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Brief process description of urea plant

Tata Chemicals owns fertilizer complex at Babrala. It has adoptedSnapgrogetti NH3 stripping process for the production of urea. It has two

identical units !! and "!# of the same nameplate capacit$ !!"% &T'( each)so the total capacit$ is ""%* &T'(.

Raw material required for urea production

The raw material condition at the batter$ limit is as under+

,mmonia+NH3 min - .%/ wt0il - !* ppm max1ater - *.%/ wt

Carbon di oxide+C0 " dr$ basis# - 2. / 4ol.minInerts 5 *.% / 4ol minH" - *.3/ 4ol minH" 0 - saturated

Utilities characteristics:

Cooling water+'ressure norm6degn# - 3.% 6 7.* 8g6cm " gInlet temperature - 3% *C0utlet temperature - 9% *C:ouling factor - *.***9 m " h * C68cal

;lectric power+,lternating current - !!

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(ew point 5 5"* *C max at 8g6cm "

'olished water+Conducti4it$ - *." s cm 5!

Silica reacti4e# - *.*" s cm 5!

:e - *.*! ppmT(S - *.! ppm0il - *.% ppmpH -

Nitrogen+Nitrogen plus noble gases - .7/ 4olC0 " - "* ppm max1ater - "* ppm max0x$gen - "** ppm max'ressure min6norm# - %6 8g6cm "

Temperature - ambient

>rea is commerciall$ manufactured b$ direct s$nthesis of gaseous carbondioxide and li?uid ammonia. The urea production in4ol4es following steps+

!. >rea s$nthesis and high pressure reco4er$". >rea purification and medium low pressure reco4er$3. >rea concentration

- 're 4acuum concentration- =acuum concentration

9. >rea 'rilling%. 1aste water treatment

1. Urea synthesis and high-pressure recovery

:or urea production raw materials li?uid ammonia and gaseous carbondioxide# are supplied b$ the ammonia plant.

The li?uid ammonia is pumped at high pressure through an e@ector into thereactor. The e@ector ser4es as dri4e less pump to rec$cle bac8 ammonia and

C0 " mixture at high pressure 8nown as carbamate into the s$nthesis loop.C0 " mixed with small but measured ?uantit$ of air is compressed in a four5stage compressor up to s$nthesis pressure. C0 " from the compressor outletis fed to the reactor. The NH 3 and C0 " react in the reactor to form anintermediate product) ammonium carbamate NH " C00NH " #. Thisintermediate product reh$drates to form urea and water. The ox$gen in theair forms a passi4e oxide la$er on the inner surface of the 4essel to pre4entcorrosion b$ carbamate and urea.

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The reaction products from the reactor o4erflow to the stripper where theuncon4erted carbamate is decomposed bac8 into the constituents with thehelp of the heat supplied b$ &' steam. The stripper is basicall$ a falling filmexchanger with urea solution on the tube side and &' steam on the shellside. The urea solution obtained at the bottom flows to &' section through ale4el control 4al4e.The 4apors from the stripper top enter the H' carbamate condenser alongwith the carbamate solution from &' section. Here the$ condense to formcarbamate solution again and in the process releasing a large amount of heatof condensation) which is utilized to generate the low5pressure steam on theshell side. =apors consisting mainl$ of inerts are sent from the carbamateseparator to the bottom of the &' decomposer to passi4ate the &' section.

2.Urea Purification

>rea purification ta8es place in two stages. 0ne is 8nown as &' section andthe other is 8nown as A' section.

edium Pressure section>rea solution from the bottom of the stripper enters the &' decomposer.(uring the expansion most of the remaining carbamate flashes forming theNH 3 and C0 " 4apors) thereb$ concentrating the urea solution. The 4aporsfrom the &' decomposer top flow to pre 4acuum concentrator shell side andfrom pre 4acuum concentrator to &' condenser shell side. 'rior to the entr$of the 4apors in the pre 4acuum concentrator the 4apors get mixed with thelow concentration carbonate solution from the A' section. The partl$condensed gas mixture from &' condenser outlet goes to &' absorber where

the C0 " is stripped from the NH 3 4apors and the pure NH 3 4apors from the &'absorber flow to the ammonia condenser. The solution at the bottom of the&' absorber pro4ides the suction for the H' carbamate pump. ,mmoniacondenser has cooling water on tube side.

:rom ammonia condenser both 4apor and li?uid flow to the ammonia recei4ertan8. The recei4er tan8 recei4es fresh ammonia from the ammonia plant. Theinert gases saturated lea4ing the recei4er enter the ammonia reco4er$ tower.Here ammonia is further condensed b$ direct contact with cold ammoniafrom the batter$ limit. The inert with residual ammonia from the tower aresent to &' inert wash tower 4ia a condenser. The ammoniacal solutioncollected at the bottom is rec$cled bac8 to the &' absorber.

