Process flow
1. Process flow of ammonia synthesis
(1) Raw gas preparation: Coal, natural gas and other raw materials are made into crude raw materials containing hydrogen and nitrogen. gas. For solid raw materials coal and coke, synthesis gas is usually produced by gasification; residual oil can be obtained by non-catalytic partial oxidation method; for gaseous hydrocarbons and naphtha, the two-stage steam reforming method is used in industry Syngas.
(2) Purification: Purify the crude raw material gas to remove impurities other than hydrogen and nitrogen, which mainly includes the conversion process, desulfurization and decarbonization process, and gas refining process.
① Carbon monoxide conversion process
In the production of synthetic ammonia, the raw gas produced by various methods contains CO, and its volume fraction is generally 12 to 40. The two components needed to synthesize ammonia are H2 and N2, so CO in the synthesis gas needs to be removed. The transformation reaction is as follows:
CO H2OH→2 CO2 =-41.2kJ/mol 0298HΔ
Since the CO transformation process is a strongly exothermic process, it must be carried out in stages to facilitate the recovery of reaction heat. And control the residual CO content at the outlet of the conversion section. The first step is high-temperature conversion, which converts most CO into CO2 and H2; the second step is low-temperature conversion, which reduces the CO content to about 0.3. Therefore, the CO shift reaction is not only a continuation of raw gas production, but also a purification process, creating conditions for the subsequent decarbonization process.
② Desulfurization and decarbonization process
The crude raw gas produced from various raw materials contains some sulfur and carbon oxides. In order to prevent the poisoning of the catalyst in the ammonia production process, it must be Ammonia is removed before the synthesis process. The first step of the steam reforming method using natural gas as raw material is desulfurization to protect the reforming catalyst. The partial oxidation method using heavy oil and coal as raw material depends on whether a sulfur-resistant catalyst is used for carbon monoxide conversion. And determine the location of desulfurization. There are many types of industrial desulfurization methods, which usually use physical or chemical absorption methods. Commonly used methods include low-temperature methanol washing method (Rectisol), polyethylene glycol dimethyl ether method (Selexol), etc.
After the crude raw material gas is converted by CO, in addition to H2, there are also components such as CO2, CO and CH4 in the converted gas, of which CO2 has the largest content. CO2 is not only a poison for the ammonia synthesis catalyst, but also an important raw material for the manufacture of nitrogen fertilizers such as urea and ammonium bicarbonate. Therefore, the removal of CO2 from shift gas must take into account the requirements of these two aspects.
Generally, solution absorption method is used to remove CO2. According to the different properties of absorbents, they can be divided into two major categories. One is the physical absorption method, such as low-temperature methanol washing method (Rectisol), polyethylene glycol dimethyl ether method (Selexol), and propylene carbonate method. One is the chemical absorption method, such as hot potash method, low heat consumption Benfield method, activated MDEA method, MEA method, etc. 4
③ Gas refining process
The raw material gas after CO conversion and CO2 removal still contains a small amount of residual CO and CO2. In order to prevent poisoning of the ammonia synthesis catalyst, it is stipulated that the total content of CO and CO2 shall not exceed 10cm3/m3 (volume fraction). Therefore, before the raw material gas enters the synthesis process, it must undergo final purification of the raw material gas, that is, a refining process.
At present, in industrial production, the final purification method is divided into cryogenic separation method and methanation method. The cryogenic separation method is mainly a liquid nitrogen washing method, which uses liquid nitrogen to absorb and separate a small amount of CO under deep freezing (lt; -100°C) conditions. It can also remove methane and most of argon, so that only inert gases can be obtained. For hydrogen-nitrogen mixed gas below 100cm3/m3, the cryogenic purification method is usually combined with air separation and low-temperature methanol washing. Methanation is a purification process that reacts a small amount of CO, CO2 and H2 to generate CH4 and H2O in the presence of a catalyst. It is required that the carbon oxide content (volume fraction) in the inlet feed gas should generally be less than 0.7.
