Delayed Coking is a semi-batch process that requires switching the feed from one drum to remove the coke formed in the on-line drum and to prepare the off-line drum for service. Approximately 40% of the vapors from the coking drum are diverted to the off-line drum to warm it up prior to routing feed into that drum. This will decrease the amount of liquid products produced in the fractionator by the same ratio. Roughly 75% of those vapors are condensed and routed to the coke drum condensate system. Typically, those condensed liquids are routed back to the fractionator where they vaporize and fractionate out into their normal products. For a 2-drum Coker, this means that the flow rate of products at a fixed quality will vary by +/- 30%. This is a fundamental consequence of the technology. A 4-drum coker will see a smaller variation in the total products from the fractionator as there are two coking drums routed to the fractionator and only one drum is warmed at a time.
If the flow rates of the products were to be kept constant, the quality of the products in terms of distillation, density, sulfur and nitrogen content would all vary with time. Fixed quality is a more desirable trait than a fixed flow rate.
