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In Kellogg’s plant capable of producing 1,000 tons per day of synthetic ammonia, the mixed gas of hydrogen, nitrogen, ammonia, etc. resulting from the reaction in the synthesis tower is mixed with fresh gas in the circulation section of the syngas compressor (103-J). After being cooled by water and ammonia, ammonia is separated before entering the synthesis tower, a process known as pre-tower ammonia separation. After undergoing upgrades to increase production and reduce energy consumption, the ammonia production capacity of many plants increased from 1000 t/d to around 1150 t/d. However, to further boost ammonia production, the synthesis system and the refrigeration system represent one of the bottlenecks limiting capacity and energy consumption. During the third capacity expansion upgrade of its facilities, Yuntianhua Co., Ltd. addressed the constraints posed by the synthesis system and the refrigeration system by making full use of existing equipment and pipelines to reduce investment. By changing the ammonia separation process from before the tower to after the tower, it successfully overcame the bottlenecks in these two systems, achieving the goal of increasing ammonia production from 1,150 t/d to 1,500 t/d. The expansion and upgrade of Yuntianhua’s ammonia synthesis plant with a capacity of 1500 t/d began in 2000; the target was to increase the ammonia production from 1150 t/d to 1500 t/d, representing an increase of 30%. Before the renovation, the ammonia synthesis plant faced many bottleneck issues that affected its operation at high load; among them, high pressure in the synthesis system and high load on the 105-J refrigeration system were the two problems that had to be addressed as part of the capacity expansion renovation. In the basic design for the modification, ammonia production increases by 30%; both the fresh gas fed into the synthesis system and the circulating gas at the 103-J outlet increase significantly. The load on the refrigeration system also rises considerably. Both the synthesis system and the refrigeration system require modifications, and if many of the high-pressure equipment in the synthesis system are replaced with ones with higher capacity and the refrigeration system is modified accordingly, the investment required will be enormous. In the input-output-based comparison of various options, the design contractor KBR proposed changing the ammonia separation process in the synthesis system from the pre-tower ammonia separation method to a post-tower method. At an ammonia production rate of 1500 t/day, the volume of fresh gas entering unit 103-J increased by 30%, while the volume of recycled gas exiting the high-pressure cylinder in unit 103-J increased only slightly compared to before the modification. Additionally, the ammonia cooler 125-C, which was used in the previous setup, was removed. In this way, the load on the refrigeration system remains roughly the same as before the modification; the cooling capacity required for ammonia separation at 1500 t/d can be achieved without any modifications, thus allowing most of the high-pressure equipment in the plant to remain unchanged as well as the refrigeration system itself, which saves a significant amount of investment. 2 Ammonia separation process before the synthesis tower and its characteristics 1) Process description. The fresh gas entering the synthesis system goes into the low-pressure cylinder 103di; after compression, the gas passes through three heat exchangers (136-C, 116-C, 129-C) in sequence, then enters the separation tank (105-F) where water is separated out. The gas subsequently enters the high-pressure cylinder, where it merges with the recycled gas from the synthesis loop at the last stage of the impeller, and after further compression, it enters the synthesis loop. The circulating gas coming out of the 103-J high-pressure cylinder enters 2 parallel water coolers (12-CA). 124－CB), and after coming out it splits into two paths; one path passes through two ammonia coolers (117－C. 118-C), while the other stream is cooled in the synthesis tower inlet/cycle gas heat exchanger (120-C). The two streams are then mixed and fed into the third-stage ammonia cooler (119-C), where they are cooled to -23°C; most of the gaseous ammonia liquefies. It is separated in the high-pressure liquid ammonia separation tank (106-F), after which the liquid ammonia is depressurized and sent to the refrigeration system. The gas, on the other hand, first exchanges heat with the cycle gas from the previous stream in 120-C, and then enters the synthesis tower inlet/outlet heat exchanger (121-C), where it is preheated to around 120°C before entering the synthesis tower. The mixed gas resulting from the ammonia synthesis reaction is cooled by the heat exchanger located at the top of the synthesis tower (122-C). It is then heated to boiler feedwater using two series-connected boiler feedwater heat exchangers (123-C and 123-C1). Subsequently, it exchanges heat with the gas entering the tower at 121-C; after being cooled, most of this gas returns to the high-pressure cylinder circulation section at 103-J, where it mixes with fresh gas, thus completing the cycle of the entire synthesis process. Another small portion is cooled by liquid nitrogen through the expanded gas ammonia cooler (125-C); after liquid ammonia is separated in the exhaust gas separation tank (108-F), the liquid nitrogen enters the ammonia