The methanation method can remove the carbon oxide (CO CO2) content in the gas to less than 10cm3/m3, but it requires the consumption of the active ingredient H2 and increases the content of the inert gas CH4. The methanation reaction is as follows:
CO 3H2→CH4 H2O =-206.2kJ/mol 0298HΔ
CO2 4H2→CH4 2H2O =-165.1kJ/mol 0298HΔ
(3) Ammonia synthesis: Compress the pure hydrogen and nitrogen mixture to high pressure, and synthesize ammonia under the action of a catalyst. Ammonia synthesis is a process that provides liquid ammonia products and is the core part of the entire synthetic ammonia production process. The ammonia synthesis reaction is carried out under relatively high pressure and in the presence of a catalyst. Since the ammonia content in the gas after the reaction is not high, generally only 10 to 20, the unreacted hydrogen and nitrogen cycle process is adopted. The ammonia synthesis reaction formula is as follows:
N2 3H2→2NH3(g) =-92.4kJ/mol
2. Catalytic mechanism of ammonia synthesis
Thermodynamic calculations show that, Low temperature and high pressure are beneficial to the ammonia synthesis reaction, but without a catalyst, the activation energy of the reaction is very high and the reaction almost does not occur. When an iron catalyst is used, the reaction process is changed and the activation energy of the reaction is reduced, causing the reaction to proceed at a significant rate. It is currently believed that a possible mechanism for the ammonia synthesis reaction is that nitrogen molecules chemically adsorb on the surface of the iron catalyst, weakening the chemical bonds between nitrogen atoms. Then, the chemically adsorbed hydrogen atoms continuously interact with the nitrogen molecules on the surface to gradually generate -NH, -NH2 and NH3 on the catalyst surface. Finally, the ammonia molecules desorb on the surface to generate gaseous ammonia. The above reaction pathway can be simply expressed as:
p>
FexNH2 + [H] absorbs FexNH3xFe NH3
Without a catalyst, the activation energy of the ammonia synthesis reaction is very high, about 335 kJ/mol. After adding the iron catalyst, the reaction proceeds in two stages to generate nitrogen compounds and nitrogen hydrides. The reaction activation energy of the first stage is 126 kJ/mol~167 kJ/mol, and the reaction activation energy of the second stage is 13 kJ/mol. Due to the change in the reaction pathway (the formation of unstable intermediate compounds), the activation energy of the reaction is reduced, so the reaction rate is accelerated.
3. Catalyst poisoning
The catalytic ability of a catalyst is generally called catalytic 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 catalyst is made, it can be used forever. In fact, during the use of many catalysts, their activity increases from small to large and gradually reaches the normal level. This is the maturity period of the catalyst. Then, the catalyst activity remains stable for a period of time, and then decreases until it ages and can no longer be used. The time the activity remains stable is the life of the catalyst, and its length varies depending on the preparation method and usage conditions of the catalyst.
During the period of stable activity of a catalyst, its activity is often significantly reduced or even destroyed due to contact with a small amount of impurities. This phenomenon is called catalyst poisoning. It is generally believed that poisoning is caused by the active centers on the catalyst surface being occupied by impurities. Poisoning is divided into two types: temporary poisoning and permanent poisoning. For example, for the iron catalyst in the ammonia synthesis reaction, O2, CO, CO2 and water vapor can poison the catalyst. However, when pure hydrogen and nitrogen mixed gas is passed through the poisoned catalyst, the activity of the catalyst can be restored, so this poisoning is temporary. On the contrary, compounds containing P, S, and As can permanently poison iron catalysts. After the catalyst is poisoned, it often loses its activity completely. At this time, even if it is treated with pure hydrogen and nitrogen mixed gas, the activity is difficult to recover. Catalyst poisoning will seriously affect the normal production.
