Steady States of Reactive Distillation: Realizability Analysis and Evaluation of Realization Conditions

Degrees-of-freedom analysis is carried out to find the working parameters of the reactive distillation model used in statics analysis. A criterion is suggested to evaluate the realizability and realization conditions for the steady states predicted by statics analysis. The steady states predicted in terms of the criterion and those realized in experiment are compared for some industrial combined processes.     

A fundamental principle and systematic procedures for process intensification in reactive distillation columns

A fundamental principle is developed for process intensification through internal mass and energy integration in reactive distillation columns and three systematic procedures are devised for process synthesis and design. For reactive distillation columns involving reactions with highly thermal effect, process intensification can be achieved with an exclusive consideration of internal energy integration between the reaction operation and separation operation involved. However, in the case of a highly endothermic reaction with an extremely low reaction rate and/or small chemical equilibrium constant, internal mass integration has also to be considered between the reactive section and stripping section. For reactive distillation columns involving reactions with negligibly or no thermal effect, process intensification can be performed with an exclusive consideration of internal mass integration. For reactive distillation columns involving reactions with moderately thermal effect, process intensification must be conducted with a careful trade-off between internal mass and energy integration. Five hypothetical and two real reactive distillation systems are employed to evaluate the principle and procedures proposed. It is demonstrated that intensifying internal mass and energy integration is really effective for process intensification. Not only can the thermodynamic efficiency be improved substantially, but also the capital investment can be further reduced.     

反应精馏技术的关键问题分析

反应精馏是为增强反应与精馏效果而将两种单元操作进行耦合的多功能新技术。因既有化学反应又有精馏过程,反应精馏过程需考虑气液相平衡、气液质量传递、催化剂内扩散(非均相反应精馏)以及动力学等因素的相互作用,导致该过程存在定态多重性、动力学复杂性。文章系统地综述了近年来国内外科研工作者对反应精馏实验和理论的研究进展,分析了反应精馏技术工业应用中存在的关键问题,并对未来的发展方向进行了展望。     

伴有多反应的精馏过程数学模拟

<正>在伴有多个反应的分离过程数学模拟中,反应量的存在使模型变量增多,非线性增强.文献[1～3]在普通精馏的基础上,提出了不同的算法来改进收敛性.他们对各种反应类型,均以动力学计算反应量,迭代变量多,算法也较复杂.Doherty针对平衡反应过程,提出了变换组成变量的概念计算反应相平衡.本文引入此概念,并对流率、焓等物理量进行了相应的变换,使变换后的反应精馏过程数学模型在形式上与普通精馏过程的模型完全一致.并以修正的Newton-Raphson法对对二甲苯和邻二甲苯的反应精馏过程进行了计算.     

A reactive distillation column with double reactive sections for the separations of two-stage consecutive reversible reactions

In this paper, a novel reactive distillation column with double reactive sections (RDC-DRS) is proposed for the separations of two-stage consecutive reversible reactions. The arrangement of two reactive sections not only allows the careful coordination of the two reaction operations involved, but also provides additional degrees of freedom for the reinforcement of internal mass integration and/or internal energy integration between the reaction operations and the separation operation involved, which could facilitate the RDC-DRS to be more advantageous than the conventional reactive distillation column with a single reactive section (RDC-SRS) in operating cost and capital investment. A representative hypothetical and real two-stage consecutive reversible reactions are chosen to evaluate the steady-state performance of the RDC-DRS. With the constraints of the same total number of stages and the same total number of reactive stages, the RDC-DRS is demonstrated to require less utility consumption than the RDC-SRS and this outcome indicates that the former could be a competitive alternative to the latter in the separations of the two-stage consecutive reversible reactions. The number of reactive sections should therefore be viewed as an important decision variable for the synthesis and design of reactive distillation columns especially in the separations of complicated reacting mixtures involving multiple reversible reactions.     

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.     

