Materials Performance

DEC 2018

Materials Performance is the world's most widely circulated magazine dedicated to corrosion prevention and control. MP provides information about the latest corrosion control technologies and practical applications for every industry and environment.

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36 DECEMBER 2018 W W W.MATERIALSPERFORMANCE.COM CHEMICAL TREATMENT A Ammonium chloride (NH 4 Cl) corro- sion is an aggressive form of local- ized, underdeposit corrosion com- monly found in overhead equipment and piping associated with crude units. This happens when ammonia (NH 3 ) and hydrochloric acid (HCl) in the vapor react to form NH 4 Cl salt. Being hygroscopic, NH 4 Cl absorbs moisture, leading to severe localized attack. Given the unpredictable na- ture of this damage mechanism, ap- propriate operational, materials, and chemical controls should to be set in place. This article summarizes case studies where this form of corrosion was observed, along with mitigation strategies. Ammonium chloride (NH 4 Cl) corrosion is primarily known to occur in overhead condensers, tower trays, interconnected piping, and heat exchangers of crude units. It happens wherever nitrogen and chlo- rides are present. 1-15 These contaminants can become a part of the crude oil feed- stock in several ways: raw crude, slop oil, and desalter water. Hydrolysis of metal chlorides in the crude leads to the forma- tion of hydrochloric acid (HCl). 11 Chlorides carried downstream of crude units lead to NH 4 Cl corrosion in overheads and pump-arounds associated with hydro- processing, catalytic reforming, f luidized catalytic cracking, and coker units. Neu- trali zers, such as ammonia (NH 3 ) and organic amines, are routinely utilized in Underdeposit Corrosion in Crude Tower Overheads SUDHAKAR MAHAJANAM AND FRED ADDINGTON, Pinnacle Advanced Reliability Technologies, Pasadena, Texas, USA AMANDA BARBA, The University of Texas Health Science Center, Houston, Texas, USA overhead systems for chloride control and to mitigate against acidic conditions. This is a delicate task, and prone to misapplica- tion, since underdosing leads to acid corro- sion, while overdosing causes localized, underdeposit corrosion when NH 4 Cl, or amine-HCl, precipitates before or above the water dew point. NH 4 Cl is hygroscopic and can precipi- tate when NH 3 and HCl are in vapor form, with salt hydration possible even above the water dew point (>300 °F [149 °C]). Another path for salt formation is dissolution of acid and base in water, followed by boiling the solution dry. The temperature at which a salt first precipitates is called the "salt point" and deposition occurs when the molar partial pressure product of NH 3 and HCl is greater than the NH 4 Cl stability constant. 1,3,6 Initially, small salt particles do not accu- mulate, but are carried away with the gas stream. However, at larger particle sizes, enough water is absorbed by the deposits to cause aggressive localized attack (>100 mpy). Hence, absorption of a small amount of water is sufficient to cause damage under " boil-dry " conditions. This has been vali- dated by relative humidity (RH) measure- ments performed experimentally, where a RH value of 20% was needed for general cor- rosion of carbon steel (CS). 14 At a critical RH value (60%), severe corrosion was observed. Above 65% RH, corrosion rates decreased. Hence, salt deposition occurring after or below the water dew point is not an issue since the salt dissolves away or is not formed at all due to the reduction of reac- tant concentration in the vapor through

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