8.2Urea 8.2.1General 1-13
Urea [CO(NH 2)2],also known as carbamide or carbonyl diamide,is marketed as a solution or in
solid form.Most urea solution produced is ud in fertilizer mixtures,with a small amount going to animal feed supplements.Most solids are produced as prills or granules,for u as fertilizer or protein supplement in animal feed,and in plastics manufacturing.Five U.S.plants produce solid urea in
crystalline form.About 7.3million megagrams (Mg)(8million tons)of urea were produced in the U.S.in 1991.About 85percent was ud in fertilizers (both solid and solution forms),3percent in animal feed supplements,and the remaining 12percent in plastics and other us.8.2.2Process Description 1-2
The process for manufacturing urea involves a combination of up to 7major unit operations.
The operations,illustrated by the flow diagram in Figure 8.2-1,are solution synthesis,solution
concentration,solids formation,solids cooling,solids screening,solids coating and bagging,and/or bulk shipping.
The combination of processing steps is determined by the desired end products.For
example,
Figure 8.2-1.Major area manufacturing operations.
plants producing urea solution u only the solution formulation and bulk shipping operations.Facilities producing solid urea employ the 2operations and various combinations of the remaining 5operations,depending upon the specific end product being produced.
In the solution synthesis operation,ammonia (NH 3)and carbon dioxide (CO 2)are reacted to form
ammonium carbamate (NH 2CO 2NH 4).Typical operating conditions include temperatures from 180to 200°C (356to 392°F),pressures from 140to 250atmospheres (14,185to 25,331kilopascals)NH 3:CO 2molar ratios from 3:1to 4:1,and a retention time of 20to 30minutes.The carbamate is then dehydrated to yield 70to 77percent aqueous urea solution.The reactions are as follows:
(1)
2NH 3
CO 2→NH 2CO 2NH 4
(2)
NH2CO2NH4→NH2CONH2H2O
桃的英文The urea solution can be ud as an ingredient of nitrogen solution fertilizers,or it can be concentrated further to produce solid urea.
The3methods of concentrating the urea solution are vacuum concentration,crystallization,and atmospheric evaporation.The method chon depends upon the level of biuret(NH2CONHCONH2) i
mpurity allowable in the end product.Aqueous urea solution begins to decompo at60°C(140°F)to biuret and ammonia.The most common method of solution concentration is evaporation.
The concentration process furnishes urea"melt"for solids formation.Urea solids are produced from the urea melt by2basic methods:prilling and granulation.Prilling is a process by which solid particles are produced from molten urea.Molten urea is sprayed from the top of a prill tower.As the droplets fall through a countercurrent air flow,they cool and solidify into nearly spherical particles. There are2types of prill towers:fluidized bed and nonfluidized bed.The major difference is that a parate solids cooling operation may be required to produce agricultural grade prills in a nonfluidized bed prill tower.
WPA2Granulation is ud more frequently than prilling in producing solid urea for fertilizer.Granular urea is generally stronger than prilled urea,both in crushing strength and abrasion resistance.There are2 granulation methods:drum granulation and pan granulation.In drum granulation,solids are built up in layers on ed granules placed in a rotating drum granulator/cooler approximately4.3meters(14feet)in diameter.Pan granulators also form the product in a layering process,but different equipment is ud and pan granulators are not commonly ud in the U.S.
The solids cooling operation is generally accomplished during solids formation,but for pan granulation process and for some agricultural grade prills,some supplementary cooling is provided by auxiliary rotary drums.
The solids screening operation removes offsize product from solid urea.The offsize material may be returned to the process in the solid pha or be redissolved in water and returned to the solution concentration process.
Clay coatings are ud in the urea industry to reduce product caking and urea dust formation. The coating also reduces the nitrogen content of the product.The u of clay coating has diminished considerably,being replaced by injection of formaldehyde additives into the liquid or molten urea before solids formation.Formaldehyde reacts with urea to from methylenediurea,which is the conditioning agent.Additives reduce solids caking during storage and urea dust formation during transport and handling.
The majority of solid urea product is bulk shipped in trucks,enclod railroad cars,or barges,but approximately10percent is bagged.
8.2.3Emissions And Controls1,3-7
Emissions from urea manufacture are mainly ammonia and particulate matter.Formaldehyde and methanol,hazardous air pollutants,may be emitted if additives are ud.Formalin TM,ud as a formaldehyde additive,may contain up to15percent methanol.Ammonia is emitted during the solution synthesis and solids production process.Particulate matter is emitted during all urea process.There have been no reliable measurements of free gaous formaldehyde emissions.The chromotropic acid procedure that has been ud to measure formaldehyde is not capable of distinguishing between gaous
formaldehyde and methylenediurea,the principle compound formed when the formaldehyde additive reacts with hot urea.
草率的拼音是什么
Table8.2-1summarizes the uncontrolled and controlled emission factors,by process,for urea manufacture.Factors are expresd in units of kilograms per megagram(kg/Mg)and pounds per ton (lb/ton).Table8.2-2summarizes particle sizes for the emissions.Units are expresd in terms of micrometers(µm).
In the synthesis process,some emission control is inherent in the recycle process where carbamate gas and/or liquids are recovered and recycled.Typical emission sources from the solution synthesi
s process are noncondensable vent streams from ammonium carbamate decompors and parators.Emissions from synthesis process are generally combined with emissions from the solution concentration process and are vented through a common stack.Combined particulate emissions from urea synthesis and concentration operations are small compared to particulate emissions from a typical solids-producing urea plant.The synthesis and concentration operations are usually uncontrolled except for recycle provisions to recover ammonia.For the reasons,no factor for controlled emissions from synthesis and concentration process is given in this ction.
