PTQ Q3 2025 Issue

operation. The pressure drop across individual sections and the entire column had increased significantly. The pressure drop across the individual sections of the column in the upper section was very high, indicating some of the trays were operating close to their flood point. Thus, it was necessary to reduce the vapour load. To achieve this, the column was forced to operate at elevated pressure. Operating the column at elevated pressure lowered the feed vaporisation and reduced the product distillate yields. Pressure drop fluctuations in the upper section of the column affected LK product qualities, hence crude charge to the unit had to be cut down. Troubleshooting process To understand the abnormal increase in pressure drop, a detailed study of the complete column operation was con - ducted. The possible reasons for the increase in pressure drop could be: • Faulty instrumentation measuring incorrect pressure readings. • Damage to the trays resulting from an upset, obstructing the passage of vapours. • Fouling of the trays, resulting in resistance to vapour flow. Upon evaluation, it was found that all pressure instru - mentation indicators were working correctly, ruling out faulty pressure instrumentation. Damage to the trays was found unlikely as the column was operating without loss in separation or heat transfer efficiency at lower throughput. Observations showed that the rise in pressure drop first occurred in the LK draw-HKPA return zone, followed by the LKPA return-LK draw zone, then the HN draw-LKPA return zone, and finally the column top-HN draw zone. This indi - cated that limitations were originating from the HKPA zone and moving upward. Tray sections near hydraulic limits can experience flooding with resultant high pressure drop and instability. The high pressure drop in the top sections, coupled with inconsisten - cies in LK product qualities, indicated that the LK section trays were operating near hydraulic limits. This was not confirmed by process simulations and tray ratings, which indicated that the existing trays and internals were suitable to handle the corresponding vapour-liquid traffic with significantly lower pressure drop than current operation. An analysis of LK prod - uct pump suction strainer deposits detected high phosphate content, indicating possible fouling of trays in these sections. The phosphorus is attributed to crude and high TAN corro - sion inhibitor injection to treat high TAN crudes. Chemical cleaning of the column top section was carried out by inject - ing an anti-foulant but did not produce the desired results. To understand the source of phosphate deposits in the crude column, NEL examined the performance of the desalter, the most crucial contaminant removal process in the refinery. Phosphate esters, whether added to the crude oil or intro - duced as high naphthenic acid crude treatment corrosion inhibitors, enter the crude column and cause fouling in the crude column. Thermally unstable partial phosphate esters exhibit acidity similar to phosphoric acid. The OH bond in these esters is thermally unstable and decomposes at tem - peratures typical of the crude distillation unit (CDU), resulting

Source and scan lines HN-LK and LK-HK f ractionation, LKPA section

Downcomer location

HN section

Flooding

HN-LK fractionation

LKPA section

LK-HK fractionation

Flooding

HKPA

Radiation intensity measured

Figure 2 Column gamma scan

PTQ Q3 2025