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Annular heating probes in oil fouling : effects of wall shear stress Feiz, Ghazal


Fouling, as the deposition of undesired material on a surface, is a common reason for energy loss in oil refineries, particularly in the preheat train section, where the surface of heat exchangers become fouled. Some of the major issues associated with fouling are the increase in pressure drop and thermal resistance. Fouling rates are known to be a function of oil properties, temperature, and fluid flow properties. There is not yet an agreement among researchers on the determining flow property (Reynolds number, velocity, and inner wall shear stress) of the oil fouling process. The present work focuses on annular flow, which is commonly used for fouling studies, and discusses the effect of inner wall shear stress on fouling rate. An experimental study was undertaken in which an asphaltene containing oil blend (ATB) was recirculated in a flow loop equipped with an annular heat transfer test section. The change in overall heat transfer coefficient was monitored at different flow conditions to calculate the fouling resistance and rate. First, a series of asphaltene precipitation tests was carried out to identify a preferred diluent to be used with the highly viscous ATB. The existing flow loop was modified to accommodate a new pump, and a new data acquisition system was installed. A new 82 cm long annular fouling probe system was devised based on a commercially available 11 cm long 1000 W electrical heater equipped with a central core thermocouple. With Probe #1, fouling was not detected in a series of runs with power up to 412 W. Surface temperatures were determined to be low by Wilson-plot calibration and heat conduction calculations. Probe #2, with an improved design, showed lower internal thermal resistance, and hence higher surface temperature at a given power. As fouling was detected in one experiment, this probe design appears promising for future fouling studies. The literature on inner wall shear stress calculation for concentric annuli was reviewed, and appropriate equations selected. Inner wall shear stress was calculated for four various fouling studies carried out at UBC. Fouling rate dependencies on velocity, Reynolds number, and wall shear stress were compared.

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