Droplet mass transfer, intradroplet nucleation and submicron particle production in two-phase flow of solvent-supercritical antisolvent emulsion

Droplet formation, droplet interaction and coagulation together with droplet mass transfer are major sub-processes in the developing technology of nanoparticle production by means of solute nucleation inside the emulsion droplet. The solvent (ethanol) droplets containing the solute form during the solvent jet dispergation in the pressurized flow of solvent CO₂. In the formed two phases flow of solvent-antisolvent emulsion, the solvent diffuses from droplets into antisolvent, while antisolvent dissolves inside solvent droplets. The solvent replacement by the antisolvent causes droplet supersaturation by solute when it occurs near the critical point of solvent-antisolvent emulsion (∼80 bar and 31°C) and the intradroplet nucleation of solute. To provide the same droplet lifetime and the uniform droplet supersaturation, the hydrodynamic relaxation time for droplets and for two phases flow have to be shorter than their relaxation time of the mass transfer. Above a critical volume fraction of solvent, droplets dissolve partially. Afterwards, i.e. downstream, antisolvent is saturated with solvent, i.e. phase equilibrium establishes within two-phase flow with uniform solute supersaturation inside droplets. Under these conditions, an additional mechanism of the supersaturation is identified, which is droplet specific (the supersaturation caused by increasing solute concentration), and is favorable for small particle production. As long as droplets move along the last quasi-equilibrium section of uniform two-phase flow with length L, the nucleation and a solute precipitation proceed within droplets, i.e. the dimension of formed particles is controlled by droplet residence time τ_res which is proportional to L and inversely proportional to stream velocity. The maximal τ_res and the precipitation time which affect nanoparticle dimension is restricted by the rate of turbulent droplet coagulation.