Mixed Fertilizer Plants
fertilizers contain two or more of the elements nitrogen, phosphorus,
and potassium (NPK), which are essential for good plant growth and high
crop yields. This document addresses the production of ammonium phosphates
(mono ammonium phosphate, or MAP, and diammonium phosphate, or DAP), nitro phosphates,
potash, and compound fertilizers. Ammonium phosphates are produced by
mixing phosphoric acid and anhydrous ammonia in a reactor to produce
a slurry. (This is the mixed-acid route for producing NPK fertilizers;
potassium and other salts are
added during the process.) The slurry is sprayed onto a bed of re-cycled
solids in a rotating granulator, and ammonia is sparged into the bed
from underneath. Granules pass to a rotary dryer followed by a rotary
cooler. Solids are screened and sent to storage for bagging or for bulk
Nitro phosphate fertilizer is made by digesting phosphate rock with nitric acid. This is the Nitro phosphate route leading to NPK fertilizers; as in the mixed-acid route, potassium and other salts are added during the process. The resulting solution is cooled to precipitate calcium nitrate, which is removed by filtration. The filtrate is neutralized with ammonia, and the solution is evaporated to reduce the water content. Prilling may follow. The calcium nitrate filter cake can be further treated to produce a calcium nitrate fertilizer, pure calcium nitrate, or ammonium nitrate and calcium carbonate. Nitro phosphate fertilizers are also produced by the mixed-acid process, through digestion of the phosphate rock by a mixture of nitric and phosphoric acids.
Potash (potassium carbonate) and sylvine (potassium chloride) are solution-mined from deposits and are refined through crystallization processes to produce fertilizer. Potash may also be dry-mined and purified by flotation.
Compound fertilizers can be made by blending basic fertilizers such as ammonium nitrate,MAP, DAP, and granular potash; this route may involve a granulation process.
Nitrogenous Fertilizer Plants
document addresses the production of ammonia,urea, ammonium sulfate, ammonium
nitrate (AN), calcium ammonium nitrate (CAN), and ammonium sulfate nitrate
(ASN). The manufacture of nitric acid used to produce nitrogenous fertilizers
typically occurs on site and is there-fore included here.
Ammonia (NH3) is produced from atmospheric nitrogen and hydrogen from a hydrocarbon source. Natural gas is the most commonly used hydrocarbon feedstock for new plants; other feedstocks that have been used include naphtha, oil, and gasified coal. Natural gas is favored over the other feedstocks from an environmental perspective.
Ammonia production from natural gas includes the following processes: desulfurization of the feedstock; primary and secondary reforming; carbon monoxide shift conversion and removal of carbon dioxide, which can be used for urea manufacture; methanation; and ammonia synthesis.
Catalysts used in the process may include cobalt, molybdenum, nickel, iron oxide/chromium oxide, copper oxide/zinc oxide, and iron.
Urea fertilizers are produced by a reaction of liquid ammonia with carbon dioxide. The process steps include solution synthesis, where ammonia and carbon dioxide react to form ammonium carbamate, which is dehydrated to form urea; solution concentration by vacuum, crystallization,or evaporation to produce a melt; forma- tion of solids by prilling (pelletizing liquid drop-lets)
or granulating; cooling and screening of solids;coating of the solids; and bagging or bulk loading. The carbon dioxide for urea manufacture is produced as a by-product from the ammonia plant reformer.
Ammonium sulfate is produced as a caprolactam by-product from the petrochemical industry,as a coke by-product, and synthetically through reaction of ammonia with sulfuric acid.
The reaction between ammonia and sulfuric acid produces an ammonium sulfate solution that is continuously circulated through an evaporator to thicken the solution and to produce ammonium sulfate crystals. The crystals are separated from the liquor in a centrifuge, and the liquor is returned to the evaporator. The crystals are fed either to a fluidized bed or to a rotary drum dryer and are screened before bagging or bulk loading.
Ammonium Nitrate, Calcium Ammonium Nitrate,and Ammonium Sulfate Nitrate
Ammonium nitrate is made by neutralizing nitric acid with anhydrous ammonia. The resulting 80-90% solution of ammonium nitrate can be sold as is, or it may be further concentrated to a 95-99.5% solution (melt) and converted into prills or granules. The manufacturing steps include solution formation, solution concentration, solids formation, solids finishing, screening, coating, and bagging or bulk shipping. The processing steps depend on the desired finished product.
