LOCATING ALL HOMOGENEOUS AZEOTROPES IN MULTICOMPONENT MIXTURES

A novel approach for enclosing all homogeneous azeotropes in multicomp onent mixtures is presented. The thermodynamic criteria for azeotropy are outlined, and mathematical equations for each criterion are develo ped. The global optimization approach is based on developing convex un derestimators which are coupled with a branch and bound framework in w hich upper and lower bounds on the solution are refined by successivel y partitioning the target region into small disjoint rectangles. The o bjective of such an approach is to enclose all global minima since eac h global minimum corresponds to a homogeneous azeotrope. Because of th e nature of the thermodynamic equations which describe the behavior of the liquid phase, the constraint equations are highly nonlinear and n onconvex. The success of this approach depends upon constructing valid convex lower bounds for each nonconvex function in the constraints. F our different thermodynamic models are studied, the Wilson, NRTL, UNIQ UAC, and UNIFAC equations. Tight convex lower bounding functions are f ound for the nonconvex terms in each model. The unique element of the proposed approach is that it offers a theoretical guarantee of enclosi ng all homogeneous azeotropes. The effectiveness of the proposed appro ach is illustrated in several example problems.