UREA

1. Introduction

Urea [57-13-6] was discovered in urine by Rouelle in 1773 and first synthesizedfrom ammonia and cyanic acid by Woehler in 1828. This was the first synthesis ofan organic compound from an inorganic compound, and it dealt a deathblow tothe vital-force theory. In 1870, urea was produced by heating ammonium carba-mate in a sealed tube.

Commercially, urea is produced by the direct dehydration of ammoniumcarbamate, NH2COONH4, at elevated temperature and pressure. Ammoniumcarbamate is obtained by direct reaction of ammonia and carbon dioxide. Thetwo reactions are usually carried out simultaneously in a high pressure reactor.Urea’s main use is as a fertilizer. Other applications are as a cattle-feed supple-ment, in the manufacture of resins, glues, solvents, and some medicinals. Urea isclassified as a nontoxic compound.

2. Properties

Urea can be considered the amide of, carbamic acid, NH2COOH, or the diamideof carbonic acid, CO(OH)2. At room temperature, urea is colorless, odorless, andtasteless. Properties are shown in Tables 1–4. Dissolved in water, it hydrolyzesvery slowly to ammonium carbamate (1) and eventually decomposes to ammoniaand carbon dioxide. This reaction is the basis for the use of urea as fertilizer.

At atmospheric pressure and at its melting point, urea decomposes to ammo-nia, biuret (1), cyanuric acid (2), ammelide (3), and triuret (4). Biuret is the mainand least desirable by-product present in commercial urea. An excessive amount(>wt%) of biuret in fertilizer-grade urea is detrimental to plant growth.

O

NH

O

NH2H2N

(1)

biuret

HNNH

NH

HN OO

(2)

cyanuric acid

HN N

HN OO

(3)

ammelide

NH2

1

Kirk-Othmer Encyclopedia of Chemical Technology. Copyright John Wiley & Sons, Inc. All rights reserved.

O

NH

O

NH

O

H2N NH2

(4)

triuret

Urea acts as a monobasic substance and forms salts with acids (4). With nitricacid, it forms urea nitrate, CO(NH2)2!HNO3, which decomposes explosivelywhen heated. Solid urea is stable at room temperature and atmospheric pres-sure. Heated under vacuum at its melting point, it sublimes without change.At 180–190"C under vacuum, urea sublimes and is converted to ammonium cya-nate, NH4OCN (5). When solid urea is rapidly heated in a stream of gaseousammonia at elevated temperature and at a pressure of several hundred kPa

Table 1. Properties of Urea

Property Value

melting point, "C 135index of refraction, n20

D 1.484, 1.602density, d20

4, g/cm3 1.3230crystalline form and habit tetragonal, needles or prismsfree energy of formation, at 25"C, J/mola #197.150heat of fusion, J/ga 251b

heat of solution in water, J/ga 243b

heat of crystallization, 70% aqueous urea solution, J/ga 460b

bulk density, g/cm3 0.74specific heat, J/(kg!K)a

at 0"C 1.43950 1.661100 1.887150c 2.109

aTo convert J to cal, divide by 4.184.bEndothermic.cExothermic.

Table 2. Properties of Saturated Aqueous Solutions of Urea

Temperature,"C

Solubilityinwater, g/100 g

solutionDensity,

g/cm3Viscosity

mPa!s(=cP)H2O vapor

pressure, kPaa

0 41.0 1.120 2.63 0.5320 51.6 1.147 1.96 1.7340 62.2 1.167 1.72 5.3360 72.2 1.184 1.72 12.0080 80.6 1.198 1.93 21.33

100 88.3 1.210 2.35 29.33120 95.5 1.221 2.93 18.00130 99.2 1.226 3.25 0.93

aTo convert kPa to mm Hg, multiply by 7.5.

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(several atm), it sublimes completely and decomposes partially to cyanic acid,HNCO, and ammonium cyanate. Solid urea dissolves in liquid ammonia andforms the unstable compound urea–ammonia, CO(NH2)2NH3, which decomposesabove 45"C (2). Urea–ammonia forms salts with alkali metals, eg, NH2CONHMor CO(NHM)2. The conversion of urea is biuret is promoted by low pressure, hightemperature, and prolonged heating. At 10–20 MPa (100–200 atm), biuret givesurea when heated with ammonia (6,7).

