Reactive dividing-wall column for the co-production of ethyl acetate and n-butyl acetate☆

Hongshi Li,Tong Li,Chunli Li*,Jing Fang,Lihui Dong

National-Local Joint Engineering Laboratory for Energy Conservation of Chemical Process Integration and Resources Utilization,Hebei University of Technology,Tianjin 300130,China

Keywords:Reactive dividing-wall columns Ethyl acetate n-Butyl acetate Coproduction Energy-saving

ABSTRACT Reactive dividing-wallcolumn(RDWC)technology plays a criticalrole in the energy saving and high efficiency of chemical process.In this article,the process of co-producing ethyl acetate(EA)and n-butyl acetate(BA)with RDWC was studied.BA was not only the product,but also acted as entrainer to remove the water generated by the two esterification reactions.Experiments and simulations of the co-production process were carried out.It was found that the experimental results were in good agreement with the simulation results.Two kinds of RDWC structures(RDWC-FC and RDWC-RS)were proposed,and the co-production process operating parameters of the two types of RDWC were optimized by Aspen Plus respectively.The optimal operating parameters of the RDWC-FC were determined as follows:0.6 of the re flux ratio of aqueous phase(RR),0.66 of the vapor split(R V)and 0.51 of the liquid split(R L).And the optimal operating parameters of the RDWC-RS were shown as follows:RR was 0.295 and R V was 0.61.Furthermore,the energy saving analysis of the co-production process was based on the annual output of 10000 tons of EA,compared with the traditional reaction distillation(RD)to prepare EA and BA,the reboiler duty of the RDWC-FC column could save 20.4%,TAC saving 23.6%;RDWC-RS reboiler energy consumption could save 17.0%,TAC 22.2%.

1.Introduction

Distillation is a main energy consumerin the production process ofpetroleum and chemical industry.At present,the traditional distillation technology has been widely applied for chemical production,which has the key problems of low thermodynamic efficiency,high energy consumption and high investment cost.Therefore,the development of novel distillation technology and equipment to achieve the chemical process of energy saving and social sustainable development is of great significance.

The dividing-wallcolumn(DWC)was conceptualized in 1930 and did not achieve the first industrialization until 1984.Due to its advantages of low energy demand,low equipment investment,high thermal efficiency and high ability to simultaneous separation formulti-componentmixture in a single column[1–5],the DWC has been studied extensively since 1992,and the number of DWC in the world had already reached more than 300 by 2016.The reactive distillation(RD)is carried out simultaneously with the reaction and separation,and promotes the reaction forward by timely removal of the product,which can improve the reaction conversion and selectivity.As shown in Fig.1,the reactive dividing-wall columns(RDWCs)integrate RD with DWC into a column,which has the both advantages of the RD and DWC.It is widely applied to the synthesis and decomposition ofesters,the hydrogenation ofhydrocarbon[6–9],the preparation of biodiesel[10–12]and so on,which has good prospects for development.

The energy-saving methods of esterification process for the preparation of ethyl acetate(EA)have been widely studied,including extractive distillation[13–15],membrane pervaporation[16–18],homogeneous/heterogeneous reactive distillation[19]and other methods.Zhang et al.[20]simulated the process of azeotropic-reactive distillation to prepare EA,in which n-butyl acetate(BA)was used as an entrainer,the reaction zone was located at the bottom of the column,the reactant ethanol(ETOH)was fed to the bottom of the reaction zone,acetic acid(AA)was fed to the top ofthe reaction zone,and the entrainerwas added to the column in advance for recycling.Compared with the traditional RD,a large amount of water generated by the reaction in the column was taken out from the side line through the BA in the process,which effectively reduced the back flow of the ester phase in the top and achieved good energy-saving effect.Raúl Delgado-delgadoa et al.[10],for the first time,published the pilottestresultsofproducing EAby RDWC.The experimental results were consistent with the steady state simulation results,which provided an important basis for verifying the accuracy of the previous simulation work.The experimental results were in good agreement with the steady-state simulation results and provided an important basis for verifying the accuracy of previous simulation work.Miguel A.Santaella et al.[6]compared different EA production routes(traditional two-column distillation,RD,reactive pressure rectification,RDWC)based on several parameters such as conversion rate,yield and E-factor.The results showed that the RDWC energy-saving effect was very good.

Fig.1.Developing process of the reactive dividing wall column.

