The presence of paraffin in a crude oil can create difficulties in storage, extraction, pumping and all classical operations in a pipeline system. Oils with high content of paraffin are usually called WCOs (waxy crude oils). One of the most important problem occurring in WCOs, for low temperatures, is the formation of paraffin deposits on pipeline walls. On the basis of available experimental results it is proposed a physical-mathematical isothermal model for the deposition of paraffin on the walls of a cylindrical circular pipe of radius R, like the ones used in experimental laboratory, usually called loops. It is known that for sufficiently high temperatures a WCO behaves like a Newtonian fluid and the presence of dissolved paraffin creates no problem. When the system is cooled below the so called cloud point or WAP (wax appearing point) paraffin begins to precipitate changing drastically the rheological features of the flow. Below the WAP paraffin crystals begin to appear and at a lower temperature they exhibit a strong tendency to aggregate giving the oil a jel-like structure. At this stage the fluid is distinctly non newtonian. Assuming that the temperature is constant and below the pour point the oil has been considered as an incompressible, laminar, bi-dimensional Bingham fluid. The first six equations that form the mathematical problem come from the usual Bingham fluid model in radial coordinates. These equations have to be coupled with the evolution of the crystalline component and the evolution of the paraffin layer. The problem is a free boundary problem and it is quite complex to study in this form. It has been performed a quasisteady approximation for the velocity field and for the inner core boundary, and this is consistent with what really happens in the experimental loops. In practice the problem has been put in a non dimensional form. Five assumptions were made. The last hypothesis ensures that the motion really takes place.
Some new results on the flow of waxy crude oils in a loop
2002
4 Seiten, 7 Quellen
Aufsatz (Konferenz)
Englisch
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