Begin Executable Code

Figure 9.19 User subroutine for ETBE.

^^^ KETBE=DEXP (10.387D0+406D.59D0/T-2 .890SSDO*DLOG (T) -0.01915<HD0*T+

C fugacity coefficient of components in the mixture

+ ,Y,NC0MP,IDX,NBOPST,KDIAG,KPHI,PHI, DPHI, KER)

WRITE(MAXWRT_MAXBUF(1),9000) T,P,KER

RATE=REALB (1) *KRATE* (ACTIV (K_ETOH) ) **2 .dO* S (ACTIV(K_IC4)-ACTIV(K_ETBE)/KETBE/ACTIV(K_ETOH))

WRITE(MAXWRT_HAXBUF(1),9030) NS TAGE,RATE,RATNET

Figure 9.19 (Continued).

99.1 mol%. The conversion of the isobutene fed is

ETBE in bottoms (706 kmol/h)(0.991)

isobutene in C4 feed (1767 kmol/h)(0.40)

9.2.3 User Subroutine for ETBE

The kinetics given above were used by Bobby Hung to develop a kinetic subroutine RAETBE.f, and this was run in Aspen Plus. Figure 9.19 lists the Fortran subroutine.

Reflux Vessel Aspen Plus

Figure 9.20 Aspen Plus steady-state conditions for ETBE.

B = 707 kmol/h 0.0007 EtOH 0.0028 ¡C = 4 0.9900 ETBE 0.0065 nC = 4

Figure 9.20 Aspen Plus steady-state conditions for ETBE.

Figure 9.20 provides the flowsheet results from the Aspen Plus simulation, which are in good agreement with the results of Al-Arfaj and Luyben.3 Figures 9.21 and 9.22 give composition and temperature profiles.

There is a peak isobutene concentration of 22.5 mol% at stage 20. This is the location where the C4 fresh feedstream is introduced. The reaction zone (stages 6-20) has a fairly high concentration of ethanol in the region between stage 16 and stage 6.

9.2.4 Chemical Equilibrium Model

Because it is reported that the ETBE reaction is equilibrium limited, not kinetically limited, we might expect that the chemical equilibrium option available in Aspen Plus could be used to look at the steady state. It cannot be used in Aspen Dynamics, but at least some useful steady-state design information might be obtained. In Section 9.19.1 a chemical equilibrium model was explored for the MTBE system and found to give a design requiring less energy than that needed when using the kinetic model. These results are what we would expect.

The equilibrium model is easy to generate, as discussed in Section 9.1.6. The reaction is changed from kinetic to equilibrium and the equilibrium constant is calculated from Gibbs free energies.

Chemical Equilibrium Model
Figure 9.21 Aspen Plus composition profiles for ETBE.
Etbe Temp Vapour Pressure Curve
Figure 9.22 Aspen Plus temperature profile for ETBE.

Thus, the same idea was applied to the ETBE simulation. However, the results were quite unexpected. The conversion dropped to less than 50%, and the concentrations of both reac-tants in the entire reaction zone were quite high. We are at a loss to explain these results.

9.2.5 Effects of Design Parameters

Attempts were made to explore the impact of several parameters on the design of the ETBE system. Convergence issues and frequent Fortran system errors severely limited this investigation.

Two design specifications were set up to hold the bottoms purity at 99 mol% ETBE and the distillate ethanol impurity at 0.7 mol% by manipulating the bottoms flowrate and reflux ratio.

Pressure. The base case design pressure is 7.5 atm. Table 9.4 gives results over a small range of pressures around this value. Decreasing pressure reduces energy consumption, but the production of ETBE decreases somewhat because of more losses of reactant in the distillate. The program would not converge for pressures lower than 7.2 atm.

TABLE 9.4 Pressure Results for ETBE

Pressure (atm)

Reboiler Heat Input (MW)

Reflux Ratio

ETBE Recovered in Bottoms (kmol/h)

Isobutene Lost in Distillate and Bottoms (kmol/h)

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