Membrane Crystallization of Sodium Carbonate for Carbon Dioxide

May 2, 2013 - Nucleation occurs inside the hollow fiber membranes and the crystals grow in the feed container. All experiments were run with recircula...
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Membrane Crystallization of Sodium Carbonate for Carbon Dioxide Recovery: Effect of Impurities on the Crystal Morphology Wenyuan Ye,† Jiuyang Lin,† Jiangnan Shen,†,‡ Patricia Luis,† and Bart Van der Bruggen*,† †

Department of Chemical Engineering, KU Leuven, Willem de Croylaan 46, B-3001 Heverlee, Belgium College of Chemical Engineering and Materials Science, Zhejiang University of Technology, Hangzhou 310014, People’s Republic of China

‡

S Supporting Information *

ABSTRACT: Membrane contactors have been proposed as an advanced tool for CO2 capture from flue gases by absorption in alkaline solutions. However, regeneration of the alkaline reagent and further CO2 sequestration are pending issues. In this paper, membrane-assisted crystallization is proposed for crystallizing Na2CO3, which allows its reuse, after CO2 absorption from flue gases. Due to the presence of compounds other than CO2 in flue gases (i.e., SO2, NOx), other compounds (Na2SO4 and NaNO3) may interfere with Na2CO3 crystallization. This was evaluated by measuring the flux through the membrane and the morphology, crystallography, and purity of the crystals. Furthermore, the presence of NaCl possibly transferred from the osmotic solution to the feed solution was evaluated. The experimental results indicate that the presence of impurities decreases the flux through the membrane due to the decrease of water activity, although there is no influence on the overall mass transfer coefficient. The presence of Na2SO4 affected the morphology of the Na2CO3 crystals while NaNO3 and NaCl had no apparent effect on the crystalline products. It was confirmed that Na2CO3·10H2O was formed during the crystallization. Moreover, the purity of Na2CO3 crystals reaches up to ca. 99.5%. Membrane-assisted crystallization was concluded to be feasible in recovering CO2 as a carbonate salt, which can possibly be reused in the industry. CO2 (about 3.5 GJ/TCO2 removed).10,11 Moreover, amines are thermally unstable and have a high toxicity and volatility, which may produce the emission of vapors to the atmosphere, leading to environmental and health related problems.12 Furthermore, the presence of other potential contaminants in the flue gas, such as SOx, NOx, H2S, fly ash and heavy metals may cause an irreversible loss of amines,11 decreasing the efficiency of CO2 capture in turn impacting plants economy.13 Alternative, more environmentally friendly (inorganic) reagents, such as alkaline solutions, and the application of membrane-based technology have been explored in recent years.14,15 The potential applicability of sodium hydroxide for CO2 capture has been shown in recent publications.16,17 A high removal efficiency of CO2 from a binary mixture containing 10% CO2 in methane has been observed.17 However, the further use or regeneration of the CO2-rich absorbents, (e.g., alkaline carbonate), is still a question mark. Achieving pure solid-state compounds with low water content that can be reused as raw materials is a challenging approach to close the cycle of CO2. For example, if sodium hydroxide is applied in the absorption process, sodium carbonate (Na2CO3) may be reused directly, for example, in the food industry, in the glass industry or the cement industry, depending on its purity. An alternative is the recovery of the original reagent (e.g., NaOH)

1. INTRODUCTION The increase of the atmospheric carbon dioxide (CO2) concentration, which is considered as the main cause of global warming, has received much attention in recent years.1 The concentration of CO2 in the atmosphere has increased considerably, from 280 ppmv in the preindustrial era to 379 ppmv in 2005.1,2 The contribution of anthropogenic sources (e.g., fossil fuel combustion, cement production, etc.) resulting from the continuous expansion of global industrial activities over the past centuries led to CO2 emissions in the atmosphere that are well out of balance with natural absorption in, for example, the oceans.3 This tendency of increasing CO2 concentrations will continue unless less carbon-intensive energy alternatives and energy saving infrastructure using new materials and technologies are realized in the near future.4 Therefore, mitigating CO2 emissions has become one of the most significant global challenges. Carbon capture and storage is to be considered essential for keeping the CO2 concentration in the atmosphere at a stable level.5 The most developed technology for CO2 capture is aminebased absorption (e.g., using monoethylamine as absorbent). Due to the high selectivity of amines toward CO2 and the relatively high CO2 loading ability (about 0.3−0.5 mol CO2/ mol MEA) at low CO2 partial pressure,6 it has a good performance for removing CO2 from postcombustion flue gases.7−9 However, it also has several disadvantages. The most critical problem is that a large amount of energy is needed for the regeneration of the amine absorbents and the removal of © XXXX American Chemical Society

