Tray Design

Of the accessories that are used in the tray column design the tray spacing, the number of passes, the type of down comers, the number of trays, the provision of down comer backups, the design of bottom seal pan, the column bottom arrangement, the tray pressure drops, the nozzle location and orientation are the key parameter in the tray column design.

Tray spacing

Usually in the columns that are employed in the chemical process industries, the vertical distance differs from 450 to 900 mm in between the adjacent trays. A tradeoff between the column diameter and the column height is involved in the spacing. The column height gradually increases when the tray spacing is greater where as the required diameter decreases on the other end.

Tray Passes

The number of passes has to be set in order so that the loads of the liquid will not exceed 70 m ³ /h per meter length.

After selecting the number of passes, the column should be adjusted if necessary so that a minimum path of about 400 mm can be arrived. About two passes are used at a maximum if the column diameter is between 1,200 and 2,100. Again, three passes are used in maximum for a diameter of about 2,100 to 3,000 mm and finally four passes are used at maximum for a diameter which exceed above 3000mm.

The conduits which have a circular, rectangular or segmental cross section which conveys the liquid from one tray to another tray that is immediately below that tray is referred to as downcomers.

Liquid velocities in downcomers:

• Minimally foaming liquids: 0.12 – 0.21 m/s
• Medium foaming liquids: 0.09 – 0.18 m/s
• Highly foaming liquids: 0.06 – 0.09 m/s

Pressure drop

In the tray design, the pressure drop of the dry-tray should be equal to the hydraulic-tray pressure drop in order to effective functioning of the trays to the best efficiency point.

Seal Pans

The clearance between the bottom down comer and the seal pan is expected to exceed the clearance when employed under the tray down comers. It is needed to be 50 mm.  The distance between the seal pan floor and the bottom tray floor is need to be 150 mm when greater than the usual tray spacing.

Nozzle orientation

The nozzle in the tray design is needed to be perpendicular to the top tray down comer for the nozzles which is used to feed the liquid into the top tray. The nozzle which feeds liquid at any of the intermediate trays should not be placed in the down comer segment.  800mm of space is needed between two trays.

Questions:

• What are the liquid velocities in down comers?
• What is nozzle orientation?