Dividing Wall Column Research Articles

Optimization and control of extractive distillation for formic acid-water separation with maximum-boiling azeotrope

It is urgent to develop sustainable chemical process through process intensification, especially for the energy-intensive distillation process. Formic acid-water forms maximum azeotrope which is purified by pressure-swing distillation in Kemira–Leonard (K-L) process and extraction plus distillation in BASF route. However, either process for this maximum azeotrope separation is energy-intensive. In the present work, by selecting the effective heavy entrainer, extractive distillation and extractive dividing wall column was firstly synthesized and optimized with genetic algorithm for formic acid-water separation. Based on optimal design, the energy consumption and CO2 emission among these proposed and existent processes were compared, and extractive distillation shows its strength for achieving high efficiency and sustainability. Then multi-loop PI control scheme and linear model predictive control based on temperature and temperature difference were developed, respectively. Dynamic response validates the control performance superiority of advanced controllers, especially for controlled variables with large overshoot, oscillations and settling time.

Intensification and performance assessment of ethanol production process by hydrogenation of methyl acetate

Fuel ethanol has attracted wide attention due to its advantages of environmental friendliness, high octane number, and excellent explosion resistance. In this paper, the design and intensification for the ethanol production process by hydrogenation of methyl acetate is developed. The conventional process containing reaction units and separation units is established. The separation units are optimized upon minimum the total annual cost (TAC). Subsequently, the dividing wall column (DWC) and extractive dividing wall column (EDWC) techniques are applied to obtain the DWC-EDWC intensified process. The results show that the TAC, CO2 emissions, and total energy consumption (TEC) of the DWC-EDWC intensified process are reduced by 9.4%, 14.9%, and 28.7%, respectively. To further improve thermodynamic efficiency, the DWC-EDWC with intermediate reboiler (DWC-IR-EDWC) intensified process is proposed. It is indicated that the DWC-IR-EDWC intensified process has more advantages in the economy. Finally, pinch technology is carried out to make full use of the heat of process streams. Compared with the conventional process, the TAC, CO2 emissions, and TEC of the modified process with heat exchange networks (HENs) are reduced by 23.7%, 29.3%, 65.4%, respectively, and the thermodynamic efficiency is increased by 9.1%.

Optimal design of the ternary azeotrope separation process assisted by reactive-extractive distillation for ethyl acetate/isopropanol/water

There are numerous issues with the separation process for ternary systems containing multi-azeotropes. Specially, for the aqueous azeotrope as ethyl acetate-isopropanol-water, the presence of water results in a ternary azeotrope, three binary azeotropes and a heterogeneous region. Reactive distillation is introduced to break the azeotropes and simplify complexity by consuming water through ethylene oxide hydration reaction. The triple-column extractive distillation process (TED) and two enhanced processes combined with reactive distillation (i.e., double-column reactive-extractive distillation process (DRED) and reactive-extractive dividing-wall column process (REDWC)) are designed for the separation in this work. A simultaneous optimization strategy is proposed to optimize the parameters through the Particle Swarm Optimization algorithm taking the total annual cost (TAC) as the optimization objective. Then, the CO2 emissions and thermodynamic efficiency are calculated to assess these three processes. Exergy analysis and entropy production are applied to quantify each component to the total irreversibility. Results show that reactive-extractive distillation effectively realize lower TACs with better environmental performance. The TACs of DRED and REDWC processes are reduced by 39.04% and 46.01% compared with TED process, respectively. The thermodynamic efficiency of DRED and REDWC processes increased from 5.40% to 42.88% and 43.64%, respectively.

Configuring topologically optimum vapor recompressed dividing-wall distillation columns to maximize operating efficiency

For dividing-wall distillation columns (DWDCs) separating a heavy-component dominated and wide boiling-point ternary (HCDWBT) mixture, a significant amount of excessive heat exists inevitably in stripping the heavy-component from the intermediate-component and it can be employed to initiate the development of vapor recompression heat pump (VRHP) assisted DWDC (VRHP-DWDC). Despite dividing wall may locate in the top, middle, and bottom, the optimum VRHP-DWDC is found to involve uniformly-two VRHP circles. While the first one serves to compress and transform the excessive heat resulted from the separation of the heavy-component from the intermediate-component, the second one to compress and transform the overhead vapor stream of the light-component pre-heated sequentially with the condensate from the first one and the bottom product stream of the heavy-component, both releasing the temperature-elevated latent heat to the pre-fractionator’s or common stripping section. The processing of two HCDWBT mixtures of benzene/toluene/o-xylene and n-pentane/n-hexane/n-heptane are selected to assess the derived optimum topological configurations of the VRHP-DWDC and their optimality is confirmed through detailed comparisons with the DWDC and two VRHP-DWDCs involving only one VRHP circle. The proposed strategy helps to tap the full potential of the VRHP-DWDC with considerably alleviated complication in process development.