Received: January 13, 2013 Revised: April 25, 2013

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Impurities of NaNO3 and Na2SO4 are expected when the Na2CO3 solution originates from an absorption step with a NaOH solution for the removal of CO2 from flue gases, due to the presence of NOx and SO2 in the gas phase. NaCl can be present due to the use of NaCl as osmotic solution in the crystallization step and transferred to the feed side of the membrane. The effects of the impurities as interfering compounds in the crystallization of Na2CO3 on the flux through the membrane, the overall mass transfer coefficient, the morphology, XRD patterns, and purity have been studied.

by desorption of CO2 from the alkaline carbonate crystals by causticization.16 Crystallization is one of the most applied separation processes for achieving solid crystalline materials from mixed solutions.18 Many factors, such as the composition of the solution and the crystallization rate, have a substantial impact on the properties of the crystals, including the morphology, the control of polymorphism, the structure, and the purity.19,20 This in turn determines the functionality of the crystals. The crystal morphology plays an essential role on downstream processing, such as filtration, drying, compaction, and storage.21 Furthermore, different polymorphisms of the crystals from the same substance were considered as different materials from the point of view of their different physicochemical characteristics. Thus, achieving a high quality of crystals is required, by means of crystallization technologies that allow an easy control of the properties. Membrane-assisted crystallization is an innovative crystallization technology in which a better control supersaturated environment can be achieved, so that crystal nucleation and growth is more uniform and predictable.21,22 Membraneassisted crystallization can be operated at room temperature, or at somewhat elevated temperatures by applying membrane distillation, using low-grade energy, waste, or alternative energy sources such as solar energy or geological energy.23 The energy consumption is lower than for conventional crystallization, such as evaporator crystallization or vacuum crystallization. Membrane-assisted crystallization enables crystals to grow slowly, which favors the crystal purity and shape.24 Four different main configurations for membrane crystallization can be used for creating the vapor pressure gradient as the driving force across the membrane.25 The simplest arrangement is the direct contact membrane process. In this system, the permeate side of the membrane consists of a condensing solution such as concentrated salt solvent (e.g., a concentrated NaCl solution), which contacts the membrane directly. Alternatively, the condensing solution can be replaced by an air gap, a sweep gas, or a vacuum. In this work, the direct contact configuration was used and a concentrated NaCl solvent was used as the hypertonic solution for driving the water vapor from feed solution to the stripping side. Membrane-assisted crystallization has been recently investigated as an advanced technology for protein separation and purification, with an excellent performance to obtain pure protein crystals with a specific structure.20,26,27 It has also been applied for the crystallization of salts.28,29 In addition, membrane-assisted crystallization and vacuum crystallization have been compared by determining the purity of NaCl crystallized from a NaCl mother liquor, doped with KCl.24 It was concluded that membrane-assisted crystallization yields a higher NaCl purity than vacuum crystallization, confirming that the slow crystallization rate in membrane-assisted crystallization enhances the product purity.24 As a consequence, membraneassisted crystallization is a potential alternative to produce Na2CO3 with high purity from solutions containing a mixture of salts in various concentrations. The technical viability for obtaining crystals of Na2CO3·10H2O by means of membrane crystallization was demonstrated by Luis et al. (2013).30 In this work, we aim at studying the possibility of crystallizing Na2CO3 crystals with high purity from a Na2CO3 solution containing impurities, such as NaNO3, and Na2SO4 in order to consider the presence of impurities from flue gases and NaCl from a realistic scenario namely the osmotic solution for crystallization.

