A thermodynamic model for predicting wax formation in crude oils

Abstract

Most crude oils contain high molecular weight components, which at low temperatures may precipitate as a wax phase. This may cause plugging of pipes and numerous other problems. This paper presents a solid-liquid equilibrium based model for the description of wax formation. The model for the Gibbs energy contains a contribution based on Flory's theory of multicomponent polymer solutions and a contribution from a metastable subcooled state which oil mixtures may attain. The latter is formulated in terms of the surface tension of the wax phase. Experimental wax appearance points (temperatures), WAP's, are reported for 17 different stabilized North Sea crude oils. The values predicted by the new model are in very good agreement with the experimental WAP's.

Cloud Points: Can We Measure or Model Them?

“…(Erickson et al, 1993;Coutinho et al, 2001) or may be described using a free energy model (Won, 1986;Hansen et al, 1988;Coutinho et al, 1996;Lira-Galena et al, 1996). For high pressure systems (live oils), on the other hand, the liquid phase is generally described using an Equation of State (Lira-Galena et al, 1996;Pan et al, 1997;Pauly et al, 2000).…”

“…Either the authors assume that the solid phase is a sum of independent pure components or pseudo-components Lira-Galena et al, 1996;Pan et al, 1997) or that it is made up of a solid solution, stable or not, of all the materials that crystallize. Some authors that consider the formation of a solid solution assume it being ideal (Erickson et al, 1993;Coutinho et al, 2001;Won, 1986;Hansen et al, 1988;Coutinho et al, 1996).…”

“…Pioneer authors generally assume that all compounds present in the mixture eventually become part of the wax phase (Won, 1986;Hansen et al, 1988) This assumption is also adopted in recent publications (Lira-Galena et al, 1996;Pan et al, 1997) in spite of the growing evidence that the wax is made up primarily of compounds with long n-alkyl chains. The majority of the nalkanes with carbon number between 18 and 60 are polymorphic and hence assume more than one crystalline form below their melting points.…”

Experimental study and a proposed new approach for thermodynamic modeling of wax precipitation in crude oil using a PC-SAFT model

“…The first important approach for modeling of wax precipitation uses a cubic equation of state (EOS) for vapor-liquid equilibrium and an activity coefficient model for solid-liquid equilibrium. These models are based on solid solution (SS) theory which assumes that all the components in the solid phase are miscible in all proportions (Won 1968(Won , 1989Hansen et al 1988;Pedersen et al 1991;Zuo et al 2001;Ji et al 2004). Chen et al (2009) proposed new correlations for the melting points and solidsolid transition temperatures of treated paraffin based on the experimental results from differential scanning calorimetry (DSC).…”

Fundamental Study of Wax Deposition Under Real Flow Conditions

“…A variety of solid paraffins models have been developed, e.g., the multisolid model, which describes solid paraffins as being multiple pure solid phases, the ideal solid solution model, which describes paraffinic solid phase as being an ideal solution, and other models that describe solid paraffins as one or more non-ideal solid phase solutions, the solid solution models (Lira-Galeana et al, 1996;Won, 1986;Hansen et al, 1988;Coutinho et al, 2006).…”