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Influences of inorganic salts on the pervaporation properties of zeolite NaA membranes on macroporous supports
Microporous and Mesoporous Materials 192 (2014) 60–68
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Influences of inorganic salts on the pervaporation properties of zeolite NaA membranes on macroporous supports Jianhua Yang ⇑, Huazheng Li, Jing Xu, Jinqu Wang, Xiangdi Meng, Ke Bai, Jinming Lu, Yan Zhang, Dehong Yin State Key Laboratory of Fine Chemicals, Institute of Adsorption and Inorganic Membrane, Dalian University of Technology, Dalian, Liaoning 116024, China
a r t i c l e
i n f o
Article history: Available online 15 November 2013 Keywords: Zeolite NaA membrane Pervaporation performance Ethanol/water/salt mixture Membrane regeneration Salt influence
a b s t r a c t Zeolite NaA membranes were hydrothermally synthesized on the macroporous supports using varyingtemperature hot dip-coating seeding methods, and then were applied to pervaporative dehydration of the alcohol/water/salt mixtures at various temperatures to investigate the influences of salts on the pervaporation performance. Five kinds of salts NaCl, NaNO3, KCl, MgCl2, and CaCl2 were chosen to examine the effects of salt concentration, cations, and anions on the membrane performance. For the model system of ethanol/H2O/NaCl mixture, the water flux decreased largely with increasing salt concentrations which resulted from the reduced water driving force/activity and negative salt physical blocking on the outer membrane surface. For different cations, potassium ion led to the largest flux drop of 58.10%, while sodium ion, calcium ion, and magnesium ion for 25.88%, 22.31%, and 24.91%, respectively. This could be mainly attributed to a joint electrostatic interaction/ion exchange action. Flushing regeneration of the NaA membranes showed that complex flux behaviors were observed for the different salts. The water flux recovered up to 91.23% and 82.47% for the NaCl and NaNO3 system, respectively, indicating that the physical salt blocking effect on the flux drop can be removed therefore is reversible. The water flux recovered to 50.00% and 116.92% for salts of KCl and CaCl2, respectively. The irreversible flux drop probably resulted from the ion exchange and ion blocking effects. This work demonstrates that inorganic salts in the alcohol/H2O mixture decreased the flux or separation selectivity and should be avoided in industrial applications. Ó 2014 Published by Elsevier Inc.
1. Introduction Pervaporation, due to the advantages of the less energy requirement and modular designable over distillation, has become established as a mature technology for the purification of organics in the petrochemical, organic chemicals, fine chemicals, pharmaceutical chemicals, and new energy industries [1–3]. With uniform pores, good mechanical strength, chemical resistance, and thermal stability, zeolite membranes have dramatic advantages over polymeric membranes, especially used in high temperature, chemically aggressive environment containing harsh solvents like dimethylformamide [4], tetrahydrofuan [5,6], etc. Among the various types of zeolite membranes, zeolite NaA membranes are extremely selective for removal of water from organic solutions by PV or vapor permeation because of their high hydrophilicity and suitable pore diameter of 0.41 nm smaller than most organic molecules and larger than water. In 1999, Mitsui Engineering and Shipbuilding Co. firstly put zeolite LTA membranes into industrialization for dehydration of alcohol/water mixtures [7]. Recently, many efforts ⇑ Corresponding author. Tel./fax: +86 411 84986147. E-mail address: [email protected] (J. Yang). 1387-1811/$ - see front matter Ó 2014 Published by Elsevier Inc. http://dx.doi.org/10.1016/j.micromeso.2013.11.013
have been made to prepare zeolite NaA membranes by different seeding methods [8–11] or synthesis procedures [12–14] on various supports [12,15,16] aiming at improving the performance [15–18] and making them feasible for the large-scale applications [19]. As reported in the literatures, the PV performance of zeolite NaA membranes are usually tested by dehydrating binary organic/ water mixtures, and they showed excellent separation performance [20–22]. Despite of the success in preparation and commercial application of zeolite NaA membranes, they have been investigated primarily for dehydration of bioethanol that contains no inorganic salts. On the other hand, the dehydration of organic solvents that used in the fields of organic chemicals, fine chemicals, pharmaceutical chemicals are highly desired. Inorganic salts are usually contained in these organic solvents. Shah et al. [23] studied the salt effects on the pervaporation performance of commercial water selective polymer membranes by dehydrating both the alcohol/water/salt mixtures and the actual pharmaceutical wastes. They found that the salt showed both negative effect on the water flux and separation selectivity at high salt concentrations over 8 wt.% for ethanol/water/sodium chloride mixtures. Similarly, the salts would inevitably interact with the charged zeolite structures
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and influence the separation performance of the zeolite NaA membranes, thus affecting the long-time application of zeolite NaA membranes in the industry. So far, to the best of our knowledge, there is no report on the pervaporative dehydration of alcohol/ water mixtures in the presence of salts for a zeolite NaA membrane. The unclear effects on the performance of zeolite NaA membranes in dehydrating organic/water/salt mixtures possibly limit the application of zeolite NaA membranes. Recently, it has been reported that 100% Na+ rejection could be theoretically achieved with reverse osmosis (RO) using a perfect zeolite LTA crystal lattice [24]. Several researchers have experimentally studied the zeolite membranes for the desalinations of seawater [25–29] or radioactive solutions [30] by pervaporation or RO, they found that the zeolite membranes showed good salt rejection and inorganic salt ions did affect the water fluxes of the membranes. Malekpour et al. [30] have achieved ion rejection of over 99% in concentrating simulated nuclear solutions containing Cs+, Sr2+ and MoO42 while the flux decreased with increasing time. They suggested that the precipitation of the permeated salt was the major reason for both the increased salt rejection and decreased fluxes. Similar results are also obtained by Duke et al. [25–27]. Duke et al. explained the results by an ion exchange/interaction mechanism. However, opposite results have been reported in pervaporative/RO seawater desalination by zeolite NaA [28] and hydroxy sodalite membranes [29]. It was found that the water fluxes in desalinating seawater form Boryeong Beach on the West Sea coast of the Korean peninsula were larger than the pure water system for the zeolite NaA membranes [28]. The surface charge was thought to be an important factor governing the water flux, and the reduced electrostatic interaction between the positive surface charge and the polar water was responsible for the higher flux. The above findings suggested that the salt influence on the permeation behavior of water molecules through the membranes is complex. It is of great value to study the salt influence on the membrane separation performance from the scientific point. Furthermore, it will attract more technological interests and broaden the industrial applications of zeolite NaA membranes. Very recently, in our previous work a varying-temperature hot dip-coating (VTHD) seeding method [2,31] was proposed to prepare NaA zeolite membranes with high PV performance on low cost macroporous supports [32]. The flux of the prepared zeolite NaA membrane reached to 2.38–2.85 kg m 2 h 1 with separation factor larger than 5000 while the total cost of the zeolite NaA membranes was estimated to be reduced by about 40%, promoting the broad application of zeolite NaA membranes. In this work, the influences of inorganic salts on the pervaporation performance of zeolite NaA membranes in dehydrating alcohol/water/salts mixtures were firstly investigated. Five kinds of salts NaCl, NaNO3, KCl, MgCl2, and CaCl2 were chosen to examine the effects of salt concentration, different cations and anions on the membrane performance. Furthermore, the membrane regeneration experiments of the NaA membranes were carried out to further clarify the influence of salt on separation characteristics.
2. Experimental 2.1. Preparation of zeolite NaA membranes Zeolite NaA membranes were prepared on the macroporous tubular alumina supports with an average pore size of 2–3 lm (OD: 13 mm, ID: 9 mm, length: 100 mm, Foshan Ceramics Research Institute, China) by VTHD seeding method. The seed layer on the support was obtained by hot dip coating with large seeds of about 2 lm, followed by rubbing off the loose and superfluous seeds, then hot dip coating the 0.4 lm NaA seeds.
