in Figure 3 (module MD1), was observed after this period. Moreover, the module MD1 was rinsed using distilled water (mode a saturated (at room temperature) wascrystals used as a feed.onDue preheating of the feed to I) aftersalt thesolution measurements; therefore, the NaCl located the to membrane surface were 363 K,dissolved. this solution became at the beginning of MD2 measurement and thesaturated supersaturation On the other unsaturated hand, the operating conditions of module (solution at 363 K)state was achieved 2–3 h of scaling MD process. A growth of NaCl crystals in the feedwas wasmost observed after facilitate after a membrane from the beginning of measurement, which probably a this period. Moreover, the module MD1 was rinsed using distilled water (mode I) after the measurements; therefore, the NaCl crystals located on the membrane surface were dissolved. On the other hand, the operating conditions of MD2 module (solution saturated at 363 K) facilitate a membrane scaling from


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Crystals 2016,of 6, measurement, 33 6 of 14 the beginning which was most probably a reason for lower module yield. The larger reason2016, for 6,lower module yield. The larger scaling observed in the MD2 module was also confirmed Crystals 33 14 scaling observed in the MD2 module was also confirmed by the fact that after 80 h of process6 of duration by the fact that after 80 h of process the distillate conductivity increased to 1500 µS/cm reason for lower module yield. The duration larger scaling observed in the MD2was module was also confirmed the distillate conductivity was increased to 1500 µS/cm (Figure 6), whereas for MD1 module and over (Figure 6),lower whereas for module and scaling over similar period, value 450 µS/cm was obtained by the for fact that after 80 hMD1 of process duration theadistillate conductivity was increased to 1500 µS/cm reason module yield. The larger observed in thethe MD2 module was also confirmed a similar period, the value 450 µS/cm was obtained (Figure 3). (Figure 3). 6), whereas for MD1 module and over a similar period, the value 450 µS/cm was obtained

by the fact that after 80 h of process duration the distillate conductivity was increased to 1500 µS/cm (Figure 6), 3). whereas for MD1 module and over a similar period, the value 450 µS/cm was obtained (Figure 7 (Figure 3). 2 2 Permeate flux flux [L/m d] [L/m Permeate [L/m d] 2d] Permeate flux

7 6 7 6 5 6 5 4 5 4 3 4 3 2 3 2 1 2 1 0 1 0 20 40 60 80 0 Time 0 20 40 [h] 60 80 0 Time 0 20during MD 40 of[h]NaCl saturated 60 Figure 4. Changes of the permeate flux (at80 363 K) solutions. Module

Figure 4. Changes of the permeate flux during MDTime of NaCl [h] saturated (at 363 K) solutions. Module MD2 MD2 assembled inside fluidized-bed crystallizer. Mode I. Figure 4. Changes of the permeate flux during MD of NaCl saturated (at 363 K) solutions. Module assembled inside fluidized-bed crystallizer. Mode I. MD2 assembled inside fluidized-bed crystallizer. Mode I. Figure 4. Changes of the permeate flux during MD of NaCl saturated (at 363 K) solutions. Module 40 MD2 assembled inside fluidized-bed crystallizer. Mode I. 40

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Time 80 100 120 [h] 140 Figure 5. Changes of the permeate flux during MD of NaCl saturated (at160 363 K) solutions. Module Time [h] MD2 assembled I—continuous concentration, mode Figure 5. Changesinside of thefluidized-bed permeate fluxcrystallizer. during MDMode of NaCl saturated (atfeed 363 K) solutions. Module Figure 5. Changes of the permeate flux during MD of NaCl saturated (at 363 K) solutions. Module MD2 II—feed with periodical rinsing with distilled(at water. MD2 assembled fluidized-bed crystallizer. Mode I—continuous feed concentration, mode Figure 5. concentration Changesinside of the permeate fluxmembrane during MD of NaCl saturated 363 K) solutions. Module

II—feed concentration with periodical membrane rinsing with distilled water. 2 2 Permeate fluxflux [L/m d][L/m Permeate [L/m d] 2d] Permeate flux

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assembled inside fluidized-bed crystallizer. Mode I—continuous feed concentration, mode II—feed II—feed concentration periodical membrane distilled water. MD2 assembled insidewith fluidized-bed crystallizer.rinsing Mode with I—continuous feed concentration, mode concentration with periodical 5 700 membrane rinsing with distilled water.

