Hybrid control mechanism of crystal morphology modification for

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Hybrid control mechanism of crystal morphology modification for ternary solution treatment via membrane assisted crystallization Xiaobin Jiang, Guannan Li, Dapeng Lu, Wu Xiao, Xuehua Ruan, Xiangcun Li, and Gaohong He Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.7b01424 • Publication Date (Web): 11 Dec 2017 Downloaded from http://pubs.acs.org on December 16, 2017

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Crystal Growth & Design

Hybrid control mechanism of crystal morphology modification for ternary solution treatment via membrane assisted crystallization Xiaobin Jianga,b, Guannan Lia, Dapeng Lua, Wu Xiaoa,b, Xuehua Ruanb, Xiangcun Lia, Gaohong He*a,b a State Key Laboratory of Fine Chemicals, Engineering Laboratory for Petrochemical Energy-efficient Separation Technology of Liaoning Province, School of Chemical Engineering, Dalian University of Technology, Dalian, Liaoning 116024, China b Petrochemical Energy-efficient Separation Technology Engineering Lab of Liaoning Province, Dalian, Liaoning 116024, China

Abstract Herein, the hybrid control mechanism of the crystal morphology modification for the treatment of a classic industrial ternary solution system (NaCl-EG-H2O) via membrane assisted crystallization was demonstrated. Solution concentration and component diffusion played the hybrid role on determining the polymorphic outcome of the crystal products. Metastable zone width under various operation temperatures and solution composition was simulated and validated by experimental results. The impact of the dominating growth mechanism (diffusion controlled growth or polynuclear growth) on the crystal morphology was also investigated. Optimized operation route aimed to simultaneously improve the crystal morphology and the separation effect was then developed based on the hybrid control mechanism. The

*

Corresponding author: e-mail: [email protected], Phone: +86-411-84986291, Fax: +86-411-84986291

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Crystal Growth & Design 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 49 50 51 52 53 54 55 56 57 58 59 60

solvent loss decreased from 4.8 to 1.2 wt%. Benefited from the improved crystal morphology, the operative duration of corresponding downstream was also shortened. Advanced membrane assisted crystallization was a promising technology towards targeted crystal morphology for high-end solid products.

1. Introduction Due to the multiple requirements of water shortages and resource reuse, the treatment of industrial wastewater and recovery of the high value-added solid products from the wastewater is a pivotal topic.1-3 Traditional treatment approaches for treating multi-component wastewater with high salinity and organic solvent (which is commonly found in petrochemical, biochemical and pharmaceutical engineering) involve vacuum evaporation crystallization (VEC), membrane based-hybrid separation technology, etc.4-8 Among these approaches, membrane distillation crystallization (MDC) that combines membrane distillation with traditional crystallization process is attractive and significant by simultaneously recycling valuable salt crystals with high purity and pure volatile solvents with high recovery. 3, 9-12

In addition, great improvement in transmembrane flux and high thermal energy

efficiency were clearly demonstrated using the developed membranes.13-17 While, for the hybrid functions presented by the microporous hydrophobic membranes, the controlled mass transfer across the membrane and the promoted heterogeneous nucleation at the membrane interface both influenced the formation of crystals. Besides the size distribution, the resulting crystal products with various shapes and morphologies strongly influence the downstream process (washing,


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centrifugation and drying, etc.) and final product quality.18, 19 Thus, revealing the control mechanism of the MDC process (which involve membrane property, operation condition, membrane separator property, etc.) on the crystal size distribution and morphology is of great importance to crystallization technology development. Similar to the reaction solidification process20-23, the control mechanism of the crystal formation is also related to two processes, diffusion and interface reaction. The size distribution and particle morphology of crystals is very sensitive to the thermodynamic property of the solution system and the kinetic parameters that generating the crystallization driving force, such as temperature, supersaturation degree, diffusion coefficient, etc. With the rapid development of the nucleation theory of the high density and viscosity solution system