In order to prevent catalyst poisoning in industry, the reactant raw materials must be purified to remove poisons, which requires additional equipment and higher costs. Therefore, the development of new catalysts with strong anti-toxic ability is an important topic.
4. The development of my country's synthetic ammonia industry
Before liberation, there were only two small-scale ammonia plants in my country. After liberation, the synthetic ammonia industry developed rapidly. In 1949, the national nitrogen fertilizer production was only 6,000 tons, but in 1982 it reached 10.219 million tons, making it one of the countries with the highest production in the world.
In recent years, my country has introduced a number of large-scale fertilizer plant equipment with an annual output of 300,000 tons of nitrogen fertilizer. The Shanghai Wujing Chemical Plant designed and built by my country is also a large-scale fertilizer plant with an annual output of 300,000 tons of nitrogen fertilizer. These fertilizer plants use natural gas, petroleum, refinery gas, etc. as raw materials. They have low energy loss and high output during production, and their technology and equipment are very advanced.
5. Research on chemically simulated biological nitrogen fixation
Currently, one of the important research topics in chemically simulated biological nitrogen fixation is the study of the structure of the active center of nitrogenase. Nitrogenase is composed of a combination of two transition metal-containing proteins, ferritin and molybdenum ferritin. Ferritin mainly plays the role of electron transport, and molybdenum ferritin, which contains two molybdenum atoms and twenty or thirty iron and sulfur atoms, is the active center that complexes N2 or other reactant (substrate) molecules and carries out reactions. Wherever you are. There are many opinions on the structure of the active center, and there is currently no conclusion. From various substrate conjugate activation and reductive hydrogenation experiments, the active center containing double molybdenum cores is more reasonable. Between 1973 and 1974, two research groups in my country proposed trinuclear and tetranuclear active center models containing molybdenum and iron, which can better explain a series of properties of nitrogenase, but their structural details have yet to be determined based on new data. Experimental results are precise.
Relevant international research results believe that nitrogen fixation under mild conditions generally includes the following three links:
① Complexation process. It uses certain organic complexes of transition metals to complex N2 to weaken its chemical bonds; ② reduction process. It uses chemical reducing agents or other reduction methods to transport electrons to the complexed N2 to break the N-N bonds in N2; ③ Hydrogenation process. It provides H to combine with negative valence N to generate NH3.
At present, a major difficulty in chemically simulating biological nitrogen fixation is that N2 is complexed but basically not activated, or complexed and activated, but the activation is not enough. Therefore, stable dinitrogen-based complexes generally can only precipitate N2 through the action of chemical reducing agents under mild conditions, and the amount of NH3 produced from the reduction of unstable dinitrogen complexes is quite small. Therefore, there is an urgent need for in-depth theoretical analysis to find ways to break through.
Some progress has been made in the biochemistry and chemical simulation of nitrogenase, which will surely promote the research of complex catalysis, especially for the search for ammonia synthesis catalysts with high catalytic efficiency. Promote.
[Edit this paragraph] Production method
The main raw materials for producing synthetic ammonia include natural gas, naphtha, heavy oil and coal (or coke).
①Ammonia production from natural gas. Natural gas is first desulfurized, then undergoes secondary conversion, and then undergoes processes such as carbon monoxide conversion and carbon dioxide removal. The resulting nitrogen-hydrogen mixture still contains about 0.1% to 0.3% (volume) of carbon monoxide and carbon dioxide. After methanation After removal, a pure gas with a hydrogen-to-nitrogen molar ratio of 3 is produced, which is compressed by a compressor and enters the ammonia synthesis loop to produce ammonia as a product. The production process of ammonia using naphtha as raw material is similar to this process.
②Ammonia production from heavy oil. Heavy oil includes various residual oils obtained from deep processing. Synthetic ammonia feed gas can be obtained by partial oxidation method. The production process is simpler than the natural gas vapor conversion method, but an air separation device is required. The oxygen produced by the air separation unit is used for gasification of heavy oil. In addition to nitrogen being used as a raw material for ammonia synthesis, liquid nitrogen is also used as a detergent for removing carbon monoxide, methane and argon.
