Direct Detection Of Double Retrograde Behavior For Equation Of State Models

J. I. RAMELLO; J.M MILANESIO; G.O. PISONI; M. CISMONDI; M.S. ZABALOY

ALICANTE

Conferencia; X IBEROAMERICAN CONFERENCE ON PHASE EQUILIBRIA AND FLUID PROPERTIES FOR PROCESS DESIGN (EQUIFASE 2015); 2015

UNIVERSIDAD DE ALICANTE - ALICANTE ESPAÑA

IntroductionBinary mixtures which are highly asymmetric, with respect to molecular size and/or shape and/orattractive forces, may present double retrograde behavior (DRB). DRB usually occurs within a narrowrange of composition, close to that of the pure lightest component. Available experimental data onDRB are scarce [2]. This may be due to the fact that DRB occurs in a very small composition range[2]. DRB was first (experimentally) identified by Chen et al. [1], for the methane + n-butane binarysystem. Raeissi and Peters [4] and Deiters [5], analyzed the DRB from a theoretical point of view,using the Gibbs-Konowalow equation, which is an extension of the Clayperon equation for multicomponentmixtures. Raeissi and Peters [3] described the DRB in the context of either isothermic orisoplethic phase equilibrium diagrams, i.e., they considered the DRB at constant temperature, and theDRB at constant composition. Models of the Equation of state (EoS) are typically used to model thephase equilibria of asymmetric mixtures, including those presenting DRB. In Ref [3] the ranges ofconditions of existence of the DRB has been studied using an EoS, by computing a number of phaseequilibrium diagrams, for binary systems. The present work shows that a single, in a way special,binary phase equilibrium diagram, computed in a single computer run, makes possible the directdetermination of the ranges of conditions where liquid-vapor DRB occurs, for a given binary system,within the universe of the EoS model chosen to describe its phase behavior. Thus, the goal of thepresent work is to propose and illustrate such methodology. The cricondenbar (CCB) is the maximumpressure at which a mixture of given composition (isopleth) can be heterogeneous. Analogously, thecricondentherm (CCT) is the maximum temperature at which a mixture of given composition can beheterogeneous. A continuous set of binary ispoleths has associated continuous sets of CCBs andCCTs. Such binary CCB and CCT sets are hyperlines. This hyperlines can be computed using an EoS.The analysis of the CCB and CCT hyperlines (and of some additional information) makes possible thedetection of the DRB. It is important to note that a CCB point is, simultaneously, a local extremum inpressure at constant composition (isopleth), and a local extremum in mole fraction at constant pressure(Isobaric Cricondencomp, CCCP). Similarly, a CCT point is, simultaneously, a local extremum intemperature at constant composition (isopleth) and a local extremum in mole fraction at constanttemperature (Isothermal Cricondencomp, CCCT). This means that a CCB line is also a CCCP line andthat a CCT line is also a CCCT line.