Number of Reactive Trays

Fig. 5.9 shows the geometric representation of these balances in the McCabe-Thiele diagram. The distillate composition xP is located at the intersection of the condenser operating line and the operating line of the RD section. By specifying xP, the number of reactive trays can be estimated from the classical staircase construction. From the intersection of the operating line of the column section with that of the reboiler line, the bottom composition is determined. 

Answer. Reduction in the number of reactive trays, more uneven distribution of catalyst and identification of suitable spots for side reactors. 

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. 

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. 

The previous section showed no surprises regarding the effect of tray holdup. In this section we look at changing the number of reactive trays. Intuition would lead us to think that the more trays the better. This is certainly the case in conventional distillation. However, as we will see, this is not the case with a steady-state reactive distillation column for this type of reaction (two reactants, two products). 

The situation is just the reverse with a large number of reactive trays. The concentrations of the reactants at the ends of the column where they are fed become large because much of the other reactant has been consumed on the many trays in the reactive zone before it reaches the opposite end. There is little B arriving near the bottom of the reactive zone, so the concentration of A is large near where it is fed. Likewise, there is little A arriving near the top of the reactive zone, so the concentration of B is large near where it is fed. 

Figure 2.6 Effect of number of reactive trays on temperature profile.

We will return to this problem in Chapter 3 and show how the design of the reactive column must be altered to handle cases with temperature dependence of the relative volatilities. As expected, the optimum column pressure decreases and the number of reactive trays increase as the values of a39o decrease. Reactive distillation becomes economically unattractive for a39o values of less than 1.2. 

However, there are two parameters that give counterintuitive results. The most important of these is the number of reactive trays. More trays does not necessarily improve the steady-state economics of a reactive distillation column. However, the effect of more reactive trays on the dynamic performance of a reactive column is distinctly different, as we will demonstrate in Chapter 10. 

The number of reactive trays is varied over a wide range, and steps 4-14 are repeated for each value of Nrx, generating its corresponding TAC. 

Chapter 2. Thus, there is an optimum number of reactive trays for each pressure that minimizes vapor boilup, energy cost, and heat exchanger cost. The left graph of Figure 3.10 shows that there is a minimum in the TAC curve at a certain number of reactive trays because of the tradeoff between increasing column cost and decreasing vapor boilup. For this case, nine reactive trays is the optimum number for all pressures. This figure also indicates that the optimum pressure is 8 bar. 

Reactive Distillation. Figure 3.18 and Table 3.8 give optimum design results for the reactive distillation process for a range of temperature-dependent relative volatilities. As the a39o parameter decreases, the optimum pressure decreases. This occurs because lower pressure helps the vapor-liquid equilibrium because it lowers temperatures and hence increases relative volatilities. However, a lower temperature is unfavorable for reaction because the reaction rates are too small. The result is a rapid increase in the required number of reactive trays. 

The chemical system considered in previous chapters featured the classical quaternary two-reactant, two-product A - - B C -h D reversible reaction. Some interesting phenomena were discussed. In particular, the effect of the number of reactive trays on energy consumption was demonstrated to be counterintuitive, that is, there is an optimum number of reactive trays that minimizes energy consumption. 

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 ... 

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%. 

Increasing the number of reactive trays Ngx improved energy consumption in the ternary system without inerts. When inerts are present, the same result is observed, as shown in Figure 5.28. Vapor boilup decreases as more reactive trays are added. 

As we discussed in Chapters 2 and 5, there is an optimal number of reactive trays at which vapor boilup is minimized in the quaternary system. This effect does not occur in the ternary synthesis reaction system (A + B C). Does it occur in the ternary decomposition reaction (A B + C) ... 

Figure 6.5 Effect of number of reactive trays (Ngx) on temperature and composition profiles where fraction of tray height is used instead of tray number 0, reboiler 1, column top. Width of reactive zone shown.

Place the reactive zone in the base of the column with an initial estimate of reactive holdup and fix the number of reactive trays (Nnx)-... 

Increasing the number of reactive trays dramatically reduces the vapor boilup, which is illustrated in the upper right graph in Figure 6.19. This is the result of approaching the reactive azeotrope toward the bottoms of the column, which can only be achieved gradually by... 

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. 

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