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Cooling Crystallization of Sodium Chloride via Hollow Fiber Devices to Convert Waste Concentrated Brines to Useful Products Lin Luo, Jian Chang, and Tai-Shung Chung Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b02818 • Publication Date (Web): 21 Aug 2017 Downloaded from http://pubs.acs.org on August 22, 2017

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Industrial & Engineering Chemistry Research

Cooling Crystallization of Sodium Chloride via Hollow Fiber Devices to Convert Waste Concentrated Brines to Useful Products

Lin Luo, Jian Chang, Tai-Shung Chung*

Department of Chemical and Biomolecular Engineering, National University of Singapore, Singapore 117585

*Corresponding author

Tel: +65-65166645; fax: +65-67791936; Email: [email protected]

ACS Paragon Plus Environment


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Abstract Cooling crystallization via hollow fiber devices emerges as a new technology suitable for salt production from concentrated seawater brine. To generate high quality NaCl crystals, a solid hollow fiber cooling crystallization (SHFCC) system was developed in this study using lab-made PVDF hollow fibers as the heat-exchanger and magnetic stirring as the downstream mixing device. Experiments were conducted to investigate the influences of downstream agitation methods and operation parameters on crystal properties. Narrowly distributed NaCl crystals with a small size of around 35 µm were successfully produced by the SHFCC system combined with ultra-sonication or magnetic stirring. Specifically, the magnetic stirring based agitation was the most effective in facilitating crystal generation with the highest rates of nucleation and crystal salt production due to the enhanced mass transfer in the diffusion-controlled crystallization process. The influences of feed solution and stirring speed on NaCl crystal production were also investigated and optimized. When treating a concentrated synthetic seawater, the SHFCC system with the optimal agitation method demonstrated a maximal nucleation rate of 8.38×108 no./m3s, a high salt production rate of 598.4 g/m2h and a high purity (~99%) of NaCl salt crystals.

Keywords Cooling crystallization; hollow fiber; agitation; SHFCC


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1. Introduction Many desalination technologies such as seawater reverse osmosis (RO) and membrane distillation (MD) have been employed and proposed to overcome the global water scarcity. However, they also create additional challenges on how to mitigate the disposal and environmental issues of concentrated RO and MD brines. To simultaneously enhance water recovery and solve brine disposal problems, new strategies are needed to harvest the values of waste concentrated brines such as salt production

1-6

and osmotic energy generation

7, 8

.

Since the mining industry is also facing problems of mineral depletion, water shortage and high energy consumption 9, harvesting salts from waste concentrated brines may be an alternative approach to produce value-added minerals with relatively low energy requirements.

To generate salts from seawater brines, many crystallization methods have been developed, such as cooling crystallization crystallization

10-12

, evaporative crystallization

13

and membrane

11, 14-18

, etc. Among them, cooling crystallization by mean of nonporous

hollow fibers (or solid hollow fiber cooling crystallization, referred to as SHFCC) has received our attention because of its unique advantages such as uniform cooling of the feed solution, high efficiency, small footprint and the potential use of cold energy. As proposed by Prof. Sirkar and his co-workers, the basic concept of SHFCC involves polymeric hollow fibers serving as heat-exchanging tubes, but the hollow fiber dimensions are orders of magnitude smaller than those conventional metal tubes for heat exchange

19-23

. As

illustrated in Figure 1, the hot concentrated brine solution is pumped through the lumen side of hollow fibers, where rapid cooling takes place by the coolant circulating on the shell side. The brine temperature exiting from the hollow fiber becomes much lower than



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its initial temperature so that supersaturation occurs. Owing to the solubility change, the excess dissolved salt crystallizes and precipitates out from the supersaturated solution at a high rate.

This technology has been tested successfully for both low solubility systems such as paracetamol in water acid in ethanol

21

20

and high solubility systems such as KNO3 in water and salicylic

. The combination of fast cooling via SHFCC (e.g. 20~30°C/s) and the

downstream mixer also realized the successful operation of paracetamol crystallization at 30~40 °C below the metastable zone limit

20

. Thus, compared with the conventional

crystallizer (e.g. mixed suspension mixed product removal (MSMPR)), the SHFCC crystallizer could produce crystals with a nucleation rate of 2~3 magnitude orders higher, crystal mean size of 3~4 times smaller and a narrower crystal size distribution 21, 22. Also, the utilization of polymeric hollow fibers provide advantages of greater corrosion resistance, geometric flexibility and ease of manufacturing and scaling-up

10

. In addition,

the extraordinarily large surface area to volume ratio of hollow fiber modules improves the overall crystallization performance in terms of heat transfer efficiency and precise temperature control of the crystallization solution

21

. In addition, the energy consumption

and overall process cost of SHFCC systems could be further reduced if utilizing other energy sources, such as the waste heat of the concentrated brine discharged from MD 24, 25, and the cold energy from the regasification of liquefied natural gas (LNG)

26, 27

. Herein,

the LNG cold energy refers to the heat adsorption from the ambient surroundings when LNG is re-gasified, which is a high quality energy source to cool other media 28.


