Revamps 2024 Issue

Diagnosing a premature flood in an atmospheric crude tower

Diagnosing and rectifying the flooding in a crude tower emphasises the importance of conducting plant tests, as well as gamma scanning investigations and analyses

Daniel Hussman, Gord Bruce, Abdullah Abufara, Komi Chandi and Joshua Donohue Parkland Refining (BC) Henry Z Kister Fluor

T his investigation of a premature flood near the diesel draw of an atmospheric crude tower demonstrates the importance of conducting plant tests, judicial gamma scanning investigations and analyses, and closely checking theories against plant data and targeted tests to diagnose fractionator problems. In a 2020 turnaround, the planned mechanical work included replacing the wash section two-pass sieve trays 8-10 with in-kind, but the tray material was changed from 410 SS to carbon steel. Afterwards, the tower operated at high crude rates for 18 months without issues. In October 2021, there was a planned turnaround of an adjacent unit, during which the tower was drained, purged, and safe-parked. Upon return to service, the tower struggled to maintain high crude rates due to increased pressure drop across the bottom two-thirds of the tower. The pressure drop pro- gressively rose over the next seven months, forcing crude rate cuts of 5,000-6,000 BPD. At higher rates, the tower flooded, causing sudden loss of overflash and diesel draw (level in the side stripper). Gamma scans showed flood ini - tiating on tray 10, right above the diesel draw tray, propa- gating up the tower when the rates were raised. Several theories requiring different remedial actions were proposed. The theories were evaluated by thoroughly checking against plant tests and data, together with quan- titative analysis of the gamma scans. Based on this evalu- ation, the team concluded that entering the tower to clear foulants or repair damage or fouling was the best path forward. Upon entry, the active areas on trays 10 and 9 (diesel draw) were found heavily fouled, to the point where the sieve holes were invisible. The foulant thickness varied across the diameter of the column and could be easily removed with a metal scraper. The deposits were clay-like and sandy. The downcomers were relatively clean. Trays above and below this zone were found to be quite clean. Process description and problem definition Figure 1 is a schematic of the crude tower with typical rates and temperatures for the 18 months before the October 2021 outage. The tower differential pressure, measured between the flash zone and the vapour space above tray

24, was steady, typically 2.3-2.5 psi, or about 0.13 psi/ tray, a little on the high side, but within the range of normal operation. The crude rate was high (30,000 BPD), the die- sel pumparound rate was high (5,700 BPD), the diesel side stripper level was steady, and the overflash was steady at about 500 BPD. After the October 2021, turnaround the tower differential pressure (dP) became erratic and crude rates had to be lim- ited to 29,000 to prevent dP excursions. The diesel pump- around rate was reduced to about 4,500 BPD with a colder outlet temperature of about 225ºF. The vapour space between trays 8 and 9 has two temperature indicators that historically read the same. Following the October 2021 outage, the one on the north- east started reading 15ºF higher than the one on the south- west. Two other temperature indicators were in the vapour space between trays 6 and 7, they kept reading the same temperatures. Things took a turn for the worse following another out- age in December 2021. The dP became more erratic, often rising to about 3.5-4 psi. Liquid appeared to be accumu- lating above the diesel draw with a loss of diesel product, wash to the lower trays, and overflash. The high dP was often accompanied by some diesel ending in the jet draw from tray 16. The accumulation appeared sudden, erratic, and unpredictable. Drawing more diesel brought the dP down, but the overflash would not re-establish until the coil outlet temperature (normally about 650-655ºF) was lowered by about 5 ° F. The dependence of the flood on the coil outlet temperature suggests strong sensitivity to the vapour loading. The frequency and severity of the flooding above the die - sel draw tray increased at higher crude charge rates and high diesel pumparound flow rates. This forced the refin - ery to gradually cut charge rates, from 28,000 BPD after the December outage to 25,000 BPD just before the tower was shut down in July 2022. Diesel pumparound flow rates were further reduced to about 2,200 BPD. Troubleshooting and testing The active areas of the trays were gamma-scanned on January 25 (flooded) and January 26 (‘normal operation’). During both scans, the crude feed rate was 28,000 BPD,

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Revamps 2024

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