Various processes for ammonia synthesis

(1) Kellogg process. The first set of large-scale ammonia synthesis plants with low energy consumption, designed by the American company Kellogg, was put into operation in 1983, with an energy consumption of 29.31 GJ per ton of ammonia produced. After the 1990s, the company, in collaboration with BP, developed more advanced ammonia synthesis processes – the KAAP and KRES combined technologies – which reduced the energy consumption per ton of ammonia to 25.96–27.21 GJ, representing a significant breakthrough in ammonia synthesis technology. The KAAP technology uses Fe-based catalysts for ammonia synthesis with high activity at low temperature and pressure. The KRES technology is a self-heating conversion technique; the equipment consists of a heat-exchanging first-stage conversion furnace and an adiabatic second-stage conversion furnace. The hot converted gas coming out of the second-stage furnace provides all the necessary heat to the first-stage furnace through heat exchange, thereby significantly reducing energy consumption. (2) Braun process. The energy-saving measures adopted by Brown Company in the United States mainly involve reducing the consumption of natural gas as fuel – that is, decreasing the load on the first-stage converter (raising the CH4 content at the outlet from 10% to around 30%) and increasing the load on the second-stage converter while adding excess air there (which generates a large amount of reaction heat to supply the heat needed for the conversion of residual CH4). This results in a lower temperature in the first-stage converter and thus less consumption of natural gas as fuel. At the same time, deep cryogenic purification is used to remove excess nitrogen, and a gas turbine drives the air compressor, resulting in an energy consumption of 28.4 GJ per ton of ammonia produced. In China, energy-saving technologies with a low water-to-carbon ratio have been applied to a section of the converter used in the imported Brown apparatus, while the ammonia synthesis process employs a three-tower and three-waste-heat boiler system to generate high-pressure steam from waste heat, thereby further reducing energy consumption. (3) ICI process. The AM-V process of the British company ICI, in addition to adopting some energy-saving measures from the Brown process, is characterized primarily by the development and use of Fe-Co catalysts for ammonia synthesis that exhibit good activity at low temperatures and pressures. In 1988, ICI developed a simplified and scaled-down LCA process, building two plants with a daily ammonia production capacity of 450 tons each; the energy consumption per ton of ammonia was 29.31 GJ, demonstrating that medium-sized ammonia synthesis plants can also achieve energy-saving levels comparable to those of large-scale plants. (4) KPK process. The KPK process is an abbreviation for KRES/PURIFIER/KAAP; it incorporates advanced technologies from Kellogg and Braun. It features the use of heat-exchange converters in place of traditional single-stage conversion furnaces, as well as ruthenium-based catalysts and cryogenic purification techniques, making it a new type of energy-efficient ammonia synthesis process.

Introduction to the Kellogg process: This facility consists of the following main components: a natural gas distribution station, a desulfurization system, a natural gas steam reforming system, a shift reaction system, carbon removal and methanation systems, a synthesis and refrigeration system, a hydrogen recovery system, a process condensate treatment system, an instrument air system, a steam pipeline network system, a boiler feedwater system, and oil systems for the four main units. 1.1.2. Tasks of various systems 1.1.2.1 Conversion system (1) The conversion unit is responsible for receiving the raw gas and fuel gas supplied to our company by the natural gas company, and it adjusts the pressure and flow rates as required by production, so that the gas can be used by various users. (2) Responsible for the metering and reception of deionized water, medium and high-pressure steam supplied from the water treatment plant to the synthesis area, and sending a portion of the deionized water, after being heated in exchangers 134-C and 106-C, to Boiler No. 4. (3) Responsible for receiving the vapor condensate from 135-J, which is sent to 103-D and 101-CA/CB via 122-J. (4) Responsible for mixing the off-gas sent from the synthesis unit to the conversion unit with fuel gas for combustion within the furnace, in order to regulate the combustion process. (5) Responsible for desulfurizing the raw natural gas and ensuring that all process parameters are within acceptable ranges. (6) Responsible for the steam pre-conversion, conversion, and reforming of the raw natural gas, in order to produce high-quality process gas suitable for further use. (7) Responsible for regulating the quality of water and high-pressure steam, as well as ensuring that all process parameters are within acceptable limits. (8) Be responsible for the proper operation of the heating furnace, the first-stage furnace, and the auxiliary boilers, and adjust the inlet temperature of the pre-conversion furnace, the outlet temperature of the heating furnace, and the temperature difference between the tubes in the first-stage furnace to keep them within specified ranges, thereby ensuring no carbon deposition, no overheating, and no overpressure. (9) Responsible for receiving hydrogen of 110-D supplied from the fertilizer and synthesis units, and adjusting the hydrogen content at the outlet of 110-D to keep it within acceptable levels. (10) Responsible for receiving a set of nitrogen gases (N5, N80, N120) supplied to the synthesis zone. (11) Be responsible for ensuring that all equipment in the conversion station operates safely and properly within the limits permitted by its design. (12) Responsible for the production and balancing of high, medium, and low-pressure steam. (13) Be responsible for preventing equipment from freezing in winter and ensuring that the equipment is not damaged by freezing. 1.1.2.2 Decarburization system (1): Removal of CO2 from low-temperature gas to produce CO2 at 98.5% (v), H2