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However, to our best knowledge, no research has ever been conducted to investigate SHFCC for salt crystallization from waste seawater brines. Since the salts from seawater (e.g. sodium chloride) are highly soluble in water, the growth of salt crystals during crystallization is diffusion controlled

13

. This indicates the downstream mixing step of SHFCC is important

because it not only improves salt diffusion but also accelerates its crystallization. Among available mixing or agitation methods, mechanical stirring and ultra-sonication are widely utilized to facilitate salt crystallization

29-31

, especially for the supersaturation limited

system. Therefore, the purposes of this work are to (1) demonstrate the possible utilization of SHFCC combined with downstream mixing steps for salt crystallization from NaCl brine and concentrated synthetic seawater, and (2) investigate the operational parameters of mixing processes and their effects on the NaCl salt crystals in terms of crystal generation rates, nucleation rates and crystal size distribution, etc. This study may open up a new direction for salt production from seawater brines by SHFCC with various mixing devices, and the optimization of operation parameters could provide useful insights on the future work of integrated MD and SHFCC systems.

2. Experimental 2.1 Materials The polyvinylidene fluoride (PVDF) Kynar® HSV900 resin was kindly supplied by Arkema Inc. N-methyl-2-pyrrolidone (NMP, >99.5%), ethylene glycol (EG, >99.5%) from Merck were utilized as the solvent and additive during hollow fiber spinning, respectively. Aqueous sodium chloride solutions were prepared by dissolving sodium chloride (NaCl, 99.5%, Merck) in deionized (DI) water at various concentrations as the model seawater and brines. The DI water was produced by a Milli-Q unit (Millipore) with a resistivity of 18 MΩ cm.



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2.2 Preparation of PVDF hollow fibers The single-bore hollow fibers were fabricated via a dry-jet wet spinning process as described elsewhere

32, 33

. Table 1 lists the dope formulation and spinning conditions. After spinning,

the fibers were soaked in water for three days to remove the residual solvents. Then the fibers were freeze-dried in a freeze-dryer (ModulyoD, Thermo Electron Corporation) overnight before further studies. To fabricate hollow fiber modules, seven pieces of dried hollow fibers with an effective length of 20 cm were assembled into a plastic module holder with two ends sealed by a slow cure epoxy resin (KS Bond EP231, Bondtec). The effective area of one hollow fiber module was around 42 cm2. Field emission scanning electronic microscopy (FESEM JEOL JSM-6700) was utilized to investigate the hollow fiber dimensions and morphologies. Before FESEM observation, dry fiber samples were fractured in liquid nitrogen and coated with platinum by a Jeol JFC-1100E Ion Sputtering device.

2.3 Solid hollow fiber cooling crystallizer Figure 2 shows the schematic of the experimental SHFCC setup for cooling crystallization of sodium chloride solutions. The feed tank was held to a constant temperature by means of a water bath on a heating plate (MR Hei-Tec, Heidolph). The hot feed was then pumped through the lumen side of the hollow fiber module by a circulation pump (Masterflex® L/S series) for heat exchange, while the coolant stream (DI water) was circulated on the shell side simultaneously by a refrigerated circulator (Vivo RT4, Julabo) with flowrates measured by flowmeters. Four Fluke 52 II digital thermocouples with an accuracy of 0.1 °C were installed at the inlets and outlets of feed and coolant streams, respectively. The inlet and outlet temperatures of the feed solution were recorded as Tf1 and Tf2, separately, while the cooling water temperatures before and after the hollow fiber module were recorded as TC1 and TC2,


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respectively. Since all downstream mixing steps, vacuum filtration and microscope observations were conducted at room temperature, all samples were cooled down to room temperature of around 23°C before the mixing and crystallization steps by adjusting the flowrates. In other words, the outlet brine temperature (Tf2) of each run was the same of around 23°C before entering the mixing steps.

2.3.1 Downstream agitation methods After rapid cooling by SHFCC, the highly concentrated feed solution became supersaturated, where the NaCl amount dissolved in water was higher than its solubility at Tf2. Different agitation methods, such as magnetic stirring and ultra-sonication, were employed to investigate their effectiveness on salt crystallization and properties. A pure NaCl solution was used as the feed solution with an initial NaCl concentration of 27.25%, which corresponded to the NaCl solubility in water at 70 ºC. The heating plate and chiller temperatures were set at 80 ºC and 18 ºC, respectively. The flowrates of the feed and cooling streams were maintained at about 10 g/min and 200 mL/min, separately. After exiting from the hollow fiber, the chilled feed solution was collected in glass bottles, immediately followed by downstream magnetic stirring or ultra-sonication. For magnetic stirring, it was conducted as a function of stirring duration at 500 rpm by means of a multi-position magnetic stirrer (MIX 15 eco, 2meg), while for sonication, it was conducted by placing the sample bottles into a laboratory ultra-sonicator in a water bath (Ultrasonic LC30H, Elma) at room temperature. For comparison, control samples were also prepared by keeping them still for different durations before the next step. After the agitation step, salt crystals in the slurry were collected by filtration using cellulose filter papers (Advantec, diameter: 90mm, retention characteristic: 5µm) with the aid of a piston vacuum pump (Micro-air, MAJP-40V). Then, the collected salt crystals were thoroughly dried for overnight in an oven for future characterizations.



