Holdup on reactive trays

In the quaternary system in Chapter 2, we demonstrated that there is an optimum number of reactive trays at which vapor boilup is minimized. Having too few or too many reactive trays increases the steady-state energy consumption. This effect is counterintuitive. Does the same occur in the ternary system  

Note the sharp liquid composition changes that occur between the top of the column on tray Nt and the reflux drum (stage Nt + 1)- This is caused by the total reflux operation. The vapor composition on tray Nt is higher for lighter component A than the liquid composition. The reverse is true for heavier component B. Therefore, the liquid leaving the total condenser is richer in A than the top tray and leaner in B. Thus, the liquid composition profiles rise for A and drop for B at the top of the column. 

These results demonstrate a fundamental difference between a two-product reactive column (quaternary system) and a one-product reactive column (ternary system). This is a good example of the complexity and challenges associated with reactive distillation. 

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These curves indicate that low ptessuie and the resulting low temperatures would result in low reaction rate, which would require large holdups on reactive trays. Alternatively, for a fixed number of trays and a fixed holdup, the concentrations of the reactants in the reactive zone would have to be large at low temperatures. This would require a large vapor boilup and reflux flowrate to keep the reactants from escaping out of the top or bottom. 

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... 

In reactive distillation, chemical reactions are assumed to occur mainly in the liquid phase. Hence the liquid holdup on the trays, or the residence time, is an important design factor for these processes. Other column design considerations, such as number of trays, feed and product tray locations, can be of particular importance in reactive distillation columns. Moreover, since chemical reactions can be exothermic or endothermic, intercoolers or heaters may be required to maintain optimum stage temperatures. Column models of reactive distillation must include chemical reaction... 

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. 

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... 

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. 

The base case considered has five stripping trays and nine reactive trays. The column operates at 8 bar and has 1000 mol of holdup on the reactive trays. Under these conditions, the bottoms composition is 0.25 mol% A and 1.75 mol% B. The resulting fi esh feeds are Fqa = 12.63 mol/s and Tqb — 12.82 mol/s. The vapor bodup requrred to achieve this purity of the product is 62.03 mol/s, and the reflux flowrate is 80.17 mol/s, which is the overhead vapor rate. Table 5.2 gives conditions for the base case. Note that the reflux composition is mostly the lightest component A, but some of the other two components are also present. Figure 5.2 presents the composition profiles. 

The base case conditions are supplied in Table 5.3. Note that the number of reactive trays in the base case has been increased to 15 from the 9 considered in the system without inerts. Likewise, the holdup on the reactive trays has been increased to 2000 mol. The effects of these design parameters are explored in the following paragraphs. The eomposition ZoA( j) of tho Fqa fresh feed is 50 mol% A and 50 mol% 1. This results in a mueh larger flowrate of this stream. The distillate is 97.2 mol% inerts. The main impurity is B at 2.04 mol%. 

Figure 5.18 Effect of reactive tray holdup on composition profiles.

Estimate the holdup on the reactive trays (Mrx). Perform a dynamic simulation using relaxation until the product specifications are met, followed by colunm sizing to find the column diameter and the corresponding reactive holdup (Mrx). Iterate the simulation runs until the reactive holdup converges. A weir height of 10 cm is assumed. 

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