Urea reacts with silver nitrate, AgNO3, in the presence of sodium hydro-xide, NaOH, and forms a diargentic derivative (5) of a pale-yellow color. Sodiumhydroxide promotes the change of urea into the imidol form (6):

OAg

NHHNAg(5)

OH

NHH2N

(6)

which then reacts with silver nitrate. Oxidizing agents in the presence of sodiumhydroxide convert urea to nitrogen and carbon dioxide. The latter reacts with

Table 3. Properties of Saturated Solutions in Urea in Ammoniaa

Temperature, "CUrea in solution,

wt %Vapor pressureof solution, kPab

0 36 40520 49 70940 68 95260 79 109480 84 1348

100 90 1267120 96 507

aRef. 2.bTo convert kPa to atm, divide by 101.3.

Table 4. Properties of Saturated Solutions of Urea in Methanol and Ethanola

Methanol Ethanol

Temperature, "C Urea, wt % Density, g/cm3 Urea, wt % Density, g/cm3

20 22 0.869 5.4 0.80440 35 0.890 9.3 0.80460 63 0.930 15.0 0.805

aRef. 3.

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sodium hydroxide to form sodium carbonate (8):

NH2CONH2 þ 2 NaOH þ 3 NaOBr! N2 þ 3 NaBrþNa2CO3 þ 3 H2O

The reaction of urea with alcohols yields carbamic acid esters, commonly calledurethanes:

NH2CNH2 + ROH

O

NH2COR + NH3

O

Urea reacts with formaldehyde compounds such as monomethylolurea,NH2CONHCH2OH, dimethylolurea, HOCH2NHCONHCH2OH, and others,depending on the mol ratio of formaldehyde, to urea and upon the pH of thesolution. Hydrogen peroxide and urea give a white crystalline powder, urea per-oxide, CO(NH2)2!H2O2, known under the trade name of Hypersol, an oxidizingagent.

Urea and malonic acid give barbituric acid (7), a key compound in medicinalchemistry:

H2N NH2

O O

HOC COH

O+

–2 H2O HN NH

O

O O

N N

OHHO

OH

(7)

malonyl urea orbarbituric acid

3. Manufacture

Urea is produced from liquid NH3 and gaseous CO2 at high pressure and tem-perature; both reactants are obtained from an ammonia-synthesis plant. Thelatter is a by-product stream, vented from the CO2 removal section of the ammo-nia-synthesis plant. The two feed components are delivered to the high pressureurea reactor, usually at a mol ratio > 2.5:1. Depending on the feed mol ratio,more or less carbamate is converted to urea and water per pass through thereactor.

The formation of ammonium carbamate and the dehydration to urea takeplace simultaneously, for all practical purposes:

2 NH3 + CO2

O

NH2CONH4

ammoniumcarbamate

ð1Þ

O

NH2CONH4 NH2CNH2 + H2O

O

urea

ð2Þ

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Reaction 1 is highly exothermic. The heat of reaction at 25"C and 101.3 kPa(1 atm) is in the range of 159 kJ/mol (38 kcal/mol) of solid carbamate (9). Theexcess heat must be removed from the reaction. The rate and the equilibriumof reaction 1 depend greatly upon pressure and temperature, because largevolume changes take place. This reaction may only occur at a pressure that isbelow the pressure of ammonium carbamate at which dissociation begins or, con-versely, the operating pressure of the reactor must be maintained above thevapor pressure of ammonium carbamate. Reaction 1 is endothermic by ca31.4 kJ/mol (7.5 kcal/mol) of urea formed. It takes place mainly in the liquidphase; the rate in the solid phase is much slower with minor variations involume.

The dissociation pressure of pure carbamate has been investigated exten-sively (10–12) and the average values are shown in Table 5.

3.1. Ammonium Carbamate. Ammonium carbamate is a white crystal-line solid which is soluble in water (2). It forms at room temperature by passingammonia gas over dry ice. In an aqueous solution at room temperature, it isslowly converted to ammonium carbonate, (NH4)2CO3, by the addition of onemol of water. Above 60"C, the ammonium carbonate solution reverts to carba-mate solution, and at 100"C, only carbamate is present in the solution. Above150"C, ammonium carbamate loses a mol of water and forms urea. The specificheat of solid ammonium carbamate is given in Table 6. Ammonium carbamatemelts at ca 150"C, and has a heat of fusion of ca 16.74 kJ/mol (4.0 kcal/mol).The conversion of carbamate to urea begins at '100"C. To obtain an appreciableamount of urea at 100"C requires 20–30 h. The rate of conversion increases withincreasing temperature (13–15); at 185"C, ca 50% of the ammonium carbamate isconverted to urea in ca 30 min.

