EPA-600/2-77-107 I
November 1977
Environmental Protection Technology Series
SOURCE ASSESSMENT:
UREA MANUFACTURE
Industrial Environmental Research Laboratory
Office of Research and Development
U.S. Environmental Protection Agency
Research Triangle Park, North Carolina 27711
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RESEARCH REPORTING SERIES
Research reports of the Office of Research and Development. U.S Environmental
Protection Agency, have been grouped into five series. These five broad
categories were established to facilitate further development and application of
environmental technology. Elimination of traditional grouping was consciously
planned to foster technology transfer and a maximum interface in related fields.
The five series are:
1. Environmental Health Effects Research
2. Environmental Protection Technology
3. Ecological Research
4. Environmental Monitoring
5. Socioeconomic Environmental Studies
This report has been assigned to the ENVIRONMENTAL PROTECTION
TECHNOLOGY series. This series describes research performed to develop and
demonstrate instrumentation, equipment, and methodology to repair or prevent
environmental degradation from point and non-point sources of pollution. This
work provides the new or improved technology required for the control and
treatment of pollution sources to meet environmental quality standards.
EPA REVIEW NOTICE
This report has been reviewed by the U.S. Environmental
Protection Agency, and approved for publication. Approval
does not signify that the contents necessarily reflect the
views and policy of the Agency, nor does mention of trade
names or commercial products constitute endorsement or
recommendation for use.
This document is available to the public through the National Technical Informa-
tion Service. Springfield. Virginia 22161.
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EPA-600/2-77-1071
November 1977
SOURCE ASSESSMENT:
UREA MANUFACTURE
by
W. J. Search and R. B. Reznik
Monsanto Research Corporation
1515 Nicholas Road
Dayton, Ohio 45407
Contract No. 68-02-1874
ROAP No. 21AXM-071
Program Element No. 1AB015
EPA Task Officer: Ronald A. Venezia
Office of Energy, Minerals, and Industry
Industrial Environmental Research Laboratory
Research Triangle Park, North Carolina 27711
Prepared for
U.S. ENVIRONMENTAL PROTECTION AGENCY
Office of Research and Development
Washington, DC 20460
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PREFACE
The Industrial Environmental Research Laboratory (IERL) of EPA
has the responsibility for insuring that pollution control tech-
nology is available for stationary sources to meet the require-
ments of the Clean Air Act, the Federal Water Pollution Control
Act, and solid waste legislation. If control technology is un-
available, inadequate, or uneconomical, then financial support is
provided .for the development of the needed control techniques for
industrial and extractive process industries. Approaches con-
sidered include: process modifications, feedstock modifications,
add-on control devices, and complete process substitution. The
scale of the control technology programs ranges from bench- to
full-scale demonstration plants.
The Chemical Processes Branch of the Industrial Processes Division
of IERL has the responsibility for investing tax dollars in pro-
grams to develop control technology for a large number (>500) of
operations in the chemical industries. As in any technical pro-
gram, the first question to answer is, "Where are the unsolved
problems?" This is a determination which should not be made on
superficial information; consequently, each of the industries is
being evaluated in detail to determine if there is, in EPA's
judgment, sufficient environmental risk associated with the
process to invest in the development of control technology. This
report contains the data necessary to make that decision for the
air emissions from urea manufacture.
Monsanto Research Corporation has contracted with EPA to investi-
gate the environmental impact of various industries which repre-
sent sources of pollution in accordance with EPA's responsibility
as outlined above. Dr. Robert C. Binning serves as Program
Manager in this overall program, entitled, "Source Assessment,"
which includes the investigation of sources in each of four
categories: combustion, organic materials, inorganic materials,
and open sources. Dr. Dale A. Denny of the Industrial Processes
Division at Research Triangle Park serves as EPA Project Officer.
In this study of urea manufacture, Dr. Ronald A. Venezia served
as EPA Task Officer.
111
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ABSTRACT
This report describes a study of air emissions from the production
of urea. The potential environmental effects from these emissions
are evaluated.
Urea production in the United States totaled 3.45 x 106 metric
tons in 1975. Major products were urea solution (38%) , granu-
lated solid material (53%), and prilled solid material (9%).
Over 75% of the urea produced is consumed in fertilizers.
Both ammonia and particulates are released to the atmosphere
during the manufacturing process. Major emission points are the
evaporator, prilling tower, and granulator. The evaporator has
the largest emission factor for ammonia, 1.73 g/kg, and the prill
tower has the largest one for particulates, 3.2 g/kg.
Source severities were determined to evaluate potential environ-
mental effects. Source severity is defined as the ratio of the
average maximum ground level concentration of an emission species
to the ambient air quality standard (particulates) or to a re-
duced threshold limit value (ammonia). Severities were between
10 and, 1 for ammonia emissions from the evaporator and gran-
ulator, and between 1 and 0.1 for ammonia emissions from the
prilling tower and particulate emissions from the evaporator,
granulator, and prilling tower.
Emissions from the evaporator and granulator are normally con-
trolled by scrubbers. Prill tower emissions are not controlled.
This report, submitted under Contract No. 68-02-1874 by Mon-
santo Research Corporation under the sponsorship of the U.S.
Environmental Protection Agency, covers the period from March
1976 through September 1977.
IV
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CONTENTS
Preface iii
Abstract iv
Figures vi
Tables vii
Abbreviations and Symbols viii
I . Introduction 1
II Summary 2
III Source Description 5
A. Product description 5
B. Urea process chemistry 9
C. Process description 12
D. Emissions from product shipment 38
IV Emissions 39
A. Selected emissions 39
B. Emission factors 40
C. Environmental effects 47
V Control Technology 60
A. Solution concentration 60
B. Solid formation 61
C. Future considerations 63
D. Potential impact of controls 64
VI Growth and Nature of the Industry 65
A. Present technology 65
B. Emerging technology 65
C. Industry production trends 66
69
References
Appendices
A. Location, population density, and capacity data for 74
urea plants in the U.S. 79
B. Derivation of averaging equations
83
Glossary 85
Conversion Factors and Metric Prefixes
v
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FIGURES
Number
1 Physical states of urea products 7
2 Geographical distribution of urea plants 8
3 Capacity distribution of urea plants 9
4 Effect of excess water on the yield of urea 11
5 Block diagram of urea production process 13
6 ' Once-through urea process 14
7 Partial recycle urea process 15
8 Total recycle urea processes 16
9 CPI-Allied gas recycle urea process 17
10 Inventa gas recycle urea process 19
11 Stamicarbon C02 Stripping urea process 19
12 Montecatini complete recycle urea process 21
13 Pechiney total recycle urea process (oil slurry) 22
14 Inventa liquid recycle urea process 23
15 Mitsui Toatsu total recycle D improved urea
process (Toyo Koatsu) ' 23
16 Stamicarbon total recycle urea process 25
17 SNAM PROGETTI Ammonia Stripping urea process 25
18 Chemico total recycle urea process 27
19 Lonza-Lummus urea process 27
20 Solidification of urea 29
21 Sketch of pan granulator 33
22 Cross section of Spherodizer 34
23 Spherodizer drum - end view 35
24 Vapor pressure and specific gravity of urea
solutions in water 46
25 Source severity for fugitive ammonia emissions
from the bulk loading of urea solutions 52
26 Source severity for fugitive particulate
emissions from the bulk loading of urea solids 53
27 Source severity distribution for ammonia
emissions from the evaporator 54
28 Source severity distribution for particulate
emissions from the evaporator 54
29 Source severity distribution for ammonia and
particulate emissions from the prilling tower 55
30 Source severity distribution for ammonia
emissions from the granulator 55
31 Source severity distribution for particulate
emissions from the granulator 56
VI
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FIGURES (continued).
Number page
32 General distribution of >(/F as a function of
distance from the source showing the two gen-
eral roots to the plume dispersion equation 57
33 Size distribution of all particulates in
prill tower exhaust 62
34 Size distribution of particles <5 ym in
prill tower exhaust 62
35 Chemico Thermo-Urea process 66
36 Urea capacity and production trends 68
TABLES
1 Emission Parameters from Solidification
Processing Steps for an Average Urea Plant 4
2 Domestic Consumption of Urea 6
3 Summary of Total Recycle Systems 28
4 Size Distribution of Urea Prilled Product 32
5 Prill Tower Characteristics 32
6 Screen Analysis of Pan Granulator Product 33
7 U.S. Spherodizer Urea Granulation Plants 35
8 Size Distribution of Screened Urea Product
from a Spherodizer Unit 36
9 Emission Data for Prilling Tower from Source
Tests at Three Urea Plants 44
10 Summary of Emission Factors for Urea Production 48
11 Emission Rates, Time-Averaged Maximum Ground
Level Concentrations, and Source Severities
for Process Operations 51
12 Population Affected by Emissions from Average
Urea Processes 57
13 Particulate Emission Burden from Urea Production
by State 58
Vll
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ABBREVIATIONS AND SYMBOLS
a --exp (B - 530/T)
AAQS —ambient air quality standard
B —constant in Equation 2; depending upon the nature of
the surface; 0.35 z B < 0.40
D —distance from an emission point, m
e —2.72
E . —emission factor, g/kg
F —hazard factor
h —emission height, m
m —total number of samples
MEA —monoethanolamine
n --number of moles
n. --number of test points in the ith sample
p1 —operating pressure in Equation 2, atm
Pl --equilibrium pressure at the operating temperature in
Equation 2, atm
P —pressure of a gas, Pa
P, --dissociation pressure in Equation 3, mm Hg
Q —mass emission rate, g/s
R —gas constant, 8.3 Pa • m3/g mole • K
s1 —surface area of ammonium carbamate deposit in
Equation 2, cm2 _
s. —standard deviation of x. for the ith sample
s —standard deviation x
s —source severity
t —short term averaging time of 3 min
t° —long term averaging time of 1,400 min (24 hr)
T —temperature, K
TLV —threshold limit value
u —average wind speed, m/s
V --gas volume, m3
w —ammonium carbamate formed per hour in Equation 2, g
x". —average of test points in the ith sample
x^ —value of a point in the ith sample
x~J —average of the averages from each sample
Xi, X2 --two general roots of the plume dispersion equation;
distance from the source where x/F - 1-0, m
x —ground level concentration
--time-averaged maximum ground level concentration
--3.14
Vlll
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SECTION I
INTRODUCTION
Urea is produced by reacting carbon dioxide gas with liquid
ammonia and dehydrating the resulting ammonium carbamate. The
solution produced can then be sold directly or solidified into
a prill or granule. The product is then sold in bulk or by the
bag.
Urea is used mainly as a direct application fertilizer or in
a mixture with other fertilizers. Urea can also substitute
for natural protein in high-protein diets consumed by feedlot
beef cattle and dairy cows. Urea also has industrial applica-
tions, primarily as a component in urea-formaldehyde resins.
Section III of this report discusses the many manufacturing
processes used to make urea solution and the various solidifi-
cation processing steps, presenting operating parameters in
many cases.
Section IV presents various emission parameters used to evaluate
the impact of the urea industry on the environment. Emission
factors for the various processing steps, as well as emission
rates, average maximum ground level concentrations, and source
severities for an average urea plant are given.
Section V discusses control technology currently used by and
projected for the urea industry. Removal efficiencies and the
potential impact of these controls on emissions are included in
this section.
The last section of this report considers projected industry
growth trends with special emphasis on its effect on emissions.
Emerging technologies in urea production are also presented in
this section.
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SECTION II
SUMMARY
Total urea production in the United States during 1975 was
3.45 .x 106 metric tons9. Major products were 1.31 x 106 metric
tons of urea solutions (as 100% urea), 3.21 x 10 5 metric tons of
prilled solid material, and 1.82 x 10 6 metric tons of granulated
solid material. Urea was produced at 50 plant sites located in
24 states and 46 counties, parishes, or boroughs.
Urea (COtNI^lz) is produced by the reaction of ammonia and carbon
dioxide to form ammonium carbamate (NI^CC^NHij) which is then
dehydrated to form urea and water. There are over 15 production
methods by which these reactions are carried out. While the
basics of these processes are the same, variations occur in
vessel design, operating conditions, and type and quantity of
recycle of unreacted material. The aqueous solution produced by
these processes contains approximately 70% urea. This solution
may either be sold directly or it may be solidified.
In the solidification process, the urea solution is first con-
centrated by either a crystallizer or evaporator, then solidifed.
If a crystallizer is used, the crystals are melted and then
formed into a solid. If an evaporator is used, it produces a
concentrated solution which is then solidified. In either case,
solid urea is formed by prilling or granulation. Additional
granular strength and packing resistance are obtained by two
methods. In over 50% of the plants, formaldehyde or a phosphate-
based additive is injected into the fluid material before solid
formation. In the second method, the sized solid particles are
coated with a clay substance. This method is used on less than
10% of all solid produced. The finished product is stored in
bulk, shipped in railroad hopper cars or trucks, or bagged in
20.4-kg or 36.3-kg bags. Some urea solution may be transported
to market via pipeline. Also some producers located near major
waterways ship solid product by barge.
An average urea plant is located in a county with a population
density of 100 persons/km2. It has a solution capacity of
117,900 metric tons/year. The urea industry as a whole produces
al metric ton = 106 grams = 2,205 pounds; conversion factors and
metric system prefixes are presented at the back of this report.
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38% of its capacity as solutions, 9% as prilled solid and 53%
as granulated solid. The actual operating schedule for a
particular urea facility is a function of the season in its
geographical region.
Emissions from urea manufacture consist of ammonia and particu-
lates (solid urea) from the following emission points:
• Solution production
Bulk loading of solutions
• Solid formation
Evaporator
Prilling tower
Granulator
Bulk loading of solids
Although the prilling tower and granulator are both emission
sources, they represent alternate rather than sequential process
steps.
As a measure of potential environmental impact, the time-averaged
maximum ground level concentration, x"max / and the source severity,
S, were determined for ammonia and particulate emissions .from
evaporation, prilling, and granulation (based on the average
plant capacity, Table 1). Emissions from the bulk loading of
solutions and solids are not presented in Table 1, since the
source severity does not exceed 1.0 outside the average plant
boundary. S for particulates is the ratio of x~max to ^ne
particulate ambient air quality standard, 260 yg/m3. For
ammonia, the air quality standard is replaced by a reduced
threshold limit value, TLV®; i.e., TLV • 8/24 • 1/100.
Those persons living in the area around an average plant where
the average ground level concentration (x") of an emission species
exceeds the ambient air quality standard or reduced TLV have been
termed the affected population. Values for affected population
also appear in Table 1.
The contribution of particulate emissions from urea plants to
total national and state particulate emissions from all sta-
tionary sources was determined based on a total particulate emis-
sion factor of 0.53 g/kg from an average plant. For the urea
industry the national emission burden is <0.1%. The state
emission burdens for all states are <1.0%.
Existing control technology for the urea industry varies from
plant to plant. In the concentration section, emissions are
controlled by condensing the evaporator overheads and sewering
or selling the product, or by passing the stream through a wet
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TABLE 1. EMISSION PARAMETERS FROM SOLIDIFICATION PROCESSING
STEPS FOR AN AVERAGE UREA PLANT3
Emission parameter Evaporator Prilling tower Granulator
Ammonia
Emission factor,
g/kg 1.73 ± 64% 0.40 ± 84% 0.25 ± 48%
Emission rate, g/s 6.73 1.56 0.97
Xmax, ug/m3 530 30.4 76.6
Source severity 8.82 0.51 1.27
Affected population,
persons/km2 245 0 22
Particulates
Emission factor,
g/kg 0.107 ± 28% 3x2± 17% 0.084 ± 29%
l,i to .
0.20 ± 25%
Emission rate, g/s 0.39 12.4 0.33 to 0.78'
Xmax' ug/m3 30.8 243 25.7 to 61.2
Source severity 0.12 0.94 0.099 to 0.24
Affected population,
persons/km2 0 39 0
Controlled emission parameters are given for the evaporator and
granulator; uncontrolled values for the prill tower, reflecting
normal industry operation.
