7/28/2019 PNRAA218

1/12

A.,PRIMARYI. SUBJECT AgrtcultNMEO AF25-0000-0000CLASSI- i.ASECONDARYFICATION Soil fertility,fertilizers,and plant nutrition2. TITLE AND SUBTITLEUrea technology,a critical reviewehli)Slack,A.V.; glouin,G.M. -

NUMBER4. DOCUMENT DATE . NUMBER OF PAGES 6. AR C1969 12p.RCONTACT NUEr

7. REFERENCE ORGANIZATION NAME AND ADDRESS3. ABTRCT

8. SUPPLEMENTARY NOTES (Sponsorin7 OrganIzation, Publishers, Avlability)r(In Fertiliser Assn.of India,14th annual general meeting: Fertiliser:ProductioLn.,& Technology National Sem.,New Dehli)!: . ... :. -L"...,,..:

9. ABSTRACT *4'r " . .' '=. . . . . .=# ..

10. CONTROL NUMVER 11. PRICE OF DOCUMENTPN-RAA- 218

12. DESCRIPTORS r ,13. PROJECT NUMBERManufacturingUrea 14.CONTRACT NUMBERPASA RA (QA) 5-69 Res.

15. TYPE OF DOCUMENT

AID 590-1 14"74)

7/28/2019 PNRAA218

2/12

UREA TECHNOLOGY;A Crit(Ea1 ReviewA. V. Slack and G.M. BlouinTehnessee ValleyAuthorityMuscle Shoals, Alabanta

The history of the fertilizer industry has beenone of shifting from one product to another in acontinuing effort to increase the nutrient content,In phosphate the shift has been from ordinarysupcrphosphate to triple superphosphate toammonium phosphate, an d in the future perhapsto ammonium polyphosphate; in nitrogen thesequence has been ammoniunammonium nitrate to urea. sulfate toToday the planning ofmajor new fertilizer complexes almost alwaysinvolves ammonium phosphate and urea as themain products,In each of these shifts a more complex anddifficult technology has been encountered and amajor research and development effort has beennecessary to overcome the new problems. This hasbeen especially true of urea, so much so that onlysince 1950 has the technology been developed farenough to give urea a significant place in worldnitrogen supply. then past 20 years importantimprovements have been made in productiontechnology. However, the development is notcomplete and there is considerable question as tothe relative merits of the techniques proposed bythe various process developers. Th e purpose of thispaper is to review critically the various phases ofcurrent urea production methods,Th e main complicating factor in making ureathat carbonic acid (l-1CO) isdoes not form a stab!eammonium salt as do nitric acid, sulfuric acid, andphosphoric acid. As a result, the simpleneutralization of this acid with ammonia cannot beutilized to produce a satisfactory nitrogenfertilizer. However, anhydrous ammonia an dcarbon dioxide can be combined directly to make

ammonium carbamate (NH 2CO 2NH 4 ), which,although also unstable, can be dehydrated byheating under pressure to form urea (NH 2 CONH 2),a stable compound. Unfortunately, the overallreaction is completely reversible and even at thehigh pressures (up to 300 atm.) and temperatures(about 2003C.) used in urea synthesis, the averagemaximum degree of conversion is less than 70% ina singlecomplicationsass through the reactor. Hence tworise as compared with makingammonium sull'ite and ammonium nitrate: thereaction must be carried out under pressure tomaximize conversion and the unconvertedreactants must be recovered and recycled foreconomic reasons. Most of the problems inherentin urea synthesis stern from these two factors, plusthe very corrosive nature of the chemical system atthe high reaction temperature and the tendency ofthe urea to decompose in tie finishing steps.Various urea producers and engineering firmshave developed ways of minimizing or avoiding theproblems in urea manuflicture. The usual practicein reviewing urea technology is to discuss eachcompany process as unit. In the presentdiscussion, however, each principal design oroperating problem will be treated as a unit and thecontribution of each company given; the maincompanies involved are Chemical ConstructionCorporation (Chemico; U. S.), CPI-Allied(Chemical Processes of Ohio, inc.-Allied ChemicalCorporation; U. S.), Lonza AG (Switzerland),Mitsui Toatsu Chemicals, Inc. (Japan), MontecatiniEdison SpA (Italy), Norsk lydro-lilektriskKvaelstof A/S (Norway), SNAM Progetti SpA(Italy), Stamicarbon NV (The Netherlands), and D.M.Weatherly Company (U. S.).

7/28/2019 PNRAA218

3/12

Reactor DesignBecause of the combined high temperature andpressure (180 0-220C. and 125-300 atm.),

