Hanxiao Du ,Lixin Xie, *,Jie Liu *,Shichang Xu

1 School of Chemical Engineering and Technology,Tianjin University,Tianjin 300072,China

2 Tianjin Key Laboratory of Membrane Science and Desalination Technology,State Key Laboratory of Chemical Engineering,Tianjin University,Tianjin 300072,China

3 School of Chemical Engineering and Technology,Hebei University of Technology,Tianjin 300130,China

4 Engineering Research Center of Seawater Utilization,Ministry of Education,Tianjin 300130,China

Keywords:Acrylic acid production Electrodialysis Concentration Membranes Absorption Mixtures

ABSTRACT The absorption process in acrylic acid production was water-intensive.The concentration of acrylic acid before distillation process was low,which induced to large amount of wastewater and enormous energy consumption.In this work,a new method was proposed to concentrate the side stream of absorption column and thus increase the concentration in bottom product by electrodialysis.The influence of operating conditions on concentration rate and specific energy consumption were investigated by a laboratory-scale device.When the voltage drop was 1 V·cP?1(1 cP=10?3 Pa?s),flow velocity was 3 cm·s?1 and the temperature was 35°C,the concentration rates of acrylic acid and acetic acid could be 203.3%and 156.6%in the continual-ED process.Based on the experimental data,the absorption process combined with ED was simulated,in which the diluted solution from ED process was used as spray water and the concentrated solution was feed back to the absorption column.The results shown that the flow rate of spray water was decreased by 37.1%,and the acrylic acid concentration at the bottom of the tower was increased by 4.56%.The ions exchange membranes before and after use 1200 h were tested by membrane surface morphology(scanning electron microscope),membrane chemical groups(infrared spectra),ion exchange capacity,and membrane area resistance,which indicated the membrane were stable in the acid system.This method provides new method for energy conservation and emission reduction in the traditional chemical industry.

1.Introduction

Acrylic acid(AA)and acrylate are the important monomers of polymerization which are widely used for synthesis in coating[1,2],adhesive[1,2],flocculant agent[3],plastic[4,5],textile[6,7],papermaking[6],rubber[5,8],inks[9],etc.Due to their excellent properties,the global demand for acrylic acid is up to 6 million t·a?1.The traditional technology to produce acrylic acid is two-step oxidation using propylene as raw materials[9].The high-temperature mixed steam of acrylic acid,acetic acid and water after pre-treatment is fed into the bottom of the absorption column.The water is sprayed continuously from the top of the tower to absorb the acid and water from the high temperature vapor.The unabsorbed exhaust gas and non-condensable gas are discharged from the top of the absorption column and burned.There are two main problems in this traditional absorption process.First of all,the concentration of acrylic acid in the bottom of the absorption tower is low(about 40%–60%),which increases the cost of acrylic acid distillation in the later process.The other is that large amount of acidic wastewater will be generated as high as 1.2 tons per ton of acrylic acid[10].The treatment of wastewater is challenging and costly because of the complex composition and strong acidity[11].Some studies have been performed in the past decades to increase the concentration of solution before the distillation process.Liu et al.used the 5A zeolite as an absorbent to absorb normal hexane(n-hexane)in the absorption process and desorption by nitrogen before entering the distillation tower[12].Min et.al used the organic solvents,such as methylbenzene and normal heptane(n-heptane)to extract methacrylic acid in an absorption tower and then distillate to dehydrate[13].However,the additional solution was needed in above methods.

A novel method to decrease the amount of wastewater and concentrate the acrylic acid was proposed in this work.Part of absorption solution was taken from a certain position in the absorption column,of which the acetic acid(HAc)and acrylic acid were concentrated outside.Then the concentrated solution was returned to the tray with the same acid concentration.The dilution water from the concentration system can be reused for absorption.At the same time,the amount of wastewater was also decreased,and the energy consumption of distillation was greatly reduced because the concentration of acrylic acid in the bottom of the absorption column was increased.

