Length

Calculating the height of the column is fairly easy if the number of trays is given. The typical distance between trays (tray spacing) is 0.61 m (2 ft). If there are NT stages, the number of trays is NT — 2 (one stage for the reflux drum and one for the reboiler).

In addition to the trays, some space is needed at the top where the reflux piping enters the vessel and at the feed tray for feed distribution piping. More significantly, space is needed at the base to satisfy two requirements: (1) liquid holdup is needed for surge capacity and (2) the liquid height in the base of the column must be high enough above the elevation of the bottoms pump to provide the necessary net positive suction head (NPSH) requirements for this pump.

Therefore, a design heuristic is to provide an additional 20% more height than that required for just the trays. So the length of the vessel can be estimated from the following equation.

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