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Ammonia (NH3) is the second largest chemical produced from syngas. Syngas is a gaseous mixture consisting primarily of carbon monoxide (CO) and hydrogen (H2), and produced from the gasification of feedstocks at temperatures in excess of 1100°F under conditions where the amount of oxygen (from air, pure oxygen, or steam) is less than what is needed for complete combustion. Syngas can be produced from many feedstocks, including natural gas, petroleum products, coal, and biomass.
In 1996, world ammonia production totaled 143 million metric tons (157 million tons). It is used primarily to make fertilizers (urea or ammonium salts such as nitrate, phosphate, and sulfate). A small fraction is used to make organic chemicals used in plastics (polyamides, caprolactam, and others) and for the production of explosives (hydrazine, nitriles, etc.). Ammonia is also converted to nitric acid and cyanides.
Ammonia synthesis process. Ammonia is manufactured from nitrogen in the air and hydrogen produced mainly by steam reforming of methane (natural gas) (figure 1). About 50% of the hydrogen produced from syngas processes is used for ammonia production. Ammonia production from syngas is a mature technology involving four steps--syngas production, gas conditioning, compression, and ammonia synthesis. The syngas production and gas conditioning operations provide pure H2 for input into the ammonia converter. Sulfur contained in the feedstock is first removed prior to steam methane reforming and secondary reformers further decrease hydrocarbons contained in the syngas. The complexity of these operations is greater for solid fuels compared to natural gas. The purified H2 is then compressed and fed to the ammonia synthesis reactor. Ammonia is recovered by cooling the syngas at process pressures to condense the ammonia. The liquid ammonia is separated from the gas, which is recycled back through the converter. Conversion rates of 10-35% per pass are typically achieved. Ammonia recovery is not efficient and the recycled gas typically contains 4% NH3 plus any inert gases that may be in the process stream. Purging some of the gas in the recycle loop minimizes inert gas concentrations and allows recovery of some additional NH3. The purged gas also contains some unconverted N2 and H2, and the H2 is recycled to the converter or used to fuel the process.
Ammonia synthesis catalysts. The ammonia synthesis reaction is exothermic and maximum production of ammonia occurs at high pressure and low temperature. Ammonia synthesis is typically performed over promoted iron (Fe) catalysts. Commercial catalyst suppliers include Haldor Topsoe, Katalco (ICI), and Sud Chemie. Efforts are ongoing to develop catalysts with high activity and long lifetimes at lower operating temperatures and pressures. Recent developments include promoted Rubidium (Ru)-based formulations attached to high surface area graphite supports, which reportedly are 40% more active than Fe catalysts.
Sulfur and chlorine are the most significant catalyst poisons. Sulfur is typically removed from the syngas prior to entering the primary reformer. Natural gas and coal particularly can high levels of H2S and CO that must be removed. Chlorine attacks both the alkali promoters used with catalysts and the active sites on the Fe catalyst.
Trace levels of arsenic and phosphorous are also strong catalyst poisons. Oxygenated compounds (e.g., CO, CO2, H2O, and O2) are temporary, reversible poisons that occur at low process temperatures.
Catalyst lifetimes can be 10 years in carefully controlled processes.
Hydrogen synthesis reactors. The basic design of an ammonia synthesis reactor is a pressure vessel with sections for catalyst beds and heat exchangers. Ammonia converters are classified by flow type (axial, radial, or cross flow) and the cooling method (quench or indirect) used.
Axial flow reactors are top-to-bottom flow reactors, characterized by a relatively simple design, but subject to large pressure drops across the catalyst bed. Radial flow converters are tall vessels with a relatively small diameter and feed gas into a region between the reactor wall and the outer surface of the catalyst bed. This design minimizes pressure drops. In a cross-flow reactor, gas is introduced along one side of the reactor and is collected radially across the reactor by a collector on the other side.
Ammonia synthesis generates large quantities of heat that must be removed to maintain sythesis conditions and various designs have been developed. Quench converters introduce a cool reactant gas at various points along the length of the catalyst bed. Interbed heat exchangers can also be used to remove heat at specific intervals along the bed, effectively separating the bed into multiple synthesis zones, or continuously along the bed with cooling tubes. These indirectly cooled designs allow for efficient reaction heat recovery that can be used in other parts of the process. Reactant gas can be circulated through the heat exchangers to preheat the ammonia synthesis gas or water that is used to produce steam.
A number of ammonia converters are commercially available (figure 2). Many designs are based on quench converters that have a series of catalyst beds with cold gas introduced between the beds for temperature control. Examples include the Topsoe radial-flow converter which uses 2 radial beds with quench gas injection between them. The Kellogg 4-bed axial flow quench converter consists of 4 catalyst beds held on separate grids with a heat exchanger located at the top of the vessel. Quench gas is introduced in the spaces between the beds. The Kellogg horizontal converter is a cross-flow converter design where gas flows through the catalyst bed perpendicular to the axis of the vessel. Numerous variations of tube cooled converters exist such as the ICI Lozenge Quench Converter which is an axial flow converter with a continuous catalyst bed divided by distributors for the addition of quench gas.
Commercial ammonia production. Originally, ammonia synthesis used coal as the feedstock, but by 1990, only 13.5% of the world ammonia capacity was based on this raw material and are located mostly in India, South Africa, and China. Natural gas is now used by most producers. World production has shifted from North America and Europe (54% of total worldwide production in 1969) to Asia (38% of the total in 1996). More than 20 commercial ammonia synthesis processes are described in the literature, and commercial vendors include ICI; Linde; Kellogg, Brown, and Root; Haldor Topsoe; Ammonia Casale; and Krupp-Uhde.
The cost of producing ammonia is highly dependent on feedstock price. The estimated cost or producing ammonia ranges from $161 to $362/tonne ($146 to $329/ton) depending on year of the analysis, assumed feedstock costs, and technology. Since 1986, the price of ammonia has fluctuated between $100-$250/tonne ($91-$227/ton). Relative to the earliest plants, energy consumption has decreased by nearly an order of magnitude (from ~100GJ/tonne NH3 to ~30 GJ/tonne NH3) (~86MBtu/ton to ~26MBtu/ton) due to improved efficiencies in syngas production (from the use of natural gas rather than coal) and improved reactor design and heat recovery. |