Posted by Bob Ellis on November 20, 2002 at 14:19:07:
In Reply to: FCC Reactor Temperature posted by Graeme Taylor on November 19, 2002 at 14:57:47:
Graeme:
It sounds like you understand the pros and cons of breaking the regulatory RX Temp TIC for the purposes of APC in a full vs partial burn FCC unit. This is a good topic that should generate a healthy discussion.
For a full-burn FCC I would try to leave the RX Temp TIC in regulatory control as long as the regulatory control response is as snappy as you describe. You will benefit from the regulatory feed disturbance rejection and should be able to identify the RCSV DP and OP, and other CV models with less difficulty. Since every unit is different, you really need to make a few healthy test moves in the RX Temperature to ensure you are able to identify clean RCSV DP and OP models.
A compelling situation where you should consider moving the SV directly in a full-burn unit is if the dynamics are dominated by the effect of increase/decrease of the delta coke-on-catalyst. In this situation you would see the “fast dynamics” as you described followed by slower dynamics, possibly starting 45-90 minutes later (?), and lining out after a much longer period of time (depending on cat circulation rate, cat inventory, feed type, etc). In this case you would probably not have the good regulatory RX Temperature control that you described. It would also be more difficult to identify the important RCSV DP and OP constraints. Again, the importance of making a few test moves and identifying good RCSV responses during the pretest cannot be overemphasized.
In the 10 or so full-burn units I have worked on, the RX Temp TIC has been a MV, and the controller was able to control against the SV constraints. I can think of one full-burn case where the engineers moved the RCSV directly, for the reasons described above (and maybe additional reasons). I am sure they will comment. I would bet 10-20% of the full-burn units would fall into this category.
Good luck.