Calcium ammonium nitrate is made by adding calcite or dolomite to the ammonium nitrate melt before prilling or granulating. Ammonium sulfate nitrate is made by granulating a solution of ammonium nitrate and ammonium sulfate.
The production stages for nitric acid manufacture include vaporizing the ammonia; mixing the vapor with air and burning the mixture over a platinum/rhodium catalyst; cooling the resultant nitric oxide (NO) and oxidizing it to nitrogen dioxide (NO2) with residual oxygen; and absorbing the nitrogen dioxide in water in an absorption column to produce nitric acid (HNO3).
Because of the large quantities of ammonia and other hazardous materials handled on site, an emergency preparedness and response plan is required.
Phosphate Fertilizer Plants
fertilizers are produced by adding acid to ground or pulverized phosphate
rock. If sulfuric acid is used, single or normal, phosphate (SSP) is produced,
with a phosphorus content of 16-21% as phosphorous pent oxide (P2O5). If
phosphoric acid is used to acidulate the phosphate rock, triple phosphate
(TSP) is the result. TSP has a phosphorus content of 43-48% as P2O5.
SSP production involves mixing the sulfuric acid and the rock in a reactor. The reaction mixture is discharged onto a slow-moving conveyor in a den. The mixture is cured for 4 to 6 weeks before bagging and shipping.
Two processes are used to produce TSP fertilizers: run-of-pile and granular. The run-of-pile process is similar to the SSP process. Granular TSP uses lower-strength phosphoric acid (40%, compared with 50% for run-of-pile). The reaction mixture, a slurry, is sprayed onto recycled fertilizer fines in a granulator. Granules grow and are then discharged to a dryer, screened, and sent to storage.
Phosphate fertilizer complexes often have sulfuric and phosphoric acid production facilities.
Sulfuric acid is produced by burning molten sulfur in air to produce sulfur dioxide, which is then catalytically converted to sulfur trioxide for absorption in oleum. Sulfur dioxide can also be produced by roasting pyrite ore. Phosphoric acid is manufactured by adding sulfuric acid to phosphate rock. The reaction mixture is filtered to re-move phosphogypsum, which is discharged to settling ponds or waste heaps.
The separation of phosphate rock from impurities and nonphosphate materials for use in fertilizer manufacture consists of beneficiation, drying or calcining at some operations, and grinding. Because the primary use of phosphate rock is in the manufacture of phosphatic fertilizer, only those phosphate rock processing operations associated with fertilizer manufacture are discussed here. Phosphate rock from the mines is first sent to beneficiation units to separate sand and clay and to remove impurities. Steps used in beneficiation depend on the type of rock. A typical beneficiation unit for separating phosphate rock mined in Florida begins with wet screening to separate pebble rock that is larger than 1.43 millimeters (mm) (0.056 inch [in.]) or 14 mesh, and smaller than 6.35 mm (0.25 in.) from the balance of the rock. The pebble rock is shipped as pebble product. The material that is larger than 0.85 mm (0.033 in.), or 20 mesh, and smaller than 14 mesh is separated using hydrocyclones and finer mesh screens and is added to the pebble product. The fraction smaller than 20 mesh is treated by 2-stage flotation. The flotation process uses hydrophilic or hydrophobic chemical reagents with aeration to separate suspended particles.
Phosphate rock mined in North Carolina does not contain pebble rock. In processing this type of phosphate, 10-mesh screens are used. Like Florida rock, the fraction that is less than 10 mesh is treated by 2-stage flotation, and the fraction larger than 10 mesh is used for secondary road building.
The 2 major western phosphate rock ore deposits are located in southeastern Idaho and northeastern Utah, and the beneficiation processes used on materials from these deposits differ greatly. In general, southeastern Idaho deposits require crushing, grinding, and classification. Further processing may include filtration and/or drying, depending on the phosphoric acid plant requirements. Primary size reduction generally is accomplished by crushers (impact) and grinding mills. Some classification of the primary crushed rock may be necessary before secondary grinding (rod milling)takes place. The ground material then passes through hydrocyclones that are oriented in a 3-stage countercurrent arrangement. Further processing in the form of chemical flotation may be required.Most of the processes are wet to facilitate material transport and to reduce dust.
