Comprehensive analysis on the economy and energy demand of pressure-swing distillation and pervaporation for separating waste liquid containing multiple components

Hongru Zhang, Yusen Chen, Haiyang Cheng, Yangyang Wang, Peizhe Cui, Shiqing Zheng,Zhaoyou Zhu, Yinglong Wang,, Yanyue Lu, Jun Gao

1 College of Chemical Engineering, Qingdao University of Science and Technology, Qingdao 266042, China

2 School of Chemistry and Chemical Engineering, Guangxi University for Nationalities, Nanning 530006, China

3 College of Chemical and Environmental Engineering, Shandong University of Science and Technology, Qingdao 266590, China

Keywords: Distillation Pervaporation Organic compounds Sequential iterative algorithms Economy and energy consumption

ABSTRACT A large amount of waste liquids containing methanol/acetone/water mixtures are produced in the synthesis of methyl methacrylate (MMA). Under the advocacy of green chemical industry, it is urgent to develop an efficient, economic and energy-saving mixture separation process. Through thermodynamic azeotropic behavior and pressure sensitivity analysis, pressure-swing distillation was determined and the optimal separation pressure of each column in the process was obtained. Due to the composition of waste liquids produced were quite different in MMA production, the pressure-swing distillation separation process was designed to fully achieve the accurate waste liquids treatment. Taking the total annual cost (TAC) as the target, the sequential iteration method was used to optimize the process, and the impact of composition on economy was compared. In order to further realize the energy-saving of the separation process,the pervaporation membrane module was introduced to pretreat the waste liquid in the pressure-swing distillation. The results showed that the TAC of the coupling process was 46%higher than that of the pressure-swing distillation process, and the thermodynamic efficiency was 30%higher.This study provides waste liquid treatment technology for enterprises and analyzes its economic and energy efficiency, which has reference significance for the development of coupled separation technology.

1. Introduction

The new materials applied in downstream optical, electronic and automotive fields have received higher attention due to the rapid development of new material industry. As an important raw material of high-performance material polymethylmethacrylate (PMMA), methyl methacrylate (MMA) has a large demand and rapid industrial development [1,2]. Acetone cyanohydrin(ACH) process is an industrial MMA production process [3]. The main raw materials are acetone and hydrocyanic acid, of which hydrocyanic acid is produced through the by-product of acrylonitrile, and the auxiliary material methanol. There are a lot of waste liquid containing methanol, acetone and water in the MMA production process, and acetone and methanol are usually used as raw materials for cost calculation. Therefore, the recovery of solvent in the mixture is crucial to the MMA production.

In addition to acetone and methanol,there is a large amount of water in the waste liquid and the ternary azeotrope is obviously affected by pressure, which is difficult to achieve efficient separation and recovery of high-value organics. Special distillation plays an important role in the chemical separation process, and has a great impact on the reaction,product quality,energy consumption,investment. Among them, the pressure-swing distillation shall be preferred for separation when the composition of azeotrope changes by more than 5% due to pressure change [4]. Its principle is based on the pressure sensitivity of special azeotropic mixture systems [5,6]. Different from azeotropic distillation [7,8] and extractive distillation [9,10], pressure-swing distillation does not introduce new components to avoid product contamination. Lüet al.[11]studied the continuous homogeneous azeotropic distillation and pressure-swing distillation for separating minimum azeotropic system, indicating that pressure-swing distillation had greater advantages in economy and energy consumption. Zhanget al.[12] studied the process of coupling liquid–liquid separation and pressure-swing distillation to separate azeotropic system in combination with thermal integration technology, and found that this process has better economy than extractive distillation,which provides a reference for the separation of mixtures with complex thermodynamic phase behavior.Thus, the pressure-swing distillation is widely used in the separation and purification of mixtures and industrial production with remarkable achievements.

The composition of waste liquid produced in each process of industrial production is obviously different, which has a great impact on the design of mixture separation scheme [13]. The change of feed composition will cause the change of material balance and process conditions of the whole column [14,15]. Due to different reaction degree,conversion rate and purification requirements in each process of MMA production,the water content in the waste liquid produced will vary greatly. Therefore, it is of great industrial value to develop the corresponding high efficiency separation process of pressure-swing distillation for different compositions. However, 45%-55% of energy consumption in chemical process is used for separation, of which distillation energy consumption accounts for 49%[16].Therefore,the energy-saving measures of distillation have always been the focus of attention.

