Reactive tray holdup

A is lost because less A is fed (upper right graph), and the concentration of A in the distiUate is lower (lower left graph). 

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Reactive distillation is also different from conventional distillation in that there are both product compositions and reaction conversion specifications. The many design degrees of freedom in a reactive distillation column must be adjusted to achieve these specifications while optimizing some objective function such as total annual cost (TAC). These design degrees of freedom include pressure, reactive tray holdup, number of reactive trays, location of reactant feedstreams, number of stripping trays, number of rectifying trays, reflux ratio, and reboiler heat input. 

In the quaternary system, increasing the reactive tray holdup decreases energy consumption. The same is true in the ternary system, as demonstrated in Figure 5.4. Thus, adding more reactive tray holdup improves the steady-state designs of both systems. There are no counterintuitive effects of reactive tray holdup. 

In the quaternary system and in the ternary system without inerts, increasing reactive tray holdup improved reactive column performance in terms of reducing energy consumption. Figure 5.17 demonstrates that the same is true for the ternary system with inerts. However, in addition to the energy benefits, there is also an improvement in yield. Less... 

Figure 5.18 Effect of reactive tray holdup on composition profiles.

Figure 5.18 gives the composition profiles for three dififerent reactive tray holdups. Increasing holdup reduces the concentrations of both reactants in the reactive zone, so less A is lost out the top. 

Increasing the reactive tray holdup decreases the vapor boilup because the products are going toward the opposite direction as the result of consuming all of reactant A, which is shown in Figure 6.19. There is no counterintuitive effect. 

Type II EtAc and IPAc. The type II flowsheet differs from type I in that the reactive zone extends to the column base of the first column (called the reactive distillation column) therefore, a much larger holdup is expected in the bottom of the reactive distillation column (Fig. 7.2). The column base holdup is taken to be 10 times that of a reactive tray holdup. We also assume the feed ratio of the reactant can be changed, which leads to the following optimization variables Nr, Ns, Nrx, fVFheavy. NFiight, and FR. 

Effect of Holdup on Reactive Trays. In all of the ideal cases considered in previous chapters, increasing the holdup on reactive trays improves performance. This corresponds to our intuition. However, in the TAME reactive column the effect of Mjtx is unexpectedly different as Table 8.6 shows. Increasing the reactive tray holdup increases the energy... 

Reactive Tray Holdup. Changing this specification in the reactor block had no effect. Even a zero holdup gave the same solution. We tried changing the holdup in the Eortran subroutine and recompiling, but the column would not converge. 

Other Parameters. We were unsuccessful in trying to change the reactive tray holdup (Fig. 9.23), the number of reactive trays, the number of stripping trays, or the location of the ethanol fresh feed. The program either would not converge or produced Fortran errors and shut down. 

In conventional distillation design, tray holdup has no effect on steady-state compositions. In reactive distillation, tray holdup (or amount of catalyst) has a profound effect on conversion, product compositions, and column composition profiles. Therefore, in addition to the normal design parameters of reflux ratio, number of trays, feed tray location, and pressure, reactive distillation columns have the additional design parameter of tray holdup. 

Now the reactions have been set up. Go to the Cl block and click Reactions. On the Specifications page tab, enter the starting and ending stages on which the reaction occurs 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 m, which corresponds to a liquid 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 guessed, tray holdups calculated, and the column is converged. Then, the diameter calculated in Tray Sizing is compared with the guessed diameter and the calculations repeated. 

Figure 9.12 (a) Specifying reactive trays and holdups, (b) Tray holdups. 

Figure 1.1 presents the flowsheet of this ideal reactive distillation column. In this situation the lighter reactant A is fed into the lower section of the column but not at the very bottom. The heavier reactant B is fed into the upper section of the column but not at the very top. The middle of the column is the reactive section and contains Nkx trays. Figure 1.2 shows a single reactive tray on which the net reaction rate of the reversible reaction depends on the forward and backward specific reaction rates (kp and kp) and the liquid holdup (or amount of catalyst) on the tray (M ). The vapor flowrates through the reaction section change from tray to tray because of the heat of the reaction. 

Another design aspect of reactive distillation that is different from conventional is tray holdup. Holdup has no effect on the steady-state design of a conventional column. It certainly affects dynamics but not steady-state design. Column diameter is determined from maximum vapor-loading correlations after vapor rates have been determined that achieve the desired separation. Typical design specifications are the concentration of the heavy key component in the distillate and the concentration of the light key component in the bottoms. However, holdup is very important in reactive distillation because reaction rates directly depend on holdup (or the amount of catalyst) on each tray. This means that the... 

Another limitation for reactive distillation is the need for reasonably large specific reaction rates. If the reactions are very slow, the required tray holdups and number of reactive trays would be too large to be economically provided in a distillation column. 

With all feed conditions and the column configuration specified (number of trays in each section, tray holdup in the reactive section, feed tray locations, pressure, and desired conversion), there is only one remaining degree of freedom. The reflux flowrate is selected. It is manipulated by a distillate composition controller to drive the distillate composition to 95 mol% C. The vapor boilup is manipulated to control the liquid level in the base. Note that the distillate and bottoms flowrates are known and fixed as the dynamic model is converged to the steady state that gives a distillate composition of 95 mol% C. The composition of the bottoms will be forced by the overall component balance to be 95 mol% D. 

The net reaction rate on a reactive tray depends on the liquid concentrations in mole fractions and liquid holdup in kilomoles on that tray. 

The holdup on the reactive trays is 1000 mol. The vapor boilup required to achieve the desired 95 mol% purities of the two products at base case conditions is 28.91 mol/s, and the corresponding reflux flowrate is 33.55 mol/s. These purities correspond to a 95% conversion. 

We now investigate the impact of changes in various parameters from those used in the base case. The first parameter studied is the holdup of liquid on the reactive trays. As we would expect, the larger the holdup, the easier it is to achieve the desired conversion. 

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