Northeastern Utah deposits are a lower grade and harder than the southeastern Idaho deposits and require processing similar to that of the Florida deposits. Extensive crushing and grinding is necessary to liberate phosphate from the material. The primary product is classified with 150- to 200-mesh screens, and the finer material is disposed of with the tailings. The coarser fraction is processed through multiple steps of phosphate flotation and then diluent flotation. Further processing may include filtration and/or drying, depending on the phosphoric acid plant requirements. As is the case for southeastern Idaho deposits, most of the processes are wet to facilitate material transport and to reduce dust.
wet beneficiated phosphate rock may be dried or calcined, depending on
its organic content. Florida rock is relatively free of organics and is
for the most part no longer dried or calcined. The rock is maintained
at about 10 percent moisture and is stored in piles at the mine and/or
chemical plant for future use. The rock is slurried in water and wet-ground
in ball mills or rod mills at the chemical plant. Consequently, there
is no significant emission potential from wet grinding. The small amount
of rock that is dried in Florida is dried in direct-fired dryers at about
120°C (250°F), where the moisture content of the rock falls from
10 to 15 percent to 1 to 3 percent. Both rotary and fluidized bed dryers
are used, but rotary dryers are more common. Most dryers are fired with
natural gas or fuel oil (No. 2 or No. 6), with many equipped to burn more
than 1 type of fuel. Unlike Florida rock, phosphate rock mined from other
reserves contains organics and must be heated to 760 to 870°C (1400
to 1600°F) to remove them. Fluidized-bed calciners are most commonly
used for this purpose, but rotary calciners are also used. After drying,
the rock is usually conveyed to storage silos on weather-protected conveyors
and, from there, to grinding mills. In North Carolina, a portion of the
beneficiated rock is calcined at temperatures generally between 800 and
825°C (1480 and 1520°F) for use in "green" phosphoric
acid production, which is used for producing super phosphoric acid and
as a raw material for purified phosphoric acid manufacturing. To produce
"amber" phosphoric acid, the calcining step is omitted, and
the beneficiated rock is transferred directly to the phosphoric acid production
processes. Phosphate rock that is to be used for the production of granular
triple super phosphate (GTSP) is beneficiated, dried, and ground before
being transferred to the GTSP production processes.
Dried or calcined rock is ground in roll or ball mills to a fine powder, typically specified as 60 percent by weight passing a 200-mesh sieve. Rock is fed into the mill by a rotary valve, and ground rock is swept from the mill by a circulating air stream. Product size classification is provided by a "revolving whizzer, which is mounted on top of the ball mill," and by an air classifier. Oversize particles are recycled to the mill, and product size particles are separated from the carrying air stream by a cyclone.
of Urea Production Processes
The commercial synthesis of urea involves the combination of ammonia and carbon dioxide at high pressure to form ammonium carbamate which is subsequently dehydrated by the application of heat to form urea and water.
2NH3 Ammonia + CO2 Carbon Dioxide is in equilibrium with NH2COONH4AmmoniumCarbamate in equilibrium with CO(NH2)2 Urea+ H2OWater Reaction 1 is fast and exothermic and essentially goes to completion under the reaction conditions used industrially. Reaction 2 is slower and endothermic and does not go to completion. The conversion (on a CO2 basis) is usually in the order of 50-80%. The conversion increases with increasing temperature and NH3/CO2 ratio and decreases with increasing H2O/CO2 ratio.
The design of commercial processes has involved consideration of how to separate the urea from the other constituents, how to recover excess NH3, and decompose the carbamate for recycle. Attention was also devoted to developing materials to withstand the corrosive carbamate solution and to optimise the heat and energy balances.
