Corrosion napthenic-acid

This input to design refers to the long-term stability of the raw material sources for the plant. It is only of importance where the raw materials can or do contain impurities which can have profound effects on the corrosivity of the process. Just as the design should cater not only for the norm of operation but for the extremes, so it is pertinent to question the assumptions made about raw material purity. Crude oil (where HjS, mercaptan sulphur and napthenic acid contents determine the corrosivity of the distillation process) and phosphate rock (chloride, silica and fluoride determine the corrosivity of phosphoric acid) are very pertinent examples. Thus, crude-oil units intended to process low-sulphur crudes , and therefore designed on a basis of carbon-steel equipment, experience serious corrosion problems when only higher sulphur crudes are economically available and must be processed. 

Some crude oils contain significant concentrations of organic acids, mainly napthenic acids, which are corrosive to carbon steel at temperatures above about 400 F (200°C), and may require the use of molybdeniun-containing austenitic stainless steels (i.e., AISI316 and 317). Determination of when to use alloys is done by analysis of the crude oil for acid content using ASTM D 974, Test Method for Acid and... 

Stainless steels are subjected to serious corrosion problems in oil refineries as they may encounter very corrosive fluids, such as hydrochloric acid and napthenic acids in the process streams. 

Most crude oils contain some napthenic 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. 

Spent caustic solutions from petroleum refining. Petrochemical refineries use caustics to remove acidic compounds such as mercaptans from liquid petroleum streams to reduce produced odor and corrosivity as well as to meet product sulfur specifications. Spent liquid treating caustics from petroleum refineries are excluded from the definition of solid waste if they are used as a feedstock in the manufacture of napthenic and cresylic acid products. U.S. EPA believes that spent caustic, when used in this manner, is a valuable commercial feedstock in the production of these particular products, and is therefore eligible for exclusion. 

---