Minyan Zhu,Yafei Hou,Na Yu,Mengqi Chen,Zhanhua Ma,Lanyi Sun*

State Key Laboratory of Heavy Oil Processing,College of Chemical Engineering,China University of Petroleum(East China),Qingdao 266580,China

Keywords:Middle-vessel batch distillation Ternary mixture Aspen Dynamics Process control

A B S T R A C T In this paper,the mixture of dimethyl carbonate, ethylmethyl carbonate and diethyl carbonate was separated by middle-vessel batch distillation with feeding in middle-vessel and process control characteristics were researched.The steady state simulation results in Aspen Plus were exported to Aspen Dynamics.Then control effect of liquid level control with High Selector,composition control(structure1,structure2)and temperature control(proportional action,proportional integration action)were proposed.Composition control structure 2 and temperature control with PI action were investigated to achieve a good control effect.

1.Introduction

Ethyl methyl carbonate(EMC),as a typical green asymmetric carbonate,has been widely used in organic synthesis,fuel additives and solvents,and its use in battery electrolyte is particularly prominent[1–3].Diethyl carbonate(DEC)is a kind of organic solvent with excellent performance,and has high industrial application value,which can be used as the electrolyte solvent of batteries and has been widely applied in synthetic resins,synthetic fibers and pharmaceutical industries[4,5].At present,the production of EMC and DEC through transesterification of dimethyl carbonate(DMC)and ethanol is a common method in industry[6,7].Since the raw materials of this process are DMC and ethanol,with non-toxic and non-polluting characteristics,and the reaction can be carried out at atmospheric pressure.The reaction temperature is not high,and the reaction conditions are mild,so it has been widely used.DMC–EMC–DEC as a common mixture of the process requires further separation to obtain high-purity products[8–10].

Batch Distillation is widely used in pharmaceutical, fine chemical and other high value–added products.Compared with continuous distillation,its advantage is that a single column can be used to achieve a high–purity products separating from mixture,especially suitable for process with small quantities and multi–components separation.A type of batch distillation column–middle–vessel batch distillation column(MVBD),is similar to a continuous distillation column,with a distillation section and a stripping section connected by a middle–vessel.MVBD was first proposed by Robinson and Gilliland in 1950[11],and later Meski and Morari[12],Davidyan et al.[13],and Hasebe[14]proposed using MVBD to separate ternary mixtures.Raw material is added into the middle vessel at one time, and concentration in middle vessel is constantly changing.The light component is distilled from the top of the column,and the heavy component is extracted from the bottom of the column,and the intermediate component is retained in the middle vessel.MVBD can reduce the degree of materials mixing and save much energy.Compared to conventional batch distillation or stripping batch distillation column on the condition of consuming same amount of energy,MVBD greatly reduces the separation time[15].

MVBD has attracted the interest of researchers.Barolo et al.[16]detailed the problems in the design and operation of MVBD,and the simulation results show that the operation under infinite reflux ratio and reboil ratio is more advantageous than limited reflux ratio and reboil ratio.Monroy and Alvarez[17]proposed a two composition control structure to adjust the purity of top product by manipulating the top reflux rate and regulate reboiler duty to adjust the purity of bottom product.Leipold et al.[18]proposed a new method of using evolutionary algorithm of multi-objective dynamic optimization to successfully deal with the optimization for MVBD. The results show that the proposed method is feasible and can significantly improve profitability.Babu et al.[19]used the Matlab programming to achieve the combination of vapor compression heat pump and MVBD,and put forward the corresponding control scheme.The simulation results show that this heat integrated MVBD can achieve cost savings and greatly reduce energy consumption.For the first time,Luyben[20]utilized Aspen Plus and Aspen Dynamics to study the separation of benzene–toluene–xylene mixture by MVBD with feeding in reboiler and several control structures were proposed to achieve a normal operation of MVBD.Zhu et al.[21]used MVBD to separate methyl formate–methanol–water mixture.The results show that the temperature control structure has a good control effect on the separation of this ternary system.

