Intermediate components, multicomponent distillation

To illustrate the procedure, we consider a fairly complex process sketched in Fig. 6.4, which shows the process flowsheet and the nomenclature used. In the continuous stirred-tank reactor, a multicomponent, reversible, second-order reaction occurs in the liquid phase A + B C + D. The component volatilities are such that reactant A is the most volatile, product C is the next most volatile, reactant B has intermediate volatility, and product D is the heaviest component a/ > ac > olb > OiQ. The process flowsheet consists of a reactor that is coupled with a stripping column to keep reactant. A in the system, and two distillation columns to achieve the removal of products C and D and the recovery and recycle of reactant B. 

For a multicomponent mixture to be split into two streams (distillate and bottoms) by distillation, it is common to specify the separation in terms of two key components of the mixture. The light key will have a specified maximum limit in the bottoms product and the heavy key will have a specified maximum limit in the distillate product. Normally, the keys are adjacent to each other in the ranking of the mixture components according to relative volatility but this is not always the case, and distributed componettts may have volatilities intermediate to those of the keys. 

Figure 3.1 provides a detailed flowsheet of the process and the notation used. The reaction occurs in a CSTR with molar holdup V. There are two fresh feedstreams Fqa and Fqb that contain pure reactants A and B, respectively, and a recycle stream Z>2 returns from a downstream distillation column. The reactor effluent contains a multicomponent mixture because complete one-pass conversion is not achieved. Two columns are needed to separate the two products from the intermediate-boiling reactants. The reactor effluent F with composition zj is fed into the first distillation column to separate product C from unreacted reactants A and B and heavy product D. Product C goes out in the distillate of the first colunm with the desired 95 mol% purity, and the other components go out in the bottoms, which is fed to the second column. This column produces a bottoms stream of D with the desired 95 mol% purity and a distillate of unreacted reactants A and B that is recycled back to the reactor with specific amounts of product impurities Xd2,c nd X/)2,d-... 

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