Nickel Insitute - Nickel Alloys in Organic Acids & Related Compounds

By-product formic acid adds acidity to the system, but does not greatly increase corrosion of the stainless steels. Table VII shows corrosion rates of various alloys in acetic- formic acid mixtures typical of those existing in a hydro- carbon oxidation unit, but without peroxides present. As in other processes for acetic acid production, the peroxides react with other components of the stream or are decom- posed with sufficient time at the higher temperatures. Thus, the cupro-nickels and Alloy 400 can be used after the process stream passes the separation column of such a system, if desired. Actual corrosion data obtained in the various parts of a hydrocarbon oxidation unit are shown in Table XXVIII. Contamination of the process stream with chlorides, metal salts, or other inorganic materials introduced in the feed streams or by leakage into the system can have disastrous effects as noted under the discussion on the effect of contaminants. c. Methanol-Carbon Monoxide Synthesis The methanol-carbon monoxide process is one of the newer and economically attractive routes for the produc- tion of acetic acid. In this process, all factors contributing to higher corrosion rates are encountered—a 50-75% acid concentration, higher temperatures, higher pressures and the use of halide salts as catalysts. The use of boron trifluoride as a catalyst did not become popular because of the exceedingly high pressures involved, but the use of iodides in combination with other metallic salts has increased in popularity throughout the world. No data directly derived from the field exposure of alloys in operating equipment of the methanol-carbon monoxide process are available. However, Togano and others have delineated the problem facing the corrosion engineer when materials selection must be made for these processes. 16-18 The reaction vessel must be made of the most resistant alloys available. In all probability, the process stream must be carried through the first two still columns before the halogen is reduced to a level sufficient to allow the use of the austenitic stainless steels. Even at this point, care must be exercised in selecting a stainless steel because the acid is not derived from an oxygenated reaction. Thus, no peroxides or oxidizing gases from their decomposition will be available to aid in passivity of the stainless steel. Certain of the higher nickel alloys do appear to have promise in this process. Tables XXIX and XXX taken from the Tagano reports show HASTELLOY alloys B and B-2 to be worthy of thorough testing along with titanium, zirconium and tantalum for the high pressure, high temperature reaction area. Once the temperature is re- duced to the normal recovery conditions, the use of nickel- molybdenum and nickel-copper alloys appears at- tractive even with the catalyst salts present. It also appears that INCOLOY alloy 825 should be evaluated with close attention to make sure that the resistance to pitting shown in Table XXX is consistent. As indicated, once the halide salts are removed, the conventional materials used for the separation and recovery of the acid can be employed.

Certain other interesting observations can be found in references 17 and 18. The catalyst system requires very high concentrations of catalyst. When half of the catalyst is a halide salt, the potential for corrosion is greatly increased. (See Effect of Contaminants.) By comparison with various other tests, particularly when appraising the austenitic stainless steels, it is apparent that the iodide ion is not as aggressive as is the chloride ion. The authors found no adverse effect of carbon monox- ide on the nickel-base alloys at the temperatures and pressures explored. Indeed, the presence of CO was reported to reduce corrosion, particularly pitting of iron- base alloys. 2. Acetic Acid Storage and Shipping Stainless steels are used for the construction of storage vessels for acetic acid to maintain the highest quality of the acid. Vessels of the dished-head type, API variety, or external support construction have been used for this purpose. The latter has been used extensively in the wine fields for the storage of wine in the past and provides the most economical method of fabricating field tanks for acetic acid storage if the size is not excessive. When choosing the stainless steel grade, consideration should be given to the temperature of storage proposed and to the grade of acid required. In the northern latitudes, it will be necessary to provide a heating coil to assure fluid

Of the 15 miles of pipe used in this storage terminal for handling organics, 3 miles are of Type 316 stainless steel. This material protects the purity of formaldehyde, acetic acid and propionic acid.

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