24-27

, the kinetic competitive

relationship between nucleation and growth in the non-perfect fluid and the heterogeneous interface turns into a complicated process28-31. As to the competition between the above processes, mesoscience has been proposed as a possible general concept for describing complex multiphase or multicomponent systems far from equilibrium32-37. Under the mescoscale control mechanism, the improved solvent separation via membrane distillation enhanced the crystallization driving force, which may deviate the nucleation and crystal growth from the steady state; meanwhile, the promoted nucleation and terminal crystal morphology at the membrane interface is determined by the diffusion rate and surface reaction rate. As the two key controlling factors during MDC process, the uncovering of the control mechanism of the solvent



separation efficiency and crystal morphology will definitely benefit the approach of the operation route design to MDC or other similar membrane based hybrid crystallization process. Hence, the proper process control to fulfill the product specification necessitates the understanding of the dynamic solution concentration and accompanying nucleation in the developed novel crystallization system. In this work, the control mechanism of crystal morphology of MDC for multi-solvent solution system was investigated. One of classic multi-component high saline wastewater, water-ethylene glycol (EG)-NaCl solution system, was introduced for the evaluation. The various nucleation barrier of MDC, polynuclear growth and diffusion controlled growth mechanisms (which have significant influence on the crystal morphology and size distribution) was emphasized and simulated. The resulting crystals with specified morphology and size distribution, the solvent separation efficiency were highlighted. The advanced operating route of MDC for crystal morphology modification and separation efficiency improvement was then outlined, validated and discussed.

2. Theory In this paper, the up limit of the metastable zone width (MSZW) is introduced. MSZW depends on various factors, temperature, seeds and the supersaturation variation rate, etc38. For the nucleation in MDC, the crystallization solution is unseeded and considered as ideal pure under static flow state at the membrane boundary layer. Thus, the supersaturation variation rate is the key model parameter of the MSZW ∆Cmax , which represents via the exceeding concentration from the


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equilibrium state. Theoretical MSZW analysis approach proposed by Sangwal and other researcher is employed39-42. As for the membrane distillation, the concentration of the feed flow keeps increasing during the solvent removal process. In the vicinity of metastability, the nucleation rate J (#·m-3·s-1) can be approximatively expressed as

J = k (∆C max )

n

 dC ∗    FC *  dt  t = = t NAN ∗    m0 − ∫ Fdτ  N A N ∗   0  

(1)

where n is the nucleation order and k is a nucleation constant; m0 is the initial mass of the solution; F (kg·s-1) is the mass rate of the solvent transmembrane; C * (mol·m-3) is the equilibrium concentration of the solution at a certain temperature. N* is the number of molecular (or ion) in the critical nucleus. NA is the Avogadro constant.

F with the membrane area A (m2) is expressed as

F = J P A = Pm ∆pA

(2)

where Jp (kg·m-2·s-1) is the permeate flux and Pm is the membrane permeability; ∆p (Pa) are the difference of partial pressure between the feed side and the permeate side of the membrane interface. t

In addition, at the initial nucleation stage, m0 ≫ ∫ Fdτ ; if the density of the solution 0

has not significantly changed during the concerting procedure, Eq. (1) can be reduced to k (∆C max ) = n

Pm ∆pAC * ρVN A N ∗

(3)

where ρ (kg·m-3) is the average density of the solution and V (m3) is the volume of the solution. Thus, the MSZW limitation ∆C max can then be expressed as



ln (∆C max ) =

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1  C ∗  1  Pm ∆pA   + ln  ln   n  kN A N ∗  n  ρV 

(4)

It is clear that the nucleation barrier is determined by the equilibrium state of the solution system ( C ∗ ), the driving force of solvent removal ( ∆p ), membrane property (Pm) and the scale ratio of the membrane separator and crystallizer (A/V). With the membrane introduced crystallization process, the heterogeneous nucleation rate BHEN is given by43

BHEN = AS max exp( −

3 16πv 2γ eff , por

3(k BT ) ln 2 S max 2

(5)

)

where Smax is the relative supersaturation of the up limit of the metastable zone, which

(1 + ∆C

is equal to

max

C ∗ ) ; as to A, for volume diffusion control process,

A = (k B T v γ ) DC ln S 12

A = (4π 3v )