corrosion of the reactor has been amajor difficultythroughout urea history. The problem goes beyondthe usual drawbacks of corrosion, becausesusceptibility of metals to attack by the reactantslimits the reactor temperature and thereforegoverns the degree of conversion and the amountof recycle necessary. In the early work, use ofsilver and lead as reactor linings was not fullysatisfactory for several reasons, including cost andproduct contamination. Passivation of stainlesssteel by oxygen, an established method forinhibiting corrosion, was then successfully appliedto urea production; this was a major step forwardand is the method generally used today. Thetechnique is covered by a 1955 patent issued toStamicarbon (I), in which a range of 0.1 to 3%02isspecified.Even with air injection, however, use of stainlesssteel still limits the reactor temperature to about1950 C. In an effort to increase conversion, byincreasing temperature, CPI-Allied has worked withzirconium linings. This has allowed temperatures ashigh as 220 0C.an d a resulting conversion (carbondioxide basis) of 85 to 90'. Difficulties in weldingzirconium caused initial lining failures; the presentpreferred method, which has been successfullyapplied, is a loose liner with a pressuie-equalizingpurge of water between liner and shell. The cost ofzirconium ison the order of four to five times thatof stainless steel but a thinner lining is normallyused (2).Titanium alloy liners have also been used,particularly by Mitsui Toatsu (3). Higher operatingtemperature apparently is questionable (2);however, the amount of air required to protect themetal is considerably less than for stainless steel,Use of these new materials has been limited,mainly because stainless steel costs less. However,the use of air to passivate stainless steel oresentssome problems. It is important to use enough air,as too little leads to rapid an d serious coi'rosion ofthe liner. There is a considerable difference ofopinion among producers as to the ninimumcritical amount; a range of 500 to 5000 ppm(oxygen in carbon dioxide) is reported. Theminilnuii depends to a considerabk extent on theteonperature in the reactor and the NH3:CO 2ratio,

as mor- oxygen is required at higher temperatureanti Ic Ner ammonia content in tle gas. There isalso an upper limit, because of ammonia loss in theinert gas purge, adverse effect on conversion,danger of explosion, and nitrogen oxide formationin the product (4).Because of these drawbacks, there may be atrend in the industry to use of titanium. Recentdecreases -* price of the material have made itmore competitive.The interior mechanical features of the reactorhave also gone through asequence of developmentstages. In the early days cooling coils were oftenplaced in the reactor to remove excess heat. Later,when higher NH3 :C0 2ratios and solution recycleprocesses came into use, reactors were generallyfree of internal parts because the additional liquidflow removed the heat. Jn recent practice internalparts have been introduced again, this time for thepurpose of improving contact between reactants.In the Stamicarbon design, for example, thereactor isactually a gas bubble contactor designedas a cascade of bubble washers, with the reactordivided into several compartments by means ofhorizontal screen plates (5). This multiple-stageeffect from bottom to top of the reactor has beenall important f'actor in increasing conversion andreactor volumetric efficiency.Some designers, however, hold that internalparts in tile reactor give more trouble than they areworth, and that although there is some theoreticaladvantage, in practice itis better to have thereactor interior free of equipment. Others claim asmall but important increase in conversion. Thefirst version of the Dutch State Mines stripperprocess, in which a relatively large amount of gaspasses through the reactor, appears to be a specialcase, requiring the screen plates mentioned above.For more co'iventional processes there ismuch lessneed for such devices but they have been used insome plants..Developments have also taken place in thefecding of reactants to tile reactor. As in ammoniaplant practice, centrifugal compressors aregradually replacing the reciprocating type becauseof lower investment and operating cost. The mainuse has been for the first stages of compression, upto 20 atmospheres or so , followed by areciprocating compressor on the higher stages. It

3.

7/28/2019 PNRAA218

4/12

has been generally considered that full use of tile;centrifugal type would not be feasible except invery large plants, 1200 tons or more per day pertrain. This situation has been changed somewhatby introduction of the stripper processes, whichoperate at lower pressure and therefore are moreamenable to use of centrifugals. Moreover, NuovoPignone (Italy) has recently announced (6)development of a centrifugal compressor capableof operating at much lower gas throughput thanprevious types. A 900-ton-per-day, single-trainplant with all-centrifugal compression is being builtin Italy and a 750-ton-per-day unit is planned inSouth America. The 900-ton unit isa stripper typeoperating at about 150 atmospheres. However,Nuovo Pignone has designed a compressor capableof operating at 350 atmospheres.

Feed pumps, both for ammonia and forcarbamate solution (see later discussion), have beenmajor problems. ,.horizontal type with guidedplunger rods has given good service in some plants.Packing wear is especially troublesome and evenwith advanced practice the packing must bereplaced frequently. Preferred design featuresinclude relatively low piston speed, case-hardenedplungers, and dual packing with a water purgebetween. A centrifugal pump, now being tested, isshowing promise.Recycle of Unconverted Reactants

Since the effluent; from the urea reactor is asolution containing urea and unconvertedcarbamate, the solution must be heated todecompose and remove the carbamate. Thedecomposition products are evolved as a hot,gaseous mixture of ammonia, carbon dioxide, andwater vapor. Recycling of this corrosive gasprdbably can qualify as the most difficult problemthat has been encountered in the development ofurea synthesis. .Early attempts by 1.G. Farben to recycle the gasby compression failed because tl:,-eciprocatingcompressors of that day were not capable of suchservice. The practice then swung to once-throughoperation with the unconverted ammonia used,without separation from the carbon dioxide, inproduction of ammonium sulfate, ammoniumnitrate, or nitric acid. This is still a fairly majorpractice in the United States, especially for makingurea - ammonium nitrate fertilizer solution, land

could well be applicable to the modern fertilizer,urca - ammonium phosphate. The major practice,however, is total recycle; the growing demand forurea as such and the difficulty in integratingmanufacture of two products has greatly reducedthe applicability of once-through andpartial-recycle operation.