Time [h] Figure 6. Changes of the maximal50permeate 100 flux (feed 150 10 g NaCl/L) and distillate electrical Time [h] conductivity during MD of NaCl saturated solutions. Figure 6. Changes of the maximal permeate flux Module (feed 10MD2. g NaCl/L) and distillate electrical conductivity during of MDthe of NaCl saturated solutions. Figure 6. Changes maximal permeate flux Module (feed 10MD2. g NaCl/L) and distillate electrical

Figure 6. Changes of the maximal permeate flux (feed 10 g NaCl/L) and distillate electrical conductivity conductivity during MD of NaCl saturated solutions. Module MD2. during MD of NaCl saturated solutions. Module MD2.


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It was found, that the permeate flux fluctuation with the time presented in Figure 4, resulted from variations the6, amount of salt crystals dosing into the crystallizer. For this reason, the MD2 Crystalsof 2016, 33 7 ofmodule 14 was thoroughly rinsed with distilled water after 80 h of studies and further studies were performed It was found, that of thesalt permeate fluxAs fluctuation the time presented in Figure 4, resulted dosing a constant amount (30 g/L). a result, with a significantly larger yield of MD process was from variations of the amount of salt crystals dosing into the crystallizer. For this reason, the MD2 obtained; however, the permeate flux was quickly decreasing to values close to zero during consecutive module was thoroughly rinsed with distilled water after 80 h of studies and further studies were measurements (Figure 5). This indicates that despite a substantial increase of the number of suspended performed dosing a constant amount of salt (30 g/L). As a result, a significantly larger yield of MD crystals, a salt was not only on their surface, butdecreasing also on the membranes. A visual process wascrystallization obtained; however, the permeate flux was quickly to values close to zero inspection MD2 module revaled that the5). salt crystals covered all the membrane surfaces, which duringof consecutive measurements (Figure This indicates that despite a substantial increase of the blocked the pores and significantly the surface fornot water number of suspended crystals, a reduced salt crystallization was only evaporation. on their surface, but also on the membranes. A visual inspection of MD2 module revaled that salt that crystals covered all the Negative effects of scaling (pore blockage) were confirmed by the fact definitely higher values membrane surfaces, which blocked the pores and significantly reduced the surface for water of the permeate flux were achieved when a scaling intensity was limited using a periodical rinsing (everyevaporation. 30 min) of the membranes with distilled water (Figure 5, mode II). However, despite membrane Negative of scaling blockage) were confirmed by the fact thatfrom definitely higher rinsing during the effects consecutive 50 h(pore of studies, the permeate flux was decreased 38 to 17 L/m2 ¨ d. values of the permeate flux were achieved when a scaling intensity was limited using a periodical Moreover, a continuous increase of the distillate electrical conductivity was observed during this rinsing (every 30 min) of the membranes with distilled water (Figure 5, mode II). However, despite period (Figure 6). The results presented in Figures 5 and 6 indicate that, performed due to mode II, membrane rinsing during the consecutive 50 h of studies, the permeate flux was decreased from 38 cyclictorepetition crystals aformation the membrane surfaces and conductivity crystals dissolution by water 17 L/m2·d. of Moreover, continuouson increase of the distillate electrical was observed rinsing favors the pore wetting. Pore wetting is confirmed also by the fact that, as a result of brine during this period (Figure 6). The results presented in Figures 5 and 6 indicate that, performed due separation, a maximal permeate flux determined for MD2 module after 150 h of exploitation decreased to mode II, cyclic repetition of crystals formation on the membrane surfaces and crystals dissolution 2 ¨ h (Figure rinsing the pore Pore the wetting confirmed alsomaintained by the fact that, as asalt result of from by 600water to 400 L/mfavors 6).wetting. However, MD2is module still a high rejection brine separation, a maximal permeate fluxsolution, determined for MD2 150 h of exploitation amounting to over 99% for saturated NaCl despite the module increaseafter of distillate conductivity to decreased from 600 to 400 L/m2·h (Figure 6). However, the MD2 module still maintained a high salt 4000 µS/cm. rejectionthe amounting to MD2 over 99% for operation, saturated NaCl solution, the increase of distillate During last 15 h of module the studies of despite MDC without the periodical rinsing conductivity to 4000 µS/cm. of the membranes with distilled water were again conducted (Figure 5, mode I—from 144 h). A further During the last 15 h of MD2 module operation, the studies of MDC without the periodical crystallization of salt on the membrane surface caused a decline of maximal permeate flux from 400 to rinsing of the membranes with distilled water were again conducted (Figure 5, mode I—from 144 h). 2 ¨ d (Figure 6). Moreover, a rapid deterioration of mechanical properties of the membranes 150 L/m A further crystallization of salt on the membrane surface caused a decline of maximal permeate flux was also period. membranes brittle,ofand fragments of polymer fromobserved 400 to 150over L/m2this ·d (Figure 6).The Moreover, a rapidbecame deterioration mechanical properties of thewere detached from their surface. membranes was external also observed over this period. The membranes became brittle, and fragments of polymer were detached from their external surface.

