Direct detection of double retrograde behavior for equation of state models

RAMELLO, JUAN IGNACIO; MILANESIO, J. M.; PISONI, G. O.; CISMONDI DUARTE, M.; ZABALOY, M.

Alicante

Conferencia; EQUIFASE 2015: X Conferencia Iberoamericana sobre Equilibrio entre Fases para el Diseño de Procesos; 2015

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 multicomponent 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.