3.2. Conversion at Equilibrium. The maximum urea conversion atequilibrium attainable at 185"C is ca 53% at infinite heating time. The conver-sion at equilibrium can be increased either by raising the reactor temperature or

Table 5. Vapor Pressure of Pure Ammonium Carbamateat which Dissociation Begins

Temperature, "C kPaa

40 3160 10680 314

100 861120 2,130140 4,660160 9,930180 15,200; 19,300b

200 20,300; 36,500b

aTo convert kPa to atm, divide by 101.3.bThe value has been extrapolated because, at temperatures above170"C, the rate of reaction (10) rapidly increases and it is difficultto determine the carbamate vapor pressure owing to the formationof water and urea and the consequent lowering of the partial pres-sure of ammonium carbamate.

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by dehydrating ammonium carbamate in the presence of excess ammonia. Excessammonia shifts the reaction to the right side of the overall equation: Water, how-ever, has the opposite effect. Actual equilibrium constants at various tempera-tures are given in Table 7. A detailed study of the effect of pressure on ureaconversion is given in Ref. 17.

4. Processing

At this time over 95% of all new urea plants are licensed by Snamprogetti,Stamicarbon, or Toyo Engineering. SNAM utilizes thermal stripping whileSTAC (Stamicarbon) and Toyo use CO2 stripping. Only these three processesare, therefore, covered in detail. Process flow sheets are included for others atthe end of this section.

STAC, with their current new design (pool reactor), feel the only size limita-tion will be vessel size. Unless both the vessel fabricator and the intended plantsite are ‘‘on water’’, a 4-m-diameter is the maximum that can be transported.

The urea produced is normally either prilled or granulated. In some coun-tries there is a market for liquid urea–ammonium nitrate solutions (32% N). Inthis case, a partial-recycle stripping process is the best and cheapest system. Theunconverted NH3 coming from the stripped urea solution and the reactor off-gasis neutralized with nitric acid. The ammonium nitrate solution formed and theurea solution from the stripper bottom are mixed, resulting in a 32–35 wt% solu-tion. This system drastically reduces investment costs as evaporation, finishing(prill or granulation), and wastewater treatment are not required.

Table 6. Specific Heat of Solid Ammonium Carbamate

Temperature, "C J/(g!K)a

20 1.6760 1.92

100 2.18140 2.43180 2.59

aTo convert J to cal, divide by 4.184.

Table 7. Reaction Equilibrium Constanta

Temperature, "C Reaction equilibriumconstant, K

140 0.695150 0.850160 1.075170 1.375180 1.800190 2.380200 3.180

aRef. 16.

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4.1. Snamprogetti Thermal Stripping Process. The first industrialurea plant based on Snamprogetti’s NH3 stripping technology was put intooperation in 1971. Since then more than 100 plants based on this technologyhave been in operation or are currently being implemented. There has been asteady increase in demand for urea. Snamprogetti has designed urea plantswith increasingly higher capacities to meet the demand. In 1998, the Prfertilplant in Argentina was designed for a single-line design capacity of 3,250 t/d.The plant started up in 2001 and has reached a capacity of 3,600 t/d urea witha record production of 1.2( 106 t of urea/year. New single-line plants are beingdesigned that will reach 5,000 t/d.

The Snamprogetti process is outlined in Figure 1. Initially SNAM utilizedNH3 as the stripping agent. Owing to the high solubility of NH3 in the synthesisliquid, an overload of NH3 occurred in the downstream recirculation sections ofthe plant. At this time only heat is supplied to the stripper to remove unreactedNH3 and CO2. Because of the high NH3 ratio, it is still necessary to have tworecirculation sections.

The synthesis recycle loop has the stripped gas going to two high pressurecarbamate condensers in series and to a high pressure separator and then backto the reactor. The flow is maintained by using an NH3-driven liquid–liquidejector. The reactor is operated at 15 MPa (150 bar) with a NH3–CO2 molarfeed ratio of 3.5. The stripper is a falling-film type and since high temperatures(200–210"C) are required for efficient thermal stripping, stainless steel tubing isnot suitable. Titanium was initially used, but it also was not satisfactory becauseof erosion near the bottom. At this time a bimetallic tube of zirconium and 25-22-2 stainless steel is used. The zirconium is corrosion-free and the only problem isthe difficulty in getting proper welds and separation of the two layers at the

Fig. 1. Snamprogretti thermal stripping urea process. BL = battery limits.

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bottom ends. Fabrication mistakes have been the only source of problems withthis vessel to date.

The stripper off-gas going to the high pressure carbamate condensers alsocontains the carbamate recovered in the medium and low pressure recirculationsections. Both of these systems are similar to those shown in the total-recycleprocess.

