Synthetic ammonia is composed of nitrogen and hydrogen under high temperature, high pressure andcatalyzerAmmonia directly synthesized in the presence of is a basic inorganic chemical process.In modern chemical industry, ammonia is the chemical fertilizer industry andBasic organic chemical industryMain raw materials of.[1]
The synthetic ammonia industry was formed in the early 20th century and began to use ammonia asexplosive Industrial raw materials, serving the war,the First World WarAfter that, they turned to serving agriculture and industry.With the development of science and technology, the demand for ammonia is increasing.[1]
2024On June 7, five departments including the National Development and Reform Commission jointly released special action plans for energy conservation and carbon reduction in four industries, including synthetic ammonia, and deployed a series of key tasks.[6]
GermanyThe chemist F. Haber (1868-1934) began to studynitrogenDirect synthesis of ammonia with hydrogen.[2]Applied for a patent in 1908, namely“Circular method”On this basis, he continued his research and improved the synthesis in 1909. The ammonia content reached 6.This is a direct synthesis method widely used in industry.In order to solve the problem of hydrogen andnitrogensynthesisConversion rateLow problem, the ammonia product is changed fromSynthetic reactionThe unreacted gas and fresh gasHydrogen and nitrogenThe mixture re participates in the synthesis reaction.
The reaction formula of synthetic ammonia is as follows:
(The reaction isReversible reaction, the reaction condition on the equal sign is "high temperature and high pressure", and the following is:“catalyzer”)
Catalytic mechanism
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Thermodynamic calculation shows that low temperature and high pressure are favorable for ammonia synthesis reaction, but without catalyst, the activation energy of the reaction is very high, and the reaction almost does not occur.When usingIron catalystWhen the reaction process is changed, theactivation energyTo make the reactionrateconduct.The mechanism of ammonia synthesis reaction is that nitrogen molecules are carried out on the surface of iron catalystchemical adsorption, makeNitrogen atomThe chemical bond between them is weakened.Then the chemically adsorbed hydrogen atom continuously interacts with the nitrogen molecule on the surface, and gradually generates - NH, - NH2 and NH3 on the catalyst surface. Finally, the ammonia molecule desorbs on the surface to generate gaseous ammonia.The above reaction pathway can be simply expressed as:
xFe+N2→FexN
FexN+[H] suction → FexNH
FexNH+[H] suction → FexNH2
FexNH2+[H] suction FexNH3xFe+NH3
Synthetic ammonia gas storage tank
In the absence of catalystactivation energyVery high, about 335kJ/mol.After the addition of iron catalyst, the reaction is carried out in two stages, namely, the formation of nitride and nitrogen hydride.The reaction activation energy of the first stage is 126kJ/mol~167kJ/mol, and that of the second stage is 13kJ/mol.Due to the change of reaction pathway (unstable intermediatechemical compound）, the activation energy of the reaction is reduced, soreaction rateAccelerated.
The catalytic ability of the catalyst is generally calledcatalytic activity。Some people believe that since the chemical properties and quality of the catalyst remain unchanged before and after the reaction, once a batch of catalysts are made, they can be used forever.In fact, during the use of many catalysts, their activity gradually reaches the normal level from small to large. This is themature period。Then, the catalyst activity remained stable for a period of time, and then declined until it became old and could no longer be used.The life of the catalyst is the time that the activity remains stable, and its length varies with the preparation method and use conditions of the catalyst.
During the period of stabilizing the activity of the catalyst, the catalyst is often exposed to a small amount of impurities and the activity is significantly reduced or even destroyed. This phenomenon is called catalyst poisoning.It is generally believed thatactive centercoverimpurityOccupy and cause poisoning.Poisoning is divided into temporary poisoning and permanent poisoning.For example, for theironCatalysts, O2, CO, CO2 andsteamAnd so onCatalyst poisoning。But using pure hydrogen and nitrogenmixed gas When passing through the poisoned catalyst, the activity of the catalyst can be recovered, so the poisoning is temporary.On the contrary, P, SAsThe iron catalyst can be permanently poisoned by the compound.After the catalyst is poisoned, it often loses its activity completely, even if pure hydrogen and nitrogen are usedmixed gas It is difficult to recover the activity after treatment.Catalyst poisoning will seriously affect the normal production.In industry, in order to prevent catalyst poisoning, the reactant raw materials should be purified to remove poisons, which will increase equipment and cost.Therefore, it is an important subject to develop a new type of catalyst with strong antitoxic ability.
technological process
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(1) Feed gas preparation: raw materials such as coal and natural gas are made into raw feed gas containing hydrogen and nitrogen.aboutsolidRaw coalAnd coke, which is usually gasified to produce syngas;Non catalytic part can be used for residual oiloxidationTo obtain syngas;For gaseous stateHydrocarbonsAnd naphtha, which are produced by two-stage steam reforming method in industry.
