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Effect of oil compatibility and resins/asphaltenes ratio on heat exchanger fouling of mixtures containing heavy oil Al-Atar, Eman

Abstract

Fouling of heat transfer equipment due to unwanted deposition of solids during heating remains a major cost penalty in oil refineries. Severe fouling is encountered during the processing of asphaltene-containing oils, and with increased reliance on heavy oils the situation has been exacerbated. Petroleum oils can be separated by solvent fractionation into saturates, aromatics, resins and asphaltenes with the latter having the highest molecular weight. Asphaltene precipitation from oils depends on the concentration of solvent components such as resins and aromatics. The available literature suggests that resins stabilize asphaltenes, minimizing their tendency to flocculate. This work was undertaken to determine how the asphaltene-resin interactions affect fouling. Fouling of asphaltenes from a heavy oil in mixtures of fuel oil and de-asphalted vacuum bottoms (DAO) was studied at asphaltene concentration of 0.04 - 3.4 %, and resin concentrations of 3.1 - 4.9 %. Experiments were performed at a bulk temperature of 85 °C, fluid velocity of 0.75 m/s and pressure of 410 kPa. Fluids under nitrogen were recirculated through an annular test section with initial surface temperature of 230 °C for periods up to 30 hours, and the fouling monitored by thermal measurement. The effects of concentration of heavy oil and de-asphalted oil are explored. High fouling rates were encountered at high pentane insolubles (asphaltene) concentrations. Fouling rates are correlated with the ratio of resins / asphaltenes. At a fixed D AO concentration, the fouling rate first increased, and then decreased as the H O concentration was raised from zero to 20 % and as the Re/As ratio decreased. The maximum initial fouling rate occurred at a ratio of = 2.5 and dropped to essentially zero for Re/As ratio > 5.8. The initial fouling rate, hot filtration insolubles concentrations and pentane insolubles concentrations were found to increase as DAO concentration was raised at a fixed Re/As ratio. This was somewhat unexpected. Pentane insolubles concentrations also increased as DAO concentration was increased at a constant asphaltene concentration, which suggests that it is not only the asphaltenes in the heavy oil that precipitate in fuel oil / DAO mixtures. The relationship of fouling to oil compatibility as determined by the method of Wiehe, was explored. Fouling rates of mixtures containing DAO did not correlate with the colloidal instability index. A fouling regime map indicated that low fouling rates were dependent on both the colloidal instability index and the resin/asphaltene ratio. Oil Co mpatibility Model predictions correlated well with the colloidal instability index and therefore were unable to predict the fouling behaviour of the mixtures. However, the Oil Compatibility Model was found to be very sensitive to small errors in titrations. Oil Compatibility Model titrations showed that the addition of DAO to a heavy oil sample resulted in asphaltene precipitation at a lower heptane concentration and required a higher toluene concentration in a toluene-heptane mixture to keep asphaltene in solution. This finding was consistent with the measured effect of DAO on fouling.

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