Xincheng Gu ,Xiaochun Zhang ,Xiangping Zhang,2, *,Chun Deng

1 Beijing Key Laboratory of Ionic Liquid Clean Process,Institute of Process Engineering,Chinese Academy of Sciences,Beijing 100190,China

2 School of Chemical Engineering,University of Chinese Academy of Sciences,Beijing 100049,China

3 Key Laboratory of Heavy Oil Processing,College of Chemical Engineering and Environment,China University of Petroleum-Beijing,Beijing 102249,China

Keywords:Ethylene oxide Separation Ethylene carbonate Process simulation

ABSTRACT Ethylene oxide (EO) is an important raw material for producing ethylene carbonate (EC).However,the traditional method for the separation of EO from mixture gas by water in the refining process is high energy consumption.In this paper,two processes of manufacturing EC from EO mixture gas were studied by process simulation.Two processes for producing EC from EO mixture as raw materials without EO purification,called the OSAC process and the Modified OSAC process,were developed and assessed systematically.Both processes use EC as the absorbent to capture EO,avoiding the separation process of EO from solution.For comparisons,the EC producing process containing EO absorption by water,EO refinement and carbonylation process were also modeled,which was called the ERC process.Three schemes were designed for the EO absorber using EC as absorbent.Compared with the initial absorber scheme,the optimal liquid–vapor ratio is reduced from 1.66 to 1.45(mass).Moreover,the mass distribution analysis for the three processes were carried out in the form of the material chain.It was found that,compared with the ERC process,the energy consumption of the OSAC and the Modified OSAC process is reduced by 56.89% and 30.03%,respectively.This work will provide helpful information for the industrialization of the OSAC process.

1.Introduction

Ethylene oxide(C2H4O,EO)is an important chemical intermediate for producing useful products,such as ethylene glycol,glycol ethers,poly (ethylene oxide).In 2018,the world production of EO reaches 2.6×107tons,while it is expected that the output will exceed 3.6 × 107tons in 2023 [1].

Generally,EO is almost obtained by partial oxidation of ethylene,either with oxygen or with air,in the presence of silverbased catalysts.The reactions are carried out in the multi-tubular packed-bed reactor in the gaseous phase at a low temperature of 450–500 K and pressure of 1.0–3.0 MPa [2].For the oxygenbased direct oxidation process,the selectivity of EO is generally ranged from 70% to 90%.However,the ethylene concentration in the reactor feed is only 20%–35%.Besides,the per pass EO conversion is roughly ranged between only 10%–15% [3,4],which results that the diluent is becoming the main part of the outlet effluent.Due to the selective absorption of ethylene oxide by water relative to other components of the mixture gas,such as methane(diluent),oxygen,and ethylene,water is usually used as the absorbent to separate EO from the reactor product both for the air-based and the oxygen-based process.Therefore,the EO concentration in the rich solution obtained from the absorption tower is extremely low,which results in the need to configure a series of distribution and the extremely high energy consumption in the EO refining process.

Many relevant studies have been carried out to solve the problem of high energy consumption for the separation and refining of EO.CHINA PETROCHEMICAL CO LTD proposed to use ethylene carbonate (EC)-water mixed solution to replace the original water absorbent for EO capture [5].According to the experimental results,the absorption of EO by the EC-water mixed absorbent is higher than that of water.Chu et al.[6]studied the process of EO absorption by EC through phase equilibrium experiment and process simulation.They found that the solution of EO in EC is much higher than CO2,CH4,and C2H4in EC.Chu’s work provides a reliable thermodynamic vapor–liquid equilibrium model of EC and EO and other components,which provides a basis for further research through process simulation.

EC exhibits a high selectivity in absorbing EO and,in addition,EC is an important product downstream of EO.Owing to the special characters of EC,such as low toxicity,biodegradability,and low saturation vapor,EC is the green substitute for many toxic organic solvents,such as hexamethylphosphoramide(HMPA),N-methyl-2-pyrrolidone (NMP) [7].Moreover,EC can also be used as an intermediate to produce DMC or as a precursor of polycarbonate,which has large market demand.

