Info

DIY Power Plant Dish System

Ultimate Guide to Power Efficiency

Get Instant Access

Condenser

Figure 17.9

process (LLK + HHK column.

Composition profile, feed locations, and fraction of total conversion (Ri/Rt) for type IR LK + HK) in (a) reactive distillation column and (b) simple distillation

The boiling point ranking indicates that both reactants (A and B) are heavier than the products (C and D). These two reactants are more concentrated toward the lower part of the reactive distillation column, and the two products can be separated easily from the top of the column. Following the rule of "placing the reactive zone at a location with high reac-tant concentration," the lower section of the column should be made reactive while the separation between reactants and products is carried out in the upper section of the column. This leads to the two-column distillation configuration shown in Figure 17.10. This configuration is actually quite similar to that of ethyl acetate and isopropyl acetate reactive distillation columns (Type II in Chapter 7). The overhead product of the reactive distillation column is fed to a simple distillation column to separate the two products.

Because the conversion is not 100% in the reactant distillation, the unconverted heavy reactants (A and B) need to be separated from the light products (C and D) in the distillation column. The boiling point ranking C(LLK), D(LK), A(HK), and B(HHK)

12.6 mol/s

Vs= 39-8 mol/s steam

12.6 mol/s

Figure 17.10 Final design for type IIp process (HK + HHK , LLK + LK).

A: 0.025 B: 0.025 C: -0.0 D: 0.95 12.6 mol/s indicates that both reactants will end up in the bottoms of the product column. We have D as the impurity in the column overhead and all of the unconverted reactants A and B will end up in the column base with no C in the bottoms. In order to keep product compositions at no less than 95%, the conversion in the reactive distillation column is set to a value greater than 95% (97.5% in this case). This results in the following product composition: 95% D (LK) and 97.5% C (LLK). Figure 17.10 gives the final design that minimizes the TAC. Table 17.3 gives the parameter values of this design.

The flowsheet gives a TAC of $665,000, which is 158% higher than that of type Ip. The TAC of the reactive distillation column is $323,000, which is slightly higher than that of the type Ip reactive distillation column (TAC = $254,000). The reactive column has five reactive trays plus a reactive reboiler and five rectifying trays. The vapor boilup to feeds ratio is around 2, and the reflux ratio is close to 1. Actually, this is a relatively simple reactive distillation column with reasonable energy consumption. The reaction takes place in the high temperature zone of the column, and we have a large reactive holdup (the holdup in the column base is 20 times the tray reactive holdup) in the column base when the temperature is the highest. In other words, the location of the reactive zone (Fig. 17.10) facilitates the reactive separation.

Figure 17.11a confirms that most of the conversion takes place in the reboiler, and the remaining reactive trays carry the reaction further to the desired conversion. The reactant composition of HHK (B) is kept high in the reactive zone and the other reactant A (HK) is maintained at an almost constant level to ensure that the forward reaction dominates. Toward the top of the reactive distillation column, two light products are separated from the two heavier reactants (Fig. 17.11a).

The TAC of the second column ($332,000) is about the same as that of the reactive distillation column. This is a simple distillation with heavy impurities. Thus, the bottoms composition is kept to the specification of 95% D (LK) with the impurity being a mixture of both A and B. The top product has a composition of 97.5% C with 2.5% D as the impurity. This column has 54 trays, and the feed is introduced into tray 43.

Reboiler Tray Number Condenser

Figure 17.11 Composition profile, feed locations, and fraction of total conversion (Ri/Rt) for type IIP process (HK + HHK , LLK + LK) in (a) reactive distillation column and (b) simple distillation column.

Reboiler Tray Number Condenser

Figure 17.11 Composition profile, feed locations, and fraction of total conversion (Ri/Rt) for type IIP process (HK + HHK , LLK + LK) in (a) reactive distillation column and (b) simple distillation column.

The distillation column has a moderate energy consumption with a boilup ratio of 3.36 and a reflux ratio of 2.07. Figure 17.11b shows the composition profile in the distillation column where the two unreacted (heavy) reactants go out the bottom of the column.

Type IIR: LLK + LK = HK + HHK. This case corresponds to the following reaction:

Was this article helpful?

0 0

Post a comment