Controlled emission factors for two types of scrubbers.
scrubber. In the solid formation section, control technology
depends on the solid formation process used. In granulation
processes, wet scrubbers are used to control emissions and
recover entrained urea product. In prilling processes, ^50% of
the industry uses some form of packed wet scrubber for emission
control. The rest of the industry exhausts emissions to the at-
mosphere. Control technology to further reduce emissions has not
been extensively proven. At least six companies are currently
trying to develop or prove effective control technology which
will reduce prill tower emissions.
Urea production is expected to increase at a rate of 4.7% to 8%
per year. As a result of this growth, assuming no additional
controls, emissions from the urea industry will be 32% to 59%
greater in 1978 than in 1972.
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SECTION III
SOURCE DESCRIPTION
A. PRODUCT DESCRIPTION
1. Physical Characteristics and Uses
Urea (CO[NH2l2) is a colorless crystal with a melting point
of 132.7°C. At 20°C, it is soluble in water 1:1 and in methanol
1:6. If urea is heated above 130°C at atmospheric pressure, the
principal products are biuret ([NH2CO]2NH) and ammonia.
Cyanuric acid (NstCOHJs) and ammonia are the principal products
above 170°C (1).
The most common end use for urea, either in liquid or solid form,
is as a fertilizer. Solid urea is combined with other fertilizer
materials to make multinutrient (nitrogen-phosphorus-potassium,
N-P-K) mixed fertilizers. Liquid urea can be used as a direct
application fertilizer or mixed with other liquid fertilizer
materials. The most popular direct application fertilizer is a
mixture of aqueous urea and ammonium nitrate solutions with a
total nitrogen content of 32%. Other fertilizers containing
urea are mixtures of various proportions of urea-ammonium
nitrate-ammonia with total available nitrogen content of 32%.
Other fertilizers containing urea are mixtures of various pro-
portions of urea-ammonium nitrate-ammonia with total available
nitrogen content of 37% to 49%. Other source assessment
(1) Mavrovic, I. Urea & Urea Derivatives. In: Kirk-Othmer
Encyclopedia of Chemical Technology, Second Edition,
Vol. 21. John Wiley & Sons, Inc., New York, New York, 1969.
pp. 37-56.
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documents (2-4) discuss the production of mixed fertilizers,
ammonium nitrate, and ammonia.
Overall urea consumption falls into three broad categories, as
shown in Table 2: fertilizers, livestock feed, and industrial
feedstock (5, 6).
TABLE 2. DOMESTIC CONSUMPTION OF UREA
Percent of
total consumption
End use Reference 5 Reference 6
Fertilizers
Solid fin 39.4
Liquid D 38.0
Livestock feeds 25 7.7
Industrial feedstock
Urea-formaldehyde resins 10 ,Q ,
Melamine j.
Other uses 4.8
Consumption statistics based on data supplied by producers to the
U.S. International Trade Commission (6) differ from other re-
ported data (5), and are based on the hypothesis that a signifi-
cant portion of the urea sold as fertilizer is actually used as
livestock feed. There is no way at present of verifying this
supposition.
(2) Rawlings, G. D., and R. B. Reznik. Source Assessment:
Fertilizer Mixing Plants. EPA-600/2-76-032c, U.S. Environ-
mental Protection Agency, Research Triangle Park, North
Carolina, March 1976. 201 pp.
(3) Search, W. J., and R. B. Reznik. Source Assessment:
Ammonium Nitrate Production. EPA-600/2-77-107i, U.S.
Environmental Protection Agency, Research Triangle Park,
North Carolina, September 1977. 78 pp.
(4) Rciwlings, G. D. , and R. B. Reznik. Source Assessment:
Synthetic Ammonia Production. EPA-600/2-77-107m, U.S.
Environmental Protection Agency, Research Triangle Park,
North Carolina, November 1977. 82 pp.
(5) Urea. Chemical Marketing Reporter, 210(5) :9, 1976.
(6) Synthetic Organic Chemicals, United States Production and
Sales, 1975. USITC Publication 804, U.S. International
Trade Commission, Washington, D.C., 1977. p. 195.
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Urea is used in livestock feed as a substitute for natural
protein in high-protein diets consumed by feedlot beef cattle and
dairy cows. It is also used in urea-formaldehyde resins, and it
is becoming more popular as a replacement for dicyandiamide,
which was previously the primary raw material in melamine
production.
2. Production
In 1975, 50 plants produced 3.45 x 106 metric tons of urea in
original solution, i.e., as 100% urea (6). The average plant
production rate was 69,000 metric tons/yr. No statistics are
available on the exact amounts of urea produced as liquid or
solid final product. It is therefore assumed that the total
liquid final product is equal to that used in liquid fertilizers,
1.31 x 105 metric tons (6). Any amount of solution consumed in
other applications is expected to be offset by the amounts of
solid product used to make liquid fertilizers. Of the solid
produced, approximately 15% (3.21 x 105 metric tons) is marketed
as solid prilled product and 85% (1.82 x 106 metric tons) as
solid granules (Figure 1). The derivation of this production and
marketing breakdown is presented in a later section of this
report.
PERCENTAGES RERECT WEIGHT PERCENT
Figure 1. Physical states of urea products.
Prilled urea is of two types: standard and microprills. These
differ in size, nitrogen content, level of coating, application,
and end use. The prill size is governed by the spraying device
used. Standard prilled urea contains 45% to 46% nitrogen.
Microprills are coated, however, and their nitrogen content drops
to 42% due to the extra weight added by the coating. Much of the
microprill production is used in animal feed because their
blending characteristics are better than those for standard
prills. Standard prills and microprills are produced in the same
manner. Molten urea is sprayed into a tower countercurrent to a
cooling stream of air. The drops of urea solidify and form a
surface crust as they fall through the air.
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Granular urea is manufactured by spraying a stream of molten
urea or urea solution (99.5+% urea) on a tumbling bed or falling
curtain of solid urea particles. The granules are thus formed in
layers;. Granular urea is used mainly as a fertilizer component
because of its high nitrogen content and its particle size. The
rugged particles result in less degradation and dustiness during
handling and transport.
3. Geographical Distribution
Table A-l in Appendix A lists the locations (city, county, state),
county population densities, and original solution capacities
for the 50 plant locations responsible for 1975 production (7).
Figure 2 shows the geographical distribution of these plants and
indicates those facilities having more than one urea processing
line. (Union Oil Company of California's plant in Kenai, Alaska,
is not shown.) The plants are located in 46 counties, parishes,
or boroughs in 24 states. The average county population density
is 100 persons/km2 with a range of 0.5 persons/km2 to
696 persons/km2.
EACH DOT REPRESENTS A PUNT FACILITY;
AN OPEN DOT (o) REPRESENTS TWO
MANUFACTURING UNITS AT THE SAME
PLANT FACILITY.
Figure 2. Geographical distribution of urea plants.
Figure 3 illustrates the capacity distribution of the urea
plants whose solution capacities are listed in Table A-l. It
can be seen that 50% of the plants have a capacity at or below
(7) itforld Fertilizer Capacity--Urea. Tennessee Valley Authority,
.Muscle Shoals, Alabama, June 7, 1976. 6 pp.
8
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Figure 3. Capacity distribution of urea plants.
75,000 metric tons/year with a statistical mean of 117,900 metric
tons/year. Approximately 58% of the plants fall below the
average size.
B.
UREA PROCESS CHEMISTRY
Urea is produced by reacting liquid ammonia and carbon dioxide
and dehydrating the resulting ammonium carbamate. A number of
variables affect the degree of completion of this process.
1. Formation of Ammonium Carbamate • .
Anhydrous ammonia and carbon dioxide react as follows to form
ammonium carbamate:
2NH
(1)
This reaction is exothermic and spontaneous, liberating 152.1 kilo-
joules of heat per mole of ammonium carbamate at constant
volume or 158.0 kj/mole to 159.5 kJ/mole at constant pressure (8).
(8) Strelzoff, S., and L. H. Cook. Nitrogen Fertilizers. In:
Advances in Petroleum Chemistry and Refining - Volume 10,
J. J. McKette, Jr., ed. John Wiley and Sons, Inc., New
York, New York, 1976. pp. 315-406.
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The rate of formation of carbamate increases approximately as
the square of the pressure, all other conditions being constant.
An equcition to approximate this rate is given (8) by:
w = as1 (p2 - p!2) (2)
where w = ammonium carbamate formed per hour, g
a = exp (B - 530/T)
s1 = surface of ammonium carbamate deposit, cm2
p = operating pressure, atm
Pl = equilibrium pressure at the operating temperature, atm
T = operating temperature, K
B = constant, depending upon the nature of the surface
(0.35 < B ^ 0.40)
The activation energy for the ammonium carbamate formation reac-
tion is approximately 10.1 kJ/mole.
At atmospheric pressure solid ammonium carbamate will sublime
at temperatures as low as 60 °C, producing a mixture of ammonia
and carbon dioxide. Ammonium carbamate vapor dissociation/evap-
oration is the reverse of Equation 1; the "vapor pressure" of
ammonium carbamate is also the dissociation pressure. This
dissociation pressure is reported (8) to follow the equation:
log Pd = 11.1448 - 2,741.9/T (3)
where P^ = dissociation pressure, mm Hg
T = temperature, °K
As a result of this high dissociation pressure, ammonium carba-
mate is most readily formed in the absence of water under high
temperature and pressure. The importance of the elevated
pressure is demonstrated by the fact that if the operating pres-
sure is not substantially higher than the equilibrium dissocia-
tion pressure at the operating temperature, the reaction in
Equation 1 will shift to the left.
2. Formation of Urea from Ammonium Carbamate
Ammonium carbamate is converted to urea and water by the follow-
ing reaction with a heat input of 32.2 kJ/mole of urea:
± CO(NH2)2 + H20 (4)
The water formed in the reaction limits the formation of urea
solutions to a maximum of 76.93% urea by weight (8).
Units are shown as they were reported (8)
10
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The rate of formation of the ammonium carbamate is highly de-
pendent upon pressure. The conversion of ammonium carbamate to
urea, on the other hand, is favored by high temperature. The
overall yield of urea is pressure sensitive.
3. Effects of Process Variables
According to Equation 1, the molar ratio of ammonia to carbon
dioxide should be 2:1. However, experiments in which two moles
of ammonia and one mole of carbon dioxide were combined in an
autoclave gave a urea yield of only 43.5%. Further experiments
have shown that the yield can be improved by two methods: (1)
raising the temperature and pressure and (2) by manipulating
the molar ratio. Experiments have shown that if the excess
ammonia is raised from 0% to 279%, the urea yield will increase
from 43.5% to 85.2%. Raising the excess CC>2 by 300%, on the
other hand, only increases the yield to 46% (8).
Excess water affects equilibrium yield (as shown in Figure 4)
as well as overall functioning of the urea producing unit. It
reduces the unit's productive capacity and increases the quantity
of water to be evaporated, thus consuming more energy.
As a result, urea manufacturing facilities try to keep the molar
ratio of water to ammonium carbamate as low as is economically
and/or technically feasible. To achieve this goal at least 20
plants utilize some of the 127 kj/mole of waste heat available
from the reactions involved.
0.2 0.4 0.6 0.8 1.0 1.2
MOLES OF WATER ADDED PER MOLE
OF AMMONIUM CARBAMATE
Figure 4. Effect of excess water on the yield of urea (8)
The quantity of urea which may be obtained from the overall
reaction
11
-------
2NH3 + C02?=> CO(NH2) 2 + H20 (5)
is dependent upon at least seven variables: temperature, pressure,
and the; concentrations of NH3, C02, CO(NH2)2, H20, and NH2C02NH4
(9) . When the reaction temperature, the ammonia/carbon dioxide
feed mole ratio, and the water/carbon dioxide feed mole ratio are
known, the equilibrium yield of urea can be calculated (8) .
Urea decomposes at 60 °C in an aqueous solution and the decompo-
sition rate accelerates as temperature rises. Dry urea, on the
other hand, is more stable, especially below 130 °C. Decompo-
sition products of urea include ammonia, biuret, cyanuric acid,
and carbon dioxide.
Biuret is formed when solid urea is heated above 130 °C at
atmospheric pressure:
2CO(NH2)2 -»• H2NCONHCONH2 + NH 3 (6)
This reaction does not occur, however, under strong partial
pressure of ammonia, such as during urea synthesis in an
autoclave .
The concentration of biuret because it is phytotoxic (i.e.,
poisonous to plants) is kept below 0.5% in fertilizer-grade
urea. Most biuret formation and decomposition occurs in the
preparation of the solid urea product. The formation of biuret
can be controlled, however, by holding the temperature below
130°C..
C. PROCESS DESCRIPTION
Urea is produced by the reaction of liquid ammonia and carbon
dioxide gas at approximately 175°C to 200°C and 19.2 MPa to
23.2 MPa to form ammonium carbamate, which is subsequently dehy-
drated to form urea and water. The solution then goes to various
solidification processes or is sold directly as shown in
Figure 5.
This document defines urea manufacturing process as the sum of
all functions which take place in the production of urea from
the point where the reactants leave the feed pipe to go into a
compressor or temporary holding tank until the solid and/or
liquid products leave the plant. This assessment therefore
includes compressors, recycle/holding tanks, reactors, con-
densers, absorbers, and various decomposers and strippers, where
applicable, for the formation of the urea solution. In the
(9) Frejacques, M. Les bases theoriques de la syntheses indus-
trielle de 1'uree. Chimie & Industire, 60(l):22-35, 1948.
12
-------
PRODUCT SHIPMENT
SOLUTION PRODUCTION
SECTION
SOLID PRODUCTION
SOLUTION
PRODUCTION
SOLUTION
CONCENTRATION
REC
SOLID
FORMATION
iTLE
FINISHING
»
BULK LOADING
co
%
O
UREA
SHIPMENT
AMMONIA
CARBON
DIOXIDE
Figure 5. Block diagram of urea production process.
solid product formation process, evaporators or crystallizers
and the actual particle forming apparatus (i.e., prilling tower
or granulator) are included. In addition, any conveying, sizing,
drying, or cooling equipment employed before the product leaves
the plant is discussed.
1. Solution Production
The basic process chemistry of urea manufacturing is relatively
simple. However, because operating parameters vary, particularly
in the initial formation of the urea solution, numerous process
designs have been utilized. Design differences occur in the
separation and recycle of component streams. There are three
major classes of urea processes, based on the type or quantity
of recycle: once-through processes, partial recycle processes
and total recycle processes.
Only a few once-through and partial recycle units have been con-
structed, the trend being toward total recycle systems (10).
At least 75% of the urea produced today is by total recycle
systems.
a. Once-Through Processes—
Figure 6 is a generalized flow diagram of a once-through urea
process. While the same variables apply to once-through processes
(10) Slack, A. V. Fertilizer Developments and Trends. Noyes
Development Corporation, Park Ridge, New Jersey, 1968.
pp. 119-145.
13
-------
EXPANSION
VALVE
r-»{x]
COMPRESSOR
REACTOR
UREA
EXCESS NH3
CARBAMATE
3, C02,
CARBAMATE
DECOMPOSER
•UREA SOLUTION
PRODUCT-CONTAINING STREAMS
RECYCLE, FEED, OR OTHER ANCILLARY STREAMS
Figure 6. Once-through urea process.
as to the two recycle systems, the minor variations in actual
process operation do not warrant individual discussion. There-
fore a representative process description will be presented.
In once-through urea processes, ammonia and carbon dioxide are
fed to the reactor and held at 175°C and 20.7 MPa for approxi-
mately 30 minutes. Approximately 40% to 45% of the carbamate
will be converted to urea and water, assuming the feed streams
of ammonia and carbon dioxide are in the molar ratio of 2:1 to
3:1, respectively (10).
The reactor effluent passes through an expansion valve where the
pressure is reduced so that residual carbamate can be decomposed
to ammonia and carbon dioxide. This process takes place in a
carbariate decomposer and the resulting mixture is separated into
two streams - aqueous urea and mixed gases (ammonia, carbon
dioxide, and water). The aqueous urea goes for further processing
or shipment. The gas stream goes directly to another fertilizer
manufacturing facility as a feed stream and is therefore not
considered a source of emissions.
b. Partial Recycle Process—
A refinement of the once-through process yields a partial recycle
process as shown in Figure 7. This process is termed partial
recycle because only excess ammonia is recovered and recycled to
the reactor.
