Heterogeneous Reactive Distillation Research Articles

A new class of control structures for heterogeneous reactive distillation processes

There are only a handful of process control structures applied to the neat operation of both homogeneous and heterogeneous reactive distillation, for two-reactants / two-products one-reaction systems. All of these control structures employ inferential temperature control (or concentration analyzers) at some location in the column to balance the reaction stoichiometry.This original study proposes a new class of control structures applicable to heterogeneous reactive distillation. The novel idea, common to all control structures, is based on monitoring the inventory of the reactant involved in the heterogeneous azeotrope. The organic reflux (or the organic reflux / aqueous distillate ratio) is used to detect the excess or deficiency of the reactant, based on which the fresh feed rate is adjusted such that the reaction stoichiometry is balanced. This control philosophy is simple and easy to implement in different ways as illustrated by several case studies. The performance of the proposed control structures depends on the system studied. For some systems, the performance is better, as good or nearly as good as that of the literature control structures. But for other systems, the performance is poor or the structure even fails to control the process, due to the insufficient feedback from inventory measurements.

Nano-catalytic heterogeneous reactive distillation for algal biodiesel production: Multi-objective optimization and heat integration

This work proposes a novel reactive distillation (RD) column for the production of algal biodiesel by the use of a heterogeneous nano-catalyst, Ca(OCH3)2. The elitist non-dominated sorting genetic algorithm (NSGA-II) is employed to optimize the design of this RD column to maximize biodiesel purity while minimizing total annual cost and CO2 emissions. The optimal point is recommended by a Pareto ranking method, namely, technique for order of preference by similarity to ideal solution (TOPSIS) with entropy information for the weighting of objectives. To improve biodiesel purity, a decanter is proposed to couple with the RD column at the bottom. Subsequently, performance of the optimal RD column is further improved by retrofitting it with the vapor recompression arrangement. In this proposed intensified configuration, a reactive stage is thermally integrated with the reboiler to constitute a side vapor recompressed RD (SVRRD) column. Finally, a comparative study is performed to quantify the performance improvement of the proposed SVRRD column over the conventional RD column. It is found that there is a 34.6% reduction in CO2 emissions with a payback period of 4.54 years for the vapor recompressor. Hence, the proposed SVRRD is attractive with better performance in terms of energetic, environmental and economic perspectives.

Temperature control of heterogeneous reactive distillation

AbstractIn this work, we explore the temperature control of heterogeneous reactive distillation. By heterogeneous reactive distillation, we mean a two‐liquid phase exists in the reflux drum and a decanter is used to separate the aqueous product from the organic reflux. Process characteristics of n‐butyl propionate are explored and a systematic procedure is proposed for the design of butyl propionate reactive distillation. The control structure design procedure consists of the following steps: (1) setting the control objective, (2) selection of manipulated variables, (3) determination of temperature‐control trays, (4) finding controller settings for regulatory control, and (5) providing feedforward compensation for production rate variation. Because specifications on the bottoms product have to be met and stoichiometric balance has to be maintained, we have a 2 × 2 control problem with two obvious inputs: reboiler duty and feed ratio. The reactive distillation exhibits unique temperature sensitivities and the nonsquare relative gain successfully identifies temperature‐control trays, which results in an almost one‐way decoupled system. Therefore, decentralized PI controllers are used at the regulatory level. As a result of the unique feature of a kinetically controlled distillation column, maintaining constant tray temperatures does not imply the same quality specification. Feedforward temperature compensation is necessary to maintain the desired product composition. The proposed design procedure is further extended to butyl acetate reactive distillation, where a similar process behavior is observed, and good control performance can be achieved using simple temperature control. © 2004 American Institute of Chemical Engineers AIChE J, 50: 2203–2216, 2004

Temperature Control of Butyl Propionate Reactive Distillation

Abstract In this work, we explore the temperature control of n-butyl propionate reactive distillation. Process characteristics of n-butyl propionate are explored and a systematic procedure is proposed for the design of butyl propionate heterogeneous reactive distillation. Control objective is to product specifications: high purity propionate and ppm level of acid. The control structure design procedure consists of the following steps: (1) selection of manipulated variables, (2) determine temperature control trays, and (3) find controller settings. Since two specifications on the bottoms product have to be met and stochiometric balance has to be maintained, we have a 2x2 control problem with two obvious inputs: reboiler duty and feed ratio. The reactive distillation exhibits unique temperature sensitivities and the non-square relative gain (NRG) successfully identifies temperature control trays. It results in an almost one-way decoupled system. Therefore, decentralized PI controllers are employed. Simulation results indicate good control performance can be achieved with simple control strategy.

Sensitivity of Thermodynamic Model Parameter to the Design of Heterogeneous Reactive Distillation: Amyl Acetate Esterification

Reactive distillation offers an attractive alternative for process intensification, especially for reaction/separation systems. In this work, we continue the effort to explore a specific class of reactive distillations: heterogeneous reactive distillation (or three-phase reactive distillation). By heterogeneous reactive distillation, we mean two-liquid phase exists in the reflux drum and a decanter is employed to separate the aqueous product from the organic reflux. For esterification reactions, propyl acetate, butyl acetate, butyl propionate and amyl acetate are good examples of this type of reactive distillation, because they all share the following characteristics: (1) a large two-liquid zone exists, (2) the minimum boiling azeotrope is located in the two-liquid zone, and (3) all tie lines slop in the direction of high purity product (typically water). Since phase-split description and location of all minimum boiling azeotropes are crucial for correct process description, the selection of thermodynamic models and corresponding binary parameters become important Unlike typical vapor-liquid equilibria (VLE) systems, the model should be able to describe VLLE (vapor-liquid-liquid equilibria) behavior as well as to locate all minimum boiling azeotropes. In an amyl acetate case study, the results clearly indicate that erroneous process designs may result if the thermodynamic model fails to capture the azeotropes or to locate reasonable two-liquid zone.

Industrial experience in the scale-up of reactive distillation with examples from C 4-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.

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.