(577s) Energy Optimization through Non-Linear Modeling and Control of Fuel Gas Process at Hovensa

Increases in prices of raw crude and utility costs, and stricter environmental regulations are some of the primary drivers for efforts towards energy optimization in refineries. An efficient optimization of a refinery's fuel gas system can provide stability to the fuel gas header pressure and account for variations in fuel gas quality, thereby leading to optimal control of charge heaters and boilers and optimization of the LPG inventories.

Hovensa LLC, with a refining capacity of 495,000 BPD of raw crude, has recently implemented a Fuel Gas Optimizer (FGO^TM). The FGO^TM is an APC platform, developed by BluESP on AspenTech's InfoPlus21 database and DMCplus controllers. FGO is a non-linear dynamic optimizer that simultaneously controls the fuel gas header pressure and the fuel gas quality, in real time. It does this my continuously monitoring the flow and quality of the fuel gas suppliers, as well as the demand from consumers. The models in the DMCPlus controller are updated online in order to mange the system non-linearities. The optimization objective of the system is adjusted to the needs of the refinery ? typically either maximize or minimize fuel gas usage.

As part of the FGO^TM implementation at Hovensa, fuel gas producers (off-gases from the FCC and Coker units, excess Hydrogen, enrichment gases, etc.) were balanced with the consumers of fuel gas (charge heaters, boilers, gas turbines, etc.). FGOTM provided a mechanism to manipulate the fuel gas production from the LPG and propane vaporizers. Additionally, by specifying the amount of fuel gas that can be consumed in each boiler, FGO effected a mechanism for controlling the fuel gas:fuel oil ratio in the fire boilers. Towards providing a real-time measure of the BTU quality of fuel gases, a multivariate model, accounting for the BTU of each producer and consumer stream, was developed by using InfoPlus21. FGO then predicted the quality of the fuel gas, and provided an on-line advisory on manipulating the number of burners on the crude heaters.

An efficient implementation of FGO^TM at Hovensa has brought stability to the fuel gas header pressure, and provided a handle to predict and account for the variations in fuel gas quality, which has led to desirable control of the operations of fire boilers and charge heaters and yielded economically attractive returns on the sale of some of the products.

Keywords: fuel gas, non-linear optimization, pressure stability, BTU quality.