尿素生产(英文)

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1、UREAPRODUCTIONUrea(NH2CONH2)isofgreatimportancetotheagricultureindustryasanitrogen-richfertiliser.THEUREAMANUFACTURINGPROCESSUreaisproducedfromammoniaandcarbondioxideintwoequilibriumreactions:2NH3+CO2→NH2COONH4(ammoniumcarbamate)NH2COONH4→NH2CONH2+H2O(urea)Theureamanufacturingprocess,shownschemati

2、callyinFigure1,isdesignedtomaximizethesereactionswhileinhibitingbiuretformation:2NH2CONH2→NH2CONHCONH2(biuret)+NH3Figure1–SchematicrepresentationofureasynthesisThisreactionisundesirable,notonlybecauseitlowerstheyieldofurea,butbecausebiuretburnstheleavesofplants.Thismeansthatureawhichcontainshighle

3、velsofbiuretisunsuitableforuseasafertiliser.ThestructureofthesecompoundsisshowninFigure2.Step1-SynthesisAmixtureofcompressedCO2andammoniaat240bargisreactedtoformammoniumcarbamate.Thisisanexothermicreaction,andheatisrecoveredbyaboilerwhichproducessteam.3Thefirstreactoracheives78%conversionofthecarb

4、ondioxidetoureaandtheliquidisthenpurified.Thesecondreactorrecievesthegasfromthefirstreactorandrecyclesolution.Figure2-SomechemicalsofinterestinureaproductionStep2-PurificationThemajorimpuritiesinthemixtureatthisstagearewaterfromtheureaproductionreactionandunconsumedreactants(ammonia,carbondioxidea

5、ndammoniumcarbamate).Theunconsumedreactantsareremovedinthreestages.Firstly,thepressureisreducedfrom240to17bargandthesolutionisheated,whichcausestheammoniumcarbamatetodecomposetoammoniaandcarbondioxide:NH2COONH4→2NH3+CO2Atthesametime,someoftheammoniaandcarbondioxideflashoff.Thepressureisthenreduced

6、to2.0bargandfinallyto-0.35barg,withmoreammoniaandcarbondioxidebeinglostateachstage.Bythetimethemixtureisat-0.35bargasolutionofureadissolvedinwaterandfreeofotherimpuritiesremains.Ateachstagetheunconsumedreactantsareabsorbedintoawatersolutionwhichisrecycledtothesecondaryreactor.Theexcessammoniaispur

7、ifiedandusedasfeedstocktotheprimaryreactor.Step3-Concentration75%oftheureasolutionisheatedundervacuum,whichevaporatesoffsomeofthewater,increasingtheureaconcentrationfrom68%w/wto80%w/w.Atthisstagesomeureacrystalsa