CA1070930A - Pollution control process for fertilizer plant - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
In a continuous process for the manufacture of fertilizer, waste heat from the process is passed in in-direct heat exchange relation with a stream of contaminated air and water entrained fertilizer solids. In this manner it is possible to remove substantially pure water vapor and air together with the fertilizer containing solution which can be used in the manufacture of the fertilizer.

Description

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B~CKGROUND OF THi:~ INVENTION
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In conventional processe~ for the production of ~
ni-trogen ~er:tili~ers, ammonium nitrate and urea.are initially ~ .
synthesized with relatively Iarge quantities of water. In the f1nal stages of drying and solidifying the fertilizer into-a;
substantially anhydrous product, water, which is contaminated with the fertiliæer and its components, is removed. Further, conventional so'idification methods result in large quantities of air being contaminated with entrained fertilizer. dust~ ;
Accordingly,.it is an ob~ect of the present invention to rem~,ve contaminants from the air discharged from the process and to eliminate a liquid water discharge from the process. . : ~;
- In an ammonium nitrate plant, ammonia and air ::
are reacted to produce nitro~en dioxidei and subsequently, .
the nitrogen dioxide and water are combined to produce nitric acid. The nitric acid is neutralized with ammonia - . .
to produce:an ammonium nitrate solution which is then con~
ducted to an evaporator for obtaining a concentrated ammonium ~ ;-nitrate mixture which is sent to solidification operation ....
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where small particles of ammonium nitrate are produced.
This operation consists of eith~r pulling or granulating the melt rom this~evapor~tor by cooling with air.
The present invention as applied to an ammonium nitrate process is pr~marily concerned with three pollu- .
tion streams,.namely, a first stream F from the neutrali~er, containing nitric oxide, nitric acid, ammonium nitrate,.
.and water, a second stream 5 from the evaporator contami~
nated by the same constituents, and a third stream T from 10- solidification stage CQntaining air and entrai.ned particlès of ammonium nitrate. Conventionally, the first and second streams are either passed to the atmosphere.or condensed ~ ::
and discharged as a liquid effluent. The third stream is ` usually exhausted to the atmosphere.
. Containments noted in the first and second.streams constitute.very serious pollutants to`waste water streams~`
Usually local pollution controi agencies place reverse limi- ~ ~
tations on t~e quantities discharged. Likewise, the en- ~ ;
trained particles and traces of nitrogen oxide and ammonia . 20 .in the third stream are serious pollutants whioh are usua-; .. lly restrlcted. .
In a urea process, a similar concept applies.
. ~nonium and carbon dioxide are reacted at hlgh tempera~
ture and pressu.re to produ e a urea water solutlon,which . :
: ~ contains large amounts of unco.nverted reactants. Most~of , , the reactants àre recovered from the urea solution and are . :~:
: recycle~ back to the~reactlon step. :The remainlng reactants are`separated from the urea:in the`evaporation step along with the water produced as a by-product of ~rea synthesis.
30. The evaporat.ed.water together with residual ammonla, carbon , . ~. 2 , .

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dioxide, and traces of entra.ined urea are condensed'and col-lected as raw process condens~e, wh.ich is then steam strip-ped as a process c~ndensa-te trea~nen't step to produc'e pro- .
cess condensate containincJ traces of a~nonia,..carbon dioxide, ' and urea. Stripped ammonia and ca.rbon dioxide are recycled to synthesi.s. The urea melt from evapora1tion is solidified in a similar mannèr to tha-t described above for ammonium.
nitrate in which either prills or'granuIe.s are produced by contact with Air.
10- . _ The present invention as applied to a urea pro- ~ :

cess is primari~y with two pollution streams, namely, a first stream Fl of process condensate rom the process condensate treatment.step containing traces of ammonia, carbon dioxide, and urea and a second stream S' o~ ai:r from '~ .:
the solidiication step containing entrained particles of ''' urea and a trace of ammonia. 'Conventionally., the first 'stream is discharged to a sewer, but-usually must be treated '~
further to reduce the urea content. This'r.eduction in ' ùrea is accomplished by hydrolyzing the urea back to am~
monia and carbon dioxide and steam stripping these out of .
the condensate to be recycled back to synthesis. Neverthe- ' :
less, aven after this treatment, the level of contaminant.s .
is still usually intolerable by local pollution:control;
' wa.ite water standards.
. The second stream, similar to the case of am- ' monium ni'trate, is conventionally discharged into the atmosphere. In the case.o granulation, the entrained urea content'is usually much greater than for milliny . . ~ -and often a single stage conventional type scrubber is .

