Mitigation of Fouling in Bitumen Furnaces by Pigging

Abstract

Processing of Athabasca oil sands bitumen requires preheating of the feed to temperatures in the range of 350−500 °C. At these temperatures significant amounts of solid foulant can be deposited on the walls of the furnaces. Ultimately, buildup of the deposit causes premature shutdown of the process, either because of excessively high tube skin temperatures or because of high pressure drop across the furnace. Steam/air de-coking, which has been practiced in the past, is unable to eliminate all the deposit since the foulant contains significant quantities of non-carbonaceous material. Currently, the preferred method to address this problem is pigging assisted by high-pressure water. The feed to the furnaces typically contains close to 0.5 wt % inorganic solids originating from the mineral matrix from which the bitumen is extracted. Elemental analysis of the feed identifies 300−450 ppm iron, much of which can be attributed to the components of the inorganic matrix, such as pyrite, iron carbonate, and iron-bearing clays. However, it is expected that soluble organic iron species, such as naphthenates and carboxylates, are more likely to decompose at the temperatures in the furnaces and to convert to iron sulfide and deposit within the furnace. Samples of foulant were collected from two furnace pigging operations, one at the Suncor Energy Inc. coker furnace and one at the Syncrude Canada Ltd. LC Finer furnace. The samples were taken at timed intervals that allowed a profile of the foulant within the cross-section of the furnace tube to be completed. After washing and drying, the foulant samples were analyzed for carbon, sulfur, and a variety of metals and inspected by optical petrography. Coker furnace foulants exhibited high concentrations of both carbonaceous material and iron sulfide. The iron sulfide concentration was highest at the wall and declined as the distance from the wall increased. The carbon content showed the reverse trend, suggesting that coke formation became the more important factor as the deposition process proceeded. This reverse trend coincided with the increase in tube skin temperature. Fluid and skin temperatures are much lower in the LC Finer furnace than in the coker furnace; therefore, the tendency to coke formation is reduced. As a result, the composition of the foulant from the LC Finer furnace was more constant and consisted almost entirely of iron sulfide. It was estimated that the amount of iron deposited as foulant corresponded to about 0.05 wt % of the total iron in the feed that passed through these furnaces.