Gas Separation-- During the 1930-1950 periodseveral recycle processes were developed in variousparts of the world. One wa s separation of theammonia and carbon dioxide by scrubbing with aselective solvent, developed first byHolzverzuckerungs AG (Inventa; Ems, Switzerland)and adapted in the United States by Chemico andVulcan-Inventa (now CPI-Allied). Chemico nolonger offers the process but CPI-Allied has builtplants in recent years incorporating boih thezirconium reactor liner and an MtEA(monoethanolamine) scrubbing system to scrubcarbon dioxide from the gas and leave ammonia forrecycling. (In the original Inventa method, ureanitrate or ammonium nitrate solution was used toscrub out ammonia.)The process has the advantage that conversion isnot reduced by recycling water to the reactor andthat the problem of recycling corrosive solution tothe reactor is avoided. Offsetting this is difficultyin recovering heat and cost of MEA makeup. Thenumber of plants using the method is relativelysmall and none of the current giant plants (800 to1500 tons per day) are of this type.Carbanate Sohtion--Du Pont (U. S.), in the1930-1950 period, worked with a recycle methodthat involved merely cooling the NH3-CO 2-H2(gas mixture to condense it (with some addition ofwater) and pumping the -resulting solution backinto the reactor. Although there are some obviousdrawbacks, this system has stood the test of timeand is now the standard for new plants. Of the ninelicensors mentioned earlier, eight offer someversion of the solution recycle process.Some years ago there was considerable variationin the design features of the various solutionrecycle processes; these differences have graduallydisappeared, until today the "conventional"methods are much the same. All use similar reactorconditions (temperature about 1850 C. andpressure about 200 atm.), maintain an NH3 :CO2mole ratio of about 4:1 in tile synthesis loop, and

4

7/28/2019 PNRAA218

5/12

get about the same cunversion (65-67%). Allreduce the reactor effluent pressure to anintermediate level and then pass the solutionthrough two or three stages of decomposition (byheating) at successively lower pressure levels. Ineach stage the evolved gas mixture iscondensed (orabsorbed in weak solution condensed in a laterstage) and the resulting solutions are worked backthrough the system to the reactor. The excessammonia (from the ex:cess used in the initialreactor feed) passes through the absorbers, iscondensed, and is fed back to the reactor.Although these major steps are common to thevarious convcntion.:l methods, there isconsiderabledifference in the carbanate solution recyclesystems--in pressure and temperature levels,equipment arrangement, and process flow. Thisphase of the development is still in a state of flux;even for a given company, the flowsheet for acurrent plant will likely be somewhat differentfrom the immediately preceding one. The generaldesign objectives are to:I. Maximize heat recovery.2. Minimize amount of carbamate solutionrecycled (smaller pumps and less power), andamount of water returned to the reactor(better conversion).3.Minimize power requirement.4. Maximize ammonia recovery (loweroperating cost and less pollution),Another major objective, of course, is to minimizeinvestment, so that the problem becomes the usualone of finding the best balance between utilityconsumlption and maintenance on the one handand investment on the other.Several parameters are involved in design of thecarbamate solution recycle system, and they are sointerrelated and interdependent that it is difficultto analyze them separately. Changing oneparameter in the direction of improvement zhflostalways changes one or more of the others in anadverse direction, and the extent of the adverseeffect can only be. determined by'somewhatcomplicated calculations. Hence it is difficult toevaluate quantitatively the various schemes thathave been developed.One important consideration is the nunber ofdecomposition ltages. Reducing the number lowersplant investment hut increases the amount of waterreturned to the reactor, makes heat recovery lessattractive (decomposer pressures generally lower),

and results in higher ammonia loss in gaseousliquid effluents. The current trend is to three stagas the optimum number. The third stage generainvolves both decomposition of carbamate aevaporation of water, with vacuum appliedremove the ammonia down to avery low level asmeans of minimizing atmospheric pollution. Tflashed gas is passed through a water-coolecondenser, the condensate stripped of ammonand the stripped condensate preferably discardrather than returned to the reactor. Suchprocedure gives only traces of ammonia in tgaseous and aqueous effluents.The point of heat recoverY1 also varies. The masource of heat in the system is carbamaformati in in the reactor. In some past designrecovery or removal of heat directly from treactor was practiced-by water coils in the reactor a cooling jacket outside. Today, however, tammonia an d carbamate solution recycled takethe reaction heat and carry it ou t of the reactizone.In the decomposition section, heat mustaddcd in order to get an adequate ratecarbamate decomposition, thus adding to the heavailable for recovery. Much of this is releaswhen the evolved gases are recondensed and it isthis point that heat is usually recovered. Trecovery is limited to the first decomposition staghowever,, because the gases are at so low a pressuin the later stages that condensation temperatureuneconomically low for heat recovery.Thus the usual heat source for recovery is hcondensed liquor from the first-stage absorbPractice varies as to the stream used to absorb theat. In some cases, water is heated and steamproduced; in other published flowsheets a procestream is involved:

Company Heat-absorbing streamChemico Solution to second deconiposer (7)Montecatinia Solution to first decomposer(8)Mitsui Toatsua Solution to crystallizer (7)Weatherly Solution to evaporator (7)aNot the latest procesz oY"fvred.Gas release pro( can have an importaeffect on the alounl of water recycled to treactor. The simplest arrangement-m1erely releaspressure, flowing the solution into a vessel, heatiit, and allowing the evolved gases to escape

