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Column riser area

Q: I am working on a refinery project with a column internals vendor for designs and review of designs. While reviewing one of the columns (a Hydrocracker unit - fractionator), I observed that the total draw-off trays are designed with a riser cross sectional area of approximately 11.5%, which is far less than general practice of 15%. The other concern is draw-off nozzle sump dimensions. Vendors have followed licensor datasheets and provided sump width 3 times the draw nozzle diameter. However, general practice is 2 times the diameter. This resulted in a reduction of an area to locate the risers on the tray and a reduction in the number of risers and, hence, the riser area. May I request your opinion and recommendations?

A: The amount of riser area is a function of vapor velocity and the allowable pressure drop across the total draw-off tray. Does this unit have a higher L/V ratio than some other units? It is especially important that the riser covers be designed to ensure the liquid leaving the cover is protected from the vapor exiting the riser so it is not entrained up the tower. A book by Henry Z. Kister, “Distillation Design”, gives some guidance for designing total collectors and the residence time required to clear the liquid of vapor before it is withdrawn. Also the size and the configuration of the piping to the nozzle outside the tower are also important and can affect the ability to draw the liquid off of the collector.