Analysis And Optimization Of Low Temperature Liquid Nitrogen Washing Process For Industrial Ammonia Synthesis

The liquid nitrogen wash device has the advantages of low energy consumption,high product purity and no pollution,so it has been widely used in the current coal-based ammonia plant,and gradually replaced the methanation process.The liquid nitrogen wash realizes the purification task of removing CO,CO₂ and other impurity gases that can cause synthesis catalyst poisoning.Therefore,as the throat passage of the ammonia feedstock gas purification system,the low temperature liquid nitrogen washing device will directly affect the production cycle of the ammonia system.The article studied the influence principle of trace impurity gas on ammonia synthesis catalyst.The main work of this project focused on using Aspen Plus process simulation software to simulate the process flow of liquid nitrogen wash for the300,000-ton synthetic ammonia project of LUXI Group CO.,LTD.,and combining with the accidents that caused the fluctuation of liquid nitrogen wash production load or forced shutdown in the actual operation,and using liquid nitrogen washing theory to solve practical production problems.Firstly,we simulated the liquid nitrogen wash system of LUXI group company by the ASPEN PLUS software,and calculated the gas-liquid balance of each plate in the liquid nitrogen washing tower.According to the calculation results,in the fourth plate,the CO content in the gas phase had been reduced to PPM level,Ar and CH4 had also been reduced to trace,and the sum of H₂ and N₂ was close to 100%.According to the material balancing calculation,it was shown that the impurity gas in the raw material gas was basically completely removed and the loss of hydrogen was less after the nitrogen washing tower,indicating that the raw material gas can be highly purified in the nitrogen washing tower.Then the influences of process parameters and the changes of composition on ammonia synthesis stable operation were analyzed,including the complex thermal synthesis gas flow,thick with nitrogen flow,washing nitrogen flow,plusing liquid nitrogen flow and so on.Finally,we chose the 5A molecular sieve.Refering to the designed liquid nitrogen amount 35.690 kmol/h,we should increase the volume of the heat exchanger to reduce temperature difference in the case of meeting equipment manufacturing standards.Then we analyzed the problems at the present situation of liquid nitrogen washing operation,such as fuel gas pipeline resistance increasing,differential pressure rising of liquid nitrogen washing tower.By controlling the methane content in the feed gas composition and promoting the regeneration of molecular sieve,we successfully solved the problem of the fuel gas pipeline resistance increasing,and put forward to control the mid trace CO in 5 PPM or less in nitrogen washing tower.Then,we analyzed the causes of the problems encountered in the practical operation of liquid nitrogen washing,and proposed solutions.Finally,we studied the economic operation of liquid nitrogen wash.By the recovery utilization of fuel gas,the saving of medium pressure nitrogen,and the saving of liquid nitrogen used,we saved 33.2 million yuan,and reduced 5200 Nm³/h fuel gas emission,and saved about 400,000 yuan for each hydrolysis process.This paper mainly combined the practical production to solve the problems.In the future,it is necessary to study the application scope of low-temperature liquid nitrogen wash,and deepen the understanding of liquid nitrogen wash process to ensure the safety,stability,long period and smooth operation of liquid nitrogen wash.