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).

The simulink block for the FCCU obtainable from cpc_fcc.mdl is to be used for development of transfer is to be used for development

of transfer function models between the outputs and the inputs of

the process. The specifications for the process are given below:

Inputs (Manipulated variables)

Regenerated catalyst feed rate (Fs) : steady state value = 294

kg/s

Air flow rate (Fa) : steady state value = 25.35 kg/s

Measured Outputs

Riser outlet temperature (T1) : steady state value = 776.9 K

Regenerator cyclone temperature (Tcy) : steady state value =

988.1 K

Regenerator bed temperature (Trg) : steady state value = 965.4 K

You should attempt to develop transfer functions between each of

the inputs and the outputs, i.e. six transfer functions in all. The

problem of control, which will be dealt with at a later stage, will

however involve the control of two outputs by manipulating the

two inputs. Suggested outputs for control are T1 and Tcy. Trg

should also be monitored during the control phase of the project.

You will develop transfer functions by running step tests, one input

at a time, and observing the output responses. You will then

attempt to fit the observed response to a transfer function model,

and comment on the goodness of fit. Note that the step inputs in

the simulink file have initial values set at steady state values, so

that you will have to input the final values of the steps in physical

variables (and not deviation variables). This is because the

simulink block has been set up to accept inputs in physical

variables. Outputs are also available as physical variables.

The simulink block has measurement noise built into it, so that you

will obtain a noisy response to your step inputs. In order to

maintain a reasonable ratio of the true value to the magnitude of

the noise for the output responses, it is recommended that you

use a step of magnitude 5 kg/s for the input Fs (regenerated

catalyst flow rate) and a step of magnitude 0.1 kg/s for the input

Fa (air flow rate).

Submissions for this stage of the project must include the transfer

functions you develop, and plots of all the output step responses.

Be sure to specify the magnitude of the step in the input

(especially if it is not a unit step) for each case. Attach a memo

with your work consisting of a description of the procedure you

followed for model development, and a discussion of your results.

You may also include a discussion on any other issues you consider

important.

Develop a MV control scheme for this Process. You can chose either a decoupling design using individual PID's with an appropriate Relative Gain Array analysis, or you may elect to convert the MV system to a State Space(SS) Model using Matlab and design a SS controller using the techniques learnt in class.