The Simulation And Optimization Of Purification Process Of Poor Natural Gas

No.1Fertilizer Plant of China Blue Chemical Ltd. produces300,000tonsammonia and520,000tons granular urea annually with natural gas coming from Ya13-1gas field as the feedstock, by the technology of hydrocarbon steam reforming.The natural gas of Ya13-1gas field has a composition of83%methane,91%total hydrocarbon, in addition to minor CO₂and N₂, etc. Its heat value isapproximately8,650Kcal/Nm³, and it is called briefly as “Rich Gas”.As the Ya13-1gas field is close to its exhaustion, and the contract for supplying nature gas for theNo.1Fertilizer plant is close to its expiration, finding the alternative feedstock for thenatural gas resources is already very urgent. In long term, an intermediate layer naturegas field may be developed to supply nature gas to the plant. But the development ofthis gas field may take7to8years.The production of natural gas in current Ledong gas field and Dongfang gasfield have excess to meet the demand of natural gas of the fertilizer plant forfollowing several years as transition. But the gas from these two gas fieldscomposites20%CO₂,16%N₂, and60⁶4%methane, and with a heat value ofapproximate5,560Kcal/Nm³. This quality gas is called as “Poor Gas”.However, ifthe current feedstock of the fertilizer plant, a rich quality gas, is directly replaced bypoor gas, a major modification on the ammonia plant is necessary, involving a largecapital investment and use of time. This will have a negative impact on productionand increase the production cost. Therefore, the company wishes to avoid majormodification to the ammonia plant by some way else.Therefore, the company likes to explore the possibility to process the poor gas tochange its composition to the rich gas, by removing most parts of the CO₂and N₂ofit. In this paper the advanced aMDEA technology for CO₂removal is adopted, basedon the comparison of the different methods. By comparing the different methods forN₂removal, the cryogenic denitrification technology is put forward.Firstly, PRO/II, a chemical process simulation system, is applied to calculatingthe aMDEA process for removing CO₂from synthesis gas of existing industrial plantusing Amine thermodynamic model. The calculation results showed that: the results of simulation are basicly in good agreements with design datas. On that basis, thispaper optimized the process of removing CO₂from poor gas, and proposed operatingparameters. The parameters are as fellows: The temperature of poor gas, leansolution and semi-lean solution are32℃,50℃and75℃for each. The liquid-gasratio of lean solution and semi-lean solution are1:1.2¹.3and1:8.5⁹.5for each. Thenumber of absorber stages is20. The pressure of the absorber is2900kPa. The feedlocation of lean solution and semi-lean solution are3and10for each. The number ofthe stripper stages is17. And the pressure is172kPa.After decarbonization and dehydration of the synthesis gas, SRKthermodynamic model is used to calculate the process of removing excess N₂fromsynthesis gas by cryogenic separation technology of existing industrial plant. Theresults showed that the computing results were coincidental with design datas. On thebasis of the simulation, the operating parameters of cryogenic rectification columnwere optimized by SRK model. Furthermore, the different flowsheets ofdenitrification are carried out, optimal denitrification process is proposed. The resultsshowed that: the number of the distillation column stages is approximately20, refluxratio is3.3³.5. The distillation pressure is the decarbonization pressure or slightlylower than it. The twin columns for denitrification process are used to lower therunning costs.