2. EXPERIMENTAL SECTION 2.1. Materials. Anhydrous Na2CO3 salt (analytical grade) supplied by VWR (Belgium) was used to simulate the alkaline solution obtained after CO2 absorption with NaOH. Anhydrous NaNO3 (Chem-Lab, Belgium, 99.5%), NaCl (AnalaR NORMAPUR, > 99.9%), and Na2SO4 (ACROS, Belgium, 99%) were used as the impurities. NaCl (AnalaR NORMAPUR, > 99.9%) was used as the osmotic solution in the membrane-assisted crystallization procedure. Ultrapure water (Milli-Q water, Millipore Mili-Q, Billerica, MA) with a conductivity of 0.056 μS·cm−1 was used in this study. All reagents were used as received. 2.2. Experimental Setup. A hollow fiber membrane contactor (MiniModule 1 × 5.5 G543, Liqui-Cel, Membrana GmbH, Germany) was used as the system to perform the membrane-assisted crystallization. The characteristics of the membrane contactor are shown in Table 1. The microporous hydrophobic membrane is the

Table 1. Characteristics of Membrane Contactor and Hollow Fibers Housing Characteristics Material Polycarbonate/polyurethane Cartridge configuration Parallel Flow Hollow Fiber Characteristics Membrane Material Porosity Effective pore size Internal diameter Outer diameter Active surface area Number of fibers

X50 Fiber Polypropylene 40% 0.04 μm 220 μm 300 μm 0.18 m2 2300

physical support that separates the mother solution (i.e., Na2CO3) from the osmotic solution flow (i.e., NaCl) under isothermal conditions. The hydrophobic nature of the membrane prevents in principle both the Na2CO3 and NaCl solution from wetting the membrane. Therefore, the transfer of water between the Na2CO3 solution and NaCl solution occurs inside the pores as vapor. The driving force for the mass transfer mainly depends on the vapor pressure gradient between the bulk solutions at both interfaces of membrane, which is created by a difference in their activities. Figure 1 shows the schematic diagram of the experimental setup for membrane-assisted crystallization. Two peristaltic pumps (Watson Marlow 503S-Belgium and Gilson Minipuls III-The Netherlands) were used to circulate the feed stream and the stripping stream from the cylindrical glasses to the membrane contactor in a counter-current mode. The effects of the velocity (both the feed and osmotic sides) on the mass transfer of water were studied. Consequently, the feed solutions were circulated at constant flow rate of 1200 μm s−1 (6.3 mL min−1) at the lumen side and the osmotic solution was circulated at 150 mL min−1 at the shell side as the optimized conditions for the membrane performance. The feed solution consisted of Na2CO3 as the desired component with a constant concentration of 1.89 mol L−1 Na2CO3 (≈ 200 g L−1), close to the saturation concentration (i.e., 215 g L−1 at 20 °C) in order to ease the crystal formation. The impurities, including NaNO3, NaCl, and Na2SO4, with different concentrations B

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Since the operation temperature is the same at both sides of the membrane module (pf* = pp* = p*, pure water vapor pressure) in this work, eq 2 can be written as:

JW = K totp*(aP − a f )

(3)

In a multicomponent system containing two solutes, the water activity can be calculated by the following equation:32 0 0 a w = (a w1 ) × (a w2 )