Currently,many research has been carried out to investigate the process of EC synthesis through process simulation.For example,Kongpanna et al.[8]studied the process of synthesizing EC by CO2and EO,and then converted it into DMC.The process of EC synthesis and transformation process demonstrated that it has great advantages in yield,environmental assessment,and economics.Recently,Yu et al.[9]studied the process of synthesizing EC from CO2and EO.According to the research results,the process can achieve nearly zero CO2emissions.However,the current process of synthesizing DMC by CO2and EC requires pure EO as raw material for subsequent conversion.These studies showed the economic and environmental advantages of EC synthesis from EO,but all these processes require pure EO as a feedstock.It should be noted that the traditional EO separation and refining process by water treatment is energy intensive.

The purpose of this paper is to investigate three different pathways for producing EC through process simulation.One is the existing route of EC synthesis via carbonylation (ERC),which involves the EO separation and carbonylation process.The second process uses EC as an absorbent to absorb EO from the reactor effluent stream,eliminating the refining process for EO,and the EO-EC solution was directly fed to the carbonylation process to prepare EC (named One Step Absorption and Conversion,OSAC).The third one added the EO refinement process on the basis of the OSAC process to separate the impurities in the EO-EC solution,called the Modified OSAC process.Firstly,all three processes were subjected to rigorous process simulation.Secondly,the structure of the absorption column was optimized,and the latter two processes used EC as the sorbent to capture EO which is completely different from the current industrial processes.Finally,the three processes were evaluated in terms of technology,energy consumption,economics,and material distribution,and will provide important information for the development of the new technologies.

2.Process Description and Methodology

2.1.Process description

The production of EC from EO mixture in this work is classified in terms of three processes,namely the ERC process(absorbing EO by water and synthesizing EC through carbonylation reaction),the OSAC process and the Modified OSAC process.The scheme of the three processes is illustrated in Fig.1.As shown in Fig.1,the technological process of ERC,OSAC and Modified OSAC are similar as a whole.Whereas,the EO absorption and EO refinement processes are very different between the ERC,OSAC and Modified OSAC.For the ERC process,EO for carbonylation reaction is pure.While EO is sent to the EC production unit in the form of the EO-EC mixture for the OSAC and Modified OSAC processes.Compared with the OSAC process,the Modified OSAC process pre-treats the EOEC mixture and reduces the amount of impurities.

The components in the ERC process,OSAC process,and Modified OSAC process including carbon dioxide (CO2),water (H2O),oxygen (O2),argon (Ar),ethylene oxide (EO),ethylene (C2H4) and ethylene carbonate (EC).The composition of raw feedstock gas is shown in Table 1,which is a typical oxygen-based EO production data.NRTL model is selected as the thermodynamics model.In addition,binary parameters between EC and other main components in the OSAC and the modified OSAC process were regressed based on the experimental data.[10]

2.1.1.ERC process

The process flowsheet of the ERC process is shown in Fig.2.The process can be divided into three sections,which are the EO capture,EO refinement and the EC production sections.The feedstock of the process is the EO containing mixture from the crude product gas of EO reactor.

Due to the poor per pass conversion,the concentration of EO in the effluent from EO reactor is quite low.Hydrocarbons including unconverted reactants and diluents take a large portion.The countercurrent scrubber with cold water is needed to recover EO from crude gas.EO is completely dissolved in the absorbent.The unabsorbed gas is recycled to the primary reactor after CO2removal and cooling.

The solution stream obtained from the scrubber needs further processing before it is sent to EO purification.The aqueous stream contains a small amount of impurities,such as CO2and CH4,etc.The ‘‘off-gas flasher+re-absorber”configuration is employed to remove a part of impurities.The vented gas from the re-absorber is pressed back to EO absorber,and the EO-rich water is fed to the EO refinement section.In this section,a variety of columns are deployed.

After treatment,the impurities in the aqueous EO solution are initially separated.The water and EO needed to be separated to obtain a pure EO product.The EO aqueous stream is first sent into the stripper to initially separate EO and water.Lean water from the bottom of the stripper is nearly free of EO,and the water is recycled to the EO absorber and re-absorber after cooling.The high concentration of EO steam enters the heavy splitter to further separate water by distillation.A part of the raw EO is withdrawn from the side stage and sent to the stripper as the reflux.The stream obtained from the top of the heavy splitter contains a small amount of CO2.A light splitter is employed to separate CO2to obtain the EO product.The process of separating and purifying EO is extremely complex and the energy consumption during the refining process is enormous.