Internal structure of synthetic ammonia plant
(2) Purification The raw feed gas is purified to remove hydrogen andnitrogenOther impurities, mainly including transformation process and desulfurizationcarbonProcess and gas refining process.[1]
① In the carbon monoxide shift process, the feed gas produced by various methods in the synthetic ammonia production containsCO, whichvolume fraction Generally, the two components required for 120 synthetic ammonia are H2 and N2, so CO in the synthetic gas needs to be removed.The conversion reaction is as follows: CO+H2O → H2+CO2 Δ H=-41.2kJ/mol
As CO conversion process is a strong exothermic process, it must be carried out in sections to facilitate recoveryreaction heatAnd control the residual CO content at the outlet of the transformation section.The first step is high temperature conversion, so that most CO is converted into CO2 and H2;The second step is low-temperature conversion, reducing CO content to about 0.3%.Therefore, CO shift reaction is not only the continuation of feed gas manufacturing, but alsopurifyThe process ofdecarbonization The process creates conditions.[5]
② Desulfurizationdecarbonization The raw feed gas produced from various raw materials in the process contains some sulfur and carbon oxides. In order to prevent catalyst poisoning in the process of synthetic ammonia production, it must be removed before the ammonia synthesis processsteamConversion methodThe first process is desulfurization to protect the conversion catalystheavy oilFor partial oxidation method with coal as raw material, the desulfurization position is determined according to whether sulfur resistant catalyst is used for carbon monoxide shift.IndustrydesulphurizationThere are many kinds of methods, usually using physical orChemical absorptionCommon methods include low temperaturemethanolRectisolPolyethylene glycol dimethyl ether method(Selexol), etc.
After CO conversion, the raw feed gas contains CO2, CO and CH4 in addition to H2, of which CO2 is the most abundant.CO2 is the catalyst for ammonia synthesispoison, and manufacturing ureaAmmonium bicarbonateAnd other important raw materials for nitrogen fertilizer.Therefore, the removal of CO2 from the change-over gas must take into account these two requirements.
Generally, CO2 is removed by solution absorption.According to the different performance of absorbents, they can be divided into two categories.One is physical absorption method, such as Rectisol,Polyethylene glycol dimethyl etherSelexol,Propylene carbonate method。One isChemical absorption method, such as hot potash method, low heat costPhilMethod, activated MDEA method, MEA method, etc.
③ The feed gas after CO shift and CO2 removal in the gas refining process still contains a small amount of residual CO and CO2.by
Main control room of synthetic ammonia production line
To prevent the poisoning of ammonia synthesis catalyst, the total content of CO and CO2 shall not be more than 10cm3/m3（volumeScore).Therefore, the final purification of feed gas, i.e. refining process, must be carried out before the feed gas enters the synthesis process.
In industrial production, the final purification methods are divided into cryogenic separation method andMethanationLaw.The cryogenic separation method is mainlyliquid nitrogenThe washing method is to use liquid nitrogen to absorb and separate a small amount of CO under the condition of deep freezing (<- 100 ℃), and can also remove methane and most of argon, so that hydrogen nitrogen mixture containing only inert gas less than 100cm3/m3 can be obtained. The cryogenic purification method is usually similar toAir separationas well asRectisolcombination.methaneChemical method is a purification process that enables a small amount of CO, CO2 to react with H2 to generate CH4 and H2O in the presence of catalyst. It requires the carbon oxide content in the inlet feed gas（volume fraction ）Generally less than 0.7MethanationThe method can convert the carbon in the gasoxideThe content of (CO+CO2) is removed below 10cm3/m3, but the effective component H2 needs to be consumed, and the content of inert gas CH4 is increased.Methanation reactionAs follows:
CO+3H2→CH4+H2O=-206.2kJ/mol0298HΔ
CO2+4H2→CH4+2H2O=-165.1kJ/mol0298HΔ
(3) Ammonia synthesis condenses the pure mixed gas of hydrogen and nitrogen to high pressure to synthesize ammonia under the action of catalyst.[1]Ammonia synthesis is the process of providing liquid ammonia products, and is the core part of the whole synthetic ammonia production process.The ammonia synthesis reaction is carried out under the condition of high pressure and the presence of catalyst. Since the ammonia content in the gas after reaction is not high, generally only 100, unreacted hydrogen is usednitrogenCircular process.Ammonia synthesis reactionThe formula is as follows:
N2+3H2→2NH3(g)=-92.4kJ/mol
Raw material composition
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The main initial raw materials of synthetic ammonia can be divided into solid raw materials, liquid raw materials and gas raw materials.asnatural gas、naphtha、Heavy oilandcoal(or coke), etc.[3]
① Ammonia production from natural gas.The natural gas is first desulfurized, then through secondary conversion, and then through carbon monoxide shift, carbon dioxide removal and other processes to obtain nitrogen hydrogen mixture, which also containscarbon monoxideandcarbon dioxideAbout 0.10.3 volume), aftermethaneAfter the chemical action is removed, the purified gas with the molar ratio of hydrogen to nitrogen of 3 is prepared and compressed by the compressor to enterAmmonia synthesisLoop to produce product ammonia.withnaphthaThe production process of synthetic ammonia as raw material is similar to this process.
