Modelling reactive distillation

R. Taylor, R. Krishna / Chemical Engineering Science 55 (2000) 5183}5229 5197Fig. 18. (a) The equilibrium stage. (b) Multi-stage distillation column.Chemical reaction equilibrium is not considered in manyof the early papers because it is more di$cult to model.There are, however, some exceptions to this statementand such works are noted below.The S equations are the summation equations x "1, y "1. (12)The enthalpy balance is given byd; H "< H H #F Hdt #¸ !(1#r )< H !(1#r )¸H !Q . (13)The superscripted H's are the enthalpies of the appropriatephase. The enthalpy in the time derivative on theleft-hand side represents the total enthalpy of the stagebut, for the reasons given above, this will normally be theliquid-phase enthalpy. Some authors include an additionalterm in the energy balance for the heat of reaction.However, if the enthalpies are referred to their elementalstate then the heat of reaction is accounted for automaticallyand no separate term is needed.Under steady-state conditions all of the time derivativesin the above equations are equal to zero.Some authors include additional equations in their(mostly unsteady-state) models. For example, pressuredrop, controller equations and so on.3.2. Steady-state algorithms and applicationsMuch of the early literature on RD modelling is concernedprimarily with the development of methods forsolving the steady-state EQ stage model. For the mostpart such methods are more or less straightforward extensionsof methods that had been developed forsolving conventional distillation problems. The numberof examples that illustrate most of the early papersusually is rather limited, both in number as well as inthe type of RD process considered (most often it is anesteri"cation reaction). Only rarely is there any attemptto compare the results of simulations to experimentaldata (some exceptions are noted below). More and moreof the more recent modelling studies are carried out usingone or other commercial simulation package: Aspen Plus,Pro/II, HYSYS, and SpeedUp are the packages mentionedmost often in the papers discussed in what follows.Tray-to-tray calculations were carried out by hand byBerman et al. (1948a); Marek (1954), and Belck (1955).Marek also presents an elaborate graphical method relatedto the McCabe}Thiele method for conventionalbinary distillation. Berman et al. (1948a) considered theproduction of dibutyl phthalate, extensive data for whichsystem had been reported by Berman et al. (1948b).We may identify several classes of computer-basedmethods that have been developed for solving theEQ stage model equations; these methods are discussedbrie#y in what follows. Readers are referred to the review