Twocolumn System With Excess B

Figure 4.4 contains a typical flowsheet in which two columns are required. Fresh reactant A is fed to the first reactive column in stream F0A. An excess of reactant B is also fed to this column. This total stream FB,tot consists of fresh feed F0B and the recycle stream D2 from recovery column, which contains mostly reactant component B with a little impurity of product component D. The amount of impurity depends on the design and operation of the recovery column and is a design optimization variable.

In this section B is selected as the excess component. If for some reason reactant A were chosen, the flowsheet would be different. This case is considered in Section 4.4. The selection of which reactant to keep in excess depends on kinetics, relative volatilities, heats of vaporization, and the costs of the two reactants. In either flowsheet, the excess reactant and one of the products are vaporized only once, so there is no obvious inherent energy difference between the two flowsheets.

The sizing relationships and economic parameters presented in Chapter 3 are used to determine the optimum design of the two-column system. The presence of the second column adds additional design optimization variables. To simplify the optimization, we assume that the reactive column has the same numbers of trays in each section as in the one-column process and operates at the same pressure. These appear to be reasonable assumptions because the TAC is fairly insensitive to the number of trays chosen in each section as long as the number of trays is kept fairly large (well above the minimum).

118 m2

D1 = 13.13 mol/s *D1 (A) = 0.0096 *D1(B) = 0.0404 *D1(C) = 0.9500 *D1(D) = 0

12.6 mol/s

118 m2

D1 = 13.13 mol/s *D1 (A) = 0.0096 *D1(B) = 0.0404 *D1(C) = 0.9500 *D1(D) = 0

12.6 mol/s

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