Increasing the recycle ratio is supposed to stop shot coke formation. However, in some cases, increased recycle ratio has no effect. Why?
Whether or not increased recycle ratio reduces shot coke formation is dependent on the composition of the incremental recycle.
Recycling 950°F-1,000°F coker gas oil will increase the formation of needle coke.
Recycling 600°F-700°F coker gas oil will reduce the formation of needle coke by raising the coke drum outlet vapor temperature. Heavy recycle is necessary to effectively suppress shot coke formation.
An additional area of concern relates to the quality of the coke produced from hydrotreated vacuum resid feeds. For a variety of reasons, premature furnace fouling, inclusion of semi-coked liquids in the quenched coke drum, and carry-over of coke fines into the fractionator, the delayed coking of 100% hydrotreated feed has not been successfully achieved in commercial operations.
Attempts lo do so have resulted in the production of high VCM coke and/or commingling of hydrotreated and virgin coker feedstocks.
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