Yangcheng Lu*,Rui Wang,Jisong Zhang,Qianru Jin,Guangsheng Luo

State Key Laboratory of Chemical Engineering,Department of Chemical Engineering,Tsinghua University,Beijing 100084,China

Keywords:Epichlorohydrin Dichloropropanol Microchemical system Synthesis Process decoupling

ABSTRACT Synthesizing epichlorohydrin(ECH)from dichloropropanol(DCP)is a complicated reaction due to the partial decomposition of ECH under harsh conditions.A microchemical system can provide a feasible platform for improving this process by conducting a separation once full conversion has been achieved.In this work,referring to a common DCP feed used in industry,the reaction performance of mixed DCP isomers with NaOH in the microchemical system on various time scales was investigated.The operating window for achieving high conversion and selectivity was on a time scale of seconds,while the side reactions normally occurred on a time scale of minutes.Plenty of Cl-ions together with a high temperature were proved to be critical factors for ECH hydrolysis.A kinetic study of alkaline mediated ECH hydrolysis was performed and the requirements for an improved ECH synthesis were proposed by combining quantitative analysis using a simplified reaction model with experimental results on the time scale of minutes.Compared with the conventional distillation process,this new strategy for ECH synthesis exploited microchemical system and decoupled the reaction and separation with potentials of higher productivity and better reliability in scaling up.

1.Introduction

Epichlorohydrin(ECH)is an important intermediate in the production of epoxide resins,chlorohydrin rubbers and other organic products[1–3].There have been several industrial manufacturing processes to produce ECH such as propylene chlorination and glycerol chlorination[4–7].One of the most common routes for the synthesis of ECH is through the elimination of hydrogen chloride from dichloropropanol(DCP).Fig.1 illustrates this process,in which the desired ring closure is generally followed with hydrolysis as the major side reaction[8].The immediate removal of ECH is necessary for limiting further hydrolysis.

Dichloropropanol has two isomers:1,3-dichloro-2-propanol(1,3-DCP)and 2,3-dichloro-1-propanol(2,3-DCP).A 1:2 ratio is normally obtained from the chlorohydrination of allyl chloride.Due to inductive effects and steric hindrance,the reaction rate of the elimination of hydrogen chloride from 1,3-DCP was reported to be 20 times faster than that from 2,3-DCP[8,9].Industrial ECH synthesis aims to convert 2,3-DCP sufficiently while avoiding subsequent hydrolysis.In this regard,the reactive distillation column is conventionally used for ECH synthesis,in which ECH can be stripped with steam once being produced.Carráet al.[8,9]determined the reaction kinetics fora complete description of ECH production process using lime milk as alkaline and provided a model for simulating a multistage unit coupling reaction with distillation.Ma et al.[10]investigated the apparent kinetics of ring closure and hydrolysis using sodium hydroxide solution as alkaline.They indicated that raising the temperature favored the desired reaction pathway if the residence time of ECH was sufficiently short.All of the analyses or simulations in these studies were based on the assumptions of ideal feed mixing and flow as well as instantaneous ECH separation.However,actual reaction output in production was frequently different from the expected results for several reasons.First,the apparent kinetic parameters determined in a conventional reactor are usually case-dependent(Ma et al.[9]even obtained two different kinetic equations according to different temperature ranges).Second,in a cascade reactive distillation column,it is impossible to realize exact idealities in mixing,two phase counter- flow or instantaneous phase equilibrium[11–13].Thus,it is still a challenge to control and enhance DCP conversion effectively.

In recent years,microreactors have shown numerous advantages over traditional reactors,such as enhanced mixing and mass transport,controlled hydrodynamic flow and good inherent safety[14–16].They have been used as effective tools for intensifying fast reactions dependent on careful controls.Examples included rapid precipitation for nanoparticle preparation[17,18],and fast and strongly exothermic reactions for chemical intermediates synthesis[19–21].Microreactors have the potential to carry out an improved ECH synthesis.

Fig.1.Synthesis of ECH.(a)Desired reaction;(b)a typical following side reaction.