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The molar composition of the starting synthesis solution was Al2O3:5SiO2:50Na2O:1000H2O. After aging at room temperature for 6 h, hydrothermal synthesis of zeolite NaA membrane was performed at 80 °C for 5 h. The details of preparation have been discussed elsewhere [32]. 2.2. Pervaporation experiments After the successful synthesis of zeolite NaA membranes, the time dependence of the separation performance (flux and separation selectivity) through the zeolite NaA membrane was examined by pervaporative separation of 90 wt.% ethanol/10 wt.% water/ 0.4 wt.% sodium chloride mixtures at 75 °C using a home-made apparatus [33]. After the NaA membranes reached the steady state, the PV performances of zeolite NaA membranes in dehydrating ethanol/ water/salts mixtures were carried out to further study the influences of salt on the performance of the zeolite NaA membranes by using the various kinds of salt concentrations and species and various feed ethanol/water concentration mixture. Dehydrations of ethanol/water/sodium chloride mixtures were performed by varying the weights of salt addition keeping ethanol and water content constant in the feed, and then by varying ethanol and water content in the feed while the salt addition was constant. The salt concentrations varied from 0.1 to 0.5 wt.% of the total feed mixtures. For the influences of cations, four kinds of salts NaCl, KCl, CaCl2, and MgCl2 were added into the feed mixtures, respectively. For the influences of different anions, NaCl and NaNO3 were added into the feed mixtures, respectively. Note, the salt concentration is referred to the ratio of the salts over the ethanol/H2O mixture. In a typical experiment, taking 90 wt.% ethanol/10 wt.% water/0.4 wt.% sodium chloride mixture for example, 1.080 g of sodium chloride was dissolved by 26.268 g of deionized water, then 243.732 g of ethanol was added into the clear mixture. Furthermore, zeolite NaA crystal powders instead of zeolite NaA membrane were used to determine the exchange degree of ions to avoid the disturbance of the salts blocked in the non-zeolitic pores. Zeolite NaA crystals were added into the 90 wt.% ethanol/10 wt.% water/0.4 wt.% potassium chloride (or calcium chloride) mixtures, and then heated at 75 °C in a reflux apparatus for the same time with the zeolite NaA membrane pervaporation tests. After the treatment, the LTA crystals were washed by deionized water thoroughly, and then dried at 50 °C for 12 h. After the pervaporative dehydrations of ethanol/water/salt mixtures, membrane regeneration was carried out. In membrane regeneration experiments, the outer side of the membranes was firstly thoroughly washed by pure water. Then the membrane module was placed into the pure water and pure water permeation experiments by pervaporation were carried out at room temperature to wash the whole membrane from the outer side to the inner side of the membranes until the pure water flux reached steady state. The membrane separation performance was evaluated by the permeation flux, separation selectivity, and ion rejection. The permeation flux was calculated by the mass of the permeate, which was collected by a liquid-nitrogen trap in a given time interval. The separation selectivity was determined as a = (yw/yo)/(xw/xo), where yw/yo is the weight ratio of water to organic solvent in the permeate and xw/xo is that of water to organic solvent in the feed. The ion rejection was defined as R = 100 (Ci,F Ci,P)/Ci,F, where Ci,F and Ci,P are the ion concentration in the feed and permeate, respectively. The removed water by membranes was compensated by adding the corresponding amount water into feed in an interval to maintain the constant feed composition. All the experiments were conducted three times to take the average results. The details of the pervaporation tests have been given elsewhere [32].
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2.3. Characterization The morphology of the as-synthesized zeolite NaA membrane was observed by scanning electron microscopy (SEM) using a QUANTA-450 at an acceleration voltage of 20 kV. The elements contained in the membranes were detected by energy dispersive X-ray (EDX) measurements with Ametec Quanta 200 FEG 132-10. The relative content of the elements was measured by the intensity of emitted characteristic X-rays of the sample elements. The collected zeolite LTA crystal powders and permeate were analyzed for cations using inductively coupled plasma mass spectrometer (ICP, Optima 2000 DV). 3. Results and discussion 3.1. Preparation of zeolite NaA membrane The SEM images of the top-surface and the cross-section of the zeolite NaA membranes obtained in the present work are shown in Fig. 1a and b, respectively. It can be seen from Fig. 1a that a compact, continuous, and pinhole-free zeolite NaA membrane composed of well inter-grown cubic-shaped crystals was formed on the macroporous support. The thickness of the NaA membrane is approximately 4–5 lm as shown in Fig. 1b. XRD patterns for the membranes (not shown here) represented the characteristic peaks of LTA zeolite together with the peaks of the support, indicating that pure zeolite NaA membranes were formed on the macroporous supports. These features are in agreement with our previous report [32]. 3.2. Influences of inorganic salts on the PV performance 3.2.1. Time dependence of the PV performance in the presence of salts species Fig. 2 shows the time dependence of the pervaporation performance of zeolite NaA membrane in dehydrating 90 wt.% EtOH/ 10 wt.% H2O/0.4 wt.% NaCl mixture at 75 °C. In the case of dehydrating 90 wt.% EtOH/10 wt.% H2O mixture the zeolite NaA membrane showed constant water flux of 2.32 kg m 2 h 1 with the separation factor of 6000 during the investigated period of 600 min. However, for dehydrating 90 wt.% EtOH/10 wt.% H2O/ 0.4 wt.% NaCl mixture, the water flux of the NaA membrane kept almost constant during the first 80 min, then gradually decreased from 2.17 to 1.72 kg m 2 h 1 from 80 to 300 min, and finally reached steady-state value of 1.64 kg m 2 h 1. In strong contrast the separation selectivity showed a reverse trend. It kept constant during the first 80 min and then gradually increased from 7000 to 17,000, finally get steady at about 17,000. The decrease in water
Fig. 2. The time dependence of the pervaporation performance of zeolite NaA membrane in dehydrating 90 wt.% EtOH/10 wt.% H2O/0.4 wt.% NaCl mixture at 75 °C.
flux and increase in separation selectivity mainly resulted from the physical blocking of the pores on the outer surface and within the zeolite NaA layer by micro-salt crystallites [23]. When the water permeated through the membrane, the salt recrystallized from the mixture (as shown in Fig. 3b), and deposited on the outer side of the membrane (as shown in Fig. 3a). Furthermore, during the permeating, ions would be retained in the pores due to the electrostatic interaction between the surface charge and ions and the zigzag pores of the membrane. The blocking effect of the deposited ions can be supported by the EDX observation in Fig. 4 which shows EDX element contents of zeolite NaA membranes after dehydrating 90 wt.% EtOH/10 wt.% H2O/0.4 wt.% NaCl (a and b) and 90 wt.% EtOH/10 wt.% H2O (c and d) mixtures, respectively. Note, even the calcium and potassium were detected in the NaA membrane, reasonably resulting from the impurity in the alkali sources (96 wt.% NaOH containing trace of K and Ca). Surprisingly, the atom ratio of sodium in the case of salt containing (11.93%) was higher than that of in the case of no salt contained (10.80%). In a word, there were more sodium ions in the zeolite layer after separating 90 wt.% EtOH/10 wt.% H2O/0.4 wt.% NaCl mixtures. This indicated that sodium ions traveled through the non-zeolitic pores and were blocked in the zeolite layer. The salt deposited on the outer surface of zeolite membranes (namely membrane fouling) and ions blocked in the pores both increased the mass transfer resistance, thereby reducing the water flux. Compared with the water molecule (2.6 Å), the ethanol molecules (4.7 Å) is bigger, therefore more ethanol molecules were blocked by the deposited microcrystals and ions, resulting in the increased separation selectivity.
Fig. 1. SEM images of the prepared zeolite NaA membrane (a, the top-surface; b, the cross-section).
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Fig. 3. Optical photos of zeolite NaA membrane after dehydrating the EtOH/H2O/0.4 wt.% NaCl mixtures and the cloudy 95 wt.% EtOH/5 wt.% H2O/0.4 wt.% NaCl mixture. The enlarged photo inserted in picture (a) shows the serious salt deposition on the outer membrane surface clearly.
Fig. 4. EDX results (a and c) and cross-sectional SEM images (b and d) of zeolite NaA membranes after dehydrating 90 wt.% EtOH/10 wt.% H2O/0.4 wt.% NaCl (a and b) and 90 wt.% EtOH/10 wt.% H2O (c and d) mixtures.
Because the deposition of salt microcrystals and ions was a chronic process, it should not play an important role at the beginning of the pervaporation. This is the reason why the membrane flux and separation selectivity kept constant in the first 80 min and then showed a gradually decreasing and increasing trend, respectively. When the salt deposition and ion blocking reached equilibrium or saturated state, the membrane flux reached steady state and kept a constant value. The slight decreased flux by the addition of salts in the feed probably resulted from a lower driving force. Since the partial pressure of water decreased when inorganic salt was added into the feed. Meanwhile, the water activity was reduced by the formation of large ionic clusters (with the effective
size from 8 to 11 Å) consisting of ions surrounded by water molecules [24], and the electrostatic interaction between the polar water and hydrated sodium ion whereby its transport through the membrane was inhibited. 3.2.2. Influences of salt concentrations on the PV performance The membranes used for the investigating effect of salt concentrations and salt species on the separation performance in the following context were reached the steady state, and the mentioned fluxes and separation factor hereafter were steady-state values. Fig. 5 shows the change of the pervaporation performance of zeolite NaA membrane with sodium chloride concentrations of
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Fig. 5. The PV performance of zeolite NaA membrane in dehydrating EtOH/H2O/ NaCl mixtures with a range of 0–0.5 wt.% sodium chloride concentrations at 75 °C.
0–0.5 wt.% in dehydrating 90 wt.% EtOH/10 wt.% H2O/NaCl mixtures at 75 °C. The concentrations of water in the feed mixture were maintained constant at about 10 wt.% to preclude any effects arising out of changes in water concentrations. It is clearly seen from Fig. 5 that the water flux decreased from 2.30 to 1.37 kg m 2 h 1 as the salt concentration in the feed was varied from 0 to 0.5 wt.%. At higher salt concentrations, a significant flux drop was observed. With the increase in salt concentration, the larger decrease in water flux at higher salt concentration also could attribute to the above reasons. With increasing the feed salt concentration, more ionic clusters were formed, the electrostatic interactions were bigger, and resulted in the water activity decreased largely thereby a significant drop in water flux. Moreover, the membrane fouling was got more severe since more salts recrystallized from the mixtures, thus resulting in larger mass transfer resistance and a lower flux. Similar results were got by Cho et al. [28], when the feed NaCl concentration was over 0.13 mol of sodium chloride per liter of water, the water flux decreased with increasing NaCl addition. It should be noted that the feed NaCl concentration in this study was from 0.1 to 0.5 wt.% of the total mixtures, which means the feed NaCl concentration was from 0.17 to 0.85 mol of sodium chloride per liter of water. It is interesting to note that, in all cases, as shown in Fig. 5, the separation selectivity increased significantly in the presence of NaCl. The higher selectivities mainly resulted from the aforementioned physical blocking by the deposited NaCl on the outer surface of the membrane. The salt precipitation on the sur-
Fig. 6. Effects of the feed ethanol concentration on the pervaporation performance in dehydrating the EtOH/H2O/NaCl mixtures at 75 °C.
