Optimization of coproduction of ethyl acetate and n-butyl acetate by reactive distillation☆

Based on a previous investigation, a simulation model was used for optimization of coproduction of ethyl acetate and n-butyl acetate by reactive distil ation. An experimental setup was established to verify the simulated results. The effects of various operating variables, such as ethanol feed location, acetic acid feed location, feed stage of reaction mixture of acetic acid and n-butanol, reflux ratio of ethyl acetate reactive distillation column, and distil-late to feed ratio of n-butyl acetate column, on the ethanol/n-butanol conversions, ethyl acetate/n-butyl acetate purity, and energy consumption were investigated. The optimal results in the simulation study are as follows:ethanol feed location, 15th stage;acetic acid feed location, eighth stage;feed location of reaction mixture of acetic acid and n-butanol, eighth stage;reflux ratio of ethyl acetate reactive distillation column, 2.0;and distillate to feed ratio of n-butyl acetate, 0.6.     

离子液体催化反应精馏合成乙酸乙酯工艺模拟

为研究离子液体在反应精馏中的作用,采用离子液体1-丁基-3-甲基咪唑硫酸氢盐（[BMIM]HSO₄）作为催化剂,对乙酸和乙醇合成乙酸乙酯的反应精馏流程进行了计算模拟。在确定了参数的酯化反应动力学的基础上,用Aspen Plus软件建立了反应精馏流程,研究了催化剂用量、精馏段理论板数、反应段理论板数、乙醇进料位置、进料摩尔比、持液量及回流比等参数对反应精馏过程的影响。研究结果表明,塔顶乙酸乙酯的质量分数随催化剂用量、精馏段理论板数、反应段理论板数和持液量增大而增大,工艺流程存在最佳回流比以及最佳进料酸醇摩尔比。得到的优化条件如下：离子液体与乙酸摩尔比为1：2.5,进料酸醇摩尔比为4：1,理论塔板数为21块,乙酸和催化剂在第7块理论塔板进料,乙醇在第19块理论塔板进料,塔板持液量0.1L,回流比为4,塔顶乙酸乙酯的质量分数可以达到98.73%。     

反应精馏法合成乙酸乙酯工艺条件的选择

乙酸乙酯是一种重要的有机化工产品,有着非常广泛的用途,生产方法多种多样。采用反应精馏技术,以乙酸和乙醇为原料,浓硫酸为催化剂,采用间歇式进料,对酯化反应生成乙酸乙酯的工艺条件进行了研究。应用气相色谱对产物组成进行了分析。通过实验,确定了该反应的最佳反应条件：醇酸体积比为1．2：1,回流比为4：1,反应时间为2．5h,催化剂用量0．8g。在此条件下,乙酸的转化率达到了93．25％,乙酸乙酯的选择性为79．09％,乙酸乙酯的收率达到了73．76％,均高于传统方法。     

The use of dilute acetic acid for butyl acetate production in a reactive distillation: Simulation and control studies

The recovery of dilute acetic acid, which is widely found as a by-product in many chemical and petrochemical industries, becomes an important issue due to economic and environmental awareness. In general, separation of acetic acid in aqueous solution by conventional distillation columns is difficult, requiring a column with many stages and high energy consumption. As a result, the primary concern of the present study is the application of reactive distillation as a potential alternative method to recover dilute acetic acid. The direct use of dilute acetic acid as reactant for esterification with butanol to produce butyl acetate in the reactive distillation is investigated. Simulation studies are performed in order to investigate effect of the concentration of dilute acetic acid and key process parameters on the performance of the reactive distillation in terms of acetic acid conversion and butyl acetate production. In addition, three alternative control strategies are studied for the closed loop control of the reactive distillation. The control objective is to maintain the butyl acetate in a bottom product stream at the desired purity of 99.5 wt%.     

侧反应器反应精馏方法生产醋酸甲酯的模拟

采用强酸性阳离子交换树脂催化醋酸与甲醇反应精馏生成醋酸甲酯,可避免硫酸作为催化剂的不足。但该非均相催化反应受平衡限制,且达到平衡时间较长,采用传统反应精馏塔难以提供足够反应空间。文中设计侧反应器与精馏塔耦合新工艺,采用Aspen Plus软件模拟研究了侧反应器数量、位置,原料进料位置,回流比,醇酸比等对反应精馏过程的影响。结果表明,当装置具有7个侧反应器,反应器间隔4块板,在优化的操作条件下,醋酸甲酯质量分数可达99.1%。     

Esterification of acetic acid with ethanol: Reaction kinetics and operation in a packed bed reactive distillation column

