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bioweb.sungrant.org » Technical » Bioproducts » Bioproducts from Syngas » Ammonia
| Products from Syngas—Ammonia (Catalyst) |
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In its simplest form, syngas (also called producer gas, town gas, blue water gas, and synthesis gas among others) is composed of carbon monoxide (CO) and hydrogen (H2), which provide the building blocks to produce a number of organic compounds. During the 1800’s, coal gasification was used for lighting and heating. The production of fuels and chemicals from syngas began in the early 20th century. Today, hydrogen, along with ammonia and methanol, constitute the major chemicals commercially produced from syngas.
In principle, syngas can be produced from any hydrocarbon feedstock, including natural gas, naphtha, residual oil, petroleum coke, coal, and biomass. Using numerous synthesis pathways, a large number of organic compounds can be produced from syngas (figure 1).
Ammonia (NH3) is the second largest chemical produced from syngas. In 1996, world production totaled 143 million tonnes (Appl, 2003). 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 is manufactured from the nitrogen in the air and hydrogen produced mainly by steam methane reforming. About 50% of the hydrogen produced from syngas processes is used for ammonia production (Wender, 1996). Ammonia production from syngas is a mature technology and numerous review articles (e.g., Appl, 2003; Jennings and Ward, 1989; Satterfield, 1991) and books and journals (e.g., Nitrogen; Ammonia Plant Safety) are available that describe the technology. |
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| Ammonia Synthesis Reactions |
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Ammonia production involves 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. To accomplish this, a number of operations are required including feedstock pretreatment, syngas generation, CO conversion, and gas purification (figure 2). Other ammonia synthesis configurations are possible, however, most commercial ammonia synthesis facilities use this process.
For natural gas plants, feedstock pretreatment involves desulfurization prior to steam methane reforming (SMR). Secondary reformers are used to further decrease the hydrocarbon content in the syngas, and following reforming, the syngas is cleaned and conditioned. These operations are more complex for solid fuel based systems.
Conditioning of the clean syngas is undertaken to increase the yield and purity of the hydrogen. CO is converted to H2 via the water gas shift reaction in high and low temperature shift reactors. Water is removed by cooling the syngas and CO2 is removed in an absorber-stripper process. Residual CO and CO2 in the product H2 is reduced to lower levels by what amounts to the reverse steam methane reforming process. A fraction of the H2 is used to convert residual CO and CO2 to CH4 in a methanation reactor (i.e. a reactor that produces methane from syngas). Purified H2 is then compressed and fed to the ammonia synthesis loop.
Conversion rates of 10-35% per pass are typically achieved. Ammonia is recovered from the synthesis loop 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. Earlier plant designs used air or water-cooling processes to recover the ammonia, but modern plants use refrigeration processes.
Ammonia recovery is not efficient and the recycled gas typically contains 4% NH3 plus any inert gases (Ar, He, CH4, etc.) that may be in the process stream. Purging some of the gas in the recycle loop before recycling 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. Some of the inert gases are dissolved in the condensed NH3 product and are separated from the liquid ammonia via flashing in a pressure letdown step (e.g., a pressure release or reduction step).
Ammonia synthesis occurs via the following reaction:
N2 + 3H2 → 2 NH3 (∆H773K = -109 kJ/mol; ∆H298K = -46.22 kJ/mol)
The reaction is exothermic and maximum conversion at equilibrium occurs at high pressure and low temperature. Ammonia is synthesized in catalytic converters at pressures ranging from 15-35 MPa (Megapascal equal to 1 million pascals; 101325 atmospheres; 1.01325 bar; or 14.7 pounds per square inch) at a minimum temperature of 430-480°C. Ammonia formation increases as the process pressure is increased. The optimum H2/N2 ratio is near 2 at high space velocities and approaches 3 at low space velocities as equilibrium becomes more dominant. In heterogeneous catalysis, space velocity is a critical parameter that defines the relation between the volumetric gas flow rate and the reactor volume (i.e., the amount of catalyst) and is inversely proportional to the residence time in the catalyst reactor). In commercial ammonia production, space velocities vary from 12,000/hr at 15 MPa to 35,000/hr at 80MPa (Appl, 2003). The balance between increasing reaction kinetics and decreasing equilibrium NH3 concentration results in a maximum temperature of ~550°C.
