Crude oils naphthenic acid corrosion

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. 

Naphthenic acid corrosion has been a problem in petroleum-refining operations since the early 1990s. Refineries processing highly naphthenic crudes must use steel alloys 316 stainless steel is the material of choice. Conversely, naphthenic acid derivatives find use as corrosion inhibitors in oil-well and petroleum refinery applications. 

Naphthenic acid corrosion Naphthenic acid pertains to a group of organic acids present in crude oil with the general chemical formula R (CH2) COOH, where R is one or more cyclopentane rings, and n is typically greater than 12. The ring structure can be tricyclic, tricyclic, or polycyclic. In addition, crude oil contains various other organic acids that contribute to acidity. These are often... 

Corrosion by naphthenic acids can be eliminated by them with neutralizing NaOH to form oil-soluble salts and the acid number of a crude containing naphthenic acids often gives an indication of its corrosivity during processing. This problem is not a major importance in refinery operations, where resistant alloys such as Type 316 stainless steel are used. 

The total acid number (TAN) of crude oils can be decreased through suitable mixing processes, i.e addition of crude oils with a low TAN into crude oils with a high TAN may obtain a mixed crude oil with a low TAN. If the TAN of this mixed crude oil is lower than the critical value for the initialization of high temperature naphthenic acid corrosion, the problems associated with this type of corrosion could be partially solved. 

Oxygen compounds are present in some crude oils, and decompose to form naphthenic acids upon distillation. These may be highly corrosive. 

Naphthenic acid is a collective name for organic acids present in some but not all crude oils. In addition to true naphthenic acids (naphthenic carboxylic acids represented by the formula X-COOH in which X is a cycloparaffin radical), the total acidity of a crude may include various amounts of other organic acids and sometimes mineral acids. Thus the total neutralization number of a stock, which is a measure of its total acidity, includes (but does not necessaiily represent) the level of naphthenic acids present. The neutralization number is the number of milligrams of potassium hydroxide required to neutralize one gram of stock as determined by titration using phenolphthalein as an indicator, or as determined by potentiometric titration. It may be as high as 10 mg KOH/gr. for some crudes. The neutralization number does not usually become important as a corrosion factor, however, unless it is at least 0.5 mg KOH/gm. 

Some crude oils contain certain organic compounds that are corrosive. In particular, these include naphthenic acids. Such crude oils cause problems in transportation, refining, and processing. The naphthenic acid content can be reduced simply with alcohol treatments, such as methanol, to form the corresponding ester. Hence, treatment temperatures will preferably be around 350° C. Pressures from about 100 to 300 kPa are typical and generally result from the system itself [1556]. 

To mitigate the effects of corrosion resulting from the presence of salts, it is advantageous to reduce the salt concentration to the range of 3 to 5 ppm. Typically, brine droplets in crude oil are stabilized by a mixture of surface-active components such as waxes, asphaltenes, resins, and naphthenic acids that are electrostatically bound to the droplets surface. Such components provide an interfacial film over the brine droplet, resulting in a diminished droplet coalescence. Adding water to the crude oil can decrease the concentration of the surface-active components on the surface of each droplet, because the number of droplets is increased without increasing component concentration. 

Organic acids present in crude oils are collectively called naphthenic acids. The concentration of these acids in different crudes is highly variable, and their corrosive effect on unprotected crude units can be very severe. 

The presence of naphthenic acids in crude oil has a great influence tend to cause operational problems on petroleum refiners, such as foaming in the desalter or other units and carrying cations through the desalting process, which can cause deactivation of catalysts and corrosion problems. 

Extraction of naphthenic acid from the oils can completely solve the corrosion problems associated with this acid. It had been reported that naphthenic acid can be transferred into the water phase by using an aqueous solution of caustic soda. This is accomplished by converting the acid into basic salt that is preferentially soluble in water. At present, this method is successful only in the laboratory practical application in crude oil processing plants has not been reported. 

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