3.3. SEM Observations 3.3. SEM Observations

After completing the MDC studies, the membrane samples were collected from modules MD1 completing the MDC studies, theof membrane samples were from collected from modules MD1 and MD2 After for SEM observations. The surfaces membranes removed the crystallizer were rinsed and MD2 for SEM observations. The surfaces of membranes removed from the crystallizer were with distilled water and the samples were subsequently dried in a natural way at ambient temperature. rinsed with distilled water and the samples were subsequently dried in a natural way at ambient The SEM observations revealed that, during the MDC studies, the membrane surface was significantly temperature. The SEM observations revealed that, during the MDC studies, the membrane surface changed in a comparison to an image of a new membrane presented in Figure 7. was significantly changed in a comparison to an image of a new membrane presented in Figure 7.

Figure 7. SEM image of the external surface of new Accurel PP S6/2 membrane.

Figure 7. SEM image of the external surface of new Accurel PP S6/2 membrane.

In accordance with predictions, the smallest changes were observed in the case of the membranes from MD1 module. The many clusters of fine deposits was of found on the In accordance with predictions, thepresence smallestof changes were observed in the case the membranes surface, the surface pores were not completely by deposit 8). The from membrane MD1 module. The but presence of many clusters of fine depositscovered was found on the (Figure membrane surface,


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but the surface pores were not completely covered by deposit (Figure 8). The SEM-EDS analysis of deposits they were mainly composed of Na, Cl, and Si, and in a smaller amount from Ca, Crystals revealed 2016,analysis 6, 33 that SEM-EDS of deposits revealed that they were mainly composed of Na, Cl, and Si, and8 of in14 a Mg, and P.amount from Ca, Mg, and P. smaller SEM-EDS analysis of deposits revealed that they were mainly composed of Na, Cl, and Si, and in a smaller amount from Ca, Mg, and P.