Conceptual design of single-feed kinetically controlled reactive distillation columns

Reactive distillation, simultaneous reaction and separation within a single unit, represents an exciting alternative to conventional processes, leading to significantly reduced in capital and operating costs. Process design for reactive distillation is facilitated by fast and effective methods for synthesis and conceptual design that take into account reactions that do not instantaneously reach equilibrium. This work presents a new methodology for synthesis and design of single-feed kinetically controlled reactive distillation columns. The design method allows rapid and relatively simple screening of different reactive distillation column configurations. Feasibility is assessed and operating conditions are determined using an extension of boundary-value methods. The approach is limited to systems with four or fewer chemical species. Both fully reactive and hybrid columns are considered. The methodology is illustrated for a metathesis reaction and for MTBE production.     

伴有平衡反应的分离过程的数学模拟——(Ⅱ)催化精馏塔的模拟计算

提出了平衡反应的非均相催化反应精馏过程的微分数学模型。应用多目标打靶法和 Newtor-Raphson 迭代对模型方程进行了求解。本算法应用于非理想系统,反应动力学未知的快速可逆反应过程。可求得沿塔最大转化率和平衡浓度分布。应用本算法对 MTBE 的催化反应过程进行了模拟计算,能快速稳定收敛。计算结果与文献数据进行比较,表现出良好的一致性。     

Analysis of residue curve maps of reactive and extractive distillation units

The aim of this work is to focus in the simultaneous analysis of reactive and extractive distillation, including the solvent recovery system. Residue curve maps can provide valuable insights and design assistance for a variety of separation processes, in which reactive distillation has been incorporated. These graphical techniques reveal the sensitivity of design options by giving us a visual representation over the whole composition space and assist the engineer to detect separation constraints. Simulation results show the reaction effect in the diagram topology: existence and position of nodes, saddles and distillation boundaries (separatrices or surfaces). Advanced distillation synthesis and design techniques, which are now supported by available software tools, were used to develop case studies for extractive distillation, entrainer influence in reactive systems and reactive distillation analysis with consecutive and/or competitive reactions.     

Towards further internal heat integration in design of reactive distillation columns—part I: The design principle

The thermodynamic efficiency of a reactive distillation column involving reactions with a highly thermal effect could sometimes be improved substantially through seeking further internal heat integration between the reaction operation and separation operation. Prudent arrangement of the reactive section and deliberate determination of feed location are the two effective methods that can complement internal heat integration within a reactive distillation column. The reactive section is suggested to properly superimpose onto both the stripping section for exothermic reactions and the rectifying section for endothermic reactions. Feed location should be determined so that the effect of internal heat integration can be maximized between the reaction and separation operations. Two reactive distillation systems, involving a highly exothermic reaction and a highly endothermic one, are employed to evaluate the proposed design philosophy and the results obtained confirm its feasibility and effectiveness. In addition, sensitivity analysis is conducted with respect to the amount of catalyst employed, relative volatilities of reacting mixtures, and thermal condition of feeds. The applicability and potentials of the design principle proposed are highlighted.     

Impact of mass transfer coefficient correlations on prediction of reactive distillation column behaviour

A significant part of the safety analysis of a reactive distillation column is the identification of multiple steady states and their stability. A reliable prediction of multiple steady states in a reactive distillation column is influenced by the selection of an adequate mathematical model.For modelling reactive distillation columns, equilibrium (EQ) and nonequilibrium (NEQ) models are available in the literature. The accuracy of the nonequilibrium stage model seems to be limited mainly by the accuracy of the correlations used to estimate the mass transfer coefficient and interfacial area.The binary mass transfer coefficients obtained from empirical correlations are functions of the tray design and layout, or of the packing type and size, as well as of the operational conditions and physical properties of the vapour and liquid mixtures.In this contribution, the nonequilibrium model was used for the simulation of a reactive distillation column. For prediction of the binary mass transfer coefficient for a sieve tray, four correlations were chosen to show their impact on the prediction of the reactive distillation column behaviour. As a model reactive distillation system, the synthesis of methyl tertiary-butyl ether (MTBE) was chosen. The steady-state analysis and the dynamic simulation of the model system were done. Qualitative differences between the steady states were predicted using the chosen correlations.     