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2.3.2 Influence of feed NaCl concentration To study the influence of feed salt concentration on crystal generation, NaCl feed solutions were prepared with different salt concentrations; namely, 26.67%, 26.93%, 27.25%, which corresponding to the NaCl solubility in water at saturation temperatures (Ts) of 40, 55 and 70 ºC, respectively. The heating plate and chiller temperatures were set at 50~80 ºC (i.e., 10 ºC higher than the Ts values) and 18 ºC, separately, and the flowrates of the feed and cooling streams were adjusted at about 10 g/min and 100~200 mL/min, respectively. Cooled feed solutions were collected in glass bottles and placed onto the multi-position stirrer for magnetic stirring at 500 rpm. After being separated by filter papers, salt crystals were completely dried in an oven overnight. For the chilled feed solution at Tf2, its relative supersaturation factor σ is calculated as below 21: ߪ=

஼ି஼ ∗ ஼∗

(1)

where C is the actual concentration of the cooled feed stream and C* is the saturated concentration at temperature Tf2.

2.3.3 Influence of stirring intensity To study the influence of stirring intensity on crystal generation by magnetic stirring, a 27.25% NaCl solution was prepared and heated to 80 ºC by the heating bath. After heat exchange, the chilled feed solution was collected in glass bottles and transferred to the magnetic stirrer. Mixing was conducted at different stirring speeds from 200 to 800 rpm. Afterwards, salt crystals were separated and dried completely for future characterizations. The Reynolds number Re of stirring vessels could be calculated as below 34, 35: ܴ݁ =

ఘே஽ మ ఓ


(2)

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where ρ and µ represents the density and viscosity of the solution, respectively, while N is the rotational speed and D is the stirrer bar diameter.

2.3.4 Concentrated synthetic seawater A concentrated synthetic seawater was prepared according to the formulation of artificial seawater from Subow 36, as listed in Table 2. The mixture was heated to 80 ºC followed by twice vacuum filtration using cellulose filter papers in order to remove the insoluble salts under the high temperature. The cooling crystallization of the concentrated synthetic seawater was then conducted under the optimized operation conditions. After magnetic stirring of the cooled feed solution, salt crystals were separated and completely dried in an oven before further characterizations.

2.4 Characterizations of salt crystals Under different operation conditions, NaCl crystals were generated in different rates and sizes as a function of time. As shown in Equation (3), the crystal generation ratio (Rc) was defined as the ratio of the experimentally obtained crystal weight (mt) to the theoretical calculated crystal weight (mtot). The theoretical crystal weight (mtot) was calculated from the NaCl solubility change before and after the cooling for a certain amount of the feed solution. ௠೟

ܴܿ = ௠

(3)

೟೚೟

During the crystal generation, NaCl crystals were withdrawn at predetermined time intervals and analyzed under an Olympus SZX 16 optical microscope with a CMAD3 digital camera. The crystal size distribution was performed on the microscope images with the aid of an image analysis software (Image-Pro® Plus 7, Media Cybernetics). To ensure the accuracy of particle sizes, the contour line of individual crystal or each particle was manually drawn, followed by the automatic measurement of the area. By assuming the crystal as an ellipse 20,



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the circle equivalent diameter (CED) was calculated, which is regarded as the crystal size in this study. At least 300 crystals were measured for each run to ensure the statistical significance of the acquired crystal size distribution.

On the basis of CED and crystal size distribution from microscope images, the nucleation rates during cooling crystallization were estimated 20. In terms of crystal samples observed by the microscope, the number of crystal particles was counted as N0, and the whole mass of these measured crystals (m0) was calculated by multiplying the volume of measured crystals with the NaCl density. Here, the crystal particle volume was calculated using the CED data and taking the particles as ellipses. Assumed that the observed crystal size distribution by the microscope was a representative of the true crystal size distribution, the total weight (mt) and number (Nt) of the experimentally obtained crystals were proportional to the weight (m0) and number (N0) of the microscope-observed crystal sample, respectively. Thus, Nt was estimated by the following equation: ே೟ ேబ

௠

= ௠೟

(4)

బ

where m0, N0 could be obtained from a microscope and software analysis, and mt was the weight of thoroughly dried crystals. The nucleation rate (r, no./m3s), defined as the number of crystals generated per m3 of the feed solution per second, was further calculated by Equation (5), where V is the volume of the collected feed solution and ∆t is the duration of the downstream mixing step. ‫=ݎ‬

ே೟ ௏∙∆௧

(5)

Also, the salt crystal production rate (p, g/m2h), defined as the amount of salt crystals produced per m2 membrane area per hour, was estimated by dividing the total weight of the collected crystals (mt) by the inner surface area of hollow fibers (A) and the total time of the heat exchange and downstream mixing of the crystallizing solution (t):


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‫=݌‬

௠೟ ஺∙௧

(6)

3. Results and discussion 3.1. SHFCC operation conditions In the SHFCC system, hollow fibers are nonporous and essentially work as heat exchangers between two streams. As shown in Figure 3, the as-spun PVDF hollow fibers have relatively dense inner and outer surfaces due to the usage of water as the internal and external coagulants. The fiber surfaces are relatively smooth, which minimizes the possibility of clogging problems inside the fiber. Since the hollow fibers were made of PVDF, which has considerable chemical and solvent resistance as well as excellent hydrophobicity. These characteristics prevent wetting and water penetration across the hollow fibers4. During the cooling crystallization, neither water permeation nor wetting was found for the hollow fiber modules. The hollow fibers have OD/ID dimensions of 1161/947 µm and a thickness of around 100 µm. For polymeric hollow fibers with a wall thickness below 100 µm, their heat transfer coefficients have been reported to be comparable with those conventional metal equipment of 647~1314 Wm-2K-1 10, 21.

