The paper presents a vapor-liquid-wax phase equilibrium model for use on reservoir fluid mixtures. The gas and liquid phases are described using the Soave-Redlich-Kwong equation of state while the wax phase is assumed to be an ideal mixture. Only part of the heavy hydrocarbons are considered to be able to potentially enter into a wax phase. A procedure is developed for estimating the fraction of the heavy hydrocarbons which may potentially form wax. The fraction increases with the content of n-paraffins and decreases with carbon number. Calculation results agree very well with experimental wax precipitation data.
Formation of solid phases is highly unwanted in oil wells, in pipelines and in process equipment. Wax is an example of the kind of solid phases which may potentially form. Simulation of the phase equilibria of hydrocarbon mixtures taking into account the possible formation of a wax phase has been treated in a number of papers. Won and Pedersen et al. have used regular solution theory for the liquid (oil) and solid (wax) phases while Hansen et al. have proposed to use polymer solution theory for the liquid phase and to treat the solid phase as an ideal mixture.
Vapor-liquid equilibria of hydrocarbon mixtures are most commonly handled using a cubic equation of state as for example the Soave-Redlich-Kwong (SRK) or the Peng-Robinson equations. A vapor-liquid-solid phase equilibrium calculation using one of the above wax models will therefore apply two different liquid phase models. A cubic equation of state is applied when considering the vapor-liquid equilibrium and either regular solution theory or polymer solution theory when considering the liquid-wax equilibrium. This is not only thermodynamically inconsistent, but may also result in convergence problems during the phase equilibrium calculations.