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.     

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.     

Investigations on the Synthesis of Methyl Acetate in a Heterogeneous Reactive Distillation Process

The production of methyl acetate in a reactive distillation process – prior art for 15 years – is often used as an example to study the basic phenomena of reactive distillation. The present paper deals with a theoretical and experimental analysis of methyl acetate synthesis in a reactive distillation column. A design method based on the interpretation of reactive distillation line diagrams is used to identify the main process parameters and to provide a foundation for experimental investigation. The significant influence of the reflux ratio on the conversion in the column is shown by mini plant experiments using supported ion exchanger in the form of Raschig rings as a heterogeneous catalyst. These experiments demonstrate the catalytic quality of this packing material. To simulate the reactive distillation column with a simple stage-to-stage method, the separation efficiency of the catalytic rings is investigated. Comparison of experimental and simulation results reveals that a simple model based on the assumption of simultaneous chemical and phase equilibrium describes the experimental data quite well over a wide range of reflux ratios. Furthermore, simulation results show that the conversion depends less on the number of reactive stages than on the use of two feed stages.     

侧反应器反应精馏过程合成醋酸甲酯的模拟

采用Aspen Plus模拟了合成醋酸甲酯的侧反应器反应精馏过程,研究进料位置、回流比、酸醇比、侧反应器数量等对反应精馏过程的影响,得到优化的工艺条件。该研究结果与传统反应精馏装置对比表明,2种精馏装置性能接近,侧反应器反应精馏技术具有工艺流程环保、塔结构简单等优点。模拟研究结果可为合成醋酸甲酯侧反应器精馏塔提供基础理论和设计数据,为在类似反应精馏体系应用该装置提供技术支持。     

Hydrolysis of methyl acetate via catalytic distillation: Simulation and design of new technological process

An equilibrium stage model was developed for the simulation of the catalytic distillation process of methyl acetate (MeOAc) hydrolysis. In the model, the influences of the reactive kinetics, residence-time, liquid holdup, and separation efficiency of the catalytic packing were considered. The model predicted the conversion of MeOAc and the mass ratio of acetic acid to water in the hydrolysis mixture. The predictions were in good agreement with experimental data. A novel process was designed based on the results of theoretic analysis and the simulation research. This new process had a higher conversion of MeOAc compared with the results in previous research and was found to be energy efficient. Optimal effect parameters and design factors of new technology on energy consumption and conversion were also determined and summarized as the following: the position of side draw is 18th to 19th stages, the catalytic distillation (CD) column pressure is at 350 kPa, the volume ratio of reflux to feed is 6–8, mole ratio of feed water to MeOAc is 3.5–4.5, and the mass ratio of side withdrawal to feed is 0.32–0.34.     

Recovery of lactic acid by batch reactive distillation

Lactic acid, being virtually a non‐boiling compound, is difficult to separate from its aqueous solution by conventional methods such as distillation. It is necessary to convert it to the relatively volatile ester and the separation of the ester, followed by hydrolysis, is recommended as an appropriate method of recovery. In the present work, we explore and investigate a novel reactive distillation strategy to perform esterification, distillation and hydrolysis in a single unit. The experiments were performed in a batch reactive distillation set‐up and the results have been explained with the help of an appropriate model. An unsteady state mathematical model based on an equilibrium stage concept was developed for batch reactive distillation. A pseudo‐homogeneous model was used for the determination of reaction kinetics. The effect of operating parameters such as feed concentration, mole ratio, catalyst loading, boil‐up rate, etc. on the recovery of lactic acid was studied with the help of simulation and experimental results. The feasibility issue of reactive distillation has been discussed based on the results obtained. Copyright © 2006 Society of Chemical Industry     

Simulation of continuous packed bed reactive distillation column for the esterification process using activity based kinetic model

A mathematical model for the continuous packed bed reactive distillation process of esterification of acetic acid with methanol is developed. The kinetic rate equation, which plays a major role for the performance of reactive distillation and it is the part of model, is required for the liquid phase reversible esterification reaction. The mineral sulphuric acid is used as the catalyst. The kinetic experiments are carried out under different temperatures in the range of 305.15 to 333.15 K and catalyst concentrations in the range of 0.1267 mole H⁺/lit to 0.6537 mole H⁺/lit. From that experimental data the kinetic model is developed and the same is used for the simulation of reactive distillation process. Equilibrium stage model, in which the vapour and the liquid leaving a stage are assumed to be in equilibrium with each other, has been used for the simulation of reactive distillation process by incorporating our kinetic model. Conversion of acetic acid as function of reflux ratio and reboiler ratio has been predicted. The liquid composition and temperature profiles versus stage number have been also predicted. Finally, the optimum operating conditions obtained from the simulation results for high pure methyl acetate by reactive distillation process.     