Fig. 7. Effects of the salt concentration on the pervaporation performance in dehydrating the 90 wt.% EtOH/10 wt.% H2O/0.4 wt.% NaCl mixtures at various temperatures (60 °C, 65 °C, 70 °C, 75 °C).
Table 1 Bare and hydrated ion size. Ion
Bare diameter (nm) [28]
Hydrated diameter (nm) [35]
Na+ K+ Ca2+ Mg2+ Cl NO3 H2O
0.24 0.30 0.20 0.14 0.32 0.36 0.26
0.72 0.66 0.82 0.86 0.66 0.68 0.26
face blocked the non-zeolitic pores and inhibited the ethanol molecular from entering the membrane pores. Thus, ethanol concentrations in the permeate for the EtOH/H2O/NaCl mixtures were lower than that of the EtOH/H2O mixtures. Considering the poor solubility of sodium chloride in ethanol, the influences of salt concentration was mainly affected by the water concentration in the feed mixtures. Experiments were therefore performed by varying the feed water concentrations while keeping the feed salt addition constant. Fig. 6 shows the change of the pervaporation performance of zeolite NaA membrane with the feed water concentrations in dehydrating EtOH/H2O/0.4 wt.% NaCl mixtures at 75 °C. For binary ethanol/water system, the water flux was observed to vary from 3.42 to 1.71 kg m 2 h 1 as the feed
Fig. 8. Comparisons of the water fluxes in dehydrating 90 wt.% EtOH/10 wt.% H2O/ 0.4 wt.% salt mixtures at 75 °C. (Inserted figure shows the variation of the ratio of the flux drop to the fresh flux with different salts.)
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Fig. 9. EDX results (a,c,e) and cross-sectional SEM images (b,d,f) of zeolite NaA membranes after dehydration of 90 wt.% EtOH/10 wt.% H2O/0.4 wt.% CaCl2 or KCl mixtures (a,b for CaCl2 and c,d,e,f for KCl).
water concentration was decreased from 30 to 5 wt.%. The reasons for the flux drop are mainly due to the increasing frictional forces between the ethanol and water molecules and the decrease of the water driving force [4]. In contrast to binary ethanol/water mixture, for the ethanol/water/sodium chloride mixtures, the water flux decreased more rapidly with decreasing water concentration, especially in low water concentration below 20 wt.%. It is probably due to the fact that with decreasing water concentration in the feed, the ratio of salt to water in the feed was increased. The increased ratio of salt to water in the feed resulted in increased electrostatic interactions between sodium ions and polar water molecules. This enhanced forces tended to result in an extra flux
drop. Additionally, the more micro-salt crystallites deposited on the outer surface of zeolite NaA membrane due to the less water in the feed, and then made the membrane fouling more severe, thus resulting in a larger flux drop. As discussed above, the fluxes for the ternary ethanol/water/sodium chloride mixtures were lower than these of the binary ethanol/water mixtures. Fig. 7 shows the temperature dependence of the pervaporation performance of zeolite NaA membrane in dehydrating EtOH/H2O/ NaCl mixtures. It can be observed that the water fluxes increased linearly with increasing temperature, and the flux improvement rates were almost the same for all mixtures. However, in pervaporation experiments performed by Cho et al. [28], a lower flux
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respectively. This was due to the changes in pore size induced by cation exchange as indicated by EDX results in Fig. 9 in which shown the EDX results (a,c,e) and cross-sectional SEM images (b,d,f) of zeolite NaA membranes after dehydration of 90 wt.% EtOH/10 wt.% H2O/0.4 wt.% CaCl2 or KCl mixtures (a,b for CaCl2 and c,d,e,f for KCl). The much stronger peaks of K+ and Ca2+ than those of Na+ strongly indicated the occurrence of cation exchange. The degrees of ion exchange for the two membranes were about 70.52% and 74.64%, respectively, which were obtained by ICP results of zeolite LTA crystals. The pore size of zeolite NaA decreases from 4 to 3 Å after the sodium ion was exchanged by potassium ion, while the pore size increases from 4 to 5 Å when the sodium ion was exchanged by magnesium and calcium. The reduced pore sizes in the case of KCl increased the mass transfer resistances, thus resulting in a larger flux drop compared to that of in the case of NaCl. On the contrary, the increased pore sizes in the case of CaCl2 resulted in a smaller flux drop compared to that of in the case of NaCl. Another important possible reason is that hydrated size of potassium is smaller than that of sodium, magnesium, and calcium (as listed in Table 1). The electrostatic interaction between the polar water and potassium are probably larger than that of between the polar water and the sodium, magnesium, and calcium, respectively, thereby decreasing the water flux significantly. It seemed that the smaller hydrated cation (potassium ion) decreased the water flux more significantly than the larger hydrated cation (sodium ion, magnesium ion, and calcium ion). The influence of the anions on the pervaporation performance was studied by comparing the fluxes for the NaCl and NaNO3 system. The flux drop in sodium nitrate system (about 27.09%) was slightly larger than in the sodium chloride system, indicating that the larger anion (nitrate ion) has more negative effects on the water flux than the smaller anion (chloride ion) due to the more reduction in pore size by the larger anion. As reported in the literature, the zeolite NaA, hydroxy sodalite and MFI membranes are well demonstrated to show good ion rejections in desalination of seawater or radioactive solutions. In this study, ion concentration in the permeate was also analyzed. The measured ion concentrations in the permeate and feed along with the ion rejections are listed in Table 2. Based on these results the zeolite NaA membrane showed excellent salt rejections and reached rejection efficiency over 99.98%, which are almost the same as reported values [28,29]. The rejection function of the NaA membrane can be evidenced by the absence of the salts in the support layer as shown in Fig. 9e. Since the Al2O3 support layer is essentially salt-free (EDX), indicating no ions passed with water through the membrane and deposited there.
Table 2 Ion concentrations in the feed and permeate and ion rejections. Ion +
Na
K+ Ca2+ Mg2+ a b c d e f
a
C
90/10/0.2 90/10/0.4 70/30/0.4 80/20/0.4 90/10/0.3 90/10/0.5 b 90/10/0.4 90/10/0.4 90/10/0.4 90/10/0.4
c
d
60 70 75 75 75 75 75 75 75 75
>2000 >4000 >4000 >4000 >3000 >5000 >4000 >4000 >4000 >4000
T (°C)
F (mg L
1
e
)
P (mg L
1
f
)
0.23 0.33 0.27 0.55 0.46 0.32 0.21 0.42 0.36 0.28
R
99.98 99.98 99.98 99.98 99.98 99.98 99.98 99.98 99.98 99.98
Concentration in the feed (wt.% ethanol/wt.% water/wt.% chloride). Concentration in the feed (wt.% ethanol/wt.% water/wt.% nitrate). Pervaporation temperature. Ion concentration in the feed (calculated values). Ion concentration in the permeate. Ion rejection (%).
improvement rate for the saline aqueous was observed in seawater desalination. They suggested that the increased Na+ adsorption on the zeolite surface with increasing temperature resulted in a decrease in water flux improvement rate. The exact mechanisms at work are still being investigated. As shown in Figs. 6 and 7, compared to the binary mixture, the presence of salts led to a smaller increase in separation selectivity with increasing feed ethanol concentration and temperature, which were similar to these of with increasing feed salt concentrations, indicating separation selectivity of zeolite NaA membrane was improved due to the presence of salt in the feed. 3.2.3. Effects of salt species on the pervaporation performance of the NaA zeolite membranes It is reported that sodium bounded to the aluminosilicate structure can be easily exchanged with the cations present in the feed during pervaporation [34], and then result in pore size changes, thereby affecting the water transport. Besides, it was reported that the anions in the feed largely affected the flux through hydroxy sodalite membrane, reverse effects on the flux were observed between chloride and nitrate ions [29]. In order to investigate the influence of different cations and anions on the water flux of the zeolite NaA membranes in dehydrating ethanol/water/salt mixtures, we changed the sodium chloride into potassium chloride, calcium chloride, magnesium chloride for the cations and sodium nitrate for anions, respectively. The bare and hydrated sizes of these ions are listed in Table 1. Fig. 8 shows comparisons of the water fluxes in dehydrating 90 wt.% EtOH/10 wt.% H2O/0.4 wt.% salt mixtures at 75 °C. It is clearly seen that a significant flux drop (approximately 58.10%) was observed in the case of potassium chloride solution. The flux decreased from 2.10 to 0.88 kg m 2 h 1 while flux drop of 25.88%, 22.31%, and 24.91% was for NaCl, CaCl2, and MgCl2 system,
3.2.4. Membrane regeneration As discussed above, the zeolite NaA membranes showed flux decreases in the presence of different salts. And the flux drop was attributed to the reduced water driving force and activity by
Table 3 Comparisons of pervaporation performances of zeolite NaA membranes in dehydrating 90 wt.% ethanol/10 wt.% water mixtures at 75 °C before and after membrane regeneration.
a b c d e
Membrane No.
