Modelling and control of a riser type fluid catalytic cracking (FCC) unit

A model for a fluid catalytic cracking (FCC) unit which describes the dynamic behaviour of the riser, particle separator vessel, and the regenerator is developed. The model consists of coupled ordinary differential equations. This facilitates the solution of the equations and makes the model particularly suitable for control studies. A sensitivity study is carried out to determine the interactions between the three controlled and manipulated variables and the elements of the Bristol(1) relative gain array matrix. The relative gain array analysis suggested that the temperature at the top of the riser, the pressure drop between the particle separator vessel and the regenerator, and the catalyst holdup in the particle separator vessel should be controlled by manipulation of the flow rates of the regenerated catalyst, flue gas from the regenerator, and the spent catalyst leaving the particle separator vessel, respectively. Three PI-controllers were used to achieve reasonable control of the process.