The plant is designed with an excellent heat-exchange system to keep over-all steam required to a minimum.

The plant wastewater containing NH3 and urea is subjected to a deso-rption–hydrolysis operation to recover almost all the NH3 and urea. In someplants, this water can then be used for boiler feedwater.

The urea solution is evaporated in a two-stage system (99.8%) if the finalproduct is prills, and a single-stage system (+95%) if granules are to be provided.

Snamprogretti list the following as the strong points of the process: Highprocess efficiency; sustainable environmental pollution; high quality product;processes condensate recovery; high reliability (easy and safe operation, low cor-rosion); easy maintenance and R&D support (18)

4.2. Stamicarbon CO2 Stripping Process. In the early 1960s, Stami-carbon introduced the first stripping process. One of the main improvements wasthe reduction of steam required pre ton or urea to 0.8–0.95 from the 1.8 tonrequired in the conventional total-recycle process (Fig. 2). Steam usage and elec-tric power required can be varied depending on the final design.

The Stamicarbon process is described in Figures 3–7 The synthesis sectionof the plant consists of the reactor, stripper, high pressure carbamate condenser,and a high pressure reactor off-gas scrubber. In order to obtain a maximum ureayield per pass through the reactor, a pressure of 14 MPa (140 bar) and a 2.95/1

Fig. 2. Typical total-recycle urea process.

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NH3##CO2 molar ratio is maintained. The reactor effluent is distributed over thestripper tubes (falling-film type shell and tube exchanger) and contacted bythe CO2, countercurrently. This causes the partial NH3 pressure to decreaseand the carbamate to decompose.

The urea solution out of the stripper bottom flows to a single-stage lowpressure recirculation section (0.4 MPa, 4 bar). The stripper off-gas is sent tothe carbamate condenser.

Fig. 3. Stamicarbon CO2 stripping process. TCW = tempered cooling water.

Fig. 4. Pool condenser type synthesis section for Stamicarbon CO2 stripping process.

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Fig. 5. Pool condenser vessel for Stamicarbon CO2 stripping process.

Fig. 6. Pool reactor synthesis section for Stamicarbon CO2 stripping process.

Fig. 7. Pool reactor vessel for Stamicarbon CO2 stripping process.

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In this condenser, part of the stripper off-gases are condensed (the heatof condensation is used to generate low pressure steam). The carbamateformed and noncondensed NH3 and CO2 are put into the reactor bottom andconversion of the carbamate into urea takes place. The reactor is sized toallow enough residence time for the reaction to approach equilibrium. Theheat required for the urea reaction and for heating the solution is supplied byadditional condensation of NH3 and CO2. The reactor which is lined with316 L stainless steel, contains sieve trays to provide good contact betweenthe gas and liquid phases and to prevent back-mixing. The stripper tubes are25-22-2 stainless steel. Some strippers are still in service after almost 30years of operation.

The noncondensable gases introduced in the CO2 (ie, passivation air) andpart of the unreacted NH3 and CO2 goes to the high pressure scrubber, whichconsists of a shell-and-tube exchanger in the bottom portion and a packed bedin the upper part. In the lower part, most of the NH3 and CO2 is condensed,the heat of condensation is dissipated in tempered cooling water. In the upperpart, the gases leaving the bottom section are countercurrently contacted withcarbamate solution returned from recirculation. The scrubber off-gas, containingnitrogen, oxygen, and very small amounts of NH3 and CO2 are vented to theatmosphere after passing through an absorber.

The carbamate solution from the scrubber flows to a high pressure ejector.The NH3 feed pressure induces enough head to convey the carbamate solutionfrom the scrubber to the carbamate condenser.

As mentioned before, because of design and operating conditions (ie, NH3/CO2 ratio, pressure, temperature, reactor volume), only one recirculation stage isrequired (0.4 MPa) (4 bar). On expansion, a large portion of the carbamate left inthe urea solution from the stripper decomposes. The remaining solution passesthrough a rectifying column, heater, and separator. The gases formed go to a car-bamate condenser and are pumped via the high pressure carbamate pump(either reciprocating or centrifugal) to the high pressure scrubber.

The urea solution is then evaporated to 99.8% for prilling (2 stages) or plus95% for granulation (1 stage).

The Stamicarbon wastewater system consisting of two desorbers, hydroly-zer, hydrolyzer heater, reflux condenser, desorber heat exchanger, and a waste-water cooler is very efficient. Also, in many plants, as the water contains lessthan 1 ppm of NH3 and of urea, it can be used as cooling water make-up, orboiler-feed water.