Figure 8. SEM image of salt deposits on the membrane surface. Module MD1. Figure 8. SEM image of salt deposits on the membrane surface. Module MD1. 8. SEM image salt deposits on the surface.surface ModuleinMD1. A similar Figure composition had a ofdeposit formed on membrane the membrane the module MD2. A similar composition hadthicker a deposit on the membrane surfaceofinthe thesurface module MD2. However, a deposit layer was and,formed in this case, covered the majority pores A similar composition had a deposit formed on the membrane surface in the module MD2. However, a deposit layerdeposit was thicker and, inthe this case, covered the majority surface (Figure 9A). The formed also fulfilled pores inside the membrane wallof at the the depth uppores to However, a deposit layer was thicker and, in this case, covered the majority of the surface pores almost 100The µmformed (Figuredeposit 9B). The internal scaling was one of the main reasons for such a significant (Figure 9A). also fulfilled the pores inside membrane wall at the depth up to (Figure 9A). The formed deposit also fulfilled the pores inside the membrane wall at the depth up to reduction of (Figure module9B). efficiency (Figures 5 and the salt caused the almost 100 µm The internal scaling was6). oneMoreover, of main reasons forcrystallization such a significant reduction almost 100 µm (Figure 9B). The internal scaling was one of main reasons for such a significant fragments(Figures to detach from the membrane walls (Figure 10).caused A boundary between the of polymer module efficiency 5 and 6). Moreover, the salt crystallization the polymer fragments reduction of module efficiency (Figures 5 and 6). Moreover, the salt crystallization caused the external membrane surface (bottom of figure) and a damaged fragment of the membrane wall was to detach from the membrane walls (Figure 10). A boundary between the external membrane surface polymer fragments to detach from the membrane walls (Figure 10). A boundary between the shownof in figure) Figure 11A. to detachment of the external surfacewall fromwas capillary, the inside the (bottom andsurface aDue damaged fragment ofand the inpores Figure 11A. Due external membrane (bottom of figure) amembrane damaged fragment ofshown the membrane wall was wall were exposed, and a magnification view of these pores was presented in Figure 11B. The to shown detachment of the external surface from capillary, the pores inside the wall were exposed, and in Figure 11A. Due to detachment of the external surface from capillary, the pores inside the a SEM-EDS analysis thatwas the presented structure indicated by arrow on area “SD” is inorganic deposit,the magnification view ofrevealed, these Figure Thewas SEM-EDS analysis revealed, wall were exposed, and pores a magnification viewinof these11B. pores presented in Figure 11B.that The which crystallized inside the membrane wall. The same deposits occurred numerously in the other structure indicated by arrow on area “SD” is inorganic deposit, which crystallized inside the membrane SEM-EDS analysis revealed, that the structure indicated by arrow on area “SD” is inorganic deposit, places of the damaged wall (lighter points at top of Figure 11A). The salt crystals growing inside the wall. Thecrystallized same deposits occurred numerously the same other deposits places ofoccurred the damaged wall (lighter which inside the membrane wall.inThe numerously in thepoints other at membrane most probably caused a detachment of membrane fragments. The changes of the topplaces of Figure 11A). The salt crystals membrane most probably detachment of the damaged wall (lightergrowing points atinside top ofthe Figure 11A). The salt crystals caused growinga inside the structural and chemical properties of polypropylene membranes caused by the salt crystallization of membrane membrane most fragments. Thecaused changes of the structural and chemical properties polypropylene probably a detachment of membrane fragments. The of changes of the were also found in other work [23]. A partial wettability due to a salt crystallization of used Accurel structural caused and chemical of polypropylene caused by[23]. the salt crystallization membranes by the properties salt crystallization were alsomembranes found in other work A partial wettability PP S6/2 was observed in work [24]. Moreover, it was also demonstrated in this work, that were also found in other work [23].Accurel A partialPP wettability due to a salt in crystallization of used Accurel due to a salt crystallization of used S6/2 was observed work [24]. Moreover, it was enhancement of hydrophobicity of PP membranes limited the penetration of salt into the membrane PPdemonstrated S6/2 was observed in work [24]. Moreover, of it hydrophobicity was also demonstrated in this work, thatthe also in this work, that enhancement of PP membranes limited pores. enhancement of hydrophobicity of PPpores. membranes limited the penetration of salt into the membrane penetration of salt into the membrane pores.

salt

salt

(A)

(B)

Figure 9. SEM images (A)of salt deposits on: (A) membrane surface, (B) inside (B) the membrane wall. Module MD2. Figure 9. SEM images of salt deposits on: (A) membrane surface, (B) inside the membrane wall. Figure 9. SEM images of salt deposits on: (A) membrane surface; (B) inside the membrane wall. Module MD2. Module MD2.


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Figure 10. SEM image of membrane cross-section. Membrane sample from MD2 module.

Figure 10.10. SEM image Membranesample samplefrom fromMD2 MD2module. module. Figure SEM imageofofmembrane membranecross-section. cross-section. Membrane

Figure 10. SEM image of membrane cross-section. Membrane sample from MD2 module.

SD

SD

SD (A)

(B)

Figure 11. SEM images(A) of membrane external surface: (A) a part of wall was(B) detached (top of figure), (B) detached wall with salt deposit (SD). Module MD2.