The reaction kinetics of the esterification of acetic acid with ethanol, catalyzed both homogeneously by the acetic acid, and heterogeneously by Amberlyst 15, have been investigated. The reactions were carried out at several temperatures between 303.15 and 353.15 K and at various starting reactant compositions. Homogeneous and heterogeneous reactions have been described using the models proposed by Pöpken et al. [T. Pöpken, L. Götze, J. Gmehling, Reaction kinetics and chemical equilibrium of homogenously and heterogeneously catalyzed acetic acid esterification with methanol and methyl acetate hydrolysis, Ind. Eng. Chem. Res. 39 (2000) 2601–2611]. These models use activities instead of mole fractions. Activity coefficients have been calculated using ASOG [K. Kojima, K. Tochigi, Prediction of Vapor–liquid Equilibria by the ASOG Method, Elsevier, Tokyo, 1979] and UNIFAC (Aa. Fredenslund, J. Gmehling, P. Rasmussen, Vapor–liquid Equilibria Using UNIFAC. A Group Contribution Method, Elsevier, Amsterdam, 1977] methods.

A packed bed reactive distillation column filled with Amberlyst 15 has been employed to obtain ethyl acetate. The influence of feed composition and reflux ratio have been analyzed.     

Experiments and dynamic modeling of a reactive distillation column for the production of ethyl acetate by considering the heterogeneous catalyst pilot complexities

Great effort has been applied to model and simulate the dynamic behavior of the reactive distillation as a successful process intensification example. However, very little experimental work has been carried out in transient conditions. The work presents a series of experiments for the production of ethyl acetate from esterification of acetic acid and ethanol in a reactive distillation pilot column. The steady-state approach performed experiments with both excess of alcohol and stoichiometric feed configuration. Predicted and measured results show good agreement and reveal a strong dependency of the structured packing catalyst activity on the pilot geometry and its operating conditions. The transient process behavior of the heterogeneously catalyzed system was deeply investigated and continuous and dynamic data were collected for an equilibrium model validation, after different perturbations on parameters. The experimental validation is shown to be essential to provide realistic hydrodynamic parameters, to understand the sensitive parameters such as heat losses and to adapt values for the catalyst holdup as a function of the system.     

Hybrid reactive distillation systems for n-butyl acetate production from dilute acetic acid

The recovery of dilute acetic acid, regarding as a waste stream in many chemical and petrochemical processes, becomes an important issue due to economic and environmental awareness. In this work, a simulation study on the direct utilization of dilute acetic acid to produce n-butyl acetate via esterification with butanol in a reactive distillation is presented by using Aspen Plus. The performance of a hybrid reactive distillation with a pretreatment unit, i.e., a conventional distillation or a pervaporation, is investigated. For a single reactive distillation system, it is found that higher overall energy of the system is required when the concentration of acetic acid is lowered. By considering the enrichment of acetic acid in the reactive distillation column feed from 35 to 65 wt.%, a hybrid pervaporation–reactive distillation requires lower energy than both the conventional distillation–reactive distillation system and the single reactive distillation.     

Design and control of reactive distillation for ethyl and isopropyl acetates production with azeotropic feeds

Reactive distillations for the production of ethyl acetate (EtAc) and isopropyl acetate (IPAc) are classified as the type-II process where the first column consists of a reactive zone and a rectifying section followed by a stripper [Tang et al., 2005. Design of reactive distillations for acetic acid esterification with different alcohols. A.I.Ch.E. Journal 51, 1683-1699]. Instead of using pure alcohols and acetic acid as reactants, this paper studies the effects of reactant purity on the design and control of reactive distillation. This offers significant economical incentives (by reducing raw materials costs), because ethanol forms an azeotrope with water at 90 mol% and isopropanol/water has an azeotrope at 68%. The purities of the acid is set to 95% for acetic acid (industrial grade), 87% for ethanol, and 65% for isopropanol. The results show that the total annual costs (TAC) increase by a factor of 5% for EtAc and 8% for IPAc production using reactive distillation. Next, the operability of the reactive distillations with azeotrope feeds is explored. Three disturbances, feed flow, acid feed purity, and alcohol feed composition, are introduced to assess control performance using dual-temperature control and one-temperature-one-composition control. Simulation results indicate good control performance can be achieved for reactive distillation with azeotropic feeds.     