The ammonia synthesis process using natural gas as a raw material involves a technological procedure for producing ammonia from gaseous feedstocks such as natural gas and refinery gas. With minor modifications, this process can also be applied using naphtha as a raw material. Seven to eight different catalysts are used in this process, and high-purity gas purification techniques are necessary; for example, the use of cobalt-molybdenum hydrogenation catalysts and zinc oxide desulfurization agents allows sulfur in natural gas to be reduced to levels below 0.1 UL/L. This not only protects the conversion catalysts but also facilitates the use of low-temperature conversion catalysts that have poor resistance to sulfur. Through high-purity decarburization methods, the total volume fraction of carbon monoxide and carbon dioxide in the gas can be reduced to levels below 5–10 UL/L. The steam reforming method is commonly used for producing ammonia from natural gas. After desulfurization, the natural gas is mixed with steam and subjected to a conversion reaction in the reaction tubes of a first-stage converter. The heat required for this conversion reaction is supplied by burning fuel outside the reaction tubes. The gas from the first-stage converter then enters a second-stage converter, where air is introduced to burn part of the hydrogen or other gases, generating heat that is used to further convert the remaining gaseous hydrocarbons. At the same time, nitrogen required for ammonia synthesis is introduced into the system. The gas from the second-stage converter then passes through medium-temperature and low-temperature converters, where carbon monoxide in the gas reacts with steam at different temperatures to produce equal amounts of hydrogen and carbon dioxide. After these processes, the crude feed gas used for ammonia synthesis is obtained, with nitrogen, hydrogen, and carbon dioxide being its main components. The crude feed gas enters the decarburization process, where carbon dioxide is removed using a potassium carbonate solution containing diethanolamine or glycine; subsequently, the small amounts of carbon monoxide and carbon dioxide remaining in the gas are removed through a methane processing step, resulting in a pure hydrogen-nitrogen mixture. This mixture is compressed to high pressure by a syngas compressor and then fed into a synthesis tower for the synthesis reaction. Since only 10–20% of the hydrogen and nitrogen in the gas react after passing through the synthesis tower once, it is necessary to cool the gas exiting the tower so that the produced ammonia can condense and be separated. The unreacted gas is sent back to the synthesis tower. Throughout the production process, heat recovery equipment is installed wherever available waste heat can be utilized, thereby forming the plant’s steam power system that is integrated into various process steps. As a result, heat energy is utilized efficiently and reasonably, leading to low energy consumption.

