Nickel Insitute - Nickel Alloys in Organic Acids & Related Compounds
TABLE LXI
Corrosion of Alloys in Adipic Acid Process
Low temperature, 100 ºC (212 ºF), reaction with am- monium vanadate and cupric ion, process involved oxidation of cyclohexanone and cyclohexanol (KA oil) in strong nitric acid. In addition to the desired adipic acid, succinic, glutaric and lighter acids were formed in the process.
Corrosion Rate
Product Washing and
Lower
Separation Still
Reactor
Scrubber
Absorber
Crystallizer
Acids Stripper
Crystallizer
Centrifuge
Drying Equip. mm/y mpy mm/y mpy mm/y mpy mm/y mpy mm/y mpy mm/y mpy mm/y mpy mm/y mpy mm/y mpy
Alloy
.23
9
.30
12
.23
9
.91 36
.18*
7*
.41 16
.03
1
.13
5
Nil
Nil
Type 304
Stainless Steel Type 316 Stainless Steel CARPENTER alloy 20Cb-3 HASTELLOY
.36
14
.05
2
.05
2
.30
12
.08*
3*
.05
2
.03
1
.05
2
Nil
Nil
.10
4
.05
2
.03
1
.13
5
Nil
Nil
.03
1
Nil
Nil
.03
1
Nil
Nil
Nil
Nil
Nil
Nil
Nil
Nil
Nil
Nil
Nil
Nil
Nil
Nil
Nil
Nil
Nil
Nil
Nil
Nil
alloy C-276 Titanium
Nil
Nil
Nil
Nil
Nil
Nil
Nil
Nil
Nil* Nil*
Nil
Nil
Nil
Nil
Nil
Nil
Nil
Nil
*Pitting indicated
Data derived from corrosion tests in a large phthalic anhydride plant are presented in Table LVIII. These show that, in addition to the higher stainless steels, the use of Alloys 400, 600 and other nickel-base alloys is permissible in many areas of the process. For equipment handling the brominated anhydride, the use of HASTELLOY alloy C- 276 has proven to be attractive. Terephthalic acid (para-phthalic acid) is produced in large quantity, primarily for the preparation of polyester resins used in the textile industry. A number of processes have been investigated to produce the acid in as pure form as economically as possible. Initially, the process required the oxidation of xylene using a bromide catalyst. Inasmuch as acetic acid is used as a dilulent in the process, the reaction mixture of a halogen and acetic acid was extremely corrosive. HASTELLOY alloy C-276 was the only contender for use in these areas. Once the bromide ion was removed, Type 316 stainless steel was found to be useful for the vast majority of the remainder of the equipment.
With the research interest to produce a simpler and more economical mode of preparation, a number of new methods have evolved. One of the more common pro- cedures is the use of oxygen along with a cobaltic ion catalyst to effect the reaction. Again, acetic acid is used as the medium for the reaction. Thus, acetic acid is the primary corrosive to be considered. Contamination of the acetic acid by the terephthalic acid (TPA) adds little to the corrosion produced. The major problem is one of han- dling an acetic acid medium at high temperatures. There are a number of steps in the process where the tempera- tures are well above those required for producing acetic acid itself. In these areas, the use of materials suitable for exposure in acetic acid at high temperatures under oxidiz- ing conditions are acceptable. (See discussion of acetic acid.) Table LX shows data generated by both laboratory and field tests designed to explore corrosion within the area of the leaching step of the process. At this point, the acetic acid medium is taken to a very high temperature to allow the rather insoluble TPA to precipitate from solution before taking the material to a crystallizer. It will be noted that Type 316 stainless steel is a borderline material for this specific area because contamination of this stream is undesirable. Titanium is favored for this most aggressive area in the process. For all of the remainder of the process, the use of Type 316 stainless steel has been found to be most satisfactory. It is necessary to avoid the presence of crevices or other areas where differential corrosion cells can be created in either titanium or the Type 316 stainless steel equipment.
TABLE LXII
Laboratory Tests for Corrosion of Alloys in Molten Adipic Acid at 170 ºC (338 ºF)
Corrosion Rate
Alloy
mm/y
mpy
1.30 .43
51 17
Type 304 Stainless Steel Type 321 Stainless Steel
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