Fighting Long Coking Cycles

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How long should it take to turn a coke drum around? The elapsed time from when one switches out of a full drum until it is empty and ready to receive resid again is called the cycle time. Most delayed coking units are designed for a 24-hour cycle time. Shorter cycles can be used in refineries where coke drum volume limits capacity. Each of the steps detailed in the Troubleshooting Checklist following this chapter can be contracted to save time.

The decided trend toward poor-quality crudes means that the tons of coke produced per barrel of resid are, on the average, increasing. Coke-drum size is therefore a more frequent bottleneck than is the capacity of the heater, combination tower, or wet gas compressor. With this in mind, a common assignment for an engineer is to determine how the operating crew can reduce cycle time.

As a basis for discussion, the details of operating on a 12-hour coke drum cycle will be reviewed. The discussion that follows applies to a 20-ft diameter coke drum producing about 1,000 T/D of anode-grade coke. The volatile combustible matter averages 1 2%. In really large-diameter coke drums (26 ft in diameter) producing hard coke with 8%-10% VCM, it is unlikely that a 12-hour coke drum filling cycle could ever be obtained. However, 16-hour cycles in 27-ft drums have become a common industry practice.

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