7/28/2019 PNRAA218

6/12

'relatively inefficient because it gives maximumevaporation of water. Tw o systems have evolved,both of which minimize water evaporation. Th efirst involves a sequence, in each stage, of (I)pressure reduction, (2) gas release, (3) heating, and(4) further gas release. Th e advantage is that gaswhich can be evolved by release of pressure alonecomes off at lower temperature than in the heateddecomposer. The partial-pressure relationshipsinvolved are such that this procedure gives lesievaporation of water.In the second system a rectifying column is usedas the decomposer; relatively cold incomingsolution flows downward in countercurrent flow tothe hotter gas evolved in a heated section (orreboiler) at the bottom. Thus, the composition ofgas leaving the column approaches equilibriumwith the incoming solution, which has a relativelylow partial pressure of water because of thereduced temperature.Both systems are used but there seems to be atrend to the rectifying column type. Heaters on themain flow line are preferred by some, however,because of the rapid heat transfer at the high flowrate.Both heat recovery and amount of waterrecycled are affected by the pressure level in thefirst decomposition stage. Higher pressure gives lesswater in the off-gas an d a higher heat level whenthe gases are condensed but makes it more difficultto evolve the gases. As pressure is increased, thetemperature level required to decomposecarbamate at an adequate rate becomes higher untilcorrosion and urea decomposition become thelimiting factors. Until recently, designers seemed tohave settled on 15 to 20 atmospheres as tileoptimum; however, Mitsui Toatsu has recently (7)a new process ("Process D") in which the firstdecomposer is operated 58t to 77 atmospheres.The evolved gases are scrubbed in a high-pressureabsorber with carbamate solution from later stagesand the ammonia gas escaping from this absorber iscondensed by contact with the incoming feedammonia rather than by cooling water, thusconserving heat is recoveredhich by producingsteam in a mixer into which the three feed streamsare introduced before the reactor. Montecatini alsohas a ne w process (7), in which the firstdecomposition takes places at about 80atmospheres. Thk evolved gases pass into ahigh-pressure condenser where about one-third oV

the feed carbon dioxide is also introduced. Thisreacts with excess annionia in the decomposeroff-gas, thus producing additional heat andavoiding th usual step of condensing the excessammonia. Th e steam raised in the condenser is at3-atmosphere pressure; part of it is used in thesolution evaporation section and the remainder iscompressed to 5-7 atmospheres and used in thesecond and third decomposition steps.The Mitsui method presumably has been testedin tile company's own plants. A 6 00-ton-per-dayplant incorporating the Montecatini process hasbeen operating since June 1968 and larger units areunder construction.Process developers have also looked for someadditional driving force, beyond the usual addition

of heat and reduction of pressure, to help volatilizethe ammonia and carbon dioxide. In an obviousanalogy to water evaporation technology, an inertstripping gas has been employed, making thedecomposer similar in principle to an air-sweptevaporator. Instead whichf air, might overloadthe reactor with inerts, ammonia or carbon dioxideis used. Trhese gases act as inerts when on e or theother is present in excess over the carbamate ratio;the addition of one reduces the partial pressure ofthe other over the solution.Such gas sweeping could be used in thedecomposers at conventional pressures to reducethe steam requirement, but designers havepreferred to use the method as a means ofgoing tomuch higher decomposition pressure, approachingand including reactor pressure level. Th e resultingprocesses have generally been called the"stripping" types as opposed to "conventional."Th e pressure level in the first decompositionstage varies considerably:-

Pressure in firstdecompositionProcess Strippinggas stage, atm.Norsk Hydro Carbon dioxide 70-100SNAM Ammonia 135Stamicarbon Carbon dioxide 135-150Weatherly Ammonia plusinert ga s 230Norsk Hydro (9) considers the intermediatepressure to be best because most of the carbamatecan:be removed at this level by the combination ofheating with steam generated in the process an d

6

7/28/2019 PNRAA218

7/12

stripping with tile feed. carbon dioxide. Tlestripped solution contains only about 8% of itsnitrogen content as carbamate, which is removed ina subsequent standard decomposer. The ga,,sesevolved in the first stage are boosted to re.ctorpressure (190-230 atm.) by centrifugalcompression, after which carbamate condensationtakes place at full pressure in a"condenser-prereactor-boiler." At this highpressure level, the pressure of the steam generatedis 5 to 8 atmospheres absolute; this is furtherincreased, by a turbocompressor, to give steam hotenough (10-15 atm. abs.) for use in thestripper-decomposer. That part of the steam notneeded in the stripper is used in the evaporator atthe 5- to 8-atmosphere level.

Th e Norsk 1lydro method has not yet beenoptimized and a full-scale demonstration has notbeen made; it is claimed, however, that capital costshould be 10 to 20% lower than for establishedprocesses an d that energy cost should be about $1less per ton of urea.In the SNAM process, the stripper operates atnear reactor pressure but the pressure is muchlower (about 147 atm.) than in conventionalprocesses; some sacrifice in conversion is made butthe lower pressure permits efficient stripping an dmakes use of centrifugal carbon dioxidecompression feasible. A carbamate pump of theusual type is not required for the high-pressure

loop. Th e small head required to move the solutionfrom condenser to reactor is supplied by a simpleejector actuated by the incoming stream of liquidammonia (only part of the ammonia is used in thestripper-decomposer) (10). Steam at 5.5 and 3.5atmospheres is recovered in the condenser.

