Nickel Insitute - Nickel Alloys in Organic Acids & Related Compounds

less steel would be satisfactory for immersion conditions in an esterification kettle, this is most unlikely. The use of Types 316 or 316L stainless steels is borderline for these applications. As the alloy content is increased, reduced rates of attack are obtained, but the economics of the selection require detailed analysis before committing one to a final decision. It will be noted that those materials high in nickel have good promise for use in the process. It has been reported that nickel-copper Alloy 400 has been used extensively for pumps, reactors, heating coils, piping and agitators for such acetic acid services in unaerated solutions where sulfuric acid is present. 22 The data and literature show that in numerous instances nickel-chromium Alloy 600 has exhibited excellent resistance to esterification environ- ments. For instance, INCONEL alloy 600 exposed in an amyl acetate preparation at 149 ºC (300 ºF) during a 28- day exposure showed corrosion of only .15 mm/y (6 mpy) while MONEL alloy 400 showed a rate of .69 mm/y (27 mpy). Some combinations of Type 316 stainless steel, Alloys 400 and 600 and the copper alloys are indicated to be the basic choices for this service. When using an austenitic stainless steel, such as the Type 316L, it can be shown that a considerable reduction of the corrosion rate can be achieved by the addition of oxidizing ions to the solution. Cupric and ferric ions are both effective for this purpose. One way of providing such an environment is the use of a copper alloy kettle with stainless steel heating coils. The stainless steel can have an adequate life in such service, whereas an all Type 316L stainless steel system would not be acceptable. The effect of the concentration of sulfuric acid in such a batch process can be noted by reference to Table LXXIII. At the temperatures of the esterification reaction, the corrosion rate of Types 316 and 316L stainless steels

increases rapidly as the sulfuric acid is concentrated in the kettle. Also, there can be some small amount of degrada- tion of the acid to provide corrosive sulfur compounds in the vapor. The severe effect of the acid conditions on a heating surface is apparent by reviewing the data of Table LXXIV. Laboratory “hot wall” tests of various alloys show the corrosion to be much higher for the materials than would be experienced in a simple boiling solution. Certainly, experience in the field has confirmed the severe corrosion to be expected on such heating surfaces in the process. For this reason, graphite calandrias are sometimes used to assure adequate resistance of the heating element. There are other, less corrosive acids available for cata- lyzing the esterification reaction. Toluene sulfonic acid (TSA) has often been used for this purpose and, in

TABLE LXXIII

Effect of Sulfuric Acid Concentration on Corrosion of Type 316 Stainless Steel in an Esterification Reaction

Conditions: Solution of 25% acetic acid, 59% butyl acetate, 10% water and 6% butanol prepared and sulfuric acid added as indicated. Tests conducted at the boiling point.

Corrosion Rate

% Sulfuric Acid Added (as 95% H 2 SO 4 )

mm/y

mpy

0.0 0.1 0.5 1.0

Nil

Nil 19

.48 5.92 17.53

233 690

TABLE LXXIV

“Hot Wall” Tests of Alloys in a Synthetic Esterification Mixture

Laboratory tests using “hot wall” apparatus for three days (<3 days for alloys showing high corrosion rates) in a mixture of 83% acetic acid–9.3% formic acid–3.8% H 2 S0 4 – 3.9% water. Comparison with conventional immersion test at boiling temperature 112 ºC (234 ºF) provided.

Conditions:

Hot Wall Solution Temperature

Hot Wall Specimen Temperature

Hot Wall Corrosion Rate

Immersion Test Corrosion Rate

Alloy

ºC

ºF

ºC

ºF

mm/y

mpy

mm/y

mpy

E-BRITS 26-1 HASTELLOY alloy G HASTELLOY alloy C-276 MONEL alloy 400 Copper (C10200) Zirconium Type 316 Stainless Steel

120 112 118 118 118 118 –

248 234 244 244 244 244 –

150 155 140 137 135 142 –

302 311 284 277 275 288 –

3.94 .99 .48 .41 1.42 .03 –

155 39 19 16 56 1 –

1.88 .20 – – – – 3.63

74 8

– – – – 143

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