Fluid mixtures at high pressures IX. Phase separation and critical phenomena in 23 ( n-alkane + water) mixtures

Abstract

The p( T) projections of the critical curves and of the (liquid + liquid + gas) (llg) three-phase curves for 23 binary mixtures of (an n-alkane + water) with alkane-carbon numbers i = 1 to 12 and i = 14, 16, 18, 20, 24, 25, 26, 28, 30, 32, and 36 were measured. The shape of the critical curves of the mixtures changes systematically with i. All critical curves are interrupted and show (gas + gas) equilibria of the second kind. The llg curves are at higher pressures than the vapour pressures of the pure substances, and all end in a (liquid + gas) upper critical end point (lg-UCEP). Up to the mixtures i = 25, along the p( T) projection of the llg lines up to the lg-UCEP, the alkane-rich liquid phase is lighter than the water-rich liquid phase. The consequence is that the critical curve starting from the lg-UCEP ends in the critical point of the alkane. As from i = 28, a barotropic phase reversal of the two liquid phases occurs on the llg curve before the UCEP is reached, so that, at the UCEP, the now lighter water-rich liquid phase becomes identical with the gas phase and the critical curve starting from the UCEP runs to the critical point of water. For a defined pseudo-binary mixture of (alkane mixture + water) with a fictitious alkane-carbon number between i = 26 and i = 28, this barotropic effect must occur directly at the UCEP. This means that all three phases here become simultaneously identical at the UCEP and the llg curve thus ends in a tricritical end point TCEP, a phase behaviour which has hitherto not been known. The three branches of the critical curve, viz. the high-pressure branch l 1l 2 and the two branches l 1g and l 2g, which start from the critical point of water or the alkane mixture, respectively, end in this tricritical end point TCEP. At TCEP the three-phase line and the three branches of the critical curve (l 1l 2, l 1g, and l 2g) have the same slope. The TCEP occurs in the border-line case between the two sub-classes a and b of phase-behaviour type III (classification according to van Konynenburg and Scott). In sub-class IIIa the critical curve starting at the UCEP ends at the critical point of the n-alkane and in sub-class IIIb at the critical point of water. For mixtures with i = 1 to i = 26, the high-pressure branch of the critical curve starting from the critical point of water runs first to lower temperatures and, with increasing pressures, via a temperature minimum back to higher temperatures. For all mixtures with i = 7 to i = 26 this branch of the critical curve additionally also passes through a pressure minimum between the critical point of water and the temperature minimum. All mixtures with i ⩾ 28 have a high-pressure high-temperature branch of the critical curve which, starting from the critical point of the alkane, first runs via a pressure maximum and pressure minimum to lower temperatures and finally, after passing through a temperature minimum, rises again. The phenomenological relations in phase behaviour on transition from type II to type III and from type IIIa to type IIIb with the characteristic intermediate stages are presented schematically with the aid of the p( T) projections of their critical curves.