where (a0w1) and (a0w2) are the water activity of a solution containing a single component. The procedure to calculate the pure water vapor pressure (p*(T), Pa) is given in Appendix A. The water activity of a single component system is also illustrated in Figure A1 in Appendix A. 2.3. Analytical Methods. Ion Chromatography. The initial crystal samples for the ion chromatography measurements were directly taken from the mother liquor. The remaining crystals were washed five times with saturated Na2CO3 solution, using a filter centrifuge with an acceleration of ac = 1000×g to remove the washing solution. Finally, the initial crystals directly taken from mother liquor and the washed crystals were dried in a vacuum oven until the weight of crystals remained constant. The dried initial crystals and washed crystals were sampled for further ion chromatography measurements, aiming to investigate whether the impurities contained in crystals were adsorbed to the surface or included inside of the crystals. Ion chromatography analysis was used to investigate possible anion impurities (Cl−, NO3−, SO42‑) in the crystals. All analyses were conducted with a Dionex ICS-2000 ion chromatograph and an analytical column IonPac AS18 after a guard column AG 18. The analyses were performed after dissolving 0.1000 ± 0.0002 g of solidstate samples in deionized water. KOH with isocratic concentration was applied as the eluent. The detection limit of the ion chromatograph for NO3− is 0.010 mg L−1, for Cl− it is 0.009 mg L−1 and for SO42‑ it is 0.010 mg L−1. Microscopy Tests. In order to evaluate the crystal morphology of the obtained crystals and explore the possible influences of impurities on the shape of crystals, a morphological assessment was performed for crystal nuclei formed initially in the solution from the membrane contactor and the grown crystals in the solution from the cylinder crystallizer by imaging with an Olympus Microscope equipped with optical head 20× and 5×, respectively. The images of these samples were processed with the HiPic 8.3.0 computer program. X-ray Diffraction Pattern. To investigate impurity patterns of crystals (e.g., (Na2SO4)2·Na2CO3, NaCl, Na2SO4, NaNO3) in crystal samples obtained from membrane-assisted crystallization, the crystals directly taken from mother liquor were detected by X-ray diffraction (XRD) on a Philips PW1830 diffractometer, equipped with a graphite monochromator and a gas proportional detector, using Cu Kα radiation at 30 mA and 45 kV, step size of 0.02° 2θ and counting time 2 s per step, over the 10−70° 2θ range; the mineral identification was performed using DiffracPlus EVA (Bruker) software. Total Water Fraction Measurement. To confirm the exact crystal forms (e.g., Na2CO3·10H2O, Na2CO3·7H2O, Na2CO3·H2O or Na2CO3) of obtained crystals, total water fraction was analyzed. A part of the crystals (∼1.5 g) was weighed in a Petri dish. Then the Petri dish with the crystals was dried at room temperature in a chamber until the constant weight was obtained. It can be assumed that the water adhered to the surface of crystals was totally evaporated. After this period, the Petri dish with dried crystals was weighed again (labeled as Winitial) and then left into an oven at 110 °C since hydrous sodium carbonate crystals are totally converted into anhydrous sodium carbonate above 109 °C.33 The weight of the dish was measured in a certain time interval until it was stable and the crystals were totally changed from transparent colorless crystal form into white powder form. The anhydrous Na2CO3 powder with the Petri dish was then weighed as Wfinal. It was supposed that all the water contained in the crystal was evaporated and anhydrous Na2CO3 was obtained. Therefore, the total water fraction can be calculated:

Figure 1. (A) Schema of one hollow fiber membrane of the membrane contactor. (B) Schematic diagram of the experimental setup. (i.e., 0.2, 0.4, and 0.6 mol L−1) were added to the feed Na2CO3 solution without pH adjustment to investigate the effect of impurities on the crystallization. All the experiments were performed at room temperature (20 ± 1 °C). The concentration of the stripping solution was controlled constantly at 300 g L−1 throughout the experiments by adding the proper amount of NaCl salt to diluted draw solution according to the amount of water permeated to the osmotic side and confirmed by constant conductivity of the osmotic solution. The stripping solution was stirred with a magnetic mixer (Fisher Scientific, Belgium) at 300 rpm to ensure the homogenization of the osmotic solution. Nucleation occurs inside the hollow fiber membranes and the crystals grow in the feed container. All experiments were run with recirculation of the feed and osmotic streams. The rate of solution extraction, given in terms of transmembrane flux, was determined by measuring the weight loss at 10 min intervals of the feed solution according to the following equation:

J(ti) = −

dVp

A dt 1 dm water,p ≈− Aρwater dt =−

1 wf (ti + 1) − wf (ti − 1) Aρwater ti + 1 − ti − 1

(1)

3

where Vp (m ) and mwater,p (kg) are the volume and weight of water permeated from the feed solution to the osmotic solution; ρwater (kg m−3) is the density of water at room temperature; wf (kg) is the weight of the cylinder containing the feed solution at time ti (h), A (m2) is the membrane area. The water flux (J, m3 m−2 h−1) is proportional to the water vapor pressure difference across the membrane, generated by the difference of the water activity in the solution of both sides of the membrane. The overall mass transfer coefficient can be obtained experimentally from the following equation:31

JW = K tot(pP* aP − pf* a f )

(4)

(2)

where Jw is the transmembrane flux obtained from the experiment, Ktot is the overall mass transfer coefficient, af and ap are the water activities, p*f and p*p are the water vapor pressures of the feed solution and draw solution, respectively. C