The synthesis is carried out with as much as 200% excess ammonia.
This has been shown to give an equilibrium yield of urea of at
least 80% (8). The reactor operating conditions are 175°C and
27.7 MPa with a residence time similar to that for the once-
through process.
This process is also similar to that of the once-through process,
with one additional step (Figure 7): the reactor effluent
containing urea, ammonium carbamate, water, and excess ammonia
14
-------
NH3 RECYCLE
C.W. jgf"
REACTOR
EXPANSION
VALVE /
r-ixj—**
LIQUID NH-
UREA
EXCESS NH3
CARBAMATE
EXCESS
AMMONIA
SEPARATOR
, C0
UREA
CARBAMATE
H;0
•A
CARBAMATE
DECOMPOSER
UREA SOLUTION
COMPRESSOR
PRODUCT-CONTAINING STREAMS
RECYCLE, FEED, OR OTHER ANCILLARY STREAMS
Figure 7. Partial recycle urea process.
passes through an expansion valve reducing the pressure to a few
hundred kilopascals depending on the particular process design.
The stream then goes to an ammonia separator where excess ammonia
is removed, condensed, and recycled to the reactor. This is
necessary to recover some of the cost of using excess ammonia.
Also, if passed directly to the carbamate decomposer, the excess
ammonia could hinder the decomposition of the carbamate.
The stream containing urea, carbamate, and water goes to a
carbamate decomposer which dissociates the carbamate to ammonia
and carbon dioxide. The aqueous urea solution is separated and
goes to further processing or shipment. The gas stream is sent
to an adjunct facility as a reactant feed stream. It is not
considered an emission point since no gas is emitted to the
atmosphere.
c. Total Recycle Process—
The total recycle process is currently believed to incorporate
the maximum operational refinements in the urea manufacturing
industry and is therefore the most widely used basic process in
the industry. Three variations of the total recycle process are
1) decomposed carbamate gases are separated and recycled in
their pure states, 2) carbamate solution is recycled to the
reactor, and 3) a combination gas/liquid recycle may occur.
Incorporated in these three basic categories are at least 10
major company designs.
Figure 8 is an extreme simplification of the basic process dif-
ferences in the three total recycle systems; gas recycle, liquid
recycle, and gas/liquid recycle. These diagrams present general
conceptual material flows and do not describe any particular
processes.
15
-------
GAS RECYCLE
J,
K
REACTOR
i
DECOMPOSER
UREA
SOLUTION
FEED-
A. Basic gas recycle process
CARBAMATE RECYCLE
-^
ft
REACTOR
CONDENSER
'
I GASES
SEPARATOR
ADDITIONAL
LIQUID
UREA
SOLUTION
FEED
B. Basic liquid recycle process
NH3 RECYCLE
r
K
t t
REACTOR
^
SEPARATOR
*
DECOMPOSER
UREA
} SOLUTION
FEED
CARBAMATE RECYCLE
C. Basic gas / liquid recycle process
— PRODUCT-CONTAINING STREAMS
RECYCLE, FEED, OR OTHER ANCILLARY STREAMS
Figure 8. Total recycle urea processes.
Figure 8a is the basic flowsheet for a gas recycle process.
The material leaving the reactor is a mixture of urea, ammonium
carbamate, water, and excess ammonia. This stream goes to a de-
composer which separates the carbamate into ammonia and carbon
dioxide. The separated gases may both be recycled, or one may be
purified at the expense of the other and returned to the process.
The latter resembles a partial recycle system as diagrammed in
16
-------
Figure 7. In the partial recycle process, however, the excess
ammonia is recovered and the ammonia or carbon dioxide in the
unreacted carbamate is lost to the process. In the total recycle
process, the entire quantity of ammonia is reused; i.e., excess
plus decomposed carbamate ammonia. Two examples of the gas
recycle process will be discussed: the CPI-Allied and Inventa
processes.
Figure 8b shows the basic flowsheet for a liquid recycle process.
This process is similar to the gas recycle process except that
the gases are condensed with the addition of water when needed,
to form a carbamate solution for recycle. Those processes which
will be discussed in this category are the Stamicarbon C02
Stripping, Montecatini, Pechiney, and Inventa processes.
Figure 8c gives the basic flowsheet for a gas/liquid recycle
process. It is characterized by ammonia recycle with carbon
dioxide being recycled in the form of carbamate. The following
processes of this type will be considered: Mitsui Toatsu (Total
Recycle D Improved), Stamicarbon, SNAM PROGETTI, Chemico, and
Lonza-Lummus.
(1) Gas recycle processes—The CPI-Allied process, as shown in
Figure 9, employs a corrosion-resistant zirconium-lined reactor
which permits higher operating temperatures, i.e., from 194°C
to 233°C at 30.3 MPa (11, 12). The conversion rate per pass is
80% to 85%.
PRODUCT-CONTAINING STREAMS
RECYCLE. FEED, OR OTHER ANCILLARY STREAMS
MAKEUP
NH.FROM STORAGE
C02 FROM
STORAGE
COMPRESSOR
c.w.
REACTOR
NH.
PRIMARY CARBAMATE
EXPANSION VALVE DECOMPOSER/
LIQUID NH,
UREA
EXCESS NH3
CARBAMATE
CO,
COMPRESSOR
LEAN SOLVENT
•-UREA SOLUTION
Figure 9. CPI-Allied gas recycle urea process.
(11) CPI-Allied Chemical Urea Process. Nitrogen, 47:32-33,
May/June 1967.
(12) Urea Processes Today. Nitrogen, 64:17-24, March/April 1970.
17
-------
This process can be used as a once-through, partial, or total
recycle system. The ammonia and carbon dioxide enter the reactor
in a ratio of 4:1 to 4.5:1. The excess ammonia inhibits the
formation of biuret at the reactor conditions. The carbon
dioxide conversion to urea is approximately 85%.
The reactor products pass through an expansion valve to a pri-
mary carbamate decomposer where 90% of the carbamate is flashed
and stripped along with water vapor. The urea solution contains
approximately 1.5% of the initial carbon dioxide feed. This
stream is sent to an ammonia separator, where excess ammonia is
stripped, and on to a secondary decomposer where any remaining
carbamate dissociates at atmospheric pressure.
The overheads from both decomposers are passed through a two-unit
series of absorbers where monoethanolamine (MEA) selectively ab-
sorbs carbon dioxide and water, leaving ammonia for recycle to
the reactor. The carbon dioxide-rich solvent is sent to a strip-
per which thermally regenerates the MEA creating a rich carbon
dioxide stream which is recycled to the reactor. The urea solu-
tion leaving the secondary decomposer passes through a centrifu-
gal Min-film evaporator unit. The product contains less than
0.7% bitiret and 0.20% water.
The Inventa process (see Figure 10) utilizes a reactor operating
at 20.2 MPa and 180°C to 200°C (13). The molar feed ratio of
ammonia to carbon dioxide is 2:1 with a maximum carbon dioxide
conversion to urea of 50% (13, 14). The reactor effluent con-
taining excess ammonia, ammonium carbamate, urea, and water
passes through an expansion valve where it is lowered to 549 kPa
and heated to 120°C in the carbamate decomposer (15). The
ammonia and carbon dioxide go to an absorber where the ammonia is
selectively absorbed and the carbon dioxide exits for recycle.
The resulting ammoniacal solution of ammonium carbamate goes to a
desorber to remove ammonia for recycle.
(2) Liquid recycle processes—Figure 11 is a flow diagram of the
Stamicarbon CC>2 Stripping urea process. Ammonia and carbon
dioxide are reacted in the molar ratio of 2.4:1 to 2.9:1 at 170°C
to 190°C and 12.1 MPa to 15.1 MPa (12). The reaction product
(185°C, 14.1 MPa) goes immediately to a high pressure stripper,
(13) Cook, L. H. Urea. Chemical Engineering Progress,
50(7):327-331, 1954.
(14) Swiss Solve Urea Problems. Chemical Engineering, 59(11):
219-220, 222, 1952.
(15) Tonn, W. H., Jr. How the Competitive Urea Processes Com-
pare Today. Chemical Engineering, 62 (10):186-190, 1955.
18
-------
PRODUCT-CONTAINING STREAMS
RECYCLE. FEED, OR OTHER ANCILLARY STREAMS
C.W.
I
COMPRESSOR
REACTOR
C02 RECYCLE
EXPANSION VALV
r—-*}
V
UREA
EXCESS NHj
CARBAMATE
H,0
1
NH3' V
co2
H20
h
\ABSORBER
) SOLVENT
—/'*
AMMONIACAL SOLUTION
LIQUID NH
CO,—
LIQUID NH.
COMPRESSOR
CO,
CARBAMATE c-w-
DECOMPOSER
• UREA SOLUTION
-AQUEOUS UREA
NITRATE
DESORBER
-H20
Figure 10. Inventa gas recycle urea process.
EXHAUST
SCRUBBER
LIQUID NH
CARBAMATE RECYCLE
II STEAM
( THIGH PRESSURE
A J CONDENSER
.^p
COMPRESSOR
EXPANSION
VALVE
PRODUCT-CONTAINING STREAMS
RECYCLE. FEED, OR OTHER ANCILLARY STREAMS
LOW PRESSURE
DECOMPOSER
c vv LOW PRESSURE
CONDENSER
FLASH TOWER
•• UREA
SOLUTION
Figure 11. Stamicarbon C02 Stripping urea process.
19
-------
operating at 14.1 MPa and 190°C (1), where the reactor stream is
stripped by incoming carbon dioxide. The stream containing 15%
unconverted carbamate is then let down for further decomposition
in the low pressure decomposer operating at SOOkPa and 120°C.
The ammonia and carbon dioxide are condensed in the low pressure
condenser operating at SOOkPa and 60°C. The solution is pumped
to the high pressure condenser where it combines with the off-gas
from the high pressure stripper and a split from the ammonia
feed line. The condensed stream from the high pressure con-
denser operating at 170°C and 14.1 MPa, goes to the reactor.
An equivalent amount of 345 kPa steam is produced in the high
pressure condenser and is used in other sections of the plant.
This process claims an ammonia and carbon dioxide consumption
of 0.57 metric ton and 0.755 metric ton per metric ton of urea
produced, respectively. Conversion efficiencies for ammonia
and carbon dioxide are 65% to 85% and 70% to 85%, respectively.
Figure 12 is a flow diagram of the Montecatini process (Montedi-
son) in which preheated liquid ammonia and carbon dioxide are
compressed to 20.2 MPa and enter the reactor operating at 195°C
(12, 16). The reactor mole ratio for NH3:C02 is 3.5:1; for
H20:C02 it is 0.6:1 (17). The effluent containing urea, excess
ammonia, ammonium carbamate, and water enters a first-stage de-
composer/separator operating at 8.1 MPa and 185°C. In this
decomposer/separator most of the ammonia is driven off along
with the carbamate decomposition products. This stream, along
with 20% to 30% of the carbon dioxide feed stream, is fed to the
first-stage carbamate condenser which operates at 8.1 MPa and
145°C.
The effluent from the first-stage condenser passes to an auxiliary
condenser operating at the same pressure but at 115°C so that
condensation is completed. The gas leaving this condenser is
washed to remove ammonia. The liquid stream is recycled to the
reactor.
The lic;uid stream leaving the first-stage decomposer/separator
proceeds to a second-stage unit operating at the same temperature
as stage one and 1.2 MPa, and finally to a third stage operating
at 202 kPa to 303 kPa before leaving the facilities. The
gaseous effluents from the stage two and three decomposer/
separators are condensed in carbamate condensers three and four,
respectively. In condenser three the gas stream is mixed with
(16) Borelli, T., and G. Nardin. Precede Montecatini Edison
pour la production d'uree de gros tonnage. Chimie et
Industrie - Genie Chimique, 104 (16):2017-2022, 1971
(17) Montecatini Edison's New Total Recycle Urea Process. Hydro-
carbon Processing, 49(8):111-112, 1970.
20
-------
DECOMPOSER / SEPARATORS
REACTOR
LIQUID NH
UREA
*-SOLUTION
AMMONIA
BEARING GAS
FROM
SOLIDIFICATION
C.W.
CARBAMATE RECYCLE
CARBAMATE
CONDENSERS
PRODUCT-CONTAINING STREAMS
RECYCLE, FEED.OR OTHER ANCILLARY STREAMS
Figure 12. Montecatini complete recycle urea process.
liquid effluent from both wash vessels and the liquid carbamate
is sent to the first-stage condenser. In condenser four an
ammonia-bearing gas stream from the solidification section is
washed with cold ammonium carbamate, and the resulting effluent
is cycled through a wash vessel before going to condenser three.
In the Pechiney total recycle process (Figure 13), a neutral
mineral is incorporated to act as a "heat sponge," as a medium
for recycling unreacted ammonia and carbon dioxide, and as a
corrosion inhibiter. Ammonia and carbon dioxide are fed to the
reactor operating at 20.2 MPa to 20.6 Mpa and 177°C to 183°C
(13, 15, 18). In the carbamate stripper and decomposer, 40% to
50% conversion of carbamate to urea is achieved; the remaining
carbamate is decomposed to ammonia and carbon dioxide which go to
an agitated slurry vessel where they recombine to form ammonium
carbamate in an oil slurry. This slurry is then recycled to the
reactor. The urea-oil mixture leaving the carbamate stripper and
decomposer goes to an oil decanter which separates the oil from
the mixture to supply the slurry vessel. The urea goes for
further processing.
(18) Urea Via the Pechiney Process.
62(4):320-323, 1955.
Chemical Engineering,
21
-------
OIL-CARBAMATESLURRY
LIQUID NH
CARBAMATE
STRIPPER
AND DECOMPOSER
UREA SOLUTION
PRODUCT-CONTAINING STREAMS
RECYCLE, FEED, OR OTHER ANCILLARY STREAMS
Figure 13. Pechiney total recycle urea process (oil slurry).
In addition to the gas recycle process previously described,
Inventa also has a liquid recycle process, as shown in Figure 14.
In this process, the reactor operates at 185°C and 23 MPa with a
molar ratio of NH3/C02 = 3.5 to 4.0/1 and a conversion of 62% to
65% (1). The reactor effluent is expanded to 505 kPa and heated
to 120°C, after which the liberated ammonia and carbon dioxide
are fed to a primary absorber (at the same pressure but at 50°C)
where an aqueous ammoniacal carbamate solution is formed. This
solution goes through a heat exchanger to remove the heat of
reaction before being sent to the desorber operating at 140°C to
150°C and 5.05 MPa where the carbamate is once again decomposed
to remove some of the water before recycling to the reactor. The
absorber gaseous effluent is passed to the secondary absorber
operating at 5.05 MPa and 90°C where it is condensed to the
carbamate solution and sent to the reactor. Water resulting from
the operation may either be used elsewhere in the plant or
sewered,, depending on the particular installation.
(3) Gas/liquid recycle processes—The Mitsui Toatsu urea process
is actucilly several processes in one: 1) once-through process
with a carbon dioxide conversion of 82% and ammonia conversion of
34%, 2) an ammonia partial recycle (56% conversion of NHs) or
3) a partial recycle with 80% carbon dioxide conversion and 70%
22
-------
ammonia conversion. There are also two total recycle processes
and an integrated process where the ammonia synthesis loop and
the urea facilities are tied together for heat economy,(19).
Shown in Figure 15 is the Mitsui Toatsu Total Recycle D Improved
urea process. Another total recycle process, labeled "C," does
not incorporate the medium pressure decomposer and absorber but
LIQUID NH
UREA SOLUTION
PRODUCT-CONTAINING STREAMS
RECYCLE, FEED, OR OTHER ANCILLARY STREAMS
Figure 14. Inventa liquid recycle urea process.
