Energy optimization of ammonia synthesis processes based on oxygen purity under different purification technologies

The energy consumption of an ammonia process plays an important role in the industrial energy consumption. For an ammonia synthesis process using pure oxygen gasification, the energy consumption of cryogenic air separation occupies a large proportion. The purity of the oxygen product from the air separation unit is mostly 99.6% in an ammonia process with pure oxygen gasification. The energy consumption of air separation will decrease significantly, if the purity of the oxygen product is reduced properly, which will lead to the decrease of the energy consumption of the total ammonia process, as the nitrogen element is needed for ammonia synthesis. However, the operating costs of the subsequent units will increase because of the increase of the syngas flow rate. So, there is an optimal oxygen purity that can minimize the energy consumption of the whole ammonia synthesis process. According to temperature, purification technology can be divided into cold and hot technologies. In this work, two different process models for ammonia production from coal with different purification technologies are developed to determine the process energy consumption with different inlet oxygen purity. In the process of ammonia production from coal with cold purification, the reduction of oxygen purity leads to the increase of energy consumption of oxygen compressor, methanol regeneration and refrigeration, and the decrease of nitrogen compressor power consumption. In the process with hot purification, the reduction of oxygen purity leads to the increase of energy consumption of oxygen compressor, preheating and MDEA regeneration, and the decrease of nitrogen compressor power consumption. The relationships between the eight kinds of energy consumption and oxygen purity are linear. Combined with the relationship between oxygen purity and the energy consumption of the air separation unit, the optimal oxygen purity which minimizes the process energy consumption is obtained. In the ammonia production process with cold purification, the optimal oxygen purity is 95%, which can save 10.3% of the energy consumption of the air separation compared with the current process using the 99.6% purity oxygen. While in the ammonia production process with hot purification, the optimal oxygen purity is 96%, which can save 8.4% of the energy consumption of the air separation.