Figure 11. SEM images of membrane external surface: (A) a part of wall was detached (top of figure), Figure 11. SEM images of (A) membrane external surface: (A) a part of wall(B) was detached (top of figure); (B)The detached wall with salt deposit (SD).in Module MD2. salt wall crystals observed manyMD2. places on the membrane surface on the lumen side (B) detached withwere salt also deposit (SD). Module Figure 11. SEM images of membrane external surface: (A) a part of wall was detached (top of figure), (Figure 12). The distillate flows inside the lumen side, which excludes the salt crystallization during (B)salt detached wallwere with salt deposit (SD).in Module The crystals observed manyMD2. places on the surface on the(Figure lumen6)side MD process. However, thealso electrical conductivity of distillate wasmembrane increased to 4000 µS/cm The salt crystals were also observed in many places onexcludes the membrane surface on the lumen (Figure 12). The distillate flows inside the lumen side, which the salt crystallization during which indicates a slight feed leakage through wetted fragments of the membrane wall. After The salt crystals were also observed in many places on the membrane surface on the lumen side side (Figure 12). The distillate flows conductivity inside the lumen side, which excludes the salt crystallization MD process. However, the electrical of distillate was increased to 4000 µS/cm (Figure completing the studies, the membranes were dried. The hydrophobic properties of PP caused, that 6) (Figure 12). The distillate flows inside the lumen side, which excludes thewas salt increased crystallization during during MD process. However, the electrical conductivity of distillate to 4000 µS/cm which indicates a slight feed leakage through wetted fragments of the membrane an NaCl solution was pushed out from the pores interior on the membrane surface, wall. where After it MD6)process. However, athe electrical conductivity of distillate was increasedof tothe 4000 µS/cm (Figure 6)After (Figure which indicates slight feed leakage through wetted fragments membrane wall. completing studies, the membranes werewere dried. The hydrophobic properties of that PP caused, evaporatedthe and as a result, the salt crystals formed. Thus, it can be concluded, the NaClthat which indicates a slight feed leakage through wetted fragments of the membrane wall. After completing studies, thepushed membranes werethe dried. hydrophobic ofsurface, PP thatitan formed on the membrane on pores the The distillate side the places in caused, which the ancrystals NaCl the solution was out surface from interior on indicate theproperties membrane where completing the studies, the membranes were dried. The hydrophobic properties of PP caused, that membrane walls were wetted. NaCl solution and was pushed outthe from the poreswere interior on the membrane surface, where it evaporated evaporated as a result, salt crystals formed. Thus, it can be concluded, that the NaCl an NaCl solution was pushed out from the pores interior on the membrane surface, where it formed membrane surface on the side indicate the inthe which the andcrystals as a result, the salt were itdistillate can be concluded, theplaces NaCl crystals formed evaporated andon as the acrystals result, the saltformed. crystals Thus, were formed. Thus, it can bethat concluded, that NaCl walls surface were wetted. on membrane the membrane on the distillate indicate theside places in which the membrane crystals formed on the membrane surface side on the distillate indicate the places in which thewalls were membrane wetted. walls were wetted.

Figure 12. SEM images of salt crystals formed on the distillate side during membrane drying.

Figure 12. SEM images of salt crystals formed on the distillate side during membrane drying. Figure 12. SEM images saltcrystals crystalsformed formed on during membrane drying. Figure 12. SEM images of of salt on the thedistillate distillateside side during membrane drying.

