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Posted by Don Snowden on September 28, 2000 at 14:33:57:
In Reply to: Control of hydrotreater stripper posted by Shashank Goel on September 28, 2000 at 01:06:13:
As Mike McCarty mentioned, it is not obvious what your exact control configuration is. If your SMCA control is only adjusting fuel gas to hold the Duty/Feed ratio, then it is not doing much (i.e this could have been implemented at the regulatory level).
But in any case, a duty controller at the regulatory level is good. I would have the advanced controller adjust the duty to control a minimum amount of separation. You can have a controlled variable which is reflux/feed ratio would work ok. The usual problem with these configurations is that someone has a TIC adjusting the fuel gas to control a lower column (or even reboiler outlet) temperature. When composition swings, then the regulatory controller doesn't put in the right amount of heat to keep the reflux above some minimum requirement to remove all of the H2s.
BTW, I don't understand the setup on your pressure controller and how the offgas supplies some "additional heat".
Hydrotreater controllers can be nice little controllers mainly since it helps Operations out. In addition, in cases where you have reboiler limits (e.g. smothering) they can supply significant benefits in feedrate credits.