A comparison of the equilibrium and nonequilibrium models for a multicomponent reactive distillation column

Apropriate models and computational algorithms were developed and a series of simulations for a reactive distillation tray column were performed in order to compare the equilibrium and nonequilibrium model. The homogeneous liquid phase reversible reaction between acetic acid and ethanol in production of ethyl acetate was employed as a test chemical reaction. Both the Newton–Raphson and homotopy-continuation methods were applied to solve the set of model equations. The homotopy-continuation method was found superior to the Newton–Raphson method in guaranteeing the desired solution for the nonequilibrium model. The comparison between the equilibrium and the nonequilibrium model reveals that the nonequilibrium model is to be preferred for the simulation of a tray column for reactive distillation because of the difficulty in the accurate prediction of tray efficiency for the equilibrium model. Close agreement in the prediction of the two models arises only when the tray efficiency can be correctly guessed for the equilibrium model. The effects of reaction rate and reflux ratio on the column performance were illustrated using the nonequilibrium model. The use of the heat generated by the exothermic reaction as an internal heat source in the column is discussed.     

Effects of Liquid Holdup in Condensers on the Start‐Up of Reactive Distillation Columns

Compared with start‐ups in conventional distillation columns, those in reactive distillation (RD) columns are much more time and energy consuming, and generate a large amount of by‐products which are not easy to deal with together. For several years, researchers have been trying out different methods to shorten the time required to lower the cost of the start‐up. In this work, a rigorous dynamic model in the ChemCAD simulator is applied to model the start‐up process for the esterification of ethyl acetate in a reactive distillation column. In the model, two sets of equations are employed: one for the fill‐up and heating stage and the other for the equilibration process which follows. In the fill‐up and heating stage, fluctuation curves of the reboiler temperatures with respect to time which are similar to those for conventional distillation columns are observed, while in the equilibration process it is found that the increase of the liquid holdup volume in the condenser reduces the time required to reach steady state for the reactive column and decreases the liquid holdup volume in the reboiler at the equilibrium state. This shows that the liquid holdup volume in the condenser has an important effect on the start‐up of reactive distillation columns.     

GRAPHICAL PROCEDURE FOR REACTIVE DISTILLATION SYSTEMS

In this paper we analyze a ternary reactive distillation system, where an equilibrium chemical reaction occurs in the liquid phase. By using a set of transformed variables proposed by Barbosa el al. (1988b) and well known graphical procedures for non-reactive systems; the minimum reflux ratio, minimum number of equilibrium stages, mass and energy balances for a reactive column and flash can be easily obtained. The procedures developed in this paper are applied to ISOBUTYLENE-METHANOL-MTBE system.

The main objective in developing this model is to obtain, as much as possible, rigorous information for the analysis of one reactive distillation column in a Process Simulator with capability of handling this type of new operation.     

Process alternatives for methyl acetate conversion using reactive distillation. 1. Hydrolysis

In a polyvinyl alcohol (PVA) plant, reaction stoichiometry indicates that equal molar of methyl acetate is generated for every mole of PVA produced. This work explores an alternative to convert methyl acetate back to acetic acid (raw materials of PVA plant), methyl acetate (MeAc) hydrolysis. The design and control of methyl acetate hydrolysis using reactive distillation is studied. Because of the small chemical equilibrium constant (∼0.013) and unfavorable boiling point ranking (MeAc being the lightest boiler), the reactive distillation exhibits the following characteristics: (1) total reflux operation and (2) excess reactant (water) design. The proposed flowsheet consists of one reactive distillation column with a reactive reflux drum, two separation columns, and one water-rich recycle stream. A systematic design procedure is used to generate the flowsheet based on the total annual cost (TAC). Two dominate design variables are: recycle flow rate (for the degree of excess in water) and the overhead impurity level of acetic acid in the product column (to avoid tangent pinch). Finally, the operability of the hydrolysis plant is evaluated. A plantwide control structure is developed followed by process identification and controller tuning. The results show that reasonable control performance can be achieved using simple temperature control for feed flow and feed composition disturbances.     

γ-戊酮酸丁酯反应精馏过程的逐步优化方案设计

Butyl-levulinate has been identified as a promising fuel candidate with high oxygen content. Its combustion in diesel engines yields very low soot and NOx emissions. It can be produced by the esterification of butanol and levulinic acid, which themselves are platform chemicals in a biorenewables-based chemical supply chain. Since the equilibrium of esterification limits the conversion in a conventional reactor, reactive distillation can be applied to overcome this limitation. The presence of the high-boiling catalyst sulfuric acid requires a further separation step downstream of the reactive distillation column to recover the catalyst for recycle. Optimal design specifications and an optimal operating point are determined using rigorous flowsheet optimization. The challenging optimization problem is solved by a favorable initialization strategy and continuous reformulation. The design identified has the potential to produce a renewable transportation fuel at reasonable cost.     

