常规间歇萃取精馏分离苯-环己烷的研究

通过一个常规间歇萃取精馏实验装置,考察了不同萃取剂在不同回流比及萃取溶剂加入速率情况下对分离苯-环己烷共沸体系的影响。结果表明,二元混合溶剂能够解决单一溶剂的选择性与溶解性相矛盾的问题;且在同等条件下,综合性能优于单一溶剂;随着溶剂加入速率和操作回流比的增加,产品的产量逐渐提高,尤其重要的是混合溶剂间歇萃取精馏技术与简单溶剂间歇萃取精馏技术相比并不复杂。     

PLANTWIDE CONTROL STUDY OF A VINYL ACETATE MONOMER PROCESS DESIGN

A simulation of a vinyl acetate monomer (VAM) process design was developed and compared with the work of Luyben and Tyreus (1998). Two incremental changes were made to the two main control substructures. Specifically, the two schemes focus on improving the liquid inventory system control and controllability of the azeotropic distillation column. The level control strategy was tested and found to produce a faster response with less oscillatory behavior. Two alternative control techniques for the azeotropic distillation column were tested, a feed-forward model predictive controller and a static feed-forward ratio controller. The model predictive controller results illustrated the large difference between the water composition analyzer sample time and the controller step size. The static feed-forward ratio controller showed excellent disturbance rejection of large feed flow variations to the azeotropic distillation column. Simulation results are presented to illustrate the effectiveness of the new control strategies.     

异丙醇-环己烷萃取精馏过程模拟与优化

利用化工流程模拟软件Aspen Plus对异丙醇-环己烷共沸物系的双塔连续萃取精馏过程进行了模拟计算与优化。首先根据溶剂相似相溶原理,先初选出糠醛和硝基苯作为备选溶剂,再通过汽液平衡试验及ChemCAD模拟筛选,确定糠醛为最适宜溶剂,选择NRTL模型作为物性方法,使用RadFrac模块进行模拟计算,并利用灵敏度分析模块对各工艺参数进行优化。结果表明,最适宜工艺方案为：萃取精馏塔理论塔板数为30,原料在第26块板进料,溶剂在第12块板进料,物质的量回流比为1.8,溶剂质量比为3∶1;溶剂回收塔理论板数为15,进料位置在第10块板,物质的量回流比为1.0。分离效果可达到环己烷质量分数为99.74%,异丙醇质量分数为99.61%。模拟和优化结果为分离过程的优化操作和设计提供了依据。     

Operation and control of batch extractive distillation for the separation of mixtures with minimum-boiling azeotrope

Batch distillation is commonly used in the fine chemicals, specialty polymer, biochemical, pharmaceutical, and food industries. For separating mixtures with minimum-boiling azeotrope, a heavy entrainer is frequently added to the top section of the batch column to aid in the separation. This process is called batch extractive distillation. Most of the papers in open literature have only studied the first operating step of the batch extractive distillation which is the recovery of the light component without mentioning the later steps for the recovery of the other component and entrainer. In this paper, two real chemical systems, one separating acetone and methanol using water as entrainer and the other separating isopropyl alcohol (IPA) and water using dimethyl sulfoxide (DMSO) as entrainer, are studied for the feasible operation of the complete batch distillation sequence. The operating variables, including the pre-load amount with the mixture, continuous feed rate of the entrainer, and reflux ratio at each operating step are determined in the operating sequence. The constant reflux ratio and constant entrainer feed rate operating policy and another policy to allow these two operating variables to be varied will be compared in order to further improve the batch operation. All dynamic simulations that are performed directly mimic industrial situations from an empty column using a rigorous dynamic simulator, Aspen Dynamics™.     

INTEGRATING THE EFFECTS OF PROCESS CONTROLLERS WITH STEADY-STATE, EQUATION-BASED SIMULATION

Process controllers have a significant influence on the steady-state, as well as the dynamic, behavior of chemical processes. Thus, the steady-state simulation of processes should include the effects of control. A new method for including controllers in steady-state simulation is presented in this paper. The method provides equations that represent the steady-state control algorithm and can be solved simultaneously with the process model to yield the steady-slate behaviour of the closed-loop system. Most importantly, the controller models include saturation effects and can be formulated and solved within an open-form model. The method is general and can be applied to single-loop controllers, to complex control designs including split range and signal select, and to several single-loop controllers in a multiloop controller design.     

