Andrussow process

The Andrussow process was patented by Dr. L. Andrussow of I.G. Farben AG in Germany in 1933 (U.S. Patent 1,934,838). Its main advantages are low converter investment, low maintenance costs and high natural gas yields when the waste gas is used as a boiler fuel. The Andrussow process produces HCN by the reaction of ammonia, air and natural gas at 1,000°C to 1,200°C in the presence of a platinum/rhodium catalyst. The reaction is  

The reaction generally takes place at a pressure of less than two bar with a very short residence time. The converter discharge gas is rapidly quenched to less than 400°C to avoid decomposition of HCN. The reaction gases are usually quenched in a steam-generating, waste-heat boiler that is directly below the catalyst129. The heat of reaction is recovered in the waste-heat boiler to generate about five (5) pounds of steam per pound of HCN produced. 

The precious metal catalyst is usually 90% platinum and 10% rhodium in gauze form. Platinum alone does not have adequate mechanical strength to endure extended exposure to the high reaction temperature. When platinum is alloyed with rhodium, the catalyst life can range from 4,000 hours to as much as 10,000 hours. A catalyst pack is usually replaced because it has been contaminated by minor impurities in the feed gases. Very little catalyst metal is consumed or lost during the life of a catalyst. 

The reaction takes place under fuel-rich conditions to maintain a nonflammable feed mixture. Typical feed composition is 13% to 15% ammonia, 11% to 13% methane and 72% to 76% air on a volumetric basis. Control of feed composition is essential to guard against deflagrations as well as to maximize the yield. The yield from methane is approximately 60% of theoretical. Conversion, yields, and productivity of the HCN synthesis are influenced by the extent of feed gas preheat, purity of the feeds, reactor geometry, feed gas composition, contact time, catalyst composition and purity, converter gas pressure, quench time and materials of construction. 

The reactions in the Andrussow process are more complex than that shown in equation 72130. Most of the heat required for HCN formation is supplied by combustion of methane. This results in an overall reaction that is exothermic even though the endotherm of the methane-ammonia reaction is 60 kcal per mole of HCN129. The converter off-gas typically has the following composition  

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Fig. 1. Andrussow process plant with ammonia recycle.

Approximately 80% of all hydrogen cyanide is manufactured by the reaction of air, ammonia, and natural gas over a platinum or platinum-rhodium catalyst at elevated temperature. The reaction is referred to as the Andrussow process. Hydrogen cyanide is also available as a by-product from aciylonitrile manufacture by ammoxidation (20%). 

Hydrogen cyanide is generally produced in industrial quantities by high temperature catalytic reaction between ammonia, methane, and air (the Andrussow process). The stoichiometry of the process is ... 

Hydrogen cyanide is an important building block chemical for the synthesis of a variety of industrially important chemicals, such as 2 hydroxy-4 methylthiobutyric acid, adiponitrile, nitrilotriacetic acid, lactic acid, and methyl methacrylate. The primary commercial routes to hydrogen cyanide are the reaction of methane and ammonia under aerobic (Andrussow Process) or anaerobic conditions (Degussa Process), or the separation of hydrogen cyanide as a by-product of the ammoxidation of propylene < ) The ammoxidation of methanol could represent an attractive alternate route to HCN for a number of reasons. First, on a molar basis, the price of methanol has become close to that of methane as world methanol capacity has increased. However, an accurate long term pricing picture for these two raw... 

One caveat pertains to the platinum oxide transport model It does not appear to be able to explain the differences in metal weight loss during ammonia oxidation and hydrogen cyanide synthesis by the Andrussow process. In the Andrussow process a mixture of methane, ammonia, and air is used to maintain a high temperature (1200°C) while generating hydrogen cyanide. Alternative processes require energy input, because HCN synthesis is an endothermic reaction. Thus, in both ammonia oxidation and HCN synthesis, platinum or alloy... 

Bodke AS, Olschki DA, Schmidt LD. Hydrogen addition to the Andrussow process for HCN synthesis. Appl Catal A General 2000 201 13-22. 