LIQUID NH,
~XJ^— —
NH3 RECYCLE
NH, C02
REACTOR
j:
*
— »
UREA
EXCESS NHj
CARBAMATE
HO
L
HIGH \
PRESSURE
ABSORBER
HIGH
PRESSURE
DECOMPOSER
CO,
MEDIUM
PRESSURE
ABSORBER
MEDIUM
PRESSURE
DECOMPOSER
LOW
PRESSURE GAS
ABSORBER CONDENSER
MOTHER LIQUOR
WATER
LOW PRESSURE
DECOMPOSER
GAS SEPARATOR
CARBAMATE RECYCLE
UREA SOLUTIONS
Figure 15.
PRODUCT-CONTAINING STREAMS
RECYCLE, FEED, OR OTHER ANCILLARY STREAMS
Mitsui Toatsu total recycle D improved urea
process (Toyo Koatsu).
(19) Urea (Mitsui Toatsu Process) - The M. W. Kellog Co.
Hydrocarbon Processing, 50(11):215, 1971.
23
-------
is otherwise the same (20). In the process shown, the reactor is
fed with a molar ratio of ammonia to carbon dioxide of 4.3:1 and
operated at 190°C and 24 MPa. Approximately 67% conversion to
urea is obtained (1).
The reactor effluent goes to the high pressure decomposer, oper-
ating at 150°C and 1.8 MPa then to the medium and low pressure
units operating at 410 kPa and 130°C, and 101 kPa and 120°C,
respectively (20). The gaseous effluents from these decomposers
go to their respective absorbers where the ammonia and carbon
dioxide contact with the condensate from the previous stage re-
sulting in a lower water content in the carbamate recycle. In
the high pressure absorber, excess ammonia is stripped for recycle,
and the condensed carbamate is recycled to the reactor.
The Stamicarbon total recycle process is shown in Figure 16 (20).
The reaction takes place at 20.2 MPa and 170°C to 190°C. The
reactor effluent is lowered to approximately 505 kPa before
going to the preseparator. The liquid stream from the presep-
arator passes through two additional separation steps before
finally leaving the process. The various ammonia and carbon
dioxide streams are condensed, and the carbamate formed is re-
cycled to the reactor. A wet scrubber is used on the gas stream
to recover ammonia for recycle.
The SNAM PROGETTI urea process, as shown in Figure 17, is similar
to the Stamicarbon CC>2 stripping process, but the stripping
is done by ammonia rather than carbon dioxide. SNAM PROGETTI
also has a process which can be integrated with ammonia synthesis
(21). The process, as shown, can operate at two different
reactor pressures, 13 MPa to 16 MPa or 20.2 MPa to 25 MPa (1, 12,
22). The normal operating temperature in the reactor is 180°C to
190°C. The normal molar feed ratio of ammonia to carbon dioxide
is 3.3 to 3.5:1 (23, 24). The feed is obtained from carbamate
(20) Yoshimura, S. Optimize New Urea Process. Hydrocarbon
Processing, 49 (6):111-115, 1970.
(21) Pagani, G. , and U. Zardi. Integrate for Lowest Urea Cost.
Hydrocarbon Processing, 5 (11) :125-129, 1972.
(22) Pagani, G., and U. Zardi. SNAM PROGRETTI Stripping Tech-
nique: One Basic Principle for Two Methods of Producing
Urea. Presented at the 74th National Meeting of the
American Institute of Chemical Engineers, New Orleans,
Louisiana, February 1973. 16 pp.
(23) Zardi, U., and F. Ortu. Recycle Carbamate Via Ejector.
Hydrocarbon Processing, 49 (4):115-116, 1970.
(24) Urea - SNAM PROGRETTI. Hydrocarbon Processing, 54(11)210,
1975.
24
-------
PRODUCT-CONTAINING STREAM
— RECYCLE. FEED. OR OTHER ANCILLARY STREAMS
RECIRCULATION SECONDARY
HEATER SEPARATOR
V
1ST STAGE
V
2ND STAGE
UREA
SOLUTION
Figure 16. Stamicarbon total recycle urea process.
LIQUID NHj
FROM STORAGE
• PRODUCT-CONTAINING STREAMS
• RECYCLE, FEED, OR OTHER ANCILLARY STREAMS
NH3 RECYCLE
_ '
CARBAMATE
CONDENSER ,
V
i UREA
1 EXCESS NH,
REACTOR \ /
^
L M
p
— -J *7t\ LIQUID NH3
rn mJ \ •-
' CA
H2
J
;
co2
RBAMATE
0
?r
VAPORIZER
CARBAMATE
1 / \ NH, KELOVI-'HV
T A A" 3
NH3, C02
/^N MIXED GASES
j r\ \ STRIPPFR
J JT H .ASH
2 .STEAM SEPARATOR
M
UREA, NH3, C02, H20 \J
COr, 1 ,
' UREA SOLUTION
COMPRESSOR
Figure 17. SNAM PROGETTI Ammonia Stripping urea process.
25
-------
recycle by using an ammonia-driven ejector and straight carbon
dioxide. Under these conditions a 60% carbon dioxide conversion
per pass can be obtained.
The effluent leaving the reactor is passed to a stripper operat-
ing at 10 MPa to 15 MPa and 160°C to 200°C (24). Most (>90%) of
the arranonia and carbon dioxide are removed in the stripper with
the remainder being removed in the flash separator. These over-
heads are collected and the carbamate is recycled; excess ammonia
is also recycled to storage.
The unique feature in the SNAM PROGETTI process is the carbamate
ejector which introduces carbamate and ammonia to the reactor
(21). The ammonia pressure drop through the ejector of 41 MPa
supplies the necessary driving force.
The Chemical Construction Corporation - U.S.A. (Chemico) has
developed the Chemico total recycle urea process shown in Figure
18. Chemico is also the United States licensed contractor for
the SNAM PROGETTI (25). In the process shown in Figure 18,
ammonia and carbon dioxide are fed in a molar ratio of 4:1 to the
reactor operating at 19 MPa to 23 MPa and 175°C to 200°C (1, 12).
Under these conditions a carbon dioxide conversion of 64% to 70%
per pass can be achieved. The reactor effluent goes through two
stages of purification and recovery of ammonia and carbon dioxide,
The first stage operates at 2.0 MPa to 2.5 MPa and 155°C. The
overheads from this stage go to the secondary absorber and the
liquid effluent goes to the second stage for further purifica-
tion. The second stage operates at 202 kPa and 120°C. The pro-
cess stream leaving the process contains 74% to 75% urea and 25%
to 26% water.
The Lonza-Lummus urea process shown in Figure 19 employs a molar
feed ratio of ammonia to carbon dioxide of 4.5:1 (1). The
reactor is operated at 30 MPa and 200°C. At these conditions,
a carbamate conversion of 70% can be achieved. The reactor
effluent is pumped to the first-stage decomposer which operates
at 1.5 MPa and 150°C (1, 26). The liquid effluent continues
through two additional purification stages. The gaseous efflu-
ents from the first-stage decomposer go to the first-stage ab-
sorber where the excess ammonia is separated and the carbamate is
recycled. The gaseous effluents from the third decomposer go
through the corresponding absorber, and the effluent from this
absorber goes to the second-stage absorber. The liquid effluent
from the second-stage absorber is recycled to the first-stage
decomposer for repurification.
(25) Urea Production. Nitrogen, 91:44, September/October 1974.
(26) The Lonza-Lummus Urea Process. Nitrogen, 33:31-32,
January 1965.
26
-------
c.w.
LIQUID"
NH3 FROM
STORAGE
RECYCLE NH,
SECONDARY ABSORBER
LIQUID NH,
CARBAMATE
RECYCLE
H20 DECOMPOSER EXPANSION VALVE DECOMPOSER
PRIMARY
ABSORBER
CARBAMATE, EXCESS NH.
c.w.
1ST STAGE
CARBAMATE, NH.
UREA
SOLUTION
2ND STAGE
PRODUCT-CONTAINING STREAMS
— RECYCLE, FEED,OR OTHER ANCILLARY STREAMS
Figure 18. Chemico total recycle urea process,
PRODUCT-CONTAINING STREAMS
RECYCLE, FEED, OR OTHER ANCILLARY STREAMS
LIQUID NH
UREA ""3
EXCESS NH3 C02
CARBAMATE
H P S THIRD-STAGE
CARBAMATE
DECOMPOSER
FROM STORAGE
CO,
COMPRESSOR
Figure 19. Lonza-Lummus urea process,
27
-------
(4) Summary of total recycle processes—A summary of the basic
operating parameters for the total recycle systems is given in
Table 3 (1, 11-26).
Two facts must be considered when interpreting Table 3:
• Much process and design information is not available
due to its proprietary nature.
• Some of the facilities have been designed by company
personnel based on basic process technology; they do
not use contractual packaged design and are not listed
in Table 3.
TABLE 3. SUMMARY OF TOTAL RECYCLE SYSTEMS
Process
CP I -Allied
Invsnta gas
recycle
Stanicarbon
C02 Stripping
Montecatini
Pecainey
Inv3nta liquid
recycle
b
Mitsui Toatsu
Stanicarbon
tatal recycle
SHAM PROGETTI
NH3 Stripping
Chenico
Lonza-Luosnus
Molar feed ratio
NH3/CO2 H20/CO2
4 to 4.5/1 -a
2/1
2.4 to 2.9/1 -d
3.5/1 0.6/1
VI
3.5 to 4.0/1
4.3/1 -a
3.3 to 3.5/1 -3
4/1
4.5/1
urea (per pass)
NH3 CO2 Carbamate
-" 85 -8
50 -
65 to 85 70 to 85 -fl
a a a
a a
40 to 50
62 to 65
-3 67 -3
-a 60 -3
-a 64 to 70 -3
70
Reactor operating
parameters
Temp. , °C
194 to 233
180 to 200
170 to 190
195
177 to 183
185
190
_
170 to 190
180 to 190
175 to 200
200
Pressure, MPa
30
20
12 to 15
20
20
23
24
20
13 to 16 or
20 to 25
19 to 23
30 to 34
Reference
12,13
1,14,15
11,13
13,16,18
14,15,17
11
11,19,20
11
11,13,21
22,23,24
11,13,25
11,26
Operating parameters for "D" Improved process only.
d. Emission Points from Solution Production—
EmissTons from the solution production step in the urea manu-
facturing process arise from individual process exhausts and
from leaking pump seals. A quick review of Figures 9 through 19
shows that the following processes have exhausts:
• Stamicarbon C02 Stripping
• Montecatini complete recycle
• Stamicarbon total recycle
• Lonza-Lummus
28
-------
Pump seals are a normal item for preventive maintenance. If a
seal starts to leak, the pump is shut down and the packing is
replaced. Therefore pump seals will not be considered further.
The only emission points to be discussed later will be process
exhausts.
2. Solid Urea Production
Approximately 62% of the urea solution produced is converted to a
solid product. This means that in 1975 2.14 x 106 metric tons of
urea were produced in solid form. Figure 20 is a flowsheet of
the entire urea solidification process. As can be seen, each
section has a number of steps which will be discussed in subse-
quent sections.
AQUEOUS UREA SOLUTION
SOLUTION
CONCENTRATION
COOLING, SCREENINGS COATING
FINAL
PRODUCT
PREPARATION
Figure 20. Solidification of urea.
29
-------
The formation of solid urea requires a critical balance between
temperature, retention time, and airflow due to several physical
characteristics; namely, melting point, heat of crystallization,
and decomposition properties. These factors lead to careful
control and at times adaptation of established techniques de-
pending upon the environment in which the material is produced.
a. Solution Concentration—
Solution concentration can be achieved by two methods as shown
in Figure 20: vacuum evaporation and crystallization. Less
than 25% of the urea solution made for solid production is con-
centrated by crystallization. This percentage is determined by
calculating the percent of urea produced for consumption in
plastics and other areas (10.1% and 4.8% of the total urea solu-
tion production) requiring an extremely low biuret content.
Feed to the solution concentration section is 70% to 75% urea
solution that already contains 0.3% to 0.7% biuret. Since
crystallizers operate at 60°C, versus 120°C to 140°C for evap-
orators, they hold to a minimum any additional biuret formation
(1, 1C).
The percentage of urea production devoted to low-biuret uses is
14.9%; this is 24% of solid urea production. This approximation
has been validated by industry sources who estimated the ratio
of crystallizers to evaporators to be 1 to 3.
(1) Crystallization and dewatering—Urea crystallization as a
solution concentration method was introduced in 1953 and has
since grown in popularity. Currently both vacuum and atmospheric
crystallizers are in use. A vacuum crystallizer operating at
8 kPa and 60°C (1) evaporates water using the sensible heat of
the urea solution and the heat of crystallization. Additional
heat can be supplied by pumping the urea solution through one
of the decomposers where it is heated by exchange. The crystal-
lizer slurry, containing about 30% (weight) urea crystals in
suspension, is placed in a continuous-type pusher centrifuge where
the crystals are separated, washed, and dried to less than 0.3%
moisture. The dewatered crystals are melted and sprayed into the
solidification device.
(2) Evaporation—A thin-film (falling-film type) evaporator
operating at atmospheric pressure is most commonly used for
evaporation. While a falling-film evaporator is standard, any
type of thin-film evaporator in which the temperature can be
controlled to between 120°C (inlet) and 140°C (outlet) can be
employed. The Tennessee Valley Authority (TVA) used a small
shell-and-tube preliminary evaporator followed by an eight-disc
rotary disc falling-film forced evaporator in its pilot-scale
30
-------
unit (27) . The use of two evaporators in series is reported to
be the trend in current plant constructions (10) . This arrange-
ment has proven most successful in producing the 98% to 99.5%
urea solution necessary for solid formation.
b. Solid Formation—
The actual solid product can be formed by two methods, prilling
and granulation, each of which has two possible modifications.
Based on an estimated 351 days of production (365 minus 14 days
planned maintenance shutdown), the daily production rate for the
industry can be calculated from 1975 yearly production data to
be ,6_,_p_9J metric tons/day. Urea granulation by the Spherodizer®
process (to be described later) has a daily capacity of
5,201 metric tons. Since Spherodizers are reported to operate at
100% capacity on an industry average (28), approximately 85%
of the solid urea produced is estimated to be produced by the
Spherodizer. Other granulation process productions are assumed
to be negligible, leaving 15% of solid urea production that em-
ploys prilling for solid formation.
(1) Prilling—The basic methodology for prilling is well estab-
lished. Concentrated urea solution is pumped to the top of a
tower 30.5 m to 33.5 m high and forced through a spray device.
Several types of devices, ranging from a single nozzle to
multiple nozzles or a spinning bucket are used. The droplets
thus formed fall countercurrent to a rising airstream which acts
as a heat transfer agent that cools and solidifies the material.
The material may fall onto a fluidized bed which acts as a cooler
or simply onto a belt conveyor which carries the material to
storage. Table 4 is a comparison of the particle sizes found
in different samples of prilled urea. Sample A was taken from a
bin at a fertilizer bulk mixing facility (2). Sample B is from
a urea plant facility and compares coated (1.2% to 2.0% clay) and
uncoated urea prills (29) . Sample C is an analysis of unscreened
prilled product from a urea plant. Sample D is a size distribu-
tion of final product from a urea prilling facility. Size distri-
butions of samples C and D were obtained from industry contacts.
Table 5 presents some characteristics of urea prilling operations
compiled from several literature sources and industry contacts.
(27) Granular Urea and Ammonium Nitrate. Nitrogen, 95:31-36,
May/June 1975.
(28) Personal communication with J. C. Reynolds, C & I/Girdler,
Inc., Louisville, Kentucky, August 1977.
(29) Reed, R. M., and J. C. Reynolds. The Spherodizer Granu-
lation Process. Chemical Engineering Progress, 69(2):62-66,
. 1973.
31
-------
TABLE 4. SIZE DISTRIBUTION OF UREA PRILLED PRODUCT
Percent retained on screens
Screen
mm
3.4
2.4
1.7
1.4
1.2
1.0
0.8
250 urn
105 urn
74 um
44 um
<44 ijm
size
mesh
6
8
10
12
14
16
20
60
150
200
325
<325
Sample B
Sample A
0
0
0
0
_a
_a
.0046
.0028
.0090
.0154
Uncoated
0
1
17
-b
-
78
_b
4C
_c
~c
Coated
0
0
lh
-
94h
5C
~c
Sample C
0
2.2
77.1
15.2
3.3
1.1
0.5d
0,6d
-fc
~b
^b
Sample D
b
_b
12
39
35
8
30
36
_e
~e
a>99% of sample retained on 250-um (60-mesh) screen.