In 1994 Stamicarbon introduced a pool condenser in the synthesis section(see Figs. 4 and 5). This allowed a 34% decrease in reactor volume and a 45%decrease in carbamate heat-exchange area, thus reducing costs considerablyfor equipment, structural steel, and construction. In view of the low ammoniaand carbon dioxide concentration in the stripped urea solution, a separate highpressure ammonia recycling stage is not necessary. The ammonia and carbondioxide still contained are recovered in the urea solution by the stripper andare recovered in a low pressure recirculation stage. This Stamicarbon 2000+technology is used by Uhde (19)

In late 1997, DSM (Stamicarbons parent company) started up a new plantutilizing, the next step, a pool reactor (see Figs. 6 and 7). The horizontal pool

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reactor further simpliefied the urea synthesis section by further reducing thenumber of high-pressure components from four to two. Salfurex steel enablesvessels and pipelines to be designed smaller, lighter, and extremely resistantto corrosion. Some parts may be eliminated completely.

In order to be 100% safe from a hydrogen explosion (sources: passivationair, CO2), a hydrogen removal system is installed before the CO2 passivationair enters the stripper.

4.3. Toyo Engineering-ACES Process. The synthesis section of theACES process (Fig. 8) consists of a reactor, a stripper, two carbamate condensers,a scrubber and operates at 17.5 MPa (175 bars). The reactor is operated at 190"Cwith a NH3/CO2 ratio of 4.0 (mol/mol). The newer version, ACES21, was designedto use less components in the urea synthesis loop in order to simplify the process.This lessens construction costs with the installation of the reactor on ground inthe CO2 stripping process. In addition the operation conditions of the synthesissection have been optimized under lower operating pressure than in the originalprocess. ACES21 consists of a reactor, a stripper and a carbamate condensor viathe HP Carbamate Ejector which provides the driving force for circulation in thesynthesis loop instead of gravity as was used in the original process. The reactoris operated at an NH3/CO2C ratio of 3.7, 182"C and 152 bar. The CO2 conversionto urea is as high as 63% at the exit of the reactor. Urea synthesis solution is fedto the stripper where unconverted carbamate is thermally decomposed andexcess ammonia and CO2 are efficiently separated by CO2 stripping. Thestripped off gas is fed to the Vertical Submerged Carbamate Condensor. Ammo-nia and CO2 condense to from ammonium carbamate and subsequently urea isformed by the dehydration of the carbamate. Uncondensed ammonia and CO2

gas is sent to a recycle carbamate solution (20).

Fig. 8. TEC ACES process. MP = medium pressure; BL = battery limits.

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Liquid NH3 is fed directly into the reactor by a centrifugal ammonia.Gaseous CO2 is sent from the centrifugal CO2 compressor to the bottom sectionof the falling-film type stripper.

The stream from the reactor consisting of a mixture of urea, unconvertedammonium carbamate, excess water, and NH3, is fed into the top of the stripper.The ACES stripper utilizes a ferrite–austenite stainless steel, as do the carba-mate condensers. The reactor and scrubber are constructed with 316 L urea-grade stainless steel.

The stripper has two functions: The upper part contains trays and sepa-rates excess NH3 in the feed (which ensures effective CO2 stripping in the falling-film type lower section). The solution then passes through the falling-film sectionwhere the ammonium carbamate is decomposed and separated by CO2 strippingand steam heating. This overhead gas mixture is fed to the parallel carbamatecondensers where the gaseous mixture is condensed and absorbed into the solu-tions from the scrubber and the 1.8 MPa (18 bar) absorber. One carbamate con-denser is used to generate low pressure steam 0.45–0.5 MPa (4.5–5 bar) and theother to heat the urea solution from the stripper bottoms. The gas–liquid streamfrom the condenser is recycled back to the reactor by gravity flow. The inertstream from the top of the reactor is purged to the scrubber for recovery of theNH3 and CO2.

The urea solution leaving the stripper bottom contains about 12 wt% of NH3

and is further purified in the 1.8 MPa (18 bar) and 0.2 MPa (2 bar) recovery sec-tions of the plant. The resultant NH3 and CO2 separated in the decomposers isabsorbed and returned to the synthesis section by the high pressure centrifugalcarbamate pump.

The urea solution stream is then fed to the vacuum concentrator unit,which operates at 17.3 kPa (130 mm Hg abs) and produces 88.7 wt% urea. Itthen goes to either two-stage evaporators if prills are made, or a single-stageunit for granule production.

As in all the processes, the process condensate and all other sources ofwaste urea–NH3–water contamination go to a waste recovery unit, whichincludes a urea hydrolyzer. The final water discharge is then below 3–5 ppm ofNH3 and urea.