13

(γ

k B T ) DC 12

and

for

interface-transfer

control

process,

44

; v (m3) is the volume of the single molecular; kB is the

Boltzmann constant, 1.381×10-23 J/K; T (K) is the system temperature. -2 γ (J·m ) is the specific surface energy of the crystal faces present in the equilibrium

form of the crystal 45, and can be obtained by

γ=

βk BT v

23

ln

1 vC *

(6)

where β is a parameter ranged from 0.2 to 0.6 (in this work, β is considered as constant equal to 0.4). In the case of HEN induced by a porous membranes surface with the contact angle θ and surface porosity ε , Eq. (6) becomes46 1

γ eff，por

(1 + cosθ )2 γ 1 2 3  =  (2 + cos θ )(1 − cos θ )  1 − ε (1 − cosθ )2  4  

(7)

When the surface nucleation is fast enough for each layer of the crystal particle to form many individual surface nuclei.


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When the rate of crystal growth exceeds 20 % of the diffusion-controlled rate, the polynuclear growth dominates the growth process47, 48. If the obtained polynuclear growth rate is high and dominating the crystal growth procedure under certain diffusion and operation condition, the obtained crystal morphology is tended to be rough with numerous small dendrites. Thus, in this work, to evaluate the possible undesired growth resulting in undesired crystal morphology, the concept of ‘polynuclear growth’ is introduced. The corresponding polynuclear growth rate υ PN (m·s-1) is given by49

υ PN

D = 3d m

 ∆C max   Cc

2

3 D  ∆E max   exp − = kT  3d m  

 ∆C max   Cc

2

[ (

 3 β ln Cc C ∗  exp − π  N ln S max  

)]

2

  (8)  

where D (m2·s-1) is the diffusion coefficient of crystallization component in the liquid phase; dm (m) is the molecular diameter; ∆C max (mol·m-3) is the up limit supersaturation for the primary nucleation; Cc (mol·m-3) is the molar density of the crystal; N is the number of the molecular or ions; Emax,s (J·mol-1) is the maximum nucleation free energy. Meanwhile, the diffusion controlled crystal growth rate υ is written as47, 48

 ∆E g  a  ∆C  RT  

υ = k ′ exp −

(9)

The parameters in Eq. (9) are obtained by fitting the experimental data (obtained from authors’ primary measurement results from laboratory), where k ′ = 3.08×10-9,

∆E g = 4.58 kJ mol-1, a=1.02, R2=0.9677. Additionally, the equations on the mass balances and population balances in the crystallizer of the investigated system can be developed based on the classic model 50



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51

, which was provided in the Appendix for the readers who would be interested.

3. Experiments Materials. Polypropylene (PP) hollow fiber membranes was utilized (provided by Zhejiang University). The key properties were listed in Table 1. NaCl-EG-H2O ternary solution with MEG/(MEG+MH2O) ranging from 20 to 80 wt% and NaCl concentration ranging from zero to saturate state was employed. Ethylene glycol (analytical pure) and sodium chloride (analytical pure) were purchased from Tianjin Kermel Chemical Reagent Co. Ltd. Hyperpure water, produced by laboratory water purification system (Liaoning RIGHTLEDER Environmental Engineering Co. Ltd., China, LTLD-P50), was of ultrapure grade with ions content less than 0.1 ppb.

Table 1 Membrane property of PP hollow fiber membrane and membrane module External

Membrane

Mean pore

Porosity

Pure water

Total membrane area in

diameter, µm

thickness, µm

size, µm

ε

contact angle, °

the module, m2

403±5

35.4±0.6

0.17±0.02

0.503±0.05

115±2

0.0098

Experimental setup. The experimental apparatus of MDC is shown in Fig. 1. (a) homemade hollow fiber membrane separator is served as solvent removal unit and nucleation generator; (b) the jacketed crystallizer is served as feed tank and crystal growth unit; (c) two water baths that control the solution temperature; (d) the buffer tank used to prevent NaCl crystallizing before entering the membrane separator; (e) the peristaltic pumps (Baoding Longer pump Co. Limited., China) is used to control the flow rate in the membrane crystallization system; (f) the condenser connected to a cooler system (g, CKDC, Nanjing FDL Co. Limited, China) to transform the permeate


