Fluid mixtures at high pressures VI. Phase separation and critical phenomena in 18 ( n-alkane + ammonia) and 4 ( n-alkane + methanol) mixtures

Abstract

The p( T) projections of (liquid + liquid) (ll), (liquid + gas) (lg), critical curves, and of (liquid + liquid + gas) (llg) three-phase curves for 18 binary mixtures of (an n-alkane + ammonia) with alkane-carbon numbers i = 1 to 12, and i = 14, 16, 18, 19, 20, and 36 were measured. The critical curves for the i = 1 and i > 18 mixtures are interrupted, whereas the values for all mixtures with carbon numbers between i = 2 and i = 18 form an uninterrupted lg-critical curve and an llg three-phase curve that terminates in a (liquid + liquid) upper critical end point (ll-UCEP). The ll-critical curves start from this ll-UCEP and rapidly approach high pressures with increase in temperature. The mixtures with i = 2 to 5 form positive azeotropes up to the critical region. The uninterrupted critical curve for (ethane + ammonia) passes through a temperature minimum and a pressure maximum, whereas those for i = 3 to 5 pass through only a temperature minimum. A temperature minimum, a pressure minimum, and a pressure maximum occur in the three lg-critical curves for i = 6 to 8. The critical curves for i = 9 to 11 pass through a pressure minimum and maximum but not through a temperature minimum. The lg-critical curves for i = 12 to 18 exhibit only a pressure maximum, but a point of inflection still exists between it and the critical point of ammonia. Pseudo-binary mixtures [{ xC 18H 38 + (1 − x)C 19H 40} + ammonia], where x is 0.50 and 0.75, were taken to demonstrate the continuous transition in phase behaviour from type II through type IV to type III. The classification is that of Scott and van Konynenburg. Types II and IV are separated in the borderline case where an ll-UCEP and an LCEP collapse into a hypercritical point; and type IV and III, in the borderline case where an LCEP and an lg-UCEP collapse into a tricritical point. To demonstrate the similar pattern of phase behaviour for (an n-alkane + a polar compound), the p( T) projections of lg-critical curves have been measured additionally for (hexane or nonane or dodecane or hexatricontane + methanol), all of which exhibit an uninterrupted lg-critical curve.