Use of Froth Initiators

1. Purpose

This procedure outlines when and where to use froth initiators.

2. General

Columns with long flow-path lengths may experience large amounts of weeping at the liquid inlet side of the tray. This often occurs on very low pressure drop columns. This is inefficient, because the liquid weeping will bypass two trays, not just one.

The use of Slotted Sieve trays (a Inflection Point Engineering product) or froth initiators will help avoid this type of weeping problem. For new columns, the use of Slotted Sieve trays is the best choice. For revamps the design engineer should consider the use of Slotted Sieve trays and froth initiators. When froth initiators are desired they should be specified at the inlet of the tray, as well as every 4 to 6 feet along the liquid flow path on the tray.

Slotted Sieve trays or froth initiators should be considered when both of the following are true:

• The trays have long liquid flow paths

• The trays are expected to be weeping if froth initiators are not used; e.g. in cases where the values of the hole F-factor are reduced to less than optimum in order to meet low pressure drop criteria.

3. Specification

It is possible to install froth initiators over a row of holes punched out for the valves, provided that the valves are left off. Tack-weld the froth initiators to the tray sections. Make the froth initiators as long as each tray panel width, to promote the easy removal of panels or manways.

It is imperative that the shop notes clearly specify that the slot shall face downstream and that the froth initiator shall be the same metal and gauge as the tray floor. The slot width shall be 1/4” or 6 mm. Using a 1/2” slot has been observed to create too violent a frothing action. Reference Figure 1 for a sample tray layout.

Figure 2 shows sketches of a typical froth initiator.

4. Example of Benefits

Consider the tremendous improvement in Denka’s EB-styrene column. This is undoubtedly the work of froth initiators, as they help create better plug flow and prevent inlet side weeping. By installing froth initiators alone, this column’s efficiency increased from 50% to 100%.

Figure 1

Panel Sections

(can run the other way)

Downcomer on

to the tray

Froth Initiators covering first row of holes

Keep these spaces to a minimum

Downcomer off the tray

24" (example)

Froth Initiators

Note: Design Engineer could well have had three froth initiators in this case

24" (example)

6'-6"

(example)

7'-0"

(example)

18'-0"

(example)

Figure 2

Slot

End covers must be put on, tack weld or press out

1½" hole in tray floor for valve

¼" slot

1"

¾"

Tray Floor

1½"

2"