Meng etal.[21]proposed a co-production simulation ofEAand BAin a reactor and column.First,ETOH and n-BUOH with an excess of AA werefirstesterified in the reactor,and then the production entered the column to be separated,the azeotropic mixture of EA was distilled overhead and the azeotropic mixture of BA and water containing a small amount of EA was withdrawn on the side ofthe column.Optimization ofthe parameters such as the number of plates and the re flux ratio of the column was carried out to determine the operating conditions for the best separation effect.Liu et al.[22]studied the acidity control of the co-production process by simulation and determined the operating conditions for controlling the contentofAAin the product.Tian etal.[23,24]carried outa simulation and experimentalresearch ofco-producing EA-BAthrough two esterification columns.The experimental results showed that the co-production process had a good energy-saving effect.

The authors group investigates the azeotrope-reactive distillation to prepare the EA.The water is separated from the side line by using BA as the entrainer to form azeotrope with water.According to this,a co-production process[22–26]that n-BUOH and ETOH react with AA at the same time to get BA and EA in the RDWC is proposed.The BA is not only the reaction product,but also the entrainer.Experiments and simulations are carried out to verify the feasibility of the process,and then the process is optimized and the energy-saving is analyzed by Aspen Plus.

2.Model Design

2.1.Properties parameters

In this esterification system,six substances are involved,and their character data is shown in Table 1.The interaction between the six substances not only forms a variety of azeotropes,but also makes the existence of liquid–liquid equilibrium phase-separation phenomenon, which involves a total of eight azeotropes,the azeotropic temperature and composition are shown in Table 2.

Table 1 The character data for AA,ETOH,n-BUOH,EA,BA and water

Table 2 Azeotropic data for ETOH,n-BUOH,EA,BA and water

2.2.Vapor–liquid equilibrium equation

For systems with gas-phase association,using a single physical method cannotaccurately calculate the interaction parameters between the systems,so a complex physical method is needed for the corresponding calculation.The Hayden-o'Connel equation of state[27]is a virial equation up to two,which can accurately predict the blend and two polymerization of polar components at less than 1 MPa,especially for mixtures containing carboxylic acids.

2.3.Thermodynamic model

The paper[25]calculated the azeotropic point and composition of the systems using the binary interaction parameters of the NRTL and UNIQUAC in the Aspen Plus database.The NRTL was found to be superior to UNIQUAC and gave a detailed azeotropic data,see Table 3,so the NRTL thermodynamic method is chosen in this paper.The binary interaction parameters of the NRTL are given in Tables 4-1 and 4-2.In addition,there is a dimer phenomenon in the vapor phase ofAA molecule,which needs the Hayden-o‘Connel state equation,so the physical property model NRTL-HOC is applied to the thermodynamic calculation.

Table 3 The data from NRTL

2.4.Reaction kinetics equation

Amberlyst15 is a new type ofstrong acidic ion exchange resin,which has the advantages of non-corrosive,reusable and selectivity,and is widely used in catalysis of various chemical reactions,so the Amberlyst15 is selected as the catalyst for esterification reaction.

2.4.1.Reaction kinetics of synthesis of EA

Kinetic equation of catalytic synthesis of EA catalyzed by Amberlyst15 catalyst[28]is expressed by Eq.(1).

Table 4-1 Parameters of NRTL activity coefficient model

Table 4-2 Parameters of NRTL activity coefficient model

where r1is the reaction rate,kmol·K-n·s-1(holdup unit);k is the exponential factor,kmol·(kg·cat)-1·s-1;x is the activity.

2.4.2.Reaction kinetics of synthesis of BA

Kinetic equation of catalytic synthesis of BA catalyzed by Amberlyst15 catalyst[29]is expressed by Eq.(2).

where r2is the reaction rate,kmol·s-1;k is the exponential factor,kmol·(kg·cat)-1·s-1;x is the activity.

2.5.Two structures of the RDWC

As is well-known,the AA content in waste acid water is strict in the industry.In order that wastewater discharge reaches the standard,the AA content should be controlled below 0.002 wt%.A structure of RDWC,RDWC-FC(Fig.2),which is the equivalent to full thermal coupling distillation column and another structure of RDWC,RDWC-RS(Fig.3),which is the equivalent to the lateral line distillation column are designed to study the process of co-producing EA and BA.The RDWC-FC is simulated by the four-column model[30](Fig.4),while the RDWC-RS(Fig.5)is simulated by the three-model for the absence of a public distillation section.

Fig.2.The process of RDWC-FC for co-producing ethyl acetate and n-Butyl acetate.

On the basis of the literature[22,24],the number of theoretical plates of each section by the simulation was determined(see Table 5).

Fig.3.The process of RDWC-RS for co-producing ethyl acetate and n-Butyl acetate.