:, o~g3~ ~ -used to remove most of the urea dust by contact with waeer.
In accordance wi-th the present invention, pol ! luting waste products generally produced in nitrogen ~erti-li2er processes can'be substantially removed from the air stream effluent so as to'render the air efiuent non-poll~ting~
Furthermore, the present invention.eliminates liquid water efluent streams from the process, which are generally con-' .
taminated, by evaporating by-product water.into the air :
stream~e.ffluent. Heat used in the evaporation of by-product water.is relatively low temperature wàste heat that ~ould '~
otherwise be transferred to a conventional cooling water system'. Thus, cooling water load for the process is re-duced thereby reducing the cooling water blowdown'efluent which is generally contaminated with treatment agents.
The present invention provides .or the use o materials ' that ordinarily would contaminate the environment and .
liquid effluent streams in conventional process are re-~ ' ~.
turned.to the process for use in the manufacture of fer- 1:
!
' tilizer.
. SUMMARY OF THE INVENTION
In accordance with illustrative embodiments demon- ~ ~ ' , strating obiects and features of the present invention, there is provided a continuous process or the manufacture of nitrogen fertilizer. Accordingly,'in the first'embodi-ment o the process:for producing ammonium nitrate, a first stream containing water vapor and contaminants pro- -duced during the manufacture of fertilizer is combined by. ~
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direct contact with a second stream containing air and 'entrained ertilizer solids, and is passed through a liquid-gas contacting medium where it is scrubbed with .

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~L~7~3V - -recirculated fertilizer solution. Most of -the entrained fertilizer soli,cls and contamin~nts,are removed from the air-vapor stream by dissolution into the recirculated fer-, .
tilizer solution. The air-vapor stream then passes through ' a second contactive medium where it is scrubbed with a ;
, rel,atively uncon,taminatèd stream of proce<;s condensate ob-tained from the evaporation.ste.p of the fertilizer process preceding the solidification s'tep. ~s the air-vapor stream passès throu~h the second medium, the remaining contamlnants are absorbed into the recirculated process condensate. Si-multaneously with the absorption of contaminants, water eva- j ;,~
porates from the recirculated process condensate into the , air-vapor stream res'ulting in cooling of.the condensate , .stream. The cooled condensate stream is recirculated to ' the evaporation condensers in the fertilizer evaporation step to be reheated by absorptlon'of the heat of condens,lng vapors 1 ' from evaporation.' Excess liquid from the stream of condensate re- ~
circulated over the second contacting medium is passed to .~ :
the stream of fertilizer solution reoirculated over the first cont'acting me'dium in order to replenish water losses' , ' due to recycling,so,lution back to the fertilizer'process ~ ,., and due' to evaporation o: water into the air-vapor stream. , The quantity of water evaporated,from both recircuiating ! :
, stream.into the clean air effluent.from the,second con~
. . , . !-.~, tacting medium is equivalent to the àmount of water gen- ~ , erated by'the fertilizer process so that no liquid waste stream is discharged. ~his water is discharged from,the ' ' ~ process as uncontaminated water.vapor in the un,co~taminatëd air eflu~n~ from. the s~con~ contacting medium.

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-`- 107093~-In the second embodiment oE the process for pro-duction of urea, a stream corresponding to the aforementione'd second stream, co~taining air and entrained fertilizer 1s passed through.both contacting mediums as described above wi'th recirculating condensa-te. Process condensate from ~ ;
evaporation con-taining ~ollutants is recovered separately .
. from the recirculated stream from'the second contacting medium, and is treated to remove ammonia before being com-' bined with that recirculated stream. In this way, the am-`10- monia concentration in the recirculated stream.is maintained sufficiently low so as-to prevent ammonia from contam~ina.ting - L
the effluent air discharge from the second contacting medium.
BRIEF DESCRIPTION OF THE DR~WINGS
' The above brie.f description as well as further objects, features, and advantages of 'the present invention 'will be more fully appreciate'd by referring to the follow-' ing description of.presently preferred but non'etheless il-' lustrative embodiments in accordance with the.pr.esent in- ¦~
vention when taken in'connection with the accompanying - . drawings, wherein: ; ' Fig. 1 is a schematic.representation of an ammon-ium nitrate process incorporating the present invention;
and Fig. 2 is a schematic representation of a urea .

