Process Design And Analysis Of Urea Synthesis Based On Natural Gas Chemical Looping Gasification

Natural gas based chemical industry has developed rapidly in recent years.The main products of natural gas based chemical industry are synthetic ammonia,methanol and H₂etc.Natural gas reforming unit is the core of natural gas chemical process,but this unit has many problems that urgently need improvement,such as large carbon emissions and high energy consumption.This paper takes the traditional natural gas to urea synthesis process as the object to study,seeks an alternative to traditional natural gas steam reforming process and innovatively design a new type of natural gas synthesis urea process.In this paper,chemical looping hydrogen production（CLH）is introduced into the process of urea synthesis based natural gas.The natural gas based chemical looping hydrogen production unit replaces the steam reforming unit and subsequent purification unit in the traditional natural gas synthesis urea process.Aspen Plus V11 process simulation software is used to model and simulate the traditional natural gas-to-urea process（NGTU）and natural gas based chemical looping hydrogen production for urea synthesis process(NG_CLHTU),including the modeling and simulation of natural gas reforming unit,CO shift unit,CO₂removal unit,methanation unit,chemical looping hydrogen production unit,synthetic ammonia unit and synthetic urea unit,as well as the flow simulation of the whole process.Based on the simulation,the key unit parameters of the process were optimized.When the reaction temperature of natural gas reforming is 900?C and the reaction pressure is 1 atm,the yield of syngas is the largest;when the CO₂/CO molar ratio of methanation unit is 1.05,the yield of methane is the highest and the conversion of hydrogen is the highest;in the chemical looping hydrogen production unit,the optimized molar ratio of Fe₂O₃/CH₄ is 1.4,the molar ratio of H₂O/Fe is 1.68,and the molar ratio of Air/Fe₃O₄ is 1.31.Under this parameter:CH₄ is completely converted,iron protoxide is completely reduced,and Fe₃O₄ is completely oxidized to Fe₂O₃.At the same time,the amount of oxygen carrier,water vapor and air in the whole hydrogen production process is the minimal,and the total circulation amount in the system is the lowest.Finally,the technical-economic-environmental performance analysis of the innovative natural gas synthesis urea system was carried out.The utilization rate of carbon and hydrogen in NG_CLHTU process is 56.65%,which is 8.57%higher than that of traditional NGTU process;for energy efficiency analysis,energy efficiency of NG_CLHTU process is 51.29%,and 6.81%higher than NGTU process;in terms of carbon emission,CO₂ emission in NG_CLHTU process is 0.72 kg/kg urea,Compared with NGTU process,CO₂ emission decreased by 22.2%;for investment analysis,NG_CLHTU process is 14.58%lower than NGTU process;in terms of cost analysis,NG_CLHTU process is 11.59%lower than NGTU process;in terms of return on investment（ROI）,NG_CLHTU process is 29.77%higher than NG_CLHTU process.The new technology has the advantages of energy saving and emission reduction.In the future,natural gas based chemical looping hydrogen production for urea synthesis will help China achieve the goal of carbon neutralization.