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3.4. Membrane Degradation 3.4. Membrane Degradation New capillary Accurel PP membranes are flexible, and they may be bent without causing New capillary PP membranes and they may be bentrigid; without causingthe damages. damages. However,Accurel the membranes usedare in flexible, the MD1 module become therefore, walls However, the membranes used in the MD1 module become rigid; therefore, the walls collapsed when collapsed when the capillaries were bent. The membranes underwent degradation in the highest the capillaries were module. bent. TheInmembranes degradation in the highest degree in the MD2 degree in the MD2 this case, aunderwent slight bending of the membranes caused their fracture. module. In this case, a slight bending of the membranes caused their fracture. Moreover, the wall Moreover, the wall fragments were detached from the membrane surface (Figure 10), and these fragments were detached from the membrane surface (Figure 10), and these fragments can be easily fragments can be easily ground into a powder. Such a state of the membranes material indicates that into adegradation powder. Such a state of the membranes indicates that significant degradation aground significant of polypropylene proceededmaterial during the studies of aMDC process. of polypropylene proceeded during the studies of MDC process. Polypropylene is a semicrystalline polymer and its mechanical properties depend to a large Polypropylene is athe semicrystalline polymer and its mechanical depend to a large degree degree on a ratio of crystalline phase to amorphous phase properties [19]. A consequence of polymer on a ratio of the to amorphous phase A consequence polymer degradation is a degradation is acrystalline change ofphase the crystallinity degree of[19]. PP and a change inofthe chemical composition change of the crystallinity degree of PP and a change in the chemical composition [19,21,24]. The FTIR [19,21,24]. The FTIR and DSC analyses were performed in order to determine the intensity of these and DSC analyses were performed in order to determine the intensity of these changes. changes. −1, The polypropylene a characteristic methyl absorption bandband at 1375 cm´1cm , and The polypropylene FTIR FTIRspectrum spectrumhad had a characteristic methyl absorption at 1375 ´ 1 −1 doublet at 1167 and 973 cm [25]. The bands that can be used to determine the crystallinity of PP and doublet at 1167 and 973 cm [25]. The bands that can be used to determine the crystallinity of PP ´1 and moreover, bands at 1219, 1167, 997, 972, 899 and 841 cm´−11 are appeared at at 1167, appeared 1167, 997 997 and and 841 841 cm cm−1, ,and moreover, bands at 1219, 1167, 997, 972, 899 and 841 cm are characteristic for izotactic PP [25,26]. A comparison of of FTIR spectra obtained for new Accurel PP S6/2 characteristic for izotactic PP [25,26]. A comparison FTIR spectra obtained for new Accurel PP membrane and samples collected from MD1 and MD2 modules was presented in Figure 13, the S6/2 membrane and samples collected from MD1 and MD2 modules was presented in Figure and 13, and FTIRFTIR spectra magnified in theinregion 400–1900 cm´1 was in Figure The 14. essential changes the spectra magnified the region 400–1900 cm−1shown was shown in 14. Figure The essential either in the shift or new bonds were not observed. Only a low intensity band was observed in the changes either in the shift or new bonds were not observed. Only a low intensity band was observed ´ 1 −1, which region of 1700–1750 cm ,cm which can indicate that that the carbonyl groups werewere created [25].[25]. ThisThis is a in the region of 1700–1750 can indicate the carbonyl groups created weak band and was also observed for Accurel PP membranes soaked in hot distilled water [19]. The is a weak band and was also observed for Accurel PP membranes soaked in hot distilled water [19]. obtained results indicate that the applied PP membranes demonstrated a good chemical The obtained results indicate that the applied PP membranes demonstrated a good resistance chemical during theduring MDC studies. Forstudies. this reason, the observed membrane not caused by not the resistance the MDC For this reason, the observeddegradation membrane was degradation was changes in the chemical composition of polymer. However, the results presented in Figures 13 and 14 caused by the changes in the chemical composition of polymer. However, the results presented in demonstrated significant decrease intensity of characteristic bands for crystallinity/izotacticity Figures 13 andthat 14 ademonstrated that a of significant decrease of intensity of characteristic bands for of iPP structure was observed in the spectra samplesincollected from modules. Thisfrom confirms crystallinity/izotacticity of iPP structure was of observed the spectra of MD samples collected MD a fact, that the changes in the polymer structure (crystallinity) were the main reason of increase of modules. This confirms a fact, that the changes in the polymer structure (crystallinity) were the main membrane embitterment duringembitterment the MDC studies. reason of increase of membrane during the MDC studies. 20 1 – new PP 2 – MD1 3 – MD2

Transmitance

15

10

5

0 400

2

3 1

1200

2000

2800

3600

-1

Wavenumber [cm ] Figure 13. FTIR spectra recorded for new PP S6/2PP membrane and for membrane Figure 13. Comparison Comparisonofof FTIR spectra recorded forAccurel new Accurel S6/2 membrane and for samples removed MD1 and membrane samplesfrom removed fromMD2 MD1modules. and MD2 modules.

Crystals 2016, 6, 33 Crystals 2016, 6, 33 Crystals 2016, 6, 33

Transmitance Transmitance

11 of 14 11 of 14 11 of 14

8 8 7 7 6 6

5 5 4 4

3 3 2 2 1 1 0 0400 400

1 – new PP 21 – MD1 new PP MD1 32 – MD2 3 – MD2

1 1 3 3 2 2

650 650

900 1150 1400 1650 1900 -1 900 1150 [cm 1400 Wavenumber ] 1650 1900 Wavenumber [cm-1] Figure 14. 14.Comparison Comparison of FTIR spectra recorded forAccurel new Accurel PP S6/2 membrane and for Figure of FTIR spectra recorded for new PP S6/2 membrane and for membrane Figure 14. samples Comparison of FTIR spectra recorded for new Accurel PP S6/2 membrane and for membrane removed from MD1 and MD2 modules. samples removed from MD1 and MD2 modules. membrane samples removed from MD1 and MD2 modules.

