Revamps 2026 Issue

Analysis Operating data analysis yielded interesting results. Public domain flow regime articles are prevalent in the literature for off-shore facilities. In fact, off-shore operators will often install slug catchers in the piping systems to avoid equipment damage, as it is impossible for them to always control the flow regime. Figure 8 illustrates the physical phe- nomena that occur on the tray immediately below the feed tray when it has to resolve the incremental flow caused by slugging. A time study on the feed line was used to determine the operating issues with the piping system. It was obvious that the slug size and pressure drop in the bottom section were inversely related. When one was high, the other was low. There was a distinct cycle that occurred between the tray being relieved of liquid load and the next cycle of slug forming. As the next slug was ready to break loose, the tray was at normal pressure drop and could accept the slug without going into a full flooding event. There are many methods in industry to estimate the slug length. The author does not have the expertise to determine the validity of each method. It was realised that many of the methods are based on a large difference in phase densities (>50 lb/cf), whereas this system has a relatively close liquid and vapour density difference of 32 lb/cf. Quick estimates range from six to 12 pipe diameters for the height of the slug. Since the tower is 54in diameter and the downcomers are close to 10in width, one can readily visualise that it would only take a liquid slug length of 12 pipe diameters to overwhelm the liquid-handling capacity of the top tray downcomer. As the tray is already operating close to 80% capacity, this slug would easily throw the tray to the liquid hold-up flooding point. It is believed that this simple anal - ysis was sufficient to substantiate tower operation and the results of the increased feed temperature trial to resolve the slug flow conditions. Conclusions Revamping a tower to a high-capacity tray is not a triv - ial issue. When the tower diameter is small and limiting, resolving every bottleneck is important. Public domain case studies are abundant with reboiler problems, feed zone problems, and transition problems that result in tray capac- ity loss. This is the first article that has identified an external pip - ing system flow regime that has a negative impact on tray stability and capacity. Resolving the feed piping slug flow regime eliminated the premature flooding instability that this tower was exhibiting and allowed for higher tower capacity. 7:58 7:49 7:40 6500 6600 6700 6900 6800 7000 Reux (bpd) Stripping DP (psid) Note: Data 1 minute averages

+15 deg F feed temperature

8:06

8:15

8:24

8:32

8:41

8:49

Figure 7 Pressure drop resolution by increasing feed temperature

Tray ood resolves

Tray oods

Slug loosing

Slug forming

Time

Point of tray ood inection Point of critical slug release

Relative size of slug Tower dP

References 1 Hanson, D., B. Miller, and B. Nowak, Results of an Alkylation Debutanizer Revamp with PFMD™ trays, Presented at the 2019 AIChE Spring Distillation Conference, April 1-4, 2019, New Orleans. 2 Kern, R., How to Size Process Piping for Two-Phase Flow, Hydrocarbon Processing , October 1969. 3 Daniels, L., Dealing with Two Phase Flows, Chemical Engineering, June 1995. 4 Fair, J. What You Need to Design Thermosiphon Reboilers, Petroleum Refiner , February 1960. 5 Anderson, R., Designing for Two Phase Flow – Part 1, Chemical Engineering, December 6, 1965. 6 Street, J., Unsteady State Gas-Liquid Slug Flow Through Vertical Pipe, AIChE Journal, Vol. 11, No 4, July 1965. Daryl Hanson is a technology advisor for Valero Energy in San Antonio. He is in the Strategic Technology and Development group where he oversees large projects, advanced optimisation, and troubleshooting in the area of fractionation and acquisition review. He manages the frac - tionation group and the heat transfer group, including fired heaters. Hanson has more than 30 years of experience and has published many technical papers. Before Valero, he worked for Glitsch, Koch-Glitsch, and Process Consulting Services. Hanson has a BS in chemical engi- neering from Texas A&M University. Figure 8 Operating regimes for slug flow interaction with tower internals

Revamps 2026