In this study, coke deposits during thermal cracking of light naphtha in the presence of sulfur‐ and sulfur/phosphorous‐containing compounds with different addition methods were investigated from the points of morphology and structure. Sulfur/phosphorous‐containing compounds were applied by continuous addition, pretreatment, and pretreatment followed by continuous addition. As for continuous addition, the amount of coke was decreased with increasing the mass concentration of sulfides in short‐term cracking periods. Catalytic coke was inhibited because of passivating the metal by sulfides at the initial stage of coking process. When phosphides were combined with the mixture of sulfides, the coke formation was further decreased as the synergistic effects of adsorption of sulfur and phosphorous onto the metal led to the decreased activity of metal surface. In the case of pretreatment with sulfur/phosphorous, the reduction in coke formation at the initial stage of cracking process was due to the adsorption of S/P‐containing radicals on the oxide film. In further cracking operation, an enrichment of Fe and Ni in the oxide layer from the pretreatment process leads to the appearance of coke filaments in coke layers. The combined addition method, the surface pretreatment with dimethyl disulfide (DMDS)/triphenyl phosphite (TPPI) followed by continuous addition of sulfides/TPPI in the feed, shows the best coking inhibition performance. The inhibition rate is up to 88.8% and 78.5% respectively when the cracking time is 1 and 3 h. The combined application strengthened the coverage of catalytic activity sites by sulfur/phosphorous‐containing radicals. The scanning electron microscope (SEM) results showed that the structural characteristics of coke deposit at the applied conditions were mainly amorphous coke. Variant coke filaments were also observed at the conditions of pretreatment and pretreatment followed by continuous addition. The analyses of Raman spectra indicated that the application methods decreased the graphitization degree of coke deposited and increased the structure defects of the coke matrix. During naphtha cracking, sulfur/phosphorous‐containing compounds reduced dehydrogenation and condensation by which hydrocarbons were degraded to coke.
Coke formation inside radiant coils is one of the main problems during thermal cracking of hydrocarbons. The on-line preparation of the coating for coking inhibition is a promising technology because it provides more flexibility to the operators on site. The SiO 2 /S coating was prepared on the inner surface of coils in an 8-year-served GK-VI industrial cracking furnace. The effects of the coating preparation process on the operation of TLE were studied. The coking rates of the tube with and without coating preparation were evaluated by the trend change of tube metal temperature. Simulations of the coating deposition process were further carried out using the computational fluid dynamics approach. The results showed that a significant temperature increase at the outlets of TLEs during coating preparation were due to the accumulation of SiO 2 and S in a loose form under the TLE operating conditions when the concentration of coating precursors was 7500 ppm (wt. %). In the three tests, coating precursors were mainly completely consumed in tubes and TLEs. For the coated tube, the run time was extended by 4-7 days because the catalytic coking was decreased. No significant changes in the distribution of products and molar yields of main products were observed. In the simulations, it was found that increasing the inlet flow rate led to a more uniform thickness and improved the mass content of sulfur in the coating. In the tube bend section, circumferential nonuniformities for the deposition were due to circumferential differences in the temperatures and mass fractions. The mass fraction of S in the coating was within the range of 0.02%-0.1%. The control step for the SiO 2 /S coating deposition was kinetic. Based on the simulation results, the optimized coating preparation parameters were determined, i.e., the inlet flow rate of 15t/h, the outlet temperature of 1093K and the inlet mass concentration of 3000 ppm (wt. %).
CeO2-based materials are widely applied in a three-way catalyst framework for catalytic combustion of diesel soot. For the application of CeO2 in coking inhibition during thermal cracking of hydrocarbons, SiO2–CeO2 and SiO2 coatings were prepared on HP40 alloy samples through sol–gel dip coating. The coatings were then characterized in terms of morphology and microstructure perspectives, and their anticoking performances were evaluated under conditions relevant to the practical applications during thermal cracking of naphtha. Results showed that the addition of ceria could improve the quality of the silica coating, which subsequently presented better anticoking properties by preventing formation of catalytic coking. Furthermore, thermogravimetric analysis (TGA) results of coke samples revealed that the ceria in the coating had the potential function as a coke oxidation catalyst. However, the silica–ceria coating lost its coking resistance after approximately ten repetitious coking/decoking cycles. Nevertheless, the combined application of anticoking coatings and addition of dimethyl disulfide (DMDS) into the naphtha feed could mitigate risks of coke formation more effectively compared with pure coatings only because of its preferential adsorption of sulfur-based radicals at the coating defects. In particular, the application of the SiO2–CeO2 coating combined with the addition of DMDS could still maintain a stable anticoking rate of above 60% throughout three cycles of coking/decoking operations.
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