The operating conditions during SHFCC are demonstrated in Figure 4, which is a concentration-temperature diagram. The NaCl solubility curve is drawn from the data of a handbook

37

. During the experiments, NaCl feed solutions with different saturation

temperatures of Ts are firstly heated to Ts+10°C, which are represented by dots below the solubility curve in Figure 4. In the SHFCC system, a rapid heat exchange happens between the feed and cooling streams with an extremely high cooling rate of 20~34 °C/s. At the outlet, the cooled feed solutions become supersaturated because their temperatures decrease to 23°C, which are indicated by tri-angle symbols above the solubility curve in Figure 4. At each



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unstable point, the amount of dissolved NaCl is higher than the NaCl solubility at that temperature. Theoretically, with or without mixing, nucleation and crystal growth happen in high rates to produce salt crystals. However, the feed stream exiting the hollow fiber module is still clear, and upon further stirring or ultra-sonication, the crystalizing solution becomes more and more turbid, indicating the crystal generation and the supersaturation depletion. Ideally, after a certain duration of mixing, all the extra dissolved NaCl would precipitate out in the form of salt crystals, while the remaining solution becomes a saturated NaCl solution at room temperature.

3.2 Downstream agitation methods Figure 5 shows the NaCl crystal generation ratio (Rc) as a function of downstream agitation method and time. Crystal samples from magnetic stirring, ultra-sonication and control conditions are labeled as stirred, sonicated and control, respectively. In the beginning at 0 min, the feed stream, which becomes just chilled and collected from the lumen side of the hollow fiber module, is visually observed to be very clear. Its Rc value is less than 0.05, indicating that nearly no salt crystals are generated after the rapid heat exchange. Afterwards, NaCl crystals precipitate out gradually in different rates after applying different agitation methods. Magnetic stirring is the fastest one in facilitating crystal generation with a Rc value approaching 1 within just 3 min. Theoretically, when the crystal generation ratio reaches 1, all the excessive dissolved salt in the supersaturated solution precipitates out in the form of crystals. A further increase in stirring time beyond 3 min cannot improve the crystal harvest any more. In contrast, the crystal generation ratios of the sonicated and control samples increase much slower. Their Rc values reach only 0.86 and 0.43 at 60 min, separately.


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Other than yield, crystal properties are another important aspect in crystallizer operations. The produced crystals are preferred to have a desired dimension with a narrow crystal size distribution

12, 21

. Figure 6a displays the evolution of crystal size (i.e., the calculated

CED based on the particle area from microscope images) with time. The crystal size of the control sample increases from the initial value of 64.3±20.3 µm at 0 min to 128.8±49.4 µm at 60 min due to the gradual crystal growth under quiescent conditions. While crystals produced by stirring and ultra-sonication exhibit obviously smaller sizes of 36.9±13.6 µm and 34.6±16.6 µm at 60 min, respectively. In addition, their crystal sizes are insensitive to the agitation duration for crystallization.

In the process of crystal generation, nucleation and nucleus growth are critical steps in determining the crystal generation ratio and size distribution

20, 29-31

. The produced crystal

size distribution (CSD) is a function of (1) the total number of nuclei formed during the nucleation stage, (2) the duration for agitation and crystallization, (3) the degree of supersaturation and (4) the final system temperature and pressure

29, 38

. In SHFCC, salt

nuclei may start to form when the chilled supersaturated feed solution flows out from the lumen side of the hollow fibers. For the control sample, the growth of nuclei and crystals is obvious since the crystal size increases gradually as a function of time. However, it has a relatively slow depletion rate of supersaturation due to the limited mass transfer rate under quiescent conditions. As a consequence, it has a low crystal generation ratio Rc.

The situation changes for the magnetic stirred sample. The agitation induced mechanical disturbances speed up the onset of nucleation in the supersaturated solution

39

. Agitation

has been reported to enhance the mass transfer and aid the diffusional transport step during nucleation, but impede or disrupt the continual growth of sub-nuclei


31, 39, 40

.


