Modelling Multicomponent Droplet and Spray Evaporation Dynamics using Functional Group Method UNIFAC

Abstract

In a multicomponent miscible liquid mixture, structural interactions among components dis- tinctly different in chemical structure and molecular size affect evaporation. To account for these effects, the UNIFAC model [1, 2] has been used to determine the activity coefficient, accounting for the non-ideality of vapour-liquid equilibrium (VLE). The phase diagram of an ethanol/isooctane mixture determined by using the UNIFAC model shows good agreement with measurement. For an ethanol/isooctane droplet, non-ideal VLE models which correct the ac- tivity of liquid components must be used to properly predict the evaporation, when ethanol is the predominantly major component (e.g. 78 vol.% of ethanol - E78 considered in this study). The separation factor, which quantifies the relative volatility between two components, directly shows the heavier hydrocarbon component isooctane evaporates faster compared to ethanol for the E78 droplet during the whole droplet life time. With UNIFAC, the evaporation dynamics of a two-component isooctane/ethanol and a four-component gasoline/ethanol spray is then investigated and compared to predictions using the ideal Raoult’s law. Compared to other ap- proaches, the group contribution method UNIFAC only requires properties of constituent func- tional groups of each component, which is particularly useful when no VLE data is available for binary-component subsystems of the multi or many-component mixture.