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.

A laboratory quartz tube reactor apparatus was used to study coke and byproduct formation in 1,2-dichloroethane, also known as ethylene dichloride (EDC), pyrolysis. The effects of metal coupons, feed purity, and temperature on the amount of coke deposited and exit gas-phase compositions were analyzed. Coke formation on nickel, chromium, iron, and stainless steel metal coupons was investigated by scanning electron microscopy/energy-dispersive X-ray spectroscopy and ion-coupled plasma atomic adsorption spectroscopy. Based on scanning electron micrographs, different metals have little effect on the nature of coke formed, and it appears that coke is formed by tar droplet formation in the gas phase with subsequent impingement on surfaces. FeCl 2 formation and migration along the reactor accompanied coke formation on the metal coupons. It was determined that the presence of metals coupons increases EDC conversion as well as coke formation. Two distinct types of coke are formed: hard coke is formed in the hot zone of the reactor and soft coke is formed at the exit. Increases in reaction temperature, CCl 4 in the feed, and presence of FeCl 2 all increase both types of coke formation, but hard coke in particular. A total of 0.3 wt % Cl 2 in the feed resulted in 60% EDC conversion at 380 °C. This also reduced by more then half the total amount of hard coke formed, as compared to the lowest amount of hard coke formed by all other runs, but increased the total byproduct formation. EDC feed obtained from a commercial vinyl chloride monomer manufacturing plant produced 28% EDC conversion at 480 °C and large amounts of coke. Chloroprene was the only chemical species that strongly correlated with total coke formation.