Assessing Thermodynamic Flexibility Boundaries via Residue Curve Maps

Residue curve maps (RCMs) are a widely exploited tool to assess whether a multicomponent mixture separation by distillation results to be feasible or not. They usually refer to a given feed composition and to the products purities as specifications; sometimes the whole distillation regions can be discussed in order to evaluate all the mixture possible splits (Petlyuk&Danilov (2001), Petlyuk (2004)). All these considerations are nevertheless related to nominal operating conditions, that is for a given and constant feed composition. If feed perturbations are likely to occur (e.g. separation downstream a fermentation process) a flexibility analysis is required to assess the operation feasibility boundaries under uncertain conditions (Swaney& Grossmann (1985)). Moreover, product recovery is often a more appealing specification from an industrial point of view since it is directly related to the process productivity. For all these reasons, this study deals with the use of RCMs to assess the thermodynamic flexibility limits of a simple distillation case study for a given bottom product recovery and purity under uncertain operating conditions. The starting binary mixture is water and n-butanol, a common and well-known mixture whose separation by simple distillation shows an heterogeneous azeotrope. One component at a time is then added up to obtain an ABE/W (acetone, butanol, ethanol and water) mixture. Due to the high nonideality of the mixture, the addition of an organic component substantially affects the equilibria. A thermodynamic flexibility analysis methodology was outlined for both the binary and ternary cases as well as for the quaternary one. After that, the separation feasibility of the binary mixture was compared to the others (Di Pretoro et al. (2019)). Beside illustrating the procedure for thermodynamic flexibility assessment via RCMs, this analysis aims to show how to enhance the separation from a flexibility point of view taking advantage of the mixture nonideality.