Extractive Distillation Scheme Research Articles

Applicability exploration and sustainable assessment of heat integration and vapor recompression heat pump to side-stream extractive distillation processes for separating ternary azeotropic system

The side-stream extractive distillation (SSED) process is attracting extensive attention due to its high energy efficiency for separating ternary azeotropic mixture. However, the correlation between the bottom temperature level of distillation columns depending on the flowrates and compositions of side streams and the introduction of heat integration (HI) and vapor recompression heat pump (VRHP) technology is ignored. In the work, a range of initialization extractive distillation processes including one conventional extractive distillation (CED) process and five SSED processes considering different flowrates, compositions, and feed locations of side streams are proposed for separating acetonitrile/isopropanol/water ternary azeotropic system. The optimization results with the minimum the total annual costs (TAC) as the optimization objective show that not all SSED schemes are superior to the CED separation process, and the SSED separation schemes can achieve up to a 2.72 % reduction in the TAC. Following that, the HI and VRHP assisted extractive distillation schemes are proposed based on the temperature levels between the top and bottom of different distillation columns and compressor restraints for the initialization extractive distillation schemes mentioned after optimization. Moreover, sustainable evaluations covering economy, energy consumption, environment, and energy utilization are implemented for the extractive distillation schemes proposed. The evaluation results indicate that the CED-HI scheme overcomes all SSED-HI and SSED-VRHP schemes in terms of economic costs, but the optimal SSED-HI processes can reduce 8.77 % of energy consumption and 4.73 % of CO2 emissions, while the energy utilization raised from 1.70 % to 1.78 % compared to the CED-HI process.

Design and multi-objective optimization of hybrid extractive distillation process for separating the toluene-methanol-water ternary azeotrope

Cleaner production and sustainable development are still hot topics for modern industry. Developing green technology with low energy consumption, low pollution, and low cost has always been the common pursuit of chemical, petroleum, pharmaceutical, and other industries. This article proposes an environmentally friendly and efficient solvent, glycerol, for the separation of the ternary azeotrope of toluene-methanol-water. Three feasible extractive distillation schemes were designed and optimized, while the double-column extractive distillation process with a decanter demonstrated superior performance when glycerol was used as the solvent. Additionally, the multi-objective genetic algorithm was employed to optimize these separation processes and determine the optimal operating conditions, taking total annual cost (TAC), CO2 emissions (ECO2), and separation efficiency (Eext) as target performance indicators to achieve the objectives of energy conservation, consumption reduction, and emission reduction. Results demonstrated that the newly proposed scheme can significantly reduce the total annual cost by up to 33.1 % and CO2 emissions by 50.5 % compared with the existing separation process. This work provides a new insight to separate and purify the multiple systems by extractive distillation in industry.

Application of diabatic extractive distillation schemes with preliminary separation of azeotropic components for separation of acetone-toluene-<i>n</i>-butanol mixture

Objectives. The study aims to investigate the effectiveness of the use of various options for organizing the process of diabatic distillation in the separation of a mixture of acetone-toluene-n-butanol by extractive distillation using dimethylformamide as an entrainer in a scheme with preliminary separation of azeotropic components.Methods. As the main research method, mathematical modeling in the Aspen Plus V. 12 software package was used. As a model for describing vapor-liquid equilibrium, the local composition Non-Random Two Liquid equation model was used. Parametric optimization of diabatic schemes was carried out according to the criterion of reduced energy costs.Results. Based on the scheme for extractive distillation of an acetone-toluene-n-butanol mixture with preliminary separation of azeotropic components, five options for organizing diabatic distillation schemes were considered, both with and without use of a compressor to reach a required flows temperature.Conclusion. It is shown that the use of diabatic schemes in the extractive distillation of a acetone-toluene-n-butanol mixture with dimethylformamide makes it possible to diminish the reduced energy costs by 8.9-43.5%. Meanwhile the maximum reduction in energy consumption is achieved in a scheme where upper vapor flows of two other columns are used to heat the azeotropic components separating column.