In our previous work,the kinetic parameters of dehydrochlorination of 1,2-DCP were determined in a microchemical system consisting of a micromixer,a delay loop and a microneutralizer[22].However,an industrial case using microchemical system as the core unit may need more considerations and evaluations because the starting material is a mixture of DCP isomers and the ECH product must eventually be separated from reaction system instead of inhibiting the hydrolysis temporarily.In this work,the reaction performance of mixed DCP(1,3-DCP/2,3-DCP:1/2)aqueous solution with sodium hydroxide solution in the microchemical system on various time scales was investigated and the kinetic characteristics of varieties of side reactions were explored to propose proper reaction conditions and to schedule the integration of reaction and separation for ECH synthesis enhancement.Compared with the traditional reactive distillation process,this new strategy for ECHsynthesis exploited microchemical system and decoupled the reaction and separation,therefore,higher productivity and better reliability in scaling up were worth expectation.

2.Experimental

2.1.Reagents

1,3-Dichloro-2-propanol(1,3-DCP,99%in mass)and 2,3-dichloro-1-propanol(2,3-DCP,97%in mass)were purchased from J&K Scientific Ltd.(Beijing).Sodium dihydrogen phosphate dihydrate(99.5%in mass),disodium hydrogen phosphate anhydrous(99.0%in mass),sodium hydroxide(NaOH)and sodium chloride(NaCl)were purchased from Beijing Chemical Works.Epichlorohydrin(ECH,99.5%in mass)was purchased from Sinopharm Chemical Reagent Beijing Co.,Ltd.All the reagents were of analytical grade and used directly without further purification.Deionized water was used throughout the experiments.

2.2.Experimental setup

The microchemical setup,as shown in Fig.2,can be found elsewhere[22,23].Brie fly,DCP solution(Feed 1)and NaOH solution(Feed 2)were delivered by two metering pumps(LB-80,Beijing Satellite Co.Ltd.)into the feeding tubes,respectively.The two solutions were preheated to the reaction temperature and then mixed in Microreactor 1 to start the reaction.The preheating zone,Microreactor1 and most of the delay loop were placed in Thermostat 1(SC-15,Ningbo Xinzhi)to guarantee a thermostatic reaction process.At the end of the delay loop,the process was extended to a low-temperature thermostat(Thermostat2,SC-15,Ningbo Xinzhi),in which phosphate buffer solution(pH=7.0)delivered by another metering pump was introduced into the reaction system through Microreactor 2.Due to the sudden decrease of temperature and concentration of hydroxyl ions,all the reactions were assumed to be quenched during the sample processing and analysis.In this microchemical system,the nominal reaction time was dependenton the length of the delay loop.

2.3.Analysis

The sample collected from the outlet of Microreator 2 was diluted with ethanol,and then 0.5 μl of sample was injected into a gas chromatograph(Shimadzu GC-2014).The GC analysis used AB-InoWax(30 m × 0.25 mm × 0.25 μm)for the column and polyethylene glycol for the stationary phase.The GC conditions included:carrier gas,N2;injection temperature,220 °C;column temperature,70 °C to 200 °C(30 °C·min-1);and detector, flame ionization detector(FID)at 280°C.The quantitative analysis by GC was based on the internal standard method.1-Octanol was used as the internal standard.The dichloropropanol conversion(X)and selectivity(S)are calculated with the following equations.

where WDCPand WECHare the mass fraction of DCP and ECHin the sample,respectively.Mpro(g)is the sampling mass,and FDCP(g)the feeding mass of DCP during sampling.The determination errors of WDCPand WECHwere about 0.5%and 2.4%,respectively.

3.Results and Discussion

3.1.Reaction performance on the time scale of seconds

Fig.2.Schematic of the microchemical system for ECH synthesis.

A microchemical system can provide a resolution of residence time in the sub-second range.For a reaction which nears completion within seconds,this is quite a valuable characteristic for process control.Our previous research showed that the dehydrochlorination of 2,3-DCP,although much slower than that of 1,3-DCP,was a fast reaction.Therefore,the reaction performance of mixed DCP isomersin a microchemical system on the time scale of seconds was first investigated.Typical flow rates were determined to be 10 ml·min-1of Feed 1 and 3 ml·min-1of Feed 2 in preliminary experiments,over which the time profile of conversion was almost consistent since the mixing performance in the microchemical system was high enough.

