T2

steam nfa nfr nfb i

Reboiler

Stage Number

Condenser

Figure 17.22 Composition profile, feed locations, and fraction of total conversion (Ri/Rt) for type IR process (LLK + HHK , LK + HK) with excess reactant design.

Reboiler

Stage Number

Condenser

Figure 17.22 Composition profile, feed locations, and fraction of total conversion (Ri/Rt) for type IR process (LLK + HHK , LK + HK) with excess reactant design.

reactant has been consumed, except for a small amount that appears as an impurity in the distillate stream that contains the LK product.

Two column sequences are possible for this ternary separation: direct and indirect (Fig. 17.19a). A similar scenario applies when the light reactant is in excess (Fig. 17.19b). The two conventional columns must separate the ternary mixture of LLK, LK, and HK.

The design procedure is similar to that of Section 17.3 except for an additional design variable: the recycle flowrate. A large recycle flowrate implies a higher reactant concentration for the excess reactant while having a greater recycle cost. Thus, it becomes a dominant design variable for excess reactant design.

Figure 17.20 shows that the heavy reactant excess design is favored over the light reactant excess design by a factor of almost 30%. As for the column sequencing, the "direct" separation sequence is preferred over the "indirect" sequence. For the separation sequence, the LLK is boiled up twice for the indirect sequence as compared to that of the direct sequence where the LLK is only boiled up once. Therefore, we expect a lower separation cost using the direct sequence.

As for heavy reactant or light reactant excess, Figure 17.9 reveals that we have a higher heavy reactant (B) concentration toward the column top as compared to the concentration of light reactant (A) toward the column base. In other words, relatively speaking, it is easier to increase the heavy reactant concentration compared to that of the light reactant. Figure 17.21 gives the TAC optimal design for type IR for the excess reactant design. The composition profile in Figure 17.22 indicates that the composition of the heavy reactant (B) is maintained at ^20% throughout the reactive zone, and this is a significant increase from the neat design (cf. Fig. 17.9). Of more importance, the TAC of the excess reactant design is only 50% of that of the neat design. This means that, for cases with poor reactant distribution, excess reactant is an attractive design alternative.

Was this article helpful?

0 0

Post a comment