process incorporating the present invention. ~ ' .;' :
BRIEF DESCRIPTION OF THE PREFERRED EMBOD~MENTS
I. A~nonium Nitrate Process ' ' Referring now'specifically to Fig. 1 of~the draw-ings, there is shown an ammonium nitrate process generally designated by the reference numeral 10, which comprises an.
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oxidation and absorption zone 12, a neutralization and ad-justment zone 14, evaporation zone 16, a prilling tower 18, and a pollution control system 20.
As is typical in the production of ammonium ni-trate, nitric acid is first made in zone 12 by introducing air and ammonia by line 22 and line 24, respectively, to produce nitrogen oxide, which is further oxidized into nitro~
gen dioxide and passed into the absorption section of zone 12 where it is absorbed in water introduced by line 26 to form nitric acid. A small amount of residual oxides including nitric oxide and nitrogen dioxide are passed from zone 12 through line 28 to a conventional power re-covery system, which is not shown in the drawings. The ~-oxidation absorption zone 12 and power recovery system comprise a conventional nitric acid plant.
A line 30 is connected from zone 12 to zone 14 for :
conveylng the nitric acid solution which is generally from ~ ~
55~ to 65% HN03. In zone 14 additional ammonia i5 intro- ~ :
duced and reacted with the nitric acid to form an ammonium 20 nitrate solution which can vary from 83% to 93% ammonium :~
nitrate according to the neutralization process used, which solution is conveyed to evaporation zone 16. ~:
In the formation of the ammonium nitrate solution ~ .......................................................... .
in zone 14g ~a.rious pollutants are also formed as a by~
product which includes a v~por mixture of nitric oxide, nitric acid, and an entrained solution of ammonium nitrat~
and w~ter. These pollutants form a first contaminant stream .~;~
which is designated by a directional arrow and reference F.
~ line 32 in flow communication between the neutralization zone 1~ and pollation system 2Q is provided for conveying the l -7-~ . -.

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first contaminant stream F to the pollution control system 20. The ammonium nitrate stream from line 34 is passed to the vacuum evaporator 16 in order to obtain a concentrated ammonium nitrate stream of from 94% to 99.5~ ammonium nitrate. The concentration will depend usually on the required density of the solid product. For example, in the case of high density prills, an alternative procedure is to carry out evaporation in two steps with final concentration to 99.5% taking place in air sweep unit 35 that is connected to evaporation zone 16 by line 36 and is passed to the prill tower 18 by means of a line 48. -Solidification of the concentrated ammonium ni-trate solution into a final product is accomplished con- ~ -ventionally by either prilling or granulation in zone 18. Both operations use air introduced through~line 51 as coolant. ~ ;~
Ammonium nitrate solution is dispersed into the moving air stream which absorbs the heat of fusion and reuseable ,~ .
heat. In the case of prilling, the solution is dispersed as droplets which are released at the top of the prill tower. These fall by gravity countercurrent to air intro-duced at the bottom and flowing up through the tower. In the case of granulation, the solution is dispersed on a bed of solid ammonium nitrate particles that move downward ~ through an inclined rotating drum. Air is introduced at ;~ one end of the drum and flows countercurrent to the bed to cool it.
In both operations, the heated air is exhausted and forms a second stream of contaminants consisting of air and entrained solid particles of ammonium nitrate, which has been designated by a directional arrow and the _,~

