Multivariable Control Configurations for Fluidized Catalytic Cracking Unit
A fluidized catalytic cracking (FCC) unit converts heavy oil into a range of lighter hydrocarbon products, including liquefied petroleum gas (LPG), fuel, gas, gasoline, light diesel, aviation kerosene, slurry oil, among which high octane number gasoline is the most valuable. It consists of a riser/reactor and regenerator system, with catalyst used to promote cracking. Due to the large throughput, high product-feed upgrade and commercial importance, the overall economic benefits of a refinery can be considerably increased if proper control and optimization strategies on operating variables (e.g. riser/ regenerator temperature, air/ catalyst flow rate, etc.) are implemented on FCC units. The control problem of FCC units is a challenging task due to its model complexity, non-linear dynamics, constrained variables and cross-coupling interaction between inputs and outputs. The performance of these units depends strongly on the structure of the control loop configuration. The conventional approach to controlling a FCC unit involves control of the reactor temperature and the flue gas oxygen concentration using the catalyst circulation rate and the regenerator airflow rate, respectively. However this scheme tends to be sluggish and exhibits large swings in regenerator temperature.
This paper presents the dynamic simulation of FCC unit highlighting the controlled and manipulated variables which best influence system performance. Relative gain array (RGA) method has been used for finding the best pairing between controlled and manipulated variables. Steady-state gain matrix and response time-constants are provided to obtain a systematic procedure for analysing multivariable control configurations of this unit.