Characterization of coke formed in vinyl chloride manufacture

Cokes formed at several locations in vinyl chloride monomer (VCM) manufactu ring have been examined by chemical analysis, scanning electron microscopy/ energy-dispersive X-ray spectroscopy (SEM/EDX), X-ray powder diffraction, M ossbauer spectroscopy, and diffuse reflectance infrared. Based on SEM micro graphs, visual inspection, and chemical analysis, it appears that coke is f ormed by tar droplet formation in the cracking furnace and subsequent conde nsation and impingement on the pipe wall surfaces. Results suggest that cok e is formed by a different mechanism in the 1,2-dichloroethane boiler. SEM results did not reveal the presence of filamentous coke, suggesting that su rface metal-catalyzed coke formation via metal carbide intermediates does n ot 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. Mossbauer data indicate that iron is primarily present as FeCl2. Chlorine is also pr esent throughout the carbon phase of the cokes. The pattern of distribution of iron through the plant suggests that iron present in the coke samples c omes from FeCl2 in the gas phase. Infrared spectra indicate that the cokes have a distinct chemical composition depending on their location in the pla nt.