③Ammonia production from coal (coke).
With the development of petrochemical and natural gas chemical industries, the method of producing ammonia from coal (coke) as raw material is rarely used in the world. However, with the changes in the energy pattern, ammonia production from coal is now being taken seriously again. In foreign countries, it is mainly Pulverized coal gasification technology has developed rapidly, and domestically, the technology of briquette coal gasification has become very mature.
Uses Ammonia is mainly used to make nitrogen fertilizers and compound fertilizers. Ammonia is used as industrial raw materials and ammoniated feeds, and its consumption accounts for about 12% of world production. Nitric acid, various nitrogen-containing inorganic salts and organic intermediates, sulfa drugs, polyurethane, polyamide fiber and nitrile rubber all need to use ammonia directly as raw materials. Liquid ammonia is commonly used as a refrigerant.
Storage and transportation: Part of commercial ammonia is transported from manufacturing plants to other places in liquid form. In addition, in order to ensure the balance of supply and demand between synthetic ammonia and ammonia processing workshops in the manufacturing plant and prevent production shutdown due to short-term accidents, a liquid ammonia warehouse needs to be set up. Depending on the size of the liquid ammonia warehouse, there are three types: non-frozen, semi-frozen and fully frozen. The transportation methods of liquid ammonia include sea transportation, barge transportation, pipeline transportation, tanker transportation, and truck transportation.
Alkali making method 1, combined alkali making method
(Hou's alkali making method)
NH3 CO2 H20 NaCl=NH4Cl NaHCO3↓ (NaHCO3 has low solubility , so it is precipitation, allowing the reaction to proceed)
2NaHCO3=Na2CO3 CO2↑ H2O ("=" should have a heating symbol)
The key point is to make alkali in Solvay Add salt solids to the filtrate of the method, and pass ammonia gas and carbon dioxide gas into the filtrate at 30 ℃ ~ 40 ℃ to make it saturated, and then cool it to below 10 ℃. According to the solubility of NH4Cl at room temperature is greater than that of NaCl, However, it is less soluble than NaCl at low temperatures, so ammonium chloride (a kind of chemical fertilizer) crystallizes, and its mother liquor can be used as the raw material for making alkali in Solvay's alkali production method.
Advantages of this method: It retains the advantages of the ammonia-alkali method, eliminates its disadvantages, and increases the utilization rate of salt to 96%; NH4Cl can be used as nitrogen fertilizer; it can be combined with ammonia synthesis plants to make the raw materials for ammonia synthesis Gas CO is converted into CO2, eliminating the process of producing CO2 from CaCO3.
Sodium carbonate has many uses. Although people have obtained sodium carbonate from saline-alkali lands and salt lakes, it still cannot meet the needs of industrial production.
In 1862, the Belgian Ernest Solvay (1838-1922) invented the "Solvay alkali production method" (also known as ammonia-alkali), which used salt, ammonia and carbon dioxide as raw materials to prepare sodium carbonate. Law). Since then, Britain, France, Germany, the United States and other countries have successively established factories for large-scale production of soda ash, and organized the Solvay Association to impose a technical blockade on countries other than members.
During World War I, there was a traffic jam in Europe and Asia. Since all the soda ash our country needs is imported from the UK, there was a severe shortage of soda ash for a while, and some national industries that used soda ash as raw materials found it difficult to survive. In 1917, patriotic industrialist Fan Xudong founded Yongli Alkali Company in Tanggu, Tianjin, determined to break the monopoly of foreigners and produce Chinese soda ash. He hired Mr. Hou Debang, who was studying in the United States, as chief engineer.
