Nickel Insitute - Nickel Alloys in Organic Acids & Related Compounds

PART IV. ESTER PREPARATIONS A. Acetic Esters

is continually removed as the process continues. The residual sulfuric acid concentration continually increases and can then create very aggressive conditions toward latter stages of a batch process run. The kettle used for this process is of major concern. The heating coils, calandria, or other heating device sustains the major corrosion in the process. On the tubes of such a heater, severe pitting, grooving and general attack develop by concentration of the acid on the hot surface, by the formation of tars on the metal and, in some instances, by the accumulation of corrosive salts from the solution. As a consequence, it is exceptionally difficult to provide defini- tive data for the corrosion of a specific alloy in the preparation of these esters. Only empirical data obtained over a lengthy period of time will provide proper guidance for the final selection of the material of construction for the coils, kettle, vapor lines, condensers and a primary column for the process. In Table LXXII, field data obtained by the exposure of numerous alloys in five different ester preparations are provided. It will be seen here that considerable variation exists in the data obtained for any one alloy. Because of the great turbulence existing and the factors enumerated above, the corrosion of an alloy in the same process during two different exposures can be greatly different. Although the data would indicate that Type 304 stain-

One major use of acetic acid is as a precursor of the various esters that become important solvents for paints and other chemical products. In the production of acetic esters, the acid is combined with other organic compounds contain- ing a hydroxyl group. The more common esters are ethyl acetate, butyl acetate, isopropyl acetate and Cellosolve acetate. Corrosion to be expected in the preparation of these esters can vary greatly depending on the operation. If acetic acid were the only corrosive contaminant present, the data provided previously for acetic acid could be used as a guide. Unfortunately, a catalyst is necessary to improve the efficiency of the process, and in most instances, the presence of this catalyst determines the corrosion to be expected. Temperatures required for the production of these esters will range from 60 to 150 ºC (140-302 ºF), depending on the boiling point of the ester. Sulfuric acid has long been used as the catalyst for synthesis of the esters. This is added as concentrated sulfuric acid in small quantities of only 0.5 to 2.0% of the total charge. In anhydrous medium, this would not be excessively corrosive. However, water is produced by the reaction between the alcohol and acid which can serve as a temporary diluent for the sulfuric acid. The water formed

TABLE LXXII

Corrosion of Alloys in Batch Acetic Ester Preparations

Conditions: Exposure of racks in same kettle during the preparation of esters using sulfuric acid catalyst. Temperature varies with ester prepared. Cupric ion present. Liquid (L) and vapor (V) exposures. Test 1–Ethyl and isopropyl acetate alternately for 50 days @ 110 ºC (230 ºF). Test 2–Isopropyl acetate for 14 days @ 110 ºC (230 ºF). Test 3–Amyl acetate for 11 days @ 115 ºC (239 ºF). Test 4–Ethyl and isopropyl acetate alternately for 81 days @ 110 ºC (230 ºF). Test 5–Butyl and methyl Cellosolve acetate alternately for 29 days @ 115 º C(239 ºF) and 150 ºC (302 ºF).

Corrosion Rate

1

2

3

4

5

L

V

L

V

L

V

L

V

L

V

Alloy

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 mm/y mpy

Type 304

–

–

–

–

.05

2

.56

22

.15

6

.28

11

.05

2

.43

17

.05

2

.30

12

Stainless Steel Type 329 Stainless Steel Type 316 Stainless Steel Type 216 Stainless Steel CARPENTER alloy 20 Cb-3 JESSOP alloy JS-700 HASTELLOY

–

–

–

–

<.03 <1

–

–

.15

6

–

–

.28

11

–

–

.03

1

.03

1

.18

7

.10

4

<.03 <1

.23

9

.18

7

.23

9

.08

3

.10

4

.03

1

<.03 <1

.23

9

.41

16

–

–

–

–

–

–

–

–

.23

9

.08

3

–

–

–

–

.13

5

.13

5

–

–

–

–

–

–

–

–

–

–

–

–

.03

1

–

–

–

–

–

–

–

–

–

–

–

–

–

–

–

–

–

–

.03

1

–

–

–

–

.03

1

.03

1

.08

3

.08

3

.05

2

.05

2

–

–

–

–

alloy G MONEL alloy 400 Copper

.86

34

.15

6

–

–

.13

5

–

–

.08

3

–

–

.03

1

–

–

–

–

1.65 65

.10

4

4.57 180

.05

2

1.42 56

.08

3

2.72 107

.43

17

.13

5

.03

1

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