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certain interval may allow the reduction of concentration polarization on the membrane surface. However, at higher velocities, the mass transfer of water molecule through the membrane tends to reach a plateau value since the mass transport is not affected anymore by the fluid dynamics of the system. Therefore, increasing the flow rate of the osmotic solution has no apparent improvement for mass transfer of water. However, a slight decrease trend appears when the velocity reaches to a certain level. This is due to the channeling effect of the draw solution in the shell side since it was observed during experiments that were performed at a speed of 200 mL min−1 or higher. This channeling effect can result in unexpectedly stagnant parts of the draw solution on the shell side, which may consequently lead to a superdiluted draw solution in these stagnant places and decrease the concentration gradient. This means that a decrease of the driving force for water vapor transportation may occur, thereby declining the mass transfer of water.35,36 Therefore, 150 mL min−1 was fixed as the optimal flow rate in the shell side in this study. In order to evaluate the effect of impurities on the mass transfer coefficient of the membrane contactor, the transmembrane flux over time and its corresponding supersaturation were studied. The transmembrane flux of the membrane contactor and the supersaturation generated in the membrane contactor are shown in Figure 3. It can be clearly seen that the transmembrane flux through the membrane contactor decreases progressively and the supersaturation of the feed solution gradually increases over time. When the supersaturation reached a certain level (around 1.30), visible crystals were first observed at the bottom of the feed solution cylinder. Then experiments were stopped in order to avoid membrane blockage. When the supersaturation of the feed solution is beyond a critical level, a sharp flux decline may occur due to the membrane blockage caused by rapid crystal growth on the membrane surface.37 As observed in Figure 3, no drastic flux decrease was observed. Thus, no membrane blockage occurred in this study. However, it is of great importance to control the supersaturation rate of the feed solution to prevent membrane blockage in the membrane crystallization system, especially on industrial scale. The average transmembrane flux of the Na2CO3 solution of 200 g L−1 with different impurities obtained in the membrane crystallizer is shown in Figure 4. It can be observed in Figure 4 that the average transmembrane flux decreases with the increase of the concentration of other compounds. The decrease was 1.0, 1.7, and 3.1% at a concentration of 0.2 mol L−1 of NaNO3, NaCl and Na2SO4, respectively, compared to the transmembrane flux of the reference solution. When a higher concentration of other compounds as impurities is present, a larger decrease was observed, that is, 6.3, 8.0, and 10.4% for NaNO3, NaCl and Na2SO4 at 0.4 mol L−1, and 13.0, 14.8, and 16.4% for NaNO3, NaCl and Na2SO4 at 0.6 mol L−1. These results were expected since adding impurities to the feed solution increases the total concentration of salts and decreases the driving force through the membrane. As reported in the literature,26 the transmembrane flux in membrane-assisted crystallization is related to the partial vapor pressure difference, which depends strongly on the difference of water activity between the draw solution and the feed solution. Since impurities are added into the feed solutions, the water activities of mixed solutions decrease. Moreover, since transport of water vapor takes place from the feed solution to the draw solution, the concentration of feed

(5)

where Winitial is the initial gross weight of the dried crystals plus the Petri dish, Wfinal is the final weight of the anhydrous Na2CO3 plus the Petri dish and Wholder is the weight of the Petri dish.

3. RESULTS AND DISCUSSION 3.1. Effect on the Membrane Mass Transfer Coefficient. In order to explore the main factors that can affect the mass transfer of water through the membrane module, the effect of the velocities at both the lumen side and shell side on the performance of mass transfer of water were extensively studied. The results in Figure 2a indicate that increasing the

Figure 2. Transmembrane flux as a function of the velocity. (a) Feed solution in the lumen side; (b) osmotic solution in the shell side.

velocity of the feed solution has a positive effect on the transmembrane flux (almost linear). This effect may be produced because the high flow rate allows a reduction of concentration polarization on the membrane surface, which in turn leads to an enhancement of the mass transfer coefficient in the boundary adjoining the membrane surface.34 However, a minimum flow rate inside the hollow fiber is necessary from the point of view of exergy analysis.30 In this work, a velocity of 1200 μm s−1 was considered to keep a high transmembrane flux. Another important factor that may affect the mass transfer of water molecule through the membrane module is the velocity of the osmotic solution at the shell side of the membrane module. Results shown in Figure 2b indicate that at a low velocity interval, the mass transfer of water molecule can be enhanced since increasing the velocity in the shell side within a D

