Introduction

Most chloride salts in the crude oil coming into a refinery are inorganic (sodium, magnesium, or calcium chloride) and are effectively removed by the desalter. The nonextractable chlorides are not removed in the desalter, but can break down from downstream heating and processing to form hydrochloric acid (HCl). They sometimes cause corrosion and fouling problems. The forms of these chlorides are still being determined, but probably include organic chlorides (either natural or added via treatment chemicals or by the disposal of slops into the crudes); inorganic chlorides encapsulated in high-melting-point waxes or asphaltenes; or chlorinated solvents used in upstream operations.

The primary locations of problems to date have been in CDUs and NHTs. Within this report, the term CDU includes atmospheric and/or vacuum distillation units. Some types of nonextractable chlorides break down (by hydrolysis and pyrolysis), primarily in the CDU atmospheric and/or vacuum heaters, causing corrosion problems in the tower overheads. With other types, only a very small percentage break down in the CDU and most of the chlorides go downstream to the NHT. There are also reports of nonextractable chlorides found in gas oils going into the applicable downstream units.

Even with as little as 1% of the nonextractable chlorides breaking down in the CDU, a major increase in the atmospheric tower overhead HCl and chloride levels can occur and cause severe corrosion and fouling problems. One refinery had new overhead condenser tubes fail in less than 14 hours with a corrosion rate of about 1,000 mm/y (40,000 mpy). All incoming chlorides are converted to HCl by the hydrotreating reaction in NHT units. One refinery with nonextractable chloride corrosion in the NHT experienced 5 to 30 mm/y (200 to 1,200 mpy) rates on the tubes and shell of the feed/effluent exchangers, reducing the service life of the exchangers to 33 days.

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