With the increasing requirements for environmental protection,it is urgent to improve equipment efficiency through technological innovation and process improvement to reduce unit energy consumption and waste discharge.The commonly used energy saving methods in distillation process include optimizing the process,improving the operating parameters, and improving the column structure[17–19].Gaoet al.[20]proposed a green and sustainable azeotrope separation method with high comprehensive and reference value based on a chemical-looping concept with the help of reversible-reaction-assisted distillation, which effectively promoted the development of distillation research. Genget al. [21]combined transesterification reaction with pressure-swing and thermal integration technology to ensure efficient conversion and separation and purification of azeotrope.This study proved the feasibility of coupling process in the reaction-separation process,and promoted the utilization of high value-added resources.

In order to adapt to technological progress, new separation technologies (membrane separation [22], supercritical extraction[23] and adsorption [24]) have been put forward. Pervaporation(PV)is a new separation purification and concentration technology,which is a membrane permeation process with phase change[25].The separation process is not limited by the component vapor–liquid balance, and can achieve the separation that are difficult to complete by traditional processes with low energy consumption[26]. Donget al. [27] proved for the first time that the prepared bis (triethoxy) acetylene derived silicone membrane has great potential in pervaporation separation of methanol/organic azeotropes after 90 days of long-term testing. Maet al. [28] studied a new bimetallic Zn/Co-ZIF membrane with ultra-high separation performance to separate methanol and methyltert-butyl ether mixture, which proved that pervaporation has great advantages in separating mixture.

The pervaporation is considered to be a promising separation technology in the separation of azeotropic mixtures,solutions with similar boiling points, thermosensitive compounds, organic and organic mixtures due to its advantages of convenient operation,high efficiency, low energy consumption and no pollution[29,30].However,pervaporation technology is difficult to compete with conventional separation technology,and high vacuum conditions lead to increased economic costs[31,32].Thus,the traditional process and pervaporation process are coupled to complement each other’s advantages and achieves good economic and energy consumption effects. Harviantoet al. [33] proposed a new mixed process that combines thermal coupled reactive distillation with pervaporation membrane to improve the production efficiency ofn-butyl acetate, and found that the coupled process significantly improved the energy efficiency and economy of the process. Hanet al. [34] applied the reaction-vapor permeation-distillation coupling process to the esterification and found that the introduction of vapor permeation process had higher feedstock conversion and better separation effect,which opened up a new idea for strengthening the distillation process.

In this work,the high efficiency separation process of pressureswing distillation was determined based on the thermodynamic phase equilibrium mechanism analysis and pressure optimization of ternary pressure sensitive mixture.In view of the different composition of waste liquid in industrial MMA production process,the influence of continuous composition change on the pressure-swing distillation separation process was studied. And the process was optimized with economic cost as the goal.Combining the selective dissolution diffusion model mechanism of pervaporation technology for aqueous mixtures, the coupling process of pervaporation pretreatment and pressure-swing distillation was developed, and the effects of the two processes in terms of economy and energy consumption in the separation process of high water mixtures were analyzed, providing reference for the development of efficient separation and energy saving processes for aqueous mixtures.

2. Analysis of Mixture Characteristics and Pressure Sensitivity

2.1. Thermodynamic phase behavior and sensitive analysis

In this work, the non-random two liquid (NRTL) method was selected to predict the thermodynamic properties and phase behavior for the ternary mixtures of methanol-acetone–water using Aspen Plus simulation software combined with the results of previous studies and the validation of the model [35–37]. The azeotropic compositions and temperatures for the ternary mixture of methanol-acetone–water under different pressures were shown in Table 1 and Fig. 1.

Fig. 1. Thermodynamic phase diagram of azeotropic behavior of the ternary mixture of methanol-acetone–water at different pressures (1 atm = 101.325 kPa).

Table 1The azeotropic characteristics for the ternary mixtures of methanol-acetone–water

It can be seen from Table 1 and Fig.1 that the ternary mixtures only have the binary azeotrope of methanol-acetone under normal pressure; there are two binary azeotropes of methanol-acetone and acetone–water when the pressure is 2 atm (1 atm =101.325 kPa) and 3 atm; when the pressure is greater than 4 atm, there are also ternary azeotropes of methanol-acetone–water besides two binary azeotropes of methanol-acetone and acetone–water. In addition, the azeotropic temperature is close and the azeotropic components change significantly with pressure, so the separation is difficult under specific conditions. Therefore, the separation of ternary azeotropes of methanol-acetone–water by pressure-swing distillation is feasible.