The simplest way to decompose the carbamate to CO2 and NH3 requires the reactor effluentto be de pressurised and heated. The earliest urea plants operated on a "Once Through" principle where the off-gases were used as feedstocks for other products. Subsequently "Partial Recycle" techniques were developed to recover and recycle some of the NH3 and CO2 to the process. It was essential to recover all of the gases for recycle to the synthesis to optimise raw material utilisation and since re compression was too expensive an alternative method was developed. This involved cooling the gases and re-combining them to form carbamate liquor which was pumped back to the synthesis. A series of loops involving carbamate decomposers at progressively lower pressures and carbamate condensers were used. This was known as the "Total Recycle Process". A basic consequence of recycling the gases was that the NH3/CO2 molar ratio in the reactor increased thereby increasing the urea yield.
Significant improvements were subsequently achieved by decomposing the carbamate in the reactor effluent without reducing the system pressure. This "Stripping Process" dominated synthesis technology and provided capital/energy savings. Two commercial stripping systems were developed, one using CO2 and the other using NH3 as the stripping gases.
Since the base patents on stripping technology have expired, other processes have emerged which combine the best features of Total Recycle and Stripping Technologies. For convenience total recycle processes were identified as either "conventional" or "stripping" processes.
The urea solution arising from the synthesis/recycle stages of the process is subsequently concentrated to a urea melt for conversion to a solid prilled or granular product. Improvements in process technology have concentrated on reducing production costs and minimising the environmental impact. These included boosting CO2 conversion efficiency, increasing heat recovery, reducing utilities consumption and recovering residual NH3 and urea from plant effluents. Simultaneously the size limitation of prills and concern about the prill tower off-gas effluent were responsible for increased interest in melt granulation processes and prill tower emission abatement. Some or all of these improvements have been used in updating existing plants and some plants have added computerised systems for process control. New urea installations vary in size from 800 to 2000t/d and typically would be 1500t/d units.
Modern processes have very similar energy requirements and nearly 100% material efficiency. There are some differences in the detail of the energy balances but they are deemed to be minor in effect.
Block flow diagrams for CO2 and NH3 stripping total recycle processes are shown in Figures 1 and 2.
Figure 1 - Block diagram of a total recycle CO2 stripping urea process.
Figure 2 - Block diagram of a total recycle NH3 stripping urea process
The total capacity of these plants is around 19,000t/d.
Description of BAT Production Processes
The process water from each process discussed in this section is purified by recovery of dissolved urea, NH3 and CO2 which are recycled to the synthesis section via a low pressure carbamate condensation system.
Carbon dioxide stripping process
NH3 and CO2 are converted to urea via ammonium carbamate at a pressure of approximately 140bar and a temperature of 180-185oC. The molar NH3/CO2 ratio applied in the reactor is 2.95. This results in a CO2 conversion of about 60% and an NH3 conversion of 41%. The reactor effluent, containing unconverted NH3 and CO2 is subjected to a stripping operation at essentially reactor pressure, using CO2 as stripping agent. The stripped-off NH3 and CO2 are then partially condensed and recycled to the reactor. The heat evolving from this condensation is utilised to produce 4.5bar steam some of which can be used for heating purposes in the downstream sections of the plant. Surplus 4.5bar steam is sent to the turbine of the CO2 compressor.
The NH3 and CO2 in the stripper effluentare vaporised in a 4bar decomposition stage and subsequently condensed to form a carbamate solution, which is recycled to the 140bar synthesis section. Further concentration of the urea solution leaving the 4bar decomposition stage takes place in the evaporation section, where a 99.7% urea melt is produced.
Ammonia stripping process
NH3 and CO2 are converted to urea via ammonium carbamate at a pressure of 150bar and a temperature of 180oC. A molar ratio of 3.5 is used in the reactor giving a CO2 conversion of 65%. The reactor effluententers the stripper where a large part of the unconverted carbamate is decomposed by the stripping action of the excess NH3. Residual carbamate and CO2 are recovered downstream of the stripper in two successive stages operating at 17 and 3.5bar respectively. NH3 and CO2 vapours from the stripper top are mixed with the recovered carbamate solution from the High Pressure (HP)/Low Pressure (LP) sections, condensed in the HP carbamate condenser and fed to the reactor. The heat of condensation is used to produce LP steam.
The urea solution leaving the LP decomposition stage is concentrated in the evaporation section to a urea melt.
Advanced cost & energy saving (ACES) process
In this process the synthesis section operates at 175bar with an NH3/CO2 molar ratio of 4 and a temperature of 185 to 190oC.
