The first step is the reactor outlet temperature needs to be increased to a
minimum temperature of 505 C (940F). I would actually increase the reactor
temperature to 510C (950F), and to 520C (970F) if possible (based upon other
operating/yields constraints and conditions).
At the current temperature of 496 C (925F), the ATB portion will not vaporize.
Cracking is a vapor phase reaction and therefore the ATB will not crack/convert.
Instead it will go unvaporized/uncracked into the reactor stripper and then pass into
the regenerator as “liquid coke”, quickly heating the regenerator. From the heat balance,
this will slow the catalyst circulation, which lowers the cat-to-oil, and thus reduces conversion.
Additionally, some of the unvaporized ATB (a “aerosol mist” of ATB) will attach to/coke onto
the cyclone outlet tubes in the low pressure region. During thermal upsets, this coke will spall
(flake-off) and plug the diplegs. This leads to large (sometimes catastrophic) catalyst losses
from the reactor to the main fractionator.
Additionally, ATB usually contains higher levels of contaminant metals (specifically Ni, V, Fe, Na)
than the gas oil feedstock. In order to control catalyst activity and the negative effects/impacts of the
contaminant metals, catalyst addition rate must be increased. Furthermore, a change in catalyst
formulation is usually required to incorporate more matrix, the matrix component that is responsible for
cracking/upgrading the heavier feed.
Finally, depending on the nature of the ATB, there may be other negative effects such as increased sulphur
(sulphur will increase environmental emissions or in the case of a flue gas scrubber, require higher usage of
caustic (NaOH), and high sulphur in the liquid products which will require treating prior to sales) and
nitrogen (nitrogen will temporarily poison the catalyst activity and reduce conversion. The nitrogen also combines
with any chloride present in the feed and leads to Ammonium Chloride salts which will deposit and plug the top
portion of the mainn fractionator and its overhead system).
Your query is very good but it is also very complex. These are just some of the highlights (or low-lights) for
processing resid in an FCCU not specifically designed for resid.
Please feel free to contact me if you would like to discuss the issue in greater detail.