Modelling and Control Analysis of Dividing Wall Distillation Columns

This work aims to study the behavior of fluid mixtures in the dividing wall column, particularly from a controllability point of view. It covers the aspects of design, modeling, and control. A ternary mixture of benzene, toluene, and o-xylene (BTX) is selected as a case study. A controllability analysis for determining and screening the candidate control combinations of the manipulated variables is carried out with the aid of a linearized model using the concept of relative gain array (RGA). The manipulated variables are the reflux (L), the distillate (D), the side stream (S), the bottom (B) and the boilup (V). Based on RGA criterion, two of the candidate combinations are selected to control the column due to the low interaction between control loops. In each combination the manipulated variables are used to control the top level, the bottom level, the top composition, the middle composition and the bottom composition. Finally, the performance of these two combinations is examined and found to be successful in resisting the disturbances.

Sliding mode control of vertical double wall dividing-wall columns

Since a multi-component distillation system has more complex process nonlinearities and time delays than a traditional three-column system, sliding mode control (SMC) is often adopted for multi-component systems. In this paper, a new type of dividing-wall column, called the vertical double-wall dividing-wall column (VDWDWC), is proposed for the separation of quaternary systems. This column has the potential for both higher separation efficiencies and lower equipment costs than the traditional three-column sequence. Therefore, SMC was adopted for the multivariable VDWDWC system and the Smith hysteresis compensation is added to reduce the impact of chattering. The results revealed that the settling time, the oscillations, and steady-state errors of the SMC controller were less than those of the PID controller. Finally, optimal parameters of the SMC controller were obtained by response surface methodology. These parameters reduced the overshoot of the controller, further indicating that SMC could control the VDWDWC system better.

Design and optimization of an inherently safe and sustainable process for the separation of anisole

Sustainable process design problems are multi-faceted approaches that pose challenges in terms of the evaluation of indicators to optimize inherent safety, economics, and control. In recent years, chemical engineering has taken advantage of recent advances in process systems engineering more notably considering intensification to generate green and more sustainable processes. This work presents the design and optimization of the anisole separation process under three intensified and one conventional schemes using a systematized methodology to find optimal designs of purification processes where the interactions between the different sustainability indicators are balanced to obtain optimal configurations. The indeces chosen in the course of process optimization (inherent safety, economic, and control) aim at generating green and more sustainable process alternatives. The results indicate that the most intensified processes, as is the case of the Dividing Wall Column (DWC), is the one with the best sustainability indicators, with better inherent safety and better operability despite it being an intensified process with fewer operational degrees of freedom. In addition, the environmental impact after the optimization of each of the processes were evaluated. Separation through DWC resulted in 2.43% savings in economic terms, 0.31% in inherent safety, improved controllability performance, and a 3.35% reduction in environmental impact, in comparison with the conventional sequence used as baseline. This indicates that in the case of anisole separation, process intensification can yield a more sustainable process design.

A multi-scale and multi-objective optimization strategy for catalytic distillation process

This work aims to establishing a multi-scale and multi-objective optimization strategy for the catalytic distillation process. Firstly, the catalyst layer efficiency factor correlation that embraces the influence of the catalyst layer structure was obtained and then coupled with the process simulation to establish a multi-scale model for the catalytic distillation process. On this basis, genetic algorithm was employed to realize the multi-objective optimization. The hydrolysis process of methyl acetate in the reactive dividing wall column was investigated. The equipment parameters and operational conditions of the catalytic distillation column, and the structure parameters of the catalyst layer are optimized simultaneously. The results indicated that the minima of the total annual cost, the gas emission cost and the exergy loss decrease by 19.26%, 23.11% and 67.27%, respectively, by comparing with the counterparts obtained from the single-factor sensitivity analysis. Furthermore, the catalyst cost decreases by 30.88% per year.

Energy and exergy analysis of a stacked complex sequence and alternatives for ternary distillation

Detailed energy and exergy analyses are conducted for a stacked complex sequence (SCS) and alternatives for a ternary zeotropic distillation problem. Multi-objective optimizations are performed using both cost and thermodynamic efficiency as objective functions. The results show that while a dividing-wall column (DWC) offers a significant improvement in both cost and thermodynamic efficiency compared to a conventional sequence, the SCS performs better by every metric considered and at every point on the Pareto front of cost and thermodynamic efficiency. These results suggest that SCSs deserve serious consideration as an alternative to thermally coupled or DWC configurations for the separation of ternary mixtures.

