Abstract
Cokes formed at several locations in vinyl chloride monomer (VCM) manufacturing have been examined by chemical analysis, scanning electron microscopy/energy-dispersive X-ray spectroscopy (SEM/EDX), X-ray powder diffraction, Mössbauer spectroscopy, and diffuse reflectance infrared. Based on SEM micrographs, visual inspection, and chemical analysis, it appears that coke is formed by tar droplet formation in the cracking furnace and subsequent condensation and impingement on the pipe wall surfaces. Results suggest that coke is formed by a different mechanism in the 1,2-dichloroethane boiler. SEM results did not reveal the presence of filamentous coke, suggesting that surface metal-catalyzed coke formation via metal carbide intermediates does not occur. Metal surfaces become chlorided which may inhibit coke formation via carbide intermediates. Chemical analysis shows the presence of metals (Fe, Cr, and Ni), predominantly iron, throughout the coke samples. Mössbauer data indicate that iron is primarily present as FeCl2. Chlorine is also present throughout the carbon phase of the cokes. The pattern of distribution of iron through the plant suggests that iron present in the coke samples comes from FeCl2 in the gas phase. Infrared spectra indicate that the cokes have a distinct chemical composition depending on their location in the plant.
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