Effect of minerals on coke formation

Petroleum coke is defined as the toluene-insoluble carbonaceous solid which can form in a variety of petroleum processing units, particularly due to heating of residual oils with a high asphaltene content. Coke formation is the objective in some high temperature (500°C) processes such as delayed coking or fluid coking, but is undesirable in refineries or heavy oil upgrading processes, where it gives rise to operating problems. It has been postulated that fine solids in the bitumen could have an effect on the formation of coke precursors which occurs during heating at temperatures above 350 °C. Most of the previous studies on coke formation concentrated mainly on the reaction mechanisms and the development of the coke phase, rather than on the coke yield. There have been very few investigations concerned with the possible effect of either naturally-occurring fines in bitumen or other additives on the coke yield. The objective of this study was to determine the formation of toluene insoluble material from Cold Lake bitumen at temperatures of about 300-400 °C. In particular, the effect of the presence of solids on the formation of toluene insolubles was investigated. The coking reactions were performed at atmospheric pressure in two different batch reactors: a non-stirred tube reactor which initially contained about 1.5 g of bitumen sample, and a stirred autoclave reactor with bitumen samples of 50 or 100 g. Both reactors were purged with 200 ml/min (NPT) of nitrogen throughout the experiments and, therefore, the volatiles which were formed during the bitumen cracking reaction were continuously swept from the remaining fluid. In the first stage of experiments, using the non-stirred tube reactor, the toluene insoluble yield and weight loss of bitumen were measured as a function of reaction time at three different temperatures (380, 390 and 400 °C) in the absence of added solids. Volatile material was released over the whole duration of the reaction. An induction time for toluene insolubles formation was observed at the three temperatures; it became shorter as the reaction temperature increased. Similar blank experiments without solids present were performed on the 50 and 100 g samples in the autoclave reactor, but at the single temperature of 390 °C. In the second stage of the experimental program, solids, including molybdenum sulphide, silica, Alberta clay, kaolinite and native clays, were added at 2 wt% to the bitumen samples prior to the coking experiments. All of the solids were characterized by scanning electron microscopy (SEM). Molybdenum sulphide, which has shown catalytic activity in cracking reactions, and was reported to produce a lower coke yield in some previous studies, did not have a statistically significant effect on the coke yield or weight loss, in either the tube reactor or the autoclave reactor experiments in this study. Two different batches of silica, having mean sizes of 3.0 and 6.5 µm were used as inert particles in the coking experiments. Only the 6.5 µm silica particles had a significant effect, reducing the coke yield from 5.8% for the blank sample to 5.1% in the non-stirred tube reactor after 4.5 h coking at 390 °C and a nitrogen purge rate of 200 ml/min. Neither kaolinite nor southern Alberta clay had any effect on either the coke yield or the weight loss when added to the bitumen at 2.0 wt%. However, native clays, which were originally separated from Athabasca bitumen, did have an influence on the coking reaction. It was found that, when this mixture was coked in the autoclave for 4.5 hour at 390 °C and 200 ml/min nitrogen purge, the coke yield decreased to 3% from the 4% obtained for the blank sample under identical conditions. No effect on volatiles formation was observed. The effect of addition of native solids was also investigated at different concentrations, and it was observed that the more of these solids added to the bitumen, the greater is the reduction in the coke yield. Some possible explanations are given for the role of the native clay in the coke formation reaction. Also the rate of the nitrogen purge rate, whose primary purpose was to remove volatiles from the reactor, had a significant impact on the coke yield both in the presence and absence of added solids. The greater the purge rate, the greater is the coke yield. The results obtained for the toluene insoluble yield and weight loss in the two types of reactors were compared. The results of the tube reactor experiments in the absence of added solids were interpreted using the reaction model of Wiehe (1993, 1997).

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