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Purification of feed gas 1. Introduction In the ammonia synthesis industry, the ammonia synthesis feed gas, after being treated through decarburization processes, still contains 0.5–3% CO and 0.5%–1% CO2. It is necessary to further treat this gas to reduce these concentrations to around 10 ppm (10-6) in order to protect the ammonia synthesis catalyst. This process of purifying the feed gas is commonly referred to as \"refining.\" Currently, the methods used in ammonia synthesis plants to remove trace amounts of CO and CO2 can be divided into thermal and cold methods. The cold-process method, also known as liquid nitrogen washing, is used in most of the new large-scale ammonia plants built at home and abroad in recent years ； There are various thermal processes, including the traditional copper acetate-ammonia solution washing method (copper washing), low-pressure methanation, methanol methanation, and molecular sieve pressure swing adsorption. Generally speaking, the cold-process technology is advanced and offers high purity, but it requires substantial investment ； The thermal process technology is relatively simple and mature, with low investment, but it falls short of the cold method in terms of purity. The copper washing process in thermal methods is gradually being replaced by other more advanced processes due to its numerous drawbacks, such as high energy consumption, low purification efficiency, and significant pollution. 2. Development of raw gas purification processes for ammonia synthesis abroad 2.1 Copper washing method The ammonium copper acetate solution washing method (referred to as copper washing) is the oldest method. It has been in use since 1913, boasting a history of nearly a hundred years, with an operating pressure of 15 Mpa. The copper washing method has long held a dominant position in small and medium-sized ammonia synthesis plants due to its mature technology and high operational flexibility. With the advancement of technology, the disadvantages of the copper-washing method for purifying feed gas have become increasingly prominent compared to other methods. The main issues are high operating, maintenance, and operational costs, as well as high material consumption (copper, acetic acid, liquid ammonia, steam). Based on the actual conditions of ammonia production plants in China, an additional cost of 50–80 yuan per ton of ammonia is required. Moreover, the purity level is low, with the concentration of CO+CO2 after purification being ≥25 ppm. Yet its most serious drawback is the severe environmental pollution it causes. Since the copper-leaching regenerated gas is washed with water to produce copper-leaching dilute ammonia solution, its concentration varies depending on the washing technique used, generally ranging from about 1% to 3%. A medium-sized nitrogen fertilizer plant generates about 10 tons of wastewater per hour. In addition to ammonia, this wastewater also contains CO2; as a result, conventional concentration methods cannot be used for its treatment because the formation of ammonium carbonate can lead to pipe blockages. Therefore, either the ammonia is regenerated through copper treatment and then released directly, or the dilute ammonia solution is discharged after copper treatment. This not only wastes valuable resources but also causes severe pollution of the atmospheric or aquatic environment. Furthermore, severe leaks of molten copper often occur during the production process, and these drawbacks are highly incompatible with modern concepts of efficient and clean production. Abroad, this outdated process was phased out as early as the 1960s, and subsequent new ammonia plants almost entirely replaced copper washing with methanation and liquid nitrogen washing methods. 2.2 Low-pressure methanation method: Compared with the copper washing method, using methanation to remove CO+CO2 from the feed gas can significantly simplify the production process, reduce construction and operation costs; the operation is also more stable, with operating costs being less than 20% of those in the copper washing method. It also occupies less space than copper washing units, and the operating pressure in the low-pressure methanation method is 1–6 MPa. After 1965, most newly built ammonia plants abroad adopted this process. However, this method also has the following drawbacks: as CO and CO2 are removed, not only is H2 consumed in amounts several times greater than that of CO and CO2, but the increase in methane content after methanation leads to an increase in the amount of ammonia that must be vented. Therefore, this process is only suitable for ammonia production plants that have undergone extensive low-temperature reforming, resulting in very low levels of CO and CO2 in the feed gas (typically CO+CO2 < 0.7%). Even so, according to the methanation reactions CO+3H2 = CH4+H2O and CO2+4H2 = CH4+2H2O, for every 0.1% increase in CO in the feed gas, 0.3% more H2 and 0.1% more CO are consumed, totaling 0.4% (as CO is converted into H2), while at the same time 0.1% more CH4 is added to the syngas. Calculations show that if 0.7% of CO+CO2 is methanized, the combined increase in raw gas consumption for these two components is approximately 10%, highlighting the large scale of this consumption. 