A recent article in RefinerLink.com, 7 Tips to Maximize Diesel Production, provides suggestions to help refiners maximize diesel production. They focus on areas to investigate to eliminate product quality giveaway and therefore assure that all material that can be sold as diesel is. This strategy makes sense whenever the alternate disposition for these incremental materials is worth less that than diesel, which is often the case nowadays.
One suggestion is to maximize T90 up to the final product specification. In other words, don’t drop virgin diesel to cracking feed. The logic behind this is simple: the FCCU will only convert diesel to LCO (a lower value form of diesel) and the hydrocracker will convert it to low-octane gasoline. Is this logic always true? Usually it is, but even when diesel price outpaces gasoline, it may still be profitable to let a little heavy diesel into the FCCU feed.
The key considerations here are the nature of the other FCC feeds and the regenerator temperatures (and catalyst-to-oil ratios) that result. Refiners cracking heavy feeds may find that adding a little light material will lower regenerator temperature and increase cat-to-oil, resulting in improved operability and conversion. The overall economic effect of improving the conversion of the non-diesel components of the FCC feed might well outweigh the economic impact of downgrading diesel to LCO.
Of course, many refiners suffer from the opposite problem. When processing severely hydrotreated or other low delta coke feeds, low regenerator temperatures and excessive cat-to-oil can be the problem. In such cases, getting every last bit of diesel out of the feed will have significant benefits in addition to the obvious elimination of product downgrade.
Rules of thumb are useful in helping to determine which variables are worthy of attention, but a complete analysis of the situation is always warranted.