Kinetic Studies of Ethylene Oxychlorination to Ethylene Dichloride and Vinyl Chloride
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Polyvinyl chloride (PVC) is the most versatile of all thermoplastics that can be used in a wide range of applications. It is the third-highest volume polymer, slightly behind polyethylene and polypropylene. Vinyl chloride, C2H3Cl, the monomer of PVC, often named vinyl chloride monomer (VCM) is one of the leading chemicals mainly used for manufacturing PVC, it is mainly produced in two ways based on the source materials used in the process. In some developing countries, which are rich in coal, VCM is primarily produced from acetylene via direct hydrochlorination, in which mercury chloride (HgCl2) is commonly used as the most efficient catalyst. While HgCl2 is extremely toxic, both acutely and as a cumulative poison. What is more, HgCl2 can be reduced by acetylene, and this will lead to the deactivation of the catalyst. More importantly, it can also contaminate the product and cause serious environmental problems. Hutchings and his co-workers proposed the gold catalyst has a promising potential to be a substituted catalyst of HgCl2. Due to the high price of Au catalyst, stability, and limited reserves of gold in the world. There is still a long way to be largely commercialized. Another important process is the balanced VCM process, where Cl2 by first chlorinating ethylene to produce ethylene dichloride (EDC), the EDC is then thermally converted to VCM by dehydrochlorination. The HCl produced in the dehydrochlorination reactor is typically captured and recycled to an oxychlorination reactor to convert C2H4, O2 to EDC, which is again converted to VCM by dehydrochlorination. The balanced VCM is a complex process including three reaction sections, separation, and recycling units. Besides, EDC dehydrochlorination also called EDC cracking, is an energy-intensive process, which is carried out at high temperatures (500−550 °C), high pressures, at a conversion of approximately 50−60% leading to an overall VCM yield of about 50% in a single pass. The control of impurities and coke formation presented the main challenges in the EDC cracking for the VCM production. The impurities such as butadiene and methyl chloride etc. generated by high-temperature radicals’ reactions must be controlled at a very low level due to the requirement of plant operation. Coke formation at the high-temperature cracker tubes is the key parameter influencing the production cost. The complexity of the process drives a search for simplifying the process to produce VCM directly from C2H4, O2, and HCl in a single pass reactor. The ethylene oxychlorination is typically catalyzed by a promoted CuCl2/γ-Al2O3 catalyst at relatively low temperatures (220−250 °C) and 2−6 bar. It is not feasible to integrate the low-temperature oxychlorination and high-temperature EDC cracking together, which loses the stability of the CuCl2 at high temperatures or reduces the activity of EDC dehydrochlorination at low temperatures. Efforts have been devoted to developing new catalysts such as lanthanum (oxy)chloride, and CeO2 with bi-functional of oxychlorination and dehydrochlorination for a high-temperature process to directly synthesize VCM. However, these catalysts are not active at low temperatures typical ethylene oxychlorination conditions around 220−250 °C. Highly active bi-functional catalysts at low temperatures are ultra-desired. In this work, ethylene oxychlorination is studied not only from the fundamental points of view but also explored the new chemistry inside. We will summarize the main conclusions in this thesis from the points of co-promoter effect; kinetic model; γ-Al2O3 support effect; and a new route of production of VCM from ethylene oxychlorination.