Base Case

Equations describing the material balances were programmed in Fortran, and all simulations were carried out on a Pentium PC. Note that the convergence of the reactive distillation is far more difficult than conventional distillation. Typically, a steady-state simulation is

2J. M. Douglas, Conceptual Design of Chemical Process, McGraw-Hill, New York, 1988.

carried out in a two-step procedure. First, the Wang-Henke method is used to converge the flowsheet (MESH equations) to a certain degree (actually to the point at which the objective function fluctuates). Second, the temperature and composition profiles are fed to a dynamic program that is integrated until temperatures and compositions converge (relaxation approach).

Saturated liquid feeds are assumed, and the two feed flowrates are 0.0126 kmol/s each of pure A and pure B. The two feeds are introduced on trays NF,A and NF,B, respectively. In the base case, the feeds are introduced to the bottom (N«x,bot) or the top (Nsx,top) of the reactive section (see Fig. 18.1 and base case in Table 18.2). Note that this is the typical feed arrangement for reactive distillation systems, which is termed the conventional feed arrangement. The conversion is specified to be 95% in this work, which corresponds to purities of 95% C in the distillate and 95% D in the bottoms.

Figure 18.2 (thickest line) shows the composition profiles of all four components at the nominal design. Reactant A has the highest concentration (xA) on the feed tray (NF,A = 9). The profile shows that xA decreases toward the upper reactive zone as a result of the reaction and also decreases toward the bottoms of the column as a result of separation. Similar behavior is observed for heavy reactant B. Both light product C and heavy product D meet the specification toward the ends of the column.

Al-Arfaj and Luyben1 show the steady-state temperature profile in which a nonmonotonic temperature profile is observed. A local temperature minimum occurs on the lower feed tray because of the presence of a significant amount of light reactant A. This behavior is not uncommon for reactive distillation columns but is rarely seen in conventional distillation systems.

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