Fig.4.The simulation process of the RDWC-FC by four radfrac models for co-producing ethyl acetate and n-Butyl acetate.

Fig.5.The simulation process of the RDWC-RS by three radfrac models for co-producing ethyl acetate and n-Butyl acetate.

Table 5 The number of theoretical plate of the RDWC-FC and RDWC-RS by simulation

3.Experiments

3.1.Experimental device

Fig.6.The experiment device of the reactive dividing-wall column.1.Voltage regulator.2.Electrically heated wire.3.Reboiler.4.Public stripping section.5.Pre-fraction section.6.Reaction section.7.Pre-fraction section.8.Public distillation section.9.Raw material feed tank.10.Re flow ratio controller.11.The top product.12.The side product.13.Valve.14.The reboiler product.

The experimental device of RDWC for the co-production of EA and BA is displayed in Fig.6,and the RDWC-FC structure is investigated.The material of the packing column is 316 L stainless steel and the column is packed with 3× 3θ ring(316 L stainless steel).The total height ofthe column is 4 m,the effective packing heightis 2.1 m,the public distillation section is 600 mm,the public stripping section is 500 mm,and the pre-fraction section and the main column section are both 1500 mm.The reaction zone is located in the pre-fraction section with a totalheightof1000 mm.The diameter ofthe public distillation section and the public stripping section are 41 mm,the main column section and the non-reaction zone of the pre-fraction section are 28 mm and the diameter of the reaction zone of the pre-fraction section is 38 mm.

The Amberlyst15 is selected as the catalyst,and the catalyst packing method is shown in Fig.7.The swing hopper is controlled by the solenoid valve,and the collected liquid is divided into two separate flows to the catalyst and packing loading section.The liquid phase contacts with the catalyst in the section filled with catalyst and enters the section filled with packing,while the vapor phase contacts with the falling liquid phase for mass transfer in the packing loading section from bottom to top.Furthermore,the height of the upper layer of the catalyst is 40 mm.

The driving process ofthe RDWC is as follows:4 L ofAA,a mixture of 2 L of BA(63 wt%),n-BUOH(8 wt%)and water(29 wt%)were preliminarily added into the reboiler.The reboiler was kept heating,and then ETOH,n-BUOH and AA were simultaneously fed at a certain rate when the ternary mixture of BA,n-BUOH and water reached the position of the liquid dispenser.The top product was withdrawn in accordance with a certain proportion,so that the top temperature was maintained at 69.5 °C,and the public distillation section T1 was stable at 71 °C.In order to ensure that the ternary mixture of BA,n-BUOH and water was taken from the side-draw,the liquid split and side line ratio were adjusted to make the outlet temperature of the side withdrawing keep at 90.5°C.

3.2.Analysis method

The samples were detected through Shimadzu GC-2014 gas chromatograph equipped with a HayeSep Q column.In the process of analysis,the column temperature was kept at 180°C.The calibration area normalization method is chosen as the detection method.

3.3.Results and discussion

The operating parameters after one hour stabilization of the system are as follows:the flow ofthe mixture ofAA and n-BUOH(92 wt%acetic acid)was 8 mL·min-1and the flow ofETOH(95 wt%)was 5 mL·min-1,re flux ratio(R)was 5,the liquid split(RL)was 0.59,and the side line ratio(RS)was 0.2.The top,side line and reboiler product composition in the experimentwere given in Table 6,the temperature measurement point of the position of the distribution was shown in Fig.6,and the temperature distribution was illustrated in Fig.8.The results of simulation and experiment agree with each other very well,hence,the model is correct and reliable,and the model optimization can be carried out.

Table 6 The results of RDWC-FC by experiment and simulation

Fig.8.The temperature distribution of RDWC-FC by experiment and simulation.

4.Optimization

The theoretical tray number of each section in the simulation process is shown in Table 5.The feed flow of ETOH(95 wt%)and n-BUOH(100 wt%)were 40 kmol·h-1and 10 kmol·h-1,respectively,and the AA excess by 20%.With the content of EA in the crude ester more than 90 wt%,and the AA contentin the side-draw below 20 μg·g-1as the restrictions,and the lowest reboiler duty as the goal,two different structures of RDWC,RDWC-FC and RDWC-RS,were studied.The optimum operating conditions were determined by the analysis of the re flux ratio of aqueous phase(RR),the vapor split(RV),RLand other factors in the process of co-producing EA and BA.The design specs,optimization,sensitivity in Aspen Plus are used to optimize the parameters.