Only one SNAM plant has been built (in Italy);however, five more are being erected, in Europe,Mexico, and Venezuela.In the Stamicarbon process, stripping is carriedout at full reactor pressure but again the pressure(135-150 atn.) is much lower than normal. Sincethe stripper-decomposer, condenser-boiler, an dreactor are all at the same pressure, no carbamatepump is needed for the high-pressure loop. In thefirst version of the process, a stackedcondenser-reactor-stripper rrangeinent was used togive the head required lor fow from condenser toreactor. The overall height, as much as 150 feet,was undesirable so a new arrangement has beendeveloped and is being incorporated into a large

'plant." In this the gas from the stripper isintroduced into the top of the condenser (a'ngwith part of the ammonia) rather than into -thereactor as in the earlier version. As a result, thecondenser does not have to be above the reactor.The method is the best proven of any of thestripping processes. Some 16 plants are inoperation in Europe an d Asia and the process ha sbeen licensed by about 20 engineering contractors.

The Weatherly method, the newest of thestripping 'type, and as yet unproven on a plantscale, .has several unique departures. The mainfeature is circulation of an ammonia-inert gasmixture through the stripper and condenser atfull reactor pressure, which is at the conventional level of about 230 atmospheres. Smallamounts of the inert gas are added to the feedgases. Heat recovery is accomplished byexchange of sensible heat in the stripper off-gasto evaporator section feed solution. There is noheat recovery in the carbamate condenser; thecondenser is cooled by introducing the feedammonia into the unit where it vaporizes. Partof the vaporiied ammonia is recondensed in awater-cooled con lenser and fed to the reactor.The remainder, containing the inert gas, issuperheated for use as stripping gas. The processcombines the reactor and stripper in a singleshell-and-tube unit; the feed reactants flow upthe shell side and then down through the tubes,Where stripping by the rising ammonia-inert gasmix takes place.A steam-driven centrifugal compressor isemployed, with the exhaust steam used forsolution heating. No steam 'is requiired for thehigh-pressure decomposition stage, of course,because the initial heat of carbamate formationprovides heat in the reactor-stripper for carbamatedecomposition on the tube side. Although the heatof carbamate condensation is not recovered, theprocess has a good energy balance because of thedirect use of reaction heat to decomposecarbamate.

For stripping processes in general, it is not yetclear which is the best stripping agent, carbondioxide or ammonia; there are several factorsinvolved.I. Carbon dioxide is theoretically the mosteffective for decomposing carbamate. Th emole fraction of carbamate in solution can beexpressed by. the equation (9):

7

7/28/2019 PNRAA218

8/12

x= Ky2 . zwhere Kisa constant cmole fraction of carbamatein lthe solutionin the gas piase

z =mole fraction of carbondioxide in the gas phaseHence reducing the partial pressure ofammonia by increasing the partial pressure ofcarbon dioxide is more effective than thereverse because the ammonia partial pressureis squared iil the equation,2. Ammonia has the advantage of higherNI-I3 :CO2 ratio in the reactor; therefore,conversion is promoted and less trouble withcorrosion and biuret formation is to beexpected. (However, with du e attention totemperature and retention time, carbondioxide appears to be satisfactory in theserespects.)3. Since ammonia gives a decomposer effluentsolution relatively high in ammonia content,additional heat is required to strip it out;however, this heat can he recovered.4. Amnmonia normally is supplied as a liquid andt Iiere fore heat must be expended invaporizing it before it can be used asstripping gas. (However, ammonia flashedfrom the reactor effluent can beused.)Because of these complications. determining therelative merits of the two stripping gases is quitedifficult. Plant experience with the five new SNAMplants due to come on-stream in 1970-1971 shouldprovide useful information for comparison.High-pressure decomposition, with or withoutstripping, appears to be a significant step forwardin urea technology. A summary of published claimsfor the various methods, some stated as guaranteedan d others unspecified, is as follows:Utility requirement,per short ton of ureaElectri,:ity, Cooling water,Process Steam, lb._ kw.-hr. ___Conventional 2400-3200 132-170 16,000-29,000Stamicarbon 2200 100 12,000SNAM 2200 10 18,500

250b 1 0bWeatherly 16,00060 0 a 130 a280 0 b 15 b 23,000Mitsui Toatsu(Process D) 1700 155: 14,500

aF Icctric-driven compressor, ' -ammoniabitau-drivcn compressor,

On this basis, and assuming utility costs typical inth United States, a saving on the order of $0.50 to$0.80 per ton of utea is indicated for the newerprocesses. As noted earlier, Norsk Hydro estimates$1.00 saving for their new process.

As to investment, the developers of the newmethods claim some reduction; Norsk Hydro, forexample, gives 10 to 20,',. However, no clear trendseems to have been established in the biddingsituations thus far. Competition is keen anddifferences in investment, if any, between theconventional and newer types appear to have beenobscured by transient factors, peculiar to anybidding situation, that have no relation to intrinsiccost factors in the processes. SuCh factors alsoseem to have nullified, in many cases, the indicatedoperating cost saving of the new methods.The stripping processes so far have not beenapplied in single plant trains larger than 600 to 700tons per day. Design and construction problems forthe high-pressure, tube-and-shell decomposer andcondenser appear to have been limiting factors.However, progress in solving these problems seemsto be under way. Single-train units of 1000- and1400-ton-per-day capacity are under construction.