Reaction-assisted separation of acetone from the azeotrope of methanol and acetone

The separation of methanol–acetone azeotrope has been a challenging issue. Methanol in the azeotrope was selectively transformed and removed in this paper. Under mild conditions, 95.5% of methanol was transformed into methyl phenylcarbonate (MPC), phenol and dimethyl carbonate (DMC), while acetone was not converted. The separation process of reaction products was simulated, and 99% of acetone with purity of 99.3 wt% was recovered. The byproducts phenol and DMC were also recovered with purity of 93 and 76 wt%, respectively. Compared to the typical pressure-swing and extractive-distillation schemes, the total number of theoretical stages and heat duty could be reduced by 27.7–38.3% and 48.6–58.4%, approximately. The results of thermodynamic experiments showed that the apparent equilibrium constant (124.4 at 23.1 °C) in the first step was much larger than that (12.9) in the second step, indicating that the intermediate MPC was very stable, and thermodynamic factors originating from the specific coplanar structure of substrate contributed to the high methanol conversion. The transesterification reactions were endothermic and spontaneous due to the positive ΔH and negative ΔG values. The spontaneity of reactions was attributed to the increase of entropy, for which the reason was the more possible spatial distributions of atoms in the products. This paper is beneficial for the separation of alcohols from various aprotic solvents.

Energy efficiency of diabatic distillation schemes for an acetone–toluene–n-butanol mixture with an entrainer in the first column

Objectives . To investigate the effectiveness of various options for organizing the process of diabatic distillation in the separation of a mixture of acetone–toluene–n-butanol by extractive distillation (ED) with dimethylformamide as an entrainer in a scheme where an entrainer is used in the first column. Methods . Mathematical modeling in the Aspen Plus v. 12.1 software package was used as the primary research method. The local Non-Random Two Liquid composition equation was used as a model for describing vapor–liquid equilibrium. Parametric optimization of diabatic schemes was carried out according to the criterion of reduced energy costs. Results . Based on ED scheme for an acetone–toluene–n-butanol mixture with an entrainer in the first column, four options for organizing diabatic distillation schemes were considered, both with and without increasing the temperature of the flows due to compression. Conclusion . It is shown that the use of diabatic schemes in the ED of an acetone–toluene– n -butanol mixture with dimethylformamide can decrease energy consumption by 11–17%. While the maximum reduction in energy consumption is achieved in a scheme using a compressor, the efficiency of schemes without a compressor is slightly lower. Nevertheless, the technological design of the latter is much simpler.

Ethyl acetate production from dilute bioethanol with low energy intensity

Ethyl acetate production by esterification and bioethanol recovery from dilute solutions (11 wt %) are energy-intensive processes. Although ethanol is a raw material for ethyl acetate production, both technologies are usually studied independently. Here, a cleaning coupled process was proposed. The dilute stream of ethanol was fed to a reactive distillation column, so reflux and columns for dehydration of ethanol were not required. This increases the steam flow at the bottoms of the reactive zone. Steam was found to displace the equilibrium allowing a total recovery of products in the reactive distillation column, ethyl acetate (top) and water (bottoms). So, it was found that, counterintuitively, steam favors the recovery of ethyl acetate, avoiding the over-recycle of raw materials such as acetic acid. An extractive distillation scheme using dimethyl sulfoxide (DMSO) was also proposed to minimize the energy requirements for the purification of a mixture of ethanol, water, and ethyl acetate. The decision tree model was used to estimate the importance of operating variables on total annualized costs. The operating conditions were selected considering steady-state multiplicities. The energy intensity was further decreased by heat integration with vapor compression from 2.4 to 2.8 to 0.9–1.4 MJ/kg-ethyl acetate. The scheme proposed in this work was found to be a novel intensified process as the energy intensity of the scheme proposed was between three and seven times lower than that reported previously in the literature.

