Ammonia production from electrolytic hydrogen

As part of the Manhattan District Project during World War II, a small plant to produce heavy water 6 Mg/a) was built by Standard Oil Development Co. at Trail, B.C. and was operated by Cominco from 1944 to 1956 (14). It was based on steam-hydrogen catalytic exchange plus steam-water equilibration coupled to water electrolysis. However, byproduct heavy water from this process is economic only if the electrolysis cost is borne by the hydrogen product, which at Trail was used for ammonia production. In any case, the small scale of operation imposed by electrolytic capacity and the large exchange tower volume have made this production method economically unattractive. 

Of this capacity of 114 tons of ammonia daily, 65 tons or 57 per cent is for operation On water-gas hydrogen, 37 tons or 32 per cent is for operation on hydrogen, a by-product of other chemical industries, while only 12 tons or less than ii per cent is for operation on electrolytic hydrogen. Of the operating rate of 80 tons per day, only 3.7 per cent is on hydrogen from the electrolysis of water, while 70 per cent is on water-gas hydrogen and over 26 per cent on by-product hydrogen. 

The water-gas shift (WGS) reaction is the exothermic conversion of carhon monoxide and water to hydrogen and carbon dioxide according to Scheme 16.1 [1, 2], The reaction is of industrial importance for purification when traces of carbon monoxide have to be removed from gaseous feed, such as in the Haber-Bosch process for ammonia production or in modem polymer electrolyte membrane (PEM) fuel cells [3]. 

As world deposits of petroleum and coal are exhausted, new sources of hydrogen will have to be developed for use as a fuel and in the production of ammonia for fertilizer. At present, most hydrogen gas is produced from hydrocarbons, but hydrogen gas can also be generated by the electrolysis of water. Figure 19-23 shows an electrolytic cell set up to decompose water. Two platinum electrodes are dipped in a dilute solution of sulfuric acid. The cell requires just one compartment because hydrogen and oxygen escape from the cell much more rapidly than they react with each other. 

When liquid air distillation is used as the source of nitrogen, the hydrogen also required for ammonia synthesis is obtained from a variety of sources. Some is obtained as the coproduct from the electrolytic production of chlorine and caustic soda (Chap. 8), some from refinery sources as a by-product of cracking processes or olefin synthesis, some from the water-gas reaction, and some is produced specifically for the purpose by the electrolysis of alkaline water (e.g., by Cominco, Trail, Eq. 11.14). 

Hydrogen is also a byproduct of electrolytic production of chlorine from caustic soda, and several small ammonia plants have used this hydrogen to make ammonia either as the only feedstock or as a supplementary source. The amount of hydrogen available from chlorine-caustic plants is too small to supply a significant portion of the ammonia needs in most situations. 

As a by-product, hydrogen is also produced by electrolytic processes such as chlorine-sodium hydroxide production, catalytic reforming processes in refineries, olefin production, and recovery from ammonia plant purge gas. 

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