Uncontrolled emission rates from prill towers may be affected by the following factors:娑罗子
(1)product grade being produced,(2)air flow rate through the tower,(3)type of tower bed,and
(4)ambient temperature and humidity.
The total of mass emissions per unit is usually lower for feed grade prill production than for agricultural grade prills,due to lower airflows.Uncontrolled particulate emission rates for fluidized bed prill towers are higher than tho for nonfluidized bed prill towers making agricultural grade prills,and are approximately equal to tho for nonfluidized bed feed grade prills.Ambient air conditions can affect prill tower emissions.Available data indicate that colder temperatures promote the formation of
smaller particles in the prill tower exhaust.Since smaller particles are more difficult to remove,the efficiency of prill tower control devices tends to decrea with ambient temperatures.This can lead to higher emission levels for prill towers operated during cold weather.Ambient humidity can also affect prill tower emissions.Air flow rates must be incread with high humidity,and higher air flow rates usually cau higher emissions.
The design parameters of drum granulators and rotary drum coolers may affect emissions.Drum granulators have an advantage over prill towers in that they are capable of producing very large particles without difficulty.Granulators also require less air for operation than do prill towers.A disadvantage of granulators is their inability to produce the smaller feed grade granules economically.To produce smaller granules,the drum must be operated at a higher ed particle recycle rate.It has been reported that, although the increa in ed material results in a lower bed temperature,the corresponding increa in fines in the granulator caus a higher emission rate.Cooling air passing through the drum granulator entrains approximately10to20percent of the product.This air stream is controlled with a wet scrubber which is standard process equipment on drum granulators.
In the solids screening process,dust is generated by abrasion of urea particles and the vibration of th
e screening mechanisms.Therefore,almost all screening operations ud in the urea manufacturing industry are enclod or are covered over the uppermost screen.This operation is a small emission source;therefore particulate emission factors from solids screening are not prented.
Emissions attributable to coating include entrained clay dust from loading,inplant transfer,and leaks from the als of the coater.No emissions data are available to quantify this fugitive dust source.
Table8.2-1(Metric And English Units).EMISSION FACTORS FOR UREA PRODUCTION
EMISSON FACTOR RATING:A(except as noted)
Appendix B explains the modifications.ND=no data.NA=not applicable.
b No ammonia control demonstrated by scrubbers installed for particulate control.Some increa in ammonia emissions exiting the control device was noted.
c References9,11.Emissions from the synthesis process are generally combine
d with emissions from th
e solution concentration process and vented through a common stack.In the synthesis process,some emission control is inherent in the recycle process where carbamate gas and/or liquids are recovered and recycled.
d EPA test data indicated a rang
e of0.005to0.016kg/Mg(0.010to0.032lb/ton).
e EPA test data indicated a range of4.01to14.45kg/Mg(8.02to28.90lb/ton).
f Reference12.The factors were determined at an ambient temperature of14to21°C
(57to69°F).The controlled emission factors are bad on ducting exhaust through a downcomer and then a wetted fiber filter scrubber achieving a98.3%efficiency.This reprents a higher degree of control than is typical in this industry.
g Only runs2and3were ud(test Series A).
h Reference11.Feed grade factors were determined at an ambient temperature of29°C(85°F)and agricultural grade factors at an ambient temperature of27°C(80°F).For fluidized bed prilling, controlled emission factors are bad on u of an entrainment scrubber.
j References8-9.Controlled emission factors are bad on u of a wet entrainment scrubber.Wet scrubbers are standard process equipment on drum granulators.Uncontrolled emissions were measured at the scrubber inlet.
k EPA test data indicated a range of0.955to1.20kg/Mg(1.90to2.45lb/ton).
m Reference10.
n Reference1.EMISSION FACTOR RATING:E.Data were provided by industry.
Table8.2-2(Metric Units).UNCONTROLLED PARTICLE SIZE DATA FOR
UREA PRODUCTION
Bagging operations are sources of particulate emissions.Dust is emitted from each bagging method during the final stages of filling,when dust-laden air is displaced from the bag by urea.Bagging operations are conducted inside warehous and are usually vented to keep dust out of the workroom area, as mandated by Occupational Safety and Health Administration(OSHA)regulations.Most vents are controlled with baghous.Nationwide,approximately90percent of urea produced is bulk loaded.Few plants control their bulk loading operations.Generation of visible fugitive particles is negligible.
Urea manufacturers prently control particulate matter emissions from prill towers,coolers, granulators,and bagging operations.With the exception of bagging operations,urea emission sources are usually controlled with wet scrubbers.Scrubber systems are preferred over dry collection systems primarily for the easy recycling of dissolved urea collected in the device.Scrubber liquors are recycled to the solution concentration process to eliminate waste disposal problems and to recover the urea collected.
围裙简笔画
Fabric filters(baghous)are ud to control fugitive dust from bagging operations,where humidities are low and binding of the bags is not a problem.However,many bagging operations are uncontrolled.
References For Section8.2
语文六年级下册
1.Urea Manufacturing Industry:Technical Document,EPA-450/3-81-001,U.S.Environmental
Protection Agency,Rearch Triangle Park,NC,January1981.
伏波将军
2. D.F.Bress and M.W.Packbier,"The Startup Of Two Major Urea Plants",Chemical
Engineering Progress,May1977.
3.Written communication from Gary McAlister,U.S.Environmental Protection Agency,Rearch
Triangle Park,NC,to Eric Noble,U.S.Environmental Protection Agency,Rearch Triangle
Park,NC,July28,1983.
冷呵呵4.Formaldehyde U In Urea-Bad Fertilizers,Report Of The Fertilizer Institute’s Formaldehyde
Task Group,The Fertilizer Institute,Washington,DC,February4,1983.