Evaluation on the separation effect and extractant recovery efficiency of extractive distillation for separating ethyl acetate/methanol with ionic liquids as extractants

For the purpose of recycling resource and cleaner production, the extractive distillation schemes with different ionic liquids as extractants are proposed to separate ethyl acetate and methanol. In fact, the evaluation of extractants should not just focus on the separation effect, the effectiveness for the recovery of extractants which has a great influence on the overall benefits of extractive distillation process also need to be paid more attention, especially for the ionic liquids (ILs). On the basis of the VLE diagram, σ-Profiles, and interaction energy based on molecular dynamics, the separation effects of the selected extractant are following the order of 1-ethyl-3-methylimidazolium acetate ([EMIM][OAc]) > 1-ethyl-3-methylimidazolium diethyl phosphate ([EMIM][DEP]) > dimethyl sulfoxide (DMSO). However, the decomposition temperature of [EMIM][DEP] is much higher than that of [EMIM][OAc], which leads to the better effectiveness for extractant recovery. Then, several recovery configurations are explored owing to different decomposition temperature of selected ILs. The result shows that the heat integration assisted the process with [EMIM][DEP] as extractant which utilizes two-stage evaporators extractant recovery configuration has a better performance than other processes and can reduce 35.07% of TAC, 37.59% of energy consumption and 37.36% of gas emission compared with conventional extractive distillation process with DMSO as extractant

Vapor-liquid equilibrium experiment and extractive distillation process design for the azeotrope ethyl propionate n-propanol using ionic liquid

Ionic liquid 1-ethyl-3-methylimidazolium acetate ([Emim][OAC]) and organic solvent ethylene glycol (EG) were evaluated for their performance to disrupt the ethyl propionate and n-propanol azeotrope through the phase equilibrium experiment. Isobaric vapor–liquid equilibrium (VLE) data of ethyl propionate + n-propanol, ethyl propionate + n-propanol + [Emim][OAC] and ethyl propionate + n-propanol + EG was measured at atmospheric pressure. The experimental apparatus and procedure were confirmed for reliability by comparing the binary experimental VLE data to published data. The NRTL model was employed to correlate the VLE data and predict the phase equilibrium behavior of systems. The results demonstrated that the separation performance of [Emim][OAC] is more excellent than EG, and the more ionic liquid is added, the better the separation performance. The simulation of the extractive distillation process using [Emim][OAC] as entrainer was developed via Aspen Plus software. The optimal operating variables were determined by reducing the total annual cost (TAC) to a minimum. Finally, the economic assessment analysis indicated that the TAC of optimal operating conditions is 803.39 k$ and the feasibility of the extractive distillation scheme to separate azeotropic systems.

Towards sustainable separation of the ternary azeotropic mixture based on the intensified reactive-extractive distillation configurations and multi-objective particle swarm optimization

The separation of ternary azeotropic systems has received significant interest as it enables the recovery of value-added organic solvents, subsequently contribute towards environmental protection. In this work, we propose a novel approach that involves the conceptual design, multi-objective optimization, and process evaluations for developing two different processes, i.e., double-column reactive-extractive distillation (DCRED) and reactive-extractive dividing wall column (REDWC), for the separation of ethanol/tert-butanol/water ternary azeotropic mixture. The conceptual design of the proposed processes was conducted using kinetic and thermodynamic analysis while optimal operating conditions of the established processes were obtained via multi-objective particle swarm optimization algorithm. Then, both developed processes were evaluated based on the total annual cost (TAC), CO2 emissions, and thermodynamic efficiency. From the steady-state simulation, DCRED and REDWC provides a TAC of 1.056 × 106 US$ and 1.117 × 106 US$, respectively. Likewise, it provides CO2 emissions of 731.27 kg/h and 733.42 kg/h, respectively. The energy efficiency of the DCRED and REDWC were found to be 1.285% and 1.055%, respectively. Relative to the conventional extractive distillation process, the TAC and CO2 emission for the proposed DCRED reduced significantly by 55.4% and 61.8%, respectively. Similar reduction was also observed for the REDWC which provides 52.8% and 61.7% lower TAC and CO2 with respect to the conventional process. In addition, the thermodynamic efficiency of the developed DCRED and REDWC processes are improved by 40.4% and 15.3% in comparison to the conventional extractive distillation scheme.