The reactor effluentis stripped at essentially reactor pressure using CO2 as stripping agent. The overhead gas mixture from the stripper is fed to two carbamate condensers in parallel where the gases are condensed and recycled under gravity to the reactor along with absorbent solutions from the HP scrubber and absorber. The heat generated in the first carbamate condenser is used to generate 5bar steam and the heat formed in the second condenser is used to heat the solution leaving the stripper bottom after pressure reduction. The inerts in the synthesis section are purged to the scrubber from the reactor top for recovery and recycle of NH3 and CO2. The urea solution leaving the bottom of the stripper is further purified in HP and LP decomposers operating at approx. 17.5bar and 2.5bar respectively. The separated NH3 and CO2 are recovered to the synthesis via HP and LP absorbers.
The aqueous urea solution is first concentrated to 88.7%wt in a vacuum concentrator and then to the required concentration for prilling or granulating.
Isobaric double recycle (IDR) process
In this process reactor pressure is about 200bar, the molar NH3/CO2 ratio is 4.5 and the reactor effluent temperature 185 to 190oC. The conversion rates to urea in the reactor are 71% for CO2 and 35% for NH3.
Unconverted materials in the effluentfrom the reactor bottom are separated by heating and stripping in two consecutive decomposers operated at reactor pressure and heated by 25bar steam. Carbamate is decomposed/stripped by NH3 in the first stripper and the remaining NH3 is evolved in the second stripper using CO2 as stripping agent. The overheads from stripper 1 are fed directly to the reactor and the overheads from stripper 2 are recycled to the reactor via the carbamate condenser. Heat of condensation is recovered as 6bar steam and used downstream in the process.
Most of the CO2 fed to the plant goes to the second stripper and the remainder goes directly to the reactor for fine temperature control when needed. About 40% of the NH3 goes to the first stripper and the remainder to the upper and lower sections of the reactor in two streams.
Unconverted carbamate, NH3 and CO2 leaving the stripper with the urea solution are recovered/vaporised in two successive distillers operating at 20bar and 6bar respectively. The vapours are condensed and recycled to the synthesis after condensation to carbamate solution.
The latent heat present in the 20bar stage off-gases is used as a heat source for the evaporation of water in the first stage evaporator.
Further concentration of the urea solution leaving the LP decomposition stage is carried out in two vacuum evaporators in series, producing urea melt for prilling or granulating.
Process Water Sources and Quantities
A 1,000t/d urea plant generates on average approximately 500m3/d process water containing 6% NH3, 4% CO2 and 1.0% urea (by weight). The principal source of this water is the synthesis reaction where 0.3tonnes of water is formed per tonne of urea e.g.
2NH3 + CO2 -----> CO(NH2)2 + H2O
The other sources of water are ejector steam, flush and seal water and steam used in the waste water treatment plant.
The principal sources of urea, NH3 and CO2 in the process water are:
The NH3 and urea in the evaporator condensate are attributable to
a. the presence of NH3 in the urea solution feed to the evaporator,
b. the formation of biuret and the hydrolysis of urea in the evaporators, both reactions liberating NH3
2CO(NH2)2 -----> H2NCONHCONH2 + NH3
CO(NH2)2 + H2O -----> 2NH3 + CO2
c. direct carry over of urea from the evaporator separators to the condensers (physical entrainment),
d. the formation of NH3 from the decomposition of urea to isocyanic acid.
CO(NH2)2 -----> HNCO + NH3
The reverse reaction occurs on cooling the products in the condensers.
Off-gases from the recovery/recirculation stage absorbed in the process water.
Off-gases from the synthesis section absorbed in the process water.
Flush and purge water from pumps.
Liquid drains from the recovery section.
The purpose of the water treatment is to remove NH3, CO2 and urea from the process water and recycle the gases to the synthesis. This ensures raw material utilisation is optimised and effluentis minimised.
Prilling and Granulation
In urea fertilizer production operations, the final product is in either prilled or granular form. Production of either from urea melt requires the use of a large volume of cooling air which is subsequently discharged to the atmosphere. A block diagram of the prilling and granulation processes is shown in Figure 3.