The rate at which ammonia syntheis occurs can be expressed by equation 1 (Temkin and Pyzhev,1940) and is widely used to design ammonia converters (Jennings and Ward, 1989).
where the first term is the rate of ammonia formation and the second term is the rate of ammonia decomposition. This rate expression is valid over a wide range of pressures with fugacities (the amount of each component in the gas phase at high temperature and pressure) substituted for pressures at very high pressure. The value for α ranges between 0.4-0.75 for commercial processes where ammonia is recycled back to the reactor. For processes using only pure reactants without ammonia recycle, the following rate equation applies (equation 2).
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| Ammonia Synthesis Catalysts |
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Ammonia synthesis is typically performed over promoted Fe-catalysts. Numerous mechanistic studies measuring surface adsorbtion have determined the rate-limiting step to be the dissociative chemisorption (i.e., compound falls apart as it chemically adsorbs to the catalyst surface) of N2 on the catalyst surface to form N atoms. Surface concentration of N atoms is governed by the equilibrium with gaseous H2 and NH3.
Ammonia synthesis catalysts are made by cooling, casting, crushing, and sieving a fused melt of magnetite ore (Fe3O4) and promoter precursors that minimize Fe sintering (i.e., the partial melting of small metal particles and their coagulation to form larger particles) during reduction and reaction. In ammonia converters, the Fe-catalysts are reduced (oxygen removed) to metallic form in situ, resulting in increased porosity (Jennings and Ward, 1989). The production and preservation of this highly porous structure during reduction of the ammonia synthesis catalysts leads to highly active catalysts. The addition of promoters facilitates the formation of highly porous metallic iron. Surface areas of freshly reduced promoted iron ammonia synthesis catalysts can be as high as 15-20 m2/g. Typical ammonia synthesis catalysts contain 2.5-4% Al2O3, 0.5-1.2% K2O, 2.0-3.5% CaO, 0-1.0% MgO, and 0.2-0.5% SiO2 (naturally occurring in the magnetite ore). Commercially available catalyst formulations are summarized in table 1.
Efforts are ongoing to develop catalysis with high activity and long lifetimes at lower synthesis temperatures and lower pressures. Recent developments include promoted Ru-based formulations on high surface area graphite supports. The Kellogg Advanced Ammonia Process is based on a Ru catalyst that is claimed to be 40% more active than Fe catalysts.
Impurities in the syngas can poison the catalysts. Oxygenated compounds like CO, CO2, H2O, and O2 are temporary, reversible poisons that occur at low process temperatures, although water vapor can become an irreversible poison at temperatures above 520°C. The typical maximum concentration permissible in the syngas stream is 10 parts per million (ppm) of total oxygenated compounds. Removal of oxygenates from the syngas gas and re-reduction of the catalyst with pure H2 and N2 restores catalyst activity.
Sulfur and chlorine are the most significant catalyst poisons. Sulfur is typically removed from the syngas stream prior to entering the primary reformer using a hydro-desulfurization unit containing Co-Mo catalysts followed by a fixed bed ZnO absorber. Syngas from coal gasification in particular, can contain high levels of H2S and CO that must be removed. Chlorine attacks the alkali promoters to form volatile alkali chlorides that can vaporize at process temperatures and also attacks the active metallic Fe sites converting them to Fe chlorides. Trace levels of arsenic and phosphorous are also strong catalyst poisons. Table 2 summarizes the various catalyst poisons.
In carefully controlled ammonia synthesis processes, 10-year catalyst lifetimes are not uncommon. Loss of activity can still occur. Deactivation by sintering is a slow, gradual process that leads to the loss of catalyst active area due to Fe crystallization and oxidation. Activity can also be reduced by trace impurities in the syngas stream or solid impurities introduced during catalyst manufacture. Careful manufacturing processes are required to avoid the solid catalyst poisons, although, some are unavoidable due to their presence as impurities in the natural magnetite (iron) used as the starting material. Extensive purification of H2 and N2 is required to minimize gas phase catalyst poisons. |
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| Hydrogen Synthesis Reactors |
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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. Figure 3 illustrates many of these reactors.
