Nitrogen

By

SOLUTION

 

 

INTRODUCTION:

 

In t1991, tthere twere tapproximately t65 tnitric tacid t(HNO3) tmanufacturing tplants tin tthe tU. tS. twith ta ttotal tcapacity tof t11tmillion ttons tof tHNO3 tper tyear. tThe tplants trange tin tsize tfrom t6,000 tto t700,000 ttons tper tyear. tAbout t70 tpercent tof tthe tnitric tacidtproduced tis tconsumed tas tan tintermediate tin tthe tmanufacture tof tammonium tnitrate t(NH4NO3), twhich tin tturn tis tused tintfertilizers. tThe tmajority tof tthe tnitric tacid tplants tare tlocated tin tagricultural tregions tsuch tas tthe tMidwest, tSouth tCentral, tand tGulftStates tbecause tof tthe thigh tdemand tfor tfertilizer tin tthese tareas. tAnother t5 tto t10 tpercent tof tthe tnitric tacid tproduced tis tused tfortorganic toxidation tin tadipic tacid tmanufacturing. tNitric tacid tis talso tused tin torganic toxidation tto tmanufacture tterephthalic tacidtand tother torganic tcompounds. tExplosive tmanufacturing tutilizes tnitric tacid tfor torganic tnitrations. tNitric tacid tnitrations tare tused tintproducing tnitrobenzene, tdinitrotoluenes, tand tother tchemical

intermediates.1 tOther tend tuses tof tnitric tacid tare tgold tand tsilver tseparation, tmilitary tmunitions, tsteel tand brass tpickling,tphotoengraving, tand tacidulation tof tphosphate trock.

 

DESIGN tBASIS

 

Nitric tacid tis tproduced tby t2 tmethods. tThe tfirst tmethod tutilizes toxidation, tcondensation, tand tabsorption tto tproduce tatweak tnitric tacid. tWeak tnitric tacid tcan thave tconcentrations tranging tfrom t30 tto t70 tpercent tnitric tacid. tThe tsecond tmethodtcombines tdehydrating, tbleaching, tcondensing, tand tabsorption tto tproduce ta thigh-strength tnitric tacid tfrom ta tweak tnitric tacid.tHigh-strength tnitric tacid tgenerally tcontains tmore tthan t90 tpercent tnitric tacid. tThe tfollowing ttext tprovides tmore tspecific tdetailstfor teach tof tthese tprocesses.

 

Weak tNitric tAcid tProduction1,3-4 t-

Nearly tall tthe tnitric tacid tproduced tin tthe tU. tS. tis tmanufactured tby tthe thigh-temperature tcatalytic toxidation tof tammoniatas tshown tschematically tin tFigure t8.8-1. tThis tprocess ttypically tconsists tof t3 tsteps: t(1) tammonia toxidation, t(2) tnitric toxidetoxidation, tand t(3) tabsorption. tEach tstep tcorresponds tto ta tdistinct tchemical treaction.

 

Ammonia tOxidation t-

First, ta t1:9 tammonia/air tmixture tis toxidized tat ta ttemperature tof t1380 tto t1470°F tas tit tpasses tthrough ta tcatalytic tconvertor,taccording tto tthe tfollowing treaction:

 

4NH3 t+ t5O2 t→ t4NO t+ t6H2O

(1)

 

 

The tmost tcommonly tused tcatalyst tis tmade tof t90 tpercent tplatinum tand t10 tpercent trhodium tgauze tconstructed tfrom tsquarestof tfine twire. tUnder tthese tconditions tthe toxidation tof tammonia tto tnitric toxide t(NO) tproceeds tin tan texothermic treactiontwith ta trange tof t93 tto t98 tpercent tyield. tOxidation ttemperatures tcan tvary tfrom t1380 tto t1650°F. tHigher tcatalyst ttemperaturestincrease treaction tselectivity ttoward tNO tproduction. tLower tcatalyst ttemperatures ttend tto tbe tmore tselective ttoward tlesstuseful tproducts: tnitrogen t(N2) tand tnitrous toxide t(N2O). tNitric toxide tis tconsidered tto tbe ta tcriteria tpollutant tand tnitrous toxidetis tknown tto tbe ta tglobal twarming tgas.

The tnitrogen tdioxide/dimer tmixture tthen tpasses tthrough ta twaste theat tboiler tand ta tplatinum tfilter.

 

 

 

 

 

 

AIR tTAIL

GAS

EMISSION tPOINT

(SCC t3-01-013-02) tEFFLUENT

STACK

 

 

 

 

 

 

 

COMPRESSOR tEXPANDER

 

 

 

NO txEMISSIONS tCONTROL

AMMONIA tVAPOR

AMMONIA tOXIDIZER

 

CATALYTIC tREDUCTION

UNITS

2 t1

 

 

 

AIR tPREHEATER

FUEL

 

 

 

NITRIC tOXIDE tGAS

 

 

 

ENTRAINED tMIST

SEPARATOR

 

 

 

WASTE tHEAT tBOILER

WATER

 

STEAM

 

 

 

 

AIR

 

 

 

 

PLATINUM tFILTER

NITROGEN tDIOXIDE

 

 

 

COOLING tWATER

 

 tABSORPTION

TOWER

 

 

 

 

SECONDARY tAIR

 

 

COOLER tCONDENSER

NO t2

 

PRODUCT t(50 t- t70%

HNO t3 t)

 

 

 

Figure t8.8-1. tFlow tdiagram tof ttypical tnitric tacid tplant tusing tsingle-pressure tprocess t(high-strengthtacid tunit tnot tshown).