The concentrated (99.7%) urea melt is fed to the prilling device (e.g. rotating bucket/shower type spray head) located at the top of the prilling tower. Liquid droplets are formed which solidify and cool on free fall through the tower against a forced or natural up-draft of ambient air. The product is removed from the tower base to a conveyor belt using a rotating rake, a fluidised bed or a conical hopper. Cooling to ambient temperature and screening may be used before the product is finally transferred to storage.
The design/operation of the prilling device exerts a majorinfluenced on product size. Collision of the molten droplets with the tower wall as well as inter-droplet contact causing agglomeration must be prevented. Normally mean prill diameters range from 1.6-2.0mm for prilling operations. Conditioning of the urea melt and "crystal seeding" of the melt, may be used to enhance the anti-caking and mechanical properties of the prilled product during storage/handling.
Depending on the process a 95-99.7% urea feedstock is used. The lower feedstock concentration allows the second step of the evaporation process to be omitted and also simplifies the process condensate treatment step. The basic principle of the process involves the spraying of the melt onto recycled seed particles or prills circulating in the granulator. A slow increase in granule size and drying of the product takes place simultaneously. Air passing through the granulator solidifies the melt deposited on the seed material.
Processes using low concentration feedstock require less cooling air since the evaporation of the additional water dissipates part of the heat which is released when the urea crystallises from liquid to solid.
All the commercial processes available are characterised by product recycle, and the ratio of recycled to final product varies between 0.5 and 1.0. Prill granulation or fattening systems have a very small recycle, typically 2 to 4%. Usually the product leaving the granulator is cooled and screened prior to transfer to storage. Conditioning of the urea melt prior to spraying may also be used to enhance the storage/handling characteristics of the granular product.
and Available Emission Abatement Techniques
Scrubbing of off-gases with process condensate prior to venting inerts to atmosphere.
Wet scrubbing of prill tower and granulation plant air to recover urea and NH3.
Connection of ammonia pump safety relief valves/seals to a flare; connection of tank vents to the plant main stack or other safe location.
Dust reduction by producing granular rather than prilled product.
Bag filtration of dust laden air from transfer points, screens, bagging machines, etc. coupled with a dissolving system for recycle to the process.
Flash melting of solid urea over-size product for recycle to the process.
Collection of solid urea spillages on a dry basis.
Treatment of process waste water/condensate for recovery of urea, NH3 and CO2.
Improved evaporation heater/separator design to minimise urea entrainment.
Provision of adequate storage capacity for plant inventory to cater for plant upset and shut-down conditions.
Provision of submerged tanks to collect plant washings, etc. from drains for recycle to the waste water treatment section.
Use of mechanical seals instead of gland packing for pumps.
Use of closed circuit gland cooling water system for reciprocating pumps.
Replacement of reciprocating machinery with centrifugal type.
Computerisation of process control to provide consistent optimum operating conditions.
Implementation of regular scheduled maintenance programmes and good housekeeping practices.
Description of Process Water BAT Treatment Systems
A block diagram for a waste-water treatment plant is shown in Figure 4.
Desorption hydrolysis system
Heated process water is fed to the top of Desorber 1 where it is stripped of NH3 and CO2 by gas streams from Desorber 2 and the hydrolyser. The liquid leaving Desorber 1 bottom is pre-heated to 190°C and fed at 17bar pressure to the top of the hydrolyser. 25bar steam is introduced to the bottom of the hydrolyser and under these conditions the urea is decomposed and the gases are counter currently stripped. The vapours go to Desorber 1. The urea free liquid stream leaving the desorber is used to heat the hydrolyser feed stream and is fed after expansion to Desorber 2 where LP steam counter currently strips the remaining NH3 and CO2 and the off-gases pass to Desorber 1. Figure 4 - Block diagram for waste water treatment plant
The off-gases from Desorber 1 are condensed in a cooled reflux condenser/separator. Part of the separated liquid is pumped back to Desorber 1 and the remainder is returned to the LP recirculation section of the urea plant. Residual NH3 in the separator off-gas is recovered in an atmospheric absorber and returned to the LP recirculation section also.
The treated water which leaves Desorber 2 is cooled and concentrations of 5mg/l free NH3 and 1mg/l urea can be attained.
