Robust Calculations Of Critical End Lines Of Ternary Systems

PISONI, GERARDO O.; CISMONDI, MARTÌN; ZABALOY, MARCELO S.

Puerto Varas

Conferencia; IX Iberoamerican Conference on Phase Equilibria and Fluid Properties for Process Design; 2012

Univ de Concepción, Chile.

At a ternary critical end point (TCEP) a critical fluid phase is at equilibrium with a non-critical fluid phase, being the system made of three components. A TCEP is the termination of a ternary three-phase line. For computing a TCEP it is necessary to add, to the critical conditions, the isofugacity conditions between the critical and non-critical phases. The resulting system of equations (SE) has only one degree of freedom. Therefore, the TCEP conditions define one or more lines (or hyperlines) of TCEPs for a given ternary system, model, and parameter values. A proper name for a line of TCEPs is “critical end line” (CEL). A ternary CEL (actually a hyperline) is a characteristic univariant line of ternary system. At a ternary CEL, a three-phase hyper-surface and a critical hyper-surface meet. CELs provide key information on the phase behavior of ternary systems. A ternary system may have several CELs. It is desirable to have available a robust algorithm for computing complete ternary CELs, thus minimizing the need for user intervention. It is also important to reliably detect the key points where CELs originate or terminate. In this work, we propose a numerical continuation method (NCM) for the fast computation of ternary CELs. As in other recent works, the present NCM makes use of a sensitivity vector for predicting the solution of the TCEP SE and for identifying the optimum variable to be specified for calculating the next point belonging to the CEL being built. We present calculated CELs for a variety of highly asymmetric systems and show the diverse topologies that these lines may define. We consider models of the equation of state (EOS) type and use them in wide ranges of conditions.