Test for the detection of the retrograde melting phenomenon in computed solid?fluid equilibria of binary asymmetric systems

S. BELÉN RODRIGUEZ-REARTES; GERARDO PISONI; MARCELO S. ZABALOY

JOURNAL OF SUPERCRITICAL FLUIDS

ELSEVIER SCIENCE BV

Lugar: Amsterdam; Año: 2015 vol. 97 p. 36 - 44

0896-8446

The phenomenon of retrograde melting at constant temperature takes place when a solid phase is melted,or made to disappear, upon compression. Analogously, retrograde melting at constant pressure happenswhen a solid phase is melted, or made to disappear, upon cooling. In this work, we propose a test thatmakes possible to establish for a calculated, already converged, binary solid?fluid equilibrium (SFE) point(of temperature TSFE, pressure PSFE, and fluid phase composition z2), whether the behavior is regular orretrograde and, in case of retrograde behavior, whether the melting phenomenon is retrograde withrespect to temperature, or to pressure, or to both. The test is based on evaluating, at TSFEand PSFE, thederivatives of the reduced tangent plane distance (tpd) with respect to temperature (dtpd∗/dT|P,z2)SFEandpressure (dtpd∗/dP|T,z2)SFE, for a tested phase of composition z2, and a trial phase consisting of the pureheavy component in solid state at TSFEand PSFE. The distinguishing feature of the present test is that thederivatives (dtpd∗/dT|P,z2)SFEand (dtpd∗/dP|T,z2)SFEare evaluated explicitly from the already convergedSFE point, i.e., the present test does not require the computation of additional SFE points. Consequently,the test is fast and can be used for all points of a calculated SFE segment of an isopleth envelope. Thisleads to the eventual identification of points along the computed SFE segment where transitions fromretrograde to regular behavior occur. We present results for a couple of highly asymmetric systems withthe help of a model that uses an equation of state for describing the fluid phases, and a standard equationfor describing the fugacity of the pure heavy component as a function of temperature and pressure.