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gas into liquid instantaneously, and a permeate collection tank (i) to store the permeate; A vacuum pump equipped with an accurate vacuum pressure gauge (h, Leybold, -0.098MPa, China) set to generate and adjust the vacuum condition in the permeate side; the video camera is equipped for the possible detection of the nucleation at the membrane surface. All the temperature and mass data can be transferred via Wi-Fi to the computer (l) installed with the developed software (which can automatically record and calculate the transmembrane mass flux). During the experiment, the feed solution mixed with saturated sodium chloride and different EG mass fraction was added into the crystallizer (b). The feed solution was transported to the buffer tank (d) and the hollow fiber membrane module (a) in the setted flow rate. Then turn on the vacuum pump (h) to launch the experiment. The obtained enriched EG solution, fresh water were analyzed by the refractometer and Inductively Coupled Plasma Optical Emission Spectrometer (ICP-OES, Optima2000DV, PerkinElmer USA, ±10 µg/L). The operations conditions utilized in this work were listed in Table 2.

Table 2 Operations conditions of MDC experiments Pressure on

Pressure

on

Feed solution

Feed flow

Residence

Initial A/V,

feed side, kPa

permeate side, kPa

temperature, oC

rate, cm/s

times, hour

m2·m-3

101

7.0

60 to 80

0.02±0.002

2 to 6

5

Characteristic of crystal products. Crystal products were collected by filtration and drying at a predetermined residence time and analyzed by optical microscope (MOTIC China Group Co. Ltd.) and image analysis software to analyze the crystal



size distribution (CSD). Scanning electron microscopy (SEM, NOVA Nano SEM 450, 3 kV) were utilized to analyze the crystal morphology. The mass of raw crystal products after centrifugation and filtration and the mass of the terminal crystal products after drying were weighed by the precision electronic balance to calculate the mother liquid entrapment in the raw crystal products.

Nucleation detection and MSZW measurement. PVM (k) (V19, Mettler Toledo Ltd., USA,) that equipped in the crystallizer was introduced to detect the nucleation and determine the MSZW. PVM (V19 version) has an expanded function to detect the nuclear in the solution (so called ‘Image-Based Process Trending Using Relative Backscatter Index, RBI’). Relative Backscatter Index (RBI), is an image-based trend, sensitive to changes in particle appearance, particle size increasing, shape and concentration. RBI signal increases significantly when the nuclear occurs in the solution system,, which can be served as the criteria of detecting nucleation besides the function of crystal morphology observation. In addition, the hollow fiber membrane module utilized in this work is home-made and all transparent. The nucleation at the membrane module can be observed by the high resolution video camera as a back-up for PVM detection. The time of the first detective nucleus from either PVM or video camera was utilized as the initial nucleation time. The corresponding concentration when the initial nuclear were detected can be calculated by the recorded transmembrane mass flux.


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Figure 1. Experimental setup of MDC processes for crystal morphology control research.

4. Results and Discussions Nucleation and growth kinetic under various MDC conditions The measured MSZW ∆Cmax of investigated multiple solution system possess diverse variation tendency compared to the one of the binary solution system. The proposed approach to simulate MSZW ∆Cmax in this article had obtained the results in accordance with the measured data (shown in Fig. 2). Both results indicated that MSZW did not monotonously increase with the increased concentrating rate or decrease with the component of the volatile solvent (water in this system). Due to the increase of less volatile solvent (EG) and dynamic removal of volatile solvent (H2O), the diffusion resistance of crystal component and the surface free energy in the solution environment acted the combined impact on the determination of the crystal



nucleation barrier.

Figure 2. Comparison of the simulative and measured results ∆C max (The operation conditions were listed in Table 2). Different from the solute (NaCl)-good solvent (H2O) binary system, the solubility and diffusion rate of NaCl both significantly declined with the increasing weak solvent (EG) contents in the ternary system, which reducing the nuclear barrier; While, with the increasing EG also reduced the partial pressure of good solvent (H2O) as the volatile components, which correspondingly slowed the concentrating rate of membrane distillation process and narrowed down the metastable zone. Thus, the conflicting mechanisms acted a complicated result on the MSZW ∆Cmax of the ternary system via the membrane distillation generated nucleation process. The measured results and the simulative results by the proposed model both demonstrated the maximum ∆Cmax when the mass fraction MEG/(MH2O+MEG) was around 0.4 to 0.6 under the investigated operation conditions.