Heated process water is fed to the top section of a distillation tower for NH3 and CO2 removal. The effluent liquid is pre-heated before entry to the hydrolyser where the urea is decomposed to NH3 and CO2. The hydrolyser and distillation tower vapours are mixed with off gases from the LP decomposer separator, cooled and recycled to the process. After effluent treatment, water suitable for boiler feed is stated to be achievable. Treated water containing 5mg/l free NH3 and 1mg/l urea is expected.
Heated process water containing NH3, CO2 and urea is fed to the top of a steam stripper operated at 3 bar for separation of NH3 and CO2. The water is then fed from the middle section to the hydrolyser operating at 16 bar. The gaseous overheads are then sent via the LP decomposer/absorber to the synthesis for recovery of NH3 and CO2.
Free NH3 and urea concentrations of 3-5mg/l for each component are expected in the treated water.
Existing emissions to water performance
The actual performance of some existing plants may vary considerably from the above with values for emissions to water of 20-230mg/l (0.01-0.61kg/t of product) of NH3 and 20-320mg/l (0.01-0.84kg/t) of urea depending on the treatment system used. Figure 5 shows the emission sources from an existing plant.
5 - Block diagram of emission sources and typical quantities for existing
Prill Tower Emissions
The prill tower is a major source of emission in urea plants. The large volumes of
discharged untreated cooling air contain particulate urea dust (1-2kg/t) as well as NH3 (0.7-1.0kg/t).
Causes of dust formation
Towers with natural draft cooling are reported to have less dust emission than towers with forced/induced draft air cooling. The lower air velocity and product mass per m 3 of tower volume reduces attrition and carryover in the natural draft towers.
Operation and maintenance items significantly affecting dust formation
Fouling of the prilling device causing wider spread in prill granulometry
High melt feed temperature causing increased evaporation.
High prill temperature at the tower base. The largest prills may not have solidified sufficiently and will fracture on impact.
Dust emission is approximately proportional to prilling tower capacity.
High air velocities and the air velocity distribution cause coarse dust to be entrained.
Weather conditions e.g. relative humidity, temperature can affect the air quality/quantity.
Unequal pressure in the prilling device causing a broad spread of prill size.
Prill tower emission abatement
Selection of the appropriate equipment for existing plants can be a complex issue. Dry dust collectors, irrigated electrostatic precipitators and irrigated dust scrubbers have been considered for dust abatement but few have been commercially proven. Wet scrubbers seem to be more attractive than dry dust collectors. Recovery of the NH3 from the emission (for example by aqueous scrubbing) is very inefficient due to the low partial pressure of the gas in the discharged air.
Existing prilling plant performance
Figure 5 shows the emission sources from an existing plant
A dust emission of 5-40kg/t of final product is suggested for granulation process operations (i e ex granulator and cooler), which is is considerably higher than for prilling.
Causes of dust formation
The following reflects some speculations about the causes of dust formation in granulation but no quantitative data is available.
-Urea vapour formation during hot spraying of the urea melt and its subsequent condensation/solidification into small (0.5-3.0mm) particles. The vaporisation becomes negligible when the melt concentration is reduced to 95%.
-Reaction product of NH3 with isocyanic acid to form Urea.
-Entrainment of fine dust in the air.
-Impact of granules with the metal surface of the drum.
-solidification of sprayed molten urea droplets prior to coating due to excessive air flow.
-High vapour pressure of sprayed molten urea.
-High or low temperature, producing soft or brittle granules.
-Inter-granular friction causing surface abrasion.
Granulator emission abatement
Air extracted from the plant is normally scrubbed with urea plant process condensate and the resultant urea solution is recycled for reprocessing. With standard wet scrubbers an efficiency of 98% can be achieved for dust removal. The low partial pressure of the NH3 in the discharged air results in low NH3 scrubbing/recovery efficiencies which can be increased by acidification but the resultant solution has to be used in other plants.
Existing granulation plant performance Figure 5 shows the emission sources from an existing plant.
Production Of Urea-Ammonium
Overview of UAN Process Technology
Ammonium nitrate (AN) and urea are used as feedstocks in the production of urea-ammonium nitrate (UAN) liquid fertilizers. Most UAN solutions typically contain 28, 30 or 32% N but other customised concentrations (including additional nutrients) are produced. Plant capacities for the production of UAN solutions range between 200 and 2000t/d. Most of the large scale production units are located on complexes where either urea or ammonium nitrate or both are produced.
