Distillation Sequences Research Articles

Energy-saving thermally coupled ternary extractive distillation process using ionic liquids as entrainer for separating ethyl acetate-ethanol-water ternary mixture

The major intrinsic obstacle of extractive distillation (ED) is high energy consumption. Thus it is significant to reduce the energy consumption of ED processes as low as possible. In this work, using green solvent ILs which have high selectivity, process intensification and optimizing operation parameters are adopted in the ED of ethyl acetate-ethanol-water mixture. For high-selective entrainers, ionic liquid (IL) 1-butyl-3-methylimidazolium acetate ([bmim][OAc]) is used as the solvent for the ED process of ethyl acetate-ethanol-water, and the physical properties of [bmim][OAc] are accurately defined in Aspen Plus by correlating the experimental data. To optimize the distillation sequence for process intensification, thermally coupled ternary extractive distillation (TCTED) process is adopted for the separation of the ternary mixture. Moreover, multi-objective generic algorithm with total annual cost (TAC), CO2 emissions (ECO2) and thermodynamic efficiency (η) as objective functions is used to optimize the operation parameters in order to evaluate the economy, the energy efficiency and environmental impact of alternative ternary extractive distillation processes. The result shows that the TCTED process can result in the reduction of both TAC and ECO2. So, this study can provide a reference for the separation of ethyl acetate-ethanol-water mixture in industry.

Optimization of Distillation Sequences with Nonsharp Separation Columns

Nonsharp distillation sequences are widely used in industrial separation processes; however, most current research has not discussed this topic, except in sequences with heat integration under special operating conditions, including complex columns. The sequence with nonsharp separation has the features of general distillation sequences, which are usually optimized by adjusting the separation sequence and the design/operation parameters of each column in the sequence, making the optimization a mixed integer nonlinear programming (MINLP) problem, which is usually hard to solve. With inclusion of nonsharp separation columns, the sequence optimization becomes even more complicated and computationally intensive. This work aimed to optimize the distillation sequence, including nonsharp distillation alongside simple columns and dividing wall columns. Inspired by the dynamic programing method for sharp distillation sequence, a framework for automatic optimization is proposed to decompose the MINLP problem into integer programming (IP) and nonlinear programming (NLP) problems. The optimization processes of sharp and nonsharp distillation sequences are compared and the solution space in terms of the possible number of distillation sequences with nonsharp separation is discussed. Two optimization cases, including an industrial one, are included to validate the proposed framework.

Synthesis of simultaneously heat integrated and thermally coupled nonsharp distillation sequences based on stochastic optimization

Synthesizing simultaneously heat integrated and thermally coupled distillation sequences (HITCDSs) is a complex combinatorial optimization problem and it is difficult to solve this problem, especially when considering nonsharp splits. Based on stochastic optimization, we propose an efficient approach to synthesize nonsharp HITCDSs. First, an implicit mathematical formulation for the HITCDS optimization is presented. Next, encoding strategies are used tactfully to represent HITCDS structures and evolving rules are developed to generate neighboring HITCDS structures. Finally, simultaneously optimizing the distillation sequences, thermal coupling structures, heat integrated networks, and operating parameters of the columns is easily attained through an improved simulated annealing algorithm. Three cases are studied to validate the high solution efficiency of the approach. The optimization results show that nonsharp HITCDSs are more energy efficient than the sharp ones. Besides, HITCDSs have lower total annual cost than conventional distillation configurations, configurations only employing heat integration, and configurations only employing thermal coupling.

Pressure swing distillation for the separation of methyl acetate‐methanol azeotrope

AbstractPressure swing distillation (PSD) process was used to separate methyl acetate‐methanol azeotrope. Through the phase diagram analysis, distillation sequence and technological process were determined. On the basis of the principle of the minimum total annual cost (TAC), design variables such as feeding position, reflux ratio, and column plate number were optimized and the optimal parameters were determined. The high‐pressure column (HPC) under various pressures were analysed. The economic results of the PSD without heat integration, with partial heat integration, and with full heat integration were compared. The comparison results indicate that the TAC of the PSD without heat integration is the highest. The PSD with partial integration and full heat integration can effectively reduce the total capital cost and total operating cost. Compared with the PSD without heat integration, the TACs of partial heat integration and full heat integration exhibit 27% and 30% savings, respectively. Therefore, the PSD with full heat integration is more economically reasonable for separating methyl acetate‐methanol azeotrope.