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To further reveal the combining impact of membrane mass transfer characteristics, the solution system property, operation temperature and the ratio of membrane separator and crystallizer on the nucleation, the solvent transmembrane flux F, supersaturation generating rate dC/dt of saturated NaCl solution under various MDC operation conditions were systematically simulated (shown in Fig. 3, the solubility of NaCl in EG-H2O mixing solution was measured data via the gravimetric method by author, the data were also provided as supporting information, listed in Table S1; the Permeate flux F, concentrating rate dC/dt were simulated based on the theory equations in above section). For the solubility of NaCl in the in EG-H2O mixing solution did not vary significantly with the increasing temperature from 40 to 80 oC (Fig. 3(a), the accelerated solvent transmembrane process was mainly due to the ascending partial pressure of the solvent vapor. Additionally, according to eq. (4), scaling up the ratio of membrane separator and crystallizer (A/V) leads to the accelerated concentrating rate (dC/dt, Fig. 3(b), which would lay profound impact on the heterogeneous nucleation rate (BHEN, Fig. 3(c), MSZW (Fig. 3(d)).



Figure 3. Solvent transmembrane flux F(a), supersaturation generating rate dC/dt (b), heterogeneous nucleation rate (c), MSZW (d) of saturated NaCl solution under various MDC operation conditions. (the labels of the corresponding feed temperature were listed at the bottom) As mentioned in the theory section, under the same MSZW ∆Cmax , the growth rate appeared to have various tendencies under the diffusion conditions (influenced by the different feed solution composition MEG/(MH2O+MEG)) and thermal kinetic conditions (influenced by the different operative feed temperature s, etc.). Shown in Fig. 4, Under the setted MDC operation, the highest υ PN was obtained when the feed solution composition MEG/(MH2O+MEG) was around 0.6 to 0.9 at different temperature. With the increasing temperature, the diffusion controlled growth rate υ increased faster and occupied a dominant role in the crystal growth, which indicated a stable


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growth procedure. It was obvious that diverse theoretical polynuclear growth indicates the different tendency of the multiple molecular (or ion) arrangement and construction pace. With the increasing υ PN , the alternation between the mononuclear and polynuclear growth mechanisms was intensified as a result of the complicated polynuclear growth process, which will definitely result in the vastly different crystal morphologies (shown in the right images of Fig. 4, operation conditions A and C， irregular particles and cube with gradient defect). On the contrary, under the proper diffusion controlled growth rate υ , even the nucleation barrier Smax (represented via Smax=1+ ∆Cmax / C ∗ in Fig. 4) was 1.04, the obtained crystal morphology was regular

cube (shown in the right images of Fig. 4, operation condition B). While, Generally, greater Smax means higher initial heterogeneous nucleation rate BHEN and smaller average crystal size. Besides the impact of membrane permeability Pm on the solvent removal and solution concentration kinetic, the increasing hydrophobicity of the concentrating ternary solution raised the heterogeneous nucleation barrier towards to the homogeneous one, which weaken the effect of microporous membrane on the promoted heterogeneous nucleation of the ternary solution (the measured hydrophobicity and diffusion property data of the ternary solution were listed in the Appendix). Thus, the interaction between the diffusion process and the crystal growth kinetic in the membrane module became extremely important. Apparently, intensified diffusion property would enhance the domination role of the surface reaction kinetic in the crystal growth and the exceeding polynuclear growth rate will lead to the growth pace changing from the



ideal steady growth pace to the disordered stochastic one. It also should be noted that the determination of the dominate role between diffusion controlled growth and polynuclear growth can not just rely on the numeric comparison between v and vPN. The in-depth theoretical research with more experimental results should be involved.

Figure 4. Comparison of diffusion controlled growth rate υ and polynuclear growth rate υ PN (left) and three classic crystal morphologies (SEM images) obtained under various feed temperature and operation conditions (right). (The operation conditions were listed in Table 2).

