The Process Study On A Revamped Rectisol With Increased Pressured CO₂ Product

Ammonia is one of the most important chemical products of economic industry in China.The unique energy structure of China determines that the synthesis process of ammonia uses coal as raw material mostly.Except for effective gases such as H₂ and CO,acid gas impurities like H₂S and CO₂ which will cause the poisoning of catalysts in the subsequent reaction also exist in the gas after coal gasification.So it is very important to purify the raw gas before the ammonia synthesis process.At present,MDEA technology and Rectisol process are the main purification crafts of the synthetic ammonia plants.Rectisol is used in this work.At low temperature,methanol has high solubility of acid gas.By using this property,the Rectisol process removes acid gas impurities such as H₂S and CO₂ from the raw gas.Based on the different solubility of different gases in methanol,Rectisol process desorbs CO₂ and H₂S from the methanol solution separately,and gets pure CO₂ product almost without sulphur,at the same time concentrates H₂S to sulphur recovery unit.At precent,the pressure of CO₂ product derived from low pressure desorption in Rectisol processe is often about 0.18 MPa.Then,this CO₂ product will be compressed（and sent tosubsequent process）,which procedure consumes large amount of compression work.In order to save the energy consumed in this process,a plant aims at performing a revamp on theoriginal Rectisol process,with purpose to enhance the pressure of CO₂ desorption from 0.18to 0.29 MPa when basically equal quantity of CO₂ gas was produced.Presently,the chemical simulation software Aspen Plus was used to simulate the original Rectisol condition.As a result the simulated values are in good agreement with the actualones,indicating the reliability of the selected PSRK physical property method and unit operation modules.After the analysis and calculation of the original Rectisol process,areform process is proposed by increasing the temperature of circulating methanol to reduce the solubility of CO₂ in the methanol and adding the twice flash evaporation,reducing the amount of N₂ to lessen the amount of CO₂ in tail gas as well as optimizing the heat transfer network and reduce energy consumption in heating of circulating methanol throughcirculating heat transfer between streams.After the revamp,the pressure of CO₂ product reaches 0.29 MPa,with almost equal quantity of before.In addition all key streams also satisfy the technological design requirements.The revamp of original process is small and easy to operate.At the same time,it reduces the usage of N₂ and saves the cost of operation.It is the first time that improve the pressure of CO₂ desorption with equal quantity of the original process in Rectisol,which may providetheoretical guidance for actual production.