There are many potential methods could be applied to concentrate the solution such as distillation[14,15],reverse osmosis(RO)[16,17],electrodialysis(ED)[18,19],etc.Compared to the distillation and RO,ED has advantages of energy conservation and membrane stability in concentrating acid solution.ED is an electricity-driven membrane process[20,21],in which the ions in the solution can be separated and concentrated due to the ion exchange membrane permselectivity[22,23].The ED is widely used in the removal of heavy metal from wastewater[20,24–26],food processing[27–29],seawater desalination[21,30,31],concentration and separation of acid.Suwal et al.[18] removed the acetic acid by ED and bipolar membrane ED,respectively,which was used as a fermentation inhibitor for corn stover hydrolysate.It was shown that the ED performed the highest acid removal and lowest system energy consumption with an operating voltage of 10 V.Zhang et al.[32] recovered acetic acid from acetaldehyde wastewater by bipolar membrane ED.After testing the stability of different kinds of ion exchange membranes in acetic acid solution,the best ED configuration for acetic acid recovery was determined and the lowest energy consumption was 0.61 kW·h·kg?1.Nagarale et al.[33]reported that the permeate rate order of different ions in the ED process was formic acid>acetic acid>propionic acid.The energy consumption was increased and current efficiency was decreased with the increase of the carbon chain,which were because of the fouling of the anion exchange membrane.These studies indicated that the ED had great potential in the concentration of organic acid.

The objective of this research was to investigate the possibility of using ED process to concentrate the artificial solution containing acrylic acid and acetic acid from the absorption column.The effects of different operating parameters such as voltage,flow,temperature and initial concentration of acrylic acid and acetic acid were investigated to determine the optimal operating conditions.The absorption process after adding the ED process was simulated by Aspen Plus?(V11.0).The performance of the ion exchange membranes was also tested to analyze the influence of acidic solution on the membrane properties.

2.Experimental

2.1.Materials

Acetic acid(99.5%,Tianjin Kemel Co.,Ltd.,China),Acrylic acid(99.5%,Tianjin Damao Co.,Ltd.,China),Ethanol absolute solution(water content<0.005%,Aladdin Reagent Co.,Ltd.,China),Sulfuric acid(98%,Tianjin Rionlon Co.,Ltd.,China)were used in the experiments.All the water used in this study were DI water(conductivity<10 μS·cm?1).

The cation exchange membranes(CEM)and anion exchange membranes(AEM)were purchased from Tianwei Co.,Ltd.The main properties of CEM and AEM were shown in Table 1.The electrode membranes were the perfluorosulfonic acid cation exchange membrane(PFSA)that purchased from Dongyue Group Co.,Ltd.

Table1 Main characteristics of CEM and AEM used in this experiments

2.2.Experiment apparatus

2.2.1.ED cell and configuration

The arrangement of the membrane and ions mass transfer was shown in Fig.1(a).There were 10 pairs of membranes in the ED membrane stack,and the effective area of each membrane was 84 cm2.Rucoated titanium plates were both used as anode and the cathode.The electrode voltage was 1.5–3 V in this work.

The lab-scale ED configuration included DC power supply (GW instek,China),ED membrane stack,pumps and solution tanks,which was assembled as shown in Fig.1(b).Three streams were pumped from three tanks into different chambers with recirculation.

2.2.2.ED experiments

The initial concentration of the concentrate stream(CS)and dilute stream (DS) was the same.The initial volume of CS and DS was 2 L and the electrode rinsing solution(0.08 g·L?1H2SO4,3 cm·s?1)was 3 L.The samples were taken from the DS and CS per half hour to test the concentration of acid by gas chromatography.The mass of solution in 1#and 2#(Fig.1(b))was measured by electronic balance(Yueping,China).The temperatures of solution were maintained a constant during the test by thermostat water bath(Tenlin,China).The voltage,current and temperature values were recorded per half hour.

The continual experiment was carried out to investigate the performance of the future industrial application.The volume of the DS solution was 3 L and the CS solution was 1 L.First,the ED system was run in batch operation for 5 h.Then the feed solution was pumped respectively into the concentration chamber and dilute chamber in a consistent flow velocity,and the production was obtained from the top hole of the tank.The concentration of acetic acid and acrylic acid was tested per hour.

2.3.Analysis and calculation

2.3.1.Acid concentration analyses by Gas chromatography

Gas chromatography(GC,Beifen,China)was used to determine the concentration of acrylic acid and acetic acid with a FID detector and a capillary column(KB-FFAP,30 m×0.32 mm×0.5 μm)[34].Since the solution contained water that did not respond under the FID detector,the internal standard method was appropriate for this sample.According to the characteristics and peak position of acrylic acid and acetic acid,ethanol was chosen as an internal standard[35].

