(119d) Gaining Insights into Parting Box Hydraulics

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.

The motive for the current tests was a hydrocarbon separation tower (designed by others, not Fluor) that was unable to achieve the design separation. A key suspect for the poor performance was the parting box whose design adhered to most good-practice guidelines. To gain insight into the parting box performance, we built a scaled-down model, geometrically similar to the parting box in the hydrocarbon tower, and tested it with water. This model was intended not only to provide insight into that tower issue, but also to provide generic insight into parting box hydraulics as tested in a realistic parting box design. The experiments conducted extended well beyond the specific design under investigation into generic parting box hydraulics. This paper describes our tests and findings.

Our tests showed the liquid in the test parting box was highly frothed up at both high and low liquid rates. The frothing was due to water jets from the feed pipe sparger entraining air into the water upon impact. This generated unexpectedly high 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.

The use of an Perforated Impingement Plate (PIP) with either 20% or 10% open area, mounted either 2” or 4” above the parting box floor, greatly improved the split of liquid to the clusters of holes at the floor of the parting box. With a PIP, the split was very good to excellent both at the maximum rates and at turndown to about 50% of the maximum rates. Lower rates were not tested.

In the tests with PIP’s, an air gap formed beneath the PIP, with tall froth above, producing premature overflows at high rates, and low and relatively less aerated froth beneath the PIP. The air gaps resulted from the water jets issuing from the feed pipe entraining air with them. This air descended through and accumulated beneath the PIP, pressuring up the space between the parting box floor and the PIP. The trapped air rose back relatively easily when the PIP had a large (20%) open area, incurring a relatively small pressure rise beneath the PIP and a relatively shorter froth backup above the PIP. The pressure buildup beneath the PIP and the froth backup above the PIP were intensified with the more restrictive 10% open area PIP.

The pressure buildup beneath the PIP helps maintain to good liquid distribution at the outlet of the parting box. A troublesome observation is that the presence of the PIP generates a high froth buildup in the parting box, and at high rates, premature overflow, at well below the maximum expected capacity. In an operating tower, with vapor ascending around the parting box, such overflow may result in entrainment and maldistribution.

Our tests placed some of the existing parting box guidelines and practices under challenge. In particular, it showed that velocities as low as 5-6 ft/s of liquid exiting the feed pipe can cause intense frothing in the parting box that may be undesirable. The air gap phenomenon with PIP’s, to our best knowledge first reported and explained here, adds a note of caution for PIP design. Premature overflows of froth above the top of the parting box have been observed due to frothing or due to air (or vapor) gaps below PIP’s.