2.3 Brown cryogenic purification process: Cryogenic purification technology involves the liquefaction and separation of certain components in a gas mixture through deep freezing, based on the principle of isentropic expansion. The methanated mixture is dried, then undergoes isentropic expansion, followed by recirculating cooling to reduce the temperature to –185°C to –175°C. At this temperature, all of the CH4 in the syngas, the residual CO, most of the Ar, and the excess nitrogen can be liquefied and separated, resulting in pure, dry (3:1) hydrogen-nitrogen syngas and achieving deep purification. Moreover, a small amount of off-gas from the synthesis system can be returned to the system for recycling. Therefore, it can be considered that the Brown cryogenic purification process compensates to some extent for the shortcomings of the methanation process. 2.4 Liquid nitrogen washing method: This process was developed by Linde in West Germany, around the same time as the methanation process, and it is also widely used in large-scale ammonia synthesis plants. In the late 1970s, China introduced a total of 9 large-scale units for producing synthetic ammonia using residue oil as raw material; for the gases resulting from the conversion process, low-temperature methanol washing was used for desulfurization and decarburization, followed by liquid nitrogen washing for further purification. The feed gas is first purified by a gas purification method using methanol as a physical solvent, namely the low-temperature methanol washing process, to remove carbon dioxide and sulfides. To prevent gas freezing, a small amount of methanol is injected before the feed gas enters the feed gas cooler, where it is cooled to –9°C. After passing through the separator, it is sent to the methanol scrubber. The purified gas coming out of the methanol scrubber tower contains CO2 at ≤20 ppm and hydrogen sulfide at ≤1 ppm, and is sent to the liquid nitrogen washing process. The raw gas leaving the methanol scrubber passes through a molecular sieve adsorber to remove trace amounts of methanol, carbon dioxide, and moisture, before entering the low-temperature section of the nitrogen washing cooling box. After being cooled to –188°C by nitrogen purging and the carbon monoxide fraction in the feed gas/nitrogen cooler, it enters the liquid nitrogen scrubber tower. In the liquid nitrogen scrubber, impurities in the gas such as carbon monoxide, methane, and most of the Ar are removed by liquid nitrogen. After purification, the hydrogen and nitrogen gases contain CO at <5 ppm, and Ar plus CH4 at <45 ppm, which fully demonstrates the advantage of high purification efficiency of this process. However, its drawback is that it consumes cooling capacity, requires large air refrigeration and separation units, and involves high investment costs. It is generally used in large-scale installations; it is more economical and reasonable to use pure oxygen to produce the feed gas in conjunction with the low-temperature methanol washing process. 2.5. Methanol methanation method: In the early 1990s, the Danish company Topsoe developed a new gas purification process. Compared to traditional processes, this new process is more attractive. This process consists of two parts: methanol synthesis, followed by the conventional methanation process. In the methanation process, CO and CO2 are converted into methanol through the following reactions: CO + 2H2 = CH3OH; CO2 + 3H2 = CH3OH + H2O. Methanation takes place on highly active copper-based catalysts. Due to the high hydrogen partial pressure in the feed gas and low partial pressures of CO and methanol, as well as the relatively low operating temperature, very few by-products are generated. Therefore, the methanolization process can produce methanol of very high purity. This process was first adopted in 1993 at the SEMACOT ammonia plant in Egypt. Its synthesis compressors are available in two pressure ranges: 22 Mpa and 45 Mpa. Initially, copper washing was used to purify CO and CO2 at 22 Mpa, while ammonia synthesis took place at 45 Mpa. Topsoo applies its methanation technology to produce methanol at a pressure of 22 Mpa, with methanation carried out concurrently; CO and CO2 are purified to 10 ppm, after which the pressure is increased to 45 Mpa for ammonia synthesis.