(56e) Evaluation of the Pid Performance for Fcc Units

Catalytic cracking are breaking reactions high mass carbon chain molecules, such as gasoils and petrol refining residues. Particularly, in Brazil's case, the development importance of such a process is reinforced by increasing heavier oil reservoir discoveries. Any conversion efficiency upgrade for heavier feedstocks, allied to large production, might represent a significant gain. This process promotes chain breaking, yielding lighter hydrocarbons, with higher commercial value, as gasoline and nafta. Catalytic cracking also we produces coke, which consists primely of noncracked carbon chains, heavy metals and aromatic compounds with characteristics near to graphite, and is the responsible for catalyst deactivation. The catalytic cracking process occurs in equipments called converters. A FCC (Fluid Catalytic Cracking) converter is composed basically by a riser, a separating vassel/stripper and a regenerator. This work has as objective the evaluation of the PID controllers on FCC units, for drastic changes on operating conditions as well as on the feed conditions. The FCC plant is represented by deterministice mathematic model using the mass and energy balances the model was validated with data from an existing refinery. Extensive simulations were carried out to verify the prediction and compared with real values in the refining process. Perturbations were done in the system to verify the response of the variables controlled by two distinct cases. In the first case, it was applied a variation on the air temperature for the regenerator. In the second, on the feeding of the load, giving a upgrade of 10% on the initial values. These variables were elected because they interfere directly in the behavior of the conversor, more specifically in the catalytic cracking reaction and in its severity. The variables analysed in all test were: temperature in the exit of the riser (or reaction temperature); first stage dense phase temperature of regeneration; second stage dense phase temperature of regeneration; second stage of the diluted phase temperature in the exit of the regenerator; diluted general phase exit temperature and reaction severity. The circuit operates in a closed loop controlled by a PID controller in the control loops: the pressure of the regenerator is controlled by manipulating the flow rate of the effluent gas, the control of the pressure of the compressor is done by controlling the flow rate, the level of the deactivated stream bed of the regenerator is controlled by the valve of flow rate of the deactivated catalyst that comes from the reactor, and manipulates the flow rate of residues feed. It can be concluded that the PID controller may be a simple and useful way to maintain the FCC units working at desired conditions even for changes in the feedstock as well as in the operating conditions. Whoever care has to be taken with the controller parameters and a extensive discussion on the performance of several funning methods is presented.