78 Some Specific Applications Of Alloy

A major chemical company producing chlorinated and fluorinated chemicals had to replace the reactor pressure vessel made out of alloy C-276 every 12-14 months, due to excessive corrosion. The process employed various hydrocarbons, ammonium fluoride, sulfuric acid, and a proprietary catalyst in which one atom of chlorine was replaced with one atom of fluorine. The presence of fluorides ruled out use of tantalum, titanium, and glass-lined vessels. Switching to a Ni-Mo alloy B-2 prolonged the life by only 20-25%. This was also unacceptable. Extensive tests made with alloy 59 and other alloys indicated that with alloy 59 the life of this ASME code reactor vessel could be increased by 300-400%. A vessel was built with alloy 59 in 1994. After 27 months of service, a minor repair of the "thermowell" weld had to be performed. It is expected that the life of this vessel will even surpass the original expectations. Since then two more vessels of alloy 59 have been ordered by this same company and are giving excellent service. Other companies in Europe have also selected alloy 59 in the production of chlorinated, fluorinated, and fine chemicals. Figure 7.3 shows one of the reactor vessel constructed out of alloy 59.

FIGURE 7.3 An 8000-gal alloy 59 pressure vessel for production of fine chemicals at Degussa-Hulls, Europe. (Fabricator: Apparatebau GmbH, Essen, Germany).

Alloy 59 was selected by a European company for hydrofluoric acid production after a field test in a rotary kiln, where this alloy gave superior performance to alloy 686, alloy C-2000, and alloy C-22.

The corrosive conditions in scrubbers of coal-fired power plants (FGD systems) and waste incinerators, both municipal and hazardous waste, have been so severe that only alloys of the Ni-Cr-Mo have given reliable performance. The presence of condensates with chloride levels over 100,000 ppm, and fluorides of over 10,000 ppm, very low pH (below 1), sulfuric acid, hydrochloric acid, hydrofluoric acid, various salts, and other contaminants create a situation where lower alloys have failed in a few days to a few weeks. Many thousands of tons of alloy 59 have been used in recent years in these systems in Europe and other parts of the world, giving satisfactory performance [39, 40]. Very recently alloy 59 (over 80 tons) was selected by Arizona Public Service for its FGD scrubber project for both the chimney and the outlet duct. This alloy was chosen over other C family alloys because of its superior corrosion-resistant properties and proven case histories. Another major specification of this alloy has been in the Syncrude Project in Canada, where oil is extracted from the tar sands. The FGD scrubber system will utilize ammonia solution to produce ammonium sulfate, which then will be used as an agrichemical. Over 700 tons of alloy 59 will be used in this project.

During manufacture and synthesis of acrylates and methacrylates, the process reaction at 130°C is carried under oxidizing conditions in the presence of acids, fatty alcohols, and paratoluene sulfonic acid. The previous material of construction, alloy 400 had failed rapidly with corrosion rates approaching 0.75 mm/yr. A test program with various alloys including 904L, 28, G-3, 625, C-276, 31, and 59 showed alloy 59 to be totally free from localized attack with corrosion rate of less than 0.025 mm/yr. Alloy 59 was selected and has operated without any problems for the last 5 years.

In citric acid production, a 6% Mo alloy failed rapidly. The reaction was treating with calcium citrate with concentrated H2SO4 around 96°C. A test program with alloy 59 led to its selection, and since then four reactors have been built. The first one, installed in 1990, continues to operate without any problems. In another citric acid plant, plate heat exchangers of alloy 20 were failing rapidly. Testing with various alloys also led to alloy 59 selection. These alloy 59 plate heat exchangers are giving very reliable service.

In a copper plant, the SO2-rich gas from the flash furnace is scrubbed with a solution of dilute 5% contaminated H2SO4 at a temperature of 45-60°C. The acid produced has a concentration of typically around 50-55% H2SO4 and a temperature of about 75°C. The chloride and fluoride contents of this acid are both high, at about 7000 ppm. Previous materials of construction (alloy 20 and rubber lined carbon steel) had failed very rapidly. Tests were carried out using alloy 59, alloy 31 and other alloys. Corrosion rates for both alloys 59 and 31 were below 0.025 mm/yr with no localized corrosion. Following these tests, alloy 31 was purchased for the scrubber internals handling the produced acid and alloy 59 for the induced draft fans. These have been in successful operation for the last 6 years with no detectable corrosion. Since then more fans of alloys 59 and 31 have been placed in service.

In a weld overlay of burner bases, where hydrogen and chlorine are burnt to produce hydrochloric acid, a two-layer electroslag alloy 59 weld overlay performed significantly better than all previously used materials including alloy C-22. In another weld overlay application with alloy 59, superheater tubes in a waste incineration plant, extended the life by significantly reducing unusually high fireside surface wastage.

In a plant, plate heat exchangers handling acetic acid derivative effluents were failing rapidly. Corrosion testing at 100°C with alloy C-276 and alloy 59 gave corrosion rates of 0.4 mm/yr for alloy C-276 vs. 0.04 mm/yr for alloy 59, a 10fold improvement. Hence alloy 59 was selected. The media consisted of sulfates, acetic acid, phosphates, and chlorides with a pH of 1.

Gold sponge is deposited from an electrolyte of dilute HCl containing impure gold. The deposited spongy gold cathodes are washed in water to remove the HCl and then dried in an oven at 150°C, where the evaporation of remaining dilute HCl electrolyte creates very severe corrosive conditions. After extensive testing, alloy 59 was selected for this application and has been performing well since 1990.

Details of some other applications are described elsewhere [41]. There are many more applications of alloy 59, too numerous to list here, but the above gives a flavor of the diversity, versatility, and usefulness of this alloy in a wide range of industries and applications.

0 0

Post a comment