2.2. Economic analysis of process

The economic cost is one of the most important factors to be considered for chemical industries.Therefore,the total annual cost(TAC) [38,39] was selected as the index of process optimization in the process design of waste liquid separation. On the basis of previous studies [40], equipment considered in economic calculation was determined. In this study, sequential iterative optimization algorithm was selected to calculate the operation cost and operation cost of the process.The corresponding formula was as follows:

The specific calculation formula of each module has been listed in Table S1(in Supplementary Material).The specific optimization path of the sequential iterative algorithm was shown in Fig. 2. On the premise of ensuring product purity, process optimization can be carried out according to the sequence of parameters in Fig. 2.The optimization variables were as follows:

Fig. 2. Optimization sequence and difference of single process and coupling process.

The constraint functions were as follows:

The purity of all the above products was 99.9%.In the optimization process,the boundaries of all design variables were as follows:

Different from the single pressure-swing distillation process,PV/pressure-swing distillation process needs to optimize not only the parameters of the distillation column, but also needs to optimize the membrane components modeled by Aspen Custom Modeler software [41,42]. The difference between the two processes has been marked in Fig. 2.

2.3. Pressure optimization

The selection of pressure in each distillation column directly affects the separation effect of mixture and the economic cost of the process for pressure-swing distillation [43]. The sequential iterative optimization algorithm was used to compare the impact of pressure changes on economic costs on the premise of ensuring product purity, and the results were shown in Fig. 3.

Fig. 3. Relationship between distillation column pressure and economy (1 atm =101.325 kPa).

It can be found from the figure that there was only a binary mixture of methanol and acetone under normal pressure, and the azeotropic behavior of the mixture under other pressures becomes complex. Taking the industrial waste liquid consisting of 2% (mol)methanol-8% (mol) acetone-90% (mol) water as an example, the economic analysis of the process was carried out.With the increase of pressure, TAC decreased first and then increased. Therefore, the three columns pressure of pressure-swing distillation was selected as atmospheric pressure first and then high pressure.

3. Design of Separation Scheme

In this work, the total feed flow for the mixtures of methanol/acetone/water was 800 kmol∙h-1. The pressure-swing distillation and PV/pressure-swing distillation were used to separate the mixture, and the economy and energy consumption of the two processes were compared.

3.1. Pressure-swing distillation

According to the composition difference of waste liquid mixture generated in each process of MMA production, the composition of mixtures was divided into 9 different proportions (18% (mol)methanol/72% (mol) acetone/10% (mol) water, 16% (mol) methanol/64% (mol) acetone/20% (mol) water, 14% (mol) methanol/56%(mol) acetone/30% (mol) water, 12% (mol) methanol/48% (mol)acetone/40% (mol) water, 10% (mol) methanol/40% (mol) acetone/50% (mol) water, 8% (mol) methanol/32% (mol) acetone/60%(mol) water, 6% (mol) methanol/24% (mol) acetone/70% (mol)water, 4% (mol) methanol/16% (mol) acetone/80% (mol) water and 2% (mol) methanol/8% (mol) acetone/90% (mol) water). Based on the pressure sensitivity analysis in Fig. 3, the pressure of columns 1 and 2 in pressure-swing distillation was selected as atmospheric pressure, and the pressure of column 3 was 4 atm. The designed process flow was shown in Fig. 4.

Fig. 4. Process of separating the mixture of methanol/acetone/water by pressureswing distillation.

Based on the sequential iterative optimization algorithm in Fig. 2, the pressure-swing distillation process under 9 different feed compositions was optimized with TAC as the target. With the water content in the waste liquid as the variable, the parameters of pressure-swing distillation were optimized. The optimization processes were shown in Figs. S1-S9, and the final result was shown in Fig. 5. The column equipment parameters were listed in Table 2 and corresponding flowsheets were shown in Fig S10-S18.

Fig. 5. The effect of feed composition on the economy of pressure-swing distillation.

Table 2The column equipment parameters of pressure-swing distillation

It can be seen from the above figure that with the increase of water content in the feed, the annual economic cost gradually decreases. With the increase of water content in the mixture and the decrease of alcohol and ketone content, the parameters of the three columns tray had no obvious change, but the reflux ratio,condenser and reboiler load had obvious changes. This reflected that the phenomenon that methanol acetone mixture existed under normal pressure and water was easy to separate. It showed that alcohol and ketone in the complex mixture of methanol/acetone/water were more difficult to separate than water, which had a great economic impact on the process.