Salts
a
M1 M2 M3 M4
NaCl NaNO3 KCl CaCl2
2.28 2.51 2.10 2.60
F. F. (kg m
2
h
1
)
b
S. F. (kg m
2
h
1
)
1.69 1.83 0.88 2.02
Fresh flux (flux of dehydration of the ethanol/water mixture without the addition of salts). Stable flux (flux of dehydration of the ethanol/water/salt mixture reached steady-state value). Regeneration flux (flux of dehydration of ethanol/water mixture after regeneration). Separation selectivity after the membrane regeneration. Ratio of (R. F.)/(F. F.).
c
R. F. (kg m
2.08 2.07 1.05 3.04
2
h
1
)
d
e
15283 12642 14816 159
91.23 82.47 50.00 116.92
Separation selectivity
Ratio (%)
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the presence of salt. Also, the membrane fouling which was caused by the physical depositions of salts on the outer surfaces of the membrane resulted in a flux drop. The flux is expected to recover once the deposited salts were removed away from the outer surface of the membranes. After the dehydration of ethanol/water/salt experiments, the membranes were thoroughly flushed with flesh water to remove the deposited salts on the outer membrane surface. In order to further remove the salts deposited in the pores, pervaporation experiment of pure water was performed for 0.5– 1 h at room temperature until the water flux reached steady state value. After pure water permeation, the membranes were dried at 50 °C overnight and were applied to dehydration of 90 wt.% ethanol/10 wt.% water mixture. Table 3 shows the pervaporation performance of zeolite NaA membranes in dehydrating 90 wt.% ethanol/10 wt.% water mixture at 75 °C before and after membrane flushing regeneration. The fluxes after membrane regeneration were largely recovered, indicating that the flushing regeneration has a positive effect on the recovery of separation performance. This was due to the removal of the deposited salts on the membrane surface that resulted in the physical blocking of pores and the flux reduction aforementioned. However, in the most cases, the regenerated flux was still lower than the flesh flux in the absence of salt, whereas in the case of CaCl2 the opposite result was obtained. Generally, ion exchange, ion adsorption or thermal expansion can change the pore structure, resulting in the variation in water flux through nanoporous inorganic membranes as well [28]. The terms of thermal expansion and ion adsorption in the present study could be neglected, because the temperatures and the feed compositions in the PV test were maintained the same before and after regeneration. The incorporation of different ions of different charge densities into zeolite structure is well-known to result in changes in the lattice dynamics and spacing [25]. Therefore, one possible reason for the lower flux is due to the ion exchange. For KCl, as mentioned earlier, the pore size of zeolite NaA decreased from 4 to 3 Å after the sodium ion was exchanged by potassium ion, and in the case of CaCl2, the pore size increased from 4 to 5 Å after the sodium ion was exchanged by calcium ion. The decreased pore size would inhibit the water transport through the membrane, thereby resulting in the reduction of flux in the case of KCl. On the contrary, in the case of CaCl2, the increased pore size naturally resulted in an increase in flux. Moreover, the increased pore size likely largely affected the separation ability of the membrane, lowering the separation selectivity of M4, since no visible cracks/defects were found on the SEM images of M4 top-surface (not shown in this study). The exact reasons for the decreased separation selectivity in the case of CaCl2 at work are still being investigated. Another possible reason for the lower flux obtained in this section was the ion blocking effect, since ion exchange was not considered in the case of NaCl and NaNO3. The micro-salt crystallites deposited on the outer surface of the membranes could be easily washed away by the flushing procedure, but the ions blocked in the pores could not be easily removed because of the polycrystalline feature of zeolite membrane and the zigzag pores of both the membrane and support.
4. Conclusions In the present study, the zeolite NaA membranes were hydrothermally synthesized on macroporous supports. The salt effects on the pervaporative separation performances of the obtained NaA membranes in dehydrating ethanol/water mixtures were investigated by using five kinds of salts NaCl, NaNO3, KCl, CaCl2, and MgCl2. The membrane flux showed time dependence in the presence of salts in the ethanol aqueous solution, firstly kept con-
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stant and gradually decreased then reached the stead state. For all the investigated salts, the water flux decreased in the presence of salt in the feed mixture, and the water flux decreased significantly with increasing salt concentrations in the feed mixture. The flux drop was attributed to a joint reduced water driving force and activity/salt and ion blocking mechanism. Comparisons of the flux in the presence of NaCl, KCl, CaCl2, MgCl2, and NaNO3 showed that different ions had varying influences on the membrane performance. For different cations, the largest potassium ion showed the largest flux drop of 58.10%, whereas the sodium ion, calcium ion, and magnesium ion showed a flux drop of 25.88%, 22.31% and 24.91%, respectively. For different anions (chloride and nitrate), the slightly larger nitrate ion showed a slightly larger flux drop (27.09%). This was attributed to a joint electrostatic interaction/ion exchange mechanism. After the flushing regeneration, the zeolite NaA membranes showed a flux recovery, suggesting the negative salt blocking effect was reversible. However, the flux was still lower than the fresh flux in the most cases, especially in the case of KCl only 50% of the fresh flux was obtained. For CaCl2, the flux was larger than the fresh flux. This could be attributed to a joint ion exchange/ion blocking mechanism. Despite of the excellent ion rejection efficiency over 99.98%, inorganic salts should be avoided in practical dehydration of solvent/water mixtures considering the irreversible flux drop of zeolite NaA membranes. Acknowledgements We are grateful to the financial support from National Natural Science Foundation of China (No. 21076029) and Program for New Century Excellent Talents in University (NCET-10-0286). References [1] M. Kondo, H. Kita, J. Membr. Sci. 361 (2010) 223–231. [2] X. Chen, J. Wang, D. Yin, J. Yang, J. Lu, Y. Zhang, Z. Chen, AlChE 59 (2013) 936– 947. [3] T.C. Bowen, R.D. Noble, J.L. Falconer, J. Membr. Sci. 245 (2004) 1–33. [4] D. Shah, K. Kissick, A. Ghorpade, R. Hannah, D. Bhattacharyya, J. Membr. Sci. 179 (2000) 185–205. [5] A. Urtiaga, E.D. Gorri, C. Casado, I. Ortiz, Sep. Purif. Technol. 32 (2003) 207–213. [6] S. Li, V.A. Tuan, R.D. Noble, J.L. Falconer, Ind. Eng. Chem. Res. 40 (2001) 4577– 4585. [7] Y. Morigami, M. Kondo, J. Abe, H. Kita, K. Okamoto, Sep. Purif. Technol. 25 (2001) 251–260. [8] A. Huang, W. Yang, J. Liu, Sep. Purif. Technol. 56 (2007) 158–167. [9] G. Shao, J. Yang, X. Zhang, G. Zhu, J. Wang, C. Liu, Mater. Lett. 61 (2007) 1443– 1445. [10] K. Kusakabe, T. Kuroda, A. Murata, S. Morooka, Ind. Eng. Chem. Res. 36 (1997) 649–655. [11] L. Tosheva, B. Mihailova, L.H. Wee, B. Gasharova, K. Garbev, A.M. Doyle, Angew. Chem. Int. Ed. 47 (2008) 8650–8653. [12] A. Huang, F. Liang, F. Steinbach, J. Caro, J. Membr. Sci. 350 (2010) 5–9. [13] A. Huang, J. Caro, Chem. Mater. 22 (2010) 4353–4355. [14] A. Huang, N. Wang, J. Caro, Chem. Commun. 48 (2012) 3542–3544. [15] Z. Wang, Q. Ge, J. Shao, Y. Yan, J. Am. Chem. Soc. 131 (2009) 6910–6911. [16] J. Zhang, W. Liu, J. Membr. Sci. 371 (2011) 197–210. [17] A. Huang, W. Yang, Sep. Purif. Technol. 61 (2008) 175–181. [18] C.H. Cho, K.Y. Oh, S.K. Kim, J.G. Yeo, Y.M. Lee, J. Membr. Sci. 366 (2011) 229– 236. [19] K. Sato, T. Nakane, J. Membr. Sci. 301 (2007) 151–161. [20] O.G. Nik, A. Moheb, T. Mohammadi, Chem. Eng. Technol. 29 (2006) 1340–1346. [21] M. Kazemimoghadam, A. Pak, T. Mohammadi, Micropor. Mesopor. Mater. 70 (2004) 127–134. [22] Y. Liu, Z. Yang, C. Yu, X. Gu, N. Xu, Micropor. Mesopor. Mater. 143 (2011) 348– 356. [23] D. Shah, D. Bhattacharyya, A. Ghorpade, W. Mangum, Environ. Prog. 18 (1999) 21–29. [24] J. Lin, S. Murad, Mol. Phys. 99 (2001) 1175–1181. [25] M.C. Duke, J.O. Abraham, N. Milne, B. Zhu, Y.S. Lin, J.C. Diniz da Costa, Sep. Purif. Technol. 68 (2009) 343–350. [26] P. Swenson, B. Tanchuk, A. Gupta, W. An, S.M. Kuznicki, Desalination 285 (2012) 68–72. [27] C. Dotrement, S. Van der Ende, H. Vandemmele, C. Vandecsteele, Desalination (1994) 91–113.