Membrane reactors in chemical production processes and the application to the pervaporation-assisted esterification

When appropriate membrane was used for the assistance of chemical and biochemical equilibrium reactions, it is possible to enhance the yield and the purity of the reaction product by selectively adding educts or selectively removing products and to a lower the energy input and the reaction time compared to conventional process. In this paper a review on membrane reactors with special emphasis on membrane-assistance of esterification reactions and a continuous tube membrane reactor for the pervaporation-assistance of the esterification are presented. The heterogeneously catalyzed esterification of ethanol and acetic acid to ethyl acetate and water was investigated as a typical chemical equilibrium reaction. The selective and simultaneous water separation from the reaction mixture of the esterification with polyvinyl alcohol pervaporation membranes is considered to be an interesting process alternative to the conventional distillation process. Compared to the distillation process, for the pervaporation-assisted process a decrease of the energy input of over 75% and of the investment and operating coasts of over 50% each was calculated.     

Composition estimation of multicomponent reactive batch distillation with optimal sensor configuration

An inferential state estimation scheme based on extended Kalman filter (EKF) with optimal selection of sensor locations using principal component analysis (PCA) is presented for composition estimation in multicomponent reactive batch distillation. The properties of PCA are exploited to provide the most sensitive dynamic temperature measurement information of the process to the estimator for accurate estimation of compositions. The state estimator is supported by a simplified dynamic model of reactive batch distillation that includes component balance equations together with thermodynamic relations and reaction kinetics. The performance of the proposed scheme is evaluated by applying it for composition estimation on all trays, reboiler, reflux drum and products of a reactive batch distillation column, in which ethyl acetate is produced through an esterification reaction between acetic acid and ethanol. This quaternary system with azeotropism is highly nonlinear and typically suited for implementation of the proposed scheme. The results demonstrate that the proposed EKF estimation scheme with optimal temperature sensor configuration is effective for inferential estimation of compositions in multicomponent reactive batch distillation.     

催化精馏合成乙酸乙酯的工业试验

以固体酸为催化剂,在中试催化精馏塔中进行了乙酸与乙醇反应制备乙酸乙酯的工业试验,试验塔的精馏段塔径为600 mm,内装陶瓷规整填料,反应精馏段塔径为1 000mm,内装7层立体催化精馏塔板.采用连续操作,考察了不同的回流比、釜酸质量分数、进料流量对反应和分离过程的影响,同时测定了塔板上气液相质量分数的分布.通过试验得到...     

Estimation of the Mass-Transfer Efficiency in Catalytic Distillation in a Packed Column

Experiments are carried out to determine, under comparable conditions, the mass-transfer efficiency of a packed column in catalytic distillation of a chemically reactive mixture and in multicomponent distillation of the same mixture that is uncomplicated by chemical transformations. The reactive distillation process involves the reversible equimolar reaction of transesterification between butyl acetate and ethanol to form butanol and ethyl acetate. It is shown that the experimental data on the packed zone of the column are satisfactorily fitted by an equilibrium cascade model. The number of stages in the cascade is found from the Fenske-Underwood equation. In the experiments performed, both in the catalytic distillation and in the multicomponent distillation uncomplicated by chemical transformations, the height equivalent to a cascade stage is the same (20 cm).     

Simulation studies on reactive distillation for synthesis oftert-amyl ethyl ether

In this work, a reactive distillation column in which chemical reactions and separations occur simultaneously is applied for the synthesis of tert-amyl ethyl ether (TAEE) from ethanol (EtOH) and tert-amyl alcohol (TAA). A rate-based kinetic model for liquid-phase etherification and an equilibrium stage model for separation are employed to study the reactive distillation. The calculation is carried out using the commercial software package, Aspen Plus. Simulations are performed to examine the effects of design variables, i.e., a number of rectifying, reaction and stripping stages on the performance of reactive distillation column. It has been found that an optimal column configuration for the TAEE production under the study is designed with no rectifying, 4 reaction and 8 stripping stages. With such an appropriate specification of the reactive distillation column, the effects of various operating variables on the TAA conversion and TAEE selectivity are further investigated and the results have shown that the reflux ratio and operating pressure are the most important factors to the operation of the reactive distillation.     

离子液体催化反应精馏合成乙酸乙酯

利用1-己基吡啶四氟硼酸盐离子液体作溶剂和催化剂,在催化反应精馏塔内合成乙酸乙酯。结果表明该离子液体具有催化活性,反应的选择性为100%,分离后的1-已基吡啶四氟硼酸盐离子液体重复使用5次,其活性没有明显变化。考察了回流比、进料比、溶剂和催化剂用量对酯化反应结果的影响,优化了反应的工艺条件,适宜的反应条件为:物料比n(乙酸):n(乙醇)=1.1:1,离子液体用量n(乙酸):n(离子液体)=8:1,回流比为3。