In 1920, Mr. Hou Debang resolutely returned to China to serve. He devoted himself wholeheartedly to the improvement of alkali production technology and equipment, and finally explored various production technologies of Solvay Process. In August 1924, Tanggu Alkali Plant was officially put into operation. In 1926, the "Red Triangle" brand soda ash produced in China won a gold medal at the World's Fair in Philadelphia, USA. The products are not only sold well in China, but also exported to Japan and Southeast Asia.
In view of the low salt utilization rate, high cost of soda production, waste liquid and waste residue that pollute the environment and are difficult to handle when producing soda ash by Solvay method, Mr. Hou Debang successfully researched it in 1943 after thousands of experiments. Joint alkali production method. This method combines the production of synthetic ammonia and soda ash, which improves the utilization rate of salt, shortens the production process, reduces environmental pollution, and reduces the cost of soda ash. The combined alkali production method was quickly adopted by the world.
The principle of Hou's alkali production method is based on the principle of ionic reaction, and the ionic reaction will proceed in the direction of decreasing ion concentration. That is to say, the metathesis reaction mentioned in many junior high and high school textbooks should produce precipitation, gas and difficult-to-ionize substances. When he wanted to make soda ash (Na2CO3), he took advantage of the fact that the solubility of NaHCO3 in the solution was small, so he made NaHCO3 first. Then use the unstable decomposition of sodium bicarbonate to obtain soda ash. To make sodium bicarbonate, a large amount of sodium ions and bicarbonate ions are needed, so ammonia gas is introduced into the saturated brine to form a saturated ammonia salt solution, and then carbon dioxide is introduced into it, and a large amount of sodium is added to the solution. ions, ammonium ions, chloride ions and bicarbonate ions, among which NaHCO3 has the smallest solubility, so it precipitates, and the remaining products can be used as fertilizer or recycled after treatment.
2. Ammonia-alkali method
In 1862, Belgian Ernest Solvay (1832-1922) used salt, ammonia and carbon dioxide as raw materials to prepare sodium carbonate, which is It is ammonia soda process.
The reaction is carried out in three steps:
NH3 CO2 H2O===NH4HCO3
NH4HCO3 NaCl===NaHCO3 NH4Cl
2NaHCO3= ==Na2CO3 CO2 H2O
The CO2 generated by the reaction can be recycled and reused, and NH4Cl can react with quicklime to produce NH3, which can be reused as raw material: 2NH4Cl CaO===2NH3 CaCl2 H2O
The ammonia-alkali method enables continuous production, improves the utilization rate of salt, and the product quality is pure, so it is called soda ash, but the biggest advantage is its low cost. In 1867, the products manufactured by Solvay set up a factory and won a bronze medal at the Paris World's Fair. This method was officially named the Solvay method. At this time, the price of soda ash dropped significantly. The news reached the United Kingdom, and the British Hutchinson Company, which was engaged in the production of alkali using the Rublan process, obtained the exclusive rights to the Solvay process for two years. In 1873, the Hutchinson Company was reorganized into the Bruneimen Company and established a large-scale factory for the production of soda ash. Later, France, Germany, the United States and other countries successively built factories. These countries initiated the organization of the Solvay Society, and the design drawings were only disclosed to member states and were strictly kept secret from the outside world. Whenever improvements or new discoveries are made, member states communicate with each other and agree not to apply for patents to prevent leakage. In addition to technology, there are also business restrictions. They adopt a regional sales approach. For example, the Chinese market is exclusively owned by the British company Benedictine. Due to such a strict organizational method, anyone who does not obtain the franchise from the Solvay Guild has no way of knowing the details of ammonia-alkali production. Over the years, manufacturers in many countries who have tried to explore the secrets of Solvay have all ended in failure. The news reached the United Kingdom, and the British Hutchinson Company, which was engaged in the production of alkali using the Lubla process, obtained the exclusive rights to the Solvay process for two years. In 1873, the Hutchinson Company was reorganized into the Bruneimen Company and established a large-scale factory for the production of soda ash. Later, France, Germany, the United States and other countries successively built factories.