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12.5 TERNARY A, B 1 C SYSTEM: HEAVY REACTANT WITH ONE-COLUMN CONFIGURATION

12.5.1 Column Configuration

Now we explore the control structure and dynamic performance of the one-column configuration for the same reaction, HK , LK + IK. The design for this configuration is based on overcoming the unfavorable boiling point ranking reaching the reactive azeotrope at the bottom of the column. If we consume most of the heavy reactant by the time it reaches the bottoms of the reactive distillation column, the intermediate-boiling product can be withdrawn from the column base at the desired purity. This can only be achieved for systems with a large chemical equilibrium constant as discussed in Chapter 6. In previous sections, the two-column configuration was shown to be controllable with simple temperature control. Will the single column configuration be dynamically controllable, especially when reaching the reactive azeotrope at one end of the reactive distillation column?

Table 12.8 gives the steady-state conditions and design parameters. For keeping the composition near chemical equilibrium, the entire lower section of the column contains reactive trays and the column base has 10 times the amount of catalyst used on each reactive tray. Figure 12.73 shows the flowsheet. Compared to the two-column configuration, this is a very tall column with 90 trays, versus 17 trays in the reactive distillation column. Figure 12.74 gives composition profiles, and Figure 12.75 gives the temperature profile.

Three control structures are considered here: one-temperature control, two-temperature control, and temperature/composition cascade control. The control objective is to keep the product purity at 98 mol% at both ends.

12.5.2 Control Structure CS1

Because we have a reactive azeotrope toward the bottoms of the column, it serves as a composition control mechanism in the lower part of the column. Figure 12.76 shows a control

TABLE 12.8 Steady-State Conditions and Design Parameters for One-Column Configuration

Fresh feed flowrate of A F0 (mol/s)

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