Ethylene dichloride (EDC) is converted to vinyl chloride monomer (VCM), the key building block for polyvinyl chloride (PVC), by energy-intensive thermal cracking. EDC cracking is carried out in furnaces which typically operate at a conversion of 55-60% and have a run length of more than a year. EDC cracking also produces small amounts of impurities such as butadiene, methyl chloride, ethyl chloride etc., whose concentrations need to be maintained below specified values to avoid operational issues in the plant. Coke is another by-product of EDC cracking and its deposition on cracker tube walls leads to increased pressure drop and high tube skin temperatures, with the consequence that eventually the furnace needs to be taken offline for decoking. The paper describes a digital operations approach that involves the development and application of rigorous models for the EDC cracking furnace for monitoring and optimising operation. The furnace model is based on detailed radical-based cracking kinetic mechanism and includes kinetics for coke deposition in the coils. Such a model, once validated against plant data, serves as a valuable tool to predict key performance indicators (KPIs) such as conversion, yields and coking rates. It can also be used for optimisation of the cracker operation to improve the VCM plant economics. The approach has successfully been used in a project with SCG Chemicals, Thailand to model and optimise their EDC crackers. The cracking and coking kinetic models were tuned to plant data and the validated model is then used to perform optimisation of the EDC cracker operation. The optimisation results identify potential for significant savings due to operation at higher conversion, without adversely affecting key impurity yields and run length.
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