Extractive distillation: Advances in conceptual design,solvent selection,and separation strategies

Extractive distillation(ED) is one of the most promising approaches for the separation of the azeotropic or closeboiling mixtures in the chemical industry. The purpose of this paper is to provide a broad overview of the recent development of key aspects in the ED process involving conceptual design, solvent selection, and separation strategies. To obtain the minimum entrainer feed flow rate and reflux ratio for the ED process, the conceptual design of azeotropic mixture separation based on a topological analysis via thermodynamic feasibility insights involving residue curve maps, univolatility lines, and unidistribution curves is presented. The method is applicable to arbitrary multicomponent mixtures and allows direct screening of design alternatives. The determination of a suitable solvent is one of the key steps to ensure an effective and economical ED process. Candidate entrainers can be obtained from heuristics or literature studies while computer aided molecular design(CAMD) has superiority in efficiency and reliability. To achieve optimized extractive distillation systems, a brief review of evaluation method for both entrainer design and selection through CAMD is presented. Extractive distillation can be operated either in continuous extractive distillation(CED) or batch extractive distillation(BED), and both modes have been well-studied depending on the advantages in flexibility and low capital costs. To improve the energy efficiency, several configurations and technological alternatives can be used for both CED and BED depending on strategies and main azeotropic feeds. The challenge and chance of the further ED development involving screening the best potential solvents and exploring the energy-intensive separation strategies are discussed aiming at promoting the industrial application of this environmentally friendly separation technique.     

Design and Control of Heterogeneous Azeotropic Distillation for Separating 2‐Methylpyridine/Water

In industrial processes, 2‐methylpyridine/water mixtures can be separated via the conventional heterogeneous azeotropic distillation (HAD) using benzene as entrainer. 2‐Methylpyridine and water can form a heterogeneous azeotrope by themselves, based on which an improved HAD process is proposed. This allows for reducing the total operating cost and total annual cost (TAC) by more than one‐tenth compared with the conventional HAD process. Two different control structures were established for the enhanced HAD process. The results indicate that applying the feedforward ratio controllers to the control structure can handle the feed disturbances and maintain product purities with smaller transient deviations and shorter settling times.     

分隔壁萃取精馏塔分离醋酸水溶液的模拟

提出了一种新的单塔萃取精馏精制醋酸水溶液的新工艺,该工艺采用分隔壁萃取精馏塔(DWC-E)替代常规萃取精馏流程的萃取精馏塔及溶剂回收塔,不仅节省了设备投资,而且降低了总能耗。利用Aspen Plus模拟软件,对DWC-E塔及常规萃取流程进行了模拟。DWC-E塔的操作条件:塔板数40块,侧线精馏段的板数10块,回流比2,溶剂摩尔比2.5,在此条件下,比较了常规萃取精馏流程与分隔壁精馏塔内温度、液相组成及汽液相流量的变化。结果表明,DWC-E塔比常规的2塔萃取精馏流程节能23.91%。     

萃取精馏分离苯/环己烷共沸体系的控制策略

将常规萃取精馏、差压热耦合萃取精馏以及隔壁塔萃取精馏技术应用于以糠醛为萃取剂的苯和环己烷共沸物分离过程。在稳态模型的基础上,利用Aspen Dynamics软件进行控制研究,对三工艺流程提出了若干控制策略。结果表明,对于常规萃取精馏过程,再沸器热负荷与进料量比值控制结构在降低控制过程超调量方面表现出明显优势;对于差压热耦合萃取精馏过程,带有压力-补偿控温策略的方案控制效果更佳;而对于隔壁塔,则选择了无隔板下方气液分离比控制的结构来作为较优的控制策略。     

Optimal operation of extractive distillation in different batch configurations

Optimal operations of extractive distillation for regular and middle-vase1 batch columns are presented based on a profit function. Detailed models are used for the rigorous dynamic optimization considering all operational decision variables, including reflux ratio, solvent feed rate, heat duties, and possible product withdrawals during the process. Optimal feed distribution and stream configuration at the middle section of the middle-vessel column are investigated. Separation of a minimum boiling azeotropic binary mixture (acetone and methanol using water as solvent) involving different separation duties and feed compositions is presented as a case study. The pereformanance of the middle-vessel column is signicantly influenced by the middle-section stream configuration, with the best profit when the stream configuration is allowed to vary during the operation. The optimal operating policy for the middle-vessel column involved the feed being charged mainly to the reboiler still with low holdup in the middle vessel during the operation.     