Recently,the published articles studying the separation of ternary mixtures by MVBD in Aspen Dynamics are fed in reboiler not in the middle vessel.Therefore,this paper studies the simulation and control strategies of separating DMC–EMC–DEC mixture by MVBD with feeding in middle vessel in Aspen Plus and Aspen Dynamics. The control effect of liquid level control structure with High Selector and composition control structure 1 are investigated,and the modified composition control structure 2 and temperature control(proportional action,proportional integration action)structure are further proposed.

2.Steady-state Simulation

The steady-state model of MVBD is established in Aspen Plus,as shown in Fig.1.The feed(FEED,composed of DMC–EMC–DEC mixture)enters to middle vessel(FLASH)and establishes liquid level.The outlet liquid stream(S14)from middle vessel enters to the top of lower column(LOWER),and the reboiler provides energy for the lower column and generates the ascending vapor stream(S4),which contacts with the liquid stream to achieve the separation of intermediate and heavy component (EMC and DEC) in lower column. After the vapor stream entering to the condenser of upper column(UPPER),the top reflux occurs and further the separation of light and intermediate components(DMC and EMC)in upper column is achieved.Finally,the bottom stream of upper column(S8)returns to middle vessel.When the batch operation is completed,the light component(DMC)is collected in reflux drum,the intermediate component(EMC)is collected in middle vessel and the heavy component(DEC)is collected in reboiler.

The Radfrac model is used for column and Flash2 model is used to replace middle vessel.Add the necessary pumps and valves in order to facilitate the subsequent dynamic control study. It can't simulate middle vessel with a certain level in steady-state simulation,so it's necessary to break the connecting stream between middle vessel and lower column.After exporting to Aspen Dynamics, the broken stream will be connected and the inlet and outlet valves will be closed for batch distillation operation.

The FEED stream feeding in middle vessel is set at 1000 kmol·h-1and its composition reference to the literature[20],is composed of 30 mol%DMC,30 mol%EMC and 40 mol%DEC.All required products purity is not less than 99mol%. A lot of work has been done on the thermodynamic model of this system.The selected UNIQUAC and binary interaction parameters are shown in Table 1[22].The theoretical stage number of upper column and lower column is 25,including condenser and reboiler.MVBD is operated at atmospheric pressure,with 1 kPa pressure drop of each stage.The distillation rate of upper column is set at 1 kmol·h-1,and reboiler duty of lower column is 2 MW.The pressure of Flash 2 model is set to ensure that S11 flow rate is as small as possible.The initial flow rate of stream S1 is 200 kmol·h-1,with the same composition as FEED stream.Table 2 is the steady-state simulation results in Aspen Plus.As can be seen,the flow rate of stream D,S11 and B is very small,because in Section 3.1 their export valves will be closed to set up a batch operating environment. Owing to the separation of these two columns,stream D is almost pure DMC,and stream B is almost pure DEC.Although there is a large difference in the flow rate between stream S1 and stream S15 in steady state,they still can be connected in the Aspen Dynamics.The initial liquid level of reflux drum,middle vessel and reboiler will be established after dynamic running,and the new flow rate of the connecting stream S14 is calculated.Table 3 is the result of equipment sizing.Feed is stored in middle vessel,and reflux drum and reboiler are set at a low level.Details can be found in the paper by Luyben[20].

Table 1Binary interaction parameters for UNIQUAC

Table 2Steady-state simulation results in Aspen Plus

Fig.1.Steady-state simulation of middle-vessel batch distillation.

Table 3Results of equipment sizing

3.Dynamics Control

3.1.Establish an initial batch distillation

After exporting to Aspen Dynamics,some modifications should be made to the process.Firstly,except pressure controller on the top,remove all the automatically added controllers;secondly,delete streams S1,S15,valve V4 and connect stream S14 to pump P1;thirdly,close the inlet and outlet valves VF,VD,VB and VV to form a closed batch process; finally,add liquid level controllers to make reflux drum,middle vessel and reboiler maintain stable initial liquid level.