Although the membranes in the MD2 module underwent a significantly larger degradation, Although the inin thethe MD2 module underwent a significantly largerlarger degradation, only themembranes membranes MD2 module underwent significantly degradation, only Although small differences in the peaks intensity were observed for asamples collected from MD1 and small differences in the peaks intensity were observed for samples collected from MD1 MD2 only small differences intensity observed for collected from and MD1 MD2 modules (Figures in 13 the nadpeaks 14). Thus, it canwere be concluded thatsamples a high feed temperature was and the modules (Figures 13 and 14). Thus, it can be concluded that a high feed temperature was the main MD2 reason modules 13innad Thus, it can be concluded a high an feed temperature the main of (Figures variations the14). crystallinity degree of PP [26]. that Moreover, internal scalingwas caused reason of variations in the crystallinity degree degree of PP [26]. Moreover, an internal scaling caused acaused larger main reason of variations in the crystallinity of PP [26]. Moreover, an internal scaling a larger degradation of the membranes in the MD2 module. degradation of the membranes in the MD2 module. a larger of the membranes the MD2confirmed module. by the results of DSC studies presented Thedegradation changes in the polymer matrix in were also The changes in the polymer matrix were also confirmed results of presented The changes in the 1. polymer matrix(T were alsocrystallization confirmedby bythe the of DSC DSC studies studies presented in Figure 15 and Table The melting m) and (T C) results temperatures were determined in Figure 15 and Table 1. The melting (T ) and crystallization (T ) temperatures were determined m C in Figure 15 and Table 1. The melting (T m ) and crystallization (T C ) temperatures were determined from both the first and the second scans. A decrease of Tm and melting enthalpy (∆Hm) was found for from first and the second second scans. scans. A A decreaseofofTTmmand andmelting melting enthalpy (∆H) m ) was found from both both the the firstsalt and the enthalpy was found for membranes after crystallization studies.decrease A decrease of melting enthalpy was (∆H alsomobserved in the for membranes after salt crystallization studies. A decrease of melting enthalpy was also observed membranes afterAsalt crystallization studies.indicated A decrease melting enthalpy was also observed the second heating. decrease of ΔHmII values thatofthe polymer crystallinity was reduced.inThe II values indicated that the polymer crystallinity was II values in the second heating. A decrease of ∆Hm second heating. A decrease of ΔH m indicated that the polymer crystallinity was reduced. The melting heat was used to determine the percent crystallinity using the reference value for reduced. heat to was used to determine the percent crystallinity the reference value melting The heatmelting was used percent crystallinity using using the value polypropylene ∆Hm,100% = 207 determine J/g [27]. Thethe new polypropylene membranes werereference characterized byfor a for polypropylene ∆H = 207 J/g [27]. The new polypropylene membranes were characterized m,100% polypropylene ∆H m,100% = 207 J/g [27]. The new polypropylene membranes were characterized by value of crystallinity degree at a level of 41.2%, and decreased to 39.2% for membranes from thea by a value of crystallinity degree a level of 41.2%,and anddecreased decreasedtoto39.2% 39.2% for membranes membranes from the value of crystallinity degree at aatlevel of that 41.2%, the MD2 module. These results confirm a fact the degree of PP degradationfor increases along from with the MD2 module. These results confirm a fact that the degree of PP degradation increases along with the MD2 module. These results exploitation time of the MD confirm module.a fact that the degree of PP degradation increases along with the exploitation exploitationtime timeof ofthe theMD MDmodule. module.

Heat flow [W/g] Heat flow [W/g]

2.0 2.0 1.8 1.8 1.6 1.6 1.4

new PP MD1 new PP MD2 MD1 MD2

1.4 1.2 1.2 1.0

Tm Tm Tm2 Tm2

1.0 0.8 0.8 0.6 0.6 0.4 0.4 0.2 0.2 0 00 0

50 50

100 150 o 100 Temperature [ C] 150

200 200

Temperature [oC]

Figure 15. DSC heating curves (I scanning) of new Accurel PP S6/2 membrane and membrane Figure15. 15. DSCheating heating curves (I scanning) of Accurel new Accurel S6/2 membrane and membrane samples removed from curves MD1 and MD2 modules. Figure DSC (I scanning) of new PP S6/2PPmembrane and membrane samples samples removed from MD1 and MD2 modules. removed from MD1 and MD2 modules.