4.1.The optimization to the RDWC-FC

As RDWC-FC structure is demonstrated in Fig.2,EA,BA and water are produced by the esterification reaction in the reaction distillation section.Next,the vapor phase of the public distillation section is condensed by the condenser and then the condensed liquid is divided into light phase and water phase in the phase separator.In order to control the contentof AA in the product,the part of the water phase should be re fluxed.Since the boiling pointofAAis high,the contentofAAin the side product is larger than that in the product at the top of the column.Therefore,itis only necessary to make the contentofAA in the crude BA less than 20 μg·g-1.The content of AA in the crude BA is positively influenced by side withdrawing position,and the content of AA in the crude BA is changed as depicted in Fig.9.The lower the position of the side withdrawing is,the higher the content of the AA in the crude BA is.Hence,the position of the side withdrawing is selected at the 25th theoretical plate.

Fig.9.Effect of the side withdrawing position on the content of acetic acid in the side-draw.

The operating variables of the system include RR,RVand RL.

4.1.1.Effect of the RR

From the simulation in Fig.10,it is clear that the contentof AA in the crude BA is significantly affected by RR.The AA content decreases with the increase of RR.When the RR is more than 0.6,the content of AA in the BA is up to standard.We can clearly see the same trend of the conversion of ETOH and n-BUOH as the AA content by the effect of the RR.Because the rise of the RR makes the increase of the concentration of the water in the column,which,to some extent,inhibits the positive esterification reaction of ETOH and n-BUOH.In order to ensure a higher conversion of ETOH and n-BUOH and the content of AA in the crude BA to standard,the RR of the column is set as 0.6.

Fig.10.Effect of the re flux ratio of aqueous phase on the content of acetic acid in the sidedraw and the conversion of ETOH and n-BUOH.

4.1.2.Effect of the RV

As can been seen from the Fig.11,a considerable increase of mass fraction of AA occurs from 0.66 to 0.68 of RV,but the AA content is close to zero when the vapor split is less than 0.66.As the RVincreases,the heat distributed to the main column decreases and the separation efficiency of the main column decreases,so that the intermediate component and the heavy component cannot be effectively separated,which results in an increase of the AA content in the side-line BA.The conversion of ETOH shows an increasing trend,while n-BUOH remains almost unchanged.This may be related to the greater the RV,the more heat distributed to the pre-fraction section.According to the figure,the RVof the structure is chosen to be 0.66 for the purpose of ensuring that a higher conversion ratio of ETOH and n-BUOH and the qualified content of AA in crude BA.

Fig.11.Effect of the vapor split on the content of acetic acid in the side-draw and the conversion of ETOH and n-BUOH.

4.1.3.Effect of the RL

Fig.12.Effect of the liquid split on the content of acetic acid in the side-draw and the conversion of ETOH and n-BUOH.

The mass fraction of AA in the side-draw sharply falls to near zero when the liquid split reaches 0.51 in Fig.12.In addition,the AA content is up to standard when the RLis more than 0.51.When the RLis less than 0.51,the liquid re flux in the pre-fraction section decreases,and the heavy component AA cannot be separated clearly in the pre-fraction section,so that the AA crosses the bottom of the public distillation section and enters the main column section,which will make the AA content in the side-draw increase.The conversion of ETOH and n-BUOH follow the same downward trend,since the increase of RLleads to the growing content of AA and BA in the pre-fraction section,and then the esterification of n-BUOH is carried out in reverse.Therefore,the optimal RLis 0.51 so thatwe can geta higher conversion ratio of ETOH and n-BUOH and a standard mass fraction of AA.

4.2.The optimization to the RDWC-RS

Fig.3 illustrates the RDWC-RS structure,whose partition is placed at the top of the distillation column,including two condensers and a reboiler.Owing to the clear separation of the light and the intermediate component in the pre-fraction column,the content of AA in the top product of the column is larger than that in the top of the main column.Thus,the AA content of EA in the top of the pre-fraction is controlled to meet the standard of AAcontentof all products.The operating variables in the system include:RR and RV.

4.2.1.Effect of the RV

Fig.13.Effect of the vapor split on the content of acetic acid in the pre-fraction and the conversion of ETOH and n-BUOH.

Fig.13 describes the effect of the vapor split on the content of AA in the pre-fraction and the conversion of ETOH and n-BUOH.The RVfrom 0.59 to 0.61 sees a sharp drop in the mass fraction of AA from 500 to nearzero untilthe RVis greater than 0.61,which AA contentis a leveling out at near zero.This is because the increase of RVcauses more heat to be distributed to the pre-fraction section,so that the separation ef ficiency of the pre-fraction section is increased,the heavy component AA is efficiently separated,and the content of AA in the overhead crude EA is decreased.The conversion of ETOH and n-BUOH are in the opposite direction.This may also be related to the heat of the pre-fraction section.Consequently,0.61 is a suitable option for RVto make sure a higher conversion ratio of reactant and qualified product.