The initial liquid level control structure is shown in Fig.2.The liquid level controller LCtop is used to manipulate top reflux rate(Rtop)to control reflux drum level;the liquid level controller LCbase is used to control the level of upper column base by manipulating bottom stream;the liquid level controller LCsump is used to control reboiler by middle reflux rate(Rmid).The control parameters of liquid level controller LCtop and LCsump are default,with gain 1 and integration time 20 min,and the integration action makes the two vessel be at a low level.The level controller LCbase with gain 2 and integration time 9999 min.Controller LCsump is reverse action.LCtop and LCbase are direct action.

Running to a steady composition and level, Click Initialization to save those established value,and the initial batch state is completed.The final liquid holdup results show 26 kmol in reflux drum with almost all the DMC,803 kmol in middle vessel with the same composition as FEED,41 kmol in reboiler with almost all the DEC.

3.2.Liquid level control structure(LLCS)with high-selector

With theknown composition and theamountof thefeed,the size of each vessel can be determined.The liquid level of top and reboiler can be calculated according to the yield of product and size of vessel[23],thereby the liquid level control structure(LLCS)is established,as shown in Fig.3.Controller LCtop is to use Rtop to control the level of reflux drum.Controller LCtop will reduce Rtop to increase the level of reflux drum firstly,and HighSelector module(HStop)is used to limit the minimum value of Rtop to prevent it from being too small;controller LCsump is to use Rmid to control the level of reboiler.Controller LCsump will increase Rmid to increase the level of reboiler,and the HighSelector module(HSmid)is used to limit the maximum value of Rmid to prevent it from being too large.Controller LCtop is set at gain 5 and integration time 50 min,direct action;controller LCsump is set at gain 5 and integration time 50 min,reverse action.Since HSmid is added,its output cannot be connected to valve V1. Therefore, it is necessary to add the flow controller FCRmid with gain 0.5,integration time 0.3 min,cascade control,and reverse action[24].

Fig.2.(a)Establish an initial liquid level control structure,(b)control panel.

Fig.3.(a)Liquid level control structure with HighSelector,(b)control panel.

Fig.S1 in Supporting Information is the results of LLCS with HighSelector.For intermediate component in middle vessel,EMC purity is continually purified due to the separation of upper and lower column.For light component in reflux drum,before 8.4 h the output signal of controller LCtop has been the minimum value of HStop,so DMC purity continues to decrease and liquid level of reflux drum increases continuously.After8.4h,liquid level of reflux drumis close to set point,so Rtop is automatically adjusted according to controller LCtop.For heavy component in reboiler, before 14 h the output signal of controller LCsump has been the maximum value of HSmid,so it behaves the continuous increase of liquid holdup in reboiler. Because the maximum value is close to the initial flow value of S14,the decrease of DEC purity occurs later(at 13 h)and the fluctuation is smaller compared with DMC purity.

After 25 h,the product purity and holdup in each vessel maintain stability.The purity of 98.93 mol%DMC in top,98.68 mol%EMC in middle and 99.92 mol%DEC in bottom is obtained with the yield of 292.0 kmol,213.6 kmol,and 367.4 kmol in each vessel,respectively.The liquid holdup in each vessel reaches the set point,but DMC and EMC purity are slightly below the set point.If the liquid level set values are changed within a certain range,the required product purity can be obtained furthermore.However,the operating time is long.Next composition control is considered.

Fig.4.Composition control structure 1.

3.3.Composition control structure 1(CCS1)

Rtop and Rmid are important manipulate variables of middle-vessel batch distillation.Luyben[20]used Rtop to control the impurity of top product and Rmid to control the impurity of middle product.In this article,the same composition control structure is established firstly,as shown in Fig.4.CCtop controller is to use Rtop to control the impurity EMC in top product below 1 mol%.If it is above 1 mol%,CCtop will increase Rtop rapidly to ensure the product purity DMC in reflux drum can be maintained at 99%.CCtop Controller is set at gain 5,integration time 50 min,direct action.CCmid controller is to use Rmid to control the impurity DEC in middle product below 1 mol%,whose controller parameters are the same as CCtop.