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The overall mass transfer coefficient Ktot of the Na2CO3 solution (200 g L−1) with different components was estimated experimentally and calculated using eq 3. As shown in Figure 5, the mass transfer coefficient is almost constant; changes are within the range of uncertainty. This

Figure 5. Overall mass transfer coefficient as a function of the concentration of impurities.

indicates that the mass transfer coefficient is irrespective of concentration of impurities and the kind of impurities. In addition, the observed fluctuation of the mass transfer coefficient may be related to the change in physical properties of the feed solution.32 The physical properties of solution such as density, viscosity and diffusivity affect the membrane film coefficient and therefore, the water vapor diffusing through the membrane.32 3.2. Effect on the Patterns of Crystals. Figures 6, 7, and 8 show the XRD patterns of crystals when impurities of NaNO3 (Figure 6), NaCl (Figure 7) or Na2SO4 (Figure 8) are present in the feed solution. It can be observed that the impurities have no obvious effect on the crystallography of Na2CO3 formed by membrane-assisted crystallization. The formed Na2CO3 in the solution is mainly Na2CO3·10H2O. This observation can be confirmed by the total water fraction measurements (see Section 3.4). From the XRD patterns in Figure 6 and Figure 7, no apparent crystals formed from impurities were observed except for the peaks of Na2CO3 crystals. These results are different from other methods such as crystallization of Na2CO3 from aqueous solutions of Na 2 CO 3 and Na 2 SO 4 by evaporation,38 in which cocrystallization of the impurities was observed. In addition, when using a common heat evaporator, different crystals can form after crystallization in the Na2CO3 solution with a high concentration of Na2SO4, since the evaporator is operated at a temperature above 100 °C.39 In general, the burkeite crystal, Na2CO3·2Na2SO4, predominantly forms as the mole ratio between Na2CO3 and Na2SO4 is lower than 5 in the solution. When this ratio is between 5 and 7, sodium dicarbonate crystals are formed. At a ratio above 7, sodium carbonate crystals are mainly found in the solution.38,39 However, the sodium carbonate crystals formed at a high ratio of 12 have a lower purity, ca. 99.0%.40 In this work, the initial ratio between Na2CO3 and Na2SO4 in the feed solution was 9.45, 4.73, and 3.15 at a Na2SO4 concentration of 0.2, 0.4, and 0.6 mol L−1, respectively, and the ratio would be much lower during the crystallization because of the concentration of the feed solution. As illustrated in Figure 8, no peaks in the XRD patterns related to compounds other than Na2CO3 appear. The reason may be that the ions from impurities cannot replace the

Figure 3. Time dependency of the flux and supersaturation of feed solutions with different impurities. (a) NaNO3; (b) NaCl; (c) Na2SO4.

Figure 4. Average transmembrane flux as a function of the concentration of impurities in a feed solution of 200 g L−1 Na2CO3.

solution increases, resulting in a decrease of the water activity over time. E

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Figure 7. XRD patterns of crystals obtained from solutions of Na2CO3 mixed with different concentrations of NaCl.

Figure 6. XRD patterns of crystals obtained from solutions of Na2CO3 mixed with different concentrations of NaNO3.

determines the shape of the crystals. This effect was also observed by Sun et al.42 during the crystallization of lithium carbonate (Li2CO3): both NO3− and Cl− caused a slight decrease of the induction time, while SO42‑ produced a noticeable increase of the induction time. Thus, NaNO3 and NaCl accelerated the growth of Li2CO3 but Na2SO4 slowed down the growth rate. Since Na+ and Li+ are part of the same group in the periodic table, they have similar properties and a similar influence of NaNO3, NaCl and Na2SO4 can be assumed on the crystallization of Na2CO3. Crystals that were sampled from the feed solution directly coming out from the outlet of the membrane contactor are shown in Figure 12. These crystals were obtained from different experiments using pure Na2CO3 solution (Figure 12a) and Na2CO3 solutions with 0.4 mol L−1 of NaNO3 (Figure 12b), NaCl (Figure 12c) and Na2SO4 (Figure 12d). In all cases, including the crystals formed in the presence of SO42‑, the same prismatic structure as Na2CO3·10H2O crystal is obtained.43 This allows claiming that impurities exerted no influence on the crystal growth in the hollow fiber membrane contactor until crystals arrived at the cylinder containing the feed solution and grew in the cylinder. This effect may happen because the hydrophobic porous membrane offers controllable supersaturation even during the crystallization process, and also benefits from the crystal growth deceleration, avoiding the step of bunching formation and thus giving more time for a better orientation of the crystal growth unit. Furthermore, the laminar flow in the hollow fiber membrane provides the opportunity for a better arrangement of the molecules, resulting in a better structured crystalline lattice and eventually, a better particle shape and purer products.24

