Nickel Insitute - Nickel Alloys in Organic Acids & Related Compounds

The middle column sections may require somewhat less highly alloyed materials, such as the iron-base nickel- chromium-copper-molybdenum alloys, while the top por- tion of the column, the condenser and all associated piping may be made of Type 316L stainless steel. Returning to the higher temperatures of the base area, the calandria circulating pump and other cast appurtenances must be of a Type CN-7M casting as a minimum, and the use of more corrosion-resistant alloys or graphite may be necessary. The remainder of all operating facilities of an acetaldehyde-based acetic acid unit can normally be con- structed of Type 316L stainless steel with Types CF-8M or CF-3M cast valves and pumps. The anhydride refining still normally presents no exceptional corrosion problems for Type 316L stainless steel. Copper, cupro-nickel alloys and Alloy 400 nickel-copper alloy can be used for any required applications once the peracetic acid is destroyed in the system by high temperatures in holding tanks or column bases and the equipment is sealed from the ingress of air. Corrosion data obtained in an acetaldehyde oxidation process unit are tabulated in Table XXVII. Additional data for a wide range of allays exposed in an acetic acid residue still of the same process are given in Table VIII.

b. Liquid Phase Oxidation of Straight- Chain Hydrocarbons

Among the important processes of today for acetic acid production are those based on the direct oxidation of straight-chain hydrocarbons, such as propane, propylene, butane, butene and higher aliphatics. The oxidation can be achieved using air or oxygen. Reaction conditions are much more severe than for the simple oxidation of an aldehyde with temperatures near 200 ºC (392 ºF) at pressures of more than 700 psi. Breaking up a hydrocarbon by such a severe oxidation obviously produces many by- products in addition to acetic acid. Among these are formic, propionic, butyric and higher acids, ketones, esters and peroxide compounds. The reaction conditions of the converter can be varied to increase or decrease the ratio of the by-products. This mix of products and by-products creates two problems not present in an aldehyde oxidation process: (1) much more separation equipment is required to recover the products and (2) the corrosion medium is more complex. Added to this is the large size of the equipment required for the large volume output of a modern single-train unit. A simplified flow diagram for a typical hydrocarbon oxidation unit is shown in Figure 4. Essentially the entire

TABLE XXIX

Corrosion of Allays in Laboratory Equivalents of the Methanol-Carbon Monoxide Reaction Medium

Conditions: Small autoclave tests for 48 hours using 50% acetic acid at autogenous pressure without

hydrate and 7 grams potassium iodide per 100 grams of acetic acid). Carbon monoxide atmosphere.

and with catalyst (7 grams cobalt acetate

Corrosion Rate Without Catalyst

With Catalyst

Temperature

Alloy

ºC

ºF

mm/y

mpy

mm/y

mpy

–

–

2.03

80

Type 304 Stainless Steel Type 310 Stainless Steel

250 300

482 572

>25.4*

>1000*

–

–

–

–

10.16

400

Type 321 Stainless Steel

250

482

>25.4

> 1000

– –

– –

Type 347 Stainless Steel Type 316 Stainless Steel

300

572 572 482 500 572 482

9.14*

360*

300 250 260

5.08

200

–

–

–

–

22.35

880

1.63

64

–

–

CARPENTER alloy 20

300

250

3.81

150

–

–

–

–

5.08

200

INCOLOY alloy 825

260

500

.36

14

–

–

HASTELLOY alloy C

280 260

536 500

–

–

1.78

70

230

446

–

–

5.08

200

<.03

<1

–

–

HASTELLOY alloy B

280

536 500

260

–

–

.36

14

230

446

– –

– –

71

28

5.84

230

Nickel 200

260

500

–

–

3.05

120

Silver

230

446

260

500

–

–

2.67

105

DURIRON

–

–

<.03

< 1

Titanium

260

500

–

–

<.03

< 1

Zirconium

260

500

–

–

<.03

< 1

Tantalum

260

500

*Pitting

Reference 17

Page 26