Corrosion control in crude unit atmospheric towers is often achieved through a combination of multiple mitigation strategies. These mitigation strategies may include modifications to unit operating conditions and the proper selection of chemical additives. The key to identifying the most effective mitigation strategies is determining the best use of diagnostic tools, such as the Baker Petrolite Ionic Model. To obtain a thorough understanding of the corrosion mechanism(s) occurring in the system, physical examination of the affected areas is also required. This article presents a case study for a US Gulf Coast refinery in which multiple corrosion control strategies were employed to address amine hydrochloride salt deposition in the atmospheric tower overhead system. Mitigation strategies included desalter wash water acidification and modified tower operations to reduce the amount of amine recycled to the crude tower, thus reducing corrosion due to salt deposition. The implementation of these strategies resulted in lower corrosion risk and continued improvement of the run length of the overhead heat exchangers.
System description
The tower overhead comprises a two stage condensing system. The unit was designed so that the first stage would operate without bulk water condensation in the first accumulator drum (‘hot and dry’ operation). The unit processes approximately 150 000 bpd (23.8 m3/d) of primarily light crude (35° API). The first stage consists of four parallel banks of shell and tube exchangers in which overhead process vapours exchange heat with raw crude on the tube side. The combined shell outlet stream is fed to a separator drum from which the hot naphtha is combined with a top pumparound and then returned to the tower as reflux.
Process vapour from the first stage feeds a set of air coolers after which the multiphase product is separated in the second accumulator drum (cold and wet operation). An oil based corrosion inhibitor is injected into the overhead line upstream of the first stage condensers (E-1s). A neutralising amine is injected via a water wash upstream of the second stage fin fans. The neutraliser formerly used was monoethanolamine (MEA) based. All equipment in the overhead system is constructed of carbon steel. Figure 1 provides a simplified diagram of the overhead system.
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