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Currently, the main production methods for hydrazine hydrate include the Raschig method, the urea method, the dinitrogenation method, the hydrogen peroxide method, and the air oxidation method. 1. Raschig: Raschig method uses ammonia as the nitrogen source, with sodium hypochlorite being used to oxidize ammonia to produce hydrazine hydrate. Chloramine is generated during this reaction process, which is why it is also known as the chloramine method. Sodium hypochlorite is produced by reacting excess 8% sodium hydroxide with chlorine gas, while ammonia gas is absorbed in pure water to form an aqueous solution. The mixing ratio of ammonia to sodium hypochlorite solution is 20:1, the reaction temperature is controlled at 170°C, and the reaction can take place under pressure and be completed within a few seconds. Adding gelatin to the reaction system helps improve the yield. The distillate obtained from the reaction tower contains, in addition to hydrazine hydrate, sodium chloride, sodium hydroxide, unreacted ammonia, and small amounts of by-products. It can be flashed at atmospheric pressure, with ammonia separated from the bottom liquid in an ammonia separation tower. The base solution enters the evaporation tower; after sodium chloride and sodium hydroxide are separated, the water is removed from the top of the tower through concentration, and hydrazine hydrate is obtained at the bottom of the tower. The hydrazine obtained by this method is a dilute aqueous solution at 1%-2%, with a maximum concentration not exceeding 4%. The overall yield is approximately 67%, and a considerable amount of heat is required to concentrate the dilute hydrazine solution; to obtain 1 kg of hydrated hydrazine, 40–110 kg of water must be evaporated. Due to the use of excessive ammonia, additional recovery devices are required, resulting in the generation of large amounts of salts such as sodium chloride and ammonium chloride. Due to severe environmental pollution, high equipment costs, and low product yields, this method has been largely phased out abroad. 2. Urea method: This method uses sodium hypochlorite as an oxidizing agent and urea as a nitrogen source to synthesize hydrazine hydrate. In this method, urea is first dissolved in water to form a urea solution, which is then reacted in a tubular oxidation reactor with a mixture of sodium hypochlorite and caustic soda in the presence of magnesium sulfate to yield crude hydrazine, namely the oxidized solution, with a hydrazine content of over 2%. Since crude hydrazine contains large amounts of impurities such as sodium chloride, sodium carbonate, and sodium hydroxide, it is subjected to vacuum distillation in a five-stage still to remove these impurities. A hydrazine-containing aqueous solution with a hydrazine content of over 6% is then obtained using a fractional distillation unit, and further concentration in an evaporator yields 40% hydrated hydrazine. This method has a mature process and its technology is easy to master. Due to the numerous side reactions, it is necessary to maintain a very low concentration of hydrazine (typically 2%-3%). As a result, large amounts of salts are generated as by-products and must be treated. Additionally, evaporating and concentrating hydrazine hydrate requires a significant amount of thermal energy; therefore, this method involves high energy and material consumption, along with considerable environmental challenges. In recent years, manufacturing enterprises in our country have been continuously reforming this process with the aim of reducing side reactions and increasing the yield of hydrazine hydrate. The main technical improvements include the production of sodium hypochlorite in a packed absorption tower ； Replacing the batch reactor with a tubular heating reactor for the synthesis of hydrazine hydrate helps to improve the yield ； Replace the intermittent evaporation using a five-layer evaporator with continuous evaporation using a new specialized evaporator ； Change the liquid-phase feed to the column to gas-phase feed for concentration, thereby reducing steam consumption ； The crude hydrazine hydrate solution is cooled to recover sodium carbonate decahydrate, and sodium chloride is recovered as a by-product; this enables the comprehensive utilization of the by-products in order to reduce production costs. 