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[28] C.H. Cho, K.Y. Oh, S.K. Kim, J.G. Yeo, P. Sharma, J. Membr. Sci. 371 (2011) 226– 238. [29] S. Khajavi, J.C. Jansen, F. Kapteijn, J. Membr. Sci. 356 (2010) 52–57. [30] A. Malekpour, M.R. Millani, M. Kheirkhah, Desalination 225 (2008) 199–208. [31] W. Xiao, Z. Chen, L. Zhou, J. Yang, J. Lu, J. Wang, Micropor. Mesopor. Mater. 142 (2011) 154–160. [32] H. Li, J. Wang, J. Xu, X. Meng, B. Xu, J. Yang, S. Li, J. Lu, Y. Zhang, X. He, D. Yin, J. Membr. Sci. 444 (2013) 513–522.
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Influences of inorganic salts on the pervaporation properties of zeolite NaA membranes on macroporous supports Jianhua Yang ⇑, Huazheng Li, Jing Xu, Jinqu Wang, Xiangdi Meng, Ke Bai, Jinming Lu, Yan Zhang, Dehong Yin State Key Laboratory of Fine Chemicals, Institute of Adsorption and Inorganic Membrane, Dalian University of Technology, Dalian, Liaoning 116024, China
a r t i c l e
i n f o
Article history: Available online 15 November 2013 Keywords: Zeolite NaA membrane Pervaporation performance Ethanol/water/salt mixture Membrane regeneration Salt influence
a b s t r a c t Zeolite NaA membranes were hydrothermally synthesized on the macroporous supports using varyingtemperature hot dip-coating seeding methods, and then were applied to pervaporative dehydration of the alcohol/water/salt mixtures at various temperatures to investigate the influences of salts on the pervaporation performance. Five kinds of salts NaCl, NaNO3, KCl, MgCl2, and CaCl2 were chosen to examine the effects of salt concentration, cations, and anions on the membrane performance. For the model system of ethanol/H2O/NaCl mixture, the water flux decreased largely with increasing salt concentrations which resulted from the reduced water driving force/activity and negative salt physical blocking on the outer membrane surface. For different cations, potassium ion led to the largest flux drop of 58.10%, while sodium ion, calcium ion, and magnesium ion for 25.88%, 22.31%, and 24.91%, respectively. This could be mainly attributed to a joint electrostatic interaction/ion exchange action. Flushing regeneration of the NaA membranes showed that complex flux behaviors were observed for the different salts. The water flux recovered up to 91.23% and 82.47% for the NaCl and NaNO3 system, respectively, indicating that the physical salt blocking effect on the flux drop can be removed therefore is reversible. The water flux recovered to 50.00% and 116.92% for salts of KCl and CaCl2, respectively. The irreversible flux drop probably resulted from the ion exchange and ion blocking effects. This work demonstrates that inorganic salts in the alcohol/H2O mixture decreased the flux or separation selectivity and should be avoided in industrial applications. Ó 2014 Published by Elsevier Inc.
1. Introduction Pervaporation, due to the advantages of the less energy requirement and modular designable over distillation, has become established as a mature technology for the purification of organics in the petrochemical, organic chemicals, fine chemicals, pharmaceutical chemicals, and new energy industries [1–3]. With uniform pores, good mechanical strength, chemical resistance, and thermal stability, zeolite membranes have dramatic advantages over polymeric membranes, especially used in high temperature, chemically aggressive environment containing harsh solvents like dimethylformamide [4], tetrahydrofuan [5,6], etc. Among the various types of zeolite membranes, zeolite NaA membranes are extremely selective for removal of water from organic solutions by PV or vapor permeation because of their high hydrophilicity and suitable pore diameter of 0.41 nm smaller than most organic molecules and larger than water. In 1999, Mitsui Engineering and Shipbuilding Co. firstly put zeolite LTA membranes into industrialization for dehydration of alcohol/water mixtures [7]. Recently, many efforts ⇑ Corresponding author. Tel./fax: +86 411 84986147. E-mail address: [email protected] (J. Yang). 1387-1811/$ - see front matter Ó 2014 Published by Elsevier Inc. http://dx.doi.org/10.1016/j.micromeso.2013.11.013
have been made to prepare zeolite NaA membranes by different seeding methods [8–11] or synthesis procedures [12–14] on various supports [12,15,16] aiming at improving the performance [15–18] and making them feasible for the large-scale applications [19]. As reported in the literatures, the PV performance of zeolite NaA membranes are usually tested by dehydrating binary organic/ water mixtures, and they showed excellent separation performance [20–22]. Despite of the success in preparation and commercial application of zeolite NaA membranes, they have been investigated primarily for dehydration of bioethanol that contains no inorganic salts. On the other hand, the dehydration of organic solvents that used in the fields of organic chemicals, fine chemicals, pharmaceutical chemicals are highly desired. Inorganic salts are usually contained in these organic solvents. Shah et al. [23] studied the salt effects on the pervaporation performance of commercial water selective polymer membranes by dehydrating both the alcohol/water/salt mixtures and the actual pharmaceutical wastes. They found that the salt showed both negative effect on the water flux and separation selectivity at high salt concentrations over 8 wt.% for ethanol/water/sodium chloride mixtures. Similarly, the salts would inevitably interact with the charged zeolite structures
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and influence the separation performance of the zeolite NaA membranes, thus affecting the long-time application of zeolite NaA membranes in the industry. So far, to the best of our knowledge, there is no report on the pervaporative dehydration of alcohol/ water mixtures in the presence of salts for a zeolite NaA membrane. The unclear effects on the performance of zeolite NaA membranes in dehydrating organic/water/salt mixtures possibly limit the application of zeolite NaA membranes. Recently, it has been reported that 100% Na+ rejection could be theoretically achieved with reverse osmosis (RO) using a perfect zeolite LTA crystal lattice [24]. Several researchers have experimentally studied the zeolite membranes for the desalinations of seawater [25–29] or radioactive solutions [30] by pervaporation or RO, they found that the zeolite membranes showed good salt rejection and inorganic salt ions did affect the water fluxes of the membranes. Malekpour et al. [30] have achieved ion rejection of over 99% in concentrating simulated nuclear solutions containing Cs+, Sr2+ and MoO42 while the flux decreased with increasing time. They suggested that the precipitation of the permeated salt was the major reason for both the increased salt rejection and decreased fluxes. Similar results are also obtained by Duke et al. [25–27]. Duke et al. explained the results by an ion exchange/interaction mechanism. However, opposite results have been reported in pervaporative/RO seawater desalination by zeolite NaA [28] and hydroxy sodalite membranes [29]. It was found that the water fluxes in desalinating seawater form Boryeong Beach on the West Sea coast of the Korean peninsula were larger than the pure water system for the zeolite NaA membranes [28]. The surface charge was thought to be an important factor governing the water flux, and the reduced electrostatic interaction between the positive surface charge and the polar water was responsible for the higher flux. The above findings suggested that the salt influence on the permeation behavior of water molecules through the membranes is complex. It is of great value to study the salt influence on the membrane separation performance from the scientific point. Furthermore, it will attract more technological interests and broaden the industrial applications of zeolite NaA membranes. Very recently, in our previous work a varying-temperature hot dip-coating (VTHD) seeding method [2,31] was proposed to prepare NaA zeolite membranes with high PV performance on low cost macroporous supports [32]. The flux of the prepared zeolite NaA membrane reached to 2.38–2.85 kg m 2 h 1 with separation factor larger than 5000 while the total cost of the zeolite NaA membranes was estimated to be reduced by about 40%, promoting the broad application of zeolite NaA membranes. In this work, the influences of inorganic salts on the pervaporation performance of zeolite NaA membranes in dehydrating alcohol/water/salts mixtures were firstly investigated. Five kinds of salts NaCl, NaNO3, KCl, MgCl2, and CaCl2 were chosen to examine the effects of salt concentration, different cations and anions on the membrane performance. Furthermore, the membrane regeneration experiments of the NaA membranes were carried out to further clarify the influence of salt on separation characteristics.
2. Experimental 2.1. Preparation of zeolite NaA membranes Zeolite NaA membranes were prepared on the macroporous tubular alumina supports with an average pore size of 2–3 lm (OD: 13 mm, ID: 9 mm, length: 100 mm, Foshan Ceramics Research Institute, China) by VTHD seeding method. The seed layer on the support was obtained by hot dip coating with large seeds of about 2 lm, followed by rubbing off the loose and superfluous seeds, then hot dip coating the 0.4 lm NaA seeds.