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Therefore, the agitation by magnetic stirring facilities the nucleation step but disrupts the crystal growth, leading to a high rate of crystal generation and a small crystal size. Likewise, ultra-sonication has similar effects on crystallization. Ultrasonic waves produce acoustic cavitation microbubbles in solutions, which repeatedly grow and collapse in synchrony. These phenomena induce large shearing forces and trigger the nucleation in supersaturated solutions

30

. However, the large shearing forces can also break down the

growing crystals and thus increase the number of nuclei, leading to the formation of monodispersed crystals of minimal sizes

29

, as shown in Figure 6b. Compared with the

magnetic stirred sample, the sonicated one exhibits a slower increase in crystal generation rate as a function of time, probably because of a low nucleation rate or crystal growth rate due to the use of a low ultrasonic power or amplitude in this study. Nucleation and crystal growth are two important steps in crystallization. When the system has a low nucleation rate, the nuclei formation is limited, leading to less crystals and a low crystal generation ratio. And, in the second possible situation, when the system has a lot crystal growth rate, a lot of nuclei may be generated in the first step, but a lot of them cannot grow up into crystals, which also results in limited crystals and a low crystal generation ratio.

The crystal size distributions and morphologies of NaCl crystals generated by different methods are presented in Figure 6b and Figure 7, respectively. Compared to the control sample, the crystal size distributions of NaCl crystals formed at 60 min under magnetic stirring and ultra-sonication conditions exhibit smaller sizes and narrower distributions. Their mean crystal sizes are 128.8 µm, 36.9 µm and 34.6 µm for control, stirred and sonicated samples, respectively. The microscope observation on salt crystals reveals that all crystals are transparent. They have a cubical shape which is the typical structure of NaCl crystals. Crystals also form clusters in small groups, especially for those produced


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by the stirring method. This phenomenon arises from the fact that mechanical agitation facilitates heterogeneous nucleation

40

, and also crystals in a stirred suspension are

susceptible to grow via an agglomeration mechanism

41

350~400 µm by traditional forced circulation crystallizers distillation crystallization

12, 43

. Compared to the crystal size of 13, 42

or 60~80µm by membrane

, SHFCC coupled with downstream magnetic stirring could

produce salt crystals of a much smaller size and narrower crystal size distribution. After comparing different agitation methods, mixing by magnetic stirring is chosen for further investigations due to its fast crystal generation, production of uniform and small crystals and ease of intensity adjustment.

3.3 Influence of feed NaCl concentration The influence of feed NaCl concentration on crystal generation is studied by cooling down feed solutions with different saturation temperatures (Ts ). As Ts increases from 40 to 70 °C, the amount of excess dissolved NaCl rises from 0.45 g to 1.54 g per 100g water in the chilled feed solutions at 23 °C (Figure 4). Accordingly, the relative supersaturation factor of the chilled feed solutions at 23 °C rises from 0.013 to 0.043 as calculated by Equation (1), indicating an increasing degree of supersaturation. In other words, a higher value of relative supersaturation factor corresponds to a higher degree of supersaturation in the chilled solution. Supersaturation is a non-equilibrium state, which could be relieved by nucleation and crystal precipitation. During crystallization, the degree of supersaturation may be considered as the driving force. Consequently, both the nucleation and growth rate have a power function depending on the supersaturation factor 41, 44-46. As shown in Figure 8a, when Ts of the feed solution increases from 40 to 70 °C, the NaCl crystal generation ratio Rc increases obviously at the same operation time. In addition, a shorter operation time is observed for Rc to reach 1 when all excess salts precipitate out.



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Furthermore, the yield crystal amounts from different feed solutions are calculated as a function of time in Figure 8b. The feed solution with a Ts of 70°C exhibits the highest and fastest crystal production, the yield crystal amount reaches around 11 mg per g feed solution at room temperature at 3 min. While the production of salt crystals from feed solutions with a lower Ts is relatively slower and much less in amount due to their limited degrees of supersaturation and lower salt concentrations.

Figure 9 shows the crystal size distributions of NaCl crystals produced from different feed solutions under the same disturbance strength of stirring. The average crystal sizes are 37.1±12.0 µm, 32.4±10.6 µm and 33.1±11.0 µm from feed solutions with Ts temperatures of 70, 55 and 40 °C, respectively. The slight difference in crystal size distributions attributes to the different degrees of supersaturation in the feed solutions. At higher supersaturation, the difference in chemical potential between NaCl liquid and solid states is larger, suggesting a higher crystallization driving force for both nucleation and nuclei growth processes, leading to a significantly faster salt production and a relatively larger salt crystal.

3.4 Influence of stirring intensity The influence of stirring intensity on crystal generation is also investigated by adjusting the stirring speed when mixing a chilled supersaturated feed solution with a Ts of 70°C. As the stirring speed increases from 200 to 800 rpm, the Reynolds number of the stirring vessel raises from 515 to 2062 as calculated by Equation (2), indicating that the vigorous mixing improves the turbulence of the fluid flow. As shown in Figure 10, the crystal generation ratio at the same operation time obviously increases with an increase in stirring rate from 200 to


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500 rpm, especially within the first 10 min, while their corresponding crystal size distributions at 3 min decrease from 55.6±15.7 µm to 37.1±12.0 µm with a smaller mean crystal size and a narrower distribution. The variations in crystal generation ratios and crystal size distributions suggest that the crystallization process is mass transport or diffusion rate limited

47

. By elevating the stirring speed, the induced shear force assists the

convective mass transfer during the crystallization process. It not only facilitates the contact of secondary nuclei but also induces crystal breakage

35, 42

. Thus, more vigorous stirring in

this region increases the crystal generation ratio but decreases the crystal size. However, when the stirring speed is further increased from 500 to 800 rpm, there are no significant differences in their crystal generation ratios and crystal size distributions. This is because the mixing intensity is sufficiently high enough so that the diffusional resistance for nuclei growth and crystallization is no longer important. In other words, a stirring speed of 500 rpm is sufficient to provide agitation for the solution and the growing crystals in this system.