4.2.2.Effect of the RR

As is exhibited in Fig.14,the RR plays an important role in the content of AA in the crude EA of the pre-fraction section.When the RR is more than 0.295,the content of AA in the side-draw is up to standard.A slight reduction in ETOH conversion is caused by the RR,because the increase of the RR promotes the esterification of ETOH and AA to a certain extent in the reverse direction.0.295 is determined as the value of RR for the sake of gaining a higher conversion ratio of ETOH and n-BUOH and a qualified content of AA.

Fig.14.Effect of the re flux ratio of aqueous phase on the content of acetic acid in the sidedraw and the conversion of ETOH and n-BUOH.

The effectsofRR,RLand RVon the reaction oftwo differentstructures were studied.The optimum operating conditions were determined.The results are shown in Table 7.

Table 7 More suitable operating conditions of RDWC-FC and RDWC-RS

4.3.Energy saving analysis of RDWC-FC and RDWC-RS

The distribution of the mass composition of the components in the RDWC-FC is shown in Fig.15.The lightcomponentofEA has the highest concentration at the top of the public distillation section,the heavy component AA at the bottom of the column reaches the highest point,and the BA and water are withdrawn at the highest concentration in the main column section.Fig.16 provides the mass fraction distribution regarding the components in the RDWC-RS.The concentration of the heavy component AA at the bottom of the column reaches a peak,and the concentration of BA and water at the top of the main column was maximum.

Fig.15.The mass fraction distribution of the RDWC-FC.

Fig.16.The mass fraction distribution of the RDWC-RS.

Table 8 compares the energy consumption and TAC of A-RD[31],RD and RDWC,details as follows:with an annual output of 10,000 tons EA as the base,relative to the traditionalA-RD(azeotropic-reactive distillation column)and RD for preparation of EA and BA,the RDWC-FC structuralreboiler duty and TAC respectively savings by 20.4%and 23.6%;the RDWC-RS can save reboiler energy demand 17.0%and save TAC 22.2%.The equipment costs of the condenser and reboiler are obtained using Eq.(3),and the column equipment cost formula is expressed in Eq. (4).And the author's group previously had done the A-RD and RD economic accounting.

Table 8 Energy consumption and TAC of A-RD,RD and RDWC

where C is the equipmentcost,CNY;A is the heatexchange area,m2;D is the column height,m;and L is the column diameter,m.

Because ofits good separation and energy saving effect,we willcarry out the next step pilot test.

5.Conclusions

Two RDWC structures for the co-production process of EA and BA were proposed in this paper.The experimentand simulation of the process were carried out by the RDWC-FC,whose results were in good agreement with each other,and the correctness of the model selection and the reliability of the simulation were verified.Based on the simulation of the two kinds of RDWC structures for the process,the optimal operating parameters about RR,RVand RLof the RDWC-FC and RR,RVof the RDWC-RS were determined.Moreover,compared with the preparation of EA and BA by A-RD and RD respectively,the co-production process of the two RDWC structures was better,which used the good water-carrying property of the product BA to make the water produced by the esterification reaction separated from the side-draw of the main column,thereby,the re flux rate of the ester phase and the components back to mix could be reduced and the thermodynamic efficiency would be improved,so as to realize the energy saving and consumption reduction of the process.

Nomenclature

A heat exchange area,m2

AA acetic acid

A-RD azeotropic-reactive distillation column

A-RDWC azeotropic-reactive dividing-wall column

BA n-butyl acetate

n-BUOH n-butanol

C equipment cost,CNY

D column height,m

DWC dividing-wall column

EA ethyl acetate

ETOH ethanol

k1exponential factor,kmol·(kg·cat)-1·s-1

L column diameter,m

NMnumber of theoretical plate of the main column section

NPnumber of theoretical plate of the pre-fraction section

NRnumber of theoretical plate of the public distillation section

Nrnumber of theoretical plate of the reaction section

NSnumber of theoretical plate of the public stripping section

R re flux ratio

RD reactive distillation

RR re flux ratio of aqueous phase

RDWC reactive dividing-wall column

RDWC-FCequivalent to full thermal coupling distillation column

RDWC-RSequivalent to the lateral line distillation column

RLliquid split

RSside line ratio

RVvapor split

r1reaction rate,kmol·K-n·s-1(holdup unit)

r2reaction rate,kmol·s-1

TAC total annual cost,CNY·a-1

x the activity