~07~30 letter S. The second contaminan-t stream is conveyed to the pollution control system 20 by means of line 52 which is connected between the solldification operation 1~ and the bottom portion of the pollution control system 20. Air sweep unit 35 (when used) is connectqd to line 52 by line 53. Evaporator 35 introduces additional water, air, and en-trained particles of ammonium nitrate to the second stream of contaminants. A third stream T of contaminated vapor flows out of the evaporation operation 16 through line 54 and consists es-;
sentially of evaporated water contaminated with ammonium nitrate and ammonia. The water represen-ts water ed to the nitric acid plant for absorption and water manufactured as a by-product in the nitric acid piant. Stream 54 connects with a vacuum condenser system which is shown as one steam ejector 24 stage with condenser 22 and an after condenser 26, but may consist of two or more ejector stages. The head of condensation of vapor flowing in line 54 is trans-ferred in the vacuum condensers 22 and 26 to a branched circulating stream 55' of dilute ammonium nitra~e and am- ;
monia from the top s~age of the pollution control scrubber 20. The heat transfer occurs in direct contact in baro-metric type ~ondensers 22 and 26 in which the condensable portion of vapors condense in lines 58 and 59, respectively, and combine in line 57 with the circulating liquid from line 55. The heat transfer may be performed indirectly by separating the condensing vapors ~rom the circulating cool-ant hy a heat transfer surface such as in conventional shell -and tube heat exchanger design. In the present case, the condensate is collected separately and may be either com-_g_ :-~
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1~7093~
bined with the coolant stream returning to the top of the scrubber 20, or'rnay be pumped ko ~he top o,the s'crubber 20. This latter ~riant may be advantageous when the`con- .
densate from line 54 is less contaminated -than the cool- ':
ant in line 55. ' ' ~ : .
' Inerts contained in vapor stream 54 are ejected from the after condenser 26 into the atmosphere throuyh line 74. If this stream still contains significant amounts of ' ' ' contaminants, it may alternatively be introduced into the bottom of scrubber 20 by extending line 57 to the scrubber ~' 20.as descri.bed above. ' . , , ', ' , In,the event that evaporation is carried out ` -'' solely by air sweep evaporator'35, the upper section of the ', scrubber 20 is elim~inated together with the.vacuum conden~
sing system 16. .Water vapor which.would otherwise flow in line 54 would then flow .in line,S together with sweep air ' into scrùbber 20. .
In the event that the vapor in line.54 from the , :
. vacuum evaporation step 16 contains a high concentration 20~ .' of'coritaminants, it may not be feasible to combine the~con-densate from this strearn with the coolant stream ln 55, or , recirculate this condensate separately at the top of the scribber 20 as previ,ously described. Combination of the con~ensate from 54 with coolant in'55 may in,crease,the le~
:.
: `,: vel of contaminants.in line 57 to such,that the sçrubbing ~`:
effect on the contaminated air is lnsufficient. q1he same would be true if th'e condensate were circul,ated separately ~;
, at the top of scrubber ~0. ..

' . . ' This problem may be overcome by two methods. 'First, .

by separating stream 58 and 59 from the barometric con:dense,.r, ::
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most contaminants should be contained in 58 which can be distributed over a third stage of scrubbing (not shown) ;~
in tower 20 located between the two stages shown, or dis-tributed over the bottom stage alternatively. Only stream 59 would be returned to the top stage, and stream 55 would be taken from the bottom solution of the top stage. In this way the relatively pure condensate from ejector motive stream and residual vapor off condenser 22 is circulated over the final stage of scrubbing.
The second method of eliminating the problems of contaminants in the vacuum condensate, is to purify the condensate before introducing it into the top stage of scrubber 20. This may be accomplished by conventional steam stripping of volatile contaminants and returning these to the neutralization operation 14.
In accordance with the present invention, the pollution control scrubber 20 comprises a tower divided into two sections by chimney plate 60. The plate 60 allows air .
and vapors to pass upward through chimney openings Çl which are covered with hats 66 so as to prevent liquid flowing down-ward from above from passing through openings 61. The plate 60 maintains liquid at sufficient levels for recirculation ~ ;
pump 63 and allows liquid to flow down into lower section through small openings 62 after overflowing wires 67. In ~`
this way the air and vapors pass upward without incurring the pressure drop of passing through the head of liquid required ~or pump 63. Other conventional designs, such as a bubble cap tray with tall caps, may be used for plate 60 instead.

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Both sections oE the tower are provided with a conventional low pressure drop with the scrubbing medium (64, 65~ properly supported and retained. Alternatively, a plate type liquid gas contactor design such as bubble cap trays or sieve trays might be utilized.
Each of the contaminated air streams 32, 53, and 52 enter the scrubber 20 below scrubbing medium 64 in the bottom section. The combined air streams pass upward first through scrubbing medium 64 where most of the contaminants are removed by a recirculating solution made up by over-flowing solution from the top section. This solution is recirculated by pump 69 which also recycles contaminants as they are removed from the entering air stream back to neutral-ization 14 by stream 71 where they may be converted into product.
After passing through the lower scrubbing medium, the air passes through the upper scrubbing medium 65. Most of the solid partlcles and a large portion of the contaminat-ing vapors are absorbed into the solution circulated in the bottom scrubbing medium 64. The remaining contaminants are removed in the top medium 65 by scrubbing with the rela-tiveIy clean solut;on made up from process condensate. This solution is circulated by pump 63 through line 55, 5S' con-, ~ densers 22 and 26 line 58, 5~, and 57, as discussed above, ~ , .
where thè heat of condensate is transferred to the solu-tion. Also~, the solution i~s replenished with relatively clean process condensate by direct contact as shown or in~

direct contact if desired as discussed above. In this way the heat of condensatïon of evaporated water is recovered by the recirculating so1ution to ~e transferred into the .

