Refinery-wide optimization
Abstract
A fuel-oriented refinery converts crude oil into various fuel products which are used for transportation and heating. It also provides feedstock for petrochemical plant. Based on a real refinery in the Gulf coast, a first-principle, nonlinear, plant-wide model was developed by integrating several single-unit models into an overall model. Detailed models were developed for two major units, crude unit and gasoline blending. The crude unit model is a non-stage-by-stage, steady-state model based on material balance and energy balance. The model calculates the yields and properties of the products based on the feed information and product specifications. The gasoline blending model calculates the complete set of gasoline specifications of three grades of gasoline from the information of gasoline blending stocks.
Existing detailed models of fluidized catalytic cracking (FCC) unit and catalytic reformer were used in this work after minor modification. Simplified first-principle models were developed for other units in the refinery. Each single-unit model was benchmarked against the industrial data obtained from the refinery. In the refinery-wide model, the outputs of models of upstream units are used as the inputs to the models of downstream units. The intermediate streams are characterized in the overall model to provide necessary information for the models of downstream units. Detailed composition information of feeds to FCC unit and catalytic reformer is calculated. General properties, volume, weight, and specific gravity are calculated for other intermediate streams.
A constrained nonlinear optimization was carried out using the developed refinery-wide model. The objective of the optimization is to maximize the daily revenue of the whole refinery. The decision variables are the collection of the process variables of each unit that has significant influence on the economy of whole refinery operation. The nonlinear and linear constraints in the optimization are the summation of constraints of each unit. Two operation modes, Summer Mode and Winter Mode, were studied. The optimal solutions obtained from refinery-wide optimization show that the revenue increase over the normal operating conditions is 4.5% for Summer Mode and 3.6%) for Winter Mode. The revenue from refinery-wide optimization is about 1.6% over single-unit optimization.