Having a lower temperature (~400 F) external coker feed day tank is essential for reliable operations. High surge volume in the fractionator bottom section increases the residence time for undesirable condensation (coking) reactions.
Similarly, the upstream vacuum distillation tower benefits from low bottom liquid residence time. In the bottom of the coker fractionator, the process objective is good mixing of coker feed and condensed natural recycle. Nominal (5 minutes) residence time (based on fresh feed) is more than adequate. Without an upstream coker feed storage tank (typically 12 hrs holdup at 50% liquid level), the coker fractionator must provide surge volume for charge pump protection. Not a good situation.