(582ac) Optimal Control of a Fluid Catalytic Cracking Unit | AIChE

(582ac) Optimal Control of a Fluid Catalytic Cracking Unit

Authors 

Ortiz-Arroyo, A. - Presenter, Universidad Autonoma de Tlaxcala
Castro, A., Universidad Autónoma de Tlaxcala
Pérez, F., Universidad Autónoma de Tlaxcala
Fluid Catalytic Cracking (FCC) is the most important refinery process in petroleum industry. Feedstock of low economic value due to high molecular weight is transformed to high value gasoline, naphtha, LPG and other products. In the fluid catalytic cracking process heavy long chain hydrocarbons are decomposed into shorter chain molecules in a zeolite used as catalyst.

Oil from the reduced viscosity unit, vacuum fractionators or a mixture is heated and steam sprayed into the reactor which is a vertical straight tubular reactor called the riser, small sized catalyst at high temperature is introduced also in the reactor. Almost all reactions take place in the first length section of the reactor lowering its temperature as most reactions are endothermic. In the last part of the reactor, the reactor have a 180o turn, entering into the disengager where deactivated by coke catalyst and gas products are separated in a cyclone system.

Hydrocarbon reactions form coke in the surface of the catalyst and when separated is fluidized into a valve which connects the disengager with the regenerator in which the coke, composed mainly by carbon is burned to CO and CO2 thus recovering its activity and feed into the riser again to form a continuous cycle.

Gasoline and naphtha have a better economic value while LPG and heavy naphtha must still be processed. Then obtaining the good process conditions and feedstock composition are very important to improve the economic profit of the process.

In this work the process is simulated inspired in the work of Santos, 2000 which is a set of ordinary differential equation including a 10 lump kinetic scheme, mass and energy balances on the riser, disengager and regenerator , non linear valves and added equations for air blowers and wet gas compressor.

The set comprised 21 differential algebraic equations and 64 algebraic equations.

In the operation of an FCC the process variables easier to change by operator are the feed temperature and the feed mass flow.

Optimal control is used to determine trajectories of control for a system expressed as a set of ordinary differential equations over a period of time to minimize a performance index. In the FCC problem it is intended to determine the trajectories of temperature and pressure that must be followed so the composition of gasoline is improved.

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