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Advances in pressure swing adsorption carbon removal technology: The pressure swing adsorption process has been in use in China for 20 years. Initially, its low technical level and low yield led to difficulties in its adoption; over time, the technology was improved to increase yields. Chengdu Tianli Chemical Technology Co., Ltd. introduced a two-stage vacuum extraction process, which resolved the issue of low yields that had hindered its widespread use. As a result, numerous pressure swing adsorption units were installed across the country to replace the older processes. With the goal of continuous innovation, and through rigorous experimentation, a revolutionary transformation was carried out on the pressure swing adsorption decarburization process, achieving significant progress in improving the recovery rate of the product and reducing energy consumption. The design of the PSA unit actually depends on the percentage of CO2 (V) required in the purified gas; the higher this value, the more can be incorporated into the pressure equalization design to recover more gas, thereby minimizing the venting pressure and gas losses. In the first-stage process, since the PSA unit is required to keep the CO2 concentration at the outlet below 0.2%, excessive equalization cycles will directly contaminate the outlet of the adsorber, making it difficult to control the CO2 concentration; therefore, the number of equalization cycles in the first-stage process should not be too high. In this way, the pressure in the French venting system is high, while the CO2 concentration in the gas is low, thereby effectively reducing gas loss. In this case, through extensive field work over a long period of time, and by moving from theory to practical testing, Chengdu Tianli Chemical Technology Co., Ltd. developed a two-stage method. The PSA unit is divided into two separate sections: a first stage and a second stage. The intermediate gas outlet temperature in the first stage is controlled at 7%-8%, and the number of pressure equalization cycles can be adjusted to reduce the back-pressure and increase the CO2 concentration in the released gas, thereby effectively reducing the amount of gas emitted. The second stage keeps the outlet CO2 level below 2%, and the gas released from the second stage is returned to the adsorber in the first stage for recovery. This addresses the issue of increasing the CO2 concentration in the exhaust air, thereby reducing effective gas loss. After its development through the two-stage method, it was widely applied in fertilizer plants. Currently, the pressure swing adsorption process is mainly used in fertilizer plants for the following two purposes. One method involves removing carbon dioxide from the shift gas to produce liquid ammonia and hydroxylamine; this approach does not recover carbon dioxide and is quite widely used. Another process is employed in urea production, where not only must the carbon dioxide content in the shift gas be reduced to below 0.2%, but it also needs to be purified to over 98.5% before being used in urea production. This process requires higher standards than the first one, as it involves purifying carbon dioxide while also ensuring an effective yield of the desired gases, which makes it somewhat more challenging. However, regardless of the type of decarbonization, to increase the yield of useful gases, it is necessary to raise the concentration of carbon dioxide removed from the air during desorption, thereby reducing the content of useful gases in it. Therefore, the design philosophy behind the aforementioned two processes is exactly the same. 1. Two-stage pressure swing adsorption process: Initially, in 1999, Hubei Yihua adopted a pressure swing adsorption decarburization process for urea production. The gas processing capacity was 6,000 NM3/H, with an electricity consumption of around 135 KWH per ton of ammonia produced. Without any gas recycling system, the hydrogen yield was 98%, while the nitrogen yield was 91%-92%. A gas recycling system increases the gas yield, but it does so at the cost of higher electricity consumption for compression. Since the variable-pressure adsorption method was used for the first urea process, there is significant room for technological improvement in this facility. Through the joint efforts of Hubei Yihua and Chengdu Tianli Chemical Technology Co., Ltd., difficulties were overcome and the pressure swing adsorption process was optimized and adjusted accordingly. For the urea decarburization process in Hubei Yihua’s Project 813, which has a gas treatment capacity of 70,000 NM3/H, the two-stage pressure swing adsorption process developed by Chengdu Tianli Chemical Technology Co., Ltd. was adopted. The PSA unit is divided into a purification system and a cleaning system. The entire process flow is summarized as follows: the semi-water gas supplied from the gas generation workshop undergoes wet desulfurization, and after pressurization it enters the medium-low shift system. The composition of the shifted gas is: 28.7% CO2, 0.2% CO, 50.4% H2, 1.1% CH2, and 19.6% N2. The product CO2 purity obtained through the PSA purification system using transformed gas is 98%; the intermediate gas is compressed followed by CO2 removal, resulting in a CO2 content of less than 0 in the purified gas. 2%; the purified gas is pressurized and then sent for copper washing, followed by further pressurization for ammonia synthesis. Pure CO2 is compressed, then dehydrated, and after further compression it is used for urea production. The unit was successfully put into operation on November 23, 2000, and qualified hydrogen, nitrogen, and carbon dioxide gases were obtained after 2 hours. The production operations showed that: 1) the use of DCS control improved stability and reduced the labor intensity. 