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Abstract: Ammonia synthesis is produced based on hydrogen extraction using pressure swing adsorption; the recovery scheme for the PSA off-gases is optimized. Gas is generated by mixing desorbed gas with coke oven gas, as well as through parallel PSA gas supply, in order to achieve the production of liquid ammonia from two different gas sources. The advantages and disadvantages of the entire process were analyzed. Keywords: coke oven gas, PSA desorbed gas, recovery, liquid ammonia, introduction. The methanol purge hydrogen extraction unit generates a large amount of desorbed gas while producing high-purity hydrogen; this desorbed gas accounts for approximately 30%~35% of the methanol purge gas. The components of the desorbed gas are CH4 (11.09%), H2 (24.11%), CO (8.93%), N2 (40.98%), and CO2 (14.89%). If discharged directly into the atmosphere, it not only leads to energy waste but also causes environmental pollution; therefore, recycling is necessary. 1. Analysis of existing facilities 1.1 Analysis of the facility’s production capacity The ammonia synthesis facility is designed to have a production capacity of 100 kt/a; its feed gas is supplied by two 100 kt/a methanol plants within the industrial park, while nitrogen is provided by the air separation units of those methanol plants. Each methanol plant generates 12,000–14,000 Nm3/h of methanol off-gas, and two such plants can produce 24,000–28,000 Nm3/h of methanol off-gas; as a result, 15,600–18,200 Nm3/h of pure hydrogen can be obtained per hour. Due to the new caprolactam production facility being built in the park, PSA will be responsible for supplying hydrogen, specifically for the purification of crude benzene, the hydrogenation of benzene to cyclohexane, and hydroxyl peroxide production; the required supply volume is 14,000 Nm3/h. After balancing, only 1600–4200 Nm3/h of hydrogen is available for ammonia synthesis; as a result, the ammonia synthesis plant will not be able to meet the production needs for caprolactam. A new plant is required to produce hydrogen and nitrogen using a mixture of coke oven gas and PSA desorbed gas. 1.2 Introduction to the original process: The methanol off-gas from the methanol plant is purified through a PSA process to produce high-purity hydrogen. This hydrogen is combined with pressurized nitrogen from the air separation unit, and together they are sent to the methanation unit where CO and CO2 are converted into CH4. After pressure augmentation, the resulting gas is fed into the synthesis process. See Fig. 1. Fig. 1 Schematic diagram of the original ammonia synthesis process. Additionally, the original plan for recovering the gas released from the PSA unit was to use it for combustion in the carbonization chamber of the coke oven; however, due to significant pressure fluctuations, this gas was not used for combustion and was instead discharged directly into a flare, resulting in a waste of resources. 2. Analysis of the modified unit 2.1 Analysis of the production capacity after modification The newly added gas generation unit is operating well at present; the amount of coke oven gas supplied is 17,500 Nm3/h, while the purified gas sent to the synthesis process amounts to approximately 26,000 Nm3/h, corresponding to about 9 tons of ammonia per hour. With this setup integrated with the existing process, the target of full production is achieved. 2.2 Process Introduction: Coke oven gas and PSA off-gas enter the gas holder, where they are pressurized by a combined coke oven gas/purified gas compressor before proceeding to the desulfurization process. Sulfur in the coke oven gas is removed through iron-molybdenum hydrogenation conversion and zinc oxide desulfurization agents. Process steam is added according to the water-carbon ratio requirements and fed into the heat-exchange conversion furnace; subsequently, oxygen-enriched air is added as per the process requirements and fed into the second-stage conversion furnace to carry out the conversion reaction. The high-temperature conversion gas is preheated using heat recovery to warm the feed gas and process condensate, after which it undergoes low- to medium-temperature conversion reactions to bring the carbon monoxide level within specified limits. It then proceeds to the MEDA decarburization process; following pressurization in the clean gas section of the coke oven gas/clean gas combined compressor, it is sent to the methanation process, where it merges with hydrogen from PSA. After being compressed by the methanation furnace and the syngas combined compressor, it enters the synthesis process. See Fig. 2. Fig. 2 Existing process of the ammonia synthesis plant 2.3 Problems encountered