Optimal modes of side-section flow in heat-pump-assisted extractive distillation systems for separating allyl alcohol–allyl acetate mixtures with butyl propionate

Objectives.To investigate the influence of side-section flow modes on the energy efficiency of apartially thermally coupled distillation sequence (PTCDS) with a vapor recompression heat pumpfor the extractive distillation of an allyl alcohol–allyl acetate mixture with n-butyl propionate andidentify modes under which the combined use of a PTCDS and heat pump are the most efficient.Methods.Mathematical modeling in the Aspen Plus V10 software package was used as the mainresearch method. The local composition equation of the non-random two-liquid model was usedas a model for describing the vapor–liquid equilibrium, while the Redlich–Kwong model was usedto consider the non-ideal vapor phase. When modeling the conventional extractive distillationscheme and PTCDS, parametric optimization was carried out according to the criterion of the totalenergy costs in the column reboilers. For the economical evaluation, Aspen Process EconomicAnalyzer V10.1 tools were used.Results.For extractive distillation of a mixture of allyl alcohol (30 wt %) and allyl acetate (70 wt %)with n-butyl propionate as an entrainer, the minimum energy consumption was achieved at thesame side-section flow mode for the variants of a PTCDS with and without a heat pump. Thereduction in energy costs relative to the conventional scheme was 20% for the sequence withouta heat pump and 38% for that with a heat pump. An economic assessment was made of thebest options in comparison with the conventional extractive distillation scheme. The PTCDS witha heat pump had an advantage over the sequence without a heat pump only for long periods ofoperation.Conclusions.For the extractive distillation of an allyl alcohol–allyl acetate mixture, the optimalmodes for the combined use of a PTCDS with a vapor recompression heat pump coincide with theoptimal modes for a PTCDS without a heat pump.

Energy saving in the extractive distillation of isobutyl alcohol–isobutyl acetate with n-butyl propionate

Objectives. Determination of the effectiveness of using various types of heat pumps in the extractive distillation of an isobutyl alcohol–isobutyl acetate mixture with n-butyl propionate as the entrainer.Methods. As the main research method, mathematical modeling was performed using the Aspen Plus V. 9 software package. As a model for describing the vapor–liquid equilibrium, the local composition equation-based UNIQUAC model was employed, and the Redlich–Kwong model was adopted to examine the non-ideal vapor phase. When modeling the conventional scheme of extractive distillation, parametric optimization was carried out according to the criterion of total energy costs in the reboilers of the columns. For economical evaluation, Aspen Process Economic Analyzer V10.1 tools were employed. Results. In comparison with the conventional extractive distillation scheme, three variants of schemes with vapor-recompression heat pumps were considered: with a heat pump placed on an extractive distillation column, on an extractive agent regeneration column, and with two heat pumps placed on both columns of the scheme. A scheme with an internal heat pump was also proposed, in which the heat pump compressor is located between sections of extractive columns that operate at different pressures: 506.6 kPa in the top sections and 101.3 in the bottom section. An economic analysis was conducted for all the considered schemes to calculate the total annual costs. It was shown that schemes with vapor-recompression heat pumps can significantly reduce the energy costs of extractive distillation by up to 39.6%; however, a significant reduction in the total annual costs is achieved only with sufficiently long operation periods of the plants. The reduction in the energy costs in the scheme with an internal heat pump was 44%, and the total annual costs were in the range of 20.2–30.1%, depending on the operating time of the plant. Conclusions. It was shown that using heat pumps in the extractive distillation of the mixture of isobutyl alcohol–isobutyl acetate with n-butyl propionate as the entrainer can significantly reduce energy costs. The scheme with an internal heat pump is the most economical of all the considered schemes.