3.5 Estimated nucleation rate and salt production rate Nucleation rates (r) of NaCl crystals and the salt crystal productivity (p) from different conditions are estimated according to Equations (5) & (6). As presented in Figure 11, the estimated nucleation rates at the mixing duration of 3 min are within the range of 1.10×107 ~ 1.02×109 no./m3s, which equals to 1.98×109 ~ 1.84×1011 no./m3 by multiplying the mixing duration of 3 min. This nucleation rate range is comparable with the data reported by Ji et al. where the nucleation rate for NaCl crystals was in the order of 109 ~ 1011 no./m3 6. Interestingly, the achieved highest nucleation rate of 1.02×109 no./m3s is 3~4 times higher than that produced by a conventional cooling crystallizer under the batch mode

12

. From

Figure 11a, it is found that the nucleation rate of the magnetic stirring method is significantly higher than those of ultra-sonication and the control experiments, due to the efficient



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agitation and mechanical disturbances of stirring. Also, as shown in Figure 11 b and c, an increase in the degree of feed supersaturation or the stirring speed accelerates the nucleation rate obviously. Since the overall nucleation rate is driven by the degree of feed supersaturation and facilitated by the agitation-enhanced diffusion, stirring the most concentrated feed solution (Ts=70°C) at the highest speed (800 rpm) could produce the highest crystal nucleation rate of 1.02×109 no./m3s. It is worth noting that the accelerated nucleation rate in the case of stirring may come from the heterogeneous nucleation 48 because mechanical agitation could enhance collisions among the embryonic nuclei, formed crystals and the container surfaces. These collisions may decrease the activation energy for heterogeneous nucleation and thus promote this type of nucleation 40.

On the other hand, a higher nucleation rate also contributes to a higher salt crystal production rate. From Figure 11a, the salt production rate is strongly affected by the agitation method. All the production rates are calculated at a total duration of 4 min, including the time for heat exchange and mixing step. Among the three methods, the magnetic stirring method achieves the highest rate of 393.8 g/m2h. In addition, more salts could be produced from the higher concentrated supersaturated solution due to the larger amount of salt dissolved inside (Figure 11b). However, the NaCl crystal production rate is less affected by the stirring speed, as illustrated in Figure 11c. Although the nucleation rate seems to be relatively low when stirring at 200 rpm, the overall crystal growth is still mainly determined by the degree of feed supersaturation. Consequently, stirring at 200, 500 and 800 rpm results in comparable salt productions.

3.6 Crystallization of concentrated synthetic seawater


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Finally, the optimal conditions of the downstream magnetic stirring method is utilized to produce crystals from a concentrated synthetic seawater through SHFCC. Table 2 shows the composition of the concentrated synthetic seawater. During SHFCC, the heating plate and chiller temperatures were set at 80 ºC and 18 ºC, separately, and Tf1 and Tf2 were recorded to be 77.5±0.5 ºC and 22.2±0.3 ºC, respectively, indicating that efficient heat exchange took place between the hot feed and coolant streams. Afterwards, the optimal magnetic stirring speed of 500 rpm was adopted for the crystal production. Figure 12a displays the crystal generation as a function of time at 3 min. The yield of crystals from one gram solution increases with mixing time and gradually reaches a plateau of 17.1 mg/g at 3 min. The trend renders to be very similar to that of crystallizing a pure 27.25% NaCl solution under magnetic stirring in Figure 5. However, there are some differences. Since the concentrated synthetic seawater is a mixture of different salts, the impurity ions affect both the nucleation and crystal growth processes. As a result, it has a relatively larger crystal size of 41.3 ± 17.5 µm (Figure 12b). Compared to crystallization from a pure 27.25% NaCl solution, the crystallization from concentrated synthetic seawater has a slightly lower nucleation rate (i.e., 8.38×108 vs. 9.28×108 no./m3s) but a remarkably higher salt production rate (i.e., 598.4 vs. 393.8 g/m2h).

The high salt production rate is attributed to the existence of impurity ions such as Mg2+. It has been reported that the MgCl2 addition in a NaCl solution will elevate the saturation temperature of the solution and decrease the NaCl solubility

45, 49

, which may outweigh its

inhibition effects on the growth rate of NaCl crystals and lead to a greater salt precipitation rate. In addition, to detect the possible incorporation of impurities, the fully dried crystals were redissolved in DI water, and the ion concentrations were measured by the inductively coupled plasma optical emission spectrometry (ICP-OES). It was found that the total amount



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of impurities in the produced salt crystals was less than 1 wt. %, including 0.3 % Mg, 0.3 % K and trace amounts of Ca, S, etc. The incorporation of impurities is difficult to be completely prevented, it may occur within the crystal lattice due to the inclusion during the crystal growth. However, a proper washing step could help to improve the crystal purity 13, 50. Overall, NaCl salt crystals with a purity of 99% are successfully produced from the concentrated synthetic seawater via SHFCC coupled with a downstream magnetic stirring method.

