Number of Rectifying Trays

Increasing the number of rectifying trays decreases the vapor boilup (lower left graph. Fig. 6.19), but not by much. The almost invariant temperature profiles from simulation confirm this insensitivity. There is no counterintuitive effect. 

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In a multi-component system, the optimum feed location depends on the light and heavy key components and their desired concentrations in the products. A feed location that is optimum for one set of specifications may be a poor selection for another. The number of rectifying trays must be sufficient to remove from the overhead as much of the components heavier than the light key as is needed to meet the required overhead composition. Similarly, the number of stripping trays must be sufficient to strip from the bottoms as much of the components lighter than the heavy key as is needed to meet the required bottoms composition. 

In selecting the feed tray for the main feed, it is desirable to maximize the number of stripping trays in order to minimize the reboiler duty required to achieve the specified methane stripping. However, the feed tray should not be too high. If it is, an excessive amount of ethane is lost in the overhead because of an insufficient number of rectifying trays. Table 9.2 shows reboiler duty requirements and ethane recoveries in the bottoms for different feed tray locations. The methane mole fraction in the bottoms is fixed in all cases at 0.001. The results indicate that the optimum feed tray is the third tray from the top. 

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. 

The number of stripping trays is assumed to be equal to the number of rectifying trays because the relative volatilities between the key components are the same in each... 

Note that the optimum number of stripping trays is larger than the optimum number of rectifying trays. This is caused by the higher temperatures in the lower part of the column, which means lower relative volatilities. 

Figure 6.6 Effect of number of rectifying trays (Nr) on temperature and composition profiles.

The number of rectifying trays Nr is varied over a wide range, and steps 6-16 are repeated for each value of Nr, generating its corresponding TAC. 

Particularly when the number of trays is small, the location of the feed tray has a marked effect on the separation in the column. An estimate of the optimum location can be made with the Under-wood-Fenske equation (13.116), by applying it twice, between the overhead and the feed and between the feed and the bottoms. The ratio of the numbers of rectifying Nr and stripping Ns trays is... 

Fix the number of stripping trays Ns and rectifying Nr trays at a small value (to be varied). 

In the quaternary system in Chapter 2, we demonstrated that increasing the number of fractionating trays in the stripping and rectifying sections tends to decrease vapor boilup. In the ternary system, the column has only stripping trays. The impact of changing the number from the base case of = 5 is shown in Figure 5.8. 

The final parameter explored in this chapter is the number of trays used in the two separation sections. In Chapter 2 we found that increasing the number of stripping and rectifying trays decreases energy consumption in the quaternary system. In Section 5.1.7 in this chapter we found that there is an optimum number of stripping trays in the ternary system without inerts. What are the effects for the ternary system with inerts ... 

The lower right graph in Figure 6.19 shows that there is an optimal feed tray location. Figure 6.21 shows the effect of feed tray locations on temperature and composition profiles. This optimum location is the top of the reactive section. Above this point, the effect of the rectifying section to keep the reactant in the reactive sections is lessened, and below this point there is an effect similar to reducing the number of reactive trays. Either position will increase the vapor boHup. 

Effect of Number of Reactive Trays. In the base case, stages 7-23 contain catalyst, so the reactive zone has 17 trays. Reactor effluent is fed on stage 28 and methanol on stage 23 (the lowest reactive stage). There are 6 rectifying trays and 11 stripping trays. 

Table 8.4 gives results over a range of values of the number of reactive trays. The numbers of stripping and rectifying trays are held constant, as are the pressme and the design specifications on the concentration of TAME in the bottoms and distillate. 

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