Economic optimization of a thermal cracker via model predictive control technology

An ethylene plant is one of the largest chemical plants and ethylene is industrially obtained through thermal cracking of hydrocarbons. The cracking furnace is the heart of such an installation and the frequent changes in feed mix, quality and prices; and the demand for its olefin products are influencing directly the production efficiency. Each reactant in the feed produces a certain distribution of products and in order to satisfy the demand constraints at the lowest cost; one needs to optimize the amounts of each reactant. A structure for an on-line economic optimizer is proposed, which provides a set of optimal set-points for the furnace. The software package is composed of a number of modules, which consist of a large scale Model Predictive Controller (MPC) and the optimization system, all built in DeltaV Control Studio and using DeltaV PredictPro technology. These powerful control applications are designed to be used with any existing control system, connecting either serially or by OPC. A simplified mathematical model was also implemented using the same real-time control environment, based on the fundamental equilibrium equations: material balance, thermal balance and mechanical energy balance.