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scrubber 20. This.heat provides the heat of-vaporization required to ev~porate excess pro~ess condensate into the air stream passing upward~through medium 65.. Thus, all excess water generatecl or added to the process is discharged as a vapor into the atmosphere, and need not otherwise be discharged as a liquid. A benefit of add.ing process con-densate to the tower is tha.t the small remaininy quantities of contaminants in the air stream leaving the bottom scrub-bing medium 64 are removed with rela*ively uncontaminated process-condensate.
In order.to maintain the proper water balance so that the.scrubbing solutions do not become depleted of water or that an excess occurs.to necessitate liquid dis-, charqe, heat e~changer 92 is added tc the scrubber solution .. . .
return line 57. .In the event that heat added by condensiny ..
vapors in condensers 22 and 26 is not sufficient to evapor-ate excess condensate in scrubber 20, heat may be added - through e~changer 92 to supply this difference. In the .
event that too much water is evaporated from scrubber 20, .
20 heat can be withdrawn from the scrubber by applyiny cooling .
. to exchanyer 92.
EXAMPLE l An exam.ple of the.invention as applied to a typi-cal ammonium nitrate production facility follows. This plant uses-a neutralization process which produces 83 .per- ~
cent ammon~um nitrate solution, part of which is sold as ;i~ ~.

solution and the balance of w~ich is evaporated in two.steps :i and formed into prills for sale.
Plant specification~

~0 Nitric a~.id production . . . 500 tons/day as 100 . IINO3 (zone 12).
, . - 13 - ; ~

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-~` 107~930 l Nitric acid st~en~th . . . . 56% ~lNO3 (zone 12).
Ammonium nitrate productio-n 635 tons/day as 100 NH4NO3 (zone 14). ' i ~mmonium nitrâte solution. ~ ,83% N~14NO3 (zone 14).
83~ solution sales . . . . . 185 tonsi/day as 100~ ' NH4NO3 (zone 14).
.
~liyh density prills made . . 450,tons/day as 100~
NH~NO3 (prill tower 12).

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Pollutant ,streams fed to Pollution , , ' ' control system 20 in lbs/hr Air Water _ .
(32) Meutralizer overhead. . . . . . . . . . . ' 2'-',0~0 (53) Air swept evaporator. . . . . . 7,4501,450 .
- (52) Prill tower' vent. . . . . . . .' 723,000 11,980 ' , . . . I
(54) Vacuum e,vàporator cond. . . .,. . . . .,. 7,000 Wash water . .i. . . . .'. . . . . . 30 Effluent streams:
.
~56) From top of 29,to'atmospher,e (87%' , ~' relative humidity) . . . . . 730l45091,185 (7i) Rècycle l~iqui,d. . . . . . . . . . . . . . ~ 1,275 , ., Nitrate , ~ ''F
pollutant, , Form ' temp.

'16'.65 Vapor 250 9.00 Vapor 330 ; 61.50 Vapor 110 ' ' 1.00 Liquid 140 ,3~ .30'. . . .'. . . . . . . . . . . . ~ ' ' ~- no~e ' Vapor ' 113 ' ~, '88.45 Liquid 120 .. . , , . ~ .
In Fig. 2 there,is illustrated a further embodi~
ment of the invention in which corresponding parts have been '~

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designated by the same reference numerals as 'part of a "100" ~, 'series. Also, the corresponding contaminant stream hasljbeen designated by the same reference letter as a prime letter.
In this fo,rm,of the invention there'is, shown a urea f,er-tllizex process generally designat,ed by the refer` ' ,~
ence number 110 which comprises a urea synthesis plant 112, a first stage evaporator 116, second stage evaporator 122, , conde,nsate treatment section 124, solidification section ' 118, and pollution control scrubber 120. , ' ,~
10, As,is found in a typical urea proces,s,, the urea synthesis plant 112 consists of a reaction section and'an ammonia, carbon dioxide recovery section. A~monia and carbon dioxide are fed to an autoclave in the reaction sec- -' tion operating at high pressure where these react to form urea and water. The degree oF conversion of carbon dioxide to urea per pass through the autoclave depends in commercial~
practice, on the ratio of ammonia'to carbon dioxide main~
tained in the reactor. Urea processes generally operate ¦~
, economically at such conditlons that conversions in the 20 ' range of 55% to 75% are obtained leaviny a considerable , portion of unreacted ammonia and carbon dioxide in the ef- !
fluent from the reactor.,' ~lost of this unreacted feed is ¦
separated from the urea water solution in the recovery sec-tion and,is recycled back to'the autoclave for further syn- ' j'~
thesis into urea. The method of separation and recycling ¦ - ' depends on the particular process, but the ùs'ual result'is, a pro~uct from the urea synthesi,s plant 112 conslsting oF
65~ to 75~ 69 weight urea dissolved in by-product water and : : :
also co'n~aining a few weight percent ammonia and carbon dioxide. Th~ughou' this discusslon where ammonia and .