during the commissioning of the modified process 1. The carbon dioxide generated as a by-product of the plant (about 4000 Nm3/h) is currently vented, resulting in a waste of resources; it is now planned to install pressurization equipment to use this carbon dioxide as a carbon source for methanol production, with the remaining carbon dioxide intended to be used as dry ice. 2. Based on the current control of operational parameters and heat balance in driving the process, the use of oxygen-enriched gas in this facility still needs to be verified; at present, pure oxygen is used only when the PSA system produces insufficient hydrogen, which represents a significant deviation from the original design ； The liquid oxygen pump was not designed with variable frequency functionality; as a result, any excess liquid oxygen used is discharged on-site, resulting in significant waste. 3. Due to the long period without operation after cleaning, the decarburization system contained solid impurities in its solution even after multiple rinses prior to commissioning; therefore, it is necessary to clean the solution pump from time to time ； Some coolers in the decarbonization system use plate heat exchangers, which are prone to clogging and require frequent cleaning. 4. The new system features three heating furnaces sharing one flue, which is unsafe and inconvenient for operation at startup. 5. The combination of the process condensate stripping step and the flow process results in a large amount of reverse-flow work required during the initial startup and heating up of the unit. 6. The design is conservative; some of the equipment is smaller in size, and the amount of catalyst used is less. 7. The device layout is unreasonable, which hinders maintenance and catalyst replacement. 8. Using a single sampling cooler at some sampling locations results in significant errors in the analysis results. 9. A water seal should be installed for the normal-pressure desorption of MEDA; a solution pump should not be used. 10. The water seals at the inlet and outlet of the gas holder are located below ground level, which hinders the drainage of condensate; they should be placed above ground level and equipped with automatic drainage systems. 3. Advantages and disadvantages of producing synthetic ammonia using two gas sources 3.1 Advantages 1. The desorbed gas from the PSA process is recovered and stored in gas tanks, which solves the environmental issue of directly releasing this gas into the atmosphere. At the same time, CH4, H2, CO, and N2, which are useful for synthetic ammonia production, are recovered, thereby increasing the output of the synthetic ammonia facility. 2. Two-stage conversion is used for the conversion of coke oven gas, which reduces the consumption of pure oxygen and improves the conversion rate of methane gas. 3. Combining the process condensate stripping unit with the hydrogen production unit makes full use of the heat generated by the equipment; it produces steam as a by-product while simultaneously stripping the process condensate, thereby reducing the discharge of ammonia-containing wastewater. 3.2 Disadvantages: 1. The pressure and composition of the desorbed gas fluctuate significantly; it cannot mix properly with coke oven gas in the gas holder, which affects subsequent stable production. Furthermore, when the amount of coke oven gas fed into the system is low (less than 6000 Nm3/h), fluctuations occur extensively in the system after the desorbed gas is added. 2. Due to the recovery of the desorbed gas, the nitrogen content in the gas holder increased, and the output of each coke oven gas compressor became insufficient, resulting in higher power consumption of the plant. 3. The control method for feeding the desorbed gas from the PSA process into the gas holder is unreasonable; a programmable valve should be used to control it according to the PSA timing, which helps to maintain a stable hydrogen-to-nitrogen ratio. 4. Conclusion: This ammonia synthesis plant is the first of its kind in China to use methanol off-gas for hydrogen production; the desorbed gas is combined with coke oven gas before being converted into hydrogen, thereby utilizing two different gas sources to produce ammonia. Based on the current operation status of the facilities, the newly added hydrogen production unit has been successfully integrated with the existing ammonia synthesis unit. With a stable supply of coke oven gas and methanol off-gas, it is possible to achieve a daily output of 300 tons of liquid ammonia, thus meeting the goals of full capacity operation and emission reduction. It also identifies the problems existing in the equipment, laying a foundation for future major repairs and technical upgrades.