4.4. Other Processes. Flow sheets for typical partial-recycle processand typical once-through urea process are given in Figures 9 and 10, respec-tively. A process for urea production that comprises a first process step inwhich ammonia and carbon dioxide are obtained, subjecting natural gas toreforming treatments, and a second step of urea production from such ammoniaand from carbon dioxide, through a formation of solution comprising ureaand ammonium carbamate in a urea synthesis reactor and subsequent decom-position of the ammonium carbamate and urea recovery has been reported(21).

5. Finishing Processes

Urea processes provide an aqueous solution containing 70–87% urea. This solu-tion can be used directly for nitrogen-fertilizer suspensions or solutions such as

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urea–ammonium nitrate solution, which has grown in popularity recently (18).Urea solution can be concentrated by evaporation or crystallization for the pre-paration of granular compound fertilizers and other products. Concentrated ureais solidified in essentially pure form as prills, granules, flakes, or crystals. Solidurea can be shipped, stored, distributed, and used more economically than insolution. Furthermore, in the solid form, urea is more stable and biuret forma-tion less likely.

5.1. Prilling. The manufacture of prills is rapidly decreasing owing toboth environmental problems and product quality as compared to granules. In

Fig. 9. Typical partial-recycle process.

Fig. 10. Typical once-through urea process.

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a prilling plant the urea solution from the recovery section is evaporated in twostages to strength. It is then pumped to the top of a 50-60 m cylindrical concretetower where it is fed into a spinning bucket containing many (+2000) small holes.The emerging small liquid droplets solidify as they fall and are cooled by a forcedor induced draft air flow. The very fine dust that is formed and exits at the top ofthe tower with the air flow is an environmental problem. Many recovery systemshave been tried (ie, water baths, dust collection, etc), but none have been verysuccessful. Snamprogetti has a patented dedusting system that reduces ureadust from 40 to 15 mg/Nm3. The addition of an inorganic acid to the urea meltjust before prilling reduces the amount of ammonia in the air.The quality pro-blem is that prill size must necessarily be small in order to obtain proper solidi-fication and cooling in the fall height that is practical (50–60). Generally, boththe crushing and impact strength of the prill is much less than for a granule.This causes many problems in handling both at the plant and in shipping.Stamicarbon has introduced a ‘‘seeding’’ system that has improved prill strength.Also, formaldehyde can be added as it not only improves the crushing strength,but is suppresses the caking tendency in storage. If it were not for environmentalconsiderations, prilling would still be a cheaper option than granulation in asmall-scale marketing area (ie, not on a global scale).

5.2. Granulation. Almost all new plants produce granules and theHydro-Agri process is used in the majority of plants (Fig. 11). This process wasdeveloped by NSM of Holland many years ago. They have no plant size limitationand will design a single-train unit for production over 3000 MTD. The C&I

Fig. 11. Hydro-Agri urea granulation process.

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Girdler drum system has been very successful, but cannot compete in today’smarket because of restrictions in the train size. Toyo has successfully developeda spout-fluid technology (Figs. 12–14) Three plants are in operation and othersare in either the design or construction stage. Stamicarbon licenses a fluid-bedlarge-scale single-train plant that is somewhat similar to the Hydro-Agri design(Fig. 15).

Fig. 12. TEC urea granulation process.

Fig. 13. Multistage spout-fluid-bed reactor. 1, spouted bed; 2, perforated plate; 3, spraynozzle; 4, air header; 5, fluidized bed.

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6. Wastewater Treatment

Under the pressure of progressively more stringent government regulations withregard to permissible levels of residual NH3 and urea content in wastewaters,the fertilizer industry made an effort to improve wastewater treatment.

Fig. 14. Single spout-fluid-bed granulator.

Fig. 15. Stamicarbon urea granulation.

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For each mol of urea produced in a total-recycle urea process, one mol ofwater is formed. It is usually discharged from the urea concentration and eva-poration section of the plant. For example, a 1200 t/d plant discharges a mini-mum of 360 t/d of wastewater. With a barometric condenser in the vacuumsection of the evaporation unit, the amount of wastewater is even higher.Small amounts of urea are usually found in wastewaters because of entrainmentcarry-over.

The problem in reducing the NH3 and urea content in the wastewaters tobelow 100 ppm is because it is difficult to remove one in the presence of the other.The wastewater can be treated with caustic soda to volatilize NH3. However, in amore efficient method, the urea is hydrolyzed to ammonium carbamate, which isdecomposed to NH3 and CO2; the gases are then stripped from the wastewater.