3.2. PV/pressure-swing distillation

The separation process is realized by the difference of the properties of each component of the mixture under certain conditions[44–46].Now,the application of membrane separation technology in water treatment can be used for both water supply and wastewater treatment [47]. Among them, PV membrane separation process plays a very important role in solving the problem of water shortage [48,49]. The PV membrane separation process uses the selective permeation membrane as the separation medium(Fig. 6), and the mechanism was shown in Fig. 7.

Fig. 6. The mechanism of mixture separation without phase change by PV membrane.

Fig. 7. Process of separating the mixture of methanol/acetone/water by PV/pressure-swing distillation.

The components on the raw material side selectively penetrate the membrane to achieve the purpose of separation or purification when the certain driving force is applied on both sides,and there is no phase change in the mixture [50,51].

Permeation flux is the mass (ΔG,kg) of material per unit area(Am, m2) penetrating the membrane in unit time (t,h), and it can be written asJ(kg∙m-2∙h-1).Jindicates the degree of permeability through the membrane, which determines the membrane area required to complete a certain separation requirements. The formula is as follows:

The pervaporation membrane is based on the dissolutiondiffusion mechanism model [52], and the corresponding diffusion coefficient (D) is mainly related to temperature and pressure, as shown in the formula below:

The pervaporation separation of methanol, acetone and water from organic/water by B-ZSM-5 zeolite membrane were analyzed by analyzing the research work of Bowen [53], and the formulas for the relationship between permeability and temperature (T,K)are as follows:

The types of PV membrane can be roughly divided into hydrophilic pervaporation membrane [54] and hydrophobic pervaporation membrane [55]. Taking the mixture composition (2% (mol)methanol/8% (mol) acetone/90% (mol) water) with the best economic performance as an example, design the coupling process to better compare the separation effect of single distillation and coupling process on aqueous waste liquid. In this work, PV membrane was used as the dehydration pretreatment process when water content in wastewater was high to reduce energy consumption for the subsequent pressure-swing distillation separation of azeotropes, and as shown in the Fig. 7:

The PV/pressure-swing distillation process was optimized with TAC as the target based on the sequential iterative optimization algorithm in Fig.2.The corresponding column equipment parameters were shown in Table 3.

Table 3The equipment parameters of PV/pressure-swing distillation

In order to obtain high-purity water as much as possible and reduce the treatment capacity of the distillation process, three PV membrane modules were used in cascade for waste liquid pretreatment.Compared with Table 2,the heat loads of the three columns in the distillation section were significantly reduced, which was because the flow to be treated was reduced by the pretreatment of PV membrane modules. The TAC of the coupled process was 3.4056 × 106USD∙a–1, including the cost of columns with 1.8133 × 106USD∙a–1and the cost membrane of module with 1.5923×106USD∙a–1,which was 46%higher than that of the single pressure-swing distillation process.The above results showed that PV membrane module plays a significant role in the dehydration of methanol/acetone/water mixture wastewater,but it will lead to an increase in economic costs.

4. Process Analysis and Comparison

In addition to annual economic cost, thermodynamic efficiency[56] is usually used as an indicator to evaluate process performance. Through the analysis of energy conversion, transmission,use and loss in the process, the size of energy consumption is revealed, and the direction and method for improving the process and energy utilization are pointed out.On the basis of the first law and the second law of thermodynamics, the relationship between the temperature, heat load and work done of the incoming and outgoing materials of the system is related,so that the energy consumption and energy utilization rate of the system can be calculated, so as to judge the space for energy conservation, so as to achieve the goal of energy conservation, production increase and economic benefit improvement [57]. The corresponding formulas of thermodynamic efficiency (η) were as follows:

In order to further analyze the difference of single pressureswing distillation and PV/pressure-swing distillation for the ternary waste liquid mixture of methanol/acetone/water,the two processes were compared in terms of economy and energy consumption, as shown in Fig. 8.

Fig. 8. The economy and energy consumption of two processes.

