PREDICTION, MONITORING, AND CONTROL OF AMMONIUM CHLORIDE CORROSION IN REFINING PROCESSES
Authors Adan Sun and Deyuan Fan, Shell Global Solutions (US) Inc.
Source CORROSION 2010, March 14 - 18, 2010 , San Antonio, TX
Copyright 2010. NACE International
Keywords ammonium chloride, dew point, dry point, corrosion prediction model, salt formation, ionic equilibria, hydroprocessing, distillation overhead systems
Corrosion by wet ammonium chloride salt is a significant concern in many refining process units. Because corrosion rates typically experienced can be extremely high when all of the conducive conditions are met, every attempt is made to prevent this corrosion from occurring. This includes process design, corrosion resistant materials, and source control of chlorides. However, despite the best effort by the industry, ammonium chloride continues to pose a serious threat to the integrity of refining equipment and the process safety of refineries because the omnipresence of chlorides in a refinery. This paper presents a recent effort to further minimize this threat. It includes advances in the fundamental understanding of ammonium chloride salt behavior, predictive modeling effort based upon such fundamentals, the integration of such models with process and operating conditions in real-time, and guidance for inspection in search of this highly elusive corrosion phenomenon.
Corrosion by wet ammonium chloride (NH4Cl) is one of the main causes of equipment and piping failures in today's refining industry. Lack of thorough understanding of this highly unpredictable and potentially rapid corrosion mechanism continues to pose a significant threat to equipment integrity and unit reliability. Ammonium chloride corrosion has affected equipment and piping in areas such as hydroprocessing effluent systems, overhead systems of crude distillation and fractionation columns of thermal and catalytic conversion units (Delayed Coking Unit (DCU) and Fluidized Catalytic Cracking Unit (FCCU)), and product stabilizer towers and recycle gas systems in catalytic reforming units 1- 6 .
The typical construction material of most of the equipment in the above-mentioned processes is carbon steel. When wet ammonium chloride salt is present, the corrosion rate of carbon steel is generally considered unacceptably high. Unfortunately, many common alloys (such as common austenitic stainless steels) are not suitable replacements because they are susceptible to chloride stress corrosion cracking (CI-SCC) in these environments 7. Use of duplex stainless steels has given mixed success with some reports of pitting corrosion and stress corrosion cracking under the combined effect of chlorides and sulfides. Thus far, only nickel alloys, such as Alloy 625 (UNS N06625) and Alloy C-276 (UNS N10276), have demonstrated satisfactory performance in these services.
Despite these significant concerns, the industry's understanding of wet ammonium chloride corrosion is still very limited. Ammonium chloride is considered an acid salt because it is formed from a strong acid (HCl) and a weak base (NH3). Dilute solution of stoichiometric NH4Cl salt (e.g. less than 0.1wt%) are generally not considered to be very corrosive. However, corrosion is most severe at or near the aqueous dew point or at the dry point where NH4Cl concentration can be very high. Corrosion by wet ammonium chloride at concentrated conditions are very rapid with rates approaching that of hydrochloric acid (i.e., greater than 1,000 mpy (25 mm/y)) on carbon steel. Surprisingly, there has been no systematic study of the wet ammonium chloride salt behaviors and corrosion rates in the various conditions typically encountered in petroleum refining processes.