Energy-saving heat integrated extraction-azeotropic distillation for separating isobutanol-ethanol-water

In the production of isobutyl alcohol by hydroxyl synthesis process, a large number of industrial effluent rich in isobutyl alcohol and ethanol would be produced, and the direct discharge could cause double harm to resources and environment. Therefore, it has important economic and environmental value to develop efficient and energy-saving separation methods to realize the recovery of high value-added chemicals. Based on the conventional four column extractive distillation scheme (CED), this paper developed a more energy-saving extractive-azeotropic distillation alternative (EAD), which reduced energy consumption and TAC by 16.14% and 22.25%. Then, the steam recompression and variable pressure heat exchange were introduced to complete the heat integration strengthening design (HI1-EAD, HI2-EAD), which saves 54.32% and 54.36% energy consumption respectively. In order to further improve the energy efficiency, the dividing wall column distillation strengthening configuration (EADWC) was embedded into the EAD scheme. Compared with the CED scheme, TAC of the EADWC scheme and its heat integration configuration (HI-EADWC) decreased by 23.21% and 36.90%, respectively. However, the high bottom temperature of the main column leads to a significant increase in the duty and equipment investment, which makes the HI-EADWC scheme not show enough attraction. Through comprehensive evaluation, the HI2-EAD alternative presents more application potential.

Comparison of extractive distillation flowsheets for methanol–tetrahydrofuran–water mixtures

Objectives. Synthesis and comparative analysis of the extractive distillation flowsheets for aqueous mixtures of solvents utilized in pharmaceutical industries using the example of a methanol−tetrahydrofuran−water system with various compositions. The ternary system contains two minimally boiling azeotropes that exist in a vapor–liquid phase equilibrium. To evaluate the selective effect of glycerol, the phase equilibria of the methanol–tetrahydrofuran–water and methanol–tetrahydrofuran–water–glycerol systems at 101.32 kPa were studied.Methods. The calculations were carried out in the Aspen Plus V.9.0 software package. The vapor–liquid equilibria were simulated using the non-random two-liquid (NRTL) equation with the binary interaction parameters of the software package database. To account for the non-ideal behavior of the vapor phase, the Redlich–Kwong equation of state was used. The calculations of the extractive distillation schemes were carried out at 101.32 kPa.Results. The conceptual flowsheets of extractive distillation are proposed. The flowsheets consist of three (schemes I–III) or four (scheme IV) distillation columns operating at atmospheric pressure. In schemes I and II, the extractive distillation of the mixtures is carried out with tetrahydrofuran isolation occurring in the distillate stream. Further separation in the schemes differs in the order of glycerol isolation: in the third column for scheme I (traditional extractive distillation complex) or in the second column for scheme II (two-column extractive distillation complex + methanol/water separation column). Sсheme III caters to the complete dehydration of the basic ternary mixtures, followed by the extractive distillation of the azeotropic methanol–tetrahydrofuran system, also with glycerol. Sсheme IV includes a preconcentration column (for the partial removal of water) and a traditional extractive distillation complex.Conclusions. According to the criterion of least energy consumption for separation (the total load of the reboilers of distillation columns), sсheme I (a traditional complex of extractive distillation) is recommended. Additionally, the energy expended for the separation of the basic equimolar mixture using glycerol as the extractive agent was compared with that expended using another selective agent: 1,2-ethanediol. Glycerol is an effective extractive agent because it reduces energy consumption, in comparison with 1,2-ethanediol, by more than 5%.

Ionic liquid extractive distillation for the recovery of diisopropyl ether and isopropanol from industrial effluent: Experiment and simulation

To recover diisopropyl ether (DIPE) and isopropyl alcohol (IPA) from industrial wastewater through extractive distillation (ED), the most suitable extrainer 1-ethyl-3-methylimidazolium acetate ([EMIM][OAc]) was screened out by comparing the selectivity of DIPE to IPA and IPA to water in ionic liquids (ILs) composed of 9 common cations and 12 common anions. Theoretical insight at the molecular level based on surface charge-density (σ-profiles) and excess enthalpy (HE) was provided to analyze the hydrogen bonding and separation mechanism between IL and the azeotrope. Isobaric vapor-liquid equilibrium (VLE) experiment of DIPE + IPA + [EMIM][OAc] and IPA + water + [EMIM][OAc] further validated the predicted results. After inputting the binary interaction parameters, the ternary ED configuration with [EMIM][OAc] as entrainer was simulated, and the thermal integration scheme was further explored. Comprehensive evaluation demonstrated that when compared to ethylene glycol (EG), the energy-integrated ED scheme with [EMIM][OAc] as entrainer saves energy consumption and the total annual cost (TAC) of 27.71% and 12.29%, respectively. This separation method provides a more energy-efficient and environmentally friendly choice for clean production of IPA.

