5.1.3 Design Parameters and Procedure
In the quaternary case, the flowrates of the two fresh feeds, the distillate, and the bottoms were all set to 12.6mol/s. Then the relaxation model adjusted the reflux flowrate to drive the purity of the bottoms product to the desired value, xB,D = 0.95 mol fraction D for the 95% conversion case. The vapor boilup was adjusted to control the base level. This approach gave the same impurity levels in both product streams and the same conversions of the two reactants fed to the system.
The ternary system without inerts has a different column structure and requires a different approach for designing the column. There are still two feedstreams and a bottoms stream, but there is no distillate. In addition, the impurity in the bottoms product will be mostly the heavier of the two reactants, component B. This means that the flowrates of the two fresh feedstreams will not be equal. Moreover, the reaction is not equimolar. Two moles of reactants produce 1 mol of product. Thus, there is a decrease in the molar liquid flowrates in the reaction section that is attributable to the nonequimolar reaction.
The procedure used for this system is to fix the production rate of product C at 12.6 mol/s and the purity of the bottoms product at 0.98 mol fraction C. This means that the bottoms flowrate is 12.6/0.98 = 12.857 mol/s. The production rate requires that 12.6 mol/s of both A and B be consumed. Thus, at least this amount must be fed to the column. In addition, however, there is a loss of reactants in the bottoms to account for the 2 mol% impurity. It is mostly B, but there is also a small amount of A. The concentrations of A and B change with the various designs. Therefore, the flowrates of the fresh feeds are slightly different in each design. At each point in time during the dynamic simulation, the fresh feed flowrates are calculated from the fixed value of the bottoms flowrate B and the present value of the bottoms composition xBj-, which changes with time until a steady-state solution is achieved.
Because B is heavier than A, the fresh feed flowrate of B is somewhat larger than that of A. The reflux flowrate is changed to drive the bottoms composition to 98 mol% C. The vapor boilup controls the level in the base. There is no distillate. The reflux drum level is not controlled.
The base case considered has five stripping trays and nine reactive trays. The column operates at 8 bar and has 1000 mol of holdup on the reactive trays. Under these conditions, the bottoms composition is 0.25 mol% A and 1.75 mol% B. The resulting fresh feeds are F0A = 12.63 mol/s and F0B = 12.82 mol/s. The vapor boilup required to achieve this purity of the product is 62.03 mol/s, and the reflux flowrate is 80.17 mol/s, which is the overhead vapor rate. Table 5.2 gives conditions for the base case. Note that the reflux composition is mostly the lightest component A, but some of the other two components are also present. Figure 5.2 presents the composition profiles.
Another change made in the equations accounts for the reduction in moles as 1 mol of product is produced by the consumption of 2 mol of reactant. The liquid rates on the reactive trays are reduced by the rate of reaction and by the vaporization caused by the heat of reaction.
Fresh feed flowrate of A (mol/s)
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