Calculation of critical point coordinates of ternary miscibility gap from experimental tie-lines

Abstract

Demixing phenomena in liquid ternary systems are widely used in chemical engineering processes. When a critical point is observed, the composition of this invariant point of major interest is experimentally difficult to determine. A simple calculation method for the critical point is proposed using the experimental tie-lines. The computing treatment is based on the application of the barycentric weighting of the binodal points in order to extend the rectilinear diameter method and the exploitation of the modulus of the experimental tie-lines. A systematic study was carried out on a large set of ternary systems taken from DECHEMA and NIST. The results are compared with those available in literature or calculated by standard methods using binary interaction parameters. Very good agreements are obtained when the experimental tie-lines are located close to the critical point.The method can be extended to wide miscibility gaps by using a polynomial deviation instead of the rectilinear diameter line. A typical case is detailed (2-butanone+2-butanol+water at various temperatures). The results lead to (i) the point by point calculation of the entire binodal curve and (ii) the thermal evolution of the critical point and the possible disappearance of the critical point when temperature increases.