单乙醇胺(MEA)间歇萃取精馏甲醇-丙酮研究

用常规的间歇萃取精馏实验装置,研究了以单乙醇胺(MEA)为萃取剂间歇萃取精馏分离甲醇—丙酮恒沸物的过程。考察了萃取剂、全回流时间、共沸物组成、溶剂与混合物的体积比、回流比等因素对萃取精馏分离甲醇—丙酮共沸体系的影响,从而得出最优的萃取条件。     

Energy-Efficient Process with a Decanter to Separate Toluene-Methanol-Water Ternary Azeotropic Mixtures

An extractive distillation process with two columns and a decanter was proposed to separate ternary toluene-methanol-water azeotropic mixtures in a previous study. Based on this process, six processes were established to explore further energy-efficient schemes to separate the ternary azeotropic system. The optimal parameters of the process were determined in terms of the minimum total annual cost (TAC). The processes with heat integration perform better than those with a side stream or thermal coupling in terms of energy savings and TAC reduction. A process in which both the feed stream of the extractive distillation column and the entrainer recovery column are preheated by the entrainer recycle stream can reduce energy consumption by up to 27.69 % and TAC by 21.36 %.     

A review of extractive distillation from an azeotropic phenomenon for dynamic control

Extractive distillation is an effective method for separating azeotropic or close boiling point mixtures by adding a third component. Various technologies for performing the extractive distillation process have been explored to protect the environment and save resources. This paper focuses on the improvement of these advanced technologies in recent years. Extractive distillation is retrieved and analyzed from the view of phase equilibrium, selection of solvent in extractive distillation, process design, energy conservation, and dynamic control. The quantitative structure–property relationship used in extractive distillation is discussed, and the future development of extractive distillation is proposed to determine how the solvent affects the relative volatility of the separated mixture. In the steady state design, the relationship between the curvature of the residue curve and parameters of the optimal steady state is also highlighted as another field worthy of further study to simplify the distillation process.     

Quantitative comparison of dynamic controllability between a reactive distillation column and a conventional multi-unit process

A comparison of the steady-state economic optimum designs of two alternative chemical processes was presented in a previous paper [Kaymak, D. B., & Luyben, W. L. (2004). A quantitative comparison of reactive distillation with conventional multi-unit reactor/column/recycle systems for different chemical equilibrium constants. Industrial & Engineering Chemistry Research, 43, 2493–2507]. A generic exothermic reversible reaction A + B ↔ C + D occurs in both flowsheets, which consist of a conventional multi-unit reactor/separator/recycle structure and a reactive distillation column. Results showed that the reactive distillation process is significantly less expensive than the conventional process for a wide range of the chemical equilibrium constant when there is no mismatch between the temperature favorable for reaction and the temperature favorable for vapor–liquid separation.

A reactive distillation column has fewer control degrees of freedom than a conventional multi-unit system. Therefore a reactive distillation column may have worse dynamic response than a conventional process. The purpose of this paper is to compare the dynamic controllability of these two alternative processes.

Three different chemical equilibrium constants are considered. Several control structures are developed for each flowsheet, and their effectiveness is evaluated. Disturbances in production rate and fresh feed compositions are considered.

The conventional multi-unit process provides significantly better control. The operability region is much larger, there is less variability in product quality and the dynamic responses are faster than those of the reactive column. Thus, these results demonstrate that there is a significant trade-off in this system between optimum economic steady-state design and dynamic controllability.     