② Ammonia production from heavy oil.Heavy oil includesresidual oil, synthetic ammonia feed gas can be produced by partial oxidation methodprocessThan natural gassteamConversion methodSimple, but air separation device is required.The oxygen produced by the air separation unit is used for heavy oil gasification, and nitrogen is used as the raw material for ammonia synthesis,Liquid nitrogenIt is also used as a detergent to remove carbon monoxide, methane and argon.
③ Ammonia production from coal (coke).With the development of petrochemical industry and natural gas chemical industry（coke）Ammonia production from raw materials is rarely used in the world.China's energy structure is characterized by more coal, less oil and less gas. Coal has become the main raw material for synthetic ammonia, while the ammonia production process from natural gas is strictly limited.[1]
Shouldactive centerWhere.aboutactive centerThere are many views on the structure of theactive centerMore reasonable.Two research groups in China proposed three cores and four cores of ferromolybdenum from 1973 to 1974active centerModel, which can be well explainedNitrogenaseHowever, its structural details need to be refined according to new experimental results.
According to relevant international research resultsNitrogen fixationIt generally includes the following three links:
② Restore process.It uses chemistryreducing agentOr other reduction methods to transport electrons to the complexed N2 to disassemble the N-N bond in N2;
③ Hydrogenation process.It provides H+to combine with negative N to generate NH3.
Chemical simulationBiological nitrogen fixationOne of the main difficulties of the work is that N2 is complexed but basically not activated, or complexed and activated but not activated enough.Therefore, stable diazo complexes can only precipitate N2 under mild conditions through the action of chemical reducing agents, and unstableDiazo complexThe amount of NH3 produced by reduction is quite small.Therefore, there is an urgent need for in-depth theoretical analysis in order to find a breakthrough way.
Biochemistry andChemistryThe simulation work has made some progress, which will strongly promote the complexationcatalysisIt will be a powerful promotion for the research of ammonia synthesis catalyst with high catalytic efficiency.[4]
Liquid ammonia is often used as refrigerant, and part of the stored and transported commercial ammonia isliquid statefrommanufacturerTransport to other places.
In addition, in order to ensure the supply and demand balance between the synthetic ammonia and the ammonia processing workshop in the manufacturing plant and prevent the shutdown due to short-term accidents, it is necessary to setliquid ammoniaLibrary.Liquid ammonia storage can be divided into non freezing, semi freezing and full freezing according to different capacities.Liquid ammonia can be transported by sea, barge, pipelinetank wagonTransport, truck transport.[3]
Current situation in China
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Before liberation, there were only two small ammonia plants in China. After liberationSynthetic ammonia industryIt has developed rapidly.1949nitrogenous fertilizerThe output was only 6000 tons, while in 1982 it reached 10.219 million tons, making it the highest in the worldcountryone of.China has introduced a batch of large-scale fertilizer plant equipment with an annual output of 300000 tons of nitrogen fertilizer.Designed and built by ChinaShanghaiWujing Chemical Plant is also a large-scale fertilizer plant with an annual output of 300000 tons of nitrogen fertilizer.These fertilizer plants use natural gas, oil, refined gas, etc. as raw materials, with low energy consumption, high output, and advanced technology and equipment.[1]
Synthetic ammonia is an important chemical raw material and agricultural fertilizer, with an annual output of about 50 million tons.About 11% of the production capacity and energy efficiency cannot reach the benchmark level, and the potential for energy conservation and carbon reduction is huge.
2024On June 7, the National Development and Reform Commission and other five departments jointly released the special action plan for energy conservation and carbon reduction in four industries of steel, oil refining, synthetic ammonia and cement, and deployed a series of key tasks.From 2024 to 2025, the steel, oil refining, synthetic ammonia and cement industries will save about 32 million tons of standard coal and reduce about 84 million tons of carbon dioxide by implementing energy-saving and carbon reduction transformation and energy consumption equipment upgrading.By the end of 2025, the production capacity above the benchmark level of energy efficiency in the steel, oil refining, synthetic ammonia and cement industries will all account for 30%, and the production capacity below the benchmark level of energy efficiency will have completed technical transformation or eliminated.[6]
