WZ2ÏÏNZ ^■Capital cost IZZ1 Operating cost

10 20 Weir Height (cm)

Figure 17.7 Effects of weir height (reactive holdup) on the TAC with 10-cm weir height as base case.

10 20 Weir Height (cm)

Figure 17.7 Effects of weir height (reactive holdup) on the TAC with 10-cm weir height as base case.

biobased esters and biodiesel, many processes fall into this class. A typical example is the recovery of lactic acid.7 The reaction can be expressed as methanol + lactic acid o water + methyl lactate LLK HHK LK HK

Unless we can consume all of the HHK before it reaches the bottom of the reactive zone and react away all of the LLK before it reaches the top of the reactive distillation, a single column configuration (e.g., Fig. 17.5) is not possible. This is typically true for the neat design.

A more likely configuration is to place the reactive zones at the opposite ends of the column where significant amounts of LLK and HHK (two reactants) are present. Then the heavy reactant (HHK) is introduced into the top of the column where a significant amount of the other reactant (LLK) is present. Similarly, the light reactant (LLK) is fed to the bottom of the column where a significant amount of the heavy reactant (HHK) is present (Fig. 17.8). The next question then becomes, how do we withdraw the product? The answer is actually quite simple. A side stream from the reactive distillation column will contain a mixture of two products. Thus, we need an additional column to separate these two products. Once the correct process configuration is set, design variables are easily identified. They are the numbers of reactive stages at two different zones, number of separation trays, and side draw location. Note that the feed locations shown in Figure 17.8 are actually the optimal ones, and we will not show the effect of feed tray locations in design for type IR. In addition, note here that the separation column is designed by setting the total number of trays to 2 times the minimum number of trays, that is, NT = 2Nm;n.

Figure 17.8 gives the process flowsheet, and Table 17.3 gives parameter values. This flowsheet results in a TAC of $1,576,000 for the reactive distillation column alone, which is 620% of that of the type Ip system. The reason is that, in order to achieve high conversion, the reflux to feed ratio in the reactive distillation is extremely high (20.5) and the vapor boilup to feed ratio is 19.1. This corresponds to a column of 30

7J. I. Choi and W. H. Hong, Recovery of lactic acid by batch distillation with chemical reactions using ion exchange resin, J. Chem. Eng. Japan. 32, 184-189 (1999).

Figure 17.8 Final design for type IR process (LLK + HHK , LK + HK).

trays where the reactive zones are located at the top and bottoms of the column with reactive holdups of 20 and 1 times the tray reactive holdup in the condenser and reboiler, respectively. Because the catalyst cost is also included in the TAC calculation, the reactive holdups in the reboiler and condenser are also design variables theoretically (not exceeding 20 times the tray holdup). In this case, the reboiler reactive holdup is varied to minimize the TAC.

A mixture of two products (LK and HK) is withdrawn from the middle of the separation section (tray 14), and this side stream is fed into a simple distillation column. The second column has a total of 48 trays with a moderate reflux ratio (2.08) and boilup ratio (3.08). Figure 17.9a displays the composition profile for the reactants (A and B) and products (C and D). As expected, the concentrations of the two reactants are drastically different in the reactive zones. For example, the LLK (reactant A) composition exceeds 60% toward the top reactive zone while the concentration of the heavy reactant (HHK, reactant B) remains below 2%, despite the fresh feed of HHK being introduced in the condenser. Similar behavior is observed in the lower reactive zone. At the side draw location, the compositions of both products are the same (stoichiometric balance), as shown in Figure 17.9a.

The product stream is fed to a distillation column for further separation to obtain 95% C and D. Figure 17.9b shows the composition profile of the distillation column with rather symmetric profiles.

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