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The Kellogg process involves conversion, transformation, decarburization, and methanation. 1. Conversion: In this process, natural gas is desulfurized so that its total sulfur content is less than 0.5 cm3/m3. It is then mixed with medium-pressure steam at a pressure of around 3.6 MPa and a temperature of around 380°C to achieve a certain water-to-carbon ratio (approximately 3.5). The mixture is sent to the convection section where it is heated to 500–520°C, after which it is directed to the top of the radiation section and distributed into various reaction tubes. The gas flows from top to bottom through the catalysts, undergoing reactions while absorbing heat; the converted gas exiting the bottom of the reaction tubes has a temperature of 800–820°C and a pressure of 3.1 MPa, with a methane content of about 9.5%. This gas gathers in the collection ducts and then rises along the upward pipes in these ducts, absorbing more heat and raising its temperature to 850–860°C. From there, it is sent via the main gas pipeline to the second-stage conversion furnace. The process air is compressed to a pressure of 3.3–3.5 MPa; a small amount of water vapor is also added to it. It then enters the process air heating coil in the convection section, where it is preheated to around 450°C. Subsequently, it enters the top of the second-stage converter, where it merges with the gas from the first stage of conversion. There, it burns in the combustion zone, releasing heat and raising the temperature to around 1200°C. As it passes through the catalyst bed, further reactions occur and heat is absorbed. The gas exiting the second-stage converter has a temperature of around 1000°C and a pressure of 3 MPa, with a residual methane content of about 0.3%. The two-stage conversion gas is fed into two parallel first waste heat boilers, and then into a second waste heat boiler; all three of these boilers generate high-pressure steam. The gas coming out of the second waste heat boiler, with a temperature of around 370°C, is sent to the shift process. The fuel, natural gas, is injected and burned from the burner at the top of the radiation section, while the flue gases flow from top to bottom, in the same direction as the gas flow within the tubes. The temperature of the flue gas leaving the radiation section is above 1000°C. After entering the convection section, it flows sequentially through the coils for the mixture gas, air, steam, raw natural gas, boiler water, and fuel natural gas, with the temperature dropping to 250°C, before being discharged into the atmosphere via an exhaust fan. The process gas coming out of the two-stage furnace enters the shell side of two first waste heat boilers (double-tube type). After heat exchange, its exit temperature is approximately 482°C; after combining, it enters the tube side of the second waste heat boiler (tube-type). The process gas, having been cooled through heat exchange, is adjusted to 371°C via a bypass line before entering the shift converter. The first and second waste heat boilers generate saturated high-pressure steam at 10.4 MPa. 2. Main equipment: The first-stage converter. The first-stage converter is a key device for producing ammonia from hydrocarbon steam; it consists of two main parts: a radiant section that includes several reaction tubes and a heating chamber, and a convective section used to recover heat. Since the reaction tube is exposed to harsh conditions of high pressure, high temperature, and gas corrosion over a long period of time, it needs to be heat- and pressure-resistant. 3. The raw gas produced by various conversion methods contains CO, with a volume fraction generally ranging from 12% to 40%. Currently, the CO conversion reaction is carried out in two steps at different temperatures. The first step is high-temperature conversion; in China it is referred to as medium-temperature conversion, and during this step most of the CO is converted into CO2 and H2. The second step is low-temperature conversion, which reduces the CO content to around 0.3%. Therefore, CO conversion is both a continuation of the production of feed gas and a purification process: CO + H2O = CO2 + H2. Before the 1960s, iron-chromium catalysts were used for high-temperature conversion; after that, with the advancement of desulfurization technologies and the total sulfur content in the gas being reduced to below 0.1 cm3/m3, copper-zinc catalysts were employed. After the 1970s, cobalt-molybdenum-based sulfur-resistant shift catalysts were used for the shift processes using residue and coal as feedstocks. Since the 1980s, various energy-saving ammonia synthesis processes have been developed, such as the Kellogg process which uses natural gas as a feedstock; these are energy-saving processes designed to reduce the water-to-carbon ratio in the first stage of conversion. To prevent the iron oxide in the ferrochrome catalyst from being excessively reduced to metallic iron and iron carbide, thereby causing Fischer-Tropsch side reactions, the natural gas steam reforming ammonia production system uses a medium-temperature shift followed by low-temperature shift process. The feed gas, containing 13%–15% CO, is cooled in a waste heat boiler before entering the medium shift reactor at a pressure of 3 MPa and a temperature of 370°C. Due to the high water vapor content in the feed gas, it is generally not necessary to add additional steam. After the reaction, the CO concentration in the gas drops to around 3%, and the temperature is between 420°C and 440°C; the gas then enters another waste heat boiler where it is cooled to 330°C, thereby generating saturated steam at 10 MPa. This steam subsequently passes through a methanation reactor and a preheater, where its temperature is reduced to 230°C, before entering the low shift reactor, where the residual CO concentration is reduced to 0.3%–0.5%. The waste heat from this reaction can also be further recovered using a decarburization lean liquid reboiler. To improve heat transfer, a small amount of water can be sprayed into the gas to achieve a saturated state; this way, when the gas enters the decarburization lean liquid reboiler, the water vapor condenses rapidly, thereby increasing the heat transfer coefficient. The gas exits the conversion system and is sent to the decarburization section to remove CO2. 