3. The same azide method: This method was first proposed by the German company Bayer, and industrial production of it began in the 1970s; hence it is also known as the Bayer method. This method involves reacting sodium hypochlorite with ammonia in the presence of each other; the resulting azino intermediate is then hydrolyzed under high pressure to produce hydrazine hydrate. Propylene, an oxidizing agent, or sodium hypochlorite is used in reaction with ammonia to produce the intermediate azonide; under mixing conditions of a molar ratio of sodium hypochlorite:propylene:ammonia of 1:2:20, a yield of 98% (based on chlorine) is achieved after full reaction. The diluted synthesis solution is passed through a pressure-driven ammonia removal tower to remove unreacted ammonia; the ammonia, after being absorbed by water, is returned to the hydrazine synthesis reactor. The liquid at the bottom of the ammonia removal tower consists of hydrazones, hydrazine, and brine, and this mixture is sent to the hydrazine synthesis tower. What evaporates from the top of the tower is a low-boiling mixture of propylhydrazine and water (with a boiling point of 95°C and a propylhydrazine content of 55.5% by mass). The liquid at the bottom of the tower is brine. The propylhydrazine distilled from the top of the tower is hydrolyzed under pressure in a pressure-driven hydrolysis tower at a pressure of 1 MPa, resulting in the formation of propylene and hydrazine hydrate. The resulting hydrazine is distilled from the top of the tower and returned to the hydrazine synthesis reactor; the liquid remaining in the reactor is a 10%-12% hydrazine aqueous solution, which is concentrated to yield 80% hydrazine hydrate. The azide method is significantly superior to the Lasi method; its synthesis yield approaches the theoretical value, and its energy consumption is about 1/3 of that of the Lasi method. If both A and B are used, the overall yield calculated on a chlorine basis is close to 90%. 4. Hydrogen peroxide method: The hydrogen peroxide method is actually an improvement on the dinitrogenation method; it involves using hydrogen peroxide as an oxidizing agent in place of sodium hypochlorite, thereby avoiding the problem of large amounts of by-product salts that arise from the use of sodium hypochlorite as an oxidizer. It is a cleaner production process, and currently, many major producers of hydrazine hydrate abroad use this method for production. In this method, methane and ammonia react to form homocoumarin in the presence of a catalyst; this compound is then oxidized by hydrogen peroxide to form an oxoisohydrazone, which subsequently yields methanocoumarazine. The latter is hydrolyzed to form hydrazine and methane, with methane being able to be reused in the cycle. The specific process involves the following steps: First, a mixture containing ethyl acetate, water, methanol, and a catalyst such as acetamide (or inorganic ammonium salts, arsenic compounds, or nitriles), is added to a reaction tower equipped with a distillation vessel. Ammonia gas is bubbled through the liquid; once a certain amount of ammonia has dissolved in the liquid, its temperature is raised to 50°C, and hydrogen peroxide is added to initiate the reaction. After the reaction is complete, the unreacted raw materials and intermediate products are separated through vacuum distillation. The pressure is first reduced gradually to 26 kPa to remove excess ammonia, after which unreacted ethyl acetate, methanol, etc., are distilled out at a temperature below 50°C. The pressure is further reduced to 6.7 kPa, and ethylenediamine is distilled out at 35°C, while the carboxyamide remains in the liquid in the vessel. The second step is to hydrolyze the hydrazine diazonide to obtain hydrazine hydrate or hydrazine salts; meanwhile, the diazonide is regenerated. The hydrolysis reaction can be carried out at atmospheric pressure and 150°C, with the hydrazine diazonide being hydrolyzed, after which hydrazine hydrate is extracted by distillation. The yield of hydrazine hydrate produced by the hydrogen peroxide method can exceed 90%; the equipment investment is low, steam consumption is minimal, the concentration of the resulting hydrazine is high, and there are no salt by-products, resulting in no environmental pollution. Currently, French companies such as Produit Chimiques and Ugine Kuhlmann, as well as Atto Chemicals; the German company Lanxess; and the Japanese company Mitsubishi Gas Chemical all possess well-developed industrial production facilities for manufacturing hydrazine hydrate using the hydrogen peroxide method. 