混合离子液体催化反应精馏合成乙酸正己酯

离子液体催化反应精馏是提高酯交换平衡反应转化率的一种绿色有效方法。以离子液体1-丙基磺酸-3-甲基咪唑三氟甲烷磺酸盐（［PSO₃HMIm］［OTf］）和离子液体1-辛基-2,3-二甲基咪唑双（三氟甲烷磺酰）亚胺盐（［OMMIm］［Tf₂N］）的混合物作为乙酸甲酯和正己醇进行酯交换反应合成乙酸正己酯的催化剂,测定了酯交换反应动力学。探讨了混合比、反应温度、反应物初始摩尔比、催化剂浓度对反应速率和乙酸甲酯转化率的影响,考察了催化剂的回收性能。利用实验数据回归得出混合离子液体催化酯交换反应动力学方程。在反应动力学的基础上进行了乙酸甲酯和正己醇的酯交换反应精馏流程模拟,分析了理论板数、回流比、进料位置及反应段塔板数、催化剂用量、持液量等参数对反应精馏塔的影响。在优化的操作条件下,获得纯度为0.9993的乙酸正己酯产品。     

Experimental study and process simulation of n-butyl acetate produced by transesterification in a catalytic distillation column

The production of n-butyl acetate by transesterification of methyl acetate with n-butanol, catalyzed by acidic ion-exchange resin NKC-9, was experimentally investigated in a homemade catalytic distillation column with a diameter of 80 mm. A non-equilibrium stage model for describing the catalytic distillation process was established and solved with Newton-Homotopy method. The effect of feeding mode, reflux ratio, material molar ratio and number of reactive sections on the conversion of methyl acetate was investigated. The experimental results agree well with those obtained by the theoretical simulation calculations, indicating the accuracy and reliability of the non-equilibrium model. It can be concluded that the non-equilibrium model established in this paper can provide theoretical guidance for the further process design and optimization.     

反应精馏合成乙酸丁酯的实验和模拟(英文)

In this paper, a reactive distillation (RD) column was applied for synthesis n-butyl acetate from n-butanol and acetic acid. The Langmuir-Hinshelwood-Hougen-Watson (LHHW) kinetic model and an equilibrium stage model for separation were employed to study the RD process. The results obtained from the equilibrium stage model agreed well with the experiments. The effects of operating variables on the n-butanol conversion and n-butyl acetate purity were further investigated. The optimal column configuration for the production of n-butyl acetate was designed with 5 rectifying stages, 8 reaction stages and 13 stripping stages by the simulation study. According to the simulation results, n-butanol conversion and n-butyl acetate purity all reached greater than 96%.     

Simulation study on a reactive distillation process of methyl acetate hydrolysis intensified by reaction of methanol dehydration

Methyl acetate (MeOAc) recovery from the polyvinyl alcohol (PVA) production is a difficult and heavy energy consuming process. In this work, a reactive distillation (RD) process of MeOAc hydrolysis intensified by methanol (MeOH) dehydration, as an auxiliary reaction, was proposed. Two different feeds with the mole ratio of MeOAc to MeOH at 1:1 and 1:9 were studied, and the effect of the operating pressure, the feed location and the reflux ratio on the RD column was analyzed. The simulations of reactive distillation were performed using a three phase non-equilibrium model implemented by gPROMS. As the limit of the reaction rate of MeOH dehydration, it is impossible to get 100% conversion of MeOAc and MeOH by a single RD column. Therefore, two novel processes for recovery of methyl acetate in PVA production were developed. The simulation results show that the high purity of dimethyl ether (DME) could be achieved with a complete conversion of MeOAc, and a large amount of energy demand and equipment costs can be reduced.     

Study on Hydrolysis of Methyl Acetate in a Catalytic Distillation Column

A nonequilibrium stage model was used to simulate countercurrent multicomponent catalytic distillation processes for methyl acetate hydrolysis. Computations of stage efficiencies or height equivalent to a theoretical plate (HETP) were entirely avoided by this model. The consistency of simulated results and experimental data in conversions and concentration of each component along a column indicates that the model predicts the actual process well. The influences of operating parameters on hydrolytic conversions, such as feed molar ratios, feed locations, feed and reflux rates, heights of reactive and stripping sections, were analyzed adequately by simulating calculations. A good operating mode was then obtained, which is helpful to the development of a new process.     

乙酸甲酯的合成与应用新进展

介绍了近几年国内外有关乙酸甲酯合成与应用的研究新进展，提出了甲醇脱氢合成法，并重点探讨和比较了反应精馏法和甲醇羰化法的合成研究，对发展乙酸甲酯工业具有指导意义。     

Conceptual Design of Double‐Feed Reactive Distillation Columns

The feasibility and feasible range of operating parameters for double‐feed reactive distillation columns are evaluated, based on the combination of pinch point map analysis for the middle‐section in the compositional space and the feed angle method as an efficient shortcut design method. Limiting bounds for operating parameters are determined where the properties of singular points change. The existence and values of such bounds may vary in double‐feed reactive distillation columns depending on the nature of the system under study. The methodology is illustrated by production of methyl acetate and ethyl acetate. An efficient method is described to identify the most promising candidates of double‐feed reactive distillation columns and to study the design flexibility in terms of operating parameters.     