Fig.S2 in Supporting Information shows the control results of CCS1 with integration time 50 min.It can be seen that,due to the significant fluctuation of Rtop and Rmid,the product concentration and holdup in each vessel are also fluctuating,as a result of poor control performance.In the literature[21],the same phenomenon also appeared,and no measures were taken to eliminate the fluctuation.We know that the integration action is effective to eliminate deviation,but it will lead to system fluctuation.Therefore,we consider to increase integration time to reduce the integration action.Both CCtop and CCmid controllers are set at integration time 150 min.

We can see from Fig.S3 in Supporting Information,the control results show that the fluctuation is eliminated and the required purity is met after 25 h.The purity of 99.00 mol%DMC in top,99.00 mol%EMC in middle and 99.97 mol%DEC in bottom is obtained with the yield of 291.9 kmol,213.1 kmol,and 367.9 kmol in each vessel,respectively.Since the liquid holding capacity in reboiler reaches a maximum of 643 kmol at about 7 h,reboiler is set to a larger size in Table 1 to avoid liquid over flowing with small vessel volume during dynamic simulation.

Although the final control results show the required purity is met,CCS1 also has the following problems.Firstly,the liquid level of middle vessel during 7–9 h is almost zero,which is an abnormal operation.EMC purity during this period is suddenly increased,which has a great relationship with this very low liquid level.Secondly,DEC purity has a larger deviation.At 7.2 h,DEC purity is as low as 59 mol%.The reason is that the initial EMC purity in middle vessel is low,so CCmid controller will increase Rmid(it can be seen from the decreased holdup of middle vessel)to improve its purity.With large liquid reflux to lower column,the separation effect of EMC and DEC is reduced,which results in a sharp drop of DEC purity. Next,we consider to modify CCS1 and attempt to control DEC purity by Rmid.

3.4.Composition control structure 2(CCS2)

The modified CCS2 is shown in Fig.5.Controller CCsump is to use Rmid to control the EMC content in bottom product below 1 mol%.If it is above 1 mol%,CCsump will reduce Rmid,so EDC product can be purified as soon as possible,as a result of setting reverse action to CCsump(gain 5,integration time 50 min).Controller CCtop is set at gain 10,integration time 60 min,direct action.

Fig.5.(a)Composition control structure 2,(b)control panel.

Fig.6.Temperature profile at the end of batch operation:(a)upper column,(b)lower column.

Fig.S4 in Supporting Information shows the control results.It can be seen that the products accumulation in each vessel is relatively smooth.As the level in middle vessel gradually reducing, the level in reflux drum and reboiler increases.After 19 h,the required purity is met.The purity of 99 mol%DMC in top,99.96 mol%EMC in middle and 99 mol%DEC in bottom is obtained with the yield of 292.0 kmol,199.4 kmol,and 380.2 kmol in each vessel,respectively.The concentration fluctuation of DMC and DEC is very small and can be quickly stabilized to the set point.For CCS1,the purity of EMC in middle vessel is increased rapidly before7h,but it is increased slowly later, in the end it reaches the purity requirement after 25 h.Thus CCS2 has a significant advantage in terms of operating time and the stability of liquid holding compared with CCS1.

3.5.Temperature control structure(TCS)

Due to the large delay in the online composition detection and the high maintenance cost of the equipment,the composition controller is generally not used in industry.Then the more popular temperature control is considered.

Fig.6 shows the temperature profile in column at the end of CCS2,and the10th stage of upper column and the17th stage of lower column with the largest slope on the curve are selected as the sensitive stage(stage number is from top to bottom).The same control structure as CCS2 is used.Controller TCupper is to use Rtop to control 10th stage temperature of upper column at 101.7°C.Since this stage has a high DMC and the initial stage temperature is below the set point,controller TCupper will firstly reduce Rtop to increase the stage temperature,as a result of setting direct action to TCupper.Controller TClower is to use Rmid to control 17th stage temperature of lower column at 127.3°C.Since this stage has a high DEC and the initial stage temperature is above the set point,controller TClower will firstly increase Rmid to decrease the stage temperature,as a result of setting direct action to TClower.The temperature control structure is established as shown in Fig.7.