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carborl dioxide are referred to as being present in a liquid phase, it is` understood that the$e exist in the liquid as ammonium carbamate, carbonate and bicar~onate, the partlcu-lar species depencling on the.concelltration of water and ammonia. ~:' .. , In a typical urea fertilizer process, water is evaporated fro~ the solution produc-t o~ the urea synthesis ~' plant to produce a urea melt product containing less than 0.5 weight p~rcent water, which is a suitable ~eed for the 10solidification pro'cessing. This is usually accomplished by evaporating the wat,er in two va.cuum evaporation stages 116 and 122. Urea solution from the u,rea synthesis 112 flows to the first sta,ge evaporator by stream 134. A re-cycle stream 171 of urea solution,containing urea recovered ' from the pollution control s,crubber.comhines'with stream 134 -and enters' the'evaporator.116. The combined urea solution feed is evaporated.to produce a,feed solution for the seco,nd.
evaporator containing ordinarily 90 to 97 weight percent urea. This feed solution flows to the second evaporator by stream 136 where the remaining water is evaporated to produce urea melt containing typically 0.3 weight percent water which flows to the solidification.section 118 by stream 148.
'. , A typical'ejector condenser system ls.shown for . the two stage evaporator system discussed above. Water l;- .
I ,evaporatea from the first ev'aporator stage 116 and contam~
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inated with ammonia, carbon dioxide and entrained ~rea flows - 'by stream :l54 to indirect condenser 123. In a typical urea . . .

synthesis.plant a vapor stream at subatmospheric' pressure is o~tained through stream.l3'5 which is contaminated with - , ,'' , ' :,,, . - 16 - . ~:

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a~nonia, carbon dioxide, and entrained urea. The stream is usually more higllly contamin~ed than the vapor froml , the first evapora~-or in stream 154 and is condensed sep'ara-tely in condellser 125 by indirect contact with cooli`ng water.' Vapors and inerts from condenser 125 flow through stream ~
137 t'o combine with vapor from t~e first evaporator 116 ' - 1?
. flowing .in stream lS~. ,The combined vapors, oE streams 137 and 154 are parti.ally condensed in condenser 123 by indirect contact with'str.eam 155 which consists of a dilute urea 10. so~ution recirculated by pump 163.' Vapors and condensate.flow from condenser 123 to condenser 126 where most of the remaining vapor is condensed indirectly aga.inst cooling water.,,Condensate from condenser ;~' 126 flows out through stream .lS9 into the condensate treatment ; ~' sec.tion 124 where the condensates from all the condenser in .. ' ~ ' " ' ' ' '. ~ :
. . the vacuum condenser system are c.ollected. Inerts'and residual .
vapor are- ejected from condenser 126 to atmospheric pres~sure~ :;
through stream 161 to condenser 127 where' the remaini~g- -vapor is condensed and sent to condens.ate treatment I24 ' u - -throùgh stream.162.

Urea solution product from the first evaporator . 1','`' ~.
116 usually contalning from'90 to 97 weight p,ercent urea and ¦ ~ ~
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from 0.2 to 1 weight.percent ammonia and carbon dioxide ~

. .lows to the second stage evaporator 122 by stream 136. 1 .' '~' Evaporator 122 operates at signif;cantly lower pressure than evaporator 116 so as to minimize the formation of degrada~
.:
tion products of urea, primarily biuret, which are acceler~

' ated by increasinc3 temperature and decreasing water content. :~:

' The second stage 122 usually.requires three steam ejection stages with three stages of condensation shown as condensers ''' . - 17 - ' ' ' `' ' : ' 1 ' ' ' ' '. ' ' ' '' ' ~~ ' ~ . ':, '. . , .
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93~)-128, 129, and 127, which are of known design.' Con~aminated condensate from these condensers together with the,conden-Ij sates from conden6er 125 and 126 flow in stream 163 into the condensate treatment sec,tion l.24. ,.
Urea melt from the second evaporator 122 contain~
ing less -than 0.5 weight percent water fl.ows in strèam 148 ,to the.urea solidification section 118 where it is converted into eithe'r granules or prills,b,y metho'ds discussed for .' ammonium nitrate production. The large quantity of air 10' used to cool and solidify the urea and leaves the solidifi-ca,tion section 118 heated. and contaminated with entrained urea dust and a small amount of ammonia. The contaminated air,effluent S' flows in stream 152 into the bottom of, the , pollution control scrubber 120 to be purified.;, , .
Proc'ess' condensate fed to the conde~sate treatment section in stream 163 is treated, conventionally by steam ' stripping, to separate ammonia and carbon dioxide whi~h are 'recycled in stream 170 to the 'ure;a synthesis plant 112. The , . . .
treated condensate typically containing 20"00 ppm of urea and '20 50 ppm of ammonia is sent in stream 172 to th~'top .of the I
scrubber 120~
In,'accordance with the present inven-tion, the pol-lution.contr~l scrubber has the same internal.configura.tion as described for the ammonium nitrate case. Briefly,.it comp.rises a tower 120 divided into two sectlons by a chi.mney '¦ : ' . plate 160, each section containing a'conventional .liquid,gas ' ! .
,, contacting medium identified as medium:-164 for,the bottom section and~165 for the top section. . , , ~.
. ' ' ' Contaminated air S' from the solidification sec- ' j , tion 118 enters the scrubber 120 through stre.am 152'under~

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neath scrubbing medium 164. Most of the entrained urea, dust is removed from the entering air by contact with urea solution rccircula~ecl over medium i64 by pump 166. Urea , is absorbed into the rccirculated urea solution and recycled to the first staye evaporator in stream 171 by pump 166 to be'convertecl into ur-ea fertilizer product. Water l~s-t from the urea solution recirculated over scrubbing medium -164 by evaporation, in the contaminated~air stream, and by recycling urea solution stream 171 i,s replenished by the dilu-te urea solution overflowing from the top sta~e of scrubber,120 through openlngs 167.
Air leaving the bottom s;crubbing medium is still contaminated with a small amount o,f entrained urea solution '~
' and traces of ammonia Vapor.,' This contaminated air stream enters the top section and passes through the top scrubbing , medium 165 where it is scrubbed w'ith a dilute solution of urea recirculat,ed over the medium by pump 163. This ' , . ~
solution absorbs residual entrained urea and ammonia vapor ' ~, to produce an uncontaminated air'effluent in stream 156 , '' ~', ' . .
suitable'for discharge into the atmosphere. ' Thq effluent '~, air stream 1S6 also contains as vapor water generated b,y and added to the process so that no liqqid effluent need be discharged from the plant. The water vapor,results from evaporation of water 'from the urea solutions'recircu~
lated over both scrubbin-g medium 164'and 165. Water genera-ted by'and added to the process is supplied to the recircu~
lating scruibbing solutions from condensate treatment sec-tion 124 throuyh stream 172.
' ~leat required for evaporating excess water from the récirculated solutions is obtained by recirculating ' ;~ ' ' 19 .~ . .
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10'7~)~3~_ the upper section scrubbing medium 165 solution by pump 163 through condenser 123. The heat removed in- condensing~vapor entering condenser 123 in streams 154 and 137 is.transferred thro~ugh-a heàt exchanger surface to raise the temperatu,re of the recirculated stream 155.' This heat is removed from~
the recirculated stream 15'5 when it returns to the scru~ber .' 120 as stream I57 by the ev`aporative effect of the contam-inated air entering the scrubber i20.. By controlling the temperature of stream 157, the equilibrium partial pressure 10 ' of water vapor over the solution in stream 157 is maintained ' sufficiently higher .than the water vapor pressure in the . effluent air stream 156 such that the required amount of water is evaporated. The result of passing the solution in s.tream 157 through' the upper scrubbing medium 165 is that , it cools as it contacts the air, and water evaporates as ' ' ~ ~ , in conventional"cooliny tower operations.
- In order ;to maintain the proper water balance . ' : :
so that the scrubbing solutions do not become depleted of ~. ' water or that an excess does not occur to necessitate a.
liquid discharge, it iY necessary to control the te~peratùre of stream 157,. This may be accomplished kY transferring I
' condenser load to'or from;condenser 123 by chanqin.g opera~
ting conditions thereof. The temperature of stream 157' ' .
. I .
'may be decreased to cause less evaporation in scrubber 120 . by either increasiAg the cooling water flow through con,den-, ser 12G or by,lntroducing,ine~ts into stream 173 so as to . , ~:

.. . reduce the pressure on the v'apor side'of condens.er 123.