The addition of corrosion inhibitors or the use of corrosion resistent coatings allows carbon steel to be used for storage and transportation equipment for the solutions. West European consumption of UAN in 1990 was 2.6 x 106t of solution, one third of which was imported.
Typical UAN solution
N content 28-32% by weight, pH 7 to 7.5, density 1280-1320kg/m3, salt-out temperature 18 to 2°C, depending on the N content and at its lowest when the Urea N/Ammonium Nitrate N ratio is about 1:1.
Continuous and batch type processes are used and in both processes concentrated urea and ammonium nitrate solutions are measured, mixed and then cooled. Block diagrams for UAN production are shown in Figures 6 and 7.
Figure 6 Block flow diagram for UAN process.
Figure 7 Block diagram of a partial recycle CO2 stripping urea process for UAN production.
In the continuous
process the ingredients of the UAN solution are continuously fed to and
mixed in a series of appropriately sized static mixers. Raw material flow
as well as finished product flow, pH and density are continuously measured
and adjusted. The finished product is cooled and transferred to a storage
tank for distribution.
In the batch process the raw materials are sequentially fed to a mixing vessel fitted with an agitator and mounted on load-cells. The dissolving of the solid raw material(s) can be enhanced by recirculation and heat exchange as required. The pH of the UAN product is adjusted prior to the addition of the corrosion inhibitor.
A partial recycle CO2 stripping urea process is also suitable for UAN solution production. Unconverted NH3 and CO2 coming from the stripped urea solution, together with the gases from the water treatment unit, are transferred for conversion into UAN solutions.
Storage and Transfer Equipment
The physical form of the feedstock dictates the handling and storage system requirements. Bunded tank areas and collection pits allow any solution spillages to be collected for recycle. Air ducting and filtration helps the recovery of air-borne dust.
Regulations specific to the storage and handling of solid or solutions of ammonium nitrate must be adhered to. Recommendations for the storage and transfer of ammonia and nitric acid are given in EFMA BAT Booklets Nos 1 and 2 respectively. Recommendations for the storage of solid ammonium nitrate can be found in Reference .
Raw material and utility inputs
Depending on Conc.
Depending on Conc.
|N-containing condensate from AN or urea plants can be used as solvent.|
|For pH adjustment of final solution.|
or NH3 gas
|For protection of carbon steel storage tanks, if necessary.|
|Cooling water, steam, electric power, instrument air.|
Typical raw materials/utilities
Urea -327.7kg/t (30% UAN solution)
Ammonium Nitrate- 425.7kg/t
Corrosion Inhibitor -1.4kg/t
Steam and electricity may approximate to 10-11KWh/t respectively but are a function of raw material type (solid or solution) and ambient temperature.
No gaseous emissions or waste arise during the non-pressure mixing of the aqueous based components.Emissions to drain are nil provided solid spillages, washings and leaks are collected in a pit or sump and recycled to the process.
Emissions do not arise if BAT is employed. Continuous monitoring of process conditions (e.g. flow, pH, density, temperature and level) ensures optimum control and no emissions. Specific national or local requirements for monitoring may exist.
The manufacture, use, storage, distribution and possession of ammonium nitrate (solid) are subject to legislation. Recommendations for its handling and storage have been issued . The plant inventory of chemicals for pH adjustment (ammonia/nitric acid) will generally be too small to cause a major hazard.
Occupational Health & Safety
The materials for consideration include urea and ammonium nitrate (solids and aqueous solutions), pH adjustment chemicals (ammonia and nitric acid) and corrosion inhibitors. Full details and data for urea are given in Section 7 of this Booklet. Information covering ammonia, nitric acid and ammonium nitrate can be found in EFMA BAT Booklets Nos 1, 2 and 6 respectively.
Summary of BAT Emission
Levels for UAN Solution Technologies
Zero gaseous and liquid emissions are achievable for new as well as for existing UAN solution technologies.
Web Site: http://www.efma.org/Publications/BAT%2095/Bat05/section04.asp