2.3.2.Membrane characterization

The ion exchange membranes before and after use were systematically characterized to investigate the properties change of membrane in an acid solution.The surface morphology of the membrane was characterized by the field emission scanning electron microscope(FE-SEM,Hitachi Regulus 8100)[36],and the chemical groups on the surface of the membrane was tested by the Fourier total reflection infrared spectrometer(FT-IR,Nicolet 6700)[37].

The ion exchange capacity(IEC)of AEM and CEM was tested using a reported method[38,39].The membrane area resistance(Rm)was measured by a lab-made device(Supplemental file,Fig.S1).The membrane was cut into 1 cm×1 cm(S)and immersed in a 0.5 mol·L?1NaCl solution for 24 h before the test.The resistance values with a membrane(R1)and without(R2) membrane were measured by the electrochemical system(Parstat 2273,Princeton),respectively.The membrane area resistance was calculated by Eq.(1)

Meanwhile,the same device was used to test the transport number as reported described[40].

Fig.1.The schematic diagram of(a)ED cell (S indicated perfluorosulfonic acid cation exchange membrane,A indicated anion exchange membrane,C indicated cation exchange membrane)and(b)lab-scale ED configuration (1#:feed tank of DS,2#:feed tank of CS,3#:feed tank of electrode rinsing solution,4#,5#,6#,10#,11#:pump to transport the solution into ED cell,7#:power supply,8#:ED cell,9#:supplementary solution for continuous experiment).

2.3.3.Acid migration

The quantity of acid migration(mc)indicates the mass of acetic acid or acrylic acid transported from the dilute chamber to the concentration chamber,which is calculated as following Eq.(2).

where ω0and ωtare the initial and final mass concentration(%)of acetic acid or acrylic acid of CS,m0and mtare the initial and final mass(g)of CS.

Concentration rate(R)is the ratio of mcto the initial mass of acid in CS,which was calculated by Eq.(3)

2.3.4.Specific energy consumption

Specific energy consumption(E/kW·h·mol?1)is the power consumed for the migration of 1 mol of organic acid,which is calculated as following Eq.(4)

where U is the voltage(V)of ED,I is the current(A)of ED,t is the time(h),and denote the mole of acetic acid and acrylic acid transported from DS to CS,respectively.

3.Results and Discussion

3.1.Influence of operating conditions on ED

3.1.1.Operating voltage

The influence of operating voltage on current density was shown in Fig.2(a).The voltage was described as voltage drop for one cell pair.The current density increased significantly as the operating voltage increased.Meanwhile,the current density increased with the treatment time at the same voltage,which indicated the resistance of ED stack continuously decreased.The variation of current density with time in this weak acid(HAc and AA)system was quite different from that of the salt solution[41],which was normally decreased with the treatment time.The mechanism behind the effect of the current density maybe related to the variation of conductivity of CS and DS[18].The test results of conductivity of DS and CS shown that the decrease of DS conductivity was less than the increase of CS conductivity (Supplemental file,Table S1).Therefore,the whole solution resistance was decreased which resulted in the increase of current density.The results confirmed this phenomenon mentioned above,which was in accordance with the previous reports[18,29].

Fig.2.Effect of operating voltage on(a)current density,(b)concentration rate,(c)specific energy consumption and mass of water migration.The ED test was conducted using mixed acid solution(2.5%AA and 2.5%HAc)with flow velocity of 2.25 cm·s?1 and at(25±1)°C.1 cP=10?3 Pa?s

Fig.3.Effect of flow velocity on(a)current density,(b)concentration rate and(c)specific energy consumption and mass of water migration.The ED test was conducted using mixed acid solution(2.5%AA and 2.5%HAc)with constant voltage drop of 1 V·cP?1 and at(25±1)°C.

With the increase of operating voltage(Fig.2(b)),the migration rate of ions was accelerated,so that the concentration rate of two kinds of acid increased[42].The concentration rate increased fast when the voltage drop was lower than 1 V·cp?1,then it increased slightly,especially the concentration rate of acrylic acid.It was mainly due to the high water osmosis (Fig.2(c)) under a high concentration difference.As shown in Fig.2(c),the mass of water migration increased with the increase of operating voltage.During the electrodialysis process,the water molecules transported with AA?,Ac?,and H+from DS to CS known as electro-osmosis[43].Meanwhile,water was also driven by osmosis due to the increasing concentration difference between DS and CS[44].Therefore,the mass of water migration increased with the current density.Besides,Yu[45]indicated that the 10%acetic acid solution could generate 0.9 mA·cm?2equivalent current density of back diffusion through the anion exchange membrane,which meant the back diffusion had a great impact on current efficiency and concentration rate[46].