First,proportional action is only considered,and the two temperature controllers are set at gain 10,integration time 9999 min.The control results of TCS with P action are shown in Supporting Information Fig.S5.

It can be seen from Fig.S5 that,for TCS with P action,the temperature of the two sensitive stages does not reach set point with 6–7 °C deviations.Although the required purity is met after 35 h,the purity of EMC and EDC does not reach a stable value.The final product yield is obtained at 286.9 kmol,209.6 kmol and 375.2 kmol in each vessel,respectively.Although Luyben[20]has proposed that the TCS with P action showed a better control effect in the separation of BTX,it does not achieve the desired control effect in this paper.

Next the integration action is added to the two temperature controllers with gain 5,integration time 50 min.

Fig.7.Temperature control structure.

Fig.8.Control results of TCS with PI action.

The control results are shown in Fig. 8. For the TCS with PI action,the temperature of the two sensitive stages reaches the set point finally,because the integration action is added to eliminate the deviation.The operating time is shorter,and stable purity of DMC and EDC can be obtained compared to the TCSwith P action. Because temperature is indirectly used to control product composition,TCS with PI action exhibits a greater fluctuation and a relatively long adjustment time in controlling the purity of DMC and DEC compared with CCS2.After 20 h,the required purity is met.The purity of 99 mol%DMC in top,99.97 mol%EMC in middle and 99 mol%DEC in bottom is obtained with the yield of 291.9 kmol,199.5 kmol,and 380.2 kmol in each vessel,respectively.

3.6.Discussion of different control structures

The control results are shown in Table 4.It can be seen that the LLCS has poor control performance in terms of operating time and product purity.After 30 h,the holdup of each vessel reaches the set point,but the composition of DMC in reflux drum and EMC in middle vessel is deviated from the purity requirements.Especially,when the feedcomposition is unknown and the final product yield cannot be determined,it is difficult to implement level control.

Table 4Summary of control results

CCS1 and CCS2 are available to obtain qualified products and similar product yields.CCS2 performs better with a shorter operating time,and level change in each vessel is relatively smooth.At a view of product composition,it is more suitable for CCS2 to separate ternary mixtures aiming at high purity intermediate product.

For the TCS,the control effect is poor when proportion action is only considered.After 35 h,the yield of DMC,EMC,DEC product reaches 286.9 kmol,209.6 kmol and 375.2 kmol,respectively.But the purity of EMC and EDC is still changeable and the operating time is longer.After the integration action is added, the control effect is improved significantly.Because temperature is indirectly used to control product composition,TCS with PI action exhibits a greater fluctuation and a relatively long adjustment time in controlling the purity of DMC and DEC compared with CCS2.The control effect of TCS with PI action is similar to CCS2 in terms of operating time,product purity and yield.However,the TCS is more convenient than the CCS in the industrial application.

4.Conclusions

In this paper,the DMC–EMC–DEC mixture was separated in middlevessel batch distillation with feeding in middle vessel. First,steady-state simulation was built in Aspen Plus and the equipment size was defined.Then,the batch operation was established in Aspen Dynamics and LLCS,CCS(CCS1,CCS2)and TCS(P and PI action)were proposed.The control results showed that the LLCS behaved poor control performance in the operating time and product purity.Both the CCS1 and CCS2 were capable of obtaining acceptable products and yields, but the CCS2 performed better in terms of operating time and the stability of liquid holding.For the TCS,the control effect was poor when proportion action was only considered.After the integration action was added,the control effect was improved significantly.The control effect of TCS with PI action was similar to the CCS2 in terms of operating time,product purity and yield,furthermore the TCS is better than the CCS in the industrial application.

Nomenclature

CCS1 composition control structure 1

CCS2 composition control structure 2

DEC diethyl carbonate

DMC dimethyl carbonate

EMC ethyl methyl carbonate

LLCS liquid level control structure

MVBD middle-vessel batch distillation column

PI proportional integration

Rtop top reflux rate

Rmid middle reflux rate

tIintegration time

Supplementary material

Supplementary data to this article can be found online at https://doi.org/10.1016/j.cjche.2017.11.014.