2) CO2 is removed at the two outlets of compression; with the same gas volume, the load on the 4M20 compressor decreases (40 KWh of electricity is saved per ton of ammonia produced), thereby resolving the overpressure issue that occurs during three-stage PC decarburization. 3) The total sulfur in the purified gas is removed in the adsorption bed, eliminating the need for secondary desulfurization. 4) CO2 in the purified gas is <0.2%, reducing the refining load. 5) The efficiency of gas production is improved due to the loss of about 3.5% of nitrogen during the adsorption process, resulting in a slight reduction in coal consumption. 6) By switching from traditional wet decarburization to dry decarburization, solvent loss is eliminated, and the adsorbent has a long service life, further reducing operating costs. 7) The process is simple, startup and shutdown are easy, and equipment maintenance costs are low. In this device, there is no gas return system; the power consumption per ton of ammonia is around 98 KWH. Without a gas return system, the hydrogen yield is 97% and the nitrogen yield is 92%. With a gas return system, the hydrogen yield is 99%, the nitrogen yield is 99%, and the nitrogen yield is 96%. The advantages of the two-stage pressure swing adsorption urea carbon removal process have been demonstrated; it is capable of keeping the carbon dioxide content in the purified gas within 0.1%-0.2% (V) even at low pressures such as 0.65 MPA, and its operation is stable and convenient. Wet carbon removal requires higher pressures (such as 1.6 MPA) in order to keep the carbon dioxide content in the purified gas below 0.2% (V), while carbonyl carbon removal needs even higher pressures (typically 2.7 MPA) to achieve the same level of carbon dioxide control. Wet carbon removal requires the hydrogen sulfide content in the shift gas to be reduced to below 20–30 mg/Nm3, whereas pressure swing adsorption using urea for carbon removal does not require desulfurization of the shift gas; it can be fed directly into the pressure swing adsorption urea carbon removal unit. The adsorbent used in urea decarboxylation with a pressure transformer is non-toxic to humans and does not cause corrosion to the decarboxylation equipment. All wet carbon removal methods incur losses to varying degrees; therefore, the additional operations involved do not consume the adsorbent. If the adsorbent becomes inactive due to operational errors, it can be restored to its original performance by heating it. In wet decarburization, after operation for a period of time, the solvent will bubble and degrade, resulting in reduced decarburization efficiency and increased operating costs. The adsorbent used for urea decarboxylation via pressure swing adsorption maintains its performance under normal operating conditions and is highly stable. In this process, the advantages of pressure swing adsorption are fully demonstrated, ensuring excellent performance and operation costs that are **better than those of wet carbon removal. 2. The full purging process involving power equipment has been eliminated. In order to further improve the yield of useful gases and reduce operating costs, the technical staff at Chengdu Tianli Chemical Technology Co., Ltd. conducted in-depth research and continuous improvements in areas such as adsorbent selection, process technology, and the durability of programmable control valves. Through repeated tests on several PSA units owned by Yihua, they developed a mature pressure swing adsorption process that eliminates the need for any power equipment. This new process builds on the two-stage pressure swing adsorption urea decarburization technology of the original patented technology; it reduces the emission of unnecessary gases and lowers the concentration of useful gases in the exhaust gases. By utilizing the kinetic energy resulting from pressure changes in the gases themselves, vacuum equipment is replaced, thereby achieving an even higher yield of useful gases and a ammonia consumption rate that is one-tenth of that in the original method. Furthermore, the new process has been successfully applied to the world’s largest capacity facility at present. The 14,600 NM3/H pressure swing adsorption desorption unit of Shandong Ruixing Biochemical Co., Ltd. is the largest and most advanced pressure swing adsorption carbon removal unit in the world, built by Chengdu Tianli Chemical Technology Co., Ltd. The unit came online in January 2005, with a successful first operation, and the purity of the produced carbon dioxide was 98.2%-98.5%. The entire unit has all its power-driven equipment removed; aside from the instruments and hydraulic pump stations, there is no electricity consumption. The PSA urea removal unit features a simple process, stable operation, low energy consumption, low maintenance costs, as well as good and consistent purification standards. The process of this device incorporates the following improvements over the previous process: 1) The purification section, through adjusted and optimized procedures, enables more gas to be returned to the purification stage while ensuring the purity of CO2 in the product gas ； 2) The purification section eliminates all power-consuming equipment; it relies entirely on the desorption and purification sections to release gas for purging purposes, thereby reducing power consumption – a significant technical breakthrough that achieves this