Vapor-liquid equilibrium experiment and process simulation of extractive distillation for separating diisopropyl ether-isopropyl alcohol using ionic liquid

The formation of azeotrope makes the separation of diisopropyl ether (DIPE) and isopropanol (IPA) a challenging task. Ethylene glycol (EG) and ionic liquid 1,3-dimethylimidazolium dimethyl phosphate ([Mmim] [DMP]) were screened as entrainers for the extractive distillation process. Isobaric vapor-liquid equilibrium data of DIPE + IPA + EG and DIPE + IPA + [Mmim] [DMP] was measured through experiment. The NRTL model was applied to correlate the experiment data and predict the effect of entrainers on the phase behavior of azeotrope in the full concentration range. The results indicate that [Mmim] [DMP] performs better ability than EG in enhancing the relative volatility of DIPE/IPA azeotrope and eliminating azeotropic point. The design and simulation of the two extractive distillation schemes were carried out after embedding the model parameters and the physical properties of IL into Aspen Plus. The optimal operating parameters were obtained though the sequential iterative procedure based on the total annual cost (TAC). The results of the economic analysis showed that the scheme 2 using the flash tank instead of the solvent recovery column has 24.582% reduction of TAC and 31.241% energy savings compared to the scheme 1.

Dynamic controllability investigation of an energy-saving double side-stream ternary extractive distillation process

The investigation of dynamic controllability for the double side-streams ternary extractive distillation (DSTED) is necessary due to its advantages in energy-efficient. However, the control strategy of the energy-saving DSTED scheme is unique and complex issues owing to the wobbly of side-stream flow rate. Herein, control strategy of the energy-saving DSTED scheme is explored for separating ternary azeotropic mixtures acetonitrile/methanol/benzene. Inspiring from the control strategy of the ordinary triple-column extractive distillation scheme, the fundamental control structure CS1 is firstly designed. The control scheme CS2 with feed/side-stream (F/SS) ratio and control structure CS3 with feedforward and F/SS are then investigated to achieve higher efficiency of process control. Finally, a robust control strategy CS4 with the ratio of reboiler duty and feed flow rate and without F/SS ratio is proposed to well maintain the product purities. Moreover, additional ±15% feed flow rate and composition disturbances are added in the CS4 to assess the stability and controllability of the energy-saving DSTED. Dynamic performances illustrate that the structure CS4 can handle as well as that of confronted ±10% disturbances when the large disturbances are occurred in the real chemical process.

SYNTHESIS OF THE THERMALLY COUPLED DISTILLATION SEQUENCES

Approaching a hypothetical thermodynamically reversible process is one of the main directions of energy saving in distillation. In practice, this is achieved by the use of systems with coupled thermal and material flows (STMP). Such systems have long been used in the separation of zeotropic mixtures. As follows from the analysis of literature data, they save energy costs by up to 30%. Recently, it has been revealed that the use of such systems is possible, expedient and energetically advantageous for separating the close-boiling and azeotropic mixtures by extractive distillation (ED). The article considers the main approaches to the synthesis of distillation schemes, including systems with STMP: 1) a method based on the construction of a "superstructure"; 2) a method based on the concept of thermodynamically equivalent configurations; 3) an evolutionary algorithm; 4) a method based on the transformation of distillation scheme graphs. As the analysis of the literature has shown, the first three methods are sufficiently well developed only for the synthesis of distillation schemes with STMP to separate zeotropic mixtures. There are a relatively small number of publications that consider the generation of ED schemes with STMP based on the concept of thermodynamically equivalent configurations for distillation of specific binary mixtures. The only current system approach to the synthesis of ED schemes with coupled thermal and material flows for multicomponent mixtures is the method of graphs (algorithm 4). At present, the first step has been taken to implement it in the form of a computer program. Thus, the article presents the current state of the problem of synthesizing highly effective, energy-saving distillation technologies, including extractive distillation, based on the analysis of publications over the past 20-25 years.

