Lucia and Taylor (2006) have shown that the boundaries in ternary residue curve maps can be defined as the locally longest residue curves in a given region of the composition triangle. Residue curves are equivalent to distillation column profiles under the assumption of equilibrium at every point in a packed column that is operated at total reflux. It is well known, however, that real distillation columns do not operate at equilibrium and the composition profiles are influenced by the rates of interphase mass transfer.
Here we provide evidence that the boundaries in nonequilibrium ternary distillation processes are the longest composition trajectories in a given region of composition space.
We further show that the minimum energy design for a specified separation can be defined by the shortest feasible stripping trajectory (when measured from the bottoms composition to the pinch point). These properties of the distillation trajectories corresponding to boundaries and to minimum boilup/reflux are the basis for optimization based algorithms that can be used for the determination of energy efficient distillation processes when mass transfer effects are important.
All this means that mass transfer effects can be considered at the conceptual design stage, instead of in later phases of process design. This can be significant in some cases because it is possible, in principle at least, for mass transfer effects to make possible designs that would not normally be considered if a purely equilibrium based model was used as the basis. More importantly, mass transfer effects can render ineffective designs based solely on equilibrium considerations.