No information available.
°Remainder as indicated is <1.0 mm (16 mesh).
This quantity indicated as the pan quantity.
Remainder is indicated as <0.8 mm (20 mesh).
TABLE 5. PRILL TOWER CHARACTERISTICS
Parameter Value
Free fall height 30.5 m to 33.5 m
Tower cross section circular or square
Cross section area 28 m2 to 113 m2
Spray temperature 135°C to 144°C
Prill exit temperature ^80°C
(2) Granulation—The basic principle of granulation involves
spraying molten urea onto fine urea particles to increase the
granule size in a layered fashion. This type of particle^forma-
tion produces a stronger particle than does prilling. This
stronger structure does not permit as much crushing in agricul-
tural machinery, thereby resulting in a better material distri-
bution on the field. As a result, a major market for solid urea
produced by granulation is the fertilizer industry.
There are two basic designs of granulation equipment: pan and
drum. These two processes are discussed in the following sections
(a) Pan granulation—Figure 21 is a sketch of a pan granu-
lator (30). This type of operation was first introduced for use
with fertilizers in 1950, and the TVA began work on the process
in 1963 (27). Powdery material is introduced at the top of a
rotating, tilted pan and tumbles to the lowest point of rotation.
As the granules are carried up by the rotating pan they are mixed
into a deep bed of granules, where the lightest particles rise
farthest before falling back through the spray to the lower side
(30) Young, R. D., and I. W. McCamy. TVA Development Work and
Experience with Pan Granulation of Fertilizers. Canadian
Journal of Chemical Engineering (Ottawa, Ontario), 45(2):
50-56, 1967.
32
-------
and back into the bed. As the granules grow they are progres-
sively displaced to the surface of the deep bed by the incoming
smaller particles. Eventually the large granules tumble over the
side of the pan.
RECYCLE ENTERS
HERE
SMALL GRANULES «-
PASS UNDER SPRAYS
/ /
S
'LARGE
GRANULES
LEAVE PAN HERE
PAN ROTATION
Figure 21. Sketch of pan granulator (30).
TVA has conducted pilot-plant studies in which the 98.5% to 99%
urea solution from an evaporator was sprayed onto a bed of re-
cycled fines at a temperature of 145°C. In this unit the pan was
tilted 1.15 rad and rotated at 1.84 rad/s. The granules leaving
the pan were at 99°C and went directly to a cooler. The screen
analysis of the material leaving the granulator is shown in Table
6.
TABLE 6. SCREEN ANALYSIS OF PAN GRANULATOR PRODUCT
Screen
mm
3.4
2.4
1.7
1.2
0.8
<0.8
size
mesh
6
8
10
14
20
<20
Percent retained
on screen
6
17
56
12
3
6
The granules leaving the pan were cooled in a conventional
countercurrent rotary cooler and finally screened to the fol-
lowing size range by vibratory screens:
33
-------
Screen size
mesh
mm
2.4
1.7
8
10
Percent retained
71
29
The sized product was further cooled in another rotary unit and
coated with approximately 0.7% (by weight) kaolin. A mixture of
90% light lubricating oil and 10% paraffin wax was sprayed as a
dust suppressant in quantities of approximately 0.3% by weight
(27) .
Norsk Hydro has developed a pan granulation process that operates
at a temperature close to the crystallization temperature of urea.
while conventional systems operate approximately 40°C lower, the
Norsk Hydro process temperature (with a maximum temperature dif-
ference of 25°C) claims a higher output because of increased
crystal growth rate. The tamped bulk density of this product is
1,000 kg/m3, as compared with a loose pour density for conventional
pan-granulated material of 700 kg/m3. At least 95% of this
material is between 4 mm and 11 mm in diameter.
(b) Drum granulation—Figure 22 is a cross-sectional
sketch of the Spherodizer®, a drum granulation unit designed by
C&I/Girdler. There are currently at least 18 Spherodizer units
in operation in the U.S. (see Table 7, having a total capacity
of 5,201 metric tons/day, accounting for 85% of the domestic
urea solid production (31).
SEED UREA PARTICLES
EXHAUST
AIR
CONCENTRATED
UREA SOLUTION
BED OF UREA
GRANULES
RETAINING DAM
COOLING AIR
Figure 22. Cross section of Spherodizer.
(31) Reynolds, J. C. and R. M. Reed. Progress Report on SPHERO-
DIZER Granulation 1975-76. In: Proceedings of The
Fertilizer Institute Environmental Symposium (New Orleans,
Louisiana), The Fertilizer Institute, Washington, D.C.,
pp. 193-215. 1976.
34
-------
TABLE 7. U.S. SPHERODIZER UREA GRANULATION PLANTS (31)
Company name
Agrico Chemical Co.
Agrico Chemical Co.
CF Industries, Inc.
CF Industries, Inc.
Collier Carbon and
Chemical Co.
Cooperative Farm
Chemical Association
TOTAL
Capacity,
metric tons/day
544
907
907
1,211
1,088
544
5,201
No. of
drums
2
3
3
4
4
2
18
Location
Donaldsonville,
Louisiana
Blytheville,
Arkansas
Donaldsonville,
Louisiana
Donaldsonville,
Louisiana
Kenai, Alaska
Lawrence ,
Kansas
Average size 289 metric tons/day
The drums are approximately 4.3 m in diameter with an average
capacity of 289 metric tons/day. Other parameters such as
length of drum and rotation speed vary at each installation.
Fine urea particulates enter the granulating section at a re-
cycle ratio (recycle:product) of 2:1. The granulating section
of the drum is separated from the cooling section by a dam, the
height of which regulates the particle residence time in the
granulating section. This granulating section is also equipped
with lifting flights (see Figure 23) which pick up the particle
material and drop it from the top of the drum, creating a falling
curtain of seed material. The seed material is undersized
product recycled to the drum.
LI (TING RIGHTS
Figure 23. Spherodizer drum - end view.
Urea melt at 138°C and 99.3% urea comes through the spray bar
onto the falling curtain and tumbling bed of particles (see
Figures 22 and 23). This application technique ensures that the
particles will be coated with thin, uniform layers. Solidifica-
tion is facilitated by passing a stream of air countercurrent to
the product flow. The air enters the cooling section at approxi-
mately 10°C and exits the granulating section at 77°C to 88°C.
35
-------
The product leaves the drum at 43°C and contains 0.06% water.
The material is then sized, with the oversized and undersized
particles being recycled through a crusher to the entrance of
the drum. The product is coated (27) with 1.2% to 2.0% clay
(based on final weight).
Urea granules produced by this process have a crushing strength
1.8 tc 2.8 times that of urea prills. The bulk density is 700
kg/m3 for loose pour and 800 kg/m3 for tamped product. Table 8
shows the size distribution of screened urea product and the
differences which can exist between two processes using dif-
ferent screen size ranges for final product sorting.
TABLE 8. SIZE DISTRIBUTION OF SCREENED UREA
PRODUCT FROM A SPHERODIZER UNIT (27)
Screen
mm
3.4
2.4
1.7
1.2
size
mesh
6
8
10
14
Percent
Sample
1.5
89.6
8.8
0.1
retained on screen
A Sample B
2.7
80.1
17.1
0.1
dProduct from process which screens to
eliminate any granules larger than 3.4 mm
(6 mesh) and smaller than 2.4 mm (8 mesh).
Product from process which screens to
eliminate any granules larger than 3.4 mm
(6 mesh) and smaller than 2.0 mm (9 mesh).
c. Final Product Preparation—
Final"product preparation consists of cooling, screening, and
coating.
(1) Cooling--Less than 5% of the urea industry uses auxiliary
coolers to lower the temperature of the solid particles between
the solid formation and screening, shipment or storage operations.
There are at least two processes in which a cooling stage is an
integral part of the solid formation process-drum granulation
and fluidized bed prilling. In the other processes, the particles
fall directly onto a conveyor belt which carries them to either
a screen or storage.
(2) Screening—The product can be screened by inclined vibrating
screens immediately after solid particle production, before
shipment, or both. The oversized material is recycled to the
36
-------
process after it is either crushed and remelted or dissolved in
solution. The fines are recycled to the process as seed material,
redissolved, or used as a product when needed for mixing with
livestock feed. The product is stored for bagging or bulk
shipment.
(3) Coating—Less than 10% of the solid urea produced is coated
because the practice of injecting formaldehyde or phosphate-based
additives into the melt prior to solid formation has increased.
The structures and quantities of the phosphate-based additives
are proprietary. The quantities of formaldehyde used are also
proprietary; however, quantities well below 0.1 wt % are used.
These additives do not alter the chemical characteristics of the
final particle and therefore leave it more compatible with prod-
uct application (e.g., feed material for urea-formaldehyde
resins) than coated particles.
When coating occurs, diatomaceous earth or kaolin is added to
the particles in quantities of 0.3 wt% to 2.0 wt%. Another
coating which has been used is a light oil-paraffin mixture that
acts as a dust suppressant in the TVA pan granulation process.
This coating process, like sizing, occurs inside a large ware-
house and requires no external vents.
d. Emission Points from Solid Production—
Potential emission points and the species emitted are listed:
Emission point
Species
Evaporator Ammonia, particulate
Crystallizer Ammonia
Crystallizer centrifuge Ammonia
Prilling tower Ammonia, particulate
Granulator . Ammonia, particulate
Emissions from all of these points will not occur at any one
plant. For example, no plant utilizes both an evaporator and
a Crystallizer, or a prilling tower and a granulator.
3. Product Shipment
a. Shipment of Bulk Solutions—
Approximately 38% of all urea is sold as a solution and shipped
in bulk by railroad tank cars or tank trucks. These tank cars
can be loaded directly from the process or from storage tanks.
At least one company has a pipeline for deliverying urea
solutions.
37
-------
Bulk solution loading, like bulk loading of solid urea, takes
place under cover of a two-sided shed. A delivery tube is placed
inside the manhole on the top of the trailer, approximately 3.7 m
off the ground. The liquid urea solution, approximately 70% urea,
is pumped into the tank car at 66°C and maintained at that
temperature because the cars are double insulated.
b. Storage, Packaging, and Bulk Loading of Solids—
Solid urea can be stored, bagged, or shipped in bulk. Storage
facilities are large warehouses where the urea is put in piles
9 m to 12 m high and at least 23 m in base diameter via an over-
head conveyor system.
The stored urea is either bagged or bulk loaded for shipment from
this stockpile. Material to be shipped in bulk is screened and
carried to an overhead conveyor for disbursement. The conveyor
drops the material into a bulk hopper train car or bulk truck at
a height of 3.7 m. This operation is done under cover of a shed
to prevent moisture contamination of the urea. In a few cases,
plants near major waterways have shipped solid product by barge.
Urea may also be shipped in bags (1). Domestic shipments are
made in 4-ply, polyethylene-coated, 20.4-kg paper bags. Ex-
porting is done in 6-ply, polyethylene-coated, 36.3-kg paper
bags. This operation is done in a large warehouse using one of
many types of automatic bagging machines.
D. EMISSIONS FROM PRODUCT SHIPMENT
In the bulk loading of solution product, air contained in the
empty ispace of the railroad tank car or tank truck is displaced
by the liquid and emitted to the atmosphere. This displaced air
contains ammonia.
Emissions from shipment of solid product consist of small urea
particles that are entrained into the air during the operation.
38
-------
SECTION IV
EMISSIONS
A. SELECTED EMISSIONS
As discussed previously, urea is formed by dehydrating the
ammonium carbamate which is produced when ammonia is reacted
with carbon dioxide. This urea may be sold as an aqueous solu-
tion or solidified to form a solid product. The manufacturing
process causes two types of emissions, ammonia and particulates.
Both materials are emitted during the solidification process,
but only ammonia is released from the solution formation process,
Particulate emissions are composed primarily (>99%) of urea,
except for those particulate emissions released from the coating
operation; these particulates consist of coating materials.
Although no analyses are available, possible impurities such as
biuret have lower vapor pressures than urea itself and therefore
would not be present in the particulate emissions at concentra-
tions higher than in the bulk product. Formaldehyde added to
urea reacts to form monomethylurea and so is not present as the
pure compound, which does have a higher vapor pressure than urea,
Since urea is normally used as a fertilizer and animal feed, it
is not considered a hazardous material; no threshold limit value
(TLV) has been assigned to it (32). Biuret is always present
as an impurity, but its concentration is controlled to less
than 0.5% in fertilizer-grade product. Although biuret is toxic
to plants, it is not hazardous to animals; no TLV has been
assigned to it. Other possible impurities include triuret,
cyanuric acid, and ammelide (1). Cyanuric acid is a hazardous
compound, but an analysis of urea shows that biuret is the main
impurity (present at concentrations between 1% and 0.1%), with
other impurities present at less than 0.1% (1).
Coating materials applied to urea may be clays, diatomaceous
earths, or a paraffin/oil mixture. Diatomaceous earth has been
(32) TLVs® Threshold Limit Values for Chemical Substances and
Physical Agents in the Workroom Environment with Intended
Changes for 1976. American Conference of Governmental
Industrial Hygienists, Cincinnati, Ohio, 1976. 94 pp.
39
-------
assigned a TLV of 1.5 mg/m3 (respirable dust, <5 ym) ; kaolin
clay has a TLV of 10 mg/m3 (32).
Proprietary phosphate-based additives are not believed to be
hazardous since phosphate compounds in general are not toxic.
Formaldehyde, also used as an additive, has been assigned a TLV
of 3 rag/in3 (32) . However/ it reacts with urea to form monomethyl
urea cind higher derivatives that do not have established TLV's.
Possible trace element contamination was not studied. Since the
raw materials for urea synthesis are carbon dioxide and ammonia
(i.e., gaseous species), the only source of trace elements would
be from equipment corrosion (ammonium carbamate solution is
highly corrosive). If a urea plant manufacturing 100,000 metric
tons/yr had trace metals present in the final product at a con-
centration of one part per billion (1 ppb), the annual metal loss
due to corrosion would be 100 g. A concentration of one part
per million would correspond to a corrosion loss of 100 kg/yr.
Equipment failure would soon result from losses of 100 kg/yr,
and it is therefore concluded that any trace metal impurities
must be present at levels below 1 ppm. By way of confirmation
the product specifications for fertilizer-grade urea include a
maximum iron concentration of 2 ppm (1).
In the subsequent discussion in this section, emissions are clas-
sified as either particulates or ammonia. Potential environ-
mental affects are evaluated based on the primary ambient air
quality standard for particulates of 260 yg/m3 and the TLV for
ammonia of 18 mg/m,3.
B. EMISSION FACTORS
Ammonia and particulates emitted per unit of product produced
for each process step in urea manufacture are discussed below
based on engineering estimates, information from industrial
sources, and data from the open literature.
1. Solution Production
The emissions from the solution production step in urea manu-
facturing come from individual process exhaust vents and fugi-
tive sources. The fugitive emissions are due to evaporation
from spills and leakage in pump seals, valves, and compressors.
Ammonia is the only emission species since only gaseous emissions
are released.
As has been noted in Section iii.c, solution production processes
utilized by the industry generally do not have process exhaust
vents to the atmosphere, except for the following four processes:
Stamicarbon_C02 stripping process, Montecatini complete recycle
process^ Stamicarbon total recycle process, and Lonza-Lummus
process. In the above total recycle processes, unreacted
40
-------
ammonia from the reactor and ammonia generated from the carbamate
decomposer are removed from the gas stream by various absorbers
for recycling back to the reactor. Tests on the gas stream
vented to the atmosphere indicate that there is no detectable
ammonia in this stream (12).
Data are not available to quantify fugitive emissions from the
solution production step. It was assumed that these emissions
are negligible for the following reasons
• OSHA regulations limit the allowable ammonia con-
centration in the work place.
• The odor of ammonia from leaks is easily detected.
Maintenance of pump seals, valves, and compressors is
regularly performed to avoid product loss and to
preserve a healthy working environment.
