(382k) An Approximate Density-Based Phase Envelope Construction Method

A new simple density-based method is proposed for phase envelope construction of constant composition complex multicomponent mixtures. The phase envelope is traced in the molar density-temperature plane (with a unique saturation temperature at given mixture molar density) and the pressure is calculated explicitly from the equation of state (EoS). The computational procedure is very easy to implement and a reduced system of only three equations must be solved for three variables (molar densities of feed and incipient phase and temperature), irrespective to the number of components in the mixture. The EoS must not be solved for volume and the elements of the Jacobian matrix have simple forms; the partial derivatives of fugacity coefficients with respect to compositions are not required. A simple correction procedure highly improves results by updating the reference conditions at each point on the phase envelope. Simple equations for calculation of maximum temperature and pressure points are presented. The proposed method was tested for a variety of mixtures, ranging from natural gases to heavy oils. For usual (closed) phase envelopes, the entire saturation curve is remarkably well reproduced. Certain unusual (open-shaped, with a bubble point branch extending to infinity) phase envelopes are also entirely traced up to very high pressures, while in certain cases the method fails at low temperatures (where the approximation of equilibrium constants become inadequate) for such mixtures, but it provides an excellent approximation for a wide temperature range. The maximum difference in saturation pressure at a given temperature is smaller than 0.1 bar for all the test mixtures. The proposed method is not dependent of the thermodynamic model (any pressure-explicit EoS can be used), it is recommended for mixtures with many components and it is particularly attractive for highly complex EoS.