Crystal morphology and size distribution modification. By realizing the importance of the diffusion property and the surface reaction on the structure formation and development of particles, the terminal crystal morphology and size distribution modification effect under various operation temperatures and solvent composition (which both influence the diffusion property of crystallization component) were systematically investigated (shown in Fig. 5 and 6). With the diverse nucleation barrier, crystal growth mechanism under the different diffusion rate,


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the obtained crystal morphology and size distribution possessed an interesting trend. With the low viscosity, good solvent (water) was the main component in the solvent composition and the operation temperature was relatively low, crystal products (located in the bottom left corner) were usually small cube with smooth surface and almost uniform size. While, with the increasing temperature, the morphology of crystal particle transferred to the irregular cube with increasing surface defect and attachment. Furthermore, it was interesting to find out that there was always a temperature range to maintain the smooth crystal surface and uniform particle size in certain solvent composition. This was in accordance with the simulative results of the diffusion controlled growth rate υ and polynuclear growth rate υ PN for there was always a operative range to maintain the diffusion controlled growth rate υ to be comparable or higher than υ PN . For the industrial aspect, as clearly shown in Fig. 5, the preferred operating route for the desired crystal product is to increase the operation temperature at the feed side along the increasing concentration of EG (also the reduction of diffusion coefficient). The route can be explained by the diffusion property and the surface reaction hybrid controlled mechanism. When the diffusion were hindered by the increasing concentration of high viscosity, non-solvent (EG), the increasing temperature did enhance the diffusion and crystal growth by providing enough driving force. Moreover, the proper operation conditions during the MDC provided effective coordination of the nucleation and crystal growth to avoid the exceedingly nucleation or the uncontrollable polynuclear events on the crystal surface. As the consequence,



even to the ternary or more complicated solution system, the acquirement of uniform crystal with desire morphology via membrane crystallization was realizable and feasible with the in-depth understanding on the combining interaction mechanism of diffusion property (which influenced the crystallization component nucleation and growth) and operation temperature (which significantly impacted the surface reaction kinetics) on the particles formation.

Figure 5. The SEM images of NaCl crystal morphologies under different feed solvent composition and operation temperature Tf. (To simplify the figure illumination, the diffusion coefficient of NaCl listed in the figure are only the one of 60 oC with the corresponding solvent composition) In addition, the analysis results of crystal size distribution furtherly indicated the


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statistical impacts of the diffusion property and surface reaction on the particles formation. The competition between nucleation and crystal growth resulted in the different average size and coefficient of variation (C.V.). With the increasing operation temperature, the maximum C.V. transferred from the lean EG solution to the rich EG solution (from MEG/(MH2O+MEG)=0.0 to 0.6). Great C.V. and small mean size indicated the persistent nucleation and the dominant role of nucleation along the crystallization process.52-54

Figure 6. Crystal particle properties under different feed solvent composition and operation temperature Tf. (Number in the figure: average crystal size and C.V. (underlined number, the number with * means the maximum C.V. among the investigated operation temperature).

Improvement of solution recovery and separation efficiency In fact, suitable operation conditions for crystal morphology modification via MDC



rely on the stable membrane distillation process. The crystallization phenomenon and possible particle deposition might foul the membrane, and decrease durability of the membranes and MDC process. Considering the potential industrial application of MDC, the stability under long time working should be emphasized when the system was operated under the saturated or supersaturated state. As reported, the proper shear stress generated in dynamic membrane crystallizer was sufficient to preserve the membrane functionality by avoiding most of the crystals deposition in the laboratories and pilot-scale.55 While, as to the mixing solvent of EG-H2O, the increasing solution viscosity during MD process may hinder the function of shear stress, which should draw the key concerns. In authors’ primary work56, the membranes maintained steady performance on the solvent removal and the crystallization modification longer than 10 hours. In the repetitive experiments, the permeate flux and rejection of EG and NaCl persisted a steady and repeatable value after a simple washing process lasted for 10 to 20 min. These results indicated the existing membrane and membrane module are capable of acting as effective crystal morphology modification devices under a considerable long duration besides its instinctive function of solvent removal. To identify the effect of crystal morphology modification on the improvement of high value-added solvent recovery, the mother liquid entrapment in the raw crystal products after centrifugation and filtration were shown in Fig. 7. The raw crystal products were sampling when the treated solution reached the corresponding compositions. As shown in Fig. 7, the solvent loss ratio in the raw crystal products were kept around 5 % before the EG mass fraction reached 80 %. The crystal particles


