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Advanced Process Control for the Refining and Petrochemical Industry
Fluidised Catalytic Cracking (FCC) Unit

Unit Feed(s): Gas oil and other higher boiling point hydrocarbons. This could range from gas oil, deasphalted oil, demetalised oil and atmospheric reduced crude. Due to the significant price differential between the products and the unit feed, a major objective of the controller is to maximise feed to the unit. The feed flowrate, temperature and composition are major disturbances to the unit.

Conversion: The FCC process converts heavy oils into more valuable gasoline and lighter products.The zeolite catalyst is in the form of very fine particles behaving as a fluid when aerated with a vapour.

Products: Products range from high octane gasoline, distillates, fuel oils to olefins for alkylation and polymerisation, LPG and petrochemicals.

Process Interactions: The main source of interaction in the FCC process comes from the fact that the reactor will produce just the amount of coke necessary for the regenerator to burn to satisfy the reactor energy demand to maintain the heat balance. To maximise the high yield octane barrels in the unit, there is interacting variables of combustion and severity. Conversion in the reactor is increased with an increase in the reactor temperature, catalyst activity and catalyst to oil ratio. The increase in temperature beyond a certain point, however, will cause overcracking and hence a reduction in the gasoline yields. On the regenerator side, maximising the regenerator temperature is favoured to acheive total combustion of the coke which will casue an increase in the catalyst circulation rate. To achieve an increase in the unit charge rate, on the other hand, the catalyst circulation rate must be increased by reducing the regenerator temperature (partial combustion). A reduction in the rate of coke burn will result in lower catalyst activity.

Stability: The time to steady state for an FCC unit is in the order of several hours and therefore stability of the unit is crucial. The pressure balance between the reactor and the regenerator is another key factor in maintaing the FCC stability.