Wax deposition in subsea pipelines is a significant economic issue in the petroleum industry. A mathematical model has been developed to predict the increase in both the deposit thickness and the wax fraction of the deposit using a fundamental analysis of the heat and mass transfer for laminar and turbulent flow conditions. It was found that the precipitation of wax in the oil is a competing phenomenon with deposition. Two existing approaches consider either no precipitation (the independent heat and mass transfer model) or instantaneous precipitation (the solubility model) and result in either an overprediction or an underprediction of deposit thickness. By accounting for the kinetics of wax precipitation of wax in the oil (the kinetic model), accurate predictions for wax deposition for both lab-scale and pilot-scale flow-loop experiments with three different oils were achieved. Furthermore, this kinetic model for wax precipitation in the oil was used to compare field-scale deposition predictions for different oils.
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.
The process of crystallization is of great interest in a wide range of industries. In the oil industry, a major interest is the deposition of wax onto subsea oil pipelines, a costly phenomenon that hinders the production of crude oil. It is known that these deposits are a volume spanning network of orthorhombic, lamellar wax crystals consisting primarily of n-alkanes that entrap some of the crude oil to form a gel. The presence of other materials in a crystallizing system can have an impact on both thermodynamic and kinetic parameters. To analyze the effects of how n-alkanes impact the crystallization of one another, three different types of apparatus (differential scanning calorimetry, densitometer, and a coldfinger apparatus) were used to explore a wide range of crystallization and deposition properties. The results of these experiments showed that longer chained n-alkanes greatly influence the crystallization properties of shorter n-alkanes, whereas shorter n-alkanes only slightly impact the crystallization properties of longer chained n-alkanes. This impact is directly related to the amount of cocrystallization that exists between the n-alkanes, which is dictated by the carbon number difference, solubility differences, and cooling rate. Cocrystallization shifts the temperature at which crystallization occurs and reduces the heat that is released by the system. Polydispersity and cocrystallization also reduce the mass and wax fraction of a deposit formed using a coldfinger apparatus.
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