Synthetic ammonia process using coal as a raw material: Most medium-sized synthetic ammonia plants in China that use coal as a raw material employ three types of catalysts—desulfurization, low-temperature shift, and methanation—to purify the gases. Small ammonia synthesis plants that use coal as raw material employ a carbonization process to remove carbon dioxide using concentrated ammonia water; the ammonium bicarbonate produced is then crystallized, and after separation, it becomes the final product. In a medium-sized ammonia synthesis plant that uses anthracite as raw material, anthracite (or coke) with a particle size of 20–75 millimeters is fed into a fixed-bed gas generator. Air and steam are introduced alternately into the furnace; the semi-water gas produced as a result of vaporization passes through a combustion chamber, where waste heat is recovered in a waste heat boiler before being stored in a gas holder. After solid particles are removed from this semi-water gas using an electrostatic precipitator, it enters the first three stages of the feed gas compressor, where its pressure is increased to 19–20 kg/cm². The gas then goes to a semi-water gas desulfurization tower, where it is washed with ADA solution (or another desulfurization solution) to remove hydrogen sulfide from the gas. Subsequently, the gas enters a saturation tower, where hot water is used to convert it into saturated steam. After being heated by gas coming from the shift reactor in a heat exchanger, the gas enters the shift reactor, where steam is used to convert carbon monoxide in the gas into hydrogen. The gas resulting from this transformation returns to the heat exchanger to exchange heat with the semi-water gas, after which it is cooled in a hot water tower before entering another desulfurization tower, where it is washed again with ADA solution to remove hydrogen sulfide generated from organic sulfur during the transformation process. After that, the gas enters the last two stages of the feed gas compressor, where its pressure is increased to 120–130 kg/cm². It then passes through a copper washing tower and an alkali washing tower, reducing the levels of carbon monoxide and carbon dioxide in the gas to below 20 ppm. The purified hydrogen-nitrogen mixture enters the final stage of the feed gas compressor, where its pressure is increased to 300–320 kg/cm². It then goes into an oil filter, where it mixes with the gas from the recycle gas compressor and any oil content is removed. After that, it enters the tubes of a condenser and an ammonia cooler, and further into the spaces between the tubes in the upper part of the condenser, where it exchanges heat with the gas inside those tubes. Finally, it enters the ammonia synthesis tower, where hydrogen and nitrogen combine to form ammonia under high temperature, high pressure, and in the presence of a catalyst. The gas exiting the tower contains 10–16% ammonia. After being separated from liquid ammonia using a water cooler and an ammonia separator, the gas is sent back to the recycle gas compressor for reuse. The separated liquid ammonia is stored in a liquid ammonia tank

Synthesis of ammonia using heavy oil as a raw material: When producing ammonia from heavy oil, the partial oxidation method is employed for gas generation. The raw gas coming out of the gasifier is first freed of carbon black; it then undergoes sulfur-tolerant conversion to carbon monoxide, followed by low-temperature methanol washing and liquid nitrogen washing before being compressed to produce ammonia. In this process, an air separation unit is necessary to supply oxygen for the gasification of the oil, while nitrogen is used in the liquid nitrogen washing step to remove any remaining carbon monoxide. The production of ammonia from heavy oil mostly employs the partial oxidation method. In China, medium-sized ammonia synthesis plants that use heavy oil as raw material generally follow one of the following two processes. The first process involves using partial oxidation to produce water gas at a pressure of 29–32 kgf/cm2; the heat generated is recovered in waste heat boilers, after which carbon black is removed and the gas enters the purification system. The gas purification steps are essentially the same as those used in ammonia synthesis from coal – the gas first undergoes desulfurization using ADA, followed by medium-temperature shift reaction and further desulfurization. Carbon removal is then carried out using diethanolamine or potassium aminoacetate methods. The gas after initial desulfurization goes through zinc oxide desulfurization, low-temperature shift reaction, and additional carbon removal before being mixed with nitrogen. A small amount of carbon monoxide and carbon dioxide is removed through methanation, resulting in a high-quality hydrogen-nitrogen mixture, which is then compressed and sent to the synthesis system. The second process is similar to the first, with the main difference being that the water gas coming out of the vaporizer enters a quench chamber where heat is recovered and carbon black is removed, before proceeding to the medium-temperature shift reactor. The gas purification steps remain the same as in the first process. Since the gas enters the medium-temperature shift reactor without prior carbon removal, it is necessary to use heavy oil with low sulfur content or medium-temperature shift catalysts that can tolerate sulfur

Coal dust – pressurized gasification – shift reaction – low-temperature methanol washing – liquid nitrogen washing – synthesis; this is currently a relatively good process for ammonia production

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