However, a kinetic model supplied by Aspen Technology is used in this chapter. It uses a kinetic expression with concentrations in terms of activities and calculates a chemical equilibrium constant using a more complex function of temperature.
Simulation of reactive distillation using the standard models in Aspen Plus has some significant problems. The reactions can be specified to be either kinetic or equilibrium. In the former case, the choices of concentration units are limited to mole fraction, molarity, or partial pressure. Unfortunately, activity is not on this list. This is a problem because activities are frequently used to describe kinetic data.
In addition, the reaction rate expression is limited to a power law in reactive distillation. Reaction rate expressions such as Langmuir-Hinshelwood-Hougen-Watson (LHHW) cannot be used. These restrictions make the use of Aspen Technology simulations tools for reactive distillation somewhat inconvenient.
In the MTBE case in which equilibrium can be assumed, this problem would seem to be of no consequence. In the steady-state design using Aspen Plus, the chemical equilibrium model can be used. However, a serious limitation arises when one attempts to export the file
Huang and S. J. Wang, Design and control of a methyl tertiary butyl ether (MTBE) decomposition reactive distillation column, IEC Res. 46, 2508-2519 (2007).
to Aspen Dynamic to conduct control studies. The reactions must be kinetic in Aspen Dynamics.
One solution to this problem is to develop a special user-generated subroutine for the reaction. Aspen Technology has such a subroutine in its library for MTBE (Program Files\AspenTech\AMSystem 2004.1\Help\ADExamples.pdf). It is written in Fortran and is called RAMTBE.f. We will use this subroutine in both the steady-state design in this chapter and in the dynamic control discussed in Chapter 15.
In the ETBE case considered later in this chapter, a user-supplied kinetic subroutine had to be developed. The efforts of Bobby Hung of the National Taiwan University are gratefully acknowledged in its development.
The first step is to set up the reaction. Figure 9.1 shows how the stoichiometry of the reaction is specified by opening the Specifications page tab under the Reactions item in the Data Browser window. Opening the Kinetics page tab gives the view shown in Figure 9.2 in which the user-supplied subroutine is selected and the reaction is specified to be in the liquid phase. Then the name of the subroutine is entered by opening the Subroutine page tab, which is displayed in Figure 9.3.
The reactive distillation block is called T1, and selecting the Reactions item opens the window illustrated in Figure 9.4. The location of the reactive stages is specified on the Specifications page tab (stages 4-10). The holdup on the reactive stage is supposedly specified as shown in Figure 9.5. As we will discuss later, this does not seem to work with the user subroutine.
The difference between using a user-supplied subroutine and one of the built-in functions is the need to compile the subroutine. Figure 9.6 demonstrates how this is done. Instead of going to Aspen User Interface, you go to Aspen Simulation Engine and use the DOS-like command aspcomp ramtbe.f to compile the subroutine. Of course, you need to have a Fortran compiler on your computer. Figure 9.7 lists the RAMTBE.f Fortran program in which the chemical equilibrium constant, activities, and reaction rates are calculated.
The Aspen Plus flowsheet is provided in Figure 9.8. The column configuration is set up by opening Setup on the list of items under the T1 block. Figure 9.9a shows the window that opens with the Configuration page tab open. The column has 17 stages with the reflux drum being stage 1 and the partial reboiler (kettle) being stage 17, using Aspen notation. Thus, there are 15 trays. The initial operating specifications are selected to be the bottoms flowrate and the reflux ratio. The convergence method is selected to be Strongly non-ideal liquid because of the azeotropes. Figure 9.9b shows the Streams page tab on which the two feed-streams are specified to enter on stage 10. The pressure of the column is entered on the Pressure page tab, as shown in Figure 9.9c.
Two design specifications are set up (Figs. 9.10 and 9.11) to achieve a bottoms composition of 99.7 mol% MTBE and a distillate composition of 0.0421 mol% MTBE. These compositions are maintained as we explore the effects of various parameters on the design in the next section.
Figure 9.12 gives the process flowsheet with many important variables, Figure 9.13 gives detailed stream information, and Figure 9.14 gives temperature and composition profiles. Notice that the isobutene composition is low throughout the column. There is a peak methanol concentration of about 25 mol% on stage 10. Temperatures in the column run from 347 K at the top to 425 K at the bottom with an operating pressure of 11 bar.
Note that there is a slight excess of methanol fed to the column. The isobutene reactant feed is 705 kmol/h and the methanol feed is 775 kmol/h. Most of the excess leaves in the distillate, giving a composition of 6.6 mol% methanol.
9.1.5 Effect of Design Parameters
The Aspen Plus file is used to explore the effects of several parameters. Convergence issues prevented a detailed study over a wide range of parameter values. This is one of the problems with commercial software. If the program does not converge, you are given little guidance as to what to do.
Pressure. The pressure used in the Aspen example is 11 bar, and this pressure was also used by Huang and Wang.1 Table 9.1 gives results for different pressures with distillate and
SUBROUTINE RAMTBE (NSTAGE, NCOMP, NR,
4 KDIAG, STOIC,
5 IHVBAS, HLDVAP,
6 NREAL, REAL,
7 NINTB, INTB,
8 IUORK, NUORK, WORK) IMPLICIT NONE
IHLBAS, HLDLIQ, TIMVAP, NINT, RATES, RATEL, NREALB, REALB,
NBOPST, TIMLIQ, INT, RATEV, NIUORK,
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