An improved thermodynamic model for wax precipitation from petroleum fluids

In this work, a solid-solution model has been developed for improving the representation of wax precipitation from petroleum fluids. The model takes into account the Poynting correction in the solid fugacity calculation. A characterization procedure has been developed for the tale boiling point (TBP) and plus fractions. The paraffin-naphthene-aromatics (PNA) distribution up to C-80 is estimated according to the properties of the TBP and plus fractions with one adjustable parameter. An optimized value for that parameter has been obtained, which can be treated as a constant for most crude oils and gas condensates or as a tunable parameter. The model has been used to predict wax appearance temperatures (WAT) over a wide range of pressures and wax contents as a function of temperatures or pressures for a great number of systems. The average deviation of the calculated WAT is within 1.5 K for the systems with the experimental PNA distribution data of TBP fractions, and 3.0 K for the systems without the experimental PNA distribution data of TBP fractions. The results of the calculated wax formation locus and amount of precipitated wax as a function of temperatures or pressures by the proposed model are in good agreement with the measured data. Moreover, the proposed model has demonstrated the capability of simulating the retrograde phenomenon of gas condensates, which indicates that the amount of precipitated wax first decreases and then increases with pressure decrease in the vapor-liquid-solid phase region at a specified temperature. (C) 2001 Elsevier Science Ltd. All rights reserved.