The TAC of the pressure-swing distillation process in Fig. 8 was 2.333 × 106USD∙a–1, the thermodynamic efficiency was 27.22%.The TAC of PV/pressure-swing distillation process was composed of two procedures: the TAC of distillation was 1.8133 × 106USD∙a–1, the TAC of membrane module was 1.5923 × 106USD∙a–1.Therefore, the addition of PV membrane module has laid a good foundation for the subsequent separation for waste liquid mixture of methanol/acetone/water by pressure-swing distillation. The removal of a large number of water components effectively reduced the difficulty of separating waste liquid by pressure-swing distillation, reduced the economic cost of the distillation part by 22.28%.However, in order to meet the purity requirements, the large membrane area required in the pretreatment and dehydration process of the membrane module led to the increase of the economic cost.Therefore,the total economic cost of the coupling process increased by 46%compared with the single pressure-swing distillation process.On the other hand,PV membrane module can achieve the separation task that is difficult to complete by traditional methods with low energy consumption because of its separation process is not limited by the gas–liquid equilibrium of components, which greatly improves the thermodynamic efficiency of the coupling process.Compared with the single pressure-swing distillation process, the thermodynamic efficiency was increased from 27.22%to 35.34%(improving 30%).The above results fully reflect that the single pressureswing distillation process has more advantages in economy than the coupling process,and the membrane module plays an important role in energy saving and consumption reduction.

5. Conclusions

In this work,pressure-swing distillation and PV/pressure-swing distillation were adopted to realize the effective separation of the methanol/acetone/water waste liquid mixture produced in the MMA synthesis process according to the thermodynamic characteristics of the mixture and the technical characteristics of the special distillation and coupling process. Through the analysis of azeotropic behavior and pressure sensitivity of the mixture, the pressure of each column and the combination sequence of pressure-swing distillation were determined. Taking TAC as the target, the sequential iterative optimization algorithm was used to optimize the processes. Combined with the different composition of waste liquid in each procedure of MMA production, the influence of different compositions on the pressure-swing distillation process was further studied. The results showed that the water in the mixture could be effectively separated due to the formation of binary azeotrope of methanol-acetone under atmospheric pressure. With the increase of water content in the material, the difficulty of separation and the economic cost of column equipment was reduced. Taking the lowest cost feed composition in a single process as an example,the PV membrane module was coupled to pretreat and dehydrate the material before the distillation process separates the complex mixture. The results showed that the coupling process had higher thermodynamic efficiency and better energy-saving effect due to the green and environment-friendly nature of PV. However, in order to realize the high purity recovery of the solvent, a large area of PV module was required, which makes the coupling process more expensive.In the conclusion, this work not only provides help for the waste liquid treatment in the MMA production process,but also provides an energy-saving scheme for the separation of other complex alcohol-ketone-water mixtures.

CRediT Authorship Contribution Statement

Hongru Zhang:Conceptualization,Methodology,Investigation,Writing – original draft.Yusen Chen:Formal analysis, Software,Data curation,Visualization.Haiyang Cheng:Visualization,Formal analysis, Software, Visualization.Yangyang Wang:Visualization,Data curation, Writing – review & editing.Peizhe Cui:Visualization, Writing – original draft, Writing – review & editing.Shiqing Zheng:Conceptualization, Supervision, Writing – original draft.Zhaoyou Zhu:Resources, Supervision,Writing– review& editing.Yinglong Wang:Conceptualization, Methodology, Investigation,Funding acquisition,Writing–original draft.Yanyue Lu:Investigation, Resources, Supervision, Project administration, Writing –review & editing, Supervision.Jun Gao:Resources, Supervision,Writing – original draft.

Declaration of Competing Interest

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

Acknowledgements

This work was supported by the National Natural Science Foundation of China (22078166).

Supplementary Material

Supplementary data to this article can be found online at https://doi.org/10.1016/j.cjche.2023.04.016.

Nomenclature

ACH acetone cyanohydrin

Ammembrane surface area, m2

Ddiffusion coefficient, kg∙m-2∙h-1

ΔGmass of permeate, kg

Jpermeation flux, kg∙m-2∙h-1

MMA methyl methacrylate

NEF1 feed position of circular logistics

NF1 feed position of column 1

NF2 feed position of column 2

NF3 feed position of column 3

NRTL nonrandom two-liquid

NT1 number of trays in column 1

NT2 number of trays in column 2

NT3 number of trays in column 3

PMMA polymethyl methacrylate

PV pervaporation

TAC total annual cost, USD∙a-1

tpermeation time, h

η thermodynamic efficiency