Based on the above discussion, no emission factor was derived
for emissions from the solution production step.
2. Solution Concentration
a. Evaporation—
Evaporation is used in over 75% of the solid production processes
as a method of solution concentration. More than 90% of the
facilities using evaporation use a thin-film evaporator operating
at atmospheric pressure. Air passes countercurrent to the
falling film of urea solution, releasing ammonia as well as par-
ticulates. Very few (<5%) of the plants allow this stream to
exhaust to the atmosphere. In some cases (approximately 50%),
the off-gas from the evaporator passes through a condenser, and
the condensate is either sewered (where allowed by water quality
standards) or sold as a dilute fertilizer solution. In the
remainder of thve~installations, the evaporator off-gas is
scrubbed to recover valuable ammonia and urea. Types of scrub-
bers used in this application as well as those used in granula-
tion are discussed in Section V.
A^number of factors affect the ammonia emission rate from
evaporators. A partial list includes:
residual ammonia in feed stream, i.e., from solution
production process
• biuret concentration
• amount of urea hydrolysis
• control efficiency of scrubber or condenser used
Occupational Health and Safety Administration.
41
-------
The level of residual ammonia in the feed stream to the evapora-
tor is one of the most significant factors, resulting in a possi-
ble range in ammonia concentration from a few hundred parts per
million to as much as 0.5%. A material balance calculation
shows that, in the absence of controls, this will correspond to
an ammonia emission factor of 0.4 g to 7 g per kg of product.
Testing has been conducted by one firm which employs wet scrubbers
to recover urea and ammonia (33). The data resulting from these
tests showed that 1.73 ± 64%a grams of ammonia and 0.107 ± 28%
grams of particulate per kilogram of product (95% confidence)
were emitted from the scrubber. The above values for ammonia
and particulate emissions thus represent controlled emission
factors. The test data also indicate an uncontrolled emission
factor of 8.55 g/kg of product for particulates. Scrubber
efficiencies are therefore on the order of 99% for particulates,
but at most 75% for ammonia. The scrubbing efficiency for
ammonia could probably be improved by using an acidic scrubbing
solution.
b. Crystallization, Dewatering, and Remelting—
A wide variety of crystallizers are used in those processes
employing crystallization as a solution concentration process
(approximately 25% of solution concentration capacity). The
emission characteristics and quantitites, being dependent upon
the particular equipment type, vary from plant to plant. In
most cases, however, any exhaust from the crystallizer passes
through a condenser and the condensate is sewered.
The slurry from the crystallizer, containing approximately 30%
crystals, is placed in a continuous centrifuge where the crystals
are dewatered to <0.3% water. While dryers are occasionally
used, centrifugation is used in more than 95% of those processes
using crystallization. After dewatering, the crystals are
melted for use in the subsequent solid formation processes.
Emissions from these operations are expected to be less than
those from evaporation. No airstream passes through the vessels
to entrain particulates and remove residual ammonia from solution.
Emission factors are given with their corresponding uncertainty
based on a Student t test of the available data.
(33) Sanders, L. Monitoring and Control of Gaseous and Particu-
late Emission from Fertilizer Complex. Paper No. 75-5.6,
presented at 69th Annual Meeting of the Air Pollution Con-
trol Association, Portland, Oregon, June 27-July 1, 1976.
14 pp.
42
-------
Operating temperatures are also lower. Although some ammonia
may escape from exhaust vents, the rest will remain in the
aqueous phase that leaves the centrifuge. There are no data
available to quantify emissions from these processes. For worst
case evaluation purposes it was assumed that the emission fac-
tors developed for evaporation are also applicable to this
emission source. In subsequent calculations, only one set of
emission factors is used for emissions from the solution con-
centration process.
3. Prilling
As discussed previously, 15% of solid urea is manufactured by
prilling—droplets of urea melt falling countercurrent to a
cooling airstream in which they cool and solidify. Ammonia and
particulate emissions result from this operation. The ammonia
is released from the possible degradation of urea to biuret
and also from excess ammonia still present in the spraying
stream. The particulates are small urea droplets formed during
spraying which solidify and leave the tower in the exiting gas
stream rather than falling to the bottom.
Table 9 shows data for ammonia and particulate emissions from
the prill towers of three plants that have conducted testing.
The ammonia data were averaged to give an emission factor of
0.40 g/kg ± 84%. For particulates, another averaging procedure
was used to obtain the average emission from a collection of
average values, since the original emissions data were not
available. The following equations are derived in Appendix B:
m _
En.x.
i i
XT ~ m
(7)
E ni
and
S =
m
m
(ni-l)si +
2 _ 2
- xm
m
ni
(8)
where
n .
x .
m =
average of the averages from each sample
number of test points in the i-th sample
average of test points in the i-th sample
total number of samples
43
-------
s_ = standard deviation of x
:s. = standard deviation of x. for the i-th sample
TABLE 9. EMISSION DATA FOR PRILLING TOWER FROM SOURCE
TESTS AT THREE UNCONTROLLED UREA PLANTS
Parameter
Actual particulate emission
factor, g/kg
Number of samples for
particulates
Ammonia concen-
tration, ppm
Airflow rate, m3/s
Actual ammonia
emission rate, g/s
Prill tower capacity,
kg urea/s
Actual ammonia
emission factor, g/kg
1
1.55 ± 6.2%
30
30
35.4
0.82
2.1
0.39
Plant number
2
1.6 ± 172%a
33
25 to 30
42.5 to 47.2
0.82 to 1.09
3.7
0.22 to 0.29
3
1.85 ± 12.8%
6
30
141.6
3.28
4.7
0.70
Average
h
$sf ± 17*°
NAC
H
28-8d
66.7°
1.5d
3.5
0.40d
Each of these three samples was the average value of several tests at different
operating parameters. It was assumed that these variations in operating parameters
do not exceed the variations experienced in normal operation.
Determined using Equations 7 and 8.
CNot applicable.
Average determined by averaging extremes of plant 2 with others single values.
eThese numbers are actual, not representative, since they are based on actual, not
.representative, source measurements and plant capacity.
The average emission factors presented in Table 9 were used for
the subsequent calculations. However, emission rates 'can fluct-
uate widely with changes in such variables as:
" type of spraying device used
• air velocity
• spray temperature
• type of product made
Additional factors that affect prilling emissions can be found
in the literature (33).
4. Granulation
The Spherodizer granulation process is used by four companies
in the U.S. to produce approximately 85% of the solid urea manu-
factured. Scrubbers are standard equipment on granulators,
since up to 20% of the product may be entrained in the cooling
44
-------
air stream (34). Therefore, the scurbber will be considered as
the emission point for the granulator.
Tests conducted by C & I/Girdler, Inc., show that scrubbers have
an average particulate removal efficiency of 99.9% (31). The
same test give an emission factor from the scrubbers of
0.20 ± 25% grams of particulate per kilogram urea produced (95% '
confidence level). Other tests have given a particulate emission
factor of 0.084 ± 29% g/kg urea and ammonia emissions of
0.25 ± 48% g/kg urea (95% confidence level) (33).
The different emission factors for particulates are a consequence
of different scrubber efficiencies. In subsequent sections both
values are used to give a range of possible emission factors.
5. Solid Product Finishing
This section includes any cooling, screening, or coating opera-
tions which may take place in the final preparation of the solid
urea product. Less than 5% of the industry uses an auxiliary
cooling step in final solid preparation. In at least two
processes, drum granulation and fluidized-bed prilling, the
cooling stage is an integral part of the solid formation, and
any material entrained in the cooling air will exit from the
solidification device. In other processes, the product falls
directly on a conveyor belt which transports it to either
screening or storage.
Solid product is screened (inside a building) before storage,
before shipment, or both. The latter is most likely if a rigid
particle size requirement is to be met. Emissions from this
building due only to screening cannot be quantified. Often the
same building houses storage and bagging facilities. Emissions
from bagging(Operations are estimated, together with those from
bulk loading of solid product, in the following subsection.
That estimate is based on the amount of small urea particles in
the prilled product that could be entrained in the air. The
possible emissions of particulates from screening are covered in
that total estimate, and further individual consideration is not
given to emissions from the screening operation.
Less than 10% of the final product is coated, primarily due to
the increased use of various additives in the melt. When
coating occurs, diatomaceous earth or kaolin is added to the
solid particles in quantities of up to 2.0 wt%. If the coating
material is applied at a rate of 2 wt% and if 10% of this
(34) Bress, D. F., and R. K. Fidler. New Concepts in Design of
Urea Plants. Paper No. 13C, presented at 74th National
Meeting of American Institute of Chemical Engineers, New
Orleans, Louisiana, March 12-15,' 1973. 22 pp.
45
-------
material is released to the atmosphere (a worst case estimate),
the emission factor for particulate emissions from this operation
will be 2 g/kg of solid urea coated.
6. Solution Product Bulk Loading
In the bulk loading of solution product, liquid is pumped into a
tank car or tank truck under cover of a shed. During the loading
operation, the liquid filling the tank displaces air which
contains ammonia vapor from entering urea solution.
Emission data are not available for this source; however, the
ammonia emission factor can be estimated by considering the
properties of 70% urea solution. Figure 24 is a diagram of the
vapor pressure and specific gravity of urea solutions in water
(8). The vapor pressure is assumed to be from ammonia rather
than urea vapor. At a solution temperature of 339°K (66°C), the
equilibrium vapor pressure of a 70% urea solution is 16.2 kPa.
The total number of moles of ammonia contained in a specified
volume, under equilibrium conditions, can be calculated from the
ideal-gas law:
where
n
P
V
R
T
pV
RT
number of moles
vapor pressure, Pa
volume, m3
8.3 (Pa) (m3)/(g/mole) (K)
temperature, °K
(9)
0.960
1.000
SPECIFIC GRAVITY
1.050 !l. 100
,1.150
1.20 1.22
10 20
30 40 50 60 '70
UREA, gper 100g of solution
80
90 100
Figure 24. Vapor pressure and specific gravity of
urea solutions in water (8).
46
-------
In this case, V is the volume displaced by a 70% urea solution.
The resultant emission factor for ammonia can be calculated as
. M
E = DM = RT _ * = PM
^ Vd Vd RTd UU)
where M = molecular weight of ammonia, g/mole
d = density of the 70% urea solution
Based on equilibrium vapor pressure (16.2 kPa) , the temperature
(339 K) , the solution density (1.175 Mg/m3, from specific
gravity in Figure 24) , and molecular weight (17 g/mole) , the
emission factor was calculated to be 0.083 g/kg of solution
product loaded. Correcting for a 100% urea basis, the emission
factor becomes 0.12 g/kg of 100% urea loaded.
7. Solid Product Bagging and Loading
Particulates are emitted from bagging for shipment and from the
bulk loading of the solid urea product into railroad cars or
barges. Data are not available to quantify the particulate
emissions from this operation. A worst case estimate was made
based on the particle size distribution given in Table 4.
According to the Table, 0.0154% of the prilled urea product is
<44 urn in size. Assuming that all of these small-size particles
are air-entrained during the bagging and loading operations, the
particulate emission factor becomes 0.15 g/kg of solid product
loaded. It should be noted that this emission factor represents
a worst case condition; the actual emission factor would be
smaller than this. It is assumed that the proportion of fines
(^44-ym size particles) in granular solids is similar to that in
prilled solids.
8 . Summary of Emission Factors
The emission factors for ammonia and particulates, emitted from
urea manufacture and discussed in the preceding subsections,
are further summarized in Table 10 for each emitting operation.
Solution production is not included in this table due to its
insignificant emissions, as noted in Section IV. B.I. All the
emission factors are based on the amount of pollutant emitted
per unit of 100% urea product.
C. ENVIRONMENTAL EFFECTS
Air emission released during the production of urea are dispersed
through the environment. This section examines the possible
effects of these emissions and evaluates their severity.
47
-------
TABLE 10.' SUMMARY OF EMISSION FACTORS FOR UREA PRODUCTION3'b
Emission factor, g/kg
Emitting operation Ammonia Particulates
Solution concentration
(controlled) 1.73 ± 64% 0.107 ± 28%"
Prilling (uncontrolled) 0.40 ± 84% 3.2 ± 17%-
Granulation 0.25 ± 48% 0.085 ± 29% f*,****^
to
0.20 ± 25%
Solid product
finishing 2
Solution product
bulk loading 0.12
Solid product bagging
and loading 0.15
Blanks indicate no emissions from the operation.
Percentages represent 95% confidence interval
based on student t test.
1. Average Plant Characteristics
For evaluation purposes, the urea industry can be described in
terms of certain average or predominant characteristics. These
characteristics based on a generalized description of process
variables gleaned from careful literature searches and conver-
sations with industry representatives. They are averages of
actual plant characteristics.
The following plant statistics have been derived from data pre-
sented in Appendix A, Table A-l:
• average plant capacity: 117,900 metric tons/yr
• average plant production rate: 69,000 metric tons/yr
• average county population density: 100 persons/km2
The average plant capacity per day was determined to be
335.9 metric tons, based on 351 days/yr operation at full
capacity. This value was used as a measure of the average maxi-
mum production rate, since most plants operate at full capacity
sometime during the course of a year. The product distribution
in the industry is 38% solution, 53% granulated solids, and 9%
prilled solids. However, no single plant makes this distribution
of products, and plants that make both solution and solids do
not generally manufacture them simultaneously. Some production
time is devoted to solutions, some to solids. Consequently, the
48
-------
average maximum production rate for each process step was also
assumed to be 335.9 metric tons/day.
In over 75% of the plants, evaporators are used to concentrate
the solution prior to solid formation. In the remaining plants
a crystallizer concentrates the solution.
Prill towers are circular with diameters of 6.1 m to 10.7 m and
tower exit gas velocities of 1.22 m/s to 1.83 m/s. Tower heights
range from 30.5 m to 33.5 m. Urea solution enters the towers
at 132°C to 144°C, and the solid prills leave at 66°C to 83°C.
The spray devices used may have a single-nozzle, multiple-
nozzle, or Tuttle bucket. In over 90% of all prilling facilities,
formaldehyde or a phosphate-based additive is placed in the urea
solution before spraying.
Plants using a granulator for solid production use a drum granu-
lator (Spherodizer). The drum is approximately 4.3 m in diameter
and has a capacity of 289 metric tons/day. The urea is sprayed
into the drum at 138°C, and solid granules leave at 43°C. As in
prilling, additives are used to improve product characteristics.
Finishing operations include cooling, screening, and coating.
Cooling as a distinct final step takes place in less than 5%
of the industry. In the remaining cases, it is an integral part
of the solid formation step, as it takes place at ambient con-
ditions as the material is being transported on a conveyor belt.
Screening can be done before storage, before shipment, or both.
If the particular product market has strict product specifica-
tions, the latter case is most likely. Coating, which is ap-
plied to less than 10% of all solid final product, is not com-
monly used, primarily because of the increased use of formalde-
hyde or phosphate-based proprietary additives.
Solution and solid product are shipped in railroad tank cars and
tank trucks. A few plants located near major waterways ship
solid product by barge. Also at least one company has a pipe-
line for delivery of urea solutions.
Emission heights for the different process steps are as follows:
Evaporator - 15.2 m
Prilling tower 30.5 m
Granulator 15.2 m
Coating, finishing,
product finishing ground level
2. Source Severity
One measure of the potential hazard of emissions from a source
is given by the source severity, S, defined as:
49
-------
s =
max
where X is the time-averaged maximum ground level concen-
tration or each pollutant emitted from a process operation, and
F is the primary ambient air quality standard for criteria pol-
lutants (particulates in this case), or a reduced threshold limit
value (TLV) for noncriteria pollutants:
F 5 TLV • 8/24 • 0.01, g/m3
The factor 8/24 adjusts the TLV for continuous rather than
workday exposure, and the safety factor of 0.01 accounts for the
fact that the general population is a higher risk group than
healthy workers.