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The molar composition of the starting synthesis solution was Al2O3:5SiO2:50Na2O:1000H2O. After aging at room temperature for 6 h, hydrothermal synthesis of zeolite NaA membrane was performed at 80 °C for 5 h. The details of preparation have been discussed elsewhere [32]. 2.2. Pervaporation experiments After the successful synthesis of zeolite NaA membranes, the time dependence of the separation performance (flux and separation selectivity) through the zeolite NaA membrane was examined by pervaporative separation of 90 wt.% ethanol/10 wt.% water/ 0.4 wt.% sodium chloride mixtures at 75 °C using a home-made apparatus [33]. After the NaA membranes reached the steady state, the PV performances of zeolite NaA membranes in dehydrating ethanol/ water/salts mixtures were carried out to further study the influences of salt on the performance of the zeolite NaA membranes by using the various kinds of salt concentrations and species and various feed ethanol/water concentration mixture. Dehydrations of ethanol/water/sodium chloride mixtures were performed by varying the weights of salt addition keeping ethanol and water content constant in the feed, and then by varying ethanol and water content in the feed while the salt addition was constant. The salt concentrations varied from 0.1 to 0.5 wt.% of the total feed mixtures. For the influences of cations, four kinds of salts NaCl, KCl, CaCl2, and MgCl2 were added into the feed mixtures, respectively. For the influences of different anions, NaCl and NaNO3 were added into the feed mixtures, respectively. Note, the salt concentration is referred to the ratio of the salts over the ethanol/H2O mixture. In a typical experiment, taking 90 wt.% ethanol/10 wt.% water/0.4 wt.% sodium chloride mixture for example, 1.080 g of sodium chloride was dissolved by 26.268 g of deionized water, then 243.732 g of ethanol was added into the clear mixture. Furthermore, zeolite NaA crystal powders instead of zeolite NaA membrane were used to determine the exchange degree of ions to avoid the disturbance of the salts blocked in the non-zeolitic pores. Zeolite NaA crystals were added into the 90 wt.% ethanol/10 wt.% water/0.4 wt.% potassium chloride (or calcium chloride) mixtures, and then heated at 75 °C in a reflux apparatus for the same time with the zeolite NaA membrane pervaporation tests. After the treatment, the LTA crystals were washed by deionized water thoroughly, and then dried at 50 °C for 12 h. After the pervaporative dehydrations of ethanol/water/salt mixtures, membrane regeneration was carried out. In membrane regeneration experiments, the outer side of the membranes was firstly thoroughly washed by pure water. Then the membrane module was placed into the pure water and pure water permeation experiments by pervaporation were carried out at room temperature to wash the whole membrane from the outer side to the inner side of the membranes until the pure water flux reached steady state. The membrane separation performance was evaluated by the permeation flux, separation selectivity, and ion rejection. The permeation flux was calculated by the mass of the permeate, which was collected by a liquid-nitrogen trap in a given time interval. The separation selectivity was determined as a = (yw/yo)/(xw/xo), where yw/yo is the weight ratio of water to organic solvent in the permeate and xw/xo is that of water to organic solvent in the feed. The ion rejection was defined as R = 100 (Ci,F Ci,P)/Ci,F, where Ci,F and Ci,P are the ion concentration in the feed and permeate, respectively. The removed water by membranes was compensated by adding the corresponding amount water into feed in an interval to maintain the constant feed composition. All the experiments were conducted three times to take the average results. The details of the pervaporation tests have been given elsewhere [32].
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2.3. Characterization The morphology of the as-synthesized zeolite NaA membrane was observed by scanning electron microscopy (SEM) using a QUANTA-450 at an acceleration voltage of 20 kV. The elements contained in the membranes were detected by energy dispersive X-ray (EDX) measurements with Ametec Quanta 200 FEG 132-10. The relative content of the elements was measured by the intensity of emitted characteristic X-rays of the sample elements. The collected zeolite LTA crystal powders and permeate were analyzed for cations using inductively coupled plasma mass spectrometer (ICP, Optima 2000 DV). 3. Results and discussion 3.1. Preparation of zeolite NaA membrane The SEM images of the top-surface and the cross-section of the zeolite NaA membranes obtained in the present work are shown in Fig. 1a and b, respectively. It can be seen from Fig. 1a that a compact, continuous, and pinhole-free zeolite NaA membrane composed of well inter-grown cubic-shaped crystals was formed on the macroporous support. The thickness of the NaA membrane is approximately 4–5 lm as shown in Fig. 1b. XRD patterns for the membranes (not shown here) represented the characteristic peaks of LTA zeolite together with the peaks of the support, indicating that pure zeolite NaA membranes were formed on the macroporous supports. These features are in agreement with our previous report [32]. 3.2. Influences of inorganic salts on the PV performance 3.2.1. Time dependence of the PV performance in the presence of salts species Fig. 2 shows the time dependence of the pervaporation performance of zeolite NaA membrane in dehydrating 90 wt.% EtOH/ 10 wt.% H2O/0.4 wt.% NaCl mixture at 75 °C. In the case of dehydrating 90 wt.% EtOH/10 wt.% H2O mixture the zeolite NaA membrane showed constant water flux of 2.32 kg m 2 h 1 with the separation factor of 6000 during the investigated period of 600 min. However, for dehydrating 90 wt.% EtOH/10 wt.% H2O/ 0.4 wt.% NaCl mixture, the water flux of the NaA membrane kept almost constant during the first 80 min, then gradually decreased from 2.17 to 1.72 kg m 2 h 1 from 80 to 300 min, and finally reached steady-state value of 1.64 kg m 2 h 1. In strong contrast the separation selectivity showed a reverse trend. It kept constant during the first 80 min and then gradually increased from 7000 to 17,000, finally get steady at about 17,000. The decrease in water
Fig. 2. The time dependence of the pervaporation performance of zeolite NaA membrane in dehydrating 90 wt.% EtOH/10 wt.% H2O/0.4 wt.% NaCl mixture at 75 °C.
flux and increase in separation selectivity mainly resulted from the physical blocking of the pores on the outer surface and within the zeolite NaA layer by micro-salt crystallites [23]. When the water permeated through the membrane, the salt recrystallized from the mixture (as shown in Fig. 3b), and deposited on the outer side of the membrane (as shown in Fig. 3a). Furthermore, during the permeating, ions would be retained in the pores due to the electrostatic interaction between the surface charge and ions and the zigzag pores of the membrane. The blocking effect of the deposited ions can be supported by the EDX observation in Fig. 4 which shows EDX element contents of zeolite NaA membranes after dehydrating 90 wt.% EtOH/10 wt.% H2O/0.4 wt.% NaCl (a and b) and 90 wt.% EtOH/10 wt.% H2O (c and d) mixtures, respectively. Note, even the calcium and potassium were detected in the NaA membrane, reasonably resulting from the impurity in the alkali sources (96 wt.% NaOH containing trace of K and Ca). Surprisingly, the atom ratio of sodium in the case of salt containing (11.93%) was higher than that of in the case of no salt contained (10.80%). In a word, there were more sodium ions in the zeolite layer after separating 90 wt.% EtOH/10 wt.% H2O/0.4 wt.% NaCl mixtures. This indicated that sodium ions traveled through the non-zeolitic pores and were blocked in the zeolite layer. The salt deposited on the outer surface of zeolite membranes (namely membrane fouling) and ions blocked in the pores both increased the mass transfer resistance, thereby reducing the water flux. Compared with the water molecule (2.6 Å), the ethanol molecules (4.7 Å) is bigger, therefore more ethanol molecules were blocked by the deposited microcrystals and ions, resulting in the increased separation selectivity.
Fig. 1. SEM images of the prepared zeolite NaA membrane (a, the top-surface; b, the cross-section).
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Fig. 3. Optical photos of zeolite NaA membrane after dehydrating the EtOH/H2O/0.4 wt.% NaCl mixtures and the cloudy 95 wt.% EtOH/5 wt.% H2O/0.4 wt.% NaCl mixture. The enlarged photo inserted in picture (a) shows the serious salt deposition on the outer membrane surface clearly.
Fig. 4. EDX results (a and c) and cross-sectional SEM images (b and d) of zeolite NaA membranes after dehydrating 90 wt.% EtOH/10 wt.% H2O/0.4 wt.% NaCl (a and b) and 90 wt.% EtOH/10 wt.% H2O (c and d) mixtures.
Because the deposition of salt microcrystals and ions was a chronic process, it should not play an important role at the beginning of the pervaporation. This is the reason why the membrane flux and separation selectivity kept constant in the first 80 min and then showed a gradually decreasing and increasing trend, respectively. When the salt deposition and ion blocking reached equilibrium or saturated state, the membrane flux reached steady state and kept a constant value. The slight decreased flux by the addition of salts in the feed probably resulted from a lower driving force. Since the partial pressure of water decreased when inorganic salt was added into the feed. Meanwhile, the water activity was reduced by the formation of large ionic clusters (with the effective
size from 8 to 11 Å) consisting of ions surrounded by water molecules [24], and the electrostatic interaction between the polar water and hydrated sodium ion whereby its transport through the membrane was inhibited. 3.2.2. Influences of salt concentrations on the PV performance The membranes used for the investigating effect of salt concentrations and salt species on the separation performance in the following context were reached the steady state, and the mentioned fluxes and separation factor hereafter were steady-state values. Fig. 5 shows the change of the pervaporation performance of zeolite NaA membrane with sodium chloride concentrations of
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Fig. 5. The PV performance of zeolite NaA membrane in dehydrating EtOH/H2O/ NaCl mixtures with a range of 0–0.5 wt.% sodium chloride concentrations at 75 °C.
0–0.5 wt.% in dehydrating 90 wt.% EtOH/10 wt.% H2O/NaCl mixtures at 75 °C. The concentrations of water in the feed mixture were maintained constant at about 10 wt.% to preclude any effects arising out of changes in water concentrations. It is clearly seen from Fig. 5 that the water flux decreased from 2.30 to 1.37 kg m 2 h 1 as the salt concentration in the feed was varied from 0 to 0.5 wt.%. At higher salt concentrations, a significant flux drop was observed. With the increase in salt concentration, the larger decrease in water flux at higher salt concentration also could attribute to the above reasons. With increasing the feed salt concentration, more ionic clusters were formed, the electrostatic interactions were bigger, and resulted in the water activity decreased largely thereby a significant drop in water flux. Moreover, the membrane fouling was got more severe since more salts recrystallized from the mixtures, thus resulting in larger mass transfer resistance and a lower flux. Similar results were got by Cho et al. [28], when the feed NaCl concentration was over 0.13 mol of sodium chloride per liter of water, the water flux decreased with increasing NaCl addition. It should be noted that the feed NaCl concentration in this study was from 0.1 to 0.5 wt.% of the total mixtures, which means the feed NaCl concentration was from 0.17 to 0.85 mol of sodium chloride per liter of water. It is interesting to note that, in all cases, as shown in Fig. 5, the separation selectivity increased significantly in the presence of NaCl. The higher selectivities mainly resulted from the aforementioned physical blocking by the deposited NaCl on the outer surface of the membrane. The salt precipitation on the sur-
Fig. 6. Effects of the feed ethanol concentration on the pervaporation performance in dehydrating the EtOH/H2O/NaCl mixtures at 75 °C.