3.1.1.Stoichiometric reaction

As depicted in Fig.1,NaOH can cause undesired ECH hydrolysis.Thus,a stoichiometric reaction seems to be a reasonable option for the desired reaction.Fig.3 shows the time profiles of DCP conversion in a series of stoichiometric reactions.In the caption of figures,[DCP]10refers to the initial DCP concentration of Feed 1 and[NaOH]20the initial NaOH concentration of Feed 2.Solid points correspond to the experiments using 3%DCP(in mass)for Feed 1.As seen,the conversion always skips from zero to a considerable value(over 0.3)within the first measurement interval(1–2 s),demonstrating that the conversion of 1,3-DCP is almost an instantaneous process.However,the discrepancy of the conversion curves at various temperatures is still remarkable with time elapsed.The increasing of conversion is always slow and smooth within the first 20 s at 50°C,while occurs rapidly within the first 5 s at 85°C.This discrepancy is attributed to the temperature-sensitive kinetics of 2,3-DCP conversion.Our previous work[22]indicated that the activation energy of dehydrochlorination of 2,3-DCP was as high as 150(±10)kJ·mol-1.Fig.3(b)illustrates the dependency of the time profile of conversion on the initial DCP concentration.Apparently,the increasing of[DCP]10can increase the DCP conversion ratio at every time intervals,indicating that the kinetic order of[DCP]is larger than 1 for the DCP dehydrochlorination reaction.A sufficient DCP conversion(＞95%)in a microchemical systemis accessible under proper conditions including DCP concentration of 5.0%(in mass),reaction temperature of 85°C,and residence time of no more than 15 s.

Since the necessary residence time in the microchemical system is quite short for a sufficient DCP conversion,it is attempted to predict the DCP conversion kinetics without considering side reactions.Assuming that 1,3-DCP converts to ECH completely instantaneously,the overall conversion of DCP can be calculated by following equations

where all the concentrations correspond to the mixture of Feed 1 and Feed 2,and the activation energy for 2,3-DCP dehydrochloration reaction is taken as 156 kJ·mol-1.The lines in Fig.3 represent the calculated values.They match with the experimental data well,indicating the assumptions are reasonable and the chemical process within the microchemical system can be well predicted or designed.

Besides of the conversion of DCP,the selectivity of DCP to ECH is another concern.Fig.4 gives selectivities at relatively high temperatures(77 °C and 85 °C)within 15 s.As seen,allthe dots are around 1.0,almost independent of the temperature,DCP initial concentration and residence time.This indicates that an operating window exists for achieving high conversion and high selectivity simultaneously in a microchemical system.

Fig.4.Selectivities of ECH from DCP under various reaction conditions.Feed 1:[DCP]10=3.0%(by mass)(solid points)or 5.0%(by mass)(hollow points),10 ml·min-1.Feed 2:[NaOH]20=3.16%(by mass)(solid points)or 5.2%(by mass)(hollow points),3 ml·min-1.

3.1.2.Effects of the molar ratio of reactants

An exactstoichiometric reaction is not always practical for industrial use since errors in feeding and controlling systems are inevitable.Moreover,when the conversion of some specific reactant like DCP is preferred,other reactants are commonly excessively introduced.Therefore,the effects of the molarratio of NaOHto DCP in terms of conversion and selectivity at a high temperature(85°C),high concentration(5%in mass)and short residence time(up to 15 s)are investigated.As illustrated in Fig.5(a),when the mole ratio of NaOH to DCP is 0.9,the conversion ratio reaches 0.9,the theoretical limit,and when the mole ratio of NaOH to DCP is 1.2,the conversion ratio reaches 0.98,2%lower than the theoretical limit.Apparently,the DCP concentration has a much more crucial influence on the reaction kinetics than the NaOH concentration.The time profiles of conversion ratio at various molar ratios of NaOH to DCP can also be predicted.The selectivity is always near 1.0 when the mole ratio of NaOH to DCP is 1.0 or less with a slight decreasing trend when the mole ratio of NaOH to DCP is over 1.0.Therefore,excessive NaOH should be avoided for achieving high selectivity in a microchemical system.