Temperature difference control and pressure-compensated temperature difference control for four-product extended Petlyuk dividing-wall columns

The four-product extended Petlyuk dividing-wall column (FPEP-DWC) saves 50% more energy than traditional distillation sequences. However, the complex structure of FPEP-DWC results in strong interactions between different control loops and highly nonlinear behaviors. For the control of the FPEP-DWC, the temperature control (TC) scheme fails to handle the disturbances inserted into the process and none of the existing control schemes can, to the best of our knowledge, avoid the use of a composition controller (which is not recommended in industrial processes). The proposed TC scheme in the current work is also unable to handle the disturbances. Therefore, a kind of temperature difference control (TDC) scheme is investigated for the FPEP-DWC to improve its controllability and operability. A pressure-compensated temperature difference control (PCTDC) is also studied for the FPEP-DWC to compensate the pressure variations. To provide an illustrative example of the FPEP-DWC fractionating, a mixture containing methanol, ethanol, 1-propanol, and n-butanol was used to verify the feasibility of the TDC and PCTDC schemes while a wide range of feed disturbances are confronted. The outcomes show that the dynamic performances of the FPEP-DWC are substantially enhanced in most cases by using the proposed TDC and PCTDC schemes, which are conducive to the industrialization of the FPEP-DWC.

Comparison of pressure-swing distillation with or without crossing curved-boundary for separating a multiazeotropic ternary mixture

The separation of tetrahydrofuran (THF)/ethanol/water mixtures from the norgestrel production is a challenging work due to the presence of multiple azeotropes, for which several pressure-swing distillation sequences are proposed in this paper, including heat-integrated schemes. Meanwhile, the potential benefits of those options with crossing curved-boundary are investigated and for this reason, a THF-rich feed composition is assumed. These sequences are optimized based on the minimal total annual cost (TAC) by using the sequential iterative search method coupled with heuristic pressure decision rules. The comparison results show that the proposed processes with boundary crossing have great economic and energy saving advantages, apart from one configuration based on the non-azeotropy property of THF/ethanol at a pressure. Also, a qualitative assessment of operational and control implications for energy integration schemes is made to draw recommendations.

Design and optimization of energy-efficient liquid-only side-stream distillation configurations using a stochastic algorithm

The liquid-only side-stream option is sometimes preferred to replace the vapor–liquid interconnection in thermally coupled distillation to achieve similar energy efficiency but easier controlling. This work enumerates all possible liquid-only type distillation sequences for ternary systems before a fair comparison of them, through a case study of equimolar BTX (benzene–toluene–xylene) mixture. The optimization problem can be formulated as a mixed integer nonlinear programming (MINLP). A self-adapting dynamic differential evolution (SADDE) algorithm is used to address its difficulties from inherent nonlinearity and nonconvexity. This stochastic optimization algorithm, written in MATLAB, is linked to Aspen Plus to ensure calculation accuracy. Evaluation and comparison of ten configuration varieties verify the outperformance of liquid-only type configurations over simple distillation sequences with main equipment in common, saving both in energy and economic cost.

Techno-economic analysis of acetone-butanol-ethanol distillation sequences feeding the biphasic condensate after in situ gas stripping separation

Distillation processes for separation of biphasic ABE solution from in situ gas stripping were developed. The distillation sequences were investigated and optimized. Results proved the energy requirement of distillation was effectively reduced when feeding the biphasic ABE mixture. After techno-economic analysis, a superior scenario that started with a decanter followed with side-by-side sequences for separation acetone and ethanol was selected. Compared with the conventional processes that detractively concentrate the biphasic ABE mixture, energy requirement and total annual cost (TAC) of the novel distillation process were decreased by 25.8% and 17.4%, respectively. $ 4102.98 K per year of TAC was estimated in a case with a capacity of 30 K tons of butanol per year.

Optimization of Distillation Sequence Based on Integration of Reaction-Separation System

On the basis of the integration of reaction and distillation systems, a systematic method is proposed for optimizing the distillation sequence with components separated individually by simple distillation columns and analyzing the effect of reactor parameters. Matrixes are employed to illustrate splits and distillation sequences; the distribution law of elements in the matrixes and the universal logical algorithm are deduced for identifying the structure of distillation sequence. With the reaction kinetics and material balance incorporated, the procedure is developed for targeting the optimal distillation sequence with the minimum vapor rate. On the basis of this method, the optimal distillation sequence at different reaction conditions can be identified, as well as the optimal reactor parameters. The proposed method is used to analyze the reaction–separation system of a styrene unit, and the results are consistent with the energy consumptions obtained by the rigorous simulation of Aspen Plus.

