Reaction-Distillation Process with External Recycling

Reaction-Distillation Process with External Recycling 

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. 

a CSTR is considered as reactor, and therefore the reaction rate r is calculated at the reactor outlet concentration, x. At infinite reflux ratio, R = , the distillate (x°) and bottom concentrations (x ) are related to each other by the Fenske equation [2] 

As the first step of conceptual process design, the distillation column is considered at infinite reflux ratio, J = with an infinite number of stages, N = oo. Under these perfect separation conditions, the distillation column will yield a bottom product that contains pure A2 x = 0. Then the distillate mole fraction only depends on the size of the reactor (Da), the recycling ratio (gj), and the chemical equilibrium concentration Combining (5.6) and (5.7) yields 

For a first-order reaction, the distillate mole fraction is given by 

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Reaction-Distillation Process with External Recycling... 

First, the role of reaction kinetics is analyzed considering RD processes for the simple reversible reaction Aj o Aj in an ideal binary mixture. The educt Aj is assumed to be the reaction component with the higher boiling point, so the product A2 is obtained in the distillate. The reaction can be carried out in an RD column sequence with an external recycling loop (Fig. 5.1), a non-RD column on top of a reactive reboiler (Fig. 5.2), or a full RD column (Fig. 5.3). More possible configurations are analyzed elsewhere [1]. 

The earliest processes employed air as the oxidant (Distillers IG Farben, Scientific Design, Union Carbide). Modem plants are nearly all supplied with oxygen (Chenuscbe Werke-Hiih, Japan Catalytic, Scientific Design second version. Shell, SNAM Progetti Societa Nazionale Metanodotti). The main concern of all these installations is the effective removal of the heat produced during the reaction. This is achieved by operation with a low ethylene conversion rate, cold product recycle, and external cooling of the catalyst bed. 

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