2 replies to this topic

[d2205]

Hello,

i'm working part time in a refinery. trying to figure out how the recycle ratio of delayed coking unit's (DCU) affects the process. from what i've read, increasing the recycle ratio not only increases the production of light distillates and lowers the heavy distillate production, but also lowers the inclination of premature coking in the furnace (by increasing pressure loss and thus residence time in furnace). but i've also been told that the new built DCUs have less recycle ratio (around 1.05). so my question would be, if a DCU production and operation could fare better with increased recycle rate, why the new ones are trying to decrease it?

Thanks in advance!

[shubh]

See if you can make sense of this

Recycle has a dramatic effect on product yields. Ghosal (1988) demonstrated that
increasing recycle increased the yield of all products except the HGO. DeBiase and Elliott
(1982) stated that increasing recycle raises coke and gas yields while the yield of C5+
liquids decreases. The coke yield of a 20 wt% CCR feed can vary between 21 and 45
wt% depending on the amount of recycle (Nelson, 1974).
Recycle also affects the quality of the coke and HGO. Higher recycle improves the
coke quality. This may be due to reducing the concentration of the more reactive species
(Heck et al., 1992). Typical practice is to adjust the recycle to control the HGO endpoint.
As recycle is reduced, the endpoint of the HGO increases. This is accompanied by an
increase in sulfiir, oxygen, nitrogen, metals, and CCR (Colin et al., 1988). If the
downstream units can handle this, coker fractionators in fiiel grade service are being
designed for "ultra low recycle." Part of this is segregating the bottom of the fractionator
to reduce condensation (Elliott, 1991).

Refer to this

[Bharat Abhyankar]

Hello d2205, Interesting question !

Recycle Ratio is decided by following:
1) Reprocessing of feed (Recycle) consumes 1.Unit capacity 2. Energy to reprocess.
2) High recycle ration not only increases light distillate yield but also increases Coke Yield.
3) Price Differential between heavy distillates and coke is so high that for a refiner "Economics" rules and prefers lower recycle ratio.

It will be an interesting case if by increasing recycle ratio type of coke made changes (Shot to Anode to Needle) and indeed rules the economics.
I would be glad if you can quote any publication or reference that describes "Effect of Recycle Ratio on premature coking in the furnace".