Multi-objective optimization of intensified processes for the purification of levulinic acid involving economic and environmental objectives

Levulinic acid obtained from acid hydrolysis of lignocellulosic biomass is considered within the twelve main chemicals from biomass in terms of economic potential due to its large number of applications. As product of hydrolysis is obtained a quite diluted stream which contains a lot of water. Consequently, the processing cost is high and it has limitations for further scaling up at industrial level using conventional separation schemes. In the downstream process proposed in this work, initially the water content is removed using a liquid-liquid extractive column, then it is purified by means of a distillation columns-based process. Recently, a set of separation designs has been implemented, including the use of decanters and intensified columns, which has reduced the cost of the process. However, these studies have only focused on optimizing the total annual cost, losing sight of the environmental impact of the process. In addition, some improvements may be applied from the process intensification point of view either by thermally couplings or considering a single or multiple walls in a column. Therefore, in this work four LA purification designs are studied, a conventional distillation sequence, a dividing wall column arrangement and a decanter, a dividing wall column configuration with a decanter and thermal coupling, and two dividing wall column and decanter. Those schemes were designed and optimized under a rigorous optimization process by means of a multi-objective hybrid algorithm, differential evolution with tabu list considering two objectives: 1) the total annual cost as the economic criterion and 2) the eco-indicator 99 as the environmental index. The results indicated that the intensified design (using a single dividing wall column) presents better results, with economic savings about 8.42% and a decrease in 10.94% the Eco-indicator 99 in comparison with the optimal conventional sequence.

Intensifying Kaibel dividing-wall column via vapor recompression heat pump

The four-product Kaibel dividing-wall column (KDWC) can save about 40% energy compared with the conventional distillation sequences. Nevertheless, the energy efficiency of KDWC are likely to be increased further by employing vapor recompression heat pump (VRHP) technology. The focus of this paper is to explore effective configurations to intensify the four-product KDWC via vapor recompression heat pump. Separation of two kinds of mixtures including n-pentane, n-hexane, n-heptane and n-octane and benzene, toluene, o-xylene and tri-methyl-benzene are chosen as illustrative cases. The standard vapor recompression heat pump assisted KDWC configuration (KDWC–SVRHP) and the vapor recompression heat pump assisted KDWC configuration with an intermediate reboiler (KDWC–VRHP-IR) are proposed in this paper and compared with the conventional distillation sequences and the original KDWC configuration. The feasibility and effectiveness of intensifying KDWC via vapor recompression heat pump technology is verified and the energy saving ability is explained by the exergy loss analysis. Results prove that KDWC–VRHP-IR is able to substantially reduce the operating cost of KDWC.

Biobutanol Purification by Liquid-Liquid Extraction Assisted Divided Wall Columns

Biobutanol is receiving a great interest from both academia and industry sectors, and some companies are already focused on revamping bioethanol plants to produce biobutanol. The recovery of fuel grade butanol by distillation was proved to be not economically sustainable. On the other side, hybrid flowsheets, obtained combining liquid-liquid extraction and distillation, were proved to be a valid alternative. Divided wall columns, as one of the most promising intensified distillation alternatives, were here explored in combination with liquid-liquid extraction. A multiple objective function taking into account the economy, the environmental impact and the process controllability was defined to screen the alternatives. Among all the configurations considered, liquid-liquid extraction combined with a DWC equipped with two reboilers and a side rectifier, reached 22% and 18% reduction of the economy and environmental index respectively in comparison with conventional schemes. At the same time, also the controllability was improved compared to the hybrid liquid-liquid assisted simple column distillation sequence considered as a reference.

