The authors have examined the complex phenomenon of deposition of solid wax during the pipelining of waxy crude oils through a cold environment. The analysis is concerned with turbulent flow regime and includes: (a) The wax transport to the pipe wall by means of molecular diffusion induced by the radial thermal gradient, (b) ablation, slowing down (or even preventing) deposition, (c) consolidation (aging) of the deposit, due to the progressive reduction of the oil fraction. The momentum boundary layer of laminar flow is calculated and consequently the thermal boundary layer and the bulk thermal profile and are obtained. The knowledge of the thermal gradient in the boundary layer is essential to provide an explicit expression of the deposition rate caused by molecular diffusion. Ablation rate is supposed to be proportional to the mechanical stress at the pipe wall. Using some approximation justified on the basis of dimensional analysis (R2/L2 << 1), the authors computed the deposit profile, calculating in particular the length of the deposit free zone and the point beyond which deposition ceases. Depending on the physical situation, the flow can enter a regime of desaturation, marked by the extinction of the suspended wax crystallized phase. The distance at which desaturation occurs has been calculated and it has been noticed that the oil may become so deprived of wax that deposition can cease completely, although this phenomenon was not found in the field cases considered. The authors have used the model to calculate the total amount of deposit exploiting field data of two real plants. Comparison with the recovered material has produced, in both cases, errors which are less than 10 %. Various details have been discussed in the Appendices, showing in particular that it is perfectly justifiable to neglect the influence of the latent heat accompanying the formation and dissolution of wax crystals.
Calculating deposit formation in the pipelining of waxy crude oils
Berechnung der Bildung von Ablagerungen in von paraffinhaltigem Öl durchströmten Pipelines
Meccanica ; 42 , 2 ; 149-165
2007
17 Seiten, 4 Bilder, 25 Quellen
Article (Journal)
English
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