Fig. 7. Effects of the salt concentration on the pervaporation performance in dehydrating the 90 wt.% EtOH/10 wt.% H2O/0.4 wt.% NaCl mixtures at various temperatures (60 °C, 65 °C, 70 °C, 75 °C).
Table 1 Bare and hydrated ion size. Ion
Bare diameter (nm) [28]
Hydrated diameter (nm) [35]
Na+ K+ Ca2+ Mg2+ Cl NO3 H2O
0.24 0.30 0.20 0.14 0.32 0.36 0.26
0.72 0.66 0.82 0.86 0.66 0.68 0.26
face blocked the non-zeolitic pores and inhibited the ethanol molecular from entering the membrane pores. Thus, ethanol concentrations in the permeate for the EtOH/H2O/NaCl mixtures were lower than that of the EtOH/H2O mixtures. Considering the poor solubility of sodium chloride in ethanol, the influences of salt concentration was mainly affected by the water concentration in the feed mixtures. Experiments were therefore performed by varying the feed water concentrations while keeping the feed salt addition constant. Fig. 6 shows the change of the pervaporation performance of zeolite NaA membrane with the feed water concentrations in dehydrating EtOH/H2O/0.4 wt.% NaCl mixtures at 75 °C. For binary ethanol/water system, the water flux was observed to vary from 3.42 to 1.71 kg m 2 h 1 as the feed
Fig. 8. Comparisons of the water fluxes in dehydrating 90 wt.% EtOH/10 wt.% H2O/ 0.4 wt.% salt mixtures at 75 °C. (Inserted figure shows the variation of the ratio of the flux drop to the fresh flux with different salts.)
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Fig. 9. EDX results (a,c,e) and cross-sectional SEM images (b,d,f) of zeolite NaA membranes after dehydration of 90 wt.% EtOH/10 wt.% H2O/0.4 wt.% CaCl2 or KCl mixtures (a,b for CaCl2 and c,d,e,f for KCl).
water concentration was decreased from 30 to 5 wt.%. The reasons for the flux drop are mainly due to the increasing frictional forces between the ethanol and water molecules and the decrease of the water driving force [4]. In contrast to binary ethanol/water mixture, for the ethanol/water/sodium chloride mixtures, the water flux decreased more rapidly with decreasing water concentration, especially in low water concentration below 20 wt.%. It is probably due to the fact that with decreasing water concentration in the feed, the ratio of salt to water in the feed was increased. The increased ratio of salt to water in the feed resulted in increased electrostatic interactions between sodium ions and polar water molecules. This enhanced forces tended to result in an extra flux
drop. Additionally, the more micro-salt crystallites deposited on the outer surface of zeolite NaA membrane due to the less water in the feed, and then made the membrane fouling more severe, thus resulting in a larger flux drop. As discussed above, the fluxes for the ternary ethanol/water/sodium chloride mixtures were lower than these of the binary ethanol/water mixtures. Fig. 7 shows the temperature dependence of the pervaporation performance of zeolite NaA membrane in dehydrating EtOH/H2O/ NaCl mixtures. It can be observed that the water fluxes increased linearly with increasing temperature, and the flux improvement rates were almost the same for all mixtures. However, in pervaporation experiments performed by Cho et al. [28], a lower flux
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respectively. This was due to the changes in pore size induced by cation exchange as indicated by EDX results in Fig. 9 in which shown the EDX results (a,c,e) and cross-sectional SEM images (b,d,f) of zeolite NaA membranes after dehydration of 90 wt.% EtOH/10 wt.% H2O/0.4 wt.% CaCl2 or KCl mixtures (a,b for CaCl2 and c,d,e,f for KCl). The much stronger peaks of K+ and Ca2+ than those of Na+ strongly indicated the occurrence of cation exchange. The degrees of ion exchange for the two membranes were about 70.52% and 74.64%, respectively, which were obtained by ICP results of zeolite LTA crystals. The pore size of zeolite NaA decreases from 4 to 3 Å after the sodium ion was exchanged by potassium ion, while the pore size increases from 4 to 5 Å when the sodium ion was exchanged by magnesium and calcium. The reduced pore sizes in the case of KCl increased the mass transfer resistances, thus resulting in a larger flux drop compared to that of in the case of NaCl. On the contrary, the increased pore sizes in the case of CaCl2 resulted in a smaller flux drop compared to that of in the case of NaCl. Another important possible reason is that hydrated size of potassium is smaller than that of sodium, magnesium, and calcium (as listed in Table 1). The electrostatic interaction between the polar water and potassium are probably larger than that of between the polar water and the sodium, magnesium, and calcium, respectively, thereby decreasing the water flux significantly. It seemed that the smaller hydrated cation (potassium ion) decreased the water flux more significantly than the larger hydrated cation (sodium ion, magnesium ion, and calcium ion). The influence of the anions on the pervaporation performance was studied by comparing the fluxes for the NaCl and NaNO3 system. The flux drop in sodium nitrate system (about 27.09%) was slightly larger than in the sodium chloride system, indicating that the larger anion (nitrate ion) has more negative effects on the water flux than the smaller anion (chloride ion) due to the more reduction in pore size by the larger anion. As reported in the literature, the zeolite NaA, hydroxy sodalite and MFI membranes are well demonstrated to show good ion rejections in desalination of seawater or radioactive solutions. In this study, ion concentration in the permeate was also analyzed. The measured ion concentrations in the permeate and feed along with the ion rejections are listed in Table 2. Based on these results the zeolite NaA membrane showed excellent salt rejections and reached rejection efficiency over 99.98%, which are almost the same as reported values [28,29]. The rejection function of the NaA membrane can be evidenced by the absence of the salts in the support layer as shown in Fig. 9e. Since the Al2O3 support layer is essentially salt-free (EDX), indicating no ions passed with water through the membrane and deposited there.
Table 2 Ion concentrations in the feed and permeate and ion rejections. Ion +
Na
K+ Ca2+ Mg2+ a b c d e f
a
C
90/10/0.2 90/10/0.4 70/30/0.4 80/20/0.4 90/10/0.3 90/10/0.5 b 90/10/0.4 90/10/0.4 90/10/0.4 90/10/0.4
c
d
60 70 75 75 75 75 75 75 75 75
>2000 >4000 >4000 >4000 >3000 >5000 >4000 >4000 >4000 >4000
T (°C)
F (mg L
1
e
)
P (mg L
1
f
)
0.23 0.33 0.27 0.55 0.46 0.32 0.21 0.42 0.36 0.28
R
99.98 99.98 99.98 99.98 99.98 99.98 99.98 99.98 99.98 99.98
Concentration in the feed (wt.% ethanol/wt.% water/wt.% chloride). Concentration in the feed (wt.% ethanol/wt.% water/wt.% nitrate). Pervaporation temperature. Ion concentration in the feed (calculated values). Ion concentration in the permeate. Ion rejection (%).
improvement rate for the saline aqueous was observed in seawater desalination. They suggested that the increased Na+ adsorption on the zeolite surface with increasing temperature resulted in a decrease in water flux improvement rate. The exact mechanisms at work are still being investigated. As shown in Figs. 6 and 7, compared to the binary mixture, the presence of salts led to a smaller increase in separation selectivity with increasing feed ethanol concentration and temperature, which were similar to these of with increasing feed salt concentrations, indicating separation selectivity of zeolite NaA membrane was improved due to the presence of salt in the feed. 3.2.3. Effects of salt species on the pervaporation performance of the NaA zeolite membranes It is reported that sodium bounded to the aluminosilicate structure can be easily exchanged with the cations present in the feed during pervaporation [34], and then result in pore size changes, thereby affecting the water transport. Besides, it was reported that the anions in the feed largely affected the flux through hydroxy sodalite membrane, reverse effects on the flux were observed between chloride and nitrate ions [29]. In order to investigate the influence of different cations and anions on the water flux of the zeolite NaA membranes in dehydrating ethanol/water/salt mixtures, we changed the sodium chloride into potassium chloride, calcium chloride, magnesium chloride for the cations and sodium nitrate for anions, respectively. The bare and hydrated sizes of these ions are listed in Table 1. Fig. 8 shows comparisons of the water fluxes in dehydrating 90 wt.% EtOH/10 wt.% H2O/0.4 wt.% salt mixtures at 75 °C. It is clearly seen that a significant flux drop (approximately 58.10%) was observed in the case of potassium chloride solution. The flux decreased from 2.10 to 0.88 kg m 2 h 1 while flux drop of 25.88%, 22.31%, and 24.91% was for NaCl, CaCl2, and MgCl2 system,
3.2.4. Membrane regeneration As discussed above, the zeolite NaA membranes showed flux decreases in the presence of different salts. And the flux drop was attributed to the reduced water driving force and activity by
Table 3 Comparisons of pervaporation performances of zeolite NaA membranes in dehydrating 90 wt.% ethanol/10 wt.% water mixtures at 75 °C before and after membrane regeneration.
a b c d e
Membrane No.