carbonate ion in the lattice due to the low energy supply to ignite at room temperature. However, it may occur at high temperature.38,39 This demonstrates that the crystals formed in the feed solution have a high purity, regardless of the concentration of other compounds that may interfere. Thus, membrane crystallization avoids the cocrystallization of impurities, leading to crystallization products with high purity, thanks to the low evaporation rate and low temperature.24,38,39 3.3. Effect on the Morphology of Crystals. The morphology of crystals obtained from Na2CO3 solutions doped with impurities of NaNO3, NaCl and Na2SO4 at different concentrations is shown in Figure 9, Figure 10, and Figure 11, respectively. The images of grown crystals in Figure 9 and Figure 10 demonstrate that the morphology of the crystal surfaces were approximately the same as the reference compound. The morphology of the crystal surface has a strong dependency on the growth step.41 Thus, it can be concluded that NO3− and Cl− impurities had no significant influence on the Na2CO3 crystal growth rate. The presence of SO42‑ (Figure 11), however, has a strong influence on the shape of the crystals and triclinic crystals are obtained. It is proved that impurities affect the crystal morphology by being adsorbed to the growing surface of crystals.41 This results in a reduction of the material delivery to the crystal surface or a decrease of the specific surface energy for crystal growth or the interruption for the surface sites and the prevention for the growing steps of crystals.41 In this case, the impurities of Na2SO4 may adsorb on a certain surface of Na2CO3 crystals when they were growing, which slows down the growth steps of that specific surface, and F

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Figure 10. Microscopy images of the grown crystals of Na2CO3 in the cylinder. (a) Reference solution; (b−d) 200 g L−1 Na2CO3 solution with 0.2, 0.4, and 0.6 mol L−1 NaCl, respectively.

Figure 8. XRD patterns of crystals obtained from solutions of Na2CO3 mixed with different concentrations of Na2SO4.

Figure 11. Microscopy images of the grown crystals of Na2CO3 in the cylinder. (a) Reference solution; (b−d) 200 g L−1 Na2CO3 solution with 0.2, 0.4, and 0.6 mol L−1 Na2SO4, respectively.

fraction: 62.94%), which further proves that the patterns of the crystal samples are Na2CO3·10H2O. The growth of the certain polymorphic form of Na2CO3 in the solution can be explained from the thermodynamics of the system. Phase diagrams (shown in Figure S1 and Table S1, Supporting Information) of soda ash-water system exhibit the main road maps explaining the thermodynamic formation of Na2CO3.44 It can be learned from the phase diagrams that the formation of Na2CO3 crystal in the solution is influenced by temperature and the concentration of Na2CO3 in the solution. In this work, the membrane crystallization procedure is carried out at a temperature of 20 ± 1 °C, The crystallization of Na2CO3 occurs when the concentration of Na2CO3 in the solution is approximately 300 g L−1 as calculated, which is lower than 30%. Therefore, Na2CO3·10H2O is formed in the solution when the critical supersaturation is reached. Na2CO3·7H2O could be formed when the concentration of Na2CO3 ranges from 32 to 45.7% within a range of temperatures between 32.5 and 35.4 °C, which is estimated from the data taken from the literature.44 The mixtures of Na 2 CO 3 ·7H 2 O/Na 2 CO 3 ·10H 2 O or

Figure 9. Microscopy images of the grown crystals of Na2CO3 in the cylinder. (a) Reference solution; (b−d) 200 g L−1 Na2CO3 solution with 0.2, 0.4, and 0.6 mol L−1 NaNO3, respectively.