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with narrow size distribution and desire surface benefited the solvent recovery via the simple centrifugation and filtration. The EG loss ratios were kept even below 2.5 wt% of the feed solution when the feed compositions ranged from 20 % to 60 %. It was a desire result for industrial application. Considering the low solvent loss, the high energy consumption unit, drying, can be omitted. When the EG mass fraction in the feed compositions reached 80 %, the exceeding viscosity of the solution (2.591 mPa·s, almost 4 times when it is 20 %) made the separation difficult and the EG loss after filtration increase to 5.1 wt%.

Figure 7. Solvent loss comparison under different feed compositions. (feed temperature Tf=65 oC; other operative conditions were listed in table 2, the corresponding SEM images of the crystal morphology were listed in the figure). Thus, as shown in Fig. 5, the alternative strategy to further improve the solvent recovery efficiency was following the formation trace of the desired crystal property (morphology and size distribution), which was carried out the MDC under a gradient increasing operation temperature route to ensure the crystallization controlled under the desire diffusion rate and surface growth rate (route A in Fig. 8). With the matching



between diffusion property of the existing solution system and the driving force of crystallization, the obtained products indeed benefit the coming up separation operation. As shown in Fig. 8(b), due to the modified crystal property, which benefited both filtration and dry separation, after the same filtration operation, EG loss during dry process reduce from 4.8 wt% (route B) to 1.2 wt% (route A). Although the MDC duration was extended from 3.9 hr. (route B) to 5.2 hr. (route A), the dry process of route A required a shorter duration to acquire the standard solid products due to the less liquid entrapment. As comparison, the MDC which was operated under a relatively low temperature (route C) manufactured part of crystals that grew under high polynuclear growth rate with a rough surface and wide size distribution. The MDC duration was extended to 5.7 hr and the EG loss was 3.3 wt%. Besides the overall operation duration, the shortened dry operation and the gradient temperature operation both helped to reduce energy consumption, which improved the separation efficiency from another aspect.

Figure 8. Schematic of the optimized operation route (a), operation duration of each separation units and the separation effect comparison (b). (The salt recovery of three routes were all kept at 85±2 %) The proposed alternative strategy was validated experimentally via batch operation in


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this article. While, the strategy can also be applied in the continuous (or semicontinuous) operation, the multistage MDC was preferred, which the operation temperature of each stage increases gradually and the operation scale (Ai/Qi, m2·m-3·h-1, the membrane area versus the flow rate for certain MDC stage) should be designed to maintain the stable solvent distillation rate and crystallization circumstance.

5. Conclusions The hybrid control mechanism and relevant optimized operation route of MDC for ternary solvent solution system was investigated. With respect to the nucleation barrier under the various MDC conditions (temperature, membrane separator and crystallizer scales, etc.) and the solution property, MSZW via membrane assisted crystallization did not monotonously increase with the increased concentrating rate or decrease with the concentration of the volatile solvent. It reached a maximum value along the operation profile under the setted condition, which proved both by the reported data and the simulative results proposed in this article. By symmetrically evaluated the combining impact of the solution system, operation conditions and the scale of the membrane separator and crystallizer on the dynamic crystallization process, the control functions of the solvent removal and component diffusion in determining the crystal product morphology were revealed. A trade off between the higher solvent removal rate (controlled by the membrane distillation conditions) and the better crystal morphology (controlled by the diffusion rate) under hindered polynuclear growth kinetic was the crucial issue. The proposed operation



route which followed the desire crystal morphology manufacture profile did decrease the solvent loss (from 4.8 wt% to 1.2 wt%) and improve the separation efficiency via shortening the operation duration and reducing energy consumption for drying. With respect to the hybrid control mechanism, the membrane assisted separation technology provides an alternative approach besides the conventional crystallization. Beyond an effective solution recovery approach, the optimized membrane crystallization process towards targeted crystal morphology and particle properties is a potentially highly controllable and feasible technology for high-end crystal production.

