Page 212 - Fluid Catalytic Cracking Handbook

180 Fluid Catalytic Cracking Handbook

With a host computer framework, the control package is all in the
software. Changing the program can still be agonizing, but the program
can be tested off-line. There is more flexibility in the computer system,
which can be used for many other purposes, including on-line heat
and weight balances.

Disadvantages of Multivariable Modeling and Control
A multivariable model is like a "black box." The constraints go in
and the signals come out. Operators do not trust a system that takes
the unit away from them. Successful installations require good training
and continual communication. The operators must know the intercon-
nections in the system.
The model may need expensive work if changes are made during a
turnaround. If the feed gets outside the range the unit was modeled
for, results can be at best unpredictable. An upset can happen for which
the system was not programmed.
The DCS-based APC is installed in a modular form, meaning operators
can understand what the controlled variable is tied to more easily.
The host computer-based system may have its own problems, includ-
ing computer-to-computer data links.
In any APC, the operators must be educated and brought into it
before they can use it. The control must be properly designed, meaning
the model must be configured and properly "tuned." The operators
should be involved early and all of them should be consulted since
all four shifts may be running the unit differently.

SUMMARY

The only proper method to evaluate the performance of a cat cracker
is by conducting a material and heat balance. One balance will tell
where the unit is; a series of daily or weekly balances will tell where
the unit is going. The heat and weight balance can be used to evaluate
previous changes or predict the result of future changes. As discussed
in the next chapter, material and heat balances are the foundation for
determining the effects of operating variables.
The material balance test run provides a standard and consistent
approach for daily monitoring. It allows for accurate analysis of yields
and trending of unit performance. The reactor effluent can be deter-