FLUIDIZED CATALYTIC CRACKING UNIT (FCCU)

A fluidized catalytic cracking unit (FCCU) is an important process in oil refineries. It upgrades heavy hydrocarbons to lighter more valuable lighter products by cracking, and is the major producer of gasoline in refineries. A simplified process schematic and instrumentation diagram is shown in the figure below.



Feed oil is contacted with recirculating catalyst and reacted in a riser tube. The feed oil vaporizes and is cracked as it flows up the riser, thus forming lighter hydrocarbons (the gasoline fraction). Large amounts of coke are formed as a byproduct. The coke deposits on the catalyst and reduces its activity. The lighter hydrocarbon products are separated from the spent catalyst in the 'reactor' . The 'reactor' is in reality just a separator (with staged cyclones), but retains its name for historical reasons. Steam is supplied to strip volatile hydrocarbons from the catalyst (not shown in the diagram). The catalyst is then returned to the regenerator, where the coke is burnt off in contact with air. This is usually done by partial combustion, though some FCCUs are operated in a complete combustion mode. The regenerated catalyst is then recirculated back to mix with the inlet feed oil from the crude unit.

FCCUs present challenging multivariable control problems. The selection of good inputs (manipulated variables) and outputs (measured variables) is an important issue, as is the pairing of chosen controlled and manipulated variables for decentralized control. In this case, the important measured variables are chosen to be the reactor temperature/riser outlet temperature (T1), the regenerator gas temperature (Tcy) and the regenerator bed temperature (Trg). The manipulated variables are the catalyst recirculation rate (Fs) and the regenerator air rate (Fa).

You are required to find transfer function models relating the given outputs to the inputs. Lee and Weekman (1976) and Grosdidier et al.(1993) discuss issues in the modeling and control of FCCU units, are good starting points for further investigation. Important issues to take notice of in each study are the selection of manipulated and controlled variables, the mode of combustion - partial/complete (the dynamics of each mode are different), and the presence of (unknown) disturbance inputs. You will later design and test a multivariable controller for the process.

Information on the operation of FCCUs and petroleum refineries in general may be obtained in the Oil and Gas Journal, and Hydrocarbon Processing.

References:

Lee, W., and V.W. Weekman, "Advanced Control Practice in the Chemical Process Industry: A View from Industry", AIChE J., 22, 27 (1976).

Grosdidier, P., A. Mason, A. Aitolahti, P. Heinonen, and V. Vanhamaki, "FCC Unit Reactor-Regenerator Control", Computers Chem. Eng., 17, 165 (1993).