(123b) Optimising EDC Cracker Operation to Enhance VCM Plant Economics

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 approximately 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 needs 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. Even though EDC conversion, yield of VCM and impurities and coking rates are key for profitable plant operation, these are not normally measured directly because of the challenging physical environment.

The paper describes the development of rigorous models for the EDC cracking furnace and their use in 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 a Thai petrochemical company 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 dynamic optimisation of the EDC cracker. The optimisation results identify potential for significant savings due to operation at higher conversion, without adversely affecting key impurity yields and run length.