Design and optimization of reactive distillation: a review

Reactive distillation process, a representative process intensification technology, has been widely applied in the chemical industry. However, due to the strong interaction between reaction and separation, the extension of reactive distillation technology is restricted by the difficulties in process analysis and design. To overcome this problem, the design and optimization of reactive distillation have been widely studied and illustrated for plenty of reactive mixtures over the past three decades. These design and optimization methods of the reactive distillation process are classified into three categories: graphical, optimization-based, and evolutionary/heuristic methods. The primary objective of this article is to provide an up-to-date review of the existing design and optimization methods. Desired and output information, advantages and limitations of each method are stated, the modification and development for original methodologies are also reviewed. Perspectives on future research on the design and optimization of reactive distillation method are proposed for further research.     

SIMULATION OF STEADY DISTILLATION PROCESS ACCOMPANIED WITH MULTIPLE CHEMICAL EQUILIBRIUM REACTIONS BASING ON TRANSFORMED VARIABLES

The equations describing the steady state operation of distillation column with simultaneous equilibrium chemical reactions have been transformed into a new set or MESH equations by introducing transrormed composition, flow rate and enthalpy variables. The new set of equations are similar to equations describing traditional non-reactive distillation, and it is general when one or several reactions occur and inert components are present.

An algorithm combining relaxation method and modified Newton-Raphson method is proposed to solve the new equations, the relaxation method is used to estimate initial values, while the modified Newton-Raphson method is used to set solution. The reactive distillation processes of MTBE synthesis with n-butane and the separation of meta- and para-xylene are simulated as numerical examples     

Enhancing mass and energy integration by external recycle in reactive distillation columns

The synthesis and design of reactive distillation columns separating reacting mixtures with the most unfavorable relative volatilities (i.e., the reactants are the heaviest and lightest components with the products being the intermediate ones) are described. The unfavorable thermodynamics poses great difficulties in combining the reaction operation and the separation operation involved and limits severely the potential of reactive distillation columns in the reduction of capital investment (CI) and operating cost. To remove the limitation, we propose two strategies for facilitating the synthesis and design of this kind of reactive distillation columns in this article. One is to arrange prudentially the reactive section so as to strengthen internal energy integration between the reaction operation and the separation operation involved; that is, while the reactive section should be placed at the bottom of the reactive distillation columns separating exothermic reactions, it should be at the top of the reactive distillation columns separating endothermic reactions. The other is to introduce an external recycle flow between the two ends of the reactive distillation columns to reinforce internal mass integration and internal energy integration between the reaction operation and the separation operation involved; that is, whereas the external recycle flow should be directed from the top to bottom of the reactive distillation columns separating exothermic reactions, it should be from the bottom to top of the reactive distillation columns separating endothermic reactions. Separation of four hypothetical ideal (i.e., two quaternary and two ternary systems, respectively) and two real nonideal (i.e., two quaternary systems) reacting mixtures is chosen to evaluate the proposed strategies. The results show that they can considerably lower energy requirement besides a further reduction in CI. © 2012 American Institute of Chemical Engineers AIChE J, 59: 2015–2032, 2013     

1-氯-1,1-二氟乙烷反应精馏工艺的定态模拟研究

提出了以偏氯乙烯为原料生产1-氯-1,1-二氟乙烷的反应精馏新工艺,并运用Aspen Plus软件对实现该工艺的反应精馏塔的定态行为进行了数值模拟研究。模拟研究结果表明：将化学反应过程和产物、副产物及残余反应物的分离过程集成在-个塔内实现是可能的,不仅能够得到高纯度的产品,同时还能提高反应选择性,减少副产物的生成量。相比于目前采用的传统生产工艺,反应精馏新工艺能够大大减少工艺环节和节省设备投资。案例研究显示,对于相同的偏氯乙烯加料量,1-氯-1,1-二氟乙烷产品的产量提高6．4％,副产物1,1,1-三氟乙烷的产量降低70．6％。     

