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Current status of pressure swing adsorption for CO purification in China. Carbon monoxide is an important basic chemical raw material that can be used in the production of various fine chemical products such as TDI, formic acid, and acetic anhydride. Therefore, it is important to obtain high-quality, low-cost carbon monoxide gas for the production of related products. For laboratory and small-scale use, carbon monoxide can be produced by methods such as the decomposition of formamide or formic acid; whereas for industrial production, carbon monoxide must be separated and extracted from various CO-containing mixtures (such as water gas and semi-water gas). Among these gas sources, carbon monoxide often coexists with hydrogen, nitrogen, methane, carbon dioxide, and water vapor; therefore, an economically viable method must be available to separate CO from these other components in order to use it in industrial processes. There are mainly three types of industrial processes for CO purification that have been developed to date: 1.1 Industrialized CO purification methods I – Cryogenic separation. Cryogenic separation is a physical separation method that has been in use for a long time; it was developed successfully by the German company Linde in 1925. This method makes use of the differences in boiling points among various gas components, and achieves the separation of gas mixtures through low-temperature distillation. China Pressure Swing Adsorption Network understands that this process is mature, capable of handling large volumes of material and achieving high recovery rates, but it also has many drawbacks. Since components such as water and CO2 in the mixture condense into solid states at low temperatures, potentially causing pipe blockages, cryogenic separation of CO requires a very complex pretreatment system ； At the same time, since the relative molecular masses of nitrogen and carbon monoxide are almost identical and their boiling points are very close to each other, it is difficult to separate them by low-temperature distillation; therefore, this process is only suitable for feed gases that do not contain N2 or contain only trace amounts of it. Overall, the cryogenic process for CO separation involves complex equipment, high capital costs, and elevated operating expenses, making it only economically viable for large-scale production. II. COSORB method: This is a solution absorption separation method that was developed in the early 1970s by the American company Tenneco. It involves the use of a toluene solution containing CuCl·AlCl3 to absorb CO, followed by desorption through heating to obtain CO as a product gas. At room temperature and a carbon monoxide partial pressure of 101.325 kPa, this absorbent can absorb approximately 1.8 mmol/ml of carbon monoxide. However, China’s pressure swing adsorption industry is aware that this method has serious drawbacks: the presence of components such as H2O, sulfides, and ammonia in the feed gas can cause side reactions with the chelating agents, reducing their absorption capacity or even rendering them ineffective. When in contact with water, the absorbent produces hydrogen chloride, which causes severe corrosion of equipment and pipelines; therefore, a complex pretreatment system is required ； Furthermore, the CO gas obtained through thermal desorption contains toluene vapor and chloride ions, requiring an additional post-treatment step. In short, due to high equipment investment, high operating costs, as well as severe corrosion and environmental pollution problems, this process has been gradually phased out abroad. III. Pressure swing adsorption method: In the 1980s, the German company Linde developed a two-stage pressure swing adsorption process for CO separation based on 5A molecular sieve adsorbents. The first stage serves as a pretreatment step to remove carbon dioxide, which is difficult to desorb from the 5A molecular sieve, while the second stage is used to purify the resulting CO product. In China as well, industrial plants have been established for the two-stage separation and purification of CO using 5A molecular sieve as the main adsorbent. However, due to the poor selectivity of the 5A molecular sieve adsorbent toward CO, N2, and methane, the purity and yield of the CO product are very low. As a result, extensive research has been conducted both domestically and internationally on CO adsorbents. Japanese company NKK, Peking University, and American company Air Products have successively developed new CO adsorbents loaded with monovalent copper, which enable effective separation of CO from N2 and CH4. Compared with the first two methods, pressure swing adsorption has a wide range of compatibility with feed gases; it does not require a complex pretreatment system, can operate at ambient temperature, features a high degree of automation and is easy to operate. There are no issues related to equipment corrosion or environmental pollution. It is not only more advanced than the COSORB method but also superior to the cryogenic method when the scale is not particularly large or when the feed gas contains a large amount of N2. It is superior to the cryogenic method when the size is particularly large or the feed gas contains a large amount of N2. 1.2 Current status of CO purification by pressure swing adsorption in China At present, there are two types of systems in China that use pressure swing adsorption technology for the separation and purification of CO: one uses the traditional 5A molecular sieve process, while the other employs an adsorbent loaded with Cu. 1.2.1 Use of 5A molecular sieve process: Although the two-stage PSA-CO system that utilizes 5A-type molecular sieve adsorbents can be used for CO separation, it has the inherent drawback of low separation coefficients for CO/N2 and CO/CH4. Moreover, such adsorbents become poisoned when they absorb CO2, thereby losing their ability to adsorb CO. Based on these characteristics of the 5A-type molecular sieve adsorbent and the composition of the feed gas, China Pressure Swing Adsorption Network believes that two factors must be taken into account during the process design of the original installation: (1) CO2 in the feed gas must be completely removed in the pretreatment step (PSA-CO-1) ； (2) It is necessary to remove as much CH4 as possible in the pretreatment step (PSA-CO-1), thereby reducing the CH4 concentration in the semi-finished gas entering PSA-CO-2. This helps to lower the washing ratio, increase the CO yield and productivity of the 5A molecular sieve, and meet the requirements regarding the CH4 content in the CO product. For the above reasons, this process has the following inherent drawbacks that cannot be improved: Ⅰ. Excessive CO loss in the PSA-CO-1 step. When the feed gas contains CH4, especially when high purity of the CO product is required or downstream users have specifications regarding the CH4 content in the CO product, a large amount of activated carbon must be used in the PSA-CO-1 step to remove CH4. Since activated carbon has a strong adsorption capacity for CO, a significant portion of CO is lost during the pre-treatment of the feed gas. Furthermore, in order to minimize CO loss while removing CH4, the operating cycle of PSA-CO-I must be made long, which forces PSA-CO-1 to be equipped with a larger amount of adsorbent. II. The operating costs of PSA-CO-2 are high, and it is difficult to improve product quality. Since the separation coefficients of CO/N2 and CO/CH4 for 5A-type molecular sieve adsorbents are very low, when the adsorbent absorbs CO, it also absorbs large amounts of CH4 and N2. In order to obtain product gas of the required high purity, a large amount of purge gas is needed during the flushing and replacement of the adsorbent; the flushing ratio (amount of purge gas/total amount of gas evacuated and desorbed) can often be as high as 60–70%. As a result, the yield of pure CO is very low, and therefore a large amount of feed gas is necessary to meet the production requirements. Furthermore, due to fluctuations in the contents of N2 and CH4 in the feed gas, even with such operations, it is difficult to maintain the purity of the resulting CO at a high level (such as above 98%). Due to the high flushing ratio, a large amount of purge gas is required, and even more gas needs to be evacuated during desorption; as a result, larger or more purge gas compressors and vacuum pumps are necessary, which leads to higher electricity consumption per unit of product. In summary, PSA units for CO purification that use 5A molecular sieve adsorbents require large amounts of feed gas and high electricity consumption per unit of product, resulting in high operating costs; moreover, the quality of the product gas cannot be effectively ensured. 1.2.2. Use of Cu-loaded adsorbents The pressure swing adsorption process for CO separation using Cu-loaded adsorbents was first industrialized in Japan, and subsequently in the United States and the United Kingdom. In China, the process of using Cu-loaded adsorbents to separate CO began to be developed and applied since the early 1990s; it was not until February 2003 that this process was successfully implemented in a large-scale facility by Peking Pioneer Technology Company.