Optimal Operation of a DWC by Self-Optimizing Control: Active Vapor Split Approach

Dividing Wall Column(DWC) offers the large potential for operating and capital cost saving in compared with conventional distillation sequence. In the studied DWC in this study, the aid of Vmin diagrams, it is shown that without a suitable value for vapor split fraction bellow the dividing wall in different operating conditions, the energy requirement increases from optimal value and it will lead to the suboptimal operation. Accordingly, a control structure based on the self-optimizing concept is designed using vapor split fraction as the control degree of freedom (active vapor split). To find the best self-optimizing Controlled Variables (CV) the exact local method is used. It is shown that the value of loss with the aid of active vapor split is lower than 0.7 percent of nominal value with using the conventional single measurement self-optimizing CV, which is a reasonably small value, and using a more complex combination of measurement as a self-optimizing CV provides a little enhancement in reducing the loss. So, including vapor split fraction in the self-optimizing control structure of the DWC can save energy and in the meantime avoid the complexity of combination of measurement. Moreover, the dynamic simulation studies show that the proposed control structure with the simple decentralized control loops can conveniently stabilize the plant and reject the effects of disturbances.

Design and control of diphenyl carbonate reactive distillation process with thermally coupled and heat-integrated stages configuration

It has been extensively proven that thermally coupled distillation columns can effectively use less energy than their conventional counterparts. Similarly, the extension to reactive systems has also shown that thermally coupled reactive distillation columns can reduce the energy consumption in comparison with their conventional reactive distillation counterparts. This work aims to show that by realizing heat integration between thermally coupled columns at different pressure, further energy savings can be attained. The diphenyl carbonate production process has been taken up to show that there is a synergistic effect when thermally coupling and heat integration are combined in the same distillation sequence. The results showed that the proposed system could attain 47% energy savings in comparison with conventional a reactive distillation sequence while keeping good rejection of throughput and feed composition disturbances.

Effect of Organic Solvents in Separation Section of Levulinic Acid Production: Synthesis of Distillation Sequences

This work highlights the system specific characteristics encountered when considering a distillation-based separation strategy for levulinic acid (LA) and the by-product formic acid (FA) in the presence of water and water-miscible sulfolane or ?-valerolactone (GVL) solvents. Simulation using Aspen Plus® was applied to identify the presence of distillation boundary surfaces and tangent pinches resulting from the non-ideal behaviour of the mixtures in question. In the applied feed composition regions, the separation performance of the GVL solvent system in particular is limited by these restricting conditions. Minimum energy requirements for the separation of valuable components are presented as the conceptual design performance indicators, allowing the benchmarking of separation strategies. Sidedraw column configurations offer advantages in enriching the dilute valuable compounds from the feed and performing difficult separations at lower solvent quantities. This is especially advantageous for the sulfolane system where both LA and FA are intermediate-boiling.

Development of the Reaction/Distillation matrix to include more complicated Reaction/Distillation systems and performance evaluation using an ethylene hydration case study

Reaction/Separation systems are very important in chemical industries and synthesis of all possible configurations for these systems is necessary for finding the best process alternative. An algorithm for synthesis of different alternatives for reaction–distillation systems is presented in this paper. The presented algorithm is a developed version of the Separation Matrix which has been introduced for generation of different distillation sequences. An initial version of the Reaction/Distillation matrix for a two-component reaction prevailing within a four-component system has already been introduced by the authors. The current research is the expansion of the previous work to include multiple reactions in a system containing any number of reactive and non-reactive components. This algorithm can also generate the sequences including components forming binary azeotropes. For performance evaluation purposes, a case study including two reactions is presented in which a binary azeotrope is formed between two components. The possible sequences for the system are generated using the introduced algorithm and the obtained sequences are simulated and compared in terms of total annual cost.

Simultaneous optimization of nonsharp distillation sequences and heat integration networks by simulated annealing algorithm

Based on stochastic optimization, a new method is proposed to synthesize heat integrated distillation sequences (HIDiSs), which are basic configurations and allow nonsharp splits with at most two middle components. Distillation sequences and heat integration networks are simultaneously optimized to minimize the total annual cost (TAC) of HIDiSs. First, the synthesis problem is formulated as an implicit mixed-integer nonlinear programming problem. Discrete variables are distillation sequences. Continuous variables include operating pressures, key component recoveries and ratios of the actual reflux ratios to the minimum reflux ratios in columns. Next, solution strategies are presented, including representing distillation sequences through a novel encoding method, randomly generating neighboring distillation sequences, automatically determining heat integration networks by the pinch method, and calculating the TAC based on shortcut design of columns. Then, the optimization problem is solved by an improved simulated annealing algorithm. Finally, correctness verification for the method is made in two case studies. The optimization algorithm is proved to be computationally efficient and capable to obtain high-quality optimal solution. The results demonstrate that heat integration between columns significantly reduces the energy consumption compared to the non-integrated distillation sequences. Moreover, nonsharp HIDiSs can further reduce the TAC compared to those with only sharp splits.

