The effects of sulfides and sulfur/phosphorus‐containing compounds on coke formation during thermal cracking of light naphtha

Abstract

AbstractIn 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.