'The temperaturé of stream 157 may be increased to cause, . ~' more evaporation in scrubber 120 by either restricting ¦

3~ the vapor flow in stream 158 from cond'enser 123 or by reduc~

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~07~)~3~ _ , ~ , ing the cooling water flow through condenser 126 so as to increase the pressure on the vapor side of condenser 123.
In the event it is desired to obtai.n a more pùre air effluent str'eam 156 Erom the scrubber 120, th~ system may be modified to provide an additiona,l stage of scr.ubbing.
By installing another chimney plate over scrubbing medium ~' 165 and extendlng the sc,rubber 120 upward -to include an ad~
ditional.scrubbing medium over the added chimney plate, another stage.is added. Process condensate taken,directly .
10~, from con'densate treatment can be added to stream recircu- , . . ;' lating over the new upper stage instead of being mixed as , , : shown in stream 172 wi'th stream 157. Excess condensate in ' , the.new'upper recirculating'stream overflows the new chimney :-plate to replenish water losses from the lower sections. By ;, .
this modification, the second stage.serves to remove con- . ':: , . taminants from the air stream which would otherwise be ;~
absorbed into the so.l'ution circulating over the.upper stage, ,~
in addition to its,primary function of evaporatin~ ~ater.
'Thus., the circulating solution of the upper new third stage is maintained relatively ~lower than the solution circulating -, .over the i'ntermediate,second stage and the air leaving the .;
second stage may be puriLi,ed ~to a ~reater extent before :.. ~ .

being,'discharged into the atmosphere.
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r Plant specification~
, Granulated urea production . 1500 tons/day as 99.;7% urea Flowrating and temperatu~es of streams flowing to and ' . Lrom pollution control scrubber,120~
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Tempera ~ure Stream and ~Qrm ` Flowrate in lbs/hrs __ , _-- _ .
. . ~ir ~ Nll 3 CO 2 Water Urea .
152 187F vapor 895,200 50 50 7,502 18,750 17.2 14:LF liquid .5 5 82,874 186 156 140F vapor 895,200 25 48 67,253 17 .155 100F liquid . 60. 591,166,414 50,377 155 145F liquid .60591~166,414 50,377 ~ ~l 91F liquld ~ 30 7 3, l~318,919 .
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Claims (10)

WHAT IS CLAIMED IS:

1. In the manufacture of fertilizer, including syn-thesis, evaporation and solidification stages, the continuous process which comprises the steps of:
a. passing a first stream including water generated as a byproduct during said synthesis stage into a direct contact heat exchange zone;
b. passing a second stream including air and en-trained fertilizer solids into said direct contact heat exchange zone;
c. directly contacting said first stream with said second stream within said direct contact heat exchange zone, where-by said solids entrained in said air are removed from said second stream and a portion of said water is evaporated from said first stream;
d. recycling from said direct contact heat exchange zone to said synthesis stage a resultant stream including the remaining portion of said water and said solids; and e. removing substantially pure water vapor and air `
from said direct contact heat exchange zone.

2. A process according to claim 1 wherein said fer-tilizer comprises ammonium nitrate and wherein said first stream comprises a contaminant stream obtained during said evaporation stage, said contaminant stream including water vapor, ammonium nitrate and ammonia.

3. A process according to claim 1 wherein said fertilizer comprises ammonium nitrate, said synthesis stage in-cludes reaction and neutralization steps, and said first con-taminant stream comprises a contaminant stream obtained during said neutralization step, said contaminant stream including water and ammonium nitrate.

4. A process according to claim 1 wherein said fertilizer comprises urea and wherein said first stream com-prises a contaminant stream removed from said evaporation stage and including water, urea and ammonia.

5. The process of claim 2 further comprising the step of introducing a third aqueous stream into said direct contact heat exchange zone, said third stream including water and ammonium nitrate, said third stream directly contacting said second stream whereby a portion of said water included in said third stream is vaporized and a portion of said solids entrained in said air are removed from said second stream.

6. The process of claim 1 in which said first stream is at a higher temperature than said second stream, said first stream giving up a portion of its heat to said second stream whereby a portion of the water included in said first stream is condensed and the water carrying capacity of said second stream is increased.

7. The process of claim 5 in which said direct contact heat exchange zone comprises a scrubbing tower, said second and third streams being introduced to a lower portion of said tower, said first stream being introduced to an upper portion of said tower.

8. The process of claim 4 wherein said second stream comprises a contaminant stream obtained during said solidification stage.

9. The process of claim 5 wherein said second stream comprises a contaminant stream obtained during said solidification stage.

10. The process of claim 9 further comprising the step of recirculating a portion of said resultant stream to said evaporation stage, said recirculated portion being added to said first stream for introduction to said direct contact heat exchange zone with said first stream.