The liquid-phase reversible reactions considered are
2M1B + MeOH ^^ TAME 2M2B + MeOH ^^ TAME 2M1B ^^ 2M2B
The kinetics for the forward and reverse reactions are given in Table 9.3. These reaction rates are given in units of kmol s"1 kg" and are converted to the Aspen-required units of kmol s"1 m"3 by using a catalyst bulk density of 900 kg/m3. The concentration units in the reaction rates are in mole fractions. The reactive stages in the column each contain
242 REACTIVE DISTILLATION TABLE 9.3 Reaction Kinetics
Af Ef Ar Er 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
The reactions and all the kinetic parameter must be set up in Aspen Plus. In the Exploring window, click on Reactions and then the second Reactions. Right-click and select New. This opens the window shown in Figure 9.9a, in which the type of reaction is selected
to be REAC-DIST. Then click on the new reaction R-1, which opens the window shown in Figure 9.9b, on which Kinetic is selected.
Clicking OK opens the window shown in Figure 9.9c, on which reactant and product components are selected. The reactant coefficients are negative, and the product coefficients are positive. The Exponent is the power-law exponent used in the reaction rate expression. Make sure to select Kinetic in the upper right corner.
The procedure is repeated for the three forward reactions and for the three reverse reactions. These are shown in Figure 9.10. All of this input is done on the Stoichiometry page tab. Clicking the Kinetic page tab and selecting one of the six reactions opens the window shown in Figure 9.11a, on which all the kinetic parameters are entered for that reaction. Remember to select Liquid for reacting phase and mole fraction for the concentration basis (see Fig. 9.11b).
It is important to point out that for reactive distillation Aspen Plus does not list activities as a standard option for the concentration basis. This is a distinct limitation because many of the reactions that occur in reactive distillation systems use activities. Special-purpose programs can be written, but these are beyond the scope of this book. Later versions of Aspen Plus should remove this limitation.
Now the reactions have been set up. Go to the C1 block and click Reactions. Enter the starting and ending stages on which reaction occurs on the Specifications page tab in Figure 9.12a and select the reaction R-1. Note that R-1 is a set of six reactions.
Clicking the Holdups page tab opens the window shown in Figure 9.12b, in which the molar or volumetric holdups on each of the reactive trays are entered. The reactive liquid volume on each tray is set at 1.22 m3, which corresponds to a liquid height of 0.055 m for a reactive column with a diameter of 5.5 m.
It is important to note that the diameter of the column is not known initially because this depends on vapor velocities that are unknown until the column is converged to the desired specifications. So column sizing in a reactive distillation column is an iterative procedure. A diameter is estimated, tray holdups calculated, and the column is converged. Then the a Browser
^Stoichiometry] ^KineticI Equilibrium | Conversion | Salt
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