Phase behaviour of l-lactide in supercritical carbon dioxide at high pressures

The solid–fluid and the liquid–vapour equilibria for the binary system carbon dioxide + l-lactide were determined in the temperature range 275–355 K and in the pressure range 10–80 MPa. This system shows the type of phase behaviour characterized by two branches of the solid l-lactide + liquid + vapour curve ended with first and second critical endpoints. In this study only the high-temperature branch of the three-phase curve was determined. The second critical end point (solid l-lactide + liquid = vapour) was found at 323.0 K and 30.3 MPa. The liquid–vapour equilibria were modelled with the Peng–Robinson equation of state coupled with the classical mixing rules. For the calculations l-lactide critical temperature, critical pressure and acentric factor were estimated from the Joback–Lyndersen group contribution method.