Distillation Tower Internals


Towers have a variety of internal devices for vapor liquid contact and feed distribution that affect the exterior layout of the vessel. There is a wide range of designs for trays, which are principal internal components of trayed tower.

Two most frequently used are the single-pass bubble can tray and the double pass bubble cap tray. Other trays (e.g. sieve or perforated trays) are similar in design to the bubble cap tray and do not affect the layout of the tower.

Tray configurations and dimensions are furnished by process engineering and are included in the process release package. Following drawing shows a process vessel sketch. Following drawing shows the tray details.

Following drawing depicts single pass and double pass trays.

Towers have the same tray configuration for the whole length of the tower. Some towers, however, especially those with enlarged sections, could change from single pass to double pass tray configurations.

Following drawing shows a typical arrangement at a tower transition section. The chimney tray, if specified, is another device that could change the tray configuration.

The chimney tray is a solid plate with a central chimney section and is usually used at draw-off sections of the tower.

Following figure identifies a typical chimney tray arrangement.

The plant layout designer must orient the trays along with the tower nozzles to suit the best exterior arrangement. The trays can be at any angle as long as the downcomers directly oppose each other.

Following diagram shows a typical example of opposing downcomers. Two main items that influence tray orientation are maintenance access ways and reboilers.

The process vessel sketch shows that the boiler draw-off nozzle is located directly below the down domer of tray 27m and the plan arrangement indicates that the reboiler is located on the west side of the tower and that the maintenance road is south of the tower.

Therefore, because the tower reboiler nozzle is generally on the same side as the reboiler and the maintenance access way is best located on the maintenance side, the trays are automatically positioned about a north/south centerline.

Following drawing illustrates a plan view of the arrangement.

The principal difference between trayed and packed towers is that the packed towers uses metal rings instead of trays for vapor-liquid contact.

The metal rings are dumped or packed into specific sections of tower, called beds and suported by cross grid bars spaced to prevent the rings from falling through.

The supports are designed to allow vapor to rise and liquid to flow down. Liquid is fed into the vessel at the top of each bed through a liquid distributor.

Unlike the trayed tower, there are no special considerations for orientation of the beds, the distributor, or the packing supports.

Following drawing shows these three packed tower components.

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