storage tank (107-F), while the gas enters the Prissen system for the recovery of hydrogen and ammonia, or is sent to the first-stage furnace as fuel. 2) Features. ①The mixed gas resulting from the reaction in the synthesis tower has the ammonia produced as a byproduct, which is separated from it. The mixed gas containing H2, N2, NH3, and a small amount of other gases enters the 103-J circulation section after heat exchange; the volume of gas in this circulation process is large, resulting in high power consumption. ②The trace amount of moisture present in the 103-J outlet gas, along with the small quantity of 103-J seal oil carried by this gas, have their temperature reduced to –23°C after passing through three stages of ammonia cooling. Effective separation occurs within unit 106-F, thereby preventing oil and moisture from entering the synthesis tower and causing catalyst poisoning; this helps to protect the catalysts in the synthesis tower. 3 Ammonia separation process after the synthesis tower and its characteristics 1) Process description. To prevent water vapor poisoning of the catalyst in the synthesis tower and reduce energy consumption, the ammonia separation process after the tower incorporates a molecular sieve dryer system between the high-pressure and low-pressure cylinders at 103-J pressure; this system adsorbs water and CO2 from the syngas, reducing the moisture content in the gas to below 0.1 mg/L. In this way, the 103-J outlet recycle gas can directly enter the synthesis tower. This molecular sieve system includes 2 molecular sieve drying vessels, 2 filters, and a regeneration system, one for operation and one for regeneration. The syngas, from which water and CO2 have been removed by the molecular sieve dryer, enters the 103-J high-pressure cylinder. There it mixes with the recycle gas returning from the synthesis loop, and is compressed to the designed pressure. After that, it passes through a newly installed oil filter to remove any trace amounts of seal oil present in the gas. Finally, after being preheated in unit 121-C, it enters the modified Topsoe S-200 type synthesis tower. The synthetic gas resulting from the reaction, coming out of the synthesis tower, flows in series through the newly added boiler feedwater heat exchangers (123-CA) and (123-C1). After being cooled, it enters the shell side of 121-C, and then passes in parallel through two water coolers with increased heat exchange area (124-CA/CB), where it is cooled to 35°C. Upon exiting, the gas is divided into two streams: one stream passes through the shell side of 120-C, while the other stream flows in series through 117-C and 118-C; these two streams merge before reaching 119-C. The gas from 119-C exits to form the mixed cycle gas, which is cooled to –23°C before entering 106-F. Liquid ammonia is separated out and passes through the level control valve LC-13; after pressure reduction, it enters the refrigeration system. The gas exits from the upper part of 106-F, enters the tube side of 120-C where it is preheated, and then returns to the circulation section of 103-J to complete the entire cycle. After the modification, the original ammonia cooler for vent gas extraction (125-C) and the exhaust gas separator (108-F) were removed. The modified vent gas is drawn from the circulating gas in the 120‑C to 103‑J cycle section, and enters either the vent gas system or the Prison hydrogen recovery system. A bypass is provided before the circulating gas reaches 120‑C to regulate the temperature of the vent gas. 2) Characteristics. ①The mixed gas resulting from the reaction in the synthesis tower undergoes three stages of ammonia cooling, during which most of the ammonia is separated. As a result, the amount of gas that returns to the 103-J circulation section decreases, and the compression work required for 103-J is reduced as well. ②The extraction of the bleed gas is simpler; it only requires diverting a stream of gas between the 120-C and 103-J cycles, with the temperature of the bleed gas being adjusted using the additional 120-C bypass. Cancel 125-C and 108-F in the ammonia separation process before the tower. Comparison of the process characteristics between hydrogen separation before the tower and ammonia separation after the tower 1) Comparison between the ammonia separation after-the-tower process and the ammonia separation before-the-tower process. The circulation gas volume in the 103-J cycle section is reduced by more than 10%, resulting in less power consumption required for 103-J and thus energy savings. 2) The ammonia separation process after the tower incorporates a molecular sieve drying system between the 103-J high-pressure and low-pressure cylinders; this system removes trace amounts of moisture and carbon oxides from the gas, reducing the dew point of the syngas to below -40°C. This prevents the formation of methanol crystals in the circulation section of the 103-J high-pressure cylinder and in the three-stage ammonia cooler, thus protecting the equipment. 3) Starting up the ammonia separation process behind the tower is more convenient than that before the tower; before starting up unit 103-J, it is possible to establish normal liquid levels in the three-stage flash tanks of the refrigeration system (110-F, 111-F, 112-F). In contrast to the ammonia separation process before the tower, there is no need to wait until the synthesis tower reaches its operating temperature of 300°C to establish the liquid levels in 111-F and 112-F. Nor is it necessary to start the ammonia injection pump (120-J) to inject ammonia into the syngas during the warming-up of the synthesis tower. 