All process licensors also feature wastewater treatment systems. Stamicar-bon guarantees the lowest NH3–urea content and has plants in operation con-firming the low NH3–urea (1 ppm NH3–1 ppm urea). This water is verysatisfactory to use as boiler feed water. See Figures 16 and 17 for this system.

7. Economic Aspects

Urea is used in the developing regions of the world and is widely traded in inter-national markets due to its relatively cheap transport costs. The two main areasof urea trade are the Black Sea and Arab Gulf. These flows determine the globalprices. The Black Sea normally supplies Europe and Latin America while theArab Gulf supplies North America and Asia/Oceania

The growth in global urea production slowed from 6.5% in 2007 to only 1.7%in 2008 when it is estimated by the International Fertilizer Industry Associationto have reached 146( 106 t. Global demand for urea is forecast to grow at 3.7%/yr

Fig. 16. Simplified flow sheet of wastewater treatment.

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to 175( 106 t in 2013. The IFA forecasted the world urea capacity will increase by47( 106 t/yr in 2013. Much of the demand will be for fertilizer. Urea demand inChina was resilient in 2008, despite downward market pressures, growing at 4%over 2007. Exports fell by 20% to an estimated 4.7( 106 t. China has introduced anew export tax system based on low and high seasons. India is the world’s secondlargest urea producer as well as major supporter. Capacity will increase in 2009-2013 by 17% growth to 25( 106 t/yr (22).

In May 2010 Yuzhny prices hovered just about break even costs of someUkrainian urea producers at $230-236/t, Baltic prices were down at $225-230/tFOB. In May 2010 urea barge prices slipped with reports of offers down to$270/short ton (New Orleans). By mid-May prices were $205-210/short ton(New Orleans) (23).

8. Health and Safety Factors

Urea can be irritating to skin, eyes, and the respiratory tract. Repeated or pro-longed contact with urea in fertilizer form on skin may cause dermatitis. It ismoderately toxic by intravenous and subcutaneous routes. It is questionableas a carcinogen. High concentrations in the blood can be damaging. It reactswith sodium hypochlorite to form explosive nitrogen trichoride. It is incompati-ble with NaNO2, P2Cl5, and nitrosyl perchlorate. Preparation of 15N-labeledurea is hazardous. When heated to decomposition, it emits toxic fumes of NOx

(24).

9. Uses

9.1. Fertilizer. Nearly 90% of urea consumed is for fertilizer. Solid ureacontaining 0.8–2.0 wt% biuret is primarily used for direct application to the soil

Fig. 17. Countercurrent hydrolyzer.

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as a nitrogen-release fertilizer. Weak aqueous solutions of low biuret urea (0.3wt% biuret max) are used as plant food applied to foliage spray. Urea is popularas a solid nitrogen fertilizer because of tis high N content (46%). Mixed withadditives, urea is used in solid fertilizers of various formulations, eg, urea–ammonium phosphate (UAP), urea–ammonium sulfate (UAS), and urea–phos-phate (urea + phosphoric acid). Concentrated solutions of urea and ammoniumnitrate (UAN) solutions (80–85 wt%) have a high nitrogen content but low crys-tallization point, suitable for easy transportation, pipeline distribution, anddirect spray application.

9.2. Animal Feeds. Urea is also used as feed supplement for ruminants,where it assists in the utilization of protein. Urea is used in the preparation oflysine, an amino acid, used widely in poultry feeds.

9.3. Resins. Urea is one of the raw materials for urea–formaldehyderesins. It is produced by the condensation reaction between urea and formalde-hyde. These resins have found outlets in adhesives for paper, board, plywood,surface coatings, molding resins, and textile processing. They are also used tocoat textiles, paper and leather (25,26). Urea (with ammonia) pyrolyzes at hightemperature and pressure to form melamine-formaldehyde resins. These resinsare used as adhesives, paints, laminates, molding compounds, and coating paperand textiles (25).

9.4. Automotive Emissions Reduction. A growing use for urea is aprocess called selective catalytic reduction to reduce NOx emissions from dieselengines, A urea solution is sprayed into the engine’s exhaust where it convertsthe nitrogen oxides to nitrogen and water.

