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The stability of residual oil is considered to be one of the major factors in coke deposition onto the inner surfaces of furnace tubes in distillation units, such as the vacuum distillation unit (VDU). This preliminary study investigated the stability of a Middle East atmospheric residue oil and the coke deposition tendency on type 304 stainless steel surfaces and chromium molybdenum steel surfaces. The latter materials are widely used as furnace tube materials. Coke deposition rates were determined using a laboratory scale test apparatus at atmospheric pressure. The oil was fed in once-through mode into the inlet of the test section of the apparatus for 24h. During the feeding of the oil, the temperature of the metal surface of the test tube was controlled within a range from 500 to 600°C. The stability of the feed oil and effluent oil was evaluated by the P-value based on the Heithaus evaluation method, which indicates the tendency of asphaltene flocculation to form sludge. A decrease of P-value indicates decreased oil stability with the formation of sludge. Coke deposition experiments were carried out at higher temperatures than the conventional VDU operation to promote coke deposition. The results showed that the coke deposition rate was slightly lower for stainless steel than for chromium molybdenum steel. The coke deposition rates increased exponentially regardless of the materials. The P-values of the effluent oil gradually increased with the effluent temperature from 415 to 450°C. This indicates an increase of oil stability with heating temperature. The changes in P-values correspond to removal of unstable aggregates of asphaltenes from the oil. The unstable aggregates are thought to be one of the predominant precursors for coke deposition. The results suggest that the analysis of P-values of the feed and effluent oils together with coke deposition is useful to estimate the quantity of deposited coke on the furnace tubes before operation.

Coke Deposition on Furnace Tubes of the Vacuum Distillation Unit Estimated by Stability of Atmospheric Residue Oil.