Fig.3.Time profiles of DCP conversion under various reaction conditions.Feed 1:[DCP]10=3.0%(by mass)(solid points)or 5.0%(by mass)(hollow points),10 ml·min-1.Feed 2:[NaOH]20=3.16%(by mass)(solid points)or 5.2%(by mass)(hollow points),3 ml·min-1.

Fig.5.Effects of the molar ratio of NaOH to DCP on ECH synthesis at 85 °C.(a)Conversion;(b)selectivity.Feed 1:[DCP]10=5.0%(by mass),10 ml·min-1;Feed 2:[NaOH]20=5.2%(by mass);the flow rate of Feed 2 is varied according to the molar ratio of NaOH to DCP;the lines represent the calculated values.

3.1.3.Effects of extra chloride ions

In industry,the ECH synthesis is usually joined directly with DCP preparation.DCP can be prepared by allyl chloride chlorohydrination orglycerolchlorination with hydrochloride(HCl)generation.Therefore,in the ECH synthesis,NaOH also acts as the neutralization agent for HCl.Considering the acid–base neutralization is generally accepted as an instantaneous reaction,itis assumed that NaClis pre-existed for a dehydrochlorination process of DCP.Therefore,some experiments were conducted using NaOH-NaCl solution instead of NaOH solution as Feed 2 to investigate the effects of extra chloride ions on ECH synthesis.The results are shown in Fig.6.Compared with the reference experiments,the influence of chloride ions on the ECH synthesis is relatively insignificant in a microchemical system.

3.2.Side reactions on the time scale of minutes

Our aforementioned results confirmed that high conversion and high selectivity could be achieved simultaneously within a sufficiently short residence time(no more than 15 s in this work)in a microchemical system.However,if ECH product continues to contact with a hot aqueous solution containing plenty of electrolytes,side reactions may consistently take place.Therefore,the side reactions in the effluent of the microchemical system must still be considered before an effective separation and chilling of ECH.Since the temperature of the effluent of the microchemical system is high,distillation appears to be an alternative choice for separating and enriching ECH.Unlike the reactive distillation process in conventional ECH synthesis,a simple distillation process can be enhanced without the compromise of conversion.However,the effects of side reactions need to be investigated on the time scale higher than that controlled in a microchemical system.Therefore,the side reactions on the time scale of minutes are investigated,during which most of conventional separation process can be completed.

3.2.1.Conversion ratio and selectivity

Fig.7 shows comparisons of the conversion ratio and selectivity after～10 s,5 min,and 10 min residence.As seen,the DCP conversion ratio has a variance with the time scale but it always corresponds to the lowest value after 10 min.The selectivity decreases substantially when the reaction time extends to 5 min or more.Considering that10 min may be long enough for the exhaustion of NaOH by reactions shown Fig.1,these results indicate in the neutral aqueous solution containing Cl-at 85°C that the reversible conversion to DCP exists in the reaction system and ECH decomposition cannot be neglected.Gaca et al.[24]have discussed the mechanisms of ECH hydrolysis in acidic and neutral medium and in the presence of chloride ions.Zong et al.[25]investigated the changing of ECH concentration in the presence of NaCl under different conditions and discussed the salt effect on the cyclization reaction.Both studies pointed out that the presence of chloride ions may promote hydrolysis side reaction,of which perhaps routes are proposed in Fig.8.Plenty of Cl-ions with H+generated from water dissociation can promote the conversion of ECH to DCP(Step 1),meanwhile,DCP reacts with OH-to generate 3-chloro-1,2-propanediol(Step 2).The overall conversion is a hydrolysis process from ECH to 3-chloro-1,2-propanediol and Clions play a catalyst-like role.It is noticed that the above mentioned hydrolysis routes are not conceivable as alkaline(NaOH)still remains in reaction system.Therefore,alkaline mediated side reaction should be considered in priority,and characterizing its kinetic performance is helpful for finding or understanding a way to guarantee the selectivity from DCP to ECH.To this end,a well-established method should be employed to eliminate the effect from other hydrolysis reaction pathways.

