Computational study of organic solvent–CO2 mixing in convective supercritical environment under laminar conditions: Impact of enthalpy of mixing

Abstract

Mixing under supercritical conditions plays an important role in a number of industrial processes including the selective separation or fractionation of certain species from oils or hydrocarbons and determining the final particle size distribution and product quality in supercritical nanoparticle synthesis routes. This paper presents a computational study of mixing in supercritical antisolvent systems in which the enthalpy of mixing is finite. In particular we focus on the mixing of an ethanol droplet immersed in a carbon dioxide stream near and far above the critical point using computational fluid dynamics (CFD) code coupled to the Peng–Robinson equation of state and appropriate mixing rules. The result of mixing is investigated under different process conditions: immediately above, above and far above the mixture critical point, all under laminar flow conditions. We show how mixing is far from being isothermal due to the large enthalpy of mixing and that significant spatially distributed temperature deviations can result. In particular, the mass and heat transfer in the droplet boundary layer exhibit large temperature gradients. Besides, this local heating/cooling may be responsible for the onset of a two-phase flow under the conditions studied; a phenomenon revealed experimentally in recent studies. Two different modes of mixing between the droplet and free stream, called “deformation” and “stripping”, are observed for the cases under study. The mixing pattern depends on the Reynolds number and significant droplet deformation is observed as the convective velocity is increased and the vorticity forming in the boundary layer between the droplet and the flow accumulates in the wake region. Our results confirm previous observations, highlighting the fact that mixing is a crucial step in nanoparticle precipitation under supercritical conditions (supercritical particle synthesis, rapid expansion of supercritical solutions, supercritical antisolvent processes, etc.) due to its effect on particle size and morphology.