Microscopy of fouling deposits in bitumen furnaces

This study investigated the deposition of foulants in a delayed coker furnace. The main components observed in several foulant samples were fine- to very coarse-grained mosaic semicoke, pyrolytic carbon, iron sulfide (FeS) and mineral matter (clays). The optical texture and reflectance (in oil) of the carbonaceous deposits were in agreement with the variations in temperature and residence time in the furnace tube. Semicoke became more prevalent, as the distance from the furnace tube wall increased and indicated areas, where the temperature was less than 500°C. Pyrolytic carbon was present primarily at the tube wall and indicated areas where temperatures were in excess of 600°C. The concentrations of iron and sulfur in the foulant both increased towards the tube wall. A continuous FeS phase was observed at the wall and in cracks within the carbonaceous deposits, while, within the bulk foulant, the FeS existed as particulates. This iron sulfide was believed to have originated from components of the bitumen feed rather than process equipment. The ratio of iron to carbon in the deposit was greater by a factor of 1000 than the ratio found in the bitumen. Bitumen feeds derived from Alberta oil sands typically contain about 0.5 wt% of toluene insoluble solids (mineral matter), if recovered by a hot water extraction process. Only a very small proportion of other minerals was deposited within the furnace tubes.