Assessment of Hybrid Processes for Bio-Butanol Purification Applying Process Simulation

Bio-butanol production based on ABE (acetone-butanol-ethanol) fermentation is facing increasing interest as a transport fuel, since it offers significant advantages to other bio-fuels. However, to ensure an economic operation two bottlenecks has to be overcome: (1) high cost of the fermentation substrate, and (2) high energy demand for butanol purification via distillation due to low solvent concentration in fermentation broth. While first bottleneck might be overcome by the use of alternative feedstock like lignocelluloses or agro-food-wastes, the latter can be targeted by introducing hybrid purification concepts, combining in-situ removal techniques with distillation. Experimental and literature data based on lab-scale size experiments operated with synthetical fermentation broth are used to parameterize an Aspen Plus® simulation to predict the energy demand for bio-butanol purification for three in-situ removal techniques coupled with distillation and to compare to a standalone distillation sequence: gas stripping, pervaporation and adsorption/desorption. Depending on the initial solvent content of fermentation broth, with 23.2 - 31.2 MJ/kg butanol the heat demand of the standalone distillation sequence is slightly below the energy content of butanol of about 36 MJ/kg. Applying gas-stripping and pervaporation before purification via distillation reduces the heat demand by 50 % to 13.6 - 16.8 MJ/kg and 12.0 - 14.5 MJ/kg butanol, respectively. Best result is shown by combining adsorption and distillation with an energy demand of 5.0 – 5.7 MJ/kg butanol. However, the advantageous low overall energy demand results from low efforts in the distillation step, only considering separation of butanol and water, but neglecting purification of acetone and ethanol obtained in ABE fermentation.

The Use of Factorial Design to Improve a Harmony Search Algorithm to Synthesize Heat-Integrated Distillation Sequences

Optimization problems often involve a large number of design variables, and the exact influence of each of these variables upon the objective function can become rather complex; there may exist local optima for the objective function, but for the typical heat-integrated distillation sequence, the matter of interest is solely the global optimum. Therefore, it is necessary to create a stochastic algorithm method which can synthesize distillation systems with multiple components. The encoding process employs and integer number series which allows the system flow sheet structure to be portrayed and then managed. Within this portrayal, the broad synthesis problem takes the form of an implicit MILP (mixed-integer linear programming) problem. This study considers the attributes of six well-known optimization algorithms: Harmony Search algorithm (HS), Artificial Bee Colony (ABC), Bat Algorithm (BA), Crow Search Optimization (CSO), Grew Wolf Optimization (GWO) and Monarch Butterfly Optimization (MBO). The optimal variables which influence the harmony search algorithm can be determined through full factorial design analysis. These variables can then be employed in the search to discover the optimal heat-integrated distillation sequence. The study investigates the attributes of the optimal configuration solution, in terms of harmony size (HS), required number of iterations, harmony memory considering the rate (HMCR), and pitch adjustment rate (PAR). The study then adopts the HS algorithm which is duly improved in order to address the problem. In comparison with alternative techniques, HS is more effective and more robust than other approaches.

A Heuristic for extractive agent flow rate in extractive distillation

Distillation is the most widely used separation process for liquids separation in the industry. Even when the volatilities of the mixture are not favorable, then enhanced distillation is used. Extractive distillation is the enhanced distillation most widely used that consists in introducing a large flow rate of a third compound called extractive agent with a high boiling point that it is collected at the column bottoms and recovered in another column and reused again. Some heuristics are available for distillation column design, e.g. the optimum reflux is around 1.2 to 1.35 times the minimum reflux (or 1.1-1.2 times for refrigerated systems). Unfortunately, there is not a similar heuristic to determine the optimum extractive agent flow rate. Based on a literature review of rigorous simulations of extractive distillation processes, a heuristic is proposed that indicates that the optimum extractive distillation flow rate is which provides a Distillation Sequence Efficiency at 78 % of its maximum value. The maximum value is calculated assuming infinite flow rate of extractive agent. The Distillation Sequence Efficiency is a shortcut method available for distillation columns sequencing.