Reactor-Column Process with Two Reactants

In the next section, we will study a two-reactant case in which the adjustment of the fresh feeds to balance the stoichiometry of the reaction is crucial from a plantwide perspective. In addition, the limiting reactant concept can be used to improve dynamic controllability. 

The dynamics and control of the reactor-column system studied in Chapter 2 (Section 2.9.3) are investigated in this section. Mathematical models of both the reactor and the column are developed, and a plantwide control structure is evaluated. 

The reactor is a CSTR with jacket cooling in which a first-order irreversible reaction takes place  

The reaction rate is, of course, the same as that used in the steady-state model  

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The simplest flow-sheet for the reaction Aj o Aj is the RD column sequence with an external recycling loop shown in Fig. 5.1. The system as a whole is fed with pure Aj. According to the assumed relative volatility of the two components a > 1, the reaction product A2 is enriched in the column distillate product whereas the bottom product contains non-converted reactant Aj, which is recycled back to the reactor (continuous stirred tank reactor, CSTR, or plug flow tube reactor, PFTR). The process has two important operational variables the recycling ratio cp = B/F, that is the ratio of recycling flow B to feed flow rate F, and the reflux ratio of the distillation column R = L/D. At steady-state conditions, D = F since the total number of moles is assumed to be constant for the reaction Aj A2. As principal design variables, the Damkohler number. 

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-... 

Let us consider one of the simplest recycle processes imaginable a continuous stirred tank reactor (CSTR) and a distillation column. As shown in Figure 2.5. a fresh reactant stream is fed into the reactor. Inside the reactor, a first-order isothermal irreversible reaction of component A to produce component B occurs A -> B. The specific reaction rate is k (h1) and the reactor holdup is VR (moles). The fresh feed flowrate is Fs (moles/h) and its composition is z0 (mole fraction component A). The system is binary with only two components reactant A and product B. The composition in the reactor is z (mole fraction A). Reactor effluent, with flowrate F (moles/h) is fed into a distillation column that separates unreacted A from product B. 

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. 

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