The value of X x is the 24-hour average maximum ground level
concentration for each emission species determined by the fol-
lowing equation (3, 35):
- 2 Q
-t v 0.17
max
ireuh2
(11)
where
Q
IT
e
u
h
= emission rate, g/s
= 3.14
= 2.72
= average wind speed, m/s
= stack height, m
= short term averaging time, 3 min
= averaging time, 1,440 min (24 hr)
For airmonia and particulates, Equation 11 gives the following (3)
0.0182 Q
"max
h2
(12)
Accordingly, the source severities for ammonia and particulates
are:
303
,. .
Ammonia
h2
(TLV for ammonia: 18 mg/m3)
(13)
(35) Turner, D. B. Workbook of Atmospheric Dispersion Estimates.
Public Health Service Publication No. 999-AP-26, U.S.
Department of Health, Education, and Welfare, Cincinnati,
Ohio, May 1970. 84 pp.
50
-------
Z2_Q (AAQS for particulates: 260 yg/m3) (14)
Particulates h2
The following equations have also been derived for ground level
emissions of pollutants from a confined area such as emissions
from the windows and doors of a building in which an operation
is taking place (3):
X" = 1.048 QD'1-811* (particulate or ammonia) , (15)
S = 17,467 QD-1-811* (ammonia only) (16)
S = 4,031 QD"1-814 (particulates only) (17)
where D is the downwind distance from the emission point. In
this case x" is the average ground level concentration at the
distance D. The highest concentration to which the general pop-
ulation is exposed occurs when D equals the distance from the
emission point to the plant boundary. For the average urea
plant this distance is estimated to be ^400 m.
Table 11 summarizes values of x"max and S for the evaporator,
prill tower, and granulator, based on an operating rate of
335.9 metric tons of urea/day. Severities exceed 1.0 for
ammonia emissions from the evaporator and granulator. This
indicates that the estimated average maximum ground level con-
centration for a representative plant is greater than the
reduced TLV. The actual emission rates are highest for ammonia
emissions from the evaporator and particulate emissions from the
prill tower.
TABLE 11. EMISSION RATES, MAXIMUM GROUND LEVEL
CONCENTRATIONS, AND SOURCE SEVERITIES
FOR PROCESS OPERATIONS3
Emission
point
Evaporator
Prill tower
Granulator
Emission
species
Ammonia
Particulate
Ammonia
Particulate
Ammonia
Emission
rate, g/s
6.73
0.392
1.56
12.44
0.972
*max' "9/m3
530
30.8
30.4
243
76.6
Source
severity
8.82
0.12
0.51
0.94
1.27
Particulate 0.327 to
0.778 25.7 to 61.2 0.099 to 0.24
Emissions from evaporation and granulation are controlled;
emissions from prilling are uncontrolled, corresponding to
current ind_us±ry.-practice..
51
-------
Maximum severities for ground level operations can be determined
by plotting S versus D, using the appropriate emission rate for
each process. This is done in Figure 25 for fugitive ammonia
emissions from the bulk loading of urea solutions and in
Figure 26 for particulate emissions from the bulk loading of
solids. In each case, the severity falls below 1.0 well within
the average plant boundary (D = 400 m). For fugitive ammonia
emissions, S = 0.15 at the plant boundary; for fugitive particu-
late emissions, S = 0.045. Fugitive emissions from coating
operations are not evaluated because: 1) coating takes place
during less than 10% of urea production, and 2) the coating rate
at an average plant is not known.
o:
LU
LU
on
UJ
o
a;
O
CO
10
6
4
1.0
0.6
0.4
0.2
0.10
0.06
0.04
0.02
0.01
20 40 60 100 200 400 600 1000 2000
DISTANCE, m
Figure 25. Source severity for fugitive ammonia emissions
from the bulk loading of urea solutions.
52
-------
1/1
LU
O
O
CO
10
6
4
1.0
E 0.6
ce
rS 0.4
0.2
0.10
0.06
0.04
0.02
0.01
I
10 20 40 60 100 200
DISTANCE, m
400 600 1000
Figure 26.
Source severity for fugitive particulate
emissions from the bulk loading of urea solids.
In addition to the single value emission parameters presented in
Table 11, it is desirable to determine the distribution of
source severities by plant size across the industry. Figures 27-
31 show these distributions for ammonia and particulate emissions
from the evaporator, prill tower, and granulator. Severity
distributions were calculated in the same way as the severities
for average processes, using the individual plant capacities
from Appendix A. An average emissions factor of 0.142 g/kg was
used for particulate emissions from the granulator.
Severities do not exceed 1.0 for particulate emissions from the
evaporator and granulator (these emissions are controlled by
scrubbers). Severities exceed 1.0 for other emissions in the
following proportions:
53
-------
100 i-
o
££
GO ^
Q=
O
•£ 60
t/l o
£ ^
Is
u. O
O UJ
£ <->
20
12
16 20
SOURCE SEVERITY
24
28
32
36
Figure 27. Source severity distribution for ammonia
emissions from the evaporator.
100
e*f ijj
O >
UJ
5^ 60
in O
40.
7
0.1
0.2 0.3
SOURCE SEVERITY
0.4
0.5
Figure 28.
Source severity distribution for particulate
emissions from the evaporator.
54
-------
0.4 0.8
1.2 1.6 2.0 2.4
SOURCE SEVERITY
2.8 3.2 3.6
Figure 29. Source severity distribution for ammonia and
particulate emissions from the prilling tower,
100
Q£ LU
O >
tn
£
60
40
20
1.0
2.0 3.0
SOURCE SEVERITY
4.0
5.0
Figure 30.
Source severity distribution for ammonia
emissions from the granulator.
55
-------
Emission
Plants with S>1
Ammonia from evaporator
Ammonia from granulator
Ammonia from prill tower
Particulates from prill tower
100%
44%
12%
40%
0.2
0.3 0.5
SOURCE SEVERITY
0.6
0.7
0.8
Figure 31.
Source severity distribution for particulate
emissions from the granulator.
3. Affected Population
The affected .population is defined as the number of persons
around an average urea plant who are_exposed to emission con-
centrations that cause the ratio of x/F to exceed 1.0. A plume
dispersion equation determines the two downwind distances for
which rhe ratio falls below 1.0 (Figure 32). The affected pop-
ulation is then determined by multiplying the annular area where
X~/F > 1.0 by the average population density (100 persons/km2)
around a representative urea plant (3, 35).
56
-------
DISTANCE FROM SOURCE
Figure 32.
General distribution of x/F as a function of distance
from the source showing the two general roots to the
plume dispersion equation.
The affected population was calculated for ammonia and particu-
late emissions from the evaporator, prilling tower, and granu-
lator. Results are shown in Table 12. The affected population
for fugitive ammonia and particulate emissions is zero.
TABLE 12. POPULATION AFFECTED BY EMISSIONS FROM
AVERAGE UREA PROCESSES
Emission point Emission species Affected population
Evaporator
Prill tower
Granulator
Ammonia
Particulate
Ammonia
Particulate
Ammonia
Particulate
247
0
22
0
Affected population is greater than zero even though
severity is less than 1.0 (S = 0.94) because different
forms of the plume dispersion equation must be used.
See References 3 and 35 for more details.
4. Particulate Emissions Burden
The environmental impact of the entire urea industry .can be
measured in terms of total industry emissions. Total particulate
emissions are listed on a state-by-state and national basis in
Table 13; they were calculated in the following manner: state
production capacities were taken from the plant listing in
Appendix A. Production data were derived by apportioning total
urea production in 1975 according to state capacities. A total
particulate emission factor was found by 1) multiplying the
emission factors for each process step by the percent of total
57
-------
TABLE 13. PARTICULATE EMISSION BURDEN FROM UREA PRODUCTION BY STATE
State
Alabama
Alaska
Arkansas
California
Florida
Georgia
Idaho
Illinois
Iowa
Kansas
Louisiana
Mississippi
Missouri
Nebraska
New Mexico
New York
North Carolina
Ohio
Oklahoma
Oregon
Tennessee
Texas
Washington
Wyoming
All other
states
U.S. TOTAL
Capacity,
10^ metric
tons/year
78
308
360
441
21
108
14
77
200
302
1,640
127
153
188
145
68
150
263
375
62
472
255
34
54
0
5,895
Estimated
1975 production,
10 3 metric tons
45.6
180.2
210.7
258.1
12.3
63.2
8.2
45.1
117.0
176.7
959.7
74.3
89.5
110.0
84.8
39.8
87.8
153.9
219.4
36.3
276.2
149.2
19.9
31.6
0
3,450
Particulate
emissions from
urea production,
metric tons/yr
24.3
95.8
112.0
137.2
6.5
33.6
4.4
24.0
62.2
94.0
510.4
39.5
47.6
58.5
45.1
21.2
46.7
81.8
116.7
19.3
146.9
79.4
10.6
16.8
0
1,830
Total particulate emissions
from all stationary sources,
10 3 metric tons
Reference 37 (Reference 36)
2,002 (1,179)
16,340 (14)
1,619 (138)
5,675 (1,006)
2,430 (226)
2,331 (404)
2,430 (55)
3,584 (1,143)
2,579 (216)
3,358 (348)
1,651 (380)
1,490 (168)
2,839 (202)
3,049 (95)
3,548 (103)
2,704 (160)
2,203 (481)
3,054 (1,766)
2,276 (94)
2,885 (169)
1,789 (410)
9,302 (549)
2,204 (162)
2,851 (75)
46,807 (8,329)
131,000a (17,872)
Percent of total
particulates resulting
from urea production
<0.01
<0.01
<0.01
<0.01
<0.01
<0.01
<0.01
<0.01
<0.01
<0.01
0.03
<0.01
<0.01
<0.01
<0.01
<0.01
<0.01
<0.01
<0.01
<0.01
<0.01
<0.01
<0.01
<0.01
0
0.0014
(<0.01)
(0.68)
(0.08)
(0.01)
( < 0 . 01 )
( < 0 . 01 )
( <0 . 01 )
(<0 . 01)
(0.03)
(0.03)
(0.13)
(0.02)
(0.02)
(0.06)
(0.04)
(0.01)
(0.01)
(<0. 01)
(0.12)
(0.01)
(0.04)
(0.01)
(<0. 01)
(0.02)
(0)
(0.010)
Approximately 75% of this total is due to fugitive particulate emissions from unpaved roads.
-------
production undergoing that operation and 2) summing these
values together.
Evaporation: 62% x 0.107 g/kg = 0.066 g/kg
Prilling: 62% x 15% x 3.2 g/kg = 0.298 g/kg
Granulation: 62% x 85% x 0.142 g/kg = 0.075 g/kg
Bagging and loading: 62% x 0.15 g/kg = 0.093 g/kg
TOTAL 0.532 g/kg
Solid urea production is 62% of the total production; 15% of the
solid urea product is prilled, 85% granular. Coating operations,
used on less than 10% of production, were not included in the
total; at most, they could increase overall particulate emissions
by 20%.
The particulate emission burden for urea production is the ratio
of particulate emissions from the urea industry to total particu-
late emissions from all sources. Different data bases can be
used to obtain a state-by-state emissions inventory. Three pos-
sibilities are a 1972 emissions summary from the National
Emissions Data System (NEDS) published by the EPA (36), a 1975
emissions listing assembled by Monsanto Research Corporation
(37), and a 1977 emissions listing by Monsanto Research Cor-
poration (38). The NEDS listing is the most conservative for
particulate emissions since a number of open sources (i.e., not
emitted from a stack) such as unpaved roads are not included in
the tabulation. A 1975 NEDS summary is scheduled to be issued in
late 1977, but it is not yet available. Therefore the 1975
emissions inventory by Monsanto Research Corporation was used to
calculate the state emission burdens in Table 13. Emission
burdens based on NEDS are also given in parentheses. On either
basis, state emission burdens are all less than 1% and the
national burden is less than 0.1%.
(36) 1972 National Emission Report; National Emissions Data
System (NEDS) of the Aerometric and Emissions Reporting
System (AEROS). EPA-450/2-74-012, U.S. Environmental
Protection Agency, Research Triangle Park, North Carolina,
June 1974. 434 pp.
(37) Reznik, R. B. Source Assessment: Flat Glass Manufacturing
Plants. EPA-600/2-76-032b, U.S. Environmental Protection
Agency, Research Triangle Park, North Carolina, March 1976.
147 pp.
(38) Eimutis, E. C., and R. P. Quill. Source Assessment: State-
by-State Listing of Criteria Pollutants. EPA-600/2-77-107b,
U.S. Environmental Protection Agency, Research Triangle
Park, North Carolina, July 1977. 138 pp.
59
-------
SECTION V
CONTROL TECHNOLOGY
The use of air emission controls in urea production is concen-
trated in the production of solid products after the aqueous
urea solution has been formed. Emissions from urea solution
production are of a fugitive nature, and controls have not been
applied because no state standards are exceeded. The discussion
in Section IV indicates that these fugitive emissions have sever-
ities less than 1.0, beyond the plant boundary.
In the solidification process there are two steps to which con-
trol technology has been applied - solution concentration and
solid formation. Controls are designed to reduce particulate
emissions, while ammonia emissions are lowered only incidentally,
A. SOLUTION CONCENTRATION
In the urea industry, solutions are concentrated by crystalliza-
tion and evaporation.
As mentioned in Section IV, emissions from crystallization are
less than those from evaporation, and no special control devices
are used. Emissions from evaporators are passed through a con-
denser or wet scrubbers. Although condensers lower the amount
of emissions, they are not designed as air pollution control
equipment, and therefore are not as effective as scrubbers.
According to industrial sources, more than 50% of urea produc-
tion facilities utilize condensers to cool exhaust gases from
the evaporator and condense the water vapor. A portion of the
ammonia vapor and entrained particulates in the exhaust is col-
lected in the condensate. The liquid condensate can then be
sewered (when permitted), treated and recycled, sold as a dilute
fertilizer solution, or sent to an adjacent facility for use in
a fertilizer mixture. No data have been reported on the removal
efficiencies of condensers, but they do not perform as well as
wet scrubbers.
Plants without condensers use wet scrubbers to remove particu-
lates and recover product. The scrubbing liquid is an aqueous
urea solution. Tests have shown that low energy scrubbers can
be over 98% efficient when operating with pressure drops of
^1.5 kPa (29). Ammonia emissions are not controlled as effect-
ively (75% removal at best; see Section IV.B.2.a). This could
60
-------
be improved by using an acidic scrubbing liquid, such as a dilute
urea-nitric acid solution. Since ammonia is removed from a gas
stream by absorption into the scrubbing solution, removal effici-
encies are strongly pH-dependent.
B. SOLID FORMATION
Solid urea is produced by either granulating or prilling the con-
centrated urea solution. Granulation units emit a concentrated
dust stream that may contain as much as 20% of the incoming urea
weight (34). High energy (3 kPa to 8kPa) wet scrubbers are used
on these exhaust streams to reduce emissions and to recover valu-
able product for recycle. Two types of scrubbers currently in u
use are a venturi scrubber with a pressure drop of 4.98 kPa to
-------
1.0
0.8
0.6
0.4
0.2
EACH SYMBOL TYPE REPRESENTS
A DIFFERENT TEST
2 5 10 20 30 40 50 60 70 80 90 95
PERCENTAGE OF PARTICLES AT OR LESS THAN GIVEN SIZE
Figure 33.
Size distribution of all particulates in
prill tower exhaust (39).
i.o
0.8
0.6
0.4
0.2
0.1
EACH SYMBOL TYPE REPRESENTS
A DIFFERENT TEST
'6.01 0.10.20.5 12 5 10 20 30 40 50 60 70 80 90 95 98 99 99.899.9 99.99
PERCENTAGE OF PARTICLES AT OR LESS THAN GIVEN SIZE
Figure 34. Size distribution of particles -5 inn
in prill tower exhaust (39).
62
-------
consist of a wetted wire mesh or other low-energy filtering
medium, and a particulate removal efficiency of greater than 50%
is rarely achieved.
Dutch State Mines' coal dust removal technology (34) developed
a simple device which claims a higher removal efficiency than
mentioned above. The dust-laden air is directed through a series
of guide vanes that are sprayed with a circulating solution of
urea. An 80% particulate removal has been estimated with a pres-
sure drop of only 37.3 Pa.
