Identifying slug flow key to successful debutaniser revamp
Conversion of a stripper tower to debutaniser operation shows how external piping in slug flow regime can negatively impact a tower
Daryl W Hanson Valero
A Valero Gulf Coast refinery recently installed a new alkylation unit that required the removal of butanes from an alkylate (gasoline) product. As part of the revamp, the new debutaniser had to be sited in a very congested area. An out-of-service tower was identified and could be replaced and used in debutaniser service. However, since it was smaller, high-capacity trays were installed. In a previous paper,1 UOP’s proprietary high-capacity Parallel Flow Multiple Downcomer (PFMD) and Enhanced Capacity Multiple Downcomer (ECMD) trays were installed in the tower. The revamped tower diameter was 54in, which is less than the 60in minimum diameter to effectively install a two-pass crossflow tray without special accom- modations (no tray manways, eliminating flow path length minimums). Installation and start-up went as expected, and the tower met the operating goals. However, starting at around 70% of design, tower pressure drop measurements exhibited periodic cycling that had not been seen before. This cycling grew in severity but did not inhibit tower performance above the design rates. Evaluation of this cycling behaviour revealed that a flow regime issue in the feed upcomer pip- ing was causing the problem. Process changes enabled a reduction in cycling severity. Small tower difficulties In small towers, there is naturally a lack of space to add downcomers to handle capacity without sacrificing a lot of efficiency. The tower area simply is not abundant enough to have the luxury of a downcomer addition. There are only a few industry examples of multi-pass trays (two-pass) in towers less than 5ft diameter. The author is aware of one depropaniser where two-pass high-capacity crossflow trays were utilised in a tower that was 42in ID (inner diameter). To handle the capacity, the end user sacrificed efficiency and manway access to install the tray. The efficiency was within the error band for a point efficiency device. When applying high-capacity trays to small columns (<6ft diameter), the practitioner typically has to settle on which variable is more important for their system: capacity or efficiency.
Repurpose of existing tower into debutaniser operation An old methyl tert-butyl ether (MTBE) unit alcohol strip- per tower was identified as a perfect location for the new debutaniser. The tower location would allow close connec- tion to existing piping for the feed and bottoms. The tower was only 54in ID, which was far smaller than the existing tower. The alcohol stripper tower would have to be refur- bished, and its certificates updated as part of the repurpose to debutaniser service. The foundation was checked for a larger tower diameter, but it was deemed too close to its limit. A process evalua- tion was performed, and high-capacity counter-flow trays in the stripping section were the limiting capacity, barely achieving the design loadings. The alcohol stripper (new debutaniser) tower was removed and sent to a local shop for refurbishing. Scope included: removing external platform and piping supports, installing new supports, replacing/removing existing nozzles, and installing new internal tower support rings/attachments. Installing new support rings was required to avoid re-spacing complications in the top of the tower and cor- rosion issues in the bottom. As such, the upper section of the tower had new rings installed at 18in spacing (exist- ing 24in), and the lower section had rings installed at 24in spacing (matching existing).
Vapour ood (Jet) Liquid ood (Downcomer) Observed ood
Constant
Time
Figure 1 Characteristic flood profiles
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Revamps 2026
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