Acknowledgment We acknowledge financial contribution from National Natural Science Foundation of China (Grant No. 21527812, 21676043, U1663223), Changjiang Scholars Program (T2012049), the Fundamental Research Funds for the Central Universities (DUT16TD19, DUT17ZD203) and Education Department of the Liaoning Province of China (No. LT2015007).

Appendix Modified mass balances and population balances models for MDC.

The mass balances and population balances models can be developed on the interpretation of the established MDC procedure. The feed flow (transporting out of the crystallizer, heating up to eliminate the fine crystals then feed to the membrane module) can decrease the crystal density in the crystallizer; the retentate steam performs as the crystal seed resource for the nuclei are generated in the membrane


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module. These two factors make population balances equation of MDC different from the conventional batch crystallization. Thus, by ignoring the agglomeration and breakage effect, the population balance equation for a well-mixed MDC nucleation is

∂f ∂ (vf ) Q Q − JA + − f = Gmδ N − N ∗ + G0δ N − N * ρV ρV ∂τ ∂N

(

)

(

)

(A-1)

where f (#/m-3) is the number density distribution function of the crystals in the crystallizer; τ is the operation duration; v is the crystal growth rate. Nuclei appear

(

)

exclusively at the critical size N* with the nucleation rate G0 (Gm) ( δ N − N * is the Dirac delta function). V is the volume of solution in the crystallizer. At the initial conditions, assumed as a Gaussian distribution57

f =

1

σ0

(

 N−N 0 exp − 2 2 σ 2π  0

)  2



(A-2)

where N 0 and σ 0 are the mean and variance of the distribution at time t=t0, respectively. The growth of each crystal is independent to the other crystals and governed by the same deterministic model. Mass balance for the liquid phase coupled with the population balance equation in crystallizer is expressed as26

dC d =− dτ N A dτ

∞

∫ N f (N )dN

(A-3)

0

Mass balance of MDC in membrane module process is expressed as τ l max

mc = ∫

∫ ρ φl f (l )dldτ 3

c

(A-4)

0 0

where mc is the mass of crystal seed generated in membrane module, l (m) is the crystal size and φ is the shape factor (as to the cube shape of NaCl crystal, φ = 1 ).



Page 26 of 32

Due to the desire separation feature of MD process, the permeate side is considered as pure water, thus,

mc mwater = X 1− X

(A-5)

where X is the initial mass fraction of the flow feeding into the membrane module. Hydrophobicity and diffusion property of NaCl (saturated) -EG-H2O solution.

The hydrophobicity and diffusion property data of the ternary solution (all measured at 60 oC) were listed in Table A-1. Table A-1 Hydrophobicity and diffusion property data MEG/(MEG+MH2O),

Diffusion coefficient,

Viscosity,

Contact angle on PP

wt%

×10-9 m2·s-1

mPa·s

membrane, o

20

16.66

0.5474

112.3

40

10.86

0.9078

109.7

60

6.79

1.5889

107.9

80

4.64

2.5908

105.7

Supporting Information The measured solubility data of NaCl in the NaCl-EG-H2O ternary solution with different EG composition at different temperatures (ranged from 40 oC to 80 oC) were listed in the supporting information.

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For Table of Contents Use Only Manuscript Title Hybrid control mechanism of crystal morphology modification for ternary solution treatment via membrane assisted crystallization

Author Xiaobin Jiang, Guannan Li, Dapeng Lu, Wu Xiao, Xuehua Ruan, Xiangcun Li, Gaohong He

TOC graphic

Synopsis The hybrid control mechanism for the crystal morphology modification during the separation process of NaCl-EG-H2O ternary solution via membrane assisted crystallization was demonstrated. The impact of the dominating growth mechanism (diffusion control or polynuclear growth) on the crystal morphology was also investigated. Optimized operation route aimed to simultaneously to reduce the solvent loss and operation duration was then developed.


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Hybrid control mechanism of crystal morphology modification for

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