(Source tClassification tCodes tin tparentheses.)

 

PROCEDURE:

The tnitric toxide tformed tduring tthe tammonia toxidation tmust tbe toxidized. tThe tprocess tstream tis tpassed tthrough tatcooler/condenser tand tcooled tto t100°F tor tless tat tpressures tup tto t116 tpounds tper tsquare tinch tabsolute t(psia). tThe tnitric toxidetreacts tnoncatalytically twith tresidual toxygen tto tform tnitrogen tdioxide t(NO2) tand tits tliquid tdimer, tnitrogen ttetroxide:

 

2NO t+ tO2 t→ t2NO2 t« tN2O4

This tslow, thomogeneous treaction tis thighly ttemperature- tand tpressure-dependent. tOperating tat tlow ttemperatures tand thightpressures tpromotes tmaximum tproduction tof tNO2 twithin ta tminimum treaction ttime.

Absorption t-

(2)

 

The tfinal tstep tintroduces tthe tnitrogen tdioxide/dimer tmixture tinto tan tabsorption tprocess tafter tbeing tcooled. tThe tmixturetis tpumped tinto tthe tbottom tof tthe tabsorption ttower, twhile tliquid tdinitrogen ttetroxide tis tadded tat ta thigher tpoint. tDeionizedtprocess twater tenters tthe ttop tof tthe tcolumn. tBoth tliquids tflow tcountercurrent tto tthe tnitrogen tdioxide/dimer tgas tmixture.tOxidation ttakes tplace tin tthe tfree tspace tbetween tthe ttrays, twhile tabsorption toccurs ton tthe ttrays. tThe tabsorption ttrays taretusually tsieve tor tbubble tcap ttrays. tThe texothermic treaction toccurs tas tfollows:

 

    3NO2 t+ tH2O t→ t2HNO3 t+ tNO

(3)

 

A tsecondary tair tstream tis tintroduced tinto tthe tcolumn tto tre-oxidize tthe tNO tthat tis tformed tin tReaction

tThis tsecondary tair talso tremoves tNO2 tfrom tthe tproduct tacid. tAn taqueous tsolution tof t55 tto t65 tpercent t(typically) tnitrictacid tis twithdrawn tfrom tthe tbottom tof tthe ttower. tThe tacid tconcentration tcan tvary tfrom t30 tto t70 tpercent tnitric tacid. tThetacid tconcentration tdepends tupon tthe ttemperature, tpressure, tnumber tof tabsorption tstages, tand tconcentration tof tnitrogentoxides tentering tthe tabsorber.

There tare t2 tbasic ttypes tof tsystems tused tto tproduce tweak tnitric tacid: t(1) tsingle-stage tpressure tprocess, tand t(2) tdual-stage tpressure tprocess. tIn tthe tpast, tnitric tacid tplants thave tbeen toperated tat ta tsingle tpressure, tranging tfrom tatmospheric tpressuretto t14.7 tto t203 tpsia. tHowever, tsince tReaction t1 tis tfavored tby tlow tpressures tand tReactions t2 tand t3 tare tfavored tby thighertpressures, tnewer tplants ttend tto toperate ta tdual- tstage tpressure tsystem, tincorporating ta tcompressor tbetween tthe tammoniatoxidizer tand tthe tcondenser. tThe toxidation treaction tis tcarried tout tat tpressures tfrom tslightly tnegative tto tabout

58 tpsia, tand tthe tabsorption treactions tare tcarried tout tat t116 tto t203 tpsia.

In tthe tdual-stage tpressure tsystem, tthe tnitric tacid tformed tin tthe tabsorber t(bottoms) tis tusually tsent tto tan texternal tbleachertwhere tair tis tused tto tremove t(bleach) tany tdissolved toxides tof tnitrogen. tThe tbleacher tgases tare tthen tcompressed tand tpassedtthrough tthe tabsorber. tThe tabsorber ttail tgas t(distillate) tis tsent tto tan tentrainment tseparator tfor tacid tmist tremoval. tNext, tthe ttailtgas tis treheated tin tthe tammonia toxidation theat texchanger tto tapproximately t392°F. tThe tfinal tstep texpands tthe tgas tin tthetpower-recovery tturbine. tThe tthermal tenergy tproduced tin tthis tturbine tcan tbe tused tto tdrive tthe tcompressor.

 

 

 

 

 

 

 

 

 

 

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