Design for Quaternary Ideal System

Both of these configurations have better economics than a reactive distillation column when there is a mismatch between favorable reaction temperatures and favorable vapor-liquid equilibrium temperatures. 

The process considered in this section is the ideal quaternary exothermic reversible liquid-phase reaction 

A range of temperature-dependent relative volatilities are considered, which are the same as those studied in Chapter 3. The relative volatilities between all adjacent components are assumed to be 2 at a temperature of 320 K. This temperature corresponds to a typical reflux-drum temperature when using coohng water in the eondenser. 

The relative volatilities are assumed to change with temperature, becoming smaller as the temperature increases. The value of the relative volatility of all adjacent components at a temperature of 390 K (aago) is used as a parameter to vary the temperature dependence. The smaller the value of the more temperature dependent the vapor- 

A temperature of 390 K is chosen because it gives reasonable reaction rates and chemical equilibrium constants for the numerical example under consideration. Different cases are studied for a range of values. When a39o = 2, the relative volatilities are independent of the temperature. When asgo = 0.95, the adjacent component switches volatilities as the temperature approaches 390 K, so the desired separation would be infeasible. 

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In Parts I and II we explored the steady-state designs of several ideal hypothetical systems. The following three chapters examine the control of these systems. Chapter 10 considers the four-component quaternary system with the reaction A + B C + D under conditions of neat operation. Chapter 11 looks at control of two-column flowsheets when an excess of one of the reactants is used. Chapter 12 studies the ternary system A + B C, with and without inerts, and the ternary system A B + C. We will illustrate that the chemistry and resulting process structure have important effects on the control structure needed for effective control of reactive distillation columns. 

Note that many real chemical systems have azeotropes associated with quaternary systems, and it may make the neat reactive distillation design infeasible. However, for the real chemical systems illustrated here, the placement of the reactive zone is the same as that of the ideal systems, despite having azeotropes. The boiling point ranking leads to an easy separation between the reactants and products. The two products leave the reactive section from opposite sides of the reactive zone while the two intermediate boilers (the reactants) are kept in the reactive zone. This is the most favorable boiling point arrangement possible for a quaternary reactive distillation system. 

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