The formation of the second melting peak was observed during the first heating of samples The formation of the second melting peak wasTm2 observed during the first collected from MD modules (Figure 15, Tm2 ). The values were lower thanheating that of of Tmsamples , which collected from MD modules (Figure 15, T m2 ). The T m2 values were lower than that of T m which indicates that the secondary structure with definitely shorter polymer chains was formed, in the indicates that the secondary structure with definitely shorter polymer chains was formed in the


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Table 1. The results of DSC analysis. MD2 cleaned—the external (degraded) layer of membrane was removed. Sample

New PP MD MD1 MD2 MD2 cleaned

I Scan (Heating)

Cooling (Crystallization)

II Scan (Heating)

Tm (˝ C)

Tm2 (˝ C)

∆Hm (J/g)

Tc (˝ C)

∆Hc (J/g)

Tm II (˝ C)

∆Hm II (J/g)

Crystallinity (%)

164.6 163.9 163.3 162.9 164.2

-* 157.9 158.3 157.3 -*

88.6 85.9 84.3 81.4 80.1

117.9 117.8 117.6 116.9 117.4

88.8 88.1 84.7 81.6 89.9

161.8 160.5 160.4 160.2 161.2

86.9 85.1 82.4 81.2 84.4

41.9 41.1 39.8 39.2 40.8

Notes: Tm , Tc —melting and crystallization temperatures; ∆Hm , ∆Hc —melting and crystallization enthalpy. *—formation of the second melting peak on a lower-temperature shoulder of the main transition was observed.

The formation of the second melting peak was observed during the first heating of samples collected from MD modules (Figure 15, Tm2 ). The Tm2 values were lower than that of Tm , which indicates that the secondary structure with definitely shorter polymer chains was formed in the polymer matrix. The lowest value of Tm2 = 157.3 ˝ C was achieved for membranes from the MD2 module, which exhibited the largest mechanical damage. In this case, almost half the thickness of wall (Figure 10) was changed into fracturing layer, easy to scraps. After the removal of this layer, the performed DSC analysis demonstrated, that such prepared membrane (Table 1, MD2 cleaned) had the parameters closed to those obtained for a new membrane. A layer of the wall, which was detached from the external membrane surface, was subjected to high feed temperature (363 K) during MDC studies and the internal scaling proceeded inside this layer (Figure 9B), which probably accelerated cracking of the polymer chains due to the synergistic effect. This conclusion is confirmed by the results presented in [28]. In this work, the same Accurel PP S6/2 membranes were used (over 180 h) for separation of feed contained 250 g NaCl/L at temperature 363 K. The DSC analysis (Table 1, sample MD) indicated that such high feed temperature caused changes in the polymer parameters, but smaller than that observed for samples from MD1 and MD2 modules. Moreover, the salt crystals were not created (feed 250 g NaCl/L) during this study, and beside high feed temperature after 180 h of MD process duration the membranes were still flexible. Thus, a limitation of salt crystallization on the membrane surface significantly reduces the degree of membrane degradation in spite of high feed temperature. 4. Conclusions The separation of salt solutions in MD process at a temperature of 363 K and the occurrence of intensive scaling caused a fast degradation of used capillary polypropylene membrane. As a result, a fracture of the membrane wall after 150 h of module exploitation was observed. Such performance restricted the practical implementation of MDC process in a single stage, even with a periodical removal of crystallized salt from the membrane surface. An attempt to reduce the scaling potential by application of heterogeneous crystallization on the surface of seeding salt crystals inside fluidized bed crystallizer was also inefficient. As a result of PP degradation during the MDC studies, more than half the thickness of the membrane wall was destroyed. Despite such significant damage, the used membranes still maintained their nonwettability and the degree of salt separation exceeding 99% was retained. Such a result confirms the conclusions from previous works indicating the suitability of PP membranes for the MD process. A limitation of salt crystallization on the membrane surface significantly reduces the degree of membrane degradation in spite of high feed temperature. This indicates that the application of


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multi-stage systems eliminating the creation of supersaturated state inside the MD module will enable enhancement of the MDC productivity by the application of higher feed temperature. The PP membranes, in a contact with hot (363 K) saturated NaCl solution maintained their good resistance to the chemical transformation. The formation of carbonyl groups on the membrane surface was only found. Acknowledgments: The National Science Centre Poland is acknowledged for the support of this work (DEC-2014/15/B/ST8/00045). Conflicts of Interest: The authors declare no conflict of interest.

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5. 6. 7. 8. 9. 10. 11.

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19.

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