Fig.6.Effects of the extra chloride ions on ECH synthesis from DCP at 85 °C.(a)Conversion;(b)selectivity.Feed 1,[DCP]10=5.0%(by mass),10 ml·min-1;Feed 2,[NaOH]20=5.2%(by mass),3 ml·min-1;hollow points in(a)are of experiments without NaCl pre-addition for reference.

Fig.7.Changing of conversion(a)and selectivity(b)on the time scale of minutes.T=85°C,[DCP]10=5.0%(by mass),[NaOH]20=5.2%(by mass);NaOH and NaCl are approximately equimolar.

Fig.8.Perhaps hydrolysis routes of ECH in neutral aqueous solution containing Cl-.

3.2.2.Sodium hydroxide mediated hydrolysis reaction

Since sodium hydroxide mediated hydrolysis is not a fast reaction,a setup as shown in Fig.9 is used to investigate its reaction kinetics.The proper stirring speed was explored to eliminate the effect of mass transfer in advance.In experiments,ECH and NaOH solutions were prepared and preheated to a preset temperature.Next,the solutions were added to a container with a magnetic stir-bar and heat jacket and the reaction was timed accordingly.So as to inhibit the hydrolysis routes shown in Fig.8,the initial molar ratio of NaOH to ECH was kept at or less than 0.1 to decrease the Cl-ion concentration.The sodium hydroxide mediated hydrolysis process was traced by determining the NaOH concentration with a pH electrode(with temperature calibration).

Fig.9.Setup for hydrolysis reaction kinetics determination.A—heat carrier inlet;B—heat carrier outlet;C—pH electrode;D—magnetic stirred rod;E—pH meter.

Fig.9 shows the hydrolysis reaction conversion ratio(based on NaOH)versus time.Considering the species in the sodium hydroxide mediated reaction,the reaction rate can be expressed as

Fig.10.Time profile of the sodium hydroxide mediated hydrolysis reaction at 50°C[NaOH]0=1.2%(by mass);ECH is absolutely excessive.

where[ECH]and[NaOH] represent the concentration of ECH and NaOH after mixing,respectively.Since ECH is in extreme excess during the reaction,[ECH]is taken as[ECH]0approximately.

If the order of NaOH is first,we derive

where XNaOHrepresents the conversion of NaOH.

Fig.10(b)shows the time profiles of ln(1-XNaOH)at various[ECH]0.The experimental results agree with Eq.(6)well,supporting the common assumption that the sodium hydroxide mediated hydrolysis reaction is first order for NaOH.Fig.10(c)is a plot of k[ECH]0αversus[ECH]0,where an approximate linearity across the base point can be found.Evidently,α is equal to unity,indicating that the sodium hydroxide mediated hydrolysis reaction is also first order for ECH.In addition,the slope of the line in Fig.10(c)is the value of k at 50°C.

The experiments of hydrolysis reaction were further carried out at varied temperature.Results are shown in Fig.11(a).Suppose that k is dependent on temperature according to the Arrhenius equation,Eq.(7)is obtained as

where k is the rate constant,k0is pre-exponential factor,E is the activation energy,R is the universal gas constant(8.314 J·mol-1·K-1)and T is the absolute temperature.k0and E can be obtained by linearity calibration of ln k vs.1/T,as shown in Fig.11(c).Finally,the following kinetic equation for the sodium hydroxide mediated hydrolysis reaction is obtained

Fig.11.Effects of temperature on the sodium hydroxide mediated hydrolysis reaction.[ECH]0=6%(by mass),[NaOH]0=1.2%(by mass);ECH is absolutely excessive.

3.3.Requirements for enhancing ECH synthesis

Since the desired reaction could be carried out in the microchemical system with high efficiency,the focus of ECH synthesis enhancement was how to inhibit side reactions.Dependent on whether reactants of the desired reaction were involved or not,the sodium hydroxide mediated hydrolysis reaction and other side reactions were discussed separately.

