Paraffin Polydispersity Facilitates Mechanical Gelation

Abstract

Incipient wax−oil gel deposits form in crude oil transport pipelines when long-chain n-paraffins precipitate at the cold interior surface of the pipe wall. The kinetics of paraffin gel formation was studied using model fluids consisting of monodisperse and polydisperse n-paraffin components dissolved in petroleum mineral oil. Classical homogeneous nucleation theory was applied to investigate the supersaturation conditions necessary for crystal formation. Differential scanning calorimetry was used to monitor paraffin crystallization rates and to provide solid-phase fraction measurements. Gelation occurs when growing n-paraffin crystals interlock and form a volume-spanning crystal network which entrains the remaining liquid oil among the crystals. Paraffin wax−oil gels exhibit a mechanical response to an imposed oscillatory stress, which is characterized by the elastic storage modulus G‘ being greater in magnitude than the viscous loss modulus, G‘ ‘. Low-temperature rheological gels can form from model fluids with n-paraffin contents as low as 0.5 wt %. Images of wax−oil gel morphologies were obtained using a cross-polarized microscope fitted with a z-drive and indicated crystal lengths of ∼10−20 μm. A microstructural gelation model based on percolation theory was introduced to provide predictions of gel formation conditions among randomly oriented paraffin crystals. The structural model provides correlations of crystal morphologies and solid fractions at the percolation threshold condition. Comparison of the initial wax contents required for gelation of monodisperse and polydisperse n-paraffin wax indicates that sharp crystal edges and ordered crystal faces hinder the paraffin crystal−crystal “anchoring” interactions which result in mechanical gelation.