Computation and analysis of binary multiphase isochores

MOLINA, MATÍAS JOSÉ; RODRIGUEZ REARTES, SABRINA BELÉN; ZABALOY, MARCELO S.

Carlos Paz, Prov. Córdoba, Argentina

Conferencia; EQUIFASE ´18 - XI Iberoamerican Conference on Phase Equilibria and Fluid Properties for Process Design; 2018

IPQA-CONICET- UNC

The isochoric method makes possible the measurement of PVT properties and phase equilibria [1]. A constant volume (V) cell is loaded with known amounts of the substances of interest (specified overall composition (z, mole fraction vector) and specified overall density ( )); next the temperature (T) is set, and once equilibrium is reached the pressure (P) is measured. A new T value is set and a new P is measured and recorded. The desired T range is covered. An isochoric (constant overall molar volume , ) curve (P vs. T) is obtained, which is also isoplethic (constant z). An isochor (IC) slope discontinuity indicates a phase transition. Vélez et al [1] computed ICs using equations of state (EOSs) through flash calculations at specified T, V and z, whose outcome is the number, composition, density and relative amounts of the equilibrium phases. Under homogeneity conditions P is directly calculated [2]. The isochoric (specified V) flash calculation can be initialized by using a global stability test [3], [4], [5]. Stability tests can be avoided when computing binary ICs if the (non isochoric) isopleth (IP) of composition z is previously calculated (e.g., red curve, solid blue curve and dotted blue curve in Fig 1). A binary IP (B-IP, specified z) is basically made of the phase envelope (B-PE) and, eventually, also of one or more three-phase lines (B-3PL), or segments of such lines, which in this work can a only be of the liquid-liquid-vapor (LLV) type. The B-PE is a two-phase line. It is the boundary between homogeneity and heterogeneity, at the specified z. In each point of a B-PE a phase of composition z (major phase) is at equilibrium with an incipient phase (infinitesimal phase size). In this work, a method for the computation of fluid binary ICs, which considers all possible IC segments, and relies on information taken from the complete B-IP (B-PE + B-3PLs), previously computed according to ref [6], is proposed.