Naphthenic acid attack

Most crude oils contain some naphthenic acid. These acids tend to concentrate in those crude oil components that boil between kerosene and heavy gas oil. Naphthenic acid corrosion is characterized by a general thinning of stainless steel vessels and tower internals at temperatures between 450 to 750°F. Rates of corrosion are greatly accelerated in areas of high velocities. Naphthenic acid corrosion is retarded by HjS. Thus, process equipment containing hydrocarbons with higher sulfur contents are, to some degree, protected from naphthenic acid corrosion. 

My experience indicates that stainless steels that have high chrome and nickel content are not very resistant to naphthenic acid attack. What is required is molybdenum. 304 stainless steel is unsuitable due to its low moly content. Even 316 stainless steel, which has about 2 percent moly, offers only marginal protection. I usually specify 317 stainless steel, which has about 3 percent moly, when designing for naphthenic acid services. 

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The neutralization number (mg of KOH to neutralize 1 g of crude) is a measure of its naphthenic acid content. A neutralization number above one indicates that naphthenic acid attack in the crude unit is going to be a problem. In addition, naphthenic acids will cause product problems. For example, virgin kerosene produced from certain crude oils contains high concentrations of naphthenic acids and, as a result, will not meet haze tests. 

A crude unit which has metallurgical protection for running high-sulfur crudes is not protected against naphthenic acid attack. For example, naphthenic acids attack 12-Cr stainless steel and carbon steel with equal vigor. Type 316 or, preferably. Type 317 is required to resist naphthenic acid corrosion, but even these alloys will pit in severe cases. Other materials effective against naphthenic acid corrosion are aluminum or alonized steel. 

Probably the most cost-effective metallurgy to use in vacuum towers exposed to naphthenic acid attack is type 316(L) S.S. The (L) denotes low carbon content. Recent operating experience has shown that 316(L) is quite a bit more resistant than type 316. While type 316 L) is not quite as rugged as type 317, it is a good deal cheaper and usually more readily available. Type 304 has, on the other hand, shown itself to be marginal in a vacuum tower exposed to naphthenic acid attack. 

Figure 1.5 Different kinds of corrosion morphologies associated with naphthenic acid attack. Region I is the IMPT random packing, region 2 is the tray and bubble caps, region 3 is the column wall flash zone, region 4 is the support grid, and region 5 is the transfer line. (Reprinted from P.P. Alvisi, V.F.C. Lins, 2011, with permission from Elsevier.)...

Dove D, Messer B, Phillips T, An Anstenitic Stainless Steel, Resistant to High Temperature Creep and Naphthenic Acid Attack in Refinery Environments , Corrosion... 

Organic acids (e.g., naphthenic acid) can cause severe corrosion above -450 F (230 C) (although attack has been seen as low as 340 F [170 C] in turbulent areas) if the neutralization number exceeds 2.0 mg of potassium hydroxide per gram (mg KOH/g). The naphthenic acid content is generally determined by... 

Naphthenic acid corrosion is quite severe in vacuum and visbreaker towers. Crudes with total acid numbers over 2.0 mg KOH/g have corroded type 316 SS in the inlet areas. At times, type 317 (UNS S31700) SS, 904L (UNS N08904), or other corrosion resistant alloys are needed. Matching weld filler metal must be used. In one case, a type 310 (UNS S31000) SS weld filler metal was used on type 316 SS lining. While the strip lining was not attacked, the weld dissolved in one year. 

At high temperatures, especially in furnaces and transfer lines, the presence of naphthenic acids may increase the severity of sulfidic corrosion. The presence of the organic acids may disrupt the sulfide film and fhereby promote sulfidic corrosion of the alloys that are normally expected to resist this form of attack. The alloys in question are 12 Cr and higher. In some cases, such as in side-cut piping, the sulfide film formed because of hydrogen sulfide is thoughl lo offer some degree of protection from NAC. 

Severe naphthenic acid corrosion (in the form of pitting) has been experienced in the vacuum towers of crude distillation units in the temperature zone of 290 to 345 °C and sometimes as low as 230 °C. Attack is often limited to the inside and the very top of the outside surfaces of bubble caps. Figure 1.5... 

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