Residence time bottom sump

Providing adequate residence time is a primary consideration in arrangements that avoid vapor in liquid outlets. Such arrangements t )-ically include bottom sumps, chimney trays, external side-drawoff drums, or surge drums. Sufficient residence time must be provided in the liquid-drawoff sump for one or more reasons ... 

Guidelines for sump residence time are scarce in the published literature. Those given by Wheeler (420) for chimney trays are the only detailed list available (Table 4.1). Wheeler (420) did not state the residence time definition on which his guidelines are based. The author believes that their application with the above definition is reasonable for chimney trays, bottom sumps, and side-draw drums. Caution is needed when these guidelines are applied, particularly when the main consideration is to buffer upsets. 

In services where the feed consists mainly of lights and a small amount of residue, in order to avoid residue accumulation in the reboiler loop and thickening of material due to excessive residence times in the bottom sump. 

They maximize bottom sump residence time when vapor disengagement is the main consideration. 

Column bottom sumps must provide sufficient residence time for vapor disentrainment (see guideline 4 for the only exception to this rule). Inadequate vapor disentrainment may lead to cavitation of the bottom pump or choking of the bottom line. 

When reboiler circulation is much larger than bottom flow rate, baflles can cause unstable bottom flow. Small variations in the reboiler heat input or column flow rates show up as large changes in reboiler sump overflow, and therefore in bottom sump level. Bottom level fluctuations cause bottom flow fluctuations, which may be troublesome if the bottom flow is the feed to another column or is used to preheat column feed. This problem is illustrated elsewhere (258) and is most severe when the bottom sump compartment is relatively small. Providing additional residence time in the bottom sump can minimize this problem. 

Additional column skirt requirement (If bottoms is not pumped) Additional column skirt height required Additional column skirt height required but less than in vertical thermosiphon Small Small, but can be large if NPSH requirement is high or if residence time in column sump is insufficient for vapor disengagement Small... 

Bottom product surge. The liquid draw compartment of kettle reboilers is much smaller than most column bottom sumps, and usually provides less liquid residence time and product surge. It is often impractical to incorporate the desired residence time (Sec. 4.4) in this draw compartment, and one needs to either live with the lower residence time or add a surge drum downstream of the reboiler. 

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