According to reaction (1),ethylene oxide from the EO refinement section is converted to ethylene carbonate combined with additional CO2.Numerous excellent catalysts have been developed for the carbonylation reaction of CO2with epoxides,which can be divided into homogeneous catalysts and heterogeneous catalysts[11–13].Though homogeneous catalysts exhibit a great effect on the fixation of CO2into the EO process,the separation of EC and catalysts is quite energy-intensive due to the high boiling point of EC [14].Therefore,in this process,the fixed-bed reactor with the heterogeneous ionic liquid catalyst [15]system was selected.Then the raw reactor product is sent through passing a flasher and light splitter to strip off impurities.Due to the huge boiling point difference between EC and other substances (CO2,H2O),the structure of the purification process is relatively simple.

2.1.2.OSAC process

The process flowsheet of the OSAC process is shown in Fig.3.Compared with the ERC process,the primary difference is that the OSAC process uses EC as an absorbent to capture EO from the raw feedstock,rather than water.

Fig.1.The process block diagrams for the ERC,the OSAC,and the Modified OSAC Process.

Table 1 The composition of the feed gas [10]

During the ethylene oxidation process,water will be generated,which has no additional impact when water is used to capture EO.However,when EC is used as the sorbent,the water is recognized as an impurity,resulting in additional energy consumption during the EC product refining process.Therefore,it is necessary to employ the flasher before the raw feed gas enters the absorber.The absorption configuration in the OSAC process is quite similar to the current process,and the structure of absorber+re-absorber is also adopted.

In the carbonylation process,the operating conditions for the OSAC are similar.In the reactor,CO2and EO are converted to EC in the liquid phase.The effluent of the reactor is sent to the flasher to separate the main part of excess CO2and other light impurities,such as CH4and C2H4.The liquid product of the CO2flasher consists mainly of EC and a small amount of CO2,and this stream is fed into the light splitter to refine the EC.The CO2obtained at the overhead of the splitter can be recycled as the reactant,while the EC obtained at the bottom of the splitter is partly fed to the system as a product and partly used as the EO absorbent.

The EC acts as an absorbent as well as a product of the carbonylation reaction in this system.Thus,compared with the ERC process,the biggest difference in structure is that there is no EO refining process in the OSAC process and the EO-EC stream is directly sent to the reactor without any purification process.The light splitter in the system not only plays the role of EC product refining but also regenerates EC as the absorbent to be recirculated.In addition,the elimination of the EO separation process will not only reduce equipment,but also avoid high concentrations of EO,allowing the system to operate under safer conditions.

In addition,another difference is that in the OSAC process,the excess CO2due to the presence of CH4,C2H4,and other impurities cannot be reused directly after the carbonylation reaction.Whereas,these inert components would accumulate in the system.Therefore,in the OSAC process,the process of separating CO2by K2CO3is added.The flowsheet diagram is shown in Fig.4.Compared with MDEA and MEA,the conventional organic CO2absorbents,the K2CO3will absorb less organic impurities in the CO2capture process,which is more suitable for this system.The gas obtained from the overhead of the CO2absorber is mainly CH4and C2H4,which will be sent to the top of the EO absorber tower for mixing,avoiding the loss of elements in the system.

Fig.2.The process flowsheet diagram of the ERC process.

Fig.3.The process flowsheet diagram of the OSAC process.

Fig.4.The Process flowsheet diagram of CO2 separation process in the OSAC process.

2.1.3.Modified OSAC

In the OSAC process,other impurities are also dissolved in the EC absorbent for the process of EO absorption,and the CO2separation process is introduced to avoid the accumulation of impurities.On the base of the OSAC process,the impurity splitter is added to separate CH4and C2H4from the EO-EC solution,as shown in Fig.5.

In the Modified OSAC process,the EO-rich solution is sent to the impurity splitter instead of being sent to the reactor directly.The stream of the EO-rich solution is divided into two parts here.One cold part enters the splitter from the overhead and another part enters the column from the bottom after preheating.This design is mainly to recover the energy of the bottom product stream to reduce the energy consumption of the splitter.The stream of the overhead of the impurity splitter mainly consists of CH4and C2H4,which is sent to the off-gas flasher.

Fig.5.The Process flowsheet diagram of the Modified ERC process.