Among the new "high-pressure decomposition"processes, the relative merits of stripping andnonstripping are ifficte to evaluate. There is nosignificant ilerence between utility requirementsclaimed, and no investment advantage for eitherhas yet been evident. Elimination of thehigh-pressure carbanale pump seems to be themain advantage of the stripper methods.Carbaniate Shirri- -I1 a further effort toreduce the amount of water recycled to thereactor, p;ocess developers have tried recycling aslurry or carbamate in a noilaqueous carrier liquid.Th e most noted example is the oil slurry recycle(originally called the l echirtcy process) used inseveral plants built some years ago. Th e methodhas not been refined to (he extent that solutionrecycle ha s in the past decade, and no ne w plantshave been reported iil some time.A more recent development is carbamate-liquidammonia slurry, developed by Montecatini andtested on a pilot-plant scale (7). Th e method isquite like the conventional one. except that thefeed animonia enters tile system at the first-stagecon'dciiser. Formation of ' a carbamate-liquidslurry a' this point makes it unnecessaryto bring in condensate from later stages to dissolve

8

7/28/2019 PNRAA218

9/12

the' carbamate, as is done in conventionalprocesses. Gases evolved from the seconddecomposition stage (the final one) enter anammonia recovery system that separates theammonia as a gas and retur;ns it to the first-stagedecomposer. Reduced power requirement andinvestment are claimed as advantages,Hot Gas Recycle- --Thc concept of recyclingunreacted amnmonia and carbon dioxide as gasesrather than in solution, the approach in the earlywork, has been revived recently by Chemico. Gasesfrom each stage of decomposition arc compressedin a centrifugal compressor to reactor pressure andcondensed to recover heat as high-pressure steam.The original concept of fill adiabatic compressionhas been altered somewhat in later patents toinclude cooling between stages by using the ho tcompressed gas from a given stage to decomposecarbamate solution (by direct contact) in the nexthigher stage.The process has the advantages that nocarbamate pumps are needed, only one condenseris involved, and practically all the heat ofcarbamate decomposition is recovered. However,there are some obvious problems that requiretesting and tests can only be made on a large scale,Theprocess thus remains unproven.The approach to ho t gas recycle taken by NorskHydro has been discussed in an earliersection,Integration with Ammonia Plant- -There havebeen various proposals, dating back severaldecades, for combining production of ammoniaand urea. The main effort has been aimed at usingenergy from the ammonia plant to reduce pumpingand heating costs in the urea unit. Mitsui Toatsuhas taken the lead in this, haV _,perated a pilotplant for a number of years. The 1rocess involves() heat exchange ftom shift converter exit gas tocarbatnate solution in decomposer reboilers, (2)compression of converter gas (112-N2-('0 2 ) to fullammonia and urea reactor pressure (300 atm.), (3)absorption of carbon dioxide at reactor pressure byliquid ammonia from the ammonia synthesis loop,and (4) feeding the resulting carbamatesolution tothe urea reactor (along with recycled carbamatesolution from the condensers). The energy savingcomes from three sources: (I) elevated pressure ofcarbon dioxide at the shift converter exit (about24 atm.), (2) heat normally required in theammonia pla:v to remove carbon dioxide from the

absorbent, and (3) pressure of liquid ammoniafrom the synthesis loop. It should be noted thatcompressing the carbon dioxide from shiftconverter exit pressure to reactor pressure stillmust be charged to the urea plant (although havingonly one compressor in both plants reducesinvestment), and that heat recovery from theconverter gas cannot be claimed for the ureaprocess because it is normally recovered in theammonia plant anyway.Utility requirements claimed, on the basis ofassuming typical consumption in the ammoniaplant (based onl conventional design) andsubtracting this from the total for the integratedplants, are (per short ton of urea) 83kilowatt-hours, 1040 pounds steam, and 24,000gallons cooling water. This represents aconsiderable saving as compared even with thestripping processes. A 5 to 7 % reduction incombined investment is also claimed.Although the process has some desirablefeatures, tying the .two products so closely togetherreduces flexibility. Moreover, there may in somecases be difficulty with the carbon balance; if thereis too much carbon dioxide for combination withammonia as urea (as perhaps with naphthafeedstock), an auxiliary carbon dioxide separationunit must be included. If there is not enoughcarbon dioxide (as perhaps ii methane reforming),provision must be made to recover the excessammonia as such or to utilize the equivalent excesshydrogen as fuel.Product Finishing

Removal of unreacted carbamate leaves a ureasolution that must be concentrated beforeconverting to the final solid form. Removal of thewater, which amounts to about 25% of thes o Iu t io n a ft e r th e third-stagedecomposer-concentrator, has been a majorproblem in urea technology development. Witheitfree ammonia present, heating ti i solutionpromotes decomposition, both to the originalreactants (hydrolysis) and to biuret. Hydrolysisproducts can be recovered but the biuret goes oninto the product where it is undesirable for someuses. Since biuret formation is a function both oftemperature and retention time, the trend has beento evaporators that minimize the levels of thesevariables. Development effort has centered on the

9.

