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Column Top and Intermediate Inlets/Outlets Arrangements
Reflux Inlet / Top
Feed Inlet
Intermediate
Feed Inlet
Downcomer Trapout & Chimney Tray
Downcomer trapouts are mainly used for partial liquid draw-off from tray columns. Downcomer trapouts seldom provide sufficient residence time for vapour disengagement, and the venting process must be completed downstream of the column outlet. Trapout must be sealed to prevent vapour from flowing up the downcomer, as showed in the Figure below (left).
And alternative to downcomer trapout is the chimney tray, shown in the Figure below (right). In a packed tower, it is also used as liquid collector or vapour distributor. The advantage of chimney trays is that they provide greater time for vapour disengagement, but consume more column height. The other disadvantage is their relatively high pressure drop.
Vapour
Outlet
Vapour outlets poses far less problem than liquid outlets. The prime consideration is avoiding the presence of entrained liquid droplets in the vapour streams leaving the columns. Mist eliminators can be installed above the top tray or a separate knock-out drum can be used.
Low points in the vapour lines should be avoided as these tend to trap liquid and accumulate liquids. The accumulated liquid can back-pressure the column causing instability and erratic operation, or can create a slug flow into downstream units.
Vapour outlet lines should be sloped (i.e. self-draining) either back to the column or into a downstream vessel.