A Systematic Method to Predict Cloud Point Temperature and Solid Precipitation

Abstract

Solid precipitations (or depositions) in crude oil operations are undesirable because they may eventually plug the process equipment and/or pipelines; as such, accurate prediction of cloud point temperature is essential in the design and operation of many oil production facilities. Determining the cloud point temperature of a mixture requires a robust solution of a multiphase, multicomponent phase equilibrium problem. Solution of the phase equilibrium problem rests on the relations set by the Gibbs criterion for stable equilibrium and the principle of mass conservation. Mathematically, two solution methods are used to characterize the equilibrium properties of a mixture at given temperature and pressure: the equal-fugacity (iteration-function formulation) and the Gibbs free energy minimization approach. In addition, an accurate solution model capable of predicting multiphase formation is required. In this study, we implement the iteration function method enhanced with a solid–liquid–vapor phase-check procedure to calculate cloud point temperatures and the amount of solid precipitations. The Peng-Robinson equation of state with a modified α function, as suggested by Gasem et al. ([2001]), was utilized to model vapor–liquid equilibria while regular solution theory was used to describe solid-liquid equilibria. We also present a systematic method to characterize the heavy components of oil. The results of this study are in excellent agreement with experimental cloud point temperature and solid precipitation data for four North Sea oils reported by Pedersen et al. ([1991]).