Fluid multiphase behavior in quasi-binary mixtures of carbon dioxide and certain 1-alkanols

Three-phase equilibria liquid–liquid–vapor of quasi-binary mixtures of near-critical carbon dioxide with certain 1-alkanols, ranging from butanol to hexanol, were determined experimentally for 11 different compositions. The phase transitions were determined visually using the Cailletet apparatus within a temperature range 270<T/K<320, and a pressure range 3.0<p/MPa<9.5. Special interest was given to determining the location of the critical endpoints for each composition. The various types of fluid phase behavior related to these experiments and their transitions into each other are discussed briefly. The double critical endpoint (DCEP) is estimated to occur at an average carbon number of N=5.71, and at a temperature and pressure of 295.9 K and 5.85 MPa, respectively. The tricritical point (TCP) is found to be within the range of 4.5<N<4.7, and the temperature and pressure are within the range of 318.5<T/K<320 and 9.2<p/MPa<9.5, respectively. A significant result of this study is that, in contrast to previous studies, the binary mixture of near-critical carbon dioxide and 1-pentanol shows type-IV fluid phase behavior, i.e., a second branch of the three-phase equilibrium l1l2g was established at somewhat higher temperatures and pressures. This three-phase region was bounded by two critical endpoints having the coordinates and T=316.02 K, p=8.752 MPa and T=317.06 K, p=8.950 MPa, respectively. The lower temperature branch of the three-phase equilibrium l1l2g ended in a critical endpoint located at T=273.45 K and p=3.413 MPa.