The Andrussow process is currently the principal HCN manufacturing process in the world,... 

The unreacted ammonia is removed from the reactor off-gas by scrubbing with sulfuric acid to make ammonium sulfate. Just like the Andrussow process, ammonia must be removed from the off-gas before HCN can be recovered because the ammonia promotes polymerization of the HCN. After ammonia is removed from the converter off-gases, the remaining gas stream is processed in a way similar to the Andrussow process129. 

In 2003 Mitsubishi Rayon was believed to be operating a commercial-scale, methanol-based HCN process and had offered to license the technology to other companies. The technology might provide a low-cost way to convert an acrylonitrile plant to HCN-only production. However methanol is a higher cost source of carbon compared to natural gas so the methanol process probably has a higher operating cost than the Andrussow process. 

In hydrogen cyanide synthesis using the Andrussow process, air, methane, and ammonia are fed over 15 to 50 layers of noble metal gauze at 1050 to 1150°C at near atmospheric pressure. 

Six processes are presently of economic significance the Andrussow process is currently the principal HCN manufacturing process in the world, the BMA process is practiced by two companies and provides high yield and selectivity by using a complex reaction system, the Fluohmic process is of interest in locations where electricity is inexpensive, the formamide process is useful for sites with inexpensive carbon monoxide, the BP (British Petroleum) acrylonitrile process produces HCN as a byproduct, and the methanol process. 

The Andrussow process (7) uses Pt(-f Rh or Ir) in the form of gauze (as in the Nila oxidation process) and its yield is 60-65% for complete conversion per pass. 

Figure 3 Micrograph of woven platinum/rhodium gauze after prolonged use in the Andrussow process. The gauze has a matte appearance, and the apertures are considerably smaller than in fresh gauze.

Some of the Group VIII metals have uses as oxidation catalysts (5). All except platinum do however tend to oxidize under vigorous conditions, such as are used in ammonia oxidation and the Andrussow process, for which only platinum and its alloys are acceptable catalysts. Under milder conditions both platinum and palladium have somewhat limited applications in liquid-phase oxidation processes, as for example in the carbohydrate field. 

These phenomena have been studied in three papers by L. D. Schmidt and his co-workers. They first report on the morphology and surface composition of gauzes after typical use in the nitric acid and Andrussow processes. 

Very similar conclusions can be reached for the Andrussow process as already discussed in Section 3 above in connection with Pan s results at 2 atm. ... 

For the Andrussow process the effect of diffusion limitation is to ensure that the composition of the gas phase at the catalyst surface is richer in NH3 and less rich in methane and oxygen than the bulk gas composition. This implies that the maximum HCN synthesis rates are achieved with surface CH4/NH3 and air/fuel ratios lower than those prevailing in the bulk gas phase. 

The balance of evidence suggests that the catalyst in ammonia oxidation has a clean surface free from S and C and that aging is a consequence of the formation of gaseous Pt02. The situation for the catalyst used in the Andrussow process is less clear. Despite much lower oxygen pressures and consequent surface contamination by C and S species, the surface restructures due, perhaps, to surface energy differences aging is linked to build-up of carbonaceous films and the presence of iron impurities. 

Figure 4.1 Hydrogen cyanide via the Andrussow process. Source [7],...

Figure 4.1 is a flow diagram of the Andrussow process [7], To avoid the decomposition of methane and ammonia, the ratio of reactants must be carefully controlled. The products are cooled where care is taken to avoid the formation of azulmic acids, polymers formed by the reaction between hydrogen cyanide, ammonia, and water. The products go to a scrubbing tower where unconverted ammonia is absorbed in sulfuric acid. The product is then absorbed in water, stripped, and distilled to produce greater than 99% HCN [8]. Yields are 70 and 60% for methane and ammonia, respectively. 

The Degussa BMA (Blausaure-Methan-Ammoniak, or hydrocyanic acid-methane-ammonia) process also is used in the production of hydrogen cyanide from methane. The difference between the Andrussow process and the Degussa process is that the latter does not use air in the synthesis of hydrogen cyanide. The reaction is as follows ... 

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