6428
Ind. Eng. Chem. Res. 2006, 45, 6428-6435
Formation and Characteristics of Carbonaceous Deposits from Heavy Hydrocarbon Coking Vapors Zhiming Fan and A. Paul Watkinson* Department of Chemical & Biological Engineering, The UniVersity of British Columbia, 2360 East Mall, VancouVer, Canada V6T 1Z3
Deposition of carbonaceous material in the cyclone exit line is a chronic problem for fluid cokers and is a key process limitation to achieving longer run length. Extensive characterization studies were conducted for laboratory deposits from a bench-scale bitumen coker using modern analytical techniques, e.g., elemental analysis, X-ray fluorescence, thermogravimetric analysis, scanning electron microscopy, diffusive reflection infrared spectroscopy, and solid-state 13C nuclear magnetic resonance. Simulated distillation was also applied to solvent extracts of deposits. Results substantiate that on the laboratory scale physical condensation rather than chemical reaction was the primary reason for fluid coker cyclone exit line fouling. Entrained liquid droplets also contribute to the deposit formation. Although there are both polyaromatic and aliphatic structures in fresh laboratory deposits, the former dominate. High-temperature operation led to deposits an increased polyaromatic content. Industrial deposits are more cokelike than laboratory deposits, and they differ in morphology. 1. Introduction Unwanted deposition of carbonaceous material is a general problem during processing of hydrocarbons. In fluid coking, an important step in the upgrading of bitumen or other heavy hydrocarbons, deposits form as the produced vapors pass through cyclone exit lines at temperatures of about 550 °C. There can be a transfer of heat from these vapors to the cyclone exit tube. Cyclone exit line fouling is a chronic problem for fluid cokers and a key process limitation to achieving longer run times. Similar problems of deposit formation from the vapor phase also occur in cracking of naphtha or other mixed hydrocarbons for olefins production, and in the delayed coking process. To operate equipment most efficiently, knowledge of the mechanisms by which the deposits are formed is essential. In the olefins industry, unwanted coke deposition occurs on the walls of the pyrolysis reactor (coil), typically at 750-1000 °C, and on the cooler or transfer line exchanger (TLE), which is a few hundred degrees lower in temperature. Under pyrolysis conditions formation of coke occurs by three mechanisms:1-4 catalytic reactions, heterogeneous radical reactions, and homogeneous radical reactions. For the transfer line exchangers, less is known about the details of the formation of the deposits, and the events that contribute to TLE fouling are still under debate. By means of 14C-labeled hydrocarbon, and with straight-run naphtha (50-180 °C) as feedstock, Kopinke et al.5 found that the temperature dependence of the deposition rate on coupons favors physical condensation as a key step. In contrast, the results of Zou et al.6 suggested that when the tube wall surface temperature was high (>350 °C), mass transfer and physical condensation could be the controlling step, whereas when the wall surface temperature was low (