Towards sustainable separation and recovery of dichloromethane and methanol azeotropic mixture through process design, control, and intensification

AbstractBackgroundHere we analysed the possibility of improving the sustainability performance for the recovery of dichloromethane and methanol from a binary azeotropic mixture using different energy‐intensified extractive distillation‐based processes: side‐stream extractive distillation (SSED), thermally coupled extractive distillation (TCED), and extractive dividing wall column (EDWC). The sustainability performance of the different processes was analysed based on three main factors: total annual cost (TAC), CO2 emissions, and condition number.ResultsThe EDWC was found to give the best improvement in terms of TAC and CO2 emissions by about 18% and 21%, relative to conventional extractive distillation (CED). These however were traded‐off by the increase in conditional number (CN) by 186 times, signifying a complex dynamic characteristic for the EDWC. Thus, the SSED was suggested as an alternative sustainable option as it also provides significant improvement in TAC and CO2 emissions by about 11%, and 18% with respect to the CED, whilst providing the least reduction in operational controllability, as evidenced by the marginal increase in the CN of about 1.5 times. We also investigated the dynamic performance of the SSED and found that the SSED provides identical dynamic performance in handling both ±10% throughput and ±5% feed composition disturbances as those of the CED.ConclusionAmong the different processes, SSED is the best sustainable alternative that provides compromised steady‐state (i.e. TAC and CO2 emissions) and dynamic (i.e. control) performance for the recovery of dichloromethane and methanol. © 2022 Society of Chemical Industry (SCI).

Dynamic controllability strategy of reactive-extractive dividing wall column for the separation of water-containing ternary azeotropic mixture

The reactive-extractive dividing wall column (REDWC), which combines conventional reactive distillation, extractive distillation, and dividing wall column, is a highly integrated process with a compact physical space and strong interrelations between variables. It is challenging to design robust control strategies for the REDWC process. However, there are no investigations reported on the control of REDWC. In this study, through analyzing open-loop controllability indicators of the steady-state process, the basic control structure (CS1) is constructed. Subsequently, four proportional-integral control schemes are further established on the CS1: the proportional control structure (CS2), the internal composition control structure (CS3), the feedforward control structure (CS4), and the three-point temperature control structure (CS5) with feedforward loops. The dynamic performance tests are conducted with ± 10 % feed flow rate and ± 10 % feed composition disturbances. The qualitative research demonstrates that the CS5 offers the best dynamic performance in terms of settling time, steady-state deviations, and overshoots without the use of expensive composition controllers. With the integral absolute error (IAE) as an evaluation criterion, it is quantitatively demonstrated that the CS5 has the best robustness and should be the preferred control scheme. Another key finding shows that the vapor split ratio and the stoichiometric ratio of the reactants are two crucial degrees of freedom to guarantee product quality in the REDWC. The major contributions of this work are to provide a meaningful reference for the control of the REDWC and even more complicated distillation.

Production of ethyl lactate by reactive dividing wall column using analysis of the statics

The sustainable development in separation-reaction processes involves a higher energy efficiency and pollutant reduction. The main interest of the analysis of the statics methodology is developed the optimal qualitative technological scheme with minimal data, the physicochemical information of the system, and the stoichiometry of the reaction. This results in a feasible intensified scheme flowsheet and a reactive distillation column (RD) that are then implemented in a reactive dividing wall column (RDWC) distillation to produce ethyl lactate from the esterification of lactic acid with ethanol. The RDWC configuration showed a reduction of 38% in the save net duty and an increase of 13% in the conversion compared with a conventional direct sequence. The analysis of the statics method presents an alternative in the design stage in a complex column, allowing for a feasible operation in different degrees of process intensification.

Multi-objective optimization of sustainable extractive dividing-wall column process for separating methanol and trimethoxysilane azeotrope mixture

The design and optimization of distillation process for separating azeotropes has attracted extensive attentions in decades. Currently, due to the high emphasis on environmental and social effect, it is necessary to incorporate relevant criteria into the objective function in addition to the economic indicators for approaching a more sustainable process. This paper aims to improve the process for separating the azeotropic mixture of methanol and trimethoxysilane via multi-objective optimization based on NSGA-II. Firstly, the performance of three processes, i.e., pressure-swing distillation, extractive distillation, and extractive dividing wall column (EDWC) distillation, is optimized and compared by using total annual cost (TAC) as the objective function, demonstrating that the EDWC distillation process is the preferred process. Subsequently, a heat pump assisted EDWC process (HP-EDWC) is proposed, and the multi-objective optimization is carried out based on NSGA-II algorithm with three criteria including TAC, carbon emission, and the thermodynamic efficiency, to obtain a set of Pareto solutions, which are then evaluated by using a multi-criteria decision-making method of TOPSIS to give the optimal solution. As a result, the values of TAC, carbon emission, and thermodynamic efficiency of the optimal HP-EDWC process are 90.30×104 $/year, 501.07 kg/h, and 26.02%, respectively.