3.4. Total water fraction of crystals. Table 2 shows the total water fraction calculated from eq 5 in the crystals collected from the feed container. It can be seen that the concentration and the kind of impurities have no influence on the water fraction of crystals. It can be also observed that all the water fractions of the crystals from mixed feed solutions are not significantly different to that of the crystals from the feed solution without added impurities and also to that obtained from a theoretical calculation for Na2CO3·10H2O (Total water G

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Figure 12. Microscopy images of Na2CO3 crystals obtained from the membrane contactor. (a) Crystal from reference solution; (b−d) 200 g L−1 Na2CO3 solution doped with 0.4 mol L−1 NaNO3, NaCl and Na2SO4, respectively (Note: The sizes of the crystals are different because they were not pictured at the same time).

NaNO3, Na2SO4). However, impurities can be adsorbed on the surface of crystals. This kind of impurities can be removed by proper postprocessing such as a certain washing process for obtaining ultrapurified crystals. Figure 13 shows the fractions of impurities present in the feed solution in the crystals without washing the surface with a saturated Na2CO3 solution and in the crystals after washing, measured with ion chromatography. The concentration of impurities WNaNO3, WNaCl and WNa2SO4 were plotted on logarithmic scale. As shown in Figure 13b and c, the concentrations of NaCl and Na2SO4 in the feed solution had an influence on the purity of the initial crystals from the cylinder without previous washing, for instance, as high as 4.41% of NaCl and 13.53% of Na2SO4 in crystals from the 0.4 mol L−1 NaCl and the 0.6 mol L−1 Na2SO4 doped solutions, respectively. However, less than 0.04% NO3− (Figure 13a) was composed in nonwashed crystals; thus, much less NO3− than Cl− and SO42‑, was adsorbed to the surface of the crystals. Nevertheless, there was no significant influence on the final washed crystals regardless of the concentration and the kind of impurities. After proper washing, the impurities in washed crystals were similar or even lower than the reference values obtained from the commercial Na2CO3 used in this work (NaCl: 0.037%; NaNO3: less than 0.034%; Na2SO4: 0.558%). The trace amount of impurities in the final crystals may be caused by the inclusion during the crystal growth period. This kind of impurities was difficult to be washed out. The final impurity of Na2SO4 (around 0.550%) in crystals was almost 10 times higher than that of NaCl (around 0.059%) and NaNO3 (less than 0.034%), even in the reference sample. This claims that SO42‑ exerted more influence than Cl− and NO3−on the final purity of Na2CO3 crystals by the way of inducing inclusions. The final purity of washed crystals from mixed solutions is shown in Table 3. Since NaNO3 was not detected in any

Table 2. Determination of the Total Water Fraction of the Obtained Crystals sample Theoretical value

Reference crystala Impurity concentration (mol L−1)

total water fraction (%) Na2CO3·10H2O Na2CO3·7H2O Na2CO3·H2O NaNO3

NaCl

Na2SO4

a

0.2 0.4 0.6 0.2 0.4 0.6 0.2 0.4 0.6

62.94 54.31 14.51 62.96 62.85 62.91 62.83 62.91 62.85 62.95 63.02 62.97 62.98

± ± ± ± ± ± ± ± ± ±

0.22 0.19 0.17 0.18 0.16 0.11 0.17 0.17 0.15 0.15

Crystals from pure Na2CO3 solution.

Na2CO3·7H2O/Na2CO3·H2O will precipitate when the concentration of Na2CO3 in the solution is higher. Thus, membrane-assisted crystallization technology can be applied to achieve a certain polymorphic form of Na2CO3 crystals (i.e., Na2CO3·10H2O) with high purity even in the feed solutions with high concentrations of impurities by controlling the supersaturation of the feed solution and the working temperature. 3.5. Effect of Impurities on the Purity of Crystals. As mentioned, from the point of view of the morphology of the nuclei (Figure 12) and grown crystals (Figures 9−11), the XRD patterns (Figures 6−8) and the total water fraction (Table 2), it can be stated that the crystals were mainly composed of Na2CO3·10H2O without any other form of crystals, such as double-salt crystals Na2CO3·2Na2SO4 or other salts (e.g., NaCl, H

dx.doi.org/10.1021/cg400072n | Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design

I

NaCl

0.4