4) In the ammonia separation process before the tower, the syngas resulting from the ammonia synthesis reaction undergoes three stages of ammonia cooling to reduce its temperature to -23 °C. Liquid ammonia is separated at 106-F before the syngas enters the synthesis tower. The sealing oil at 103-J, which is carried along with the circulating gas, is effectively separated at 106-F, preventing oil from entering the synthesis tower and thus providing good protection for the catalysts in that tower. In the ammonia separation process after the tower, the outlet recycled gas passes through an oil filter and, after heat exchange with 121-C, enters the synthesis tower directly. The 103-J sealing oil carried in the gas can more easily enter the synthesis tower compared to the ammonia separation process before the tower, leading to catalyst poisoning in the upper part of the synthesis tower; the situation worsens in case of oil leakage from 103-J or failure of the oil filter. 5 Modification details: The implementation of the post-tower ammonia separation process in the ammonia synthesis tower was carried out in conjunction with an increase in ammonia production by 1,500 t/d. The main modifications included: 1) Two molecular sieve drying systems and one regeneration system were added between the high-pressure and low-pressure cylinders No. 103. 2) An oil filter (169-F) was added at the outlet of the 103-J high-pressure cylinder. 3) A 103-J high-pressure cylinder anti-surge water cooler (134-C) was added. 4) Replace 124-CA/CB with a heat exchanger having a larger heat exchange area. 5) Cancel the original 125-C and 108-F. 6) Remove the original boiler feedwater heat exchanger (123-C) and replace it with the new 123-CA; install a temperature control and interlock bypass system between 123-CA and the original 123-C1 boiler water. 7) 103-J underwent modifications to its high and low pressure cylinder rotors as well as internal components to increase production capacity. 8) To meet the increased production needs, the synthesis tower was changed from the original four-layer axial radial tower of Cassali to a Topsoe S-200 type two-layer, two-exchanger radial tower. 9) Some pipes, valves, and control instruments were replaced due to increased production volume and modifications to the process route. 6 Operation status of the upgraded unit and existing issues: The production-enhancement upgrade began with a shutdown on November 1, 2002, and the unit resumed ammonia production on January 29, 2003 – a process that took over two months. Due to factors such as limited natural gas supply, the unit managed to achieve an ammonia production rate of 1,500 t/day for the first time on February 26. After further optimization of the system, the process load was increased again to 1,500 t of ammonia per day in March. Based on the operation of the synthesis system and the refrigeration system after the upgrade, the power consumption of unit 103-J increased compared to before the upgrade, while the load of unit 105-J remained similar to that before the upgrade. Both systems were able to meet and even exceed their designed capacity, and their operation and maintenance were simpler than before, indicating that the ammonia separation process using towers in the synthesis system was a complete success. ①During the startup phase, since the outlet of the 103-J cycle gas in the ammonia separation process behind the tower is not cooled by water or ammonia, the time required to warm up the synthesis tower is **reduced** compared to before the modification. ② In the early stages of operation of the synthesis system, the requirements for the refrigeration system are simpler; after the 105-J unit is brought online, it is possible to establish the liquid levels in the three flash tanks, which facilitates the operation and maintenance of the refrigeration system. ③The extraction and control of vent gas are more convenient than before the modification, preventing the liquid in the 108-F section from affecting the operation of the Prison washing system. However, there are also some issues: ① Controlling the temperature of the reactor bed is more difficult than before the modification; firstly, the leakage rate in the cold shock valve MIC-14 on the second layer is high, making it hard to control the temperature increase during operation ； Secondly, the Topso S-200 tower affects the inlet temperature of the first layer when adjusting the inlet temperature of the second layer; therefore, it is necessary to take into account the temperatures of both layers during such adjustments. ②In the event of faults with the 106-F level control valve LC-13 or errors in the level indication, which result in liquid being carried into the system, for the ammonia separation process prior to the modification, liquid ammonia entering the synthesis tower will cause a drop in the bed temperature of the tower, a reduction in reactions, and an increase in the pressure within the synthesis system. For the modified ammonia separation process after the tower, liquid ammonia will be introduced into the 103-J circulation section, causing damage to the equipment in that section. Therefore, more careful operation and maintenance of 106-F are required after the modification, in order to prevent liquid from entering 106-F and ensure the safe operation of 103-J.