4. Conclusion In this study, a solid hollow fiber cooling crystallizer is proposed and demonstrated to produce salt crystals from NaCl brines and concentrated synthetic seawater. Experiments were conducted to investigate the influences of operating parameters and downstream agitation methods on the crystal properties. The following conclusions can be drawn from this work: (1) In the newly developed SHFCC system, PVDF hollow fibers are successfully spun and applied as heat exchangers. By controlling the temperatures and flowrates of both feed and coolant streams, extremely high cooling rates of 20~34 °C/s can be achieved. As a result, supersaturation is created and downstream agitation can be conducted at 20~50 °C below the saturation temperature of the feeds. (2) Different agitation methods; namely, magnetic stirring, ultra-sonication and control conditions, have been systematically studied and compared in terms of NaCl crystal generation ratio and crystal size distribution. Both ultra-sonication and magnetic stirring produce uniform crystals with a small size of around 35 µm. However, magnetic stirring induces the fastest crystal generation (Rc=1 at 3 min), the highest estimated nucleation


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rate (9.28×108 no./m3s) and crystal salt production rate (393.8 g/m2h) due to the enhanced mass transfer in the diffusion-controlled crystallization process. (3) A higher NaCl concentration in the feed solution can bring a higher crystal generation rate in terms of nucleation and crystal generation ratios due to the increased degree of supersaturation. A magnetic stirring speed of 500 rpm is sufficient to overcome the diffusional resistance during crystallization and produce both high nucleation and salt precipitation rates. (4) The utilization of concentrated synthetic seawater as a feed solution has been investigated for salt production via SHFCC with a downstream magnetic stirring at 500 rpm. The crystal nucleation rate and salt production rates are found to be as high as 8.38×108 no./m3s and 598.4 g/m2h, respectively, and the yielded NaCl salt crystals also has a high purity of ~99%, indicating the success of NaCl salt production via SHFCC.

Acknowledgments We thank Singapore National Research Foundation under its Energy Innovation Research Programme for supporting the project entitled, “Using Cold Energy from Re-gasiﬁcation of Liqueﬁed Natural Gas (LNG) for Novel Hybrid Seawater Desalination Technologies” (Grant number: R-279-000-456-279). Special thanks are due to Dr. Jian Zuo, for his kind help and valuable suggestions.

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List of Figures Figure 1 Basic concept of the solid hollow fiber cooling crystallization (SHFCC) Figure 2 Experimental setup of the SHFCC crystallizer Figure 3 Morphologies of PVDF hollow fibers Figure 4 Cooling operation conditions of different NaCl feed solutions Figure 5 NaCl crystal generation ratios under different agitation methods as a function of time (feed solution: TS=70°C) Figure 6 a) NaCl crystal size changes as a function of time and b) crystal size distributions (CSD) under different agitation methods (samples at 60 min) Figure 7 Morphologies of NaCl crystals generated under stirring, ultra-sonciation and control conditions (samples at 60 min) Figure 8 a) NaCl crystal generation ratios and b) the amount of crystal produced from different feed solutions as a function of time (agitation conditions: stirred at 500 rpm) Figure 9 Crystal size distribution of NaCl crystals produced from different feed solutions (sample conditions: stirred at 500 rpm for 3 min) Figure 10 a) NaCl crystal generation ratios and b) crystal size distributions of NaCl crystals produced under different stirring speeds as a function of time (sample conditions: feed Ts=70°C, stirred for 3 min) Figure 11 Estimated nucleation rates and NaCl crystal production rates as a function of a) agitation method, b) the relative supersaturation factor of the feed solution and c) the stirring speed (all nucleation rates are calculated at a mixing duration of 3 min, all crystal production rates are calculated at a total duration of 4 min) Figure 12 a) Yield crystal amount as a function of time and b) crystal size distribution of produced crystals from the concentrated synthetic seawater (sample conditions: stirred at 500 rpm for 3 min)



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List of Tables Table 1 Spinning conditions of PVDF hollow fibers Table 2 Synthetic seawater composition


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Heat exchange

Coolant

Hot feed: NaCl brine or concentrated synthetic seawater

Coolant

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42


Solid hollow fiber cooling crystallization

Salt crystals

Table of contents graphic ACS Paragon Plus Environment

Hot feed

Coolant

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48

Coolant


Hollow fiber

Figure 1 Basic concept of the solid hollow fiber cooling crystallization (SHFCC)


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Hollow fiber membrane module

1

Tf1 Feed tank Circulation Pump

Coolant tank

Tf2

Crystal generation

Tc2 Circulation Pump Chiller

Figure 2 Experimental setup of the SHFCC crystallizer


Vacuum filtration


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Cross section

100µm

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Inner surface

Outer surface

1µm

1µm

Figure 3 Morphologies of PVDF hollow fibers

3 ACS Paragon Plus Environment

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38.5

NaCl concentration (g/100g water)

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NaCl feeds before cooling Supersaturated solutions

