Because of its many uses, ammonia is one of the most highly-produced inorganic chemicals. There are literally dozens of large-scale ammonia production plants worldwide. The worldwide production in 2004 was 109,000,000 metric tons. China produced 28.4% of the worldwide production followed by India with 8.6%, Russia with 8.4%, and the United States with 8.2%. About 80% or more of the ammonia produced is used for fertilizing agricultural crops. Ammonia is also used for the production of plastics, fibers, explosives, and intermediates for dyes and pharmaceuticals.
Before the start of World War I, most ammonia was obtained by the dry distillation of nitrogenous vegetable and animal products; by the reduction of nitrous acid and nitrites with hydrogen; and also by the decomposition of ammonium salts by alkaline hydroxides or by quicklime, the salt most generally used being the chloride (sal-ammoniac).
The Haber process, which is the production of ammonia by combining hydrogen and nitrogen, was first patented by Fritz Haber in 1908. In 1910 Carl Bosch, while working for the German chemical company BASF, successfully commercialized the process and secured further patents. It was first used on an industrial scale by the Germans during World War I. Since then, the process has often been referred to as the Haber-Bosch process.
Modern ammonia-producing plants
A typical modern ammonia-producing plant first converts natural gas (i.e., methane) or LPG (liquified petroleum gases such as propane and butane) or petroleum naphtha into gaseous hydrogen. The method for producing hydrogen from hydrocarbons is referred to as "Steam Reforming". The hydrogen is then combined with nitrogen to produce ammonia.
Starting with a natural gas feedstock, the processes used in producing the hydrogen are:
- The first step in the process is to remove sulfur compounds from the feedstock because sulfur deactivates the catalysts used in subsequent steps. Sulfur removal requires catalytic hydrogenation to convert sulfur compounds in the feedstocks to gaseous hydrogen sulfide:
- H2 + RSH → RH + H2S(gas)
- The gaseous hydrogen sulfide is then absorbed and removed by passing it through beds of zinc oxide where it is converted to solid zinc sulfide:
- H2S + ZnO → ZnS + H2O
- Catalytic steam reforming of the sulfur-free feedstock is then used to form hydrogen plus carbon monoxide:
- CH4 + H2O → CO + 3H2
- The next step then uses catalytic shift conversion to convert the carbon monoxide to carbon dioxide and more hydrogen:
- CO + H2O → CO2 + H2
- The carbon dioxide is then removed either by absorption in aqueous ethanolamine solutions or by adsorption in pressure swing adsorbers (PSA) using proprietary solid adsorption media.
- The final step in producing the hydrogen is to use catalytic methanation to remove any small residual amounts of carbon monoxide or carbon dioxide from the hydrogen:
- CO + 3H2 → CH4 + H2O
- CO2 + 4H2 → CH4 +2H2O
To produce the desired end-product ammonia, the hydrogen is then catalytically reacted with nitrogen (derived from process air) to form anhydrous liquid ammonia. This step is known as the ammonia synthesis loop (also referred to as the Haber-Bosch process):
- 3H2 + N2 → 2NH3
The steam reforming, shift conversion, carbon dioxide removal and methanation steps each operate at absolute pressures of about 25 to 35 bar, and the ammonia synthesis loop operates at absolute pressures ranging from 60 to 180 bar depending upon which proprietary design is used. There are many engineering and construction companies that offer proprietary designs for ammonia synthesis plants. Haldor Topsoe of Denmark, Uhde GmbH of Germany, and Kellogg Brown & Root of the United States are among the most experienced companies in that field.
Sustainable ammonia production
Ammonia production depends on plentiful supplies of natural gas, a finite resource, to provide the hydrogen. Due to ammonia's critical role in industrial agriculture and other processes, sustainable production is desirable. This is possible by using renewable energy to generate hydrogen by electrolysis of water. This would be straightforward in a hydrogen economy by diverting some hydrogen production from fuel to feedstock use. For example, in 2002, Iceland produced 2,000 tons of hydrogen gas by electrolysis, using excess electricity production from its hydroelectric plants, primarily for the production of ammonia for fertilizer. In practice, natural gas will remain the major source of hydrogen for ammonia production as long as it is cheapest.
- Today's Hydrogen Production Industry
- Energy Use and Energy Intensity of the U.S. Chemical Industry, Report LBNL-44314, Lawrence Berkeley National Laboratory (Scroll down to page 39 of 40 PDF pages for a list of the ammonia plants in the USA)
- Ammonia: The Next Step includes a detailed process flow diagram.