Discussion on energy issues in ammonia synthesis towers
Thread Content
For the dimethyl process currently used in small and medium-sized fertilizer plants, if the gasification and purification units ahead of these plants are upgraded, and purified gas is supplied directly to the ammonia synthesis unit after low-methane separation and liquid nitrogen washing, what change will occur in the processing capacity of the ammonia synthesis unit? If the same processing capacity is achieved, by how much can the synthesis pressure be reduced?
Both processes involve the removal of hydrogen sulfide and carbon dioxide from the gas, so the ammonia synthesis capacity remains unchanged; as for the synthesis pressure, it depends on the design pressure of the synthesis system.
Small and medium-sized fertilizer plants are products of the last century; they generally produce less than 180,000 tons of synthetic ammonia per year. Using low-temperature methanol washing or liquid nitrogen washing for purification is not suitable, as it would require high investments. At that time, domestic process packages did not yet exist, so one had to purchase imported process packages from companies like Linde and Lurgi, which were very expensive. Moreover, the towers, pipes, and valves had to be made of low-temperature steel, which was also extremely costly. Therefore, the dual-methane process mentioned by the original poster is not a purification process but rather a gas refining process: methanation and methylation (or alcoholization). This is a patented technology owned by Hunan Ammonium Alcohol, and it is suitable for process streams with low water vapor content but high CO levels at the outlet of the shift reactor.
After the purification system is modified, it is necessary for the manufacturer to carefully calculate the capacity of the synthesis tower. After all, the methane content changes significantly during the initial period; the methane content in the gas exiting the double-washing unit is on a percentage scale, while that in the gas exiting the double-cleaning unit is on a ppm scale. An increase in the amount of useful gas will naturally lead to a decrease in the pressure within the synthesis tower. However, the production capacity of the tower itself will also decrease, as the reaction becomes more intense and more heat is generated. Not to mention whether the heat load of cooling equipment such as the waste heat boiler and jacket coolers can be met; even the internal components of the synthesis tower need to be replaced, as the composition of the gas changes and the reaction temperatures at different stages must be adjusted accordingly. Additionally, it’s important to consider whether the circulation pumps and chillers can handle the new conditions. There are also some considerations to keep in mind: 1. Changes in the flow rate inside the heat exchange tubes of the waste heat boiler after the modification can affect its service life； 2. Such modifications are designed with cost savings in mind; no additional openings are created, and only the existing pipe openings are utilized. Therefore, the inlet airflow velocity of the f3 pipeline after the modification will have a certain impact on the system resistance ； 3. If other equipment and pipelines in the system are not modified, when the system’s reaction pressure decreases, the flow rates of these equipment and pipelines will increase, which has a certain impact on the system’s resistance.
A lot has been explained upstairs; the yield of ammonia synthesis is primarily determined by the composition of the effective gases, as well as by the system pressure and the efficiency of the components inside the synthesis tower. After treatment with liquid nitrogen, the methane content decreases, the composition of the effective gases in the synthesis tower increases, and the system pressure drops. System pressure is inversely proportional to the volume of gas, so this requires detailed calculations