萃取精馏分离苯乙酮与α-苯乙醇的模拟研究

苯乙酮与α-苯乙醇属近沸点物系,用普通精馏方法很难将二者进行有效地分离。在对已报道的分离方法进行比较分析的基础上,提出了采用萃取精馏方法来分离苯乙酮和α-苯乙醇。文中首先通过定性判断和基团贡献法定量地估算选择了该二元物系合适的萃取剂为丙三醇。然后采用Aspen Plus化工模拟软件中的RadFrac模块进行了萃取精馏塔的模拟,分别考察了溶剂与原料进料位置、回流比、溶剂比对分离效果的影响。结果表明:对于处理量为1 000 kg/h的待分离物系,操作压力为5 kPa,在塔板数为30的条件下萃取精馏塔在原料进料位置为第19块塔板,溶剂进料位置为第6块塔板,回流比为3∶1(质量比),溶剂流率为800 kg/h的优化条件下,可以使塔顶苯乙酮质量分数达到99.8%,且塔釜几乎不含苯乙酮。模拟结果对进一步的实验研究和工业生产具有一定的指导意义。     

萃取精馏分离异丙醇-水共沸体系的模拟与优化

对异丙醇-水共沸体系的萃取精馏过程进行模拟与优化。以乙二醇为萃取剂,基于UNIFAC模型,使用Aspen Plus化工模拟软件中的RadFrac模块进行萃取精馏模拟,并利用灵敏度分析模块对各工艺参数进行灵敏度分析与优化。结果表明,以乙二醇做萃取剂分离异丙醇-水共沸体系是可行的。对于处理流量5000kg·h-1的异丙醇-水共沸溶液,精馏塔具有22块塔板时,原料进料位置在第16块塔板,萃取液进料位置在第3块塔板,摩尔回流比为1.4,萃取剂与原料的进料比为2∶1,塔顶异丙醇质量分数可达0.9981,萃取精馏塔的分离效果和热负荷达到最优。模拟和优化的结果对工业化设计和生产具备指导意义。     

Batch extractive distillation as a hybrid process: separation of minimum boiling azeotropes

The batch extractive distillation is compared with the hybrid process (absorption+distillation) by feasibility studies and rigorous simulation. A new method is presented for the assessment of feasibility of the hybrid batch extractive distillation. The limiting values of the operational parameters are determined. Calculations are presented for the separation of the minimum boiling azeotropic mixtures of acetone-methanol and ethanol-water by the application of water and ethylene glycol as heavy solvents, respectively.     

Optimization of the Design and Operation of Extractive Distillation Processes

The achievement of the optimal operating point of extractive distillation systems involves determining the values of the process variables, such as the solvent flowrate, the reflux ratio of the extractive, and recovery columns. From the point of view of design, the optimum involves defining the number of stages of extractive and recovery columns, as well as the feed stage positions of these columns. The above-mentioned columns are coupled through a recycle stream, which makes obtaining the optimal operating and design points a more complex task. This study arose from a new procedure for the analysis of extractive distillation columns, in which the solvent mole fraction in the solvent feed stage is the primary variable to be analyzed. The procedure allows for determining the values of the process and design variables that provide the global minimum for the total annual cost and the specific energy consumption of the extractive distillation processes (extractive and recovery columns). Furthermore, it is possible to determine the minimum solvent flowrate and the minimum reflux ratio for separation. Obtaining anhydrous ethanol using ethylene glycol as solvent is the case study of this work.     

Heterogeneous extractive batch distillation of chloroform-methanol-water: Feasibility and experiments

A novel heterogeneous extractive distillation process is considered for separating the azeotropic mixture chloroform-methanol in a batch rectifying column, including for the first time an experimental validation of the process. Heterogeneous heavy entrainer water is selected inducing an unstable ternary heteroazeotrope and a saddle binary heteroazeotrope with chloroform (ternary diagram class 2.1-2b). Unlike the well-known heterogeneous azeotropic distillation process and thanks to continuous water feeding at the column top, the saddle binary heteroazeotrope chloroform-water is obtained at the column top, condensed and further split into the liquid-liquid decanter where the chloroform-rich phase is drawn as distillate. First, feasibility analysis is carried out by using a simplified differential model in the extractive section for determining the proper range of the entrainer flowrate and the reflux ratio. The operating conditions and reflux policy are validated by rigorous simulation with ProSim Batch Column^® where technical features of a bench scale distillation column have been described. Six reproducible experiments are run in the bench scale column matching the simulated operating conditions with two sequentially increasing reflux ratio values. Simulation and experiments agree well. With an average molar purity higher than 99%, more than 85% of recovery yield was obtained for chloroform and methanol.