As the operating voltage increased,the specific energy consumption(Fig.2(c))increased,which increased more quickly at the higher voltage.Although the voltage increased,the number of migrated ions increased disproportionately,which might result in an increase of specific energy consumption.

3.1.2.Flow velocity

The influence of different flow velocity on the current density during the ED operation was quite different as shown in Fig.3(a).When the flow velocity was 1.5 cm·s?1,the current density gradually decreased during the first 60 min,then remained constant.When the flow velocity was 2.25 cm·s?1and 3 cm·s?1,the current density became higher with the increase of treatment time.The different trend of current density variation was attributed to the difference of residence time(Tr)in a membrane cell.The value of Trwas longer at a lower flow velocity.With the ions transferred,the number of ions near the surface of the membrane became less [47].The resistance of the solution became larger,so the current density reduced under a constant voltage.As the flow velocity increased,the solution flow turbulence excessed which reduced the diffusion boundary layer.Although the ions were constantly transferred from DS to CS,they could be replenished quickly.

As presented in Fig.3(b),the concentration rate increased with the flow velocity increasing.It may be attributed to the fact that the diffusion boundary layer near the surface of the ion exchange membrane became thin,which reduced the concentration polarization [44].Therefore,the resistance of mass transfer became low,which accelerated the ions migration near the membrane surface[48].As shown in Fig.3(c),the mass of water migration increased dramatically to 70 g in 3 h at 3 cm·s?1.

The specific energy consumption(Fig.3(c))was little influenced by the flow velocity from 1.5 cm·s?1to 3 cm·s?1.Although the current density increased,the concentration rate increased more,which resulted in a decrease in power consumption.When the flow velocity was 3 cm·s?1,the specific energy consumption was lowest,about 0.0163 kW·h·mol?1.

3.1.3.Temperature

The effects of temperature on the ED concentration process were investigated from 20°C to 35°C.The higher temperature would lead to large ions permeability,lower membrane resistance[49],and lower solution viscosity[19,50,51].In addition,the values of pKaof acetic acid and acrylic acid increased[52],which induced a larger amount of ions.All the above effects would induce to the increase of the current density and the concentration rate as shown in Fig.4(a)and(b).The mass of water migration was increased with the increase of temperature(Fig.4(c)),which was in accordance with the variation of current density.The mass of water migration was 75 g in 3 h at 35°C.

As shown in Fig.4(c),the specific energy consumption decreased significantly from 20°C to 25°C and then decreased slightly.At 35°C,the specific energy consumption was about 0.0139 kW·h·mol?1.Since the temperature of the solutions from the absorption column in the industrial process was more than 40 °C,an additional heat exchanger was required to cool down before entering into the ED.Considering both the cost of the heat exchanger and ED performance,the operating temperature of 35°C may be more reasonable.

Fig.5.Effect of the initial concentration of AA(group A)on(a)current density,(b)the mass of acid transport and(c)specific energy consumption and mass of water migration,and the initial concentration of HAc(group B)on(d)current density,(e)the mass of acid transport and(f)specific energy consumption and mass of water migration.The ED test was conducted with constant voltage drop of 1 V·cP?1,the flow velocity of 3 cm·s?1 and at(35±1)°C.

3.1.4.Initial concentration of acid

Fig.6.Continuous-mode experiments of acid concentration by ED(a)current density and(b)(c)(d)were all indicated the concentration of AA and HAc in DS and CS.The initial mass concentration of AA was 2.5%.The initial concentration of HAc was 0.5%(b),1.5%(c) and 2.5%(d),respectively.The ED test was conducted with constant operating voltage of 1 V·cP?1,the flow velocity of 3 cm·s?1 and at(35±1)°C.

Fig.7.Aspen flowsheet illustrating absorption process.