Salts
a
M1 M2 M3 M4
NaCl NaNO3 KCl CaCl2
2.28 2.51 2.10 2.60
F. F. (kg m
2
h
1
)
b
S. F. (kg m
2
h
1
)
1.69 1.83 0.88 2.02
Fresh flux (flux of dehydration of the ethanol/water mixture without the addition of salts). Stable flux (flux of dehydration of the ethanol/water/salt mixture reached steady-state value). Regeneration flux (flux of dehydration of ethanol/water mixture after regeneration). Separation selectivity after the membrane regeneration. Ratio of (R. F.)/(F. F.).
c
R. F. (kg m
2.08 2.07 1.05 3.04
2
h
1
)
d
e
15283 12642 14816 159
91.23 82.47 50.00 116.92
Separation selectivity
Ratio (%)
J. Yang et al. / Microporous and Mesoporous Materials 192 (2014) 60–68
the presence of salt. Also, the membrane fouling which was caused by the physical depositions of salts on the outer surfaces of the membrane resulted in a flux drop. The flux is expected to recover once the deposited salts were removed away from the outer surface of the membranes. After the dehydration of ethanol/water/salt experiments, the membranes were thoroughly flushed with flesh water to remove the deposited salts on the outer membrane surface. In order to further remove the salts deposited in the pores, pervaporation experiment of pure water was performed for 0.5– 1 h at room temperature until the water flux reached steady state value. After pure water permeation, the membranes were dried at 50 °C overnight and were applied to dehydration of 90 wt.% ethanol/10 wt.% water mixture. Table 3 shows the pervaporation performance of zeolite NaA membranes in dehydrating 90 wt.% ethanol/10 wt.% water mixture at 75 °C before and after membrane flushing regeneration. The fluxes after membrane regeneration were largely recovered, indicating that the flushing regeneration has a positive effect on the recovery of separation performance. This was due to the removal of the deposited salts on the membrane surface that resulted in the physical blocking of pores and the flux reduction aforementioned. However, in the most cases, the regenerated flux was still lower than the flesh flux in the absence of salt, whereas in the case of CaCl2 the opposite result was obtained. Generally, ion exchange, ion adsorption or thermal expansion can change the pore structure, resulting in the variation in water flux through nanoporous inorganic membranes as well [28]. The terms of thermal expansion and ion adsorption in the present study could be neglected, because the temperatures and the feed compositions in the PV test were maintained the same before and after regeneration. The incorporation of different ions of different charge densities into zeolite structure is well-known to result in changes in the lattice dynamics and spacing [25]. Therefore, one possible reason for the lower flux is due to the ion exchange. For KCl, as mentioned earlier, the pore size of zeolite NaA decreased from 4 to 3 Å after the sodium ion was exchanged by potassium ion, and in the case of CaCl2, the pore size increased from 4 to 5 Å after the sodium ion was exchanged by calcium ion. The decreased pore size would inhibit the water transport through the membrane, thereby resulting in the reduction of flux in the case of KCl. On the contrary, in the case of CaCl2, the increased pore size naturally resulted in an increase in flux. Moreover, the increased pore size likely largely affected the separation ability of the membrane, lowering the separation selectivity of M4, since no visible cracks/defects were found on the SEM images of M4 top-surface (not shown in this study). The exact reasons for the decreased separation selectivity in the case of CaCl2 at work are still being investigated. Another possible reason for the lower flux obtained in this section was the ion blocking effect, since ion exchange was not considered in the case of NaCl and NaNO3. The micro-salt crystallites deposited on the outer surface of the membranes could be easily washed away by the flushing procedure, but the ions blocked in the pores could not be easily removed because of the polycrystalline feature of zeolite membrane and the zigzag pores of both the membrane and support.
4. Conclusions In the present study, the zeolite NaA membranes were hydrothermally synthesized on macroporous supports. The salt effects on the pervaporative separation performances of the obtained NaA membranes in dehydrating ethanol/water mixtures were investigated by using five kinds of salts NaCl, NaNO3, KCl, CaCl2, and MgCl2. The membrane flux showed time dependence in the presence of salts in the ethanol aqueous solution, firstly kept con-
67
stant and gradually decreased then reached the stead state. For all the investigated salts, the water flux decreased in the presence of salt in the feed mixture, and the water flux decreased significantly with increasing salt concentrations in the feed mixture. The flux drop was attributed to a joint reduced water driving force and activity/salt and ion blocking mechanism. Comparisons of the flux in the presence of NaCl, KCl, CaCl2, MgCl2, and NaNO3 showed that different ions had varying influences on the membrane performance. For different cations, the largest potassium ion showed the largest flux drop of 58.10%, whereas the sodium ion, calcium ion, and magnesium ion showed a flux drop of 25.88%, 22.31% and 24.91%, respectively. For different anions (chloride and nitrate), the slightly larger nitrate ion showed a slightly larger flux drop (27.09%). This was attributed to a joint electrostatic interaction/ion exchange mechanism. After the flushing regeneration, the zeolite NaA membranes showed a flux recovery, suggesting the negative salt blocking effect was reversible. However, the flux was still lower than the fresh flux in the most cases, especially in the case of KCl only 50% of the fresh flux was obtained. For CaCl2, the flux was larger than the fresh flux. This could be attributed to a joint ion exchange/ion blocking mechanism. Despite of the excellent ion rejection efficiency over 99.98%, inorganic salts should be avoided in practical dehydration of solvent/water mixtures considering the irreversible flux drop of zeolite NaA membranes. Acknowledgements We are grateful to the financial support from National Natural Science Foundation of China (No. 21076029) and Program for New Century Excellent Talents in University (NCET-10-0286). References [1] M. Kondo, H. Kita, J. Membr. Sci. 361 (2010) 223–231. [2] X. Chen, J. Wang, D. Yin, J. Yang, J. Lu, Y. Zhang, Z. Chen, AlChE 59 (2013) 936– 947. [3] T.C. Bowen, R.D. Noble, J.L. Falconer, J. Membr. Sci. 245 (2004) 1–33. [4] D. Shah, K. Kissick, A. Ghorpade, R. Hannah, D. Bhattacharyya, J. Membr. Sci. 179 (2000) 185–205. [5] A. Urtiaga, E.D. Gorri, C. Casado, I. Ortiz, Sep. Purif. Technol. 32 (2003) 207–213. [6] S. Li, V.A. Tuan, R.D. Noble, J.L. Falconer, Ind. Eng. Chem. Res. 40 (2001) 4577– 4585. [7] Y. Morigami, M. Kondo, J. Abe, H. Kita, K. Okamoto, Sep. Purif. Technol. 25 (2001) 251–260. [8] A. Huang, W. Yang, J. Liu, Sep. Purif. Technol. 56 (2007) 158–167. [9] G. Shao, J. Yang, X. Zhang, G. Zhu, J. Wang, C. Liu, Mater. Lett. 61 (2007) 1443– 1445. [10] K. Kusakabe, T. Kuroda, A. Murata, S. Morooka, Ind. Eng. Chem. Res. 36 (1997) 649–655. [11] L. Tosheva, B. Mihailova, L.H. Wee, B. Gasharova, K. Garbev, A.M. Doyle, Angew. Chem. Int. Ed. 47 (2008) 8650–8653. [12] A. Huang, F. Liang, F. Steinbach, J. Caro, J. Membr. Sci. 350 (2010) 5–9. [13] A. Huang, J. Caro, Chem. Mater. 22 (2010) 4353–4355. [14] A. Huang, N. Wang, J. Caro, Chem. Commun. 48 (2012) 3542–3544. [15] Z. Wang, Q. Ge, J. Shao, Y. Yan, J. Am. Chem. Soc. 131 (2009) 6910–6911. [16] J. Zhang, W. Liu, J. Membr. Sci. 371 (2011) 197–210. [17] A. Huang, W. Yang, Sep. Purif. Technol. 61 (2008) 175–181. [18] C.H. Cho, K.Y. Oh, S.K. Kim, J.G. Yeo, Y.M. Lee, J. Membr. Sci. 366 (2011) 229– 236. [19] K. Sato, T. Nakane, J. Membr. Sci. 301 (2007) 151–161. [20] O.G. Nik, A. Moheb, T. Mohammadi, Chem. Eng. Technol. 29 (2006) 1340–1346. [21] M. Kazemimoghadam, A. Pak, T. Mohammadi, Micropor. Mesopor. Mater. 70 (2004) 127–134. [22] Y. Liu, Z. Yang, C. Yu, X. Gu, N. Xu, Micropor. Mesopor. Mater. 143 (2011) 348– 356. [23] D. Shah, D. Bhattacharyya, A. Ghorpade, W. Mangum, Environ. Prog. 18 (1999) 21–29. [24] J. Lin, S. Murad, Mol. Phys. 99 (2001) 1175–1181. [25] M.C. Duke, J.O. Abraham, N. Milne, B. Zhu, Y.S. Lin, J.C. Diniz da Costa, Sep. Purif. Technol. 68 (2009) 343–350. [26] P. Swenson, B. Tanchuk, A. Gupta, W. An, S.M. Kuznicki, Desalination 285 (2012) 68–72. [27] C. Dotrement, S. Van der Ende, H. Vandemmele, C. Vandecsteele, Desalination (1994) 91–113.
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