Simple reactive distillation

SIMPLE REACTIVE DISTILLATION 8.2.1 Extending the Residue Curve Map... 

Analogously to batch distillation and the RCM, the simplest means of reactive distillation occurs in a still where reaction and phase separation simultaneously take place in the same unit. Additionally, we can choose to add a mixing stream to this still, and the overall process thus consists of three different phenomena chemical reaction, vapor liquid equilibrium, and mixing. Such a system is referred to as a simple reactive distillation setup. This setup is shown in Figure 8.1 where a stream of flowrate F and composition Xp enters a continuously stirred tank reactor (CSTR) in which one or more chemical reaction(s) take place in the liquid phase with a certain reaction rate r =f(kf, x, v) where v represents the stoichiometric coefficients of the reaction. Reactants generally have negative stoichiometric coefficients, while products have positive coefficients. For example, the reaction 2A + B 3C can... 

Example 8.1 illustrates how one may go about synthesizing a simple reactive distillation process, or at the vary least allows one to assess whether the proposed process is at all feasible. It should be noted that if either k ox p turned out to be negative, the proposed process would be infeasible since these parameters only have realistic meaning when they are positive quantities. Moreover, if the k and p were found to be positive but the node at the desired composition is found to be an unstable node, the simple process would not be feasible either. If a saddle node were found, one has to be certain of the initial composition within the reactor because there is only a single profile which will end at the desired composition. Any other initial composition would result in an entirely different ending composition because the profile will veer away from the saddle point. Lasdy, the reader should also be aware that, in the case of the three component system, there are two independent equations that may be written, implying that one may solve for two unknown process... 

The variation of efficiencies is due to interaction phenomena caused by the simultaneous diffusional transport of several components. From a fundamental point of view one should therefore take these interaction phenomena explicitly into account in the description of the elementary processes (i.e. mass and heat transfer with chemical reaction). In literature this approach has been used within the non-equilibrium stage model (Sivasubramanian and Boston, 1990). Sawistowski (1983) and Sawistowski and Pilavakis (1979) have developed a model describing reactive distillation in a packed column. Their model incorporates a simple representation of the prevailing mass and heat transfer processes supplemented with a rate equation for chemical reaction, allowing chemical enhancement of mass transfer. They assumed elementary reaction kinetics, equal binary diffusion coefficients and equal molar latent heat of evaporation for each component. 

The reactor system may consist of a number of reactors which can be continuous stirred tank reactors, plug flow reactors, or any representation between the two above extremes, and they may operate isothermally, adiabatically or nonisothermally. The separation system depending on the reactor system effluent may involve only liquid separation, only vapor separation or both liquid and vapor separation schemes. The liquid separation scheme may include flash units, distillation columns or trains of distillation columns, extraction units, or crystallization units. If distillation is employed, then we may have simple sharp columns, nonsharp columns, or even single complex distillation columns and complex column sequences. Also, depending on the reactor effluent characteristics, extractive distillation, azeotropic distillation, or reactive distillation may be employed. The vapor separation scheme may involve absorption columns, adsorption units,... 

Figure 4.33 illustrates the PSPS and bifurcation behavior of a simple batch reactive distillation process. Qualitatively, the surface of potential singular points is shaped in the form of a hyperbola due to the boiling sequence of the involved components. Along the left-hand part of the PSPS, the stable node branch and the saddle point branch 1 coming from the water vertex, meet each other at the kinetic tangent pinch point x = (0.0246, 0.7462) at the critical Damkohler number Da = 0.414. The right-hand part of the PSPS is the saddle point branch 2, which runs from pure THF to the binary azeotrope between THF and water. 

The outline of this chapter is as follows First, some basic wave phenomena for separation, as well as integrated reaction separation processes, are illustrated. Afterwards, a simple mathematical model is introduced, which applies to a large class of separation as well as integrated reaction separation processes. In the limit of reaction equilibrium the model represents a system of quasilinear first-order partial differential equations. For the prediction of wave solutions of such systems an almost complete theory exists [33, 34, 38], which is summarized in a second step. Subsequently, application of this theory to different integrated reaction separation processes is illustrated. The emphasis is placed on reactive distillation and reactive chromatography, but applications to other reaction separation processes are also... 

Cuille and Reklaitis (1986) and Albet et al. (1991) used similar model to simulate batch reactive distillation process. Egly et al. (1979), Reuter et al. (1989), Mujtaba (1989) and Mujtaba and Macchietto (1992, 1997) used a modified version of this model based on constant molar holdup in their studies. Sorensen and Skogestad (1996c), Sorensen et al. (1996b), Balasubramhanya and Doyle III (2000) used simple models for studying control strategies in batch reactive distillation. 

Hence, manufacturing several fatty esters in a reactive distillation setup by using heterogeneous catalysis is possible. The equipment is simple and efficient in operation. Making use of a fast catalyst remains the key aspect in the technology. 

FIGURE 8.17 A simple two product single feed reactive distillation columa... 

Reactive Distillation Process Development in the Chemical Process Industries 39 In simple equilibrium reactions, the reaction equation can be described like this... 

In a first approach, the use of the external function in GAMS is compared to an Aspen Plus simulation based on the same thermodynamic data. In this simulation for a reactive distillation with fixed stage numbers and feed stages, the acetic acid dimerization is neglected and a simple reaction kinetic model based on the liquid mole fractions is applied for comparison purposes. The results show exactly the same condenser and reboiler heat duties for both calculations. The perfect agreement between these two calculations which is also confirmed by the concentration profiles clearly demonstrates that the use of the external functions in GAMS gives as thermodynamically accurate results as Aspen Plus. 

We now consider the design of industrial DCRs. First, we consider a simple design without going into details of the role of distillation boundaries for both straight distillation and reactive distillation. This will be followed by a mathematical analysis of these factors. 

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