Types of Reactive Distillation Systems

There are many types of reactive distillation systems because there are several types of reactions that are carried out in reactive columns. There are also several types of process 

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The results presented here are rather unconventional because chemical processes are known to be quite nonlinear but not to this degree in such a consistent manner. Two measures are used to differentiate the degree of nonlinearity for these three types of reactive distillation systems. One obvious choice is the fraction of trays that exhibit sign reversal. 

There is only one reactant that is fed to the column. The two products are removed out of the two ends of the column. Olefin metathesis is an example of this type of reactive distillation column. Figure 9.2 illustrates this system and gives an effective control scheme. A C5 olefin reacts to form a light C4 olefin, which is removed in the distillate, and a heavy C6 olefin, which is removed in the bottoms. The two temperature controllers are used to maintain conversion and product quality. The production rate is set by a feed flow controller. 

Reactive distillation is attractive in those systems where certain chemical and phase equilibrium conditions exist. We will discuss some of its limitations in Section 1.4. Because there are many types of reactions, there are many types of reactive distillation columns. In this section we describe the ideal classical simation, which will serve to outline the basics of reactive distillation. 

Our emphasis is on rigorous simulations, not approximate methods. Rigorous models are used for steady-state design and dynamic analysis of a variety of different types of reactive distillation columns. Several types of ideal systems are studied as well as several real chemical systems. 

Steady-state analysis indicates that the type I and type III systems are more economical than the type II system. This chapter explores the dynamic controllability of these three flowsheets. Of more importance, we want to devise a systematic approach to the control of these three types of reactive distillations. AU of the results are obtained from steady-state and dynamic simulations using Aspen Plus and Aspen Dynamics. 

In this section a systematic approach is proposed to design the control structures for these three types of reactive distillation flowsheets. Because all five reactive distillation systems (Table 7.5) have almost equal molar feedflows (neat flowsheet), the stoichiometric balance has to be maintained. Here we adjust the feed ratio to prevent accumulation of unreacted reactants attributable to stoichiometric imbalance. The next issue is, how many product compositions or inferred product purities should be controlled For the esterification reactions with A -f B C + D with a neat flowsheet, controlling one-end product purity implied a similar purity level on the other end, provided the product flowrates are equal. Thus, a single-end composition (or temperature control) is preferred. This leads to 2 x 2 multivariable control, as opposed to a 3 x 3 multiple-input-multiple-output system. The... 

Matouq et al [3.31] tested two types of catalysts an ion exchange-resin (the form of Amberlyst 15) and a heteropolyacid (HPA) in the production of MTBE from methanol and -butyl alcohol (TBA). Both were shown, active, but the ion-exchange resin showed poor selectivity, producing substantial amounts of by-product isobutylene (IB). Matouq et al. [3.31] tested the production of MTBE using the ion-exchange resin in a reactive distillation column. It was difficult to test the HPA catalyst in the reactive distillation system, however, because its particle size was too small and was carried out by the liquid phase. Matouq et al. [3.31] proposed, instead, the use of a PVMR incorporating a PVA membrane. As shown in Figure 3.9, in the proposed system the PVMR is coupled with a con-... 

Chapter 9 covers the treatment of fluidized-bed reactors, based on two-phase models and new empirical correlations for the gas interchange parameter and axial diffusivity. These models are more useful at conditions typical of industrial practice than models based on theories for single bubbles. The last chapter describes some novel types of reactors including riser reactors, catalyst monoliths, wire screen reactors, and reactive distillation systems. Examples feature the use of mass and heat transfer correlations to help predict reactor performance. 

A comparison of the processes shown in Figs. 11.5-1 and 11.5-2 demonstrates the high potential of reactive distillation for process simplification. This type of processes is generally applieable to systems with reversible chemical reactions, e.g., to esterification and etherification of alcohols, to alkylations, to dimerization of olefins, and to hydrogenation of aromatics (Sundmacher and Kienle 2003). 

The first three chapters have explored in a fair amount of detail the four-component quaternary system with the reaction A + B C + D. This system has two reactants and two products. In the next two chapters we will study various aspects of two other types of ideal chemical systems. In Chapter 4 we investigated the impact of a number of kinetic and design parameters on the ideal ternary system with the reaction A + B C with and without inerts in the system. In Chapter 5 we study systems with the reaction A4=> B + C in which there is only one reactant but two products. We will illustrate that the chemistry has an important effect on how the many kinetic and design parameters impact the reactive distillation column. 

Now that we have demonstrated that the one-column neat reactive distillation system can be controlled but may require a composition measurement to handle all types of disturbances, we want to see how the two-column system with an excess of reactant handles disturbances. The case considered is the 20% excess of B fed to the reactive coliunn. Figure 11.25 contains the flowsheet of the reactive column/iecovery column system. The two product streams are distillate Di from the reactive column containing product C and bottoms B2 from the recovery column containing product D. The distillate D3 from recovery... 

Control systems were explored in Chapters 10 and 11 for quaternary reactive distillation systems with the classical two-reactant, two-product, reversible A - - B -O C - - D reaction. In this chapter we study the control of several types of ternary systems. Their steady-state designs were discussed in Chapters 5 and 6. 

The analysis presented here clearly indicates that the reactive distillation systems, regardless of the types of flowsheet, exhibit severe nonlinearities that include significant sign reversal, extremely large values of Allgower s nonlinearity measure (< ) ), and input multiplicity. Under these circumstances, control structure design becomes important. 

CATALOG OF TYPES OF REAL REACTIVE DISTILLATION SYSTEMS... 

A ternary system with a hyperbola-type PSPS is used to investigate the influence of membrane permeation (Fig. 4.32). The applied parameters (ct,A and Kg) and the corresponding eigenvalues of the matrix [A] are summarized in Tab. 4.3. For comparison, again the PSPS for the reactive distillation process is given in Fig. 4.32(a). The effect of a selective membrane with a diagonal [/e]-matrix is illustrated in Fig. 4.32(b, d). 

The kinetics of a chemical reaction have a significant influence on the products that can be attained from a RD process. The attainable products of counterinfinite reflux ratio can be obtained as singular points of a reactive reboiler batch process (bottom product) or a reactive condenser batch process (distillate product). The compositions of both products are located on a unique singular point curve. This curve is independent of any special type of reaction kinetics. However, the locations of the top and bottom products on this curve depend on the structure of the rate equation and on the intensity of the reaction (Damkbhler number) in the considered reaction system. 

First simulation results on steady state multiplicity of etherification processes were obtained for the MTBE process by Jacobs and Krishna [45] and Nijhuis et al. [78]. These findings attracted considerable interest and triggered further research by others (e. g., [36, 80, 93]). In these papers, a column pressure of 11 bar has been considered, where the process is close to chemical equilibrium. Further, transport processes between vapor, liquid, and catalyst phase as well as transport processes inside the porous catalyst were neglected in a first step. Consequently, the multiplicity is caused by the special properties of the simultaneous phase and reaction equilibrium in such a system and can therefore be explained by means of reactive residue curve maps using oo/< -analysis [34, 35]. A similar type of multiplicity can occur in non-reactive azeotropic distillation [8]. 

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