Entrainer based economical design and plantwide control study for Tetrahydrofuran/Water separation process

In this work, the control structure design for a continuous extractive distillation scheme using entrainers for separating THF–water azeotropic mixture into high purity product (THF) has been studied. The selection of suitable entrainer for the undertaken process was purely based on economical design criteria where total annualised cost (TAC) for different entrainers were first calculated and further compared. Once the suitable entrainer was selected based on the design economics, later the plantwide control structure was evaluated for disturbance rejection capabilities and smooth process operations in the face of feed throughput and composition change. For economical design operations, the recycle rate of entrainers has to be kept around a minimum recycle rate. However, operating below minimum entrainer recycle rate leads the product purity to fall sharply in conventional control structure designs due to THF–Water azeotropic constraints. The presented control structure design in this works ensures that product purity is not lost even in case of sever disturbances in feed throughput (by ±10%) or a changed feed composition (by ±5%).

ACETONE-CHLOROFORM-n-BUTANOL MIXTURE SEPARATION BY THE EXTRACTIVE DISTILLATION IN SCHEMES OF TWO-OUTLET COLUMNS

Extractive distillation of acetone-chloroform-n-butanol mixture with dimethylformamide in two-outlet column schemes is considered. Optimal parameters according to the total energy consumption criterion in the column boilers of the three extractive distillation schemes for this mixture separation are determined. Calculations were carried out in a design-verification version at 1000 kg/hr of the initial mixture with the concentrations of acetone, chloroform and n-butanol 71.3, 14.7 and 14.0% wt., respectively. Dimethylformamide concentration in the entrainer flow was set to 99.99 wt%. The main component concentration in the product stream was 99.9 wt% for chloroform and 99.5 wt%. for acetone and n-butanol. The parameters to be optimized were: the number of plates in the columns, the temperature and flow rate of dimethylformamide, reflux ratios, distillate flow rates and the position of the feed plates in the columns. The optimum location of the entrainer feed plate was found additionally in the extractive distillation column. Separation product concentrations served as the constraints of the optimization. The optimization was carried out in Aspen Plus with the use of a combination of Sensitivity Analysis and sequential quadratic programming (SQP). It is established that scheme P5 has the lowest energy consumption. In the first column of this scheme, n-butanol is separated, and then the azeotrope-forming components (acetone and chloroform) are separated by the extractive distillation subsystem. Energy consumptions for two other schemes (P1 and P2), in which dimethylformamide is used in the first column of the sequence, are significantly higher than for scheme P5 - by 69.1% and by 49.3%, respectively. The data obtained will be used: to synthesize and optimize the extractive distillation schemes including the subsystems with coupled thermal and material flows to separate the acetone-chloroform-n-butanol mixture; to estimate the energy efficiency of those schemes and to obtain the criterion for estimating the energy efficiency of systems with coupled thermal and material flows in the extractive distillation of multicomponent mixtures.

Optimal retrofit design of extractive distillation to energy efficient thermally coupled distillation scheme

Design and optimization procedures employing the response surface methodology (RSM) for retrofitting the conventional extractive distillation sequence to a thermally coupled extractive distillation scheme (TCEDS—SR) is presented. The optimum TCEDS—SR structure can be found in a practical manner with minimal simulation runs. Furthermore, the RSM allows the interactions between variables to be identified and quantified. The separation of close boiling point mixtures and azeotropic mixtures was examined to test the proposed method. The predictions agreed well with the results of a rigorous simulation. The results showed that a retrofit of the extractive distillation sequence to TCED—SR can achieve significant energy savings compared to the conventional extractive distillation sequence. © 2012 American Institute of Chemical Engineers AIChE J, 59: 1175–1182, 2013