5. Air oxidation method: Japan has reported a process for producing hydrazine hydrate using air oxidation. Thorium oxide or thorium oxide-silica is used as a catalyst ； In the liquid-phase method, zinc chloride, ammonium chloride, or ion exchange resin is used as a catalyst. In the presence of this catalyst, imine is first oxidized using air, leading to dehydration and condensation of diphenylmethane and ammonium to form diphenylmethylimine. Subsequently, under the action of copper chloride as a catalyst, the imine is oxidized and coupled to produce diphenylmethanenitrile, which is then hydrolyzed to yield hydrazine, with diphenylmethane being recovered in the process. The air oxidation method is currently the most advanced approach for producing hydrated hydrazine; its basic raw materials are merely ammonia and air, while other materials such as diphenylmethane and copper chloride can be reused in the synthesis process. The sources of these raw materials are relatively easy to obtain. However, this method has not yet been put into industrial use. 6. Comparison of process technologies: Among the various methods for synthesizing hydrazine hydrate, the Lassie method has low raw material costs; at large production scales, its total cost is lower than that of the urea method. However, this method causes significant pollution, requires high investment in equipment and consumes a lot of energy, which is why few manufacturers, both domestically and internationally, use it for production today. The advantage of the urea method is its low investment and simple equipment; it is the most economical production method for small-scale production (less than 1,000 tons per year). Almost all hydrazine hydrate manufacturers in our country use this method, and they have achieved continuous production; it is the most mature process, its technology is easy to master, and the synthesis yield is higher than that of the Raschig method. However, due to the higher cost of the raw materials used compared to other methods, it cannot compete with them on a large scale, and this method has been largely phased out abroad. Although many domestic enterprises have carried out extensive technological innovations in the urea process, given its numerous drawbacks, many production companies in China currently manufacture both urea and hydrazine hydrate. It has significant advantages over the azido coupling method; since hydrazine decomposition is avoided in this process, the synthesis yield approaches the theoretical value, and the energy consumption is about 1/3 of that of the Raschig method. In the co-nitrogenation method, hydrazine acts as a co-nitrogenator and forms a low-boiling azeotrope with water, which is removed from the top of the tower, while the remaining water and salts remain at the bottom of the tower. In Rasifa, the hydrazine forms a high-boiling azeotrope with water, which remains at the bottom of the tower; therefore, a large amount of water must be vaporized. Moreover, to prevent salt from precipitating in the tower bottom, desalination by evaporation is required in advance, resulting in high energy consumption. In the context of energy shortages and rising prices, the energy-saving advantages of the same-nitrogen method are particularly important. Furthermore, the equipment investment for the azide method is lower than that for the Raschig method. The disadvantage of the diazotization method is that it requires the treatment of organic by-products and consumes propane. Overall, the azine method is superior to the Raschig method and has seen rapid development in recent years. The economic comparison between the hydrogen peroxide method, the Raschig method, and the dinitrogenation method depends mainly on the relative prices of chlorine and sodium hydroxide. The yield of this method can reach 75%, making it quite attractive when there is a cheap source of hydrogen peroxide available. This method uses methylethyl isomer; although its cost is higher than that of propyl isomer, the methylethyl azide produced is insoluble in water and easy to separate, eliminating the need for distillation, thus resulting in lower energy consumption compared to the azide-based method. Furthermore, this method produces no salt by-products, causes no environmental pollution, has minimal excess ammonia, and the phase separation operation used for nitrogen recovery results in lower energy consumption compared to other methods, thereby improving the quality of the product. Furthermore, using hydrogen peroxide in place of chlorine can prevent a range of problems caused by chlorine and sodium chloride, such as corrosion and contamination. The technology for this method is not yet mature in China, and it requires the use of hydrogen peroxide; currently, there are no facilities in the country that use this method to produce hydrazine hydrate.