Petroleum coke sold for fuel may have a high VCM; coke sold for calcining must have a low (10%—13%) VCM. Making coke too soft (high VCM) leads to potential foamover problems and increased coke yields at the expense of more valuable gas oil. Hard coke (low VCM) takes longer to cut.

The VCM of coke is primarily a function of coke drum overhead vapor temperature. Many refiners attempt to control coke VCM by monitoring heater-outlet (i.e., transfer-line) temperature. This is incorrect because the temperature at which the coking reaction takes place is a function of the vapor-line temperature. A very high transfer-line temperature (930°F) will not make hard coke if the drum insulation has deteriorated and the vapor line is only running at 770°F.

A one-unit (1 %) change in VCM can be effected by changing the over­head vapor temperature by 7° F. This rule of thumb holds in the range of 10%-15% coke VCM. The vapor-line temperature is increased by dropping heavy gas oil into the heater charge or by raising the heater outlet tempera­ture

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