Two groups of experiments were carried out to investigate the influence of the initial concentration of acid.In group A,the concentration of acetic acid was fixed at 2.5%,the concentration of acrylic acid was tested at 0%,1.5%,2.5%,3.5%and 5%,respectively.In group B,the concentration of acrylic acid was fixed at 2.5%,the concentration of acetic acid was tested at 0%,1.5%,2.5%,3.5%and 5%,respectively.

It could be seen from Fig.5(a)and(d)that the current density of the two groups increased with the increase of acid concentration.The conductivity of solution became high due to the increase of acid concentration[53].Since the acetic acid and acrylic acid were basically presented in molecular form,the current density was not significantly with the increase of acid concentration.It could be seen that the increase of current density in group A(Fig.5(a))was larger than that of group B(Fig.5(d)),which means the increase of the same weight percentage of acrylic acid could result in greater increasing of the current density.As the results shown in the above sections(Sections 3.1.1,3.1.2 and 3.1.3),the concentration rate of acrylic acid was faster than that of acetic acid.The migration of different ions in the membrane was influenced by ion valence,molecule size and the molar concentration of the species[54].The molecular dimensions of acrylic acid and acetic acid were similar.But the pKaof acrylic acid and acetic acid was 4.25 and 4.75 at 25°C,respectively.Therefore,the amount of AA?in the solution was more than that of Ac?,and the current density was greatly affected by the initial concentration of acrylic acid.

Fig.5(b)and(e)presented that the variation of the total quantity of acid migration in the two groups was different from the increase of initial acid concentration.In group A,the mcof total acid increased dramatically then keep substantially constant when the concentration of acrylic acid was over 3.5%.In group B,the mcof total acid increased dramatically when the concentration of acetic acid was below 1.5%,then increased slightly.Due to the increase of initial acid concentration,more ions could be transported from DS to CS which increased the mcof total acid at lower concentration[41].However,with the increase of initial acid concentration,water transport caused by electro-osmosis and concentration osmosis became significant.As presented in Fig.5(c,f)that the mass of water migration increased sharply when the concentration of acrylic acid exceeded 3.5% and acetic acid exceeded 2.5%,respectively.In addition,Fig.5(b) shown that the mcof acetic acid reached the maximum when the concentration of acrylic acid was 3.5%.In group A,the ionization of acetic acid was suppressed due to the increase of the acrylic acid concentration.The decrease of the amount of the Ac?was the reason for decreased mcof acetic acid.

Table2 Absorption process specification

Table3 Process parameters and results of the simulation without and with ED

The specific energy consumption of the two groups was significantly reduced initially,followed by a slight increase(Fig.5(c)and(f)).The lowest specific energy consumption was 0.0128 kW·h·mol?1in group A when the concentration of acrylic acid was 3.5%,while the lowest one in group B was 0.0149 kW·h·mol?1when the concentration of acetic acid was 2.5%.

3.2.Continual-ED experiments

The continual experiments were performed based on the results of batch experiments to investigate the conditions of actual application.To increase the concentration rate,the volume rate between DS and CS was increased to 3:1.Referring to the concentration distribution of acrylic acid and acetic acid in different trays of the industrial absorption column,the investigated acrylic acid concentration was 2.5%,and the acetic acid concentration was 0.5%,1.5%,and 2.5%.The continuous experiments included two stages:(1)batch mode for 5 h and(2)continual mode for 5 h.At the first stage,the acid concentration of CS increased quickly.It was demonstrated in Fig.6(a)that the current density increased dramatically in the first 3 h.Then it increased very slowly in the next 4 h.The current density of three groups was separately stabilized at 6.07 mA·cm?2,6.43 mA·cm?2,and 6.67 mA·cm?2at the end of the first stage.At the second stage,raw solution was added into the tank with a specific flow rate and solution after treated started discharging from the tank at the same time.As shown in Fig.6(b-d),the concentration of acid fluctuated in the first hour of the second stage and became stable later.When the initial concentration of acetic acid was 0.5%,1.5%,and 2.5%,the concentration of acrylic acid discharged from CS was 5.46%,6.79%,and 7.43%,respectively.The concentration rate was increased significantly after increased the volume rate.When the ED process was conducted the mixed acid solution(2.5%acrylic acid and 2.5%acetic acid)in the continual mode,the concentrating rates of acetic acid and acrylic acid were 156.6%and 203.3%,respectively.