Axial flow reactors are essentially top-to-bottom flow reactors. The design is relatively simple, however, a fairly large pressure drop develops across the catalyst bed. A radial flow configuration feeds gas into an annular region between the reactor wall and the outer surface of the catalyst bed. Gas flows through the bed and exits out a central collection tube. This design minimizes pressure drops across a shallow bed with a large surface area. Radial flow converters tend to be tall vessels with relatively small diameters. The cross-flow configuration has a similar principle in that 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 is an exothermic process that generates large quantities of heat that must be removed to maintain sythesis conditions. Various designs have been developed to remove excess heat. 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 (Zardi 1982; Appl, 2003). Many designs are based on quench converters that have a series of catalyst beds with cold gas introduced between the beds for temperature control. Early designs based on axial flow were easier to build but not as efficient as the more complex radial flow converters that have more recently been designed. The Topsoe radial-flow converter uses 2 radial beds with quench gas injection between them. A similar radial flow design uses an inter-bed heat exchanger in the first catalyst bed. Cold ammonia synthesis gas is first introduced from the bottom of the converter through the second catalyst bed, then through the heat exchanger in the first catalyst bed. The cold gas flow through the second bed also provides indirect heat exchange. An additional heat exchanger is located at the bottom of the reactor to cool the reacted gases. 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. It is available in both quench and indirectly cooled versions. Numerous variations of tube cooled converters exist. The ICI Lozenge Quench Converter is an axial flow, continuous catalyst bed divided by lozenge distributors (i.e., tubes in the catalyst bed) for quench gas addition.
Commercial production of ammonia from syngas. 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 (Appl, 2003). 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. Since 1986, the price of ammonia has fluctuated between $100-$250/tonne (Appl, 2003). Table 3 summarizes the costs of producing ammonia estimated by three studies.
Relative to the earliest plants, energy consumption has decreased by nearly an order of magnitude (from ~100 GJ/tonne NH3 to ~30 GJ/tonne NH3) due to improved efficiencies in syngas production (from the use of natural gas rather than coal) and improved reactor design and optimized heat integration (Jennings and Ward, 1989). |
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| References |
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Ammonia Casale (http://www.casale.ch/ammonia/).
Appl, M. (2003). "Ammonia." Ullmann's Encyclopedia of Industrial Chemistry Release 2003, 6th Edition, Wiley-VCH Verlag GmbH & Co.KGaA.
Dietz, C.K., Lutz, I.H., Roman, A. (1978) Ammonia Production from Brava Cane. Energy From Biomass and Wastes Symposium, August 14-18, Washington, D.C., 575-604.
Eggeman, T. (2001). "Ammonia." Kirk-Othmer Encyclopedia of Chemical Technology, John Wiley & Sons, Inc.
Haldor Topsoe (http://www.topsoe.com/tempfiles/267.asp).
ICI (http://www.synetix.com/ammonia/index.htm).
Ilg, J., and Kandziora, B. (1997). "Linde ammonia concept." Ammonia Plant Safety & Related Facilities, 37, 341-352. (http://www.linde-process-engineering.com/en/p0001/p0017/p0020/p0020.jsp),
Jennings, J. R., and Ward, S. A. (1989). "Ammonia Synthesis." Catalyst Handbook, M. V. Twigg, ed., Wolfe Publishing, Ltd., London, England, 384-440.
Kellogg, Brown, and Root (http://www.mwkl.co.uk/pdf/Ammonia_brochure_1.pdf
and http://www.mwkl.co.uk/pdf/KAAPplus.pdf).
Krupp-Uhde (http://www.uhde.biz/kompetenzen/technologien/duengemittel.en.html).
Lauriente, D.H. (2001). "Ammonia." Chemical Economics Handbook Marketing Research Report, SRI International, Menlo Park, CA. Report number 756.6000.
Satterfield, C. N. (1991). Heterogeneous Catalysis in Industrial Practice, Krieger Publishing Company, Malabar, FL.
Spath, P.L. and D.C. Dayton, Preliminary screening—technical and economic assessment of synthesis gas to fuels and chemicals with emphasis on the potential for biomas-derived syngas, National Renewable Energy Laboratory, NREL/TP-510-34929, December, 2003.
Wender, I., (1996),Reactions of synthesis gas, Fuel Processing Technology, 48(3):189-297.
Zardi, U. (1982). "Review These Developments in Ammonia and Methanol Reactors." Hydrocarbon Processing, 61(8), 129-133. |
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