H-M(E) AND V-M(E) THERMODYNAMIC SURFACES FOR BINARY-MIXTURES IN THE NEAR-CRITICAL REGION

Even for such simple mixtures as (argon+methane), the excess enthalpy H-m(E) and the excess volume V-m(E) in the near critical region are ab out two orders of magnitude higher than for the liquid mixture at low temperatures and pressures near ambient conditions. Mixtures for which the critical temperatures are close together, and for which the criti cal pressures are far apart, have similar H-m(E) (x,p,T) and V-m(E) (x ,p,T) surfaces, and near critical isotherms show double maxima in the supercritical fluid region. Mixtures for which the critical pressures are close together, and the critical temperatures are far apart, also have similar H-m(E) (x,p,T) and V-m(E) (x,p,T) surfaces, but isobars o n the surfaces are 'S' shaped. The shapes of these near-critical exces s-function surfaces can be understood from an inspection of the enthal py, or residual enthalpy curves of the mixture and of the pure compone nts. Examples of both are given. Attention is drawn to the large value that these excess functions can have close to a pure component critic al point. (C) 1997 Elsevier Science B.V.