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Tray Column & Tray Types
The function of a tray is to facilitate contact between the vapour phase and liquid phase so that mass transfer between the 2 phases can take place.
Each tray can be considered as being made up of 3 sections: weir, bubbling area, and downcomer as shown in the Figure below.
The bubbling area is the place where vapour-liquid contact takes place. Vapour flows through the openings on the tray from below, and into the liquid flowing across the tray, forming a foaming, turbulent mixture. In the simplest design, sieves are provided for vapour-liquid contact. Alternatively, valves or bubble-caps may be used. During this foaming process, a high interfacial area for efficient vapour-liquid mass transfer is produced.
The function of a weir is to maintain a desired liquid level on the tray. Downcomers are used to guide liquid flow from an upper tray to a lower tray. This is shown in the Figure below.
Trays are supported in a vessel by means of support rings or beams.
Above the liquid (before the next tray) is the vapour disengagement space, where the vapour separates from the liquid after contact and continue its up-flow to the next tray above. Ideally, the vapour should carry no liquid droplets (entrainment) to the tray above. The liquid overflows the weir into the tray below, its flow path being guided by the downcomer. Click here for more information on tray components.
We will discuss the operating principles of 3 common types of trays: sieve, valve and bubble-cap.
Other Tray Types Many other types of trays are available. These are all of proprietary designs, and it is best to consult the manufacturer's literature. Example include the Max-Frac® High Capacity Tray, Nye® Tray, etc from Koch.
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The principle factors to consider when comparing the performance of bubble-cap, sieve and valve trays are:
Cost: Bubble-caps are appreciably more expensive than sieve or valve trays, and the relative cost will depend on the material of construction used. Due to its simple design, sieve trays are normally the cheapest.
Operating Range: This refers to the range of vapour and liquid rates over which the tray will operate satisfactorily. The ratio of the highest to the lowest flowrates is often referred to as the "turndown" ratio. Bubble-cap trays have a positive liquid seal and can therefore operate efficiently at very low vapour rates. Sieve trays rely on the flow of vapours through the holes to hold the liquid on the tray, thus cannot operate at very low vapour rates. Valve trays are intended to give greater flexibility than sieve trays at a lower cost than bubble-caps Thus, bubble-cap trays have the widest operating range, followed by valve tray, and sieve tray.
Pressure Drop: This factor will be important in vacuum operations. The tray pressure drop will depend on the detailed design of the tray, but in general, sieve trays give the lowest pressure drop, followed by valves, with bubble-caps giving the highest.
Maintenance: For dirty services, bubble-caps are not suitable as they are most susceptible to plugging. Sieve trays are the easiest to clean.
In summary, sieve trays are the cheapest and are satisfactory for most applications. Valve trays should be considered if the specified turndown ratio cannot be met with sieve trays. Bubble-caps should only be used where very low vapour rates have to be handled and a positive liquid seal is essential at all flow rates.
NOTE: Tray design sizing is not covered here. The tray manufacturers have their own sizing handbooks that pertain to their products.
[ For more general info on tray sizing, refer to Chp. 4, "Process Plant Design", J.R. Backhurst & J.H. Harker, or Chps. 4, 5 & 6, "DistillationTray Fundamentals", M.J. Lockett ]
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