38.0 37.5

Ts=70°C 37.0

Ts=55°C 36.5

Ts=40°C 36.0

NaCl solubility curve 35.5 10

20

30

40

50

60

70

80

90

Temperature (°C)

Figure 4 Cooling operation conditions of different NaCl feed solutions



Control Stirred Sonicated

1.0

Crystal generation ratio, Rc

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0.9

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Stirred

0.8 0.7

Sonicated

0.6 0.5 0.4 0.3

Control

0.2 0.1 0.0 0

0.5

1

3

5

10

30

60

Duration (min)

Figure 5 NaCl crystal generation ratios under different agitation methods as a function of time (feed solution: TS=70°C) ACS Paragon Plus Environment

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Stirred Sonicated Control

b)

Stirred Sonicated Control

180

15

150

120

Number (%)

a) Crystal mean size (m)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48


90

60

10

5

30

0

0 0

10

20

30

40

50

60

30

Duration (min)

60

90

120

150

180

Crystal size (m)

Figure 6 a) NaCl crystal size changes as a function of time and b) crystal size distributions (CSD) under different agitation methods (samples at 60 min)


210

240

270

300


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48

Stirred

x50

Sonicated

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Control

100µm 100µm

100µm

Figure 7 Morphologies of NaCl crystals generated under stirring, ultra-sonication and control conditions (samples at 60 min)


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1.1

b) 12

1.0 0.9

70°C

0.8 0.7 0.6 0.5

55°C

0.4

40°C

0.3

Ts=40°C Ts=55°C Ts=70°C

0.2 0.1 0.0 0

0.5

1

3

5

10

30

60

Yield crystal amount (mg/g feed)

a) Crystal generation ratio

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48


Ts=40°C Ts=55°C Ts=70°C

10 8 6 4 2 0 0.5

Duration (min)

1

3

5

10

30

Duration (min)

Figure 8 a) NaCl crystal generation ratios and b) the yield crystal amount from different feed solutions as a function of time (agitation conditions: stirred at 500 rpm)


60 l


Ts=40°C Ts=55°C Ts=70°C

20

Number (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48

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15

10

5

0 10

20

30

40

50

60

70

80

90

Crystal size (m)

Figure 9 Crystal size distribution of NaCl crystals produced from different feed solutions (sample conditions: stirred at 500 rpm for 3 min)


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a)

1.1

b)

1.0

20

200rpm-Stirred 500rpm-Stirred 800rpm-Stirred

0.9 15

0.8 0.7 0.6 0.5 0.4

200rpm-Stirred 500rpm-Stirred 800rpm-Stirred

0.3 0.2

Number (%)

Crystal generation ratio

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48


10

5

0.1 0.0

0

0

0.5

1

3

5

10

30

60

10

Duration (min)

20

30

40

50

60

70

80

Crystal size (m)

Figure 10 a) NaCl crystal generation ratios and b) crystal size distributions of NaCl crystals produced under different stirring speeds as a function of time (sample conditions: feed Ts=70°C, stirred for 3 min)


90

100

12

Estimated nucleation rate NaCl salt production

400

10 300

8

Feed solution: Ts=70 Stirring: stir at 500 rpm

6

200

4 100

2 0

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NaCl salt production (g/m2h)

a)

0 Stirring

Sonication

Control

Method

12

12

400

8

300

6

Generation step: Stirring at 500 rpm

4 2


0 0.01

0.02

0.03

0.04

200

100

0 0.05

10

c)

400

8

300

6

Feed solution: Ts=70

200

4 2


0

100


b)


10

Estimated nucleation rate (108 no./m3s)


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48



0 200

400

600

800

Stirring speed (rpm)

Relative supersaturation factor

Figure 11 Estimated nucleation rates and NaCl crystal production rates as a function of a) agitation method, b) the relative supersaturation factor of the feed solution and c) the stirring speed (all nucleation rates are calculated at a mixing duration of 3 min, all crystal production rates are calculated at a total duration of 4 min) ACS Paragon Plus Environment

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20

b)

20

15 15

Number (%)

a) Yield crystal amount (mg/g feed)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48


10

10

5

5

0

0 0

0.5

1

3

5

10

30

10

20

Duration (min)

30

40

50

60

70

80

90

100 110 120

Crystal size (m)

Figure 12 a) Yield crystal amount as a function of time and b) crystal size distribution of produced crystals from the concentrated synthetic seawater (sample conditions: stirred at 500 rpm for 3 min)



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Table 1 Spinning conditions of PVDF hollow fibers Spinning parameters

PVDF hollow fibers

Dope composition (wt %)

PVDF/EG/NMP=15.2/5/79.8

Dope flowrate (mL/min)

1.6

Bore fluid

Water

Bore fluid flowrate (mL/min)

1.5

External coagulant

Water

Air gap (cm)

3

Take-up speed (m/min) Spinning temperature (°C)

1.76 Ambient (23±2°C)


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Table 2 Synthetic seawater composition

Artificial seawater36 Concentrated synthetic seawater

Salt/water

NaCl

MgCl2

MgSO4

CaCl2

KCl

NaHCO3

NaBr

g/kg

26.52

2.45

3.31

1.14

0.73

0.20

0.08

g/100g

36.69

3.39

4.57

1.58

1.00

0.28

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