Pressure selection for the ammonia synthesis process

The low-pressure ammonia synthesis systems currently in use in China all rely on foreign technology, such as those from Topsoe and Cassali; the pressure levels range from 15 MPa to 18 MPa.

Low-pressure ammonia synthesis refers to an ammonia synthesis system with a system pressure below 15 MPa. For general production capacities of 300,000 tons per year or more, low-pressure ammonia synthesis is recommended ； For production capacities below 300,000 t/a, it is recommended to use an ammonia synthesis system with a pressure of 20–24 MPa, and the relevant technology can be fully localized in China ； Compared with 20–24 MPa ammonia synthesis systems, the comprehensive energy consumption per ton of ammonia produced in low-pressure ammonia synthesis does have certain advantages, but they are not significant. Low-pressure ammonia synthesis systems are characterized by stable operation and longer operational cycles, with their main benefits arising from these longer periods. At present, in China, only Beijing Global Engineering Company is capable of designing low-pressure ammonia synthesis systems with a production capacity of 300,000 tons per year, and such systems operate satisfactorily ； Abroad, the main ones include Casali, Topso, Kellogg, etc., all of which offer relatively reliable technology.

I think the key to the level of synthesis pressure is the activity of the ammonia synthesis catalyst. Synthetic ammonia production capacities of 300,000 tons per year and above all operate at low pressure; this is mainly because the internal components of the ammonia synthesis tower allow for the use of catalysts with small particle sizes, which ensures their effectiveness and thus a high yield of ammonia. In this way, the volume of circulating gas will not be high, and thus the power consumption of the circulation pump will not increase significantly; overall, the hydrogen and nitrogen compressors together with the circulating gas compressor remain energy-efficient.

The pressure in the ammonia synthesis system not only affects the power consumption of the synthesis cycle machine; more importantly, it influences the overall power consumption of the system. To achieve a low pressure level, large reaction towers, large heat exchangers, a large amount of catalyst, and high-quality catalysts are necessary. The lower the pressure in the ammonia synthesis system, the lower the overall power consumption of the system. But it requires a large initial investment. In the old 32MP system, the power consumption of the entire system is lowest at a pressure of 24–26MP.

The pressure levels for ammonia synthesis are mainly 15 MPa, 22 MPa, and 30.4 MPa. For traditional processes, if the methane content in the fresh gas used in the synthesis step is high, it is more appropriate to choose a higher pressure level, namely 30.4 MPa, in order to keep the inert gas content in the gas fed into the reactor at around 25%； If the methane content in the fresh gas used in the synthesis process is moderate, it is reasonable to choose a pressure at a relatively moderate level, namely 22 MPa, in order to keep the inert gas content in the gas entering the tower at around 16% ； If the methane content in the fresh gas used in the synthesis process is essentially zero (for example, through processes such as liquid nitrogen washing), it is reasonable to choose a relatively lower pressure level, namely 15 MPa, in order to keep the inert gas content in the gas fed into the tower at around 2-3% ；

The pressure in the ammonia synthesis system not only affects the power consumption of the synthesis cycle machine; more importantly, it influences the overall power consumption of the system. To achieve a low pressure level, large reaction towers, large heat exchangers, a large amount of catalyst, and high-quality catalysts are necessary. The lower the pressure in the ammonia synthesis system, the lower the overall power consumption of the system. But it requires a large initial investment. The old system with 32MP has a pressure range of 24–26MP, and it consumes the least electricity across the entire system

Consideration must be given to circuit selection, catalyst type, and raw gas composition.

The 6th floor said: For a pressure level of 15 MPa, the inert gas content in the gas fed into the tower is controlled to be around 2-3%； What is the impact of a higher content of inert gases?

To make a choice based on the lowest combination of investment costs and operating expenses, it is necessary to consider factors such as the parameters of the selected process and the method of recovering vent gas; it is difficult to make a clear decision based on just one parameter, namely pressure. 1# lanhuagen

High-pressure ammonia synthesis: pressure of 100–1000 atm, temperature of 300–550 degrees. Medium-pressure synthesis: pressure of 150–270 atm, temperature of 300–500 degrees

How to reduce the content of inert gases? How to choose new catalysts? Waiting to address the issue of choosing low-pressure ammonia synthesis as the top priority