October 1, 2019 at 4:45 pm #30787
can someone please explain or know any literatures explaining the chemistry behind recycling naphtha/LCGO (distillate recycle)? How does the yield shift occur? By lower the partial pressure? increasing concentration of products in recycle boiling range, suppressing further formation? help to minimize cracking?
I just couldn’t find any good explanation regarding the chemistry (to molecular level).
thanks for your help
October 3, 2019 at 2:39 pm #30789
Distillate recycle was developed by Conoco and is part of the licensed Delayed Coker from Bechtel Hydrocarbon Technology Solutions. I think the patents on this technology have expired, but the details remain a trade secret, which is why I believe there is not much available to explain this technology in any detail.
Distillate recycle does a number of things; 1) it dilutes the resid with hydrocarbons that do not have coke precursors which lowers the probability of laying down coke in the heater and allows a lower velocity steam rate as there is velocity hydrocarbon in the feed, 2) inclusion of this distillate into the feed increases the coke drum outlet temperature and the higher average drum temperature reduces the coke yield and increases the liquid product yield, 3) the concentration of distillate in the reaction mass is higher so the reaction rate of the thermal decomposition reactions to convert that boiling range material into other hydrocarbons are higher, 4) the distillate recycle increases the vapor velocity in the coke drum so that has to be taken into consideration when evaluating an increase in throughput, 5) similarly, the distillate recycle requires heater duty and can become a constraint on increased throughput.
The yield shift occurs as the material that is recycled goes through additional thermal decomposition into other hydrocarbons. For hydrocarbons boiling at temperatures less than 650 F, the products are smaller hydrocarbons. These lighter hydrocarbons carry heat through the coke drum and allow increased concentrations of heavy hydrocarbons to be in the vapors leaving the coke drum at the same temperature and pressure than if there was no distillate recycle. These heavy hydrocarbons have coke precursors in them that are not converted to coke in the coke drum which lowers the coke yield.
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