Cracking of Ammonia

Since ammonia (NH3) contains 75% atomic hydrogen, it can also be a good H2 source for fuel cells after it goes through the following reaction in the presence of suitable catalysts. 

If we assume that the initial molar concentration of NH3 is 1, and n moles of it cracks to form N2 and H2, then 

Enthalpy Change, Gibbs Free Energy Change, and Equilibrium Constant during NHj Cracking at Different Temperatures 

Equilibrium Contents during NH3 Cracking at Different Temperatures 

Since there is no CO formation in the NH3 cracking process, WGS and Prox reactors are not needed, further simplifying the process. However, since NH3 is basic and can react with Nafion in the catalyst layers and the PEM, a filter containing some acidic materials should be used to trap any unconverted ammonia before the product mixture enters the PEMFC. 

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Cracking of ammonia (NDf) Complete dissociation of ammonia is achieved by heating it with catalyst at 550-580°C at 55-60 atm pressure. The product syngas can be burnt in air or oxygen to produce D2O. 

Despite the relatively low heat of decomposition of ammonia, 3 x 10 J/kg, cracking of ammonia represents the costliest operation. Hence, bithermal process for the ammonia-hydrogen exchange was proposed. 

Hydrogen is made available by combining a storage cartridge with a catalytic ammonia cracker in which part of the released ammonia is used as an energy source for both the endothermic cracking of ammonia and the release of additional ammonia from the storage material (see Fig. 19.12). 

The relative ease with which hydrogen reflux can be obtained by thermal cracking of ammonia... 

Although H2 does not exist naturally on earth, it can be generated by a variety of methods such as electrolysis of water, thermal splitting of water, reforming of hydrocarbons, cracking of ammonia, reaction of certain chemicals with water, and biological processes using enzymes and bacteria. 

Nickel powder with very small grain size can adsorb up to 17 times of its own volume of hydrogen. This powder is used as a catalyst for many processes, e.g. hydrogenation of vegetable oils and cracking of ammonia to nitrogen and hydrogen. 

As a protection gas for stainless steels during heat treatment (1050°C), a mixture of 75% hydrogen and 25% nitrogen is very much used. It is prepared by cracking of ammonia at 900°C and atmospheric pressure with a nickel catalyst present. The process thus compels the Haber synthesis to go backwards. 

Stress corrosion cracking may be caused by a number of reasons including presence of trace compounds, especially chlorine, poor heat treatment after welding, etc. Some examples of stress corrosion on loop equipment are given in [629-633]. Stress corrosion cracking of ammonia storage tanks is dealt with in Sect. 6.5.6.1. 

FIG. 23-3 Temperature and composition profiles, a) Oxidation of SOp with intercooling and two cold shots, (h) Phosgene from GO and Gfi, activated carbon in 2-in tubes, water cooled, (c) Gumene from benzene and propylene, phosphoric acid on < uartz, with four quench zones, 260°G. (d) Mild thermal cracking of a heavy oil in a tubular furnace, hack pressure of 250 psig and sever heat fluxes, Btu/(fr-h), T in °F. (e) Vertical ammonia svi,ithesizer at 300 atm, with five cold shots and an internal exchanger. (/) Vertical methanol svi,ithesizer at 300 atm, Gr O -ZnO catalyst, with six cold shots totaling 10 to 20 percent of the fresh feed. To convert psi to kPa, multiply by 6.895 atm to kPa, multiply by 101.3. 

Virtuallv evety alloy system has its specific environment conditions which will prodiice stress-corrosion cracking, and the time of exposure required to produce failure will vary from minutes to years. Typical examples include cracking of cold-formed brass in ammonia environments, cracking of austenitic stainless steels in the presence of chlorides, cracking of Monel in hydrofluosihcic acid, and caustic embrittlement cracking of steel in caustic solutions. 

Finally, any living organism dies. Decomposition may generate ammonia at local concentrations high enough to produce stress-corrosion cracking of brass condenser tubes (Fig. 6.1). 

Figure 6.1 Stress-corrosion cracking of a brass condenser tube caused by ammonia from decomposing slime masses lodged on internal surfaces.

A specific corrodent. One of the unusual and interesting features of SCC is the specificity of the corrodent. A particular alloy system is susceptible to SCC only when exposed to certain corrodents, some or all of which may be unique to that particular alloy system. For example, austenitic stainless steels (300 series) are susceptible to cracking in chloride solutions but are unaffected by ammonia. Brasses, on the other hand, will crack in ammonia but remain unaffected by chlorides. The corrodent need not be present at high concentrations. Cracking has occurred at corrodent levels measured in parts per million (ppm). 

A routine inspection of the tube bundle during a plant outage revealed fine cracks of the type shown in Fig. 9.11. Scattered longitudinal cracks were observed along the lengths of most tubes. The external surface was covered with a thin film of black copper oxide and deposits. The bundle had been exposed to ammonia levels that produced 14 ppm of ammonia in the accumulated condensate. 

Cracking of the ester at about 500°C leads to the formation of the undecylenic acid ester together with such products as heptyl alcohol, heptanoic acid and heptaldehyde. Undecylenic acid may then be obtained by hydrolysis of the ester. Treatment of the acid by HBr in the presence of a peroxide leads to w-bromoundecanoic acid together with the 10-isomer, which is removed. Treatment of the w-bromo derivative with ammonia leads to w-aminoundecanoic acid, which has a melting point of 50°C (Figure 18.8). 

Storage tanks containing ethylene oxide are usually inerted with nitrogen. One plant used nitrogen made by cracking ammonia. The nitrogen contained traces of ammonia, which catalyzed an explosive decomposition of the ethylene oxide. Similar decompositions have been set off by traces of other bases, chlorides, and rust. 

Nitrogen compounds These also arise from both natural and synthetic sources. Thus ammonia is formed in the atmosphere during electrical storms, but increases in the ammonium ion concentration in rainfall over Europe in recent years are attributed to increased use of artiflcial fertilisers. Ammonium compounds in solution may increase the wettability of a metaland the action of ammonia and its compounds in causing season cracking , a type of stress-corrosion cracking of cold-worked brass, is well documented. 

The behaviour of a wide range of a, a-0 and /3 brasses in various corrosive environments was studied by Voce and Bailey and the results summarised by Whitaker . Penetration by mercury and by molten solder was intercrystalline in all three types of brass. In moist ammoniacal atmospheres the penetration of unstressed brasses of all types was intercrystalline. Internal or applied stresses accelerated the intercrystalline penetration of a brasses and initiated some transcrystalline cracking, and also caused severe transcrystalline cracking of /3 alloys and transcrystalline cracking across the 0 regions in the two-phase brasses. Immersion in ammonia solution, however, caused intercrystalline cracking of stressed 0 brasses. 

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