Optimization of dividing wall columns based on online Kriging model and improved particle swarm optimization algorithm

Dividing wall columns (DWCs) can effectively improve the thermodynamic efficiency of traditional distillation columns. However, DWCs have intricate structures and strong internal interactions. Numerous structural and operational variables are interrelated. This work presents an improved cellular particle swarm optimization based on the online Kriging model (KCPSO) algorithm, and applies it to the optimization of DWC with the objective of minimizing the total annual cost. The algorithm uses the information of particle swarm search to act on the online Kriging model, and reacts on the particle search through the information of the online Kriging model. Calculations demonstrate that the KCPSO algorithm is superior to standard particle swarm optimization (PSO) and cellular PSO (CPSO) algorithms due to its higher quality of iteration. The KCPSO algorithm can effectively overcome the difficulty of early convergence of the CPSO algorithm and the problem that the PSO algorithm is prone to falling into local optimal solutions.

Are process-intensified extractive distillation always energetically more efficient?

The thermally coupled, dividing wall column, and side-stream configurations are some of the energy-intensified techniques widely applied to the extractive distillation for improving the energy efficiency. Today, several heuristics are available for analysing the energy-saving efficiency of these intensified techniques for ideal distillation system but there is no heuristic available for analysing the complex distillations system. Therefore, it has not yet been established what variables affect the energy-savings and under what particular conditions the intensified processes for these complex distillation systems present energy-savings. In this work, we aim to provide insights and preliminary conclusions that open a field of study for a more detailed and in-depth analysis for future researches through three different case studies where their corresponding intensified configurations do not provide any energy-saving relative to the conventional extractive distillation (CED). Based on our preliminary analysis, we attributed this to the poor values of interconnecting flowrate or composition and high column internal vapour flowrate. Our results also revealed the possibility of extending several heuristics, previously developed for evaluating the energy-saving efficiency in ideal distillation system, for complex distillation. Lastly, several recommendations are given that were derived from our simulation results and the analyses we carried out on existing publications.

Comparison of different extractive distillation processes for chloroform/n‐hexane separation: design and control

AbstractBackgroundWhen extracting total glycosides from Panax notoginseng, a waste liquid containing chloroform and n‐hexane will be produced. These waste liquids are not separated and recycled in time, which will not only waste resources but also cause environmental pollution. Chloroform and n‐hexane form an azeotrope which cannot be separated by ordinary distillation methods. This study proposes three extractive distillation processes to separate the chloroform/n‐hexane azeotrope: conventional extractive distillation (ED), side‐stream extractive distillation (SSED) and extractive dividing wall column (EDWC). The total annual cost (TAC) and carbon dioxide (CO2) emissions of the three options were compared, and the optimal process was selected. Furthermore, this research developed dynamic control of the optimal process.ResultsCompared with ED, from the perspective of economic indicators, the SSED process showed 18.9% reduction in TAC yet the EDWC process showed 33.2% reduction in TAC. In terms of CO2 emission indicators, the EDWC process also is more advantageous. Three control structures thus were developed for EDWC: applying ±5% or ±10% feed flow disturbance, and feed composition disturbance. The results show that CS3 with temperature composition cascade control structure can perfectly control all disturbances.ConclusionThis paper designs an economical, environmentally friendly and controllable chloroform/n‐hexane azeotropic separation process, which can provide a reference for the industrial application of chloroform/n‐hexane separation. © 2022 Society of Chemical Industry (SCI).

Separating Mixture Glycols in DWC with Flow Rate-Composition Cascade Control Structure

Dividing wall column (DWC) is an advanced technology which could decrease energy consumption. As the vertical wall inserted in DWC increases the degree of freedom of systems, the control becomes more difficult than the conventional columns. In this study, the mixture glycols (1, 2-propanediol+ 1, 3-butanediol+ 1, 4-butanediol) were taken as feed, an optimized DWC structure was proposed, and three control structures for DWC were tested by ± 10% feed disturbances. The temperature control structure (CS1) was hard to control the system while composition control structure (CS2) and flow rate-composition cascade control structure (CS3) had good performance on controlling the DWC. It showed that CS3 was superior to CS2, as CS3 could stabilize the DWC within 4 hours, and the maximum deviations of the three products, namely 1, 2-propanediol, 1, 3-butanediol and 1, 4-butanediol, were 0.07%, 1.44%, and 0.46% respectively, while CS2 could realize the stability within 7 hours, and the corresponding numbers were 0.35%, 2.78%, and 0.41%, respectively.

