Direct Detection Of Double Retrograde Behavior For Equation Of State Models

RAMELLO, JUAN I.; MILANESIO JUAN M.; PISONI, GERARDO O.; CISMONDI, MARTÌN; ZABALOY, MARCELO S.

Alicante

Conferencia; X IBEROAMERICAN CONFERENCE ON PHASE EQUILIBRIA AND FLUID PROPERTIES FOR PROCESS DESIGN.; 2015

Universidad de Alicante

Binary mixtures which are highly asymmetric, with respect to molecular size and/or shape and/or attractive forces, may present double retrograde behavior (DRB). DRB usually occurs within a narrow range of composition, close to that of the pure lightest component. Available experimental data on DRB 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 binary system. 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 multi-component mixtures. Raeissi and Peters [3] described the DRB in the context of either isothermic or isoplethic phase equilibrium diagrams, i.e., they considered the DRB at constant temperature, and the DRB at constant composition. Models of the Equation of state (EoS) are typically used to model the phase equilibria of asymmetric mixtures, including those presenting DRB. In Ref [3] the ranges of conditions of existence of the DRB has been studied using an EoS, by computing a number of phase equilibrium 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 direct determination 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 the present work is to propose and illustrate such methodology. The cricondenbar (CCB) is the maximum pressure at which a mixture of given composition (isopleth) can be heterogeneous. Analogously, the cricondentherm (CCT) is the maximum temperature at which a mixture of given composition can be heterogeneous. A continuous set of binary ispoleths has associated continuous sets of CCBs and CCTs. 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 the detection of the DRB. It is important to note that a CCB point is, simultaneously, a local extremum in pressure 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 in temperature at constant composition (isopleth) and a local extremum in mole fraction at constant temperature (Isothermal Cricondencomp, CCCT). This means that a CCB line is also a CCCP line and that a CCT line is also a CCCT line.