Northshore CK356N6C: Low-phosphorus corrosion control on high-TAN crude in a vacuum unit

Following its 2019 turnaround, the refinery’s 5.1 million tonnes/year crude unit resumed processing a heavy offshore crude slate. Vacuum sidestream total acid number (TAN) was reported at 3–5 mg KOH/g, with elevated sidestream iron indicating a need to investigate and manage naphthenic-acid corrosion.

The refinery needed to bring sidestream iron within its technical-agreement limits while operating on existing metallurgy. Downstream hydroprocessing units also imposed a phosphorus restriction, which constrained inhibitor selection.

Northshore CK356N6C is Cestoil’s low-phosphorus high-temperature corrosion inhibitor. The supplied case study lists product phosphorus at ≤ 3 wt% and describes a protective inhibitor–iron complex film. Product phosphorus content is distinct from the phosphorus concentration reaching downstream units.

Cestoil sampled process streams and reviewed corrosion history, sidestream temperatures and metallurgy with the refinery’s technical department. The program used three vacuum-tower injection points: the first and second intermediate pumparounds and the wash-oil reflux. These streams together represented approximately 73% of the crude flow, as reported.

The team monitored iron and TAN on vacuum sidestreams 2, 3 and 4 from 7 April to 1 July 2019, and reviewed trends with the refinery throughout the run. Dose, crude-slate changes and injection distribution were part of the service program.

Dose figures are based on crude flow. The source does not provide an individual dose for each injection point. Injection-stream temperatures should not be read as a measurement of metal temperature at every protected location.

During film formation, sidestream-3 iron was reported at 2.2–4.1 mg/kg and sidestream-4 iron at 2.9–4.3 mg/kg. Within a week of switching to maintenance dosing, the reported ranges were 0.4–0.9 mg/kg i 0.9–1.7 mg/kg, respectively. Two one-day excursions on 27 and 30 May cleared by the next analysis.

Average sidestream TAN remained around 3.0–3.4 mg KOH/g, with peaks above 4 in April and mid-June, while iron stayed lower. The refinery interpreted the trend as evidence of treatment performance under continued acidic-feed conditions.

These figures compare two treatment periods, rather than an untreated baseline with a treated period. Iron is an indirect corrosion indicator: a 72% decrease in iron is not a direct measurement of a 72% reduction in corrosion rate. Inspection or probe data and operating conditions should be reviewed alongside the iron trends. Percentage changes are rounded as reported in the source.

The refinery’s technical department recorded the agreement as met and concluded that CK356N6C supported high-temperature naphthenic-acid corrosion control and longer operating cycles.

The supplied summary does not list the numerical iron limits, the number of analysed days for each stream or downstream phosphorus measurements. The percentages refer to days with analysis, not every calendar day.

The reported objective was to control sidestream iron within the refinery’s agreement while meeting its downstream phosphorus restriction. The refinery recorded that objective as achieved at a 12–15 ppm maintenance dose on crude, using the existing injection system and vacuum-section metallurgy.

The customer report provides no monetary savings figure. This case therefore does not assign a value to avoided maintenance, equipment replacement or downtime.

Allow for an initial film-forming period before evaluating maintenance performance. In this trial, the initial dose was approximately twice the maintenance dose. Review any dosing plan against the unit’s metallurgy, stream temperatures, crude TAN and downstream phosphorus limit.

The refinery recommended linking dose to crude TAN as crude slates change and evaluating carrier-stream injection to improve dispersion. The documented trial used neat injection; the supplied report does not quantify a dose saving from the proposed carrier-stream approach.

Track iron against TAN on the same sidestream, together with chemical dose, crude slate and operating conditions. Pair stream analysis with inspection or probe measurements when assessing protection and long-term operating value.