Types of Trays (Tower Internal)

Tower Trays

Types of Trays:

Fractional distillation requires mass and heat transfer between vapor and liquid flowing counter currently through a fractionating tower. A large number of devices to ensure a more or less thorough contact between the rising vapors and down-coming liquid had been developed. Bubble-cap trays, valve trays, sieve trays, and grid trays are examples of devices.

A. Trays Having Separate Liquid Down comers

The great majority of commercial fractionations are carried out in columns where the liquid flows horizontally across each tray. The liquid contacts the rising vapor and is separated from the vapor before flowing through down comers onto the tray below. In nearly all cases, the down comers are segmental parts of the column, and are provided with a liquid-overflow weir to assure a minimum height of liquid on each tray. Inlet weirs for the liquid entering onto a tray are used in some designs. Various types of trays with separate liquid down comers which are in more common use today are illustrated.

Bubble-Cap Trays are so widely used in the petroleum and chemical industries that they are generally considered to be "the standard." All new types of trays are compared with "a bubble-cap tray,"

The outstanding characteristic of a will designed bubble-cap tray is probably its ability to perform satisfactorily over wide ranges of liquid and vapor rates. In which the vapor passes up through short pipes, called risers, covered by a cap with a serrated edge, or slots. The bubble-cap tray is the traditional, oldest type of cross-flow tray, and many different designs have been developed. Standard cap designs would now be specified for most applications.

The most significant feature of the bubble-cap is that the use of risers to ensure that a level of liquid is maintained on the tray at all vapours flow-rates.

Although there are many styles and dimensions of caps (Figure 3.4) in use, the round bell shaped bubble-cap is quite practical and efficient (Figure 3.5).

Dimensions, it is available in sizes of 3,4,5,6 and 7 inches, but the most popular and most adaptable size is about 4 inches O.D. The 3-inch and 6-inch are also in common use for the smaller and larger diameter towers.

Slots are the working part of the cap. Slots are usually rectangular of trapezoidal in shape. The rectangular slots give slightly greater capacity while the trapezoidal slots gives slightly better performance at low vapor rates.

Shroud Ring, it is recommended to give structural strength to the prongs or ends of the cap.

Sieve or Perforated have been in use longer than bubble-cap trays but have not received the same wide acceptance.  This is partly because of inadequate performance data with respect to liquid and vapor capacities. Recently more attention has been given to sieve trays, and it appears as though they will find increased use by industry.  The vapor passes up through perforations in the tray and the liquid is retained on the tray by the vapor flow. There is no positive vapor liquid seal, and at low flow rates liquid will "weep" through the holes, reducing the tray efficiency.  The perforations are usually small holes, but larger holes and slots are used.

Valve Trays (Floating Cap) Valve trays are proprietary designs.  They are essentially sieve trays with large diameter holes covered by movable flaps, which lift as the vapor flow increases.

As the area for vapor flow varies with the flow rate, valve trays can operate efficiently at lower flow rates than sieve trays the valves closing at low vapour rates. Is somewhere between a bubble-cap and a sieve tray in operating principle. It is a bubble-cap tray where the vapor, makes one 90-degree turn to enter the liquid horizontally, there are no risers, and the caps have no teeth. It can also be considered as a modified sieve tray where the vapor emerges horizontally into the liquid instead of vertically, and the perforations have variable area.

Float-Valve Trays is a recent development worthy of consideration, although there is very little published information about its operation. It is a valve-type tray with floating rectangular caps positioned by end-brackets. One edge, the heavy edge, of each cap is turned upward 90 degrees. At low vapor rates the light edge of each cap opens first, and at higher vapor rates the heavy edge opens. Like the Flexitray, it can be considered as somewhere between a bubble-cap and a sieve tray in operating principle.

Uniflux Trays is a third newcomer to the field.  It has had considerable industrial use already, but there is very little published information about its operation. It is a bubble-cap tray, modified so as to reduce fabrication costs considerably and to have possible other advantages. The tray is made of a number of S-sections, with the vapor making a 180-degree turn between the riser and the cap, and then emerging from one side (the downstream side) of the cap, after a 90-degree turn through the cap slots. In this way the vapor emergence should help the liquid flow across the tray, reducing hydraulic gradient.

B. Trays Having No Liquid Down comers

Traditionally, fractionating columns that have no liquid down comers have been packed columns, where the ascending vapor contacted the descending liquid in true countercurrent action. Recent developments have substituted perforated trays, where the liquid and vapor both pass through the same openings, for the continuous packing. There is not the same degree of differential contacting here as for the continuously packed columns, but there are many possible advantages for this modified "packing," compared with either conventional packed columns or the conventional tray columns.

Turbo grid Trays consists of a flat grid of parallel slots extending over the entire cross sectional of the column. The slots can be stamped perforations in a flat metal plate, or can consist of the spaces between horizontal bars. Liquid level on each tray is maintained by dynamic balance of liquid and vapor rates. The Turbo grid tray has had considerable industrial applications already, but there is little published information about its operation.

Ripple Trays is the latest arrival in the field of liquid-vapor contacting devices.  It is made by corrugating a conventional sieve plate into sinusoidal waves. The perforations extend over the entire cross sectional area of the column. Liquid level is maintained on each tray by a dynamic balance of the fluid flows, being a very recent development, there is little published information about its use and operation.