Zuzana Švandová

Impact of mathematical model selection on prediction of steady state and dynamic behaviour of a reactive distillation column

The objective of this paper was to compare the prediction of the equilibrium (EQ) and nonequilibrium (NEQ) models during safety analysis of a reactive distillation column focusing on the identification of hazardous situations or particular operability problems. The safety and operability analyses are based on application of the HAZOP procedure integrated with a mathematical model with the aim to determine the column response to deviations from normal operation conditions or during a nonstandard procedure, e.g. the start-up of the reactive distillation column. A significant part of the safety and operability problems analysis is the identification of multiple steady states and their stability. A reactive distillation column can in general exhibit multiple steady states which reduce the column operability and controllability during perturbations of the manipulated variables and particularly during the start-up and shut down procedures. The EQ and NEQ models were compared focusing on prediction of the multiple steady states phenomenon and of the consequences which can result from this phenomenon.

Theoretical study on transesterification in a combined process consisting of a reactive distillation column and a pervaporation unit

Steady state analysis of a combined hybrid process consisting of a reactive distillation column, pervaporation unit, and a distillation column is presented. This process configuration was first presented by Steinigeweg and Gmehling (2004) for the transesterification of methyl acetate and butanol to butyl acetate and methanol. This system is characteristic for its low reaction rate and complex phase equilibrium. Steinigeweg and Gmehling (2004) have shown that the combination of reactive distillation and pervaporation is favourable since conversions close to 100 % can be reached with a reasonable size of the reactive section in the reactive distillation column. The aim of this paper is to show that although high conversion can be achieved, very complicated steady state behaviour must be expected. The presented analysis is based on mathematical modelling of a process unit, where the steady-state analysis, including continuation and bifurcation analyses, was used. Multiple steady states were predicted for the studied system; three steady states with conversions higher than 98 %. However, not all predicted steady states met the maximal allowed temperature condition in the reactive section (catalyst maximal operation temperature of 393 K). The presence of multiple steady states reduces the operability and controllability of the reactive distillation column during its start-up and during the occurrence of any variation of operating parameters because the system can be shifted from one steady state to another one (concurrent exceeding the maximal allowed temperature) with unwanted consequences, e.g. production loss. Therefore, design and subsequent operation of such a complicated system is an ambitious task requiring knowledge of any possible system behaviour.

Multiple Steady States in a CSTR with Total Condenser: Comparison of Equilibrium and Nonequilibrium Models*

Comparison of equilibrium and nonequilibrium models of a CSTR with total condenser focused on the multiple steady states and dynamic behaviour was carried out. The steady-state behaviour of the model system, MTBE synthesis from methanol and isobutene in a reactive distillation column, was studied in terms of the input parameters, i. e. feed flow rate of methanol or butenes, reflux ratio, and mass of catalyst. The dynamic behaviour of the system during the start-up was investigated and perturbations of manipulated variables were found to cause transitions between the parallel steady states.

HAZOP study of a fixed bed reactor for MTBE synthesis using a dynamic approach

A methodology for hazard investigation based on the integration of a mathematical model approach into hazard and operability analysis is presented. This approach is based on mathematical modelling of a process unit where both steady-state analysis, including analysis of the steady states multiplicity and stability, and dynamic simulation are used. The dynamic simulation serves for the investigation of consequences of failures of the main controlled parameters, i.e. inlet temperature, feed temperature and feed composition. This simulation is also very useful for the determination of the influence of failure duration on the reactor behaviour. On the other hand, the steady state simulation can predict the reactor behaviour in a wide range of failure magnitude and determine the parametric zones, where shifting from one steady state to another one may occur. A fixed bed reactor for methyl tertiary-butyl ether synthesis was chosen to identify potential hazard and operational problems of a real process.

A three-phase nonequilibrium model for catalytic distillation

Nonequilibrium model for steady state simulation of catalytic distillation is presented. Mathematical model takes into account both mass and heat transfers across the gas liquid interface and through the liquid-solid (catalyst) interface. Equations describing the mentioned phenomena are based on the effective diffusivity approach. The resulting system of nonlinear algebraic equations was implemented in the FORTRAN programming language and solved by the BUNLSI (Ferraris & Tronconi, 1986) solver. The described model was verified using the experimental data obtained from a continuous distillation column equipped with catalytic packing. As an experimental model system, synthesis of propyl propionate from propan-1-ol and propionic acid was chosen. Comparison of experimental and simulation data is presented, and appropriateness of the developed model for other types of catalytic distillation processes is discussed.

Steady state and dynamic simulation of a hybrid reactive separation process

Modelling and simulation of hybrid reactive separation system in steady state and in dynamic regime was carried out. The investigated hybrid process consisted of a reactive distillation column and a pervaporation membrane located in the distillate stream to remove water from the process. Heterogeneously catalysed esterification of propionic acid with propan-1-ol to propyl propionate and water was chosen as the model chemical reaction. Esterification reactions are a typical example of equilibrium-limited reactions producing water as a by-product. Using just a pervaporation membrane brings the biggest benefit in increasing the yield of one of the reactants due to the removal of water. To study reactive separation processes, a model of the hybrid system in steady state and in dynamic regime was developed. Steady state behaviour of the model was studied for different hybrid system configurations. The effect of catalyst amount doubling was also investigated. Dynamic behaviour of the system during the step changes of propionic acid feed flow rate and during the membrane module failure was investigated. For this reason, the conversion of propionic acid, purity of the product stream, mole fraction of water, and the temperature in three different parts of the reactive distillation column were monitored.

Impact of Mass Transfer on Modelling and Simulation of Reactive Distillation Columns

1.1 Reactive distillation In chemical process industries, chemical reaction and purification of the desired products by distillation are usually carried out sequentially. In many cases, the performance of this classic chemical process structure can be significantly improved by an integration of reaction and distillation in a single multifunctional process unit. This integration concept is called ‘reactive distillation’ (RD); when heterogeneous catalysts are applied, the term ‘catalytic distillation’ is often used. As to the advantages of this integration, chemical equilibrium limitations can be overcome, higher selectivity achieved, by-product formation reduced, heat of the reaction can be used for distillation in-situ, hot spots and run-away effect can be avoided, and azeotropic or closely boiling mixtures can be separated more easily than in a non-RD process. Some of these advantages are realised using a reaction to improve the separation; others are realised using separation to improve the reaction (Sundmacher & Kienle, 2002). Technological advantages as well as financial benefit resulting from this integration are important. Simplification or elimination of the separation system can lead to significant capital savings, increased conversion and total efficiency, which then result in reduced operating costs (Taylor & Krishna, 2000).

Influence of the reactive distillation column configuration on its performance: A computational study

Comparison of the performance of a reactive distillation column with three different hardware configurations is presented. As a reaction system the methyl tertiary-butyl ether (MTBE) synthesis has been chosen. The sieve tray columns with catalyst (encased inside wire gauze envelopes) placed along the liquid flow path differ in the number of reactive trays. The column simulations have been performed using the nonequilibrium model. The steady state behaviour of the three different hardware configurations was studied regarding the three input parameters; feed flow rate of methanol, feed flow rate of butenes, and reflux ratio. It has been shown that by varying the location of the methanol feed stage, the columns exhibit significantly different solution diagrams using the butenes feed flow rate as a continuation parameter. Using dynamic simulations, different perturbations of the manipulated variables were found to cause transitions between multiple steady states and these were also investigated. The major objective of this paper is to demonstrate the importance of the hardware choice in the performance of a reactive distillation column e.g. during the start-up or if occasional variations of the operating parameters occur.