Many other systems have been or are being tested on prill tower
exhausts, including the following:
• Monsanto Enviro-Chem Systems' Brink® High
Velocity or High Efficiency Mist Eliminators
• BECO scrubbers (e.g., Model V-2000)
• Mist-Air scrubbers
• Wet electrostatic precipitators
• Anderson 2000 CHEAF scrubber
• Fluid Ionics Hydroprecipitol
• TRW-Charged Droplet scrubber
Under normal operating conditions these scrubbers or other wet
devices are expected to remove a part of the ammonia which may
exist in the stream. Exact removal efficiencies depend upon
operating and design parameters and local conditions.
C. FUTURE CONSIDERATIONS
The application of additional controls to urea production will
be slow compared with their application to a sister industry,
ammonium nitrate manuacturing, because: the applicability of
several of the systems for use on prill towers has yet to be
proven, and in other processes, the installed control technol-
ogy is adequate to meet emission regulations and reduce product
losses.
The major problem area in the industry is the control of prill
tower particulate emissions to meet strict opacity regulations.
Because of the high quantity of extremely small particles (Fig-
ures 33 and 34) in the exhaust stream, collection efficiencies
to reduce emissions to the levels needed to meet the opacity
requirements are significantly higher than those needed to
meet mass emission regulations.
63
-------
In other areas, such as the bulk loading of solutions and evapo-
ration, available techniques could further reduce emissions. For
example, acidic scrubbing solutions can be used for better con-
trol of ammonia emissions. Other possibilities are the use of
floating roofs and other techniques developed by the petroleum
industry to reduce hydrocarbon losses from storage tanks. These
controls are not presently employed because the emissions from
these points do not violate state standards and because product
losses are not excessive.
D. POTENTIAL IMPACT OF CONTROLS
The greatest potential impact of control technology would result
from better control of particulate emissions from prill towers
and ammonia emissions from evaporators and granulators. If par-
ticulate emissions from the prill tower were controlled by 95%,
the resulting average source severity would be 0.05. A reduction
in ammonia emissions from the evaporator and granulator by an
additional 99% would result in average severities of 0.088 and
0.013, respectively.
64
-------
SECTION VI
GROWTH AND NATURE OF THE INDUSTRY
A. PRESENT TECHNOLOGY
Urea production technology is well established with no major
process changes anticipated. Each company has its own modifica-
tion of the basic design based upon individual plant operation.
B. EMERGING TECHNOLOGY .
Energy conservation is the first area of emerging technology for
the urea industry. There are two major new energy conserving
processes. Mitsui Toatsu has developed a process for heat ex-
change between the urea process and the ammonia process to
effect overall heat conservation (7). Several other companies
including SNAM PROGETTI are investigating similar processes (17).
A second energy conserving process is the Mavrovic "Heat Recycle"
urea process (40, 41). This process uses an isothermal reactor,
specialized decomposers, and strippers designed to achieve
higher yield, lower utility costs, and ease of control. In-
ternal heat recovery and exacting operation are key elements in
this process.
A second area of emerging technology lies in derivations from
traditional flow and processing elements. One of these is the
Chemico Thermo-Urea process (Figure 35) which uses a multistage
centrifugal compressor to decompose carbamate and recycle it to
the reactor. This process also recovers steam, which is a
utility savings.
Esso Research and Engineering Co. has also developed a process
that departs from the traditional. This process synthesizes
urea at traditional temperatures; however, it incorporates the
decomposer/stripper and the reactor into a single unit.
(40) The Mavrovic "Heat Recycle: Urea Process. Nitrogen,
78:19-21, July/August 1972.
(41) Mavrovic, I. Improved Urea Process is Developed. Chemical
Engineering Progress, 70(2):69, 71, 73, 1974.
65
-------
These new processes do not affect the overall environmental air
impact of urea manufacture, since they are all involved with the
solution production stage.
STEAM [~
SEPARATOR
MIXED GAS
COOLER
REACTOR
VENT
f
STEAM ..
r— '
-— C02GAS
-CONDENSATE
• LOW PRESSURE STEAM
INTERMEDIATE PRESSURE STEAM
HIGH PRESSURE STEAM
UREA SOLUTION
REACTOR
CONDENSATE
PUMP
INERT GAS
PURGE
MULTISTAGE
DECOMPOSITION
SEPARATION
UNIT
O
NH3 STORAGE
c.
Figure 35. Chemico Thermo-Urea process.
INDUSTRY PRODUCTION TRENDS
Figure 36 illustrates the actual growth in the urea industry
from 1960 through 1975 and projected growth through 1980.
Historical production information is available from the U.S.
Tariff Commission and the U.S. International Trade Commission,
(42-56). Projected capacities are based on known plans for con-
struction (7). Projected production is based on 87.1% of
projected capacity. (The 87.1% value was determined by averaging
production/capacity ratios from 1960 through 1975.)
(42) Synthetic Organic Chemicals, United States Production and
Sales, 1960. TC Publication 34, United States Tariff Com-
mission, Washington, D.C., 1961. p. 58.
(43) Synthetic Organic Chemicals, United States Production and
Sales, 1961. TC Publication 72, United States Tariff Com-
mission, Washington, D.C., 1962. p. 56.
(44) Synthetic Organic Chemicals, United States Production and
Sales, 1962. TC Publication 114, United States Tariff Com-
mission, Washington, D.C., 1963. p. 59.
(45) Synthetic Organic Chemicals, United States Production and
Sales, 1963. TC Publication 143, United States Tariff Com-
mission, Washington, D.C., 1964. p. 58.
(continued)
66
-------
Urea production is anticipated to increase at a rate of 4.7% to
8% per year (5). This growth will be promoted by three factors:
1) domestic melamine production has switched from dicyandiamide
to urea as a raw material, 2) prilled urea is being promoted as
a fertilizer export, and 3) research is being conducted in
forest fertilization using urea as the fertilizer. As a result
of this growth, emissions from the industry in 1978 should be
32% to 59% greater than in 1972.
(46) Synthetic Organic Chemicals, United States Production and
Sales, 1964. TC Publication 167, United States Tariff Com-
mission, Washington, D.C., 1965. p. 59.
(47) Synthetic Organic Chemicals, United States Production and
Sales, 1965. TC Publication 206, United States Tariff Com-
mission, Washington, D.C., 1965. p. 59.
(48) Synthetic Organic Chemicals, United States Production and
Sales, 1966. TC Publication 248, United States Tariff Com-
mission, Washington, D.C., 1968. p. 58.
(49) Synthetic Organic Chemicals, United States Production and
Sales, 1967. TC publication 295, United States Tariff Com-
mission, Washington, D.C., 1969. p. 56.
(50 Synthetic Organic Chemicals, United States Production and
Sales, 1968. TC Publication 327, United States Tariff Com-
mission, Washington, D.C., 1970. p. 213.
(51) Synthetic Organic Chemicals, United States Production and
Sales, 1969. TC Publication 412, United States Tariff Com-
mission, Washington, D.C., 1971. p. 203.
(52) Synthetic Organic Chemicals, United States Production and
Sales, 1970. TC Publication 479, United States Tariff Com-
mission, Washington, D.C., 1972. p. 211.
(53) Synthetic Organic Chemicals, United States Production and
Sales, 1971. TC Publication 614, United States Tariff Com-
mission, Washington, D.C., 1973. p. 204.
(54) Synthetic Organic Chemicals, United States Production and
Sales, 1972. TC Publication 681, United States Tariff Com-
mission, Washington, D.C., 1974. p. 203.
(55) Synthetic Organic Chemicals, United States Production and
Sales, 1973. ITC Publication 728, United States Inter-
national Trade Commission, Washington, D.C., 1975. p. 201.
(56) Synthetic Organic Chemicals, United States Production and
Sales, 1974. USITC Publication 776, United States Inter-
national Trade Commission, Washington, D.C., 1976. p. 199.
67
-------
2 4 -
S 3 -
1960
1965
1975
1980
Year
Figure 36. Urea capacity and production trends (42-56)
aCapacity projection based on announced or anticipated
expansions or new facilities. Production projection based on
an 87.1% operating rate.
68
-------
REFERENCES
1. Mavrovic, I. Urea & Urea Derivatives. In: Kirk-Othmer
Encyclopedia of Chemical Technology, Second Edition,
Vol. 21. John Wiley & Sons, Inc., New York, New York, 1969.
pp. 37-56.
2. Rawlings, G. D., and R. B. Reznik. Source Assessment:
Fertilizer Mixing Plants. EPA-600/2-76-032c, U.S. Environ-
mental Protection Agency, Research Triangle Park, North
Carolina, March 1976. 201 pp.
3. Search, W. J., and R. B. Reznik. Source Assessment:
Ammonium Nitrate Production. EPA-600/2-77-107i, U.S.
Environmental Protection Agency, Research Triangle Park,
North Carolina, September 1977. 78 pp.
4. Rawlings, G. D., and R. B. Reznik. Source Assessment:
Synthetic Ammonia Production. EPA-600/2-77-107m, U.S.
Environmental Protection Agency, Research Triangle Park,
North Carolina, November 1977. 82 pp.
5. Urea. Chemical Marketing Reporter, 210(5):9, 1976.
6. Synthetic Organic Chemicals, United States Production and
Sales, 1975. USITC Publication 804, U.S. International
Trade Commission, Washington, D.C., 1977. p. 195.
7. World Fertilizer Capacity--Urea. Tennessee Valley Authority,
Muscle Shoals, Alabama, June 7, 1976. 6 pp.
8. Strelzoff, S., and L. H. Cook. Nitrogen Fertilizers. In:
Advances in Petroleum Chemistry and Refining - Volume 10,
J. J. McKette, Jr., ed. John Wiley and Sons, Inc., New
York, New York, 1976. pp. 315-406.
9. FreJacques, M. Les bases theoriques de la syntheses indus-
trielle de 1'uree. Chimie & Industrie, 60(l):22-35, 1948.
10. Slack, A. V. Fertilizer Developments and Trends. Noyes
Development Corporation, Park Ridge, New Jersey, 1968.
pp. 119-145.
11. CPI-Allied Chemical Urea Process. Nitrogen, 47:32-33,
May/June 1967.
69
-------
12. Urea Processes Today. Nitrogen, 64:17-24, March/April 1970,
13. Cook. L. H. Urea. Chemical Engineering Progress,
50(7):327-331, 1954.
14. Swiss Solve Urea Problems. Chemical Engineering, 59(11):
219-220, 222, 1952.
15. Tonn, W. H., Jr. How the Competitive Urea Processes Com-
pare Today. Chemical Engineering, 62 (10) :186-190, 1955.
16. Borelli, T., and G. Nardin. Procede Montecatini Edison
pour la production d'uree de gros tonnage. Chimie et
Industrie - Genie Chimique, 104 (16):2017-2022, 1971.
17. Montecatini Edison's New Total Recycle Urea Process. Hydro-
carbon Processing, 49 (8):111-112, 1970.
18. Urea Via the Pechiney Process. Chemical Engineering,
62 (4):320-323, 1955.
19. Urea (Mitsui Toatsu Process) - The M. W. Kellog Co.
Hydrocarbon Processing, 50(11):215, 1971.
20. Yoshimura, S. Optimize New Urea Process. Hydrocarbon
Processing, 49 (6):111-115, 1970.
21. Pagani, G., and U. Zardi. Integrate for Lowest Urea Cost.
Hydrocarbon Processing, 5 (11):125-129, 1972.
22. Pagani, G., and U. Zardi. SNAM PROGRETTI Stripping Tech-
nique: One Basic Principle for Two Methods of Producing
Urea. Presented at the 74th National Meeting of the
American Institute of Chemical Engineers, New Orleans,
Louisiana, February 1973. 16 pp.
23. Zardi, U., and F. Ortu. Recycle Carbamate Via Ejector.
Hydrocarbon Processing, 49 (4):115-116, 1970.
24. Urea - SNAM PROGRETTI. Hydrocarbon Processing, 54(11)210,
1975.
25. Urea Production. Nitrogen, 91:44, September/October 1974.
26. The Lonza-Lummus Urea Process. Nitrogen, 33:31-32,
January 1965.
27. Granular Urea and Ammonium Nitrate. Nitrogen, 95:31-36,
May/June 1975.
70
-------
28. Personal communication with J. C. Reynolds, C & I Girdler,
Inc., Louisville, Kentucky, August 1977.
29. Reed, R. M., and J. C. Reynolds. The Spherodizer Granu-
lation Process. Chemical Engineering Progress, 69(2):62-66,
1973.
30. Young, R. D., and I. W. McCamy. TVA Development Work and
Experience with Pan Granulation of Fertilizers. Canadian
Journal of Chemical Engineering (Ottawa, Ontario), 45(2):
50-56, 1967.
31. Reynolds, J. C. and R. M. Reed. Progress Report on SPHERO-
DIZER Granulation 1975-1976. In: Proceedings of The
Fertilizer Institute Environmental Symposium (New Orleans,
Louisiana), The Fertilizer Institute, Washington, D.C.,
1976. pp. 193-215.
32. TLVs® Threshold Limit Values for Chemical Substances and
Physical Agents in the Workroom Environment with Intended
Changes for 1976. American Conference of Governmental
Industrial Hygienists, Cincinnati, Ohio, 1976. 94 pp.
33. Sanders, L. Monitoring and Control of Gaseous and Particu-
late Emission from Fertilizer Complex. Paper No. 75-5.6,
presented at 69th Annual Meeting of the Air Pollution Con-
trol Association, Portland, Oregon, June 27-July 1, 1976.
14 pp.
34. Bress, D. F., and R. K. Fidler. New Concepts in Design of
Urea Plants. Paper No. 13C, presented at 74th National
Meeting of American Institute of Chemical Engineers, New
Orleans, Louisiana, March 12-15, 1973. 22 pp.
35. Turner, D. B. Workbook of Atmospheric Dispersion Estimates.
Public Health Service Publication No. 999-AP-26, U.S. Depart-
ment of Health, Education, and Welfare, Cincinnati, Ohio,
May 1970. 84 pp.
36. 1972 National Emission Report; National Emissions Data
System (NEDS) of the Aerometric and Emissions Reporting
System (AEROS). EPA-450/2-74-012, U.S. Environmental
Protection Agency, Research Triangle Park, North Carolina,
June 1974. 434 pp.
37. Reznik, R. B. Source Assessment: Flat Glass Manufacturing
Plants. EPA-600/2-76-032b, U.S. Environmental Protection
Agency, Research Triangle Park, North Carolina, March 1976.
147 pp.
71
-------
38. Eimutis, E. C., and R. P. Quill. Source Assessment: State-
by-State Listing of Criteria Pollutants. EOA-600/2-77-107b,
U.S. Environmental Protection Agency, Research Triangle
Park, North Carolina, July 1977. 138 pp.
39. Personal communication with T. R. Metzger, Monsanto Enviro-
Chem Systems, Inc., St. Louis, Missouri, 11 May 1976.
40. The Mavrovic "Heat Recycle" Urea Process. Nitrogen,
78:19-21, July/August 1972.
41. N.avrovic, I. Improved Urea Process is Developed. Chemical
Engineering Progress, 70(2) :69, 71, 73, 1974.
42. Synthetic Organic Chemicals, United States Production and
Sales, 1960. TC Publication 34, United States Tariff Com-
mission, Washington, D.C., 1961. p. 58.
43. Synthetic Organic Chemicals, United States Production and
Sales, 1961. TC Publication 72, United States Tariff Com-
mission, Washington, D.C., 1962. p. 56.
44. Synthetic Organic Chemicals, United States Production and
Sales, 1962. TC Publication 114, United States Tariff Com-
mission, Washington, D.C., 1963. p. 59.
45. Synthetic Organic Chemicals, United States Production and
Sales, 1963. TC Publication 143, United States Tariff Com-
mission, Washington, D.C., 1964. p. 58.
46. Synthetic Organic Chemicals, United States Production and
Sales, 1964. TC Publication 167, United States Tariff Com-
mission, Washington, D.C., 1965. p. 59.
47. Synthetic Organic Chemicals, United States Production and
Sales, 1965. TC Publication 206, United States Tariff Com-
mission