Modelling reactive distillation

5216 R. Taylor, R. Krishna / Chemical Engineering Science 55 (2000) 5183}5229Prevost (1998) brie#y addressed some numerical problemsthat can be encountered in the simulation of reactiveabsorption using NEQ models. We have already notedthe success (using e!ective di!usivities and enhancementfactor approaches to mass transfer with reaction) of NEQmodels of amine-based gas treating processes (Careyet al., 1991; Altiqi et al., 1994).6. Reactive distillation designDesign and simulation of RD operations are verydi!erent types of calculation, calling for verydi!erent approaches. Most conceptual design models arebased on the EQ stage model described above. However,as discussed below, some recent developments haveopened up the possibility of using NEQ models for RDdesign, and a limited number of papers suggest that NEQmodels already have a place in industrial RD designpractice.6.1. Conceptual designBarbosa and Doherty (1988c) developed the "xedpointmethod for the design of single-feed RD columns.The method is based on the following assumptions: the column is adiabatic; the molar heat of vaporisation is constant; the heat of mixing is negligible; sensible heat e!ects can be ignored; the heat of reaction is negligible compared to theenthalpy of the vapor; the feed is a saturated liquid; phase equilibrium is achieved on each stage; the column operates with a partial condenser.These assumptions ensure that the vapor and liquid #owsinside the column are constant, thereby permitting theenergy balances to be solved essentially independent ofthe remaining stage equations.The "xed-point method uses material balance equationswritten around a stage in each section of the column(above and below the feed stage) and thecorresponding end of the column. With reference to Fig.18(a), for example, we have, for the rectifying sectionX " rH #1 > ! 1 > (32) rH rH and for the stripping section the material balance readsX "sH sH#1 > ! 1sH#1 X . (33) Note that these equations have been written in terms ofthe transformed composition variables de"ned by Eq. (3).The transformed re#ux and stripping ratios are de"nedas follows:rH "¸ (ν !ν x )D(ν !ν y ) , sH "