7/28/2019 PNRAA218

10/12

film type, both falling film (air swept)and risingfilm (vacuum with high recirculation rate). Thespinning disk, air-swept type is also effective bu tmay not scale up as wull as the others. The vacuumtype has the disadvantage that volatilized ammonia(residual ammonia from the decomposers orammonia formed by hydrolysis) is collected in theevaporator condensate and therefore may cause awater pollution problem (unless recovered), whichusually is more troublesome than the air pollutionresulting from use of an air-swept evaporator. Th elatter, however, requires a supply of dehumidifiedair reheated to about 140' C.With good evaporator design and operation,biuret i'ormation can be kept acceptably low.There i%, however, some biuret already in thesolution, formed mainly in the carbamatedecomposers. This can vary with equipment design;the reboiler type of decomposer for example, issaid (4) to give 0.5 to 0.6% bitret in the ureasolution (based on urea content) as compared with0.3 to 0.4% (typically) for other types. A goodevaporator installation will add only 0.3 to 0.4% tothis. A typical before-and-after analysis is (II): .

Before AfterUrea evaporation73 evaporationwt.%wt.%substute99Water 26.7 0.3Biuret 0.3 0.7ure 0. 10 .Ternperature 1200C. 1400C.Very little biuret formation takes place duringthe prilling operation,It should be noted that there is wide variation inreported values for biuret formation in thereactor-decomposer section of the plant. As low as0.2% is claimed, with only 0.3% more in theevaporator to give 0.5% in the prilled product.Others report as high as 0.5% in the solution to theevaporator.Getting the biuret content of prills under 1.0%has been a major accomplishment, but fortechnica!-grade urea even this has no t beenconsidered low enough. As a result, the crystalremelt method-in which the urea solution iscrystallized in a vacuum crystallizer and thecrystals are centrifuged, (tried, melted, andprilled-has been developed. The product containsonly 0.25 to 0.35% biuret because the biuret in

solution does not cocrystallize with the urea. it isremoved from tile crystallizer in a purge stream ofmother liquor that is generally fed back ta thesynthesis reactor where conditions are such thatconversion back to urea takes place. The streammay go directly to the reactor or indirectlythrough the absorber system; SNAM feeds it backto the stripper (at synthesis pressure).The crystal remelt method seems to be growingin popularity, even though investment is on theorder of 8 to 10% higher than for'evaporation-prilling.Since reducing biuret content is expensive, thereshould be some clear-cut gain to offset theadditional cost. The advantage or necessity of lowbiuret, however, is aot at all clear, and severalproducers feel that its importance is exaggerated.There are three major types of products toconsider.I. Fertilizer grade. Although 0.9% is a typicalspecification, and most producers try to staybelow 1.0 , there is good evidence that tip to2.5% is acceptable (with a few minorexceptions such as foliar spraying) (12).2. Feed grade. In the United States, largequantities of urea "microprills" are use as a!nr protein in animial feeds. Forthis use, high biuret is actually desirable;biuret has been shown to be superior to ureabecause it is less likely to be toxic to animals.3. Technical grade. Industrial users have

generally desired low-biuret urea for use inproducts such is plastics. There is someiluestioi.. however, as to whether this isnecessary. Impurities ;uch as oil and ironsalts are regarded by some to be much moredetrimental and since the levels of theseimpurities, as well as that of biuret, aregoverned by plant operating conditions,biuret may have incurred guilt by association.In current practice, the iron and oi l areremoved by filtering the urea solution beforeevaporation, in some cases with air added inthe decomposers to ensure oxidation of ironto the insoluble ferric form. With thispurification, urea containing as much as 0.8%biuret has been sold as technical grade in theUnited States. In contrast, some producersi n s t a I I e q it i p me in t b o t h forevaporation-prilling and crystalremelt-prilling, as well as handling facilities10

7/28/2019 PNRAA218

11/12

for crystals (0.1% biurct), in order 'to meet,be: 'any, customer demand tliat :mayencountered.Prilling the urea melt has become a fairly wellstandardized operation. Most producers prefer

multiple spray heads rather than spinning baskets,apparently blecause a wider range of particle sizes(microprills to agricultural prills) is possible withthe sprays. Th e main development probably hasbeen use of a fluidized bed in the base of thetower, both to cool the product in a convenientwa y and to prevent sticking of prills on the towerbottom. Alternatively, a rake-type prill removalmechanism in a flat-bottom tower is preferred tothe cone-bottom, gravity-flow type.Some progress has been made in developingfinishing equipment smaller in size than prillingtower installations and which gives a larger productgranule. TVA has tested pan granulation on apilot-plant scale (13), and Cominco has developedand is using a "sphcrodizer" type of operation(14).Th e finished prills or granules should containnot more than 0.3% moisture for good storageproperties. General specifications in the UnitedStates are: Comosiion %bCapacity,

soainmiaonia is evolved in storage, ventilation of,thebuilding is necessary.Economic Considerations,.

Developments in the past decade or so haveresulted in a major reduction in the cost ofIproducing urea. Utility requirements have beenreduced by recovering heat of carbamatecondensation and by using feed gases to help stripout carbamate. Maintenance requirements havebeen reduced by better control of corrosion and byimproved design of pumps and compressors.Finally, investment per ton of urea has beenreduced by improved design and by building largerproduction units. Today urea is gaining at theexpense of tile major nitrogen fertilizer,,ammonium nitrate, both because of its superiorproperties and because its productk,n cost has beenreduced to a comparable level even though theprocess is much more complicated.Plant investment varies widely, of course,depending on location and bidding situation. In theUnited States, average battery limit investment(Gulf Coast location) appears to be as follows (7):

Battery limitComposition, by wt. short tons/day investment, U. S.SN IbO Biuret NllJa Oil Fe Ash303,000Fert.grade a 46.3 0.3 0.9 0.015 _b 0.0002 _bTech.grade

a46.3 0.3 0.4 0.010 0.002 0.0001 0.002Uniidmioed.Theallnconiditioiied.

bNot specified.

