Reactive Holdup

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)- 

Guess the number of trays in the rectifying section (A ) and fix the feed tray location (Np) location at the column base (tray 0). 

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

Guess a total number of trays and feed tray location. 

TABLE 6.4 Steady-State Conditions and Design Parameters for TAC Optimum Case 

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In previous work (Filipe et al. 2007) the multi-objective optimization of a distillation column was performed and the Pareto front relating the total number of stages, reactive holdup and cost, identified. In this work a study on how the Pareto optimal designs could be adapted for real implementation is presented. Different design details, such as reactive holdup and feed quality, are investigated and the sensitivity of the solutions assessed to quantify the effect on the column expected performance. 

Figure 2. Variation of the stream purity, eapacity and energy versus the total reactive holdup...

The sensitivity to feed quality and the scope for the real implementation of designs involving extreme feed qualities is investigated in this section with case B. The details for this case are 26 stages, 14 reactive stages (5-18), total reactive holdup equal to 1.1 kmole, two feeds (stages 9 and 20) with feed qualities of -2 and 2, respectively. 

Figure 6.4 Effect of reactive holdup on temperature and composition profiles.

Increasing the reactive holdup in the column base decreases the vapor boilup but by a very small fraction, as shown in the upper left graph in Figure 6.10. However, adding... 

Thus far in this chapter, the holdup per tray is kept constant at 1000 mol when the number of reactive trays is increased in an attempt to improve the dynamics of the CS7-RR structure. This means that the suboptimal design (5/10/5) has a total of 10,000 mol of reactive holdup and the optimal design (5/7/5) has a total of 7000 mol. The question that arises is what if the total holdup of the optimal design is kept the same and just distributed over a larger number of trays ... 

The reactive holdup on tray j Mj) is computed from the column geometry by assuming a weir height of 10 cm, and the reactive holdups in the reboiler and condenser are taken to be 20 times the tray holdup. 

Effects of weir height (reactive holdup) on the TAC with 10-cm weir height as... 

The assumptions made in this work include 1) ideal vapor-hquid equilibrium, 2) equal molar feed (neat process), 3) the reactive holdup set by the column diameter, and 4) a sequential approach for optimization. It is interesting to note that the reactive zone can be placed at the upper section, lower section, middle, or both ends of the reactive distillation column, depending on the sequences of the relative volatilities. The principle is actually quite simple place the reactive zone where the reactants are most abundant and introduce the feeds to facilitate the reaction (considering the composition effect). 

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