Economic, environmental, exergy (3E) analysis and multi-objective genetic algorithm optimization of isopropyl acetate production with hybrid reactive-extractive distillation

Isopropyl acetate (IPAC) is an important fine chemical intermediate with a great demand for industrial production. In view of the high energy consumption in the traditional reactive distillation (RD) process of IPAC production, this study further proposed three enhanced processes of reactive-extractive distillation (RED), reactive-extractive dividing wall column (REDWC) and reactive-extractive distillation coupled pervaporation (REDPV) process. Firstly, the surface charge density distribution between components is calculated by COSMO-SAC model to screen a variety of potential solvents. Combined with the empirical screening method, relative volatility is used to determine the optimal solvent. Then, the conceptual design of RD, RED and REDWC three processes are completed based on reaction kinetics, and the operating conditions of processes are optimized by multi-objective genetic algorithm. Furthermore, REDPV process using pervaporation module instead of solvent recovery column is proposed. Finally, the total annual cost (TAC), gas emissions and exergy efficiency are used to evaluate the proposed process. The results show that compared with RD process, the TAC and gas emission of RED process are reduced by 42.69% and 49.82%. While those of REDWC process are reduced by 45.06% and 52.93%. The thermodynamic efficiency of these two processes is increased by 63.28% and 62.67%. Compared with RED process, REDPV process further reduces the cost by 14.45% and the gas emissions by 34.55%, but obtain the lowest thermodynamic efficiency.

An enhanced mesoscale model for laminar and turbulent three-dimensional simulations of the gas phase flow with concentration mixing in structured packing columns

This work presents a mesoscale model (MSM) simulating the laminar and turbulent gas phase fluid dynamics and concentration mixing in structured packings, used in absorption and distillation columns or static mixers. The MSM aims to provide computationally efficient predictions of the three-dimensional distributions of pressure, velocity and concentration in the structured packing’s gas phase. The MSM is successfully calibrated and validated with virtual experiments obtained from DNS (direct numerical simulations) of a single structured packing section featuring concentration mixing under uniform and maldistributed velocity inlet profiles. The packing section scenarios exhibit a variety of physical effects, like backflow, high-velocity fronts and blockage at the packing’s borders to the wall, which are all predicted correctly by the MSM. The physical effects have an impact on concentration mixing and velocity smoothing and are particularly interesting for applications of structured packings in dividing-wall columns or static mixers. Finally, the MSM is set up to simulate cylindrical columns with alternately rotated structured packing sections, including wall regions and wipers. MSM simulations of three packed columns of varying diameter are compared against experimental data and correctly predict the pressure drop dependence on the diameter, demonstrating the MSM’s potential for scale-up predictions.

Optimization and model-based control of sustainable ethyl-methyl carbonate and diethyl carbonate synthesis through reactive distillation

As excellent solvents of lithum-ion electrolyte, ethyl-methyl carbonate (EMC) and diethyl carbonate (DEC) are synthesized through transesterification of dimethly carbonate (DMC) and ethanol. Process intensification, optimization and the corresponding control scheme of the synthesized process are essential when sustainability is becoming a necessity. In the present work, the intensified flowsheet for EMC and DEC production process was firstly proposed and optimized with conventional reactive distillation (RD) and reactive dividing wall column (RDWC), respectively. Compared with RD process, RDWC could achieve 25.3% and 8.2% total annualized cost reduction for EMC and DEC production process, respectively. And carbon dioxide emission could be decreased by 29.4% and 11.7% by further integrating RD and separation column into RDWC arrangement for DEC and EMC production. Then multi-loop PI control schemes were developed as benchmark and linear model predictive controller was designed to handle the persistent feed disturbance. The superiorities of model-based controllers for the strongly interactive and coupled system are demonstrated by the dynamic response comparisons in terms of settling time, overshoot and integral of squared error (ISE), especially for the controlled variables with poor control performance under conventional multi-loop PI control structure. To summarize, the comprehensive control performance index-ISE with MPC is reduced by 55.9%–99.1%, as compared to those under PI control strategy.