For the downstream,the purified EO-EC solution is sent to the carbonylation reactor to react with CO2directly,which is similar to the OSAC process.The stream from the carbonylation reactor is sent to the CO2flasher,and the gas from the flasher is recycled after scrubbing.In the Modified process,the light splitter is also employed to purify the EC,but the gas product from the top is not recycled directly to avoid impurity accumulation.

Compared to the OSAC process,a splitter to separate impurities from the EO-EC solution is added in the Modified OSAC process.The recycle CO2with less impurities can be obtained from the CO2flasher,and the recycle CO2can be reused directly without refining by absorption–desorption.

2.2.Thermodynamic Properties and VLE model

The thermodynamic model plays an important role in stimulating the absorption and desorption process.When modeling the OSAC process,the NRTL model is selected to predict the vapor–liquid equilibrium behavior.The binary parameters of the NRTL model for the main components in the system are shown in Table 2.[6]

The T-x diagram for EC and EO,CH4,C2H4is shown in Fig.6,which is calculated by Aspen Properties.The data is calculated at the pressure of 2.0 MPa,which is consist of the operating pressure of the EO absorber.In Fig.6,the x-axis is the molar composition of EO,CH4and C2H4respectively,and the line is the bubble point temperature at different compositions.From Fig.6,it can be seen that the bubble point of EO-EC is much higher than the others.In addition,in the OSAC system,since CO2will be used as a reactant in the downstream carbonylation reaction,CO2is no longer regarded as an impurity during the absorption process.

Fig.6.The T-x diagram for EC and EO,CH4 ,C2 H4 systems at 2.0 MPa.

2.3.Mass distribution analysis

The mass distribution between operating units is a very important parameter when analyzing a chemical process.The carbon chain analysis has been widely used to systematically study processes,such as the IGCC process [16],biomass conversion process[17].Here,the material chain is employed to explore the connection between sections in the studied processes.The material chain method is similar to the carbon chain.Whereas,in this work,instead of expressing all substances in the form of carbon elements,the substances in the system are divided into EO,EC,CO2and water and the others by the material chain method.The others are consisting of CH4,C2H4,etc.,which are contained in the feed gas as impurities.Water is also listed in the analysis,because water acts as the absorbent in the ERC process system.In addition,the concentration of EO is a very important parameter.Thus,EO is also clearly marked during the analysis.By such an effective means,it is possible to clearly show the variation of EO concentration in different manufacture pathways.

Table 2 Binary parameters of the NRTL model for main components [6]

3.EC Absorber Configuration Design

In the OSAC process,the structure of the EO absorber has a great impact on the absorption ratio of EO.For the process of separating EO from the raw materials,the following two problems need to be solved.

First,the concentration of EO in the C2H4oxidation reactor effluent is quite low,which is only 4.07 %(mol).Diluent and unreacted reactants occupy the main parts.Usually,when the target component is too low in the raw material,it will need to increase the amount of absorbent to improve the absorption effect.However,with the increase of absorbent,the energy consumption of the separation process and the concentration of impurities dissolved in the absorbent will be increased.

Second,in view of safety,the required amount of EO in the recycle gas is extremely low to prevent EO from entering the circulating compressor and other equipment.Therefore,EO should be separated as much as possible in the absorption process.

Thus,the structure of the absorption tower for EO capture is designed.The separation target for the absorber was determined to be less than 10×10-6of EO in the recycle gas obtained from the top of the tower,which will be fed to the EO reactor after CO2separation.

3.1.Scheme 1

Scheme 1 is the basic design,which is shown in Fig.7(A).The feed gas is sent into the column from the bottom countercurrent scrubbed by cold EC.According to the simulation results,to achieve the separation target (the EO concentration in the recycle gas from the absorber overhead is less than 10×10-6),the liquid–vapor ratio is determined to be 1.66 (mass).

The design of 20 theoretical plates is adopted.The changing trend of EO concentration in the vapor phase in the tower is shown in Fig.7(B).It can be found that the EO concentration decreased steadily,and the separation target is achieved.

3.2.Scheme 2

In scheme 2,the design of partial rich solvent recycles after cooling is added to the basic design,as shown in Fig.8(A).Lean absorbent countercurrent contacts with feed gas in the column.The exotherm caused by the absorption process raises the temperature of the absorbent.The rich EO solvent from the bottom is divided into two portions.One part is sent downstream,and the other is returned to the absorber after cooling to 323.15 K.