goal of lower power use ； 3) The pressure equalization method in the purification section was adjusted and optimized, achieving good results that enabled more thorough recovery of the useful gas ； 4) The purification section is equipped with an appropriate number of pressure equalization cycles based on the requirements for the purified gas; this is done to ensure a stable supply of gas for its own purging process as well as to meet the specifications for the purified gas ； 5) The method used by the purification section to equalize pressure has also been adjusted and optimized, enabling more thorough recovery of the useful gas ； 6) A completely new process called purging has been introduced in the purification section; it uses its own gas to purge and desorb substances, thereby replacing power-driven equipment entirely, and practice has shown that its effectiveness is even better than that of such equipment. Thanks to the above improvements and optimizations, this device operates stably after being put into use; the amount of gas released is reduced, as is the amount of useful gases in that released gas. Moreover, the carbon dioxide level in the gas used for urea production remains stable, while the carbon dioxide level in the purified gas can be controlled flexibly. In actual operation, the power consumption per ton of ammonia is around 4 kWh, with about 1 T of cooling water per hour. The ammonia production volume has increased by about 2% compared to the original level. In August 2005, the Tianjin Alkali Plant of Bohai Chemical Group, which operates a pressure swing adsorption carbon removal unit with a capacity of 4000 NM3/H for producing soda ash, made detailed adjustments to its process compared to the previous full-purge process. This device presents relatively high challenges, as the composition of the raw materials is 24.13% CO2, 36.9% H2, 22.33% CH2, and 4.03% N2. It can be seen that the CO content is quite high, while the CO2 content is lower than that of CO. For decarburization processes, such a composition is most unfavorable, as the partial pressure of CO2 is lower than that of CO; as a result, a large amount of CO is absorbed, which directly leads to a fairly low yield of CO in the entire system. The issue of CO yield has always been a challenge; for many years it has been difficult to overcome low yields, especially when dealing with the aforementioned gas components. Chengdu Tianli Chemical Technology Co., Ltd. optimized the process further based on the Shandong Ruixing facility. After the commissioning of the pressure-swapping and carbon removal unit at Tianjin Alkali Plant, capable of handling 40,000 NM3/h of shifted gas, the yield of CO and H2 reached 98%. By continuously summarizing experience, promptly correcting any inappropriate approaches, and adjusting and optimizing the processes. The pressure swing adsorption desorption decarburization unit designed by Hubei Yihua Group’s Guizhou Xingyi Fertilizer Plant for treating shift gas at a rate of 12,000 NM3/H (for urea production) has a shift pressure of 2.0 MPA; the adsorption pressures for the first and second stages of pressure swing adsorption are 1.8–1.9 MPA. The carbon dioxide content in the purified gas is 0.5% (by volume). The off-gas from the second stage is almost entirely recycled back to the first stage, while the carbon dioxide concentration in the off-gas from the first stage increases due to adjustments made in the new process, thereby significantly reducing the amount of useful gas emitted. In addition to the devices mentioned above, there are several other plants in China that are currently in operation using this full-purge process: Sanning Chemical Co., Ltd. in Zhijiang, Hubei, which operates a pressure swing adsorption carbon removal unit capable of handling 30,000 NM3/h of shifted gas (for the production of liquid ammonia); Fengcheng Fertilizer Factory in Liaoning, which has a pressure swing adsorption carbon removal unit handling 21,000 NM3/h of shifted gas (for liquid ammonia production); and Anxiang Fertilizer Factory in Hunan, which uses a pressure swing adsorption carbon removal unit handling 1,500 NM3/h of shifted gas (for liquid ammonia production). Among these, the plants with an annual production capacity of over 160,000 tons include Wulasan Fertilizer Factory in Inner Mongolia, which operates a pressure swing adsorption carbon removal unit handling 96,000 NM3/h of shifted gas, and Dangyang Huahua Chemical Co., Ltd. in Hunan, which has a pressure swing adsorption carbon removal unit handling 88,000 NM3/h of shifted gas. The carbon dioxide content in the purified gas is generally around 0.2% (by volume), while the hydrogen recovery rate is greater than 99.5% (by volume) and the nitrogen recovery rate is greater than 98% (by volume). There is no power consumption associated with these devices. With the continuous technological innovations in recent years, the two-stage pressure swing adsorption patent technology has been applied across the country. Compared with traditional NHD methods, improved carbon-propane methods (PC), improved MDMEA methods, and improved thermal potassium-alkali methods, it offers advantages such as a simpler process, greater flexibility in operation, lower operating costs, and a higher degree of automation. It is possible to replace the existing wet carbon removal systems with this technology, and the entire investment can be recovered within less than two years. As energy shortages become more severe, the advantages of the two-stage pressure swing adsorption patent technology in terms of energy savings and coal reduction become even more apparent.