TERNARY REACTIVE DISTILLATION SYSTEMS

The chemical system considered in previous chapters featured the classical quaternary two-reactant, two-product A - - B C -h D reversible reaction. Some interesting phenomena were discussed. In particular, the effect of the number of reactive trays on energy consumption was demonstrated to be counterintuitive, that is, there is an optimum number of reactive trays that minimizes energy consumption. 

In this chapter we explore a similar but somewhat different chemical system. The reaction is A + B C, which is reversible, liquid phase, and exothermic. There is only one product instead of two. This seemingly small change in the chemistry alters the effects of several important design parameters of a reactive distillation column. Because there are three components, we label the system ternary. 

This is a very important class of reactions. The majority of commercial reactive distillation columns deal with this type of reaction. The large volume gasoline additive chemicals produced by reactive distillation, such as MTBE, ETBE, and TAME, are this type. These three examples involve an alcohol and an iso-olefin (either C4 or C5). The reactants are methanol and isobutene for MTBE, ethanol and isobutene for ETBE, and methanol and isoamylenes for TAME. 

Although there are only three components involved in the reaction, in many of the A -h B C systems there are more than three components in the column because the feed-streams contain other components. These components are inert from the standpoint of the reaction, but they are not inert from the standpoint of their effect on the vapor-liquid equilibrium in the column. These inert components are present in the olefin feedstreams that contain the reactive iso C4 and C5 olefins in the examples cited. The reason for their presence is the great difficulty in separating the desired iso-olefin from the other components. For example, in the MTBE and ETBE cases, isobutene is produced in a catalytic cracker in a refinery along with a number of other C4 components (isobutane, w-butane, and n-butene). 

Reactive Distillation Design and Control. By William L. Luyben and Cheng-Ching Yu Copyright 2008 John Wiley Sons, Ihc. 

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The design of two real reactive distillation systems was explored. Both the MTBE and ETBE systems are basically ternary systems with inerts. The control of these two systems will be studied in Chapter 15. 

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 TAME reactive distillation system with a two-column methanol recovery system was successfully simulated in Aspen Dynamics. The system features two recycles (methanol and water) and three feedstreams (C5, methanol, and water). The system is essentially a ternary system with inerts, but the complex vapor-liquid equilibrium results in the formation of azeotropes that result in losses of methanol out of the top of the reactive column with the inerts. Therefore, a methanol recovery system must be included in the plant design and control. 

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). 

Ma, Xu, Liu, and Sun (2010) used perfluorosulfonic acid-poly(vinyl alcohol)-Si02/ poly(vinyl alcohol)/polyacrylonitrile (PFSA-PVA-Si02/PVA/PAN) bifunctional hollow-fiber composite membranes. The catalytic and the selective layer of the membrane were independently optimized. These membranes were synthesized by dipcoating. The performance of these bifunctional membranes was evaluated by dehydrating the ternary azeotropic composed of a water, ethanol, and ethyl acetate system (top product of a reactive distillation process of esterification of acetic acid with ethanol), obtaining separation factors of water/ethanol up to 379. An extensive assessment on the esterification reaction of ethanol-acetic acid was later published (Lu, Xu, Ma, Cao, 2013). In this case, the reaction equilibrium was broken in less than 5 h, and a 90% conversion of acetic acid was achieved after 55 h. 

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. 

We hrst discuss the ternary system without inerts to gain some insights into how changing from a two-product system to a one-product system impacts reactive distillation design. Unlike the quaternary column with distillate and bottoms products, the ternary column without inerts has only one product stream leaving the column. The effects of several design variables are shown to be quite different in the ternary system than those we observed in the quaternary system. 

Then we explore the ternary system with inerts present in one of the feedstreams. The reactive distillation column now has two streams leaving the column. One contains product C and the other contains the inerts. 

These results demonstrate a fundamental difference between a two-product reactive column (quaternary system) and a one-product reactive column (ternary system). This is a good example of the complexity and challenges associated with reactive distillation. 

Feed tray location is an important design parameter in reactive distillation, especially in the ternary decomposition system. As shown in the bottom graph in Figure 6.3, there... 

This system is fundamentally a ternary system with inerts. The heavy component is TAME, which leaves the reactive distillation column in the bottoms. 

The phase equihbrium of this system is complex because of the existence of azeotropes. The inert components in the C5 feedstream include isopentane, n-pentane, 1-pentene, and 2-pentene. Essentially all of these inerts go out the top of the reactive distillation column. To illustrate the vapor-hquid equilibrium issues involved in the separation, we consider the ternary system iCs, methanol, and TAME. 

In this chapter we examine two reactive distillation column systems that are used for the production of real chemical components. The two systems are quite similar and are basically ternary systems with inerts that have characteristics similar to those discussed in Chapter 5 for ideal components. 

In Parts I and II we explored the steady-state designs of several ideal hypothetical systems. The following three chapters examine the control of these systems. Chapter 10 considers the four-component quaternary system with the reaction A + B C + D under conditions of neat operation. Chapter 11 looks at control of two-column flowsheets when an excess of one of the reactants is used. Chapter 12 studies the ternary system A + B C, with and without inerts, and the ternary system A B + C. We will illustrate that the chemistry and resulting process structure have important effects on the control structure needed for effective control of reactive distillation columns. 

In Chapter 9 we explored the steady-state designs of both the MTBE and the ETBE reactive distillation columns using Aspen Plus. In this chapter we export the files into Aspen Dynamics as pressure-driven dynamic simulations and then look at dynamics and control. The control structures evaluated on both systems are based on those developed in Chapter 12 for ternary systems with inerts. 

Sundmacher and Qi (Chapter 5) discuss the role of chemical reaction kinetics on steady-state process behavior. First, they illustrate the importance of reaction kinetics for RD design considering ideal binary reactive mixtures. Then the feasible products of kinetically controlled catalytic distillation processes are analyzed based on residue curve maps. Ideal ternary as well as non-ideal systems are investigated including recent results on reaction systems that exhibit liquid-phase splitting. Recent results on the role of interfadal mass-transfer resistances on the attainable top and bottom products of RD processes are discussed. The third section of this contribution is dedicated to the determination and analysis of chemical reaction rates obtained with heterogeneous catalysts used in RD processes. The use of activity-based rate expressions is recommended for adequate and consistent description of reaction microkinetics. Since particles on the millimeter scale are used as catalysts, internal mass-transport resistances can play an important role in catalytic distillation processes. This is illustrated using the syntheses of the fuel ethers MTBE, TAME, and ETBE as important industrial examples. 

In the ternary reaction system without inerts, the column has only a bottoms product in which heavy product C is removed and has only stripping and reactive zones. In the ternary reaction system with inerts, the column has both distillate and bottoms streams. Figure 5.10 gives the flowsheet of the reactive column. 

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