On the multiple solutions of the reactive distillation column for production of fuel ethers

A systematic study of the operating conditions and parameter sensibility of multiple steady states in a reactive distillation column for production of fuel ethers is presented. The multiplicity analysis is performed using bifurcation diagrams for several scenarios using the process simulator Aspen One Aspen Plus. Our results indicate that the main variables that affect both the presence and behavior of multiple solutions in the reactive distillation column are the reflux ratio, the reboiler duty and the thermal condition in the feed stream. In particular, we have determined that the vapor–liquid stream in the feed has a major influence on the occurrence of this phenomenon and this parameter especially establishes the type of multiplicity that can occur in the reactive distillation column for production of fuel ethers.     

伴有平衡反应的分离过程的数学模拟(Ⅰ)板式反应精馏塔的模拟计算

本文提出了相平衡和化学平衡同时存在的多相复杂可逆反应的反应精馏计算方法。模型方程由物料衡算方程,化学平衡方程,相平衡方程,归一方程,焓衡算方程组成。模型分三层迭代求解。该算法可适用于非理想系统、反应动力学未知的快速可逆反应过程。并且对不同操作条件下的计算均能稳定收敛,可以进行操作条件的调优。应用该算法模拟计算未达到平衡的反应过程,可以预测各板上的最大转化率和各板组成。     

Catalytic cyclic distillation – A novel process intensification approach in reactive separations

Cyclic distillation uses a periodic operation mode that leads to key benefits, such as increased column throughput, lower energy requirements and much higher separation performance. By integrating chemical reactions with cyclic distillation, a novel process intensification approach is possible – catalytic cyclic distillation – which outperforms classic reactive distillation.This paper is the first to describe the catalytic cyclic distillation process and to develop a rigorous mathematical model. By means of a case study involving a simple reaction system, the model is used to demonstrate the key benefits of this operation mode. In addition, the synthesis of dimethyl ether by catalytic cyclic distillation is considered, for which a design algorithm is suggested. Investigation of the column behavior reveals the coexistence of two periodic states, one of them being unstable.     

Analysis of the possibility of implementing the principle of redistribution of concentration fields for reactive distillation processes

The evolution of the phase portrait for the dynamic system of open evaporation combined with a chemical reaction for a fixed liquid amount was studied as dependent on the rate constants of the reaction. A three-component azeotropic mixture whose vapor-liquid equilibrium diagram is characterized by two distillation regions was considered. The possibility of the open evaporation trajectory to transfer through the separatrix between the distillation regions was shown, as well as the possibility of implementing the principle of redistribution of the concentration fields due to chemical reactions occurring in reactive distillation columns.Translated from Teoreticheskie Osnovy Khimicheskoi Tekhnologii, Vol. 39, No. 2, 2005, pp. 115–119.Original Russian Text Copyright © 2005 by Solokhin, Nazanskii, Timofeev.     

Recovery of Formic Acid and Acetic Acid from Waste Water Using Reactive Distillation

Processing of biobased feedstock materials may lead to formation of multicomponent azeotropic mixtures. Reactive separations provide an opportunity to circumvent azeotropes by changing the substance properties through chemical reactions. Exemplarily several effluents from black liquor processing contain aqueous mixtures of low molecular weight fatty acids such as formic acid and acetic acid. These mixtures form inseparable azeotropes. Separation of the system formic acid–acetic acid–water by esterification with methanol was investigated. Reactive distillation experiments in batch and continuous mode confirmed complete removal of formic acid in a first step. Acetic acid may then be isolated by distillation or by reactive distillation.