(215a) Improved Feed Entry Improves Parting Box Hydraulics

By Henry Kister, Garry Jacobs, and Abraham A. Kister (presenter), Fluor, Aliso Viejo, CA

Achieving good liquid distribution is crucial for the performance of packed towers and multi-pass tray towers. With the focus on liquid distribution, one device that has received surprisingly little attention in the published literature is the parting box. While good-practice guidelines are available, the published literature is short of work on the hydraulics of parting boxes and tested guidelines for its design.

To gain insight into the parting box performance, we built a scaled-down model, geometrically similar to the parting box in a hydrocarbon tower, and tested it with water. This model was intended not only to provide insight into the tower issue, but also to provide generic insight into parting box hydraulics as tested in a realistic parting box design.

Tests in our previous paper (Kister Distillation Symposium, New Orleans, April 2018) showed the liquid in the test parting box was highly frothed up. The frothing was due to water jets from the feed sparger entraining air into the water upon impact. This generated excessive froth heights in the parting box, and premature overflow at high rates. The liquid split to the five clusters of holes at the floor of the parting box was good at the maximum rates but quickly deteriorated as the liquid rate was turned down.

This paper addresses the question of whether lowering the hole velocities would mitigate the frothing. In the current series of tests we used two alternative pipe spargers with bigger holes, one at a time, which reduce the discharge velocities. This extends our hydraulic insights over a wider range of sparger hole velocities.

To avoid frothing, many parting box designs equip the pipe sparger perforations with dip tubes that bring the liquid below the liquid surface, often a short distance above the floor of the parting box, and thus avoid entrainment of vapor into the parting box liquid. This is a more expensive design, often considered ideal. Our tests showed that this fully eliminated the frothing in all but the lowest rate, but again the liquid heights in the parting box were considerably higher than those calculated from the orifice equation. Improvements to circumvent this problem and provide better parting box hydraulics were devised, water tested, and will be discussed.