3.3.Aspen simulation

Fig.8.Surface SEM and FT-IR images of CEM(a,b,c)and AEM(d,e,f).The morphology of membrane before and after use 1200 h was shown in(a,d)and(b,e),respectively.

The whole absorption process combined with ED was simulated by Aspen plus based on the results of continual experiments.The flowsheet of the absorption process was illustrated in Fig.7.The stream of GAS was introduced from reaction process,which was absorbed by the stream of WATER.The stream of PRODUCT was piped to distillation process,and the stream of TOP was burned outside the column.The stream of SIDING was taken from the fifth tray to ED process,and the stream of CS and DS was sent back to the column from suitable trays.The stream of REFLUX was used for cooling the tower.After combining with ED,the process parameters,such as the flow rate of sideline production,the flow rate of absorption water and the reflux ratio at the bottom of the tower,were optimized to satisfy the design requirements that the concentration of acrylic acid at the top flow of the tower was lower than 0.03%.The simulation specification and main results were shown in Tables 2 and 3,respectively.The specific results of each flow were shown in Table S2(Supplementary Material).The acrylic acid concentration in the bottom product increased by 4.6%after adding ED,while the flowrates of spray water and the product at the bottom of the tower were reduced by 37.1%and 7.3%,respectively.The energy consumption in later distillation and the amount of wastewater treatment were reduced by increasing the concentration of acrylic acid and decreasing the volume of the product.

3.4.Membrane properties analysis

The morphology,surface chemical composition and performance of the CEM and AEM were tested after 1200 h concentration experiment.As shown in the SEM images in Fig.8((a)-(b),(d)-(e)),the surface of the CEM and AEM was still compact after using,and the membrane structure did not change significantly.The FT-IR spectra of AEM and CEM were tested in the range of 4000–600 cm?1.There was no significant change in the FT-IR spectra of CEM as shown in Fig.8(c),which meant the acid solution barely influenced the chemical groups of the membrane surface.In the spectra curves of AEM(Fig.8(f)),two additional peaks were shown,which presented the characteristic peak of Ac?and AA?at 1098 cm?1(—C—O—stretching vibration)[55]and 1650 cm?1(—C=C—stretching vibration)[56].This result was consistent with the conclusion of Nagarale[33].

The results of the ED performance of CEM and AEM before and after utilization were shown in Table 4.The transport number and IEC were slightly reduced,and the area electric resistance was increased.The changes of membrane properties might be attributed to the partial neutralization of active groups of AEM and CEM by Ac?and AA?.The fouling of acrylic acid and acetic acid reduced the ability of the membrane to transfer ions[57].A decreasing transport number and IEC caused a decrease in the selectivity and permeability of the ion exchange membrane[58].The increase of membrane area resistance meant an increase in the resistance of ions transport through the membrane[56].It can be seen that the acid solution had a greater effect on AEM than CEM,which would attribute to the permselectivity of the ion exchange membrane.A large amount of AA?and Ac?migrated through the AEM,while a very small part of AA?and Ac?passed through the CEM because of co-ion transference[59].The membrane performance was degraded but still acceptable.

Table4 The results of ED performance of AEM and CEM

4.Conclusions

In this work,a simple and efficient method to concentrate the solution and reduce amount of wastewater generated by the absorption process of acrylic acid production was proposed and investigated.The influences of different operating conditions on the concentration of acrylic acid were discussed.It was shown that the concentration rate increased with the increase of voltage,flow velocity,temperature and the initial concentration of acid.The specific energy consumption decreased with the increase of flow velocity,temperature and initial concentration of acid,but increased with the increase of voltage.With the operating voltage of 1 V·cP?1,flow velocity of 3 cm·s?1and temperature of 35°C,the specific energy consumption was 0.0139 kW·h·mol?1.Specifically,the weak acid system showed different variation of current density with time from salt solution because of the ionization.The output concentration of DS and CS was stable in the continual experiments.In addition,process simulation showed that the acrylic acid concentration of the bottom product increased by 4.56%,and the product flow at the bottom of the tower was reduced by 7.3%.The scale and energy consumption of the distillation of the acrylic acid could be reduced,as well as the amount of wastewater treatment before discharge.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was supported by the National Key Research and Development Program of China(2016YFC0401202)and Key Research and Development Program of Hebei Province(18394008D).

Supplementary Material

Supplementary data to this article can be found online at https://doi.org/10.1016/j.cjche.2020.07.020.