NaOH is the reactant of either the sodium hydroxide mediated hydrolysis reaction or the desired reaction.Therefore,the sodium hydroxide mediated hydrolysis reaction will be inhibited by the fast desired reaction as long as the molar ratio of NaOH to DCP is no more than 1:1.In such cases,assuming that only a small proportion of ECH is hydrolyzed,the rate ratio of the desired reaction and the sodium hydroxide mediated hydrolysis reaction at any point of DCP conversion ratio can be approximately predicted,as shown in Fig.12.An overall optimization strategy involves:(1)increasing the initial DCP concentration[DCP]0[Fig.12(a)]and temperature[Fig.12(b)],and/or(2)using excessive DCP[Fig.12(c)].Fig.12(d)indicates that the optimization on reaction conditions can strongly inhibit the effect of sodium hydroxide mediated hydrolysis on the desired reaction.Moreover,as long as most of the NaOH is consumed in the microchemical system,it may be able to ignore the effect of sodium hydroxide mediated hydrolysis.This requirement can be met within a residence time of 10–20 s under optimized reaction conditions.

Unlike the sodium hydroxide mediated hydrolysis reaction,other hydrolysis reaction pathways are not terminated even if NaOH is exhausted.Thus,it is exactly necessary to separate ECH from the reaction system before considerable hydrolysis reactions occur as well.Synthetically viewing the time dependency of selectivity on various time scales, finishing ECH separation within 1 min may achieve acceptable selectivity(＞95%)at a temperature around 85 °C.Fig.13 gives an illustration on the whole process including a microchemical system and a separation unit for ECH synthesis.However,more details on other hydrolysis reactions of ECH need further exploration for a more accurate process design.The process could also benefit from technologies involving flash or highly efficient separation for the ECH synthesis system as well.

Fig.12.Prediction on the rate ratio of desired reaction to sodium hydroxide mediated hydrolysis reaction.Ref 1:T=85 °C,[DCP]0=[NaOH]0=0.2 mol·L-1;Ref 2:T=95 °C,[DCP]0=0.51 mol·L-1,[NaOH]0=0.5 mol·L-1;the curves in each sub figure are calculated by using the same parameters with the exception of(a)initial concentrations,(b)temperature,or(c)excessive amount of DCP.

Fig.13.Schedule of ECH synthesis process using a microchemical system.

4.Conclusions

In this work,the reaction between mixed DCP(1,3-DCP/2,3-DCP:1/2)and NaOH in aqueous solution in a microreactor was investigated.It was observed that＞95%conversion of DCP can be achieved using 5.0%(by mass)of DCP at 85°C with a residence time of 15 s in a microchemical system.On the time scale of seconds,extra chloride ions introduced by DCP feed did not show a significant effect on the reaction performance.The effects of side reactions were limited and the time profile of DCP conversion ratio could be accurately predicted.

By extending the time scale to minutes,the effects of side reactions become obvious at high temperature like 85°C.Besides a sodium hydroxide mediated pathway,plenty of Cl-ions,independent of sodium hydroxide,led to hydrolysis reactions with a high impact on the main reaction.Through specifically designed experiments,the kinetics of the sodiumhydroxide mediated hydrolysis of ECH was determined.Both calculated and experimental results indicated:(1)a practical direction to inhibit the effect of the sodium hydroxide mediated hydrolysis of ECH was to increase the initial DCP concentration and temperature as well as introduce a slight excess of DCP;and(2)as long as most of NaOH was consumed in the microchemical system,the side reactions caused by the sodium hydroxide mediated hydrolysis were insignificant in following.

Systematically viewing the time dependency of selectivity on various time scales, finishing ECH separation within 1 min could achieve acceptable selectivity(＞95%)at a temperature around 85°C.Compared with the traditional reactive distillation process,this new strategy for ECH synthesis exploited microchemical system and decoupled the reaction and separation with potentials of higher productivity and better reliability in scaling up.However,both the hydrolysis reactions of ECH without alkaline and the technologies eligible for flash and high efficient separation of ECH synthesis system need further exploration for more accurate process design and more effective production.