Thermodynamic Fundamentals of Reactive Distillation

Abstract

Reactive distillation means the simultaneous implementation of reaction and distillation in a counter currently operated column. The effective phase equilibrium is very complex since the chemical equilibrium is superimposed on the vapor liquid equilibrium. The paper focuses on the calculation and presentation of this equilibrium using so called reactive distillation lines. This leads to a clear explanation for the existence of reactive azeotropes and to a simple method for the determination of the concentration of these azeotropes. The effectiveness of this method is demonstrated on several mixtures with different reaction mechanisms.One of the classic tasks of Chemical Engineering is the downstream processing of products from a chemical synthesis. As most chemical reactions are reversible, and hence do not achieve complete conversion, the reactor effluent consists of both reactants and products. Pure product is required downstream, i.e. the remaining reactants have to be separated and recycled back into the reactor. The separation of reactor effluent, mainly through distillation, is often more complicated than the reaction itself, especially if an azeotrope is present. This process combination can be substantially simplified through the implementation of a counter-current column, in which both the reaction and distillation occur. In the accompanying text, the thermodynamic background of this so called reactive distillation will be presented. The following simplifying assumptions have been made:The reaction is reversible, i.e. the reactants are not completely converted into the desired products. The reaction occurs in the liquid phase only. The reaction requires a catalyst, either homogeneous or heterogeneous. In the presence of catalyst, the reaction can be considered instantaneous. Without catalyst, no reaction can take place.These assumptions simplify the true conditions, however, allow for the basic mechanisms to be presented and more easily understood. Understanding these mechanisms will also enable the comprehension of more complex systems.

Tributyl Citrate Production via Reactive Distillation—Conceptual Design and Pilot-Scale Validation

“…Once the RD process constrains and specifications are established, the analysis of the reduced concentration diagram enables a conceptual understanding of the operation. − Initially, Figure presents the chemical equilibrium conditions ( i.e., chemical equilibrium line) for the entire range of CA/BuOH feed ratios and their corresponding SD values in the color scale; this corresponds to the parabolic line. The composition of any mixture of CA and BuOH potentially fed to the column is located in the vertical right side of the rectangle.…”

“…The gray arrows ( i.e., stoichiometric lines) represent the pathway followed by the corresponding feed mixture toward the chemical equilibrium composition. In this case, there is no change in the number of moles, and then, the reaction pathways are parallel horizontal lines moving toward the left ( i.e., product side) . Therefore, the gray area within the chemical equilibrium envelope corresponds to the attainable regions when using a batch reactor.…”

Mixed-Integer Nonlinear Programming Optimization of Reactive Distillation Processes

“…3 The system considered exhibits two (nonreactive) binary minimum azeotropes with a distillation boundary in between. The residuum lines run between the a-b-minimum azeotrope and the high boiler c and the intermediate boiler b, respectively.…”

Process Design Approach for Reactive Distillation Based on Economics, Exergy, and Responsiveness Optimization

“…The nonlinear coupling of chemical reaction, phase equilibria, and transport phenomena can, however, lead to undesired, systemdependent features, such as multiple steady states 2-17 and reactive azeotropes. [18][19][20][21][22][23][24][25] The results of a case study on the interactions between economics, exergy efficiency, and process responsiveness in the conceptual design of RD processes are discussed in this contribution. In the wider context of conceptual process design, sustainability issues like feedstock selection, (re)use of catalyst, and solvents are of key importance.…”