!ow Pressure RecoveryThe urea solution from &' decomposer bottom enters the A' decomposerafter expansion through a le4el controller. Conse?uentl$ most of the residualcarbamate is decomposed and in the process urea solution getsconcentrated. The remaining carbamate is decomposed in falling filmexchanger) which is a part of A' decomposer.

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The 4apors from the A' decomposer enter the A' condenser where the$ getcooled and li?uefied. 'rior to entr$ of A' off gases in A' condenser the 4aporsget mixed with a?ueous solution from wastewater section. The 4apors thusformed get condensed in A' condenser goes to carbonate solution tan8 fromwhere it is sent bac8 to &' condenser. The inert gases in the tan8 containconsiderable amount of ammonia and thus are absorbed in cool condensatebefore being sent to the 4ent stac8.The urea solution at the bottom of A' decomposer is sent to pre 4acuumconcentrator through a le4el controller 4al4e.

".Urea concentration

Pre #acuum concentratorThe urea solution from A' decomposer enters the pre 4acuum concentrator)which is an exchanger with urea solution from the A' section on tube sideand &' decomposer off5gases on the shell side. The &' decomposer off5gases gi4e their heat and the urea solution gets concentrated in the process.The pre 4acuum concentrator operates under 4acuum and the 4apors fromthe top are condensed in the pre 4acuum condenser. The 4apors condensingin the pre 4acuum condenser are collected at the bottom in 4acuum s$stemtan8 and the li?uid is sent to the wastewater section.

The concentrated urea solution at the bottom is sent to the 4acuumconcentrated section.

#acuum concentrationThe urea solution from the pre 4acuum section bottom gets further

concentrated up to the concentration re?uired for prilling in two 4acuumconcentrators in series. The heat for concentrating the urea solution issupplied b$ low5pressure steam. The 4acuum is maintained in both thestages with the help of e@ectors and condensers. The urea solution from theoutlet of second 4acuum concentrator has . / w6w# concentration and ispumped to prilling section. The 4apors from both the stages get condensed inthe condensers and are sent to the wastewater section.

$.Urea Prilling

The urea melt from the second 4acuum concentrator is sent to the prillingbuc8et. The urea melt comes out from the buc8et in the form of li?uid dropsand the$ fall along the prilling tower. The drops get solidified and cooled b$the countercurrent flow of air from the bottom of prilling tower. The solidifiedurea melt drops 8nown as urea prills) fall on the prilling tower bottom. Theseprills are collected b$ a rotating scrapper and are sent to the bagging plantwith the help of belt con4e$ors. The heated air containing few ppm of NH 3 isreleased from the top into the atmosphere.

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%.&aste water treatment

The li?uid waste containing water) urea) NH 3 and C0 " from the process) thecondensate from 4acuum and pre 4acuum concentrator e@ector6 condensersare collected in a wastewater tan8. :rom here it is transferred to thedistillation tower where it is fed in the upper portion of the distillation tower.The stripping medium is low5pressure steam fed at the bottom of the tower.The NH 3 stripped wastewater is then pumped to the h$drol$ser where theurea in the wastewater is h$drol$zed to be con4erted bac8 into NH 3 and C0 " .The h$drol$zation ta8es place with the help of the superheated steam. Thewastewater from the h$drol$ser outlet contains onl$ few ppm of urea but stillhas NH 3 in it. So the wastewater from the h$drol$ser outlet is again fed tothe lower portion of the distillation column where the remaining NH 3 isstripped off and the wastewater at the distillation tower at the bottomcontains onl$ few ppm of NH 3 and urea. This treated water is sent to 0 >plant for reuse. The 4apors from the distillation tower and h$drol$ser getcondensed in the o4erhead condenser and are rec$cled bac8 to the A' sectionand a part of it is used as reflux for distillation column. The distillationcolumn is a tra$ed column.

'team and (ondensate system:The steam networ8 pro4ided in the urea plant are+!. Superheated steam

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". :irst 4acuum concentrator3. Second 4acuum concentrator9. (istillation tower bottom%. 're4acuum 6 4acuum s$stem e@ectors7. Steam tracing

. Steam @ac8ets2. &' start up lines

. &' inert gas wash tower 4ent line!*. Steam condensate collector tan8!!. Blow down 4ents) &;5!9) &;5!%) &;5"!!". 'S= flushing!3. &ech seal6 @ac8et flushing of '5*2) '5* ) '5"*!9. >tilities points

! ' *etwor+:This steam is produced b$ boosting AS pressure with the help of ane@ector with &S as moti4e fluid. The steam is utilized in the &'decomposer.

(ondensate collection:Turbine condensate is exported to 0 > for steam generation. &S HScondensate 8nown as &C HC respecti4el$ is utilized for AS generation inH' carbamate condenser shell.

AS condensate 8nown as AC from all the sources in the plant is collectedin a common tan8 as steam condensate tan8. :rom where it is used forflushing purpose and the excess condensate is exported to 0 >.

lushing *etwor+:Condensate in the steam condensate tan8 is utilized to pro4ide flushingwater re?uirement in all the sections of the plant. There are three t$pes of networ8+

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