In this design,two variables need to be considered.One is the proportion of the recycle solvent,and the other is the feed stage of the recycle solvent.Analysis of the two variables is carried out simultaneously.The results are summarized in Fig.8(B).When the amount of recycling absorbent increases,the EO concentration in the recycle gas from the overhead of the tower decreases significantly.At the same time,it can be found that it is beneficial to EO absorption when the recycle absorbent enters from the lower section of the tower.

As the results shown in Fig.8(B),the content of EO in the lean gas decreased significantly with the increase of the cycling ratio,and a better absorption effect could be achieved when the cycling ratio reached 0.7.The concentration of EO in the lean gas is lower when the feed position of the circulating absorber is at 17.The fraction of the recycle absorbent is determined to be 0.7 and the feed stage of the recycle absorbent is set at 17.The liquid–vapor ratio of this scheme is 1.61 (mass),which is better than the basic design.

3.3.Scheme 3

In the scheme3,the pump around is set to the absorption column,and the configuration is demonstrated in Fig.9(A).In the initial design,the temperature of the liquid phase in the column is shown as the red line in Fig.9(B).Due to the low specific heat capacity of EC (1.27 kJ·kg-1·K-1,roughly a quarter of water) [16],the temperature increases obviously in the lower section of the column,which is not beneficial to the absorption process.To deal with this problem,two sets of pump-around are added to the tower.The liquid phase is collected from the upper stage and returned to the tower from the next layer after cooling to 323.15 K.The two sets of pump-around are set at 11 and 17 stages of the tower,respectively.According to the blue line in Fig.9(B),the temperature of the absorbent in the tower is significantly reduced with the cooling devices.To achieve the separation target,the liquid–vapor ratio is set at 1.45 (mass).

According to the results of the above three designs,the design of scheme 3 achieves the lowest liquid–vapor ratio under the same separation target.The use of less absorbent will correspondingly reduce energy consumption during the subsequent separation process.Therefore,the structure of scheme 3 is chosen to study the EO absorber.

4.Results and Discussion

4.1.Techeconomic analysis

For the systematic assessment,the processes of the manufacturing of EC from EO containing mixture were modeled in Aspen Plus.The main results are presented in Table 3.The plant scale of all these manufacturing EC processes is 10 × 104ton per year,and the operating time per year is set at 8000 h.

As shown in Table 3,for the ERC process,508.88 kg·h-1EO in the mixture gas is fed and 497.29 kg·h-1EO is separated and converted.97.72% of EO in the mixture gas is used to synthesize the target product.When the consumption of CO2is considered,the raw material utilization efficiency is 96.92%,achieving the effective utilization of feedstock.Nevertheless,the consumption of heat utility in the ERC process is the highest of the studied three processes.Compared with other processes,the carbonylation reaction of the ERC process needs pure EO as the reactant,and the process of separation of water and EO is energy-intensive,which results in the total energy consumption of the ERC process is much higher than the other two processes.In addition,due to the lower flow rate of liquid EC in the effluent of the carbonylation reactor,the amount of CO2absorbed in the liquid phase is lower,which is the reason that the CO2consumption of the ERC processing is much lower.

Fig.7.The structure and concentration analysis for scheme 1.(A) The basic design for the absorber;(B) The changing trend of the EO concentration.

Fig.8.The structure and parameter analysis for scheme 2.(A)The structure of the EO absorber with recycling absorbent;(B)The effects of the feed stage and recycle fraction on the EO separation.

Fig.9.The structure and parameter analysis for scheme 3.(a)The structure of the EO absorber with pump-around;(b)The effects of pump-around on the stage temperature.

The product output of the OSAC process is slightly higher than that of the ERC process.The flow rate of the converted EO is 506.15 kg·h-1,which is 1.78% higher than the ERC process.Since no separation of EO from the EC is required after the EO absorption process,loss of EO due to solvent regeneration or removal of impurities is avoided.Usually,it is easy to refine EC through distillation.However,in the OSAC process,the energy consumption of the EC refinement is much higher than that in the conventional process.Due to the design of the CO2separation process in the OSAC process,the energy consumption of this operating section is considerable.However,the total energy consumption for heating of the OSAC process is very competitive,which is just 43.11% of the mature ERC process.