9.5. Clathrates. Reagent-grade urea is used in some pharmaceuticalpreparations. In these applications, urea must meet the purity specificationsissued by the ACS. Urea has the remarkable property of forming crystalline com-plexes or adducts with straight-chain organic compounds. These crystalline com-plexes consist of a hollow channel, formed by the crystallized urea molecules, inwhich the hydrocarbon is completely occluded. Such compounds are known asclathrates. The type of hydrocarbon occluded, on the basis of its chain length,is determined by the temperature at which the clathrate is formed. This propertyof urea clathrates is widely used in the petroleum-refining industry for the pro-duction of jet aviation fuels and for dewaxing of lubricant oil. The clathrates arebroken down by simply dissolving urea in water or in alcohol.

9.6. Other. Urea is used in some pesticides, as a viscosity modifier forcasein or starch-based paper, coatings, in pharmaceuticals, pesticides, tooth-paste, cosmetics, flame-proofing agents, and fabric softeners.

BIBLIOGRAPHY

‘‘Urea’’ in ECT 1st ed., Vol. 14, pp. 458–466, by F. A. Wolff and D. J. O’Flynn, E. I. du Pontde Nemours & Co., Inc.; ‘‘Urea’’ in ECT 2nd ed., Vol. 21, pp. 37–56, by I. Mavrovic, Con-sulting Engineer; ‘‘Urea’’ in ECT 3rd ed., Vol. 23, pp. 548–575, by Ivo Mavrovic, Consul-tant and A. Ray Shirley, Jr., Applied Chemical Technology; in ECT 4th ed., Suppl. Vol.,pp. 597-621, by Ivo Mavrovic, Consultant, A. Ray Shirley, Jr., Applied Chemical Technol-ogy, G. R. ‘‘Buck’’ Coleman, Consultant.

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CITED PUBLICATIONS

1. E. Blasiak, Technology of Nitrogen Compounds, Vol. 2, State Technical Publisher,Warsaw, 1956, pp. 596–642.

2. E. Janecke, Z. Elektrochem. 36, 645 (1930).3. A. Seidell, Solubility of Inorganic Compounds, 2nd ed., D. Van Nostrand Co., New

York.4. D. F. Du Toit, Proc. Koninkl. Akad. Wetenschap, Amsterdam 16, 555 (1913).5. R. Escales and H. Kopke, Chem. Zig. 33, 595 (1911).6. U.S. Pat. 3,232,984 (Feb. 1, 1966), J. A. Finneran (to Pullman, Inc.).7. U. S. Pat. 3,255,246 (June 7, 1966), (to E. I. du Pont de Nemours & Co., Inc.).8. Knop and Hufner in A. E. Werner, ed., Chemistry of Urea, Longmans, Green and Co.,

London, 1923, p. 161.9. C. Matignon and M. Fr!ejacques, Bull. Soc. Chim. France 29, 21 (1921); 31, 394

(1922).10. T. R. Briggs and V. Migrdichian, J. Phys. Chem. 28, 1121 (1924).11. E. Briner, J. Chim. Phys. 4, 267 (1906).12. N. F. I. Isambert, Compt. Rend. 92, 919 (1881).13. F. Fichter and B. Becker, Ber. 44, 3470 (1911).14. C. Matignon and M. Fr!ejacques, Bull. Soc. Chim. 31, 394 (1922).15. K. K. Clark, V. L. Gaddy, and C. E. Rist, Ind. Eng. Chem. 25, 1092 (1933).16. Eur. Chem. News (Jan. 17, 1969).17. S. Kawasumi, Bull. Chem. Soc. Jpn. 24, 148; 25, 227; 26, 218; 27.18. Urea Process Technology, Snamprogetti. Milan, Italy, www.samprogett.it/media.,

accessed August 2010.19. Technology: Profile Urea, Uhde GmbH, www.udhe.eu/competence/technolgy,

accessed August 2010.20. Toyo: Leader in Urea Process Technology, Toyo Engineering Corp. (TEC), Chiba,

Japan, 2006.21. U. S. Pat. 7,732,640 (June 8, 2010), D. Romiti and P. Sticchi (to Urea Casale S. A.)22. Urea Uses and Market Data, ICIS Reporter, www.icis,com, updated to May 2010.23. Urea Prices and Pricing Information, ICIS Reporter, www.icis.com, updated to May

2010.24. ‘‘Urea,’’ in R. J. Lewis, ed., Dangerous Properties of Industrial Materials, Wiley

Interscience, Hoboken, N.J., on line 2008.25. H. Holk, Handbook of Paper and Board, Wiley-VCH, Weinheim, Germany, 2006.26. Z. W. Wicks, Organic Coatings, Wiley, Hoboken, N.J., 2007.

IVO MAVROVIC

Consultant

A. RAY SHIRLEY JR.Applied Chemical Technology

G. R. ‘‘BUCK’’ COLEMAN

Consultant

Updated by Staff

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