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The effects of oxygen on Synthetic crude oil fouling

University of British Columbia

Fouling may be defined as a process that leads to the formation of any undesirable deposit on the heat exchanger surfaces, which increases resistance to heat transmission or pressure drop. The effect of fouling is significant in the preheat exchanger networks of petroleum refineries, resulting in major economic penalties. Gum formation by autoxidation is reported to be one cause of fouling. This thesis involves a study of the effects of trace levels of dissolved oxygen on the fouling behaviour of synthetic crude oil. To measure dissolved oxygen content, a sophisticated technique based on gas chromatography / mass spectrometry was employed. The method was first used to determine the solubility of oxygen as a function of temperature in some pure hydrocarbons, some hydrocarbon distillates, and the synthetic crude oil. The synthetic crude oil is produced from bitumen by Syncrude Canada Limited at its plant in Mildred Lake, Alberta. Fouling in field conditions was simulated in a lab scale fouling rig. The synthetic crude oil was recirculated through an electrically heated annular probe, which operated at constant heat flux. The dissolved oxygen content of the oil was varied by sparging gas mixtures containing different air/nitrogen ratios continuously into the feed tank of the fouling loop. Fouling was detected by the increase in wall temperature of the probe. The fouling resistance increased roughly linearly with time after a short induction period for the relatively short run times of up to 24 hours which were typical of oxygenated experiments. A longer experiment of 130 hours under nitrogen saturation revealed asymptotic behaviour. The initial fouling rates showed a strong effect of the dissolved oxygen concentration in the range 0 - 9 ppmw which could be approximated by an exponential dependence. Above 9 ppmw of bulk dissolved oxygen, which corresponded to saturation by a mixture of 10% air - 90% nitrogen, the initial fouling rates were independent of dissolved oxygen concentration. Induction times were small or absent. Initial fouling rates were up to 3 times higher in the presence of oxygen than under inert gas blanketing, the above experiments were done at a bulk temperature of 75 °C and at an initial surface temperature of 290 °C. Experiments at different surface temperatures from 261 °C to 297 °C, under a constant dissolved oxygen content of 4.5 ppm, showed a strong effect on initial fouling rates. This was indicated by an apparent activation energy of 44 kJ/mol. Increasing the bulk temperature from 75 to 125 °C resulted in an increase of about 60% in initial fouling rate. When the annular velocity was increased from 0.44 m/s to 0.77 m/s, the initial fouling rate was reduced by a factor of two. Effects on fouling of dissolved oxygen content, wall and bulk temperature, and velocity were compared to previous results in the literature. The deposits formed at high dissolved oxygen content (100% air sparge) tended to be polymeric in nature. With 100%) nitrogen sparge fouling appeared to be particulate. The levels of total insolubles in the oil at the end of the fouling runs varied from about 200 ppmw for a 133 hour run (with zero ppmw dissolved oxygen) to 900 ppmw after a 16 hour run with 18 ppmw dissolved oxygen. The suspended solids formed were composed mostly of aromatics and/or poly cyclics. The morphology of particulates and deposits was investigated using scanning electron microscopy.

Thesis/Dissertation

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2009-04-30

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http://hdl.handle.net/2429/7747

Chemical and Biological Engineering

University of British Columbia

UBCV

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