In the Modified OSAC process,the input and output are quite similar to the process of the OSAC.But,the CO2fed into the system is higher than other processes.The main loss of CO2is caused by absorbed CO2by EC.Although pure EO is not obtained in this process,the energy consumption of the impurity separation from the EO-rich solution cannot be ignored.The total energy consumption of the Modified OSAC process is between the OSAC and ERC processes.Compared with the OSAC process,the CO2separation unit is replaced by the impurity splitter,which leads to less equipment employed.

Table 3 Mass and energy balance for the ERC,the OSAC and the Modified OSAC process

Fig.10.The material chain for the ERC Process.

Fig.11.The Material chain for the OSAC process.

The equipment investment cost analysis for the three processes was carried out by Aspen Process Economic Analyzer.The total equipment investment cost analysis results are listed in Table 3,and the detailed results listed in Table S1.According to the calculations,the Modified OSAC process has the lowest equipment investment cost at 1833.7×103USD.The total equipment cost of the ERC process is 3596.9×103USD for the water purification EO process,which is the major part of the ERC process.The total equipment investment cost of the OSAC process is 2239.4×103USD,accounting for 60% of the total investment,which is lower than the ERC process,but higher than the Modified OSAC process.

Fig.12.The material chain for the modified ERC process.

4.2.Mass distribution analysis

The mass distribution analysis is carried out by the material chain analysis.To make the expression clearer,the flow rate of imported EO in the feed gas is set to 100 kg·h-1,and other flow rates are adjusted according to the flow rate of imported EO.All data were obtained from the calculation results in Aspen Plus.

The results of the material chain for the ERC process is shown in Fig.10.From Fig.10,it can be seen that there is a small amount of EO loss in the EO refinement section of the ERC process.In the actual industry,this part of the EO aqueous solution is usually used as a raw material for the synthesis of ethylene glycol.In the process of carbonylation production,the conversion of EO and CO2achieves full-atom utilization.Although there is a small loss of CO2,the utilization efficiency of CO2can reach 96.48%.

The material chain of the OSAC process is displayed in Fig.11.Since there is no section for EO purification,EO maintains a low concentration in the system.EC in this system acts as both the absorbent and the target product.A large amount of EC circulates in the system,and only 4.87%is sent out of the system as a product.Thus,optimizing the process of EC absorption of EO and reducing the amount of absorbent could be regarded as the future optimization focus of this process.

The material chain results of the Modified OSAC process are very similar to those of the OSAC process,which is shown in Fig.12.Through the separation of the light-component impurities by the EO refinement unit,the EO-EC solution contains only trace amounts of CH4and CO2,which is acceptable for the carbonylation reaction.The main component in the vent gas from the light splitter in the EC refinement section is CO2.The trace amount of CH4entering the carbonylation system from the EO-EC solution is an inert component.To avoid the accumulation of CH4in the system,the vent gas is discharged to a storage tank rather than being recycled.

5.Conclusions

In this work,systematic research was carried out for the three EC synthesis processes,namely,the ERC,the OSAC,and the Modified OSAC process.The energy and material results of these processes were obtained through process simulation in Aspen Plus.In the ERC process,EO absorption by water is employed,which is very mature,but the energy consumption of the separation of EO-water does not have any advantage.The heat utility consumed by the EO refinement occupies 98.96%of the total consumption.In the OSAC and the Modified OSAC process,the process models were established based on the NRTL model obtained from previous work.The structure of the EO absorber was designed to reduce the amount of absorbent,and the liquid–vapor ratio is reduced from 1.66 to 1.45 (mass).Compared with the ERC process,the energy consumed for heating for the OSAC and Modified OSAC process is reduced by 56.89% and 30.03%,respectively.From the perspective of energy consumption,both the OSAC process and the Modified OSAC process have great advantages.Through the assessment and comparisons based on the process simulation,using EC to capture EO and directly carry out the carbonylation reaction is a very feasible route.

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 is financially supported by the National Key Research and Development Program of China (2018YFB0605802),and National Natural Science Foundation of China (No.21978293,U1704251).

Supplementary Material

Supplementary data to this article can be found online at https://doi.org/10.1016/j.cjche.2020.11.017.