As shown in the Figure below, the bubble-cap consist a slotted cap on a central riser.
The gas flows up through the riser, reverse flow under the cap, passes downward through the annulus between riser and cap, and finally passes into the liquid through a series of openings or "slots" in the lower side of the cap. See the Figure below.
This device has a built-in liquid seal (i.e. the riser) which prevents liquid drainage at low gas rates. This design does not rely on the velocity of the upcoming vapour to hold the liquid on the tray. See the Figure below.
Because of its high cost and complexity, most modern column designs favour the use of sieve or valve trays over bubble-cap trays. Bubble-caps should only be used where very low vapour rates have to be handled, or adequate residence time is necessary for separation and/or chemical reaction, or in applications where a positive liquid seal is essential at all flow rates.
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