基于优化的反应精馏塔简捷设计

基于G ibbs自由能最小原理达到反应和相平衡的假设,提出了一种适用于初步设计的新的设计方法。以甲基叔丁基醚和碳酸二乙酯的反应精馏过程为例,应用Aspen软件模拟,经分析发现,通过改变理论级数和回流比,可得到合理的塔内温度分布,从而使塔底产品组成得到最大程度的提高,为此提出基于优化的设计法:以产品组成最大为目标函数,以回流比、精馏段和提馏段理论板数为决策变量,采用遗传算法进行优化计算。结果表明,2个设计实例均能在保证低能耗的同时得到较高的产品纯度,说明该设计方法可以设计出较好的反应精馏塔。     

Multiple Analysis in a Reactive Distillation Column for the Synthesis of tert‐Amyl Methyl Ether

tert‐Amyl methyl ether (TAME) is produced via reactive distillation. A simulation is set up and controlled on Aspen HYSYS v 8.0 for generating the highest purity of TAME. This simulation includes a plug‐flow reactor and a reactive distillation column. Emphasis was put on finding the optimal operating conditions of the reactive distillation column in order to get the maximum purity of TAME. The operational parameters were reflux ratio, number of reactive stages in the distillation column, and condenser pressure. The results indicated the optimal reflux ratio and condenser pressure which could be adapted to industrial scale.     

Performance of a three-phase fluidized bed as a reactive distillation device

A new reactive distillation device, the multistage gas/liquid/solid three-phase fluidized bed, has been developed. The flow regimes of the multistage three-phase fluidized bed have been studied and the regimes can be divided into the liquid leakage regime, the dispersed bubble regime, and the coalesced bubble regime. Liquid velocity has a much smaller effect on phase holdups in this device than in conventional three-phase fluidized beds. The three phase fluidized bed is used as a reactive distillation device for the hydrolysis of methyl acetate. Much higher reaction conversion than the equilibrium value and high catalyst-contacting efficiency are obtained. Different methods of feeding the water into the reactive distillation section are studied.     

Feasibility analysis,synthesis, and design of reactive distillation processes: A focus on double‐feed processes

A new approach for the feasibility analysis of reactive distillation processes based on the reactive extractive curve maps (rExCM) concept is introduced. A method dedicated to reactive distillation feasibility analysis, and design has been developed in our team since 1999. From minimal information concerning the physicochemical properties of the system, three steps lead to the design of the unit and the specification of its operating conditions. Currently, the procedure permits the conceptual design of hybrid reactive column configuration with one or two feed plates, for any number of equilibrium reactions (provided that the degree of freedom of the system is equal to 2) occurring in liquid or vapor phase. This contribution focuses on the most recent developments: the generalization of the feasibility analysis step to double‐feed processes thanks to the introduction of the rExCM concept. This methodology is illustrated through two examples: the emblematic methyl acetate example and the production of dimethyl methyl glutarate. © 2011 American Institute of Chemical Engineers AIChE J, 58: 2346–2356, 2012     

Design and control of transesterification reactive distillation with thermal coupling

In the study, the design and control strategies of a reactive distillation process with partially thermal coupling for the production of methanol and n-butyl acetate by transesterification reaction of methyl acetate and n-butanol are investigated. Since methanol and methyl acetate formed an azeotrope, the products of a reactive distillation column include n-butyl acetate and the mixture of methanol and methyl acetate, which must be separated by an additional column. Partially thermal coupling can be used to eliminate the condenser of the second column. Not only energy reduction but also better operability and controllability can be obtained for the thermally coupled reactive distillation process. Proper selection and pairing of controlled and manipulated variables chosen for three control objectives were determined by using steady-state analysis. A simple control scheme with three temperature control loops is sufficient to maintain product purities and stoichiometric balance between the reactant feeds.     

Industrial experience in the scale-up of reactive distillation with examples from C4-chemistry

The objective of this paper is to illustrate process design from the industrial point of view for a heterogeneous reactive distillation exemplified by the decomposition of MTBE. Based on thermodynamics a plausible column concept is suggested. Open questions concerning scale-up of structured catalytic packings are discussed on the basis of experiments in the lab and pilot plant scale. Lab scale experiments were modelled satisfactorily with an equilibrium stage approach. In order to perform the scale-up from lab to pilot scale with the equilibrium stage approach the reaction rate constant had to be reduced significantly. Incomplete catalyst wetting due to maldistribution effects or mass transfer phenomena might be possible reasons.