5. Decarburization: In the synthetic ammonia feed gas produced either from solid fuels or hydrocarbon feedstocks, 18% to 35% CO2 remains after CO conversion. The presence of CO2 not only poisons the ammonia synthesis catalyst but also poses difficulties in the purification process. When the liquid ammonia washing method is used to remove small amounts of CO and CO2 from the feed gas, CO2 tends to solidify into dry ice at low temperatures, thereby blocking pipes and equipment ； When the copper-ammonia solution washing method is used, CO2 can react with the ammonia in the copper-ammonia solution to form ammonium carbonate crystals, which block pipes and equipment; therefore, CO2 in the raw gas for ammonia synthesis must be removed. Furthermore, CO2 is also a raw material for producing products such as urea, dry ice, soda ash, and ammonium bicarbonate, so it must be recycled. 5.1 Benfield decarburization: The Benfield method is used in the Kelllogg process to remove CO2 from the shift gas. (1) Basic principle: The absorbent in the Benfield process is a solution of potassium carbonate at 25%~40% (by mass), to which a diethanolamine activator (at a concentration of about 2.5~3%) is added. In addition, a corrosion inhibitor KVO3 (at a concentration of 0.6~0.7%) and an antifoaming agent are also included. The absorption reaction mechanism involves the absorption of CO2 by an aqueous K2CO3 solution; this is a gas-liquid phase reaction, and the absorption process takes place in four steps: carbon dioxide from the gas phase diffuses to the solution interface ； Carbon dioxide dissolves in the solution at the interface ； The dissolved carbon dioxide reacts chemically with the potassium carbonate solution in the interfacial liquid layer ； The reaction products diffuse into the liquid phase matrix. During the absorption process, the chemical reaction rate is the slowest, and it constitutes the controlling step in this process. The aqueous solution of K2CO3 is weakly basic, and its reaction with CO2 is as follows: file:///C:/Users/muzil/AppData/Local/Temp/msohtmlclip1/01/clip_image001.png This is a reversible reaction; the potassium bicarbonate formed as a product releases CO2 under reduced pressure and heating, thereby allowing the K2CO3 solution to be regenerated for reuse. (2) The low-temperature shift gas, with a process pressure of 2.6 MPa and a temperature of 240°C, is cooled by water to its saturation temperature before entering the reboiler at the bottom of the regeneration tower; there it cools down to around 125°C, releasing a large amount of heat of condensation that serves as the heat source for regeneration. The raw gas coming out of the reboiler passes through a separator to have its moisture removed, and then enters the bottom of the absorption tower, where it comes into countercurrent contact with the absorption liquid from top to bottom; carbon dioxide in the gas is absorbed in this process. The purified gas, containing about 0.1% CO2, is sent to the methanation process after the entrained liquid droplets are removed in a separator. The main equipment for the decarburization process is the absorption tower and the regeneration tower, which can be classified as packed towers or tray towers. Due to the stable and reliable operation of packed towers, packed towers are used for the absorption towers and regeneration towers in most factories, while tray towers are used less frequently. Commonly used packing includes Pall rings made of stainless steel, carbon steel, and polypropylene, as well as saddle-shaped packing made of porcelain. (1) The absorption tower uses absorption at both ends; the amount of solution entering the upper part of the tower is about 1/4 of the total amount of solution. Meanwhile, most of the CO2 in the gas is absorbed in the lower part of the tower, which is why the diameter of the tower is smaller at the top and larger at the bottom. The inner diameter of the upper tower is approximately 2.5 m, the inner diameter of the lower tower is approximately 3.5 m, and the height of the tower is approximately 42 m. The upper and lower towers are both equipped with packing. To ensure that the solution wets the surface of the packing evenly, in addition to a liquid distributor installed at the top of the packing layer, the packing in both the upper and lower columns is divided into two layers, with a liquid redistributor placed between these two layers. (2) The regeneration tower is divided into upper and lower sections; the inner diameter of both sections is 4.27 m, and the height of the tower is approximately 49 m. The upper tower is equipped with Pall ring packing made of polypropylene, while the lower tower uses Pall ring packing made of carbon steel. 6. Methanation 6.1 The gas coming out of the CO2 absorption tower separator, with a temperature of around 71°C, enters the heat exchanger at the outlet of the low-pressure stage of the syngas compressor, where it is preheated to 113°C. It then proceeds to the medium-temperature heat exchanger, where it is heated to the temperature required for the reaction (the design value being 316°C), before entering the methanation reactor. After the reaction, the temperature of the gas rises to 363°C; it is first cooled to 149°C by a boiler feedwater preheater, and then enters a water cooler where its temperature is reduced to 40°C. The levels of CO and CO2 in the methanated gas drop below 10 cm3/m3. After water is separated out using a separator, the gas is sent to a compressor. 6.2 Main Equipment The main equipment for methaneation production is the methanation reactor. Structure of the methanation reactor: The methanation reactor is a cylindrical, vertical device. Due to the high hydrogen partial pressure in the gases inside the reactor, hydrogen corrosion is severe; therefore, the reactor shell is made of low-alloy steel. Above and below the catalyst are layers of alumina balls and steel wires, which help to turn over the catalyst layer as the gas flows past and facilitate gas distribution. Thermocouples are installed at various locations in the catalyst layer as well as at the gas inlet and outlet to measure temperature ； The methanation reactors in large ammonia synthesis plants generally have an inner diameter of 3 meters and a height of 5 meters, with 25–30 cubic meters of catalyst contained within them. In medium-sized plants, the methanation reactors usually have an inner diameter of 2.2 meters and a height of 6.6 meters, with 15–25 cubic meters of catalyst inside (the amount depends on the designed space velocity).