Design Of Tame Reactive Distillation Systems

In this chapter we take a look at an important example of a reactive distillation column operating in a plantwide environment. The reactive column is part of a multiunit process that includes other columns for recovery of one of the reactants. The process may give the impression that the reactive column is not operating in neat mode because of the need for reactant recovery. We will show that this is really not the case. The recovery of reactant is made necessary by the presence of azeotropes that unavoidably remove one of the reactants from the reactive column.

The example is the production of TAME, which is used as a high-octane gasoline blending component. With the phase-out of lead and the reduction of aromatics in gasoline, the production of various oxygen-containing compounds such as alcohols and esters has grown in importance. The largest volume component in the past was MTBE, but it is being phased out because of groundwater contamination problems. Therefore, TAME is becoming more important. It is produced by the reaction of methanol with unsaturated C5 isoamylenes (2-methyl-1-butene and 2-methyl-2-butene). The liquid-phase reversible reactions considered are

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

The process flowsheet has a prereactor, a reactive distillation column, and a methanol recovery section. A methanol recovery section is required because the inert C5 components coming in with the reactive isoamylenes in the C5 fresh feed form azeotropes with methanol. The result is that a significant amount of methanol is present in the distillate from the

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

reactive column. This requires that more methanol must be fed into the reactive column than just that amount required by the stoichiometry of the reactions.

The designs of two different types of methanol recovery sections are compared. In the first, pressure-swing azeotropic distillation is used. In the second, extractive distillation is used.

The material in this chapter is based on articles by Subawalla and Fair,1 AlArfaj and Luyben,2 and Luyben.3 Rigorous steady-state simulation of the process is performed using Aspen Plus. All columns use rigorous RadFrac models. Details of how to use Aspen Plus for simulating conventional as well as reactive distillation columns are provided in work by Luyben.4

8.1 CHEMICAL KINETICS AND PHASE EQUILIBRIUM 8.1.1 Chemical Kinetics

The three reactions are first order in their respective reactants and products. Concentration units are mole fractions.

R1: <1 = Mcat(kF1x2M1BxMeOH — kB 1xTAMe) (8-2)

Kinetic parameters for the three liquid-phase reversible reactions are given in Table 8.1. These reaction rates are given in units of kilomoles per second per kilogram of catalyst and are converted to the Aspen required units of kilomoles per second per cubic meter by using a catalyst bulk density of 900kg/m3. The reactive stages in the column each contain 1100 kg of catalyst. This corresponds to 1.22 m3 on each tray, which gives a weir height of 0.055 m for a reactive column with a diameter of 5.5 m.

TABLE 8.1 Reaction Kinetics for TAME Reactions

Af Ef Ag Eg DHrx

Reaction (kmol s-1 kg-1) (kJ/mol) (kmol s-1 kg-1) (kJ/mol) (kJ/mol)

R1 1.3263 x 108 76.103737 2.3535 x 1011 110.540899 -34.44

R2 1.3718 x 1011 98.2302176 1.5414 x 1014 124.993965 -26.76

R3 2.7187 x 1010 96.5226384 4.2933 x 1010 104.196053 -7.67

'H. Subawalla and J. R. Fair, Design guidelines for solid-catalyzed reactive distillation systems, Ind. Eng. Chem. Res. 38, 3693 (1999).

2M. A. Al-Arfaj and W. L. Luyben, Plantwide control for TAME production using reactive distillation, AIChE J. 50, 1462 (2004).

3W. L. Luyben, Comparison of pressure-swing and extractive-distillation methods for methanol recovery systems in the TAME reactive-distillation process, Ind. Eng. Chem. Res. 44, 5715-5725 (2005).

4W. L. Luyben, Distillation Design and Control Using Aspen Simulation, Wiley, Hoboken, NJ, 2006.

8.1.2 Phase Equilibrium Using Aspen Plus

The phase equilibrium 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-liquid equilibrium issues involved in the separation, we consider the ternary system iC5, methanol, and TAME.

Pure Component and Binary Information. The normal boiling points of these three components are 301, 338, and 359 K, respectively. The reactive column operates at 4 bar, at which the boiling points are 348, 377, and 413 K, respectively. This pure component information can be easily obtained in Aspen Plus by going to the top tool bar and clicking Tools, Conceptual Design, and TernaryMap Analysis. The window shown in Figure 8.1 opens in which components, pressures, and a physical property model (UNIFAC) are specified. Then click Pure Components in the Output list, as shown in Figure 8.2. To find the azeotropes, click Azeotropes on the list (Fig. 8.3). Compositions and temperatures of the azeotropic mixtures are listed for the two binary azeotropes at 4 bar.

iC5/methanol: 339 K, 25.01 mol% methanol methanol/TAME: 376 K, 84.43 mol% methanol

Note that the UNIFAC physical property package predicts that the iC5/methanol azeotrope is heterogeneous (two liquid phases).

Reactive Distillation
Figure 8.1 Select components and pressure.
Figure 8.2 Pure component vapor pressures.

The boiling point of the first azeotrope (339 K) is lower than the boiling point of the lightest component iC5 (348 K). This means that the overhead from the column will have a composition close to the azeotropic composition.

Various types of phase diagrams can be generated in Aspen Plus. Binary Txy diagrams are produced by going to the top tool bar and clicking Tools, Analysis, Property, and Binary. The window shown in Figure 8.4 opens in which components and pressures are specified. Clicking the Go button at the bottom produces the Txy diagram shown in Figure 8.5 for iC5/methanol. A detailed table of results is given in the Binary Analysis Results window, which is provided in Figure 8.6. To generate other plots for the same components, click the Plot Wizard button at the bottom of this window and select the type of plot desired. Figure 8.7 gives the xy diagram, and Figure 8.8 shows the composition dependence of the activity coefficients. Figure 8.9 displays the Txy diagram for methanol/TAME at 4 bar.

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