Nickel Insitute - Nickel Alloys in Organic Acids & Related Compounds

acetic acid streams and the contaminant may control the corrosion rate. Acrylic acid per se is not required as an end product in large quantities as is the ester. Consequently, many of the commercial processes are designed to prepare the ester from a basic organic molecule without isolating the acrylic acid. Regardless of the route to the final product, however, all processes produce the acid as an intermediate with subsequent esterification. Acrylic aid, or the acid-ester in one sequence, has been produced by at least nine different processes. Three basic reactions have been used predominantly. 27 These are the acetylene-carbon monoxide, the nitrile and the propylene oxidation processes. Today, the direct oxidation of pro- pylene to acrolein and finally to the acid in a one or two- step process is the most popular. The acetylene-carbon monoxide process relies on the catalytic activity of nickel carbonyl in the presence of a strong acid (hydrochloric) to prepare the acid. Obviously, the presence of the HCl controls the corrosive conditions existing in the process. Even at the low reaction tempera- tures, 30-52 ºC (86-126 ºF), the reaction step is most corrosive and is conducted in glass, ceramics and TEFLON* equipment. Following this step, the nickel- base molybdenum alloys can be used, and as the mineral acid is removed, the alloy content of the materials of construction can be reduced until Type 316 stainless steel is acceptable to handle the acrylic acid. The nitrile procedure for production of the acid suffers the same drawback from a materials standpoint with sulfuric acid used to produce an organic sulfate which can be released to the acid or directly reacted to form the ester. Ammonium acid sulfate is formed as a by-product, and the higher process temperatures, 150 ºC (300 ºF) region, generate SO 2 and SO 3 which must be contended with in equipment design. The problems encountered are essen- tially the same whether using acrylonitrile or ethylene oxide and hydrogen cyanide as the starting materials. Table XXXIX provides typical corrosion data for a nitrile type process operation through the distillation of the crude acid. Sets of data are given for two exposures in the same equipment (top head of reactor condenser) to show the wide variation in corrosion rates experienced during different periods of operation. Such large changes in the corrosive environment may be found wherever a mineral acid is mixed with an organic acid. When using metallic materials of construction in such processes, the operation must be conducted with particular care to maintain favorable conditions for a maximum life of the equipment. A few hours of adverse operating conditions can severely damage equipment under such circum- stances. Monitoring of the corrosion by continuous or sequential testing is also advised to detect periods of unusual corrosion. This process is obviously most corro- sive until the decomposition products of the inorganic acid and salts are removed. The usual materials, primarily the austenitic stainless steels, are then used to process the acrylic acid.

FIG 11–Isocorrosion Chart for MONEL alloy 400 in Formic Acid

B. Acrylic Acid

Acrylic acid is the most common 3-carbon acid encoun- tered in industry. The great reactivity of this unsaturated acid makes the material and its esters useful in the preparation of a wide variety of resinous products used in manufacturing plastics, paints, textiles, paper and pol- ishes. There are probably some one billion pounds of the esters produced in the USA today, of which 75% is ethyl acrylate. The significant fact about handling acrylic acid is that temperatures are maintained as low as possible to prevent homopolymerization of the acid. Distillation in vacuum stills, dilution with innocuous solvents, storage of the product at the lowest convenient temperatures and reaction of the acid in polymerization processes at low temperatures are common process conditions. Consequently, exposure conditions in most acrylic acid applications are less severe than in the saturated acid processes. As with propionic acid, the acrylic acid can be consid- ered as equivalent to acetic acid in aggressiveness at a given temperature. However, the contaminants in acrylic acid process streams can be different from those found in

This HASTELLOY alloy C-276 tube bundle is used in the reboiler in the manufacture of acrylic monomers. It was found to be the most economical material of construction for this severely corrosive service. Courtesy Stellite Division, Cabot Corporation.

*Trademark of E.I. duPont de Nemours & Co.

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