(325e) Elucidation of Anomalies in Reactive Distillation: The Node-Bumping Effect

The Residue Curve Map (RCM) technique is a tool for the synthesis and analysis of distillation columns. A Residue Curve is the locus of the compositions of the residual liquid during an isobaric batch distillation process, where the vapour removed is assumed to be in equilibrium with the liquid in the still. A Residue Curve Map is the family of all residue curves which could be obtained by using feed materials of different initial compositions in the still.     A Reactive Residue Curve Map (RRCM) is similarly defined as the locus of compositions of the residual liquid during an isobaric batch process in which boiling and reaction occur simultaneously. The vapour that could be obtained can be obtained by mass balance. In this work, an equation is derived (this is the RRC equation) to model the liquid composition of a particular ideal three component system in a kinetically controlled simple reactive distillation process. From the RRC equation a dimensionless parameter Damkohler number (Da), is defined, which measures the amount of reaction relative to the product removal.  A global RRCM viz. for both positive and negative compositions is then plotted for some arbitrarily chosen increasing Damkohler numbers. When Da is zero (separation alone) there are three nodes in the composition space. We notice that when Da is greater than zero the number of nodes increases from 3 to 5. Nodes that previously lay outside the Mass Balance Triangle (MBT) move inside the MBT at Da values greater than a critical value (0.5). Two singularities lie on top of each other at this critical value of Da.  For this example, above this value of Da the saddle node that was at the corner moves into the composition space and the one that was outside at lower Da now becomes a fixed node on the corner of the MBT. We call this phenomenon node bumping. The composition space is thus divided into two distillation regions at Da values greater than this critical value.  By monitoring changes in Da it is shown how to avoid multiple steady states where changes in composition of initial condition could result in profiles running in for what could be described as the incorrect direction (bifurcation regions). Tracking compositions outside the MBT also provides understanding as to why „reactive azeotropes‟ can be formed.