Northshore CK356N6C: Controle de corrosão com baixo teor de fósforo para petróleo com alto TAN em unidade de vácuo

Após a parada de manutenção de 2019, a da refinaria Unidade de destilação de petróleo bruto de 5,1 milhões de toneladas/ano retomou o processamento de uma carga de petróleo pesado offshore. O número de acidez total (TAN) da corrente lateral de vácuo foi registrado em 3–5 mg KOH/g, com níveis elevados de ferro na corrente lateral indicando a necessidade de investigar e gerenciar a corrosão por ácido naftênico.

A refinaria precisava adequar o teor de ferro na corrente lateral aos limites estabelecidos no acordo técnico, mantendo a metalurgia existente. Unidades de hidroprocessamento a jusante também impunham restrições quanto ao teor de fósforo, o que limitava a escolha do inibidor.

Northshore CK356N6C é o inibidor de corrosão para altas temperaturas e baixo teor de fósforo da Cestoil . O estudo de caso fornecido indica o teor de fósforo do produto em ≤ 3 % em massa e descreve uma película protetora de complexo inibidor-ferro. O teor de fósforo do produto difere da concentração de fósforo que chega às unidades a jusante.

Cestoil coletou amostras de correntes de processo e analisou o histórico de corrosão, as temperaturas das correntes laterais e a metalurgia em conjunto com o departamento técnico da refinaria. O programa utilizou três pontos de injeção na torre de vácuo: os circuitos de recirculação (*pumparounds*) intermediários primeiro e segundo, e o refluxo de óleo de lavagem (*wash-oil*). Essas correntes, em conjunto, representavam aproximadamente 73 % da vazão de petróleo cru, conforme relatado.

A equipe monitorou o ferro e o TAN nas correntes laterais de vácuo 2, 3 e 4 provenientes de 7 de abril a 1º de julho de 2019, e analisou as tendências com a refinaria ao longo da campanha. A dosagem, as alterações na composição do petróleo e a distribuição da injeção faziam parte do programa de serviços.

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 e 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.