Carburization of Ethylene Pyrolysis Furnace Tube in a Petrochemical Plant

Carburization phenomena of the ethylene pyrolysis furnace tube made of 25Cr35NiNb alloy were investigated after service for approximate 30,000 h in a petrochemical plant. Three different zones, namely, oxidation zone, carburization zone, and aging product zone, were observed in the cross section of furnace tube by microscopic analysis. The oxidation zone near the inner surface has a thickness of about 700 μm and possesses the characterization of many loose pores and voids. The inner surface has a continuous thin Cr2O3 layer and the grain boundaries near the inner wall are mainly composed of mixed oxides of Cr2O3 and SiO2. In addition, segregation of impurities S and P was detected at grain boundaries. The dark gray SiO2 distributes in the front of the oxidation zone. Carbides distribute at grain boundaries in the carburization zone with lots of voids and micro cracks. The width of grain boundary is broadened. According to the experimental results, the initiation of cracks may be promoted by carbides at grain boundaries. The uneven distribution of the Cr element was found in the carburization zone, and direct experimental evidence of Cr element diffusion was detected. In the aging product zone, the carbides evolution was observed. The chromium carbides are composed of Cr7C3 and Cr23C6. The microstructure and composition evolutions of the furnace tube and their effects on service life of ethylene pyrolysis furnace tube were also discussed.

Carburization of Ethylene Pyrolysis Furnace Tube in a Petrochemical Plant

Carburization phenomena of the ethylene pyrolysis furnace tube made of 25Cr35NiNb alloy was investigated after service for approximate 30,000h in a petrochemical plant. Three different zones, namely oxidation zone, carburization zone and aging product zone, were observed in the cross section of furnace tube by microscopic analysis. The oxidation zone near the inner surface has a thickness of about 700μm and possesses the characterization of many loose pores and voids. The inner surface has a continuous thin Cr2O3 layer and the grain boundaries near the inner wall are mainly composed of mixed oxides of Cr2O3 and SiO2. In addition, segregation of impurities S and P were detected at grain boundaries. The dark grey SiO2 distributes in front of the oxidation zone. Carbides distribute at grain boundaries in the carburization zone with lots of voids and micro cracks. The width of grain boundary is broadened. According to the experimental results, the initiation of cracks may be promoted by carbides at grain boundaries. The uneven distribution of the Cr element was found in the carburization zone, and direct experimental evidence of Cr element diffusion was detected. In the aging product zone, the carbides evolution was observed. The chromium carbides are composed of Cr7C3 and Cr23C6. The microstructure and composition evolutions of the furnace tube and their effects on service life of ethylene pyrolysis furnace tube were also discussed in this paper.

Study on Optimal Reinjection Strategy of Geothermal Field: A Case Study from Tianjin, North China

Based on general characteristics of the Wumishan reservoir, the hydrogeology model is founded. And the problem of no more space for adding reinjection wells, the mathematical descriptions and influent of hydrothermal transportation, thermal break-through and the flow feed path with the different fracture zone were emphasied. According to simulat ion of the different models, a new ideal strategy about the reinjection wells was put forward, which can be set in the periphery of geothermal zone. And more heat can be extracted from the reservoir rocks through the reinjection activities. The pressure and temperature field of the reservoir will be controlled even though the injected cool water moves forward to the product zone, the water level and temperature field can be more stable with the reinjection models.