Urea is often conditioned with clay, especially inhumid areas. About 2.5% of a kaolin clay isnormally used. Microprills for aninmal feeding aremore subject to caking because of their smallersize; clay or cereal powder is used as conditioner,someti,nes up to 10% by weight.Adequate amounts of clay-type conditionerslower the nitrogen content of urea from about46,3% to 45%. There are many materials claimedto be effective conditioners when added in suchsmall amounts (less than 0.5%) that tile resultinggrade is 46. nitrogen. Among these are amines,aldehydes, ketones, fatty acids, acid amides,polymeric surfactants. oils, and waxes.A dchumidified building is quite desirable whenthe product is stored in bulk in l humid areas. Since

300 3,900,000600 5,900,0001200 9,000,000

Th cost f'or 200-ton-per-day, solutionot o thle 75%plant being built by TVA is $2,300,000 (batterylimits).For application in other areas, a factor should beapplied. For example, plant cost in India and thePersian Gulf area is generally considered to beabout 30% higher than in the United States.Production cost for the smaller plants in theUnited States appears to average S12 to S13 perton, including depreciation bu t excluding ammoniacost. An increase in plant size from 300 to 600tons per day decreases production cost byabout$2 l.,r ton, and going from 600 to 1000 tols perday gives a further reduction of about SI.50 pertoi. The main advantage of the larger plants, ofcourse, is the lower investment per ton of urea an dthe resulting lower cash flow required to make theplant financially attractive. Single-train units aslarge as 150 0 ions per da y are planned an d one wasput into ope.ration recently. S 11.

7/28/2019 PNRAA218

12/12

For tile future, high-pressuremay be . favored because of therequirement, particularly in areascost is relatively high. So far,conventional type has held its

decompositionlower uti!itywhere energyhowever, theow n in biddingsituations in the United States. Whether this hasbeen due to extraneous rather than intrinsic cost

factors remains to be seen.The hot gas recycle and ammonia plantcombination processes are farther away fromrealization but offer potential savings both ininvestment and operating cost. Several yearsprobably will elapse, however, before they areproven commercially.

REFERENCES

L Van Wacs, J. P. Nt . U.S. Pat. 2,727,069 (Dec. 13, 1955). James, G. R. "Urea Reactor Lining Mterials." Proc 181hA nual Aecting of rertilizer Idustri, Round Table(Washington, 1). (.), 18-21. Office of Secretary-Treasurer,Baltimore, Maryland, 21212, 1968.3. Reed, R. M., and Reynolds, J. C. "Urea Plant, ImprovementsGalore." Chem.Ing. lhogr. 61 (l):62-65 (1965).4. Mavrovic, I. Urea Constullant, New York, New York; privatecommunication, September 1969.S. Kaasenbrood, P. J.C. "The Urea Stripping Process-TheTechnical Manufacture of Urea, With"Carbon Dioxide UsedBoth as Reactant and as Stripping Gaq." Paper preented at58th National Niceting of tie American Chemical Society,New York, New Yoik, September 7-12, 1969.6. Ferrara, P. L ,"Urca Synthesis Pressures Achieved byCentrifugal Compressors." Paper presented ateeting 158h Nationklf the American Chemical Society, New York, NewYork, September 7-12, 1969.7. Company brochures and private communications withroducers, contractors, and consultants, 1969.8, Fauser, G. (editor). Chent. Fertilizers, pp. 49-62. Pergamon

Press, New York, New York, 1969.9, Nilsen, P. If., Ylvisaker, L,and Terjesen, S.G."Developmentof a Hot Ga s Recycle Urea Process." Paper presented at 1581hNational Meeting of the American Chemical Society, NewYork, New York, September 7-12, 1969.10. Zardi, U., and Orto, F."Stripping Technique: A New Deal inUrea Production." 'ape'r presented at I.581h National Meetingof the American Ctemical Society, New York, New York,September 7-L2, 1969.11. Mavrovic, 1."Urca"in Kirk-OthmerEncyclopedia of ChemicalTcchnology, (3d edition). Interscience Publishers (Division ofJohn Wiley and Sons), New York, New York. in press.12. Mitsui. S. :jjfiientUse ffUrea .'eriilizer in Japan, pp. 34-38.University ofrTokyo, January 1965.13. "Pan Granulation of Urea." Nitrogen,, No. 44, pp. 31-32(nov./Dec. 1966).14. Pelitti, E., Reed, R. Nt., and Ifildred, G. C. RecentDevelopments in the Granulalion of Nitrogenous Fertilizers."Paper presented at 1581h National Meeting of the AmericanChemical Society, New York, New York, Sept.ember 7-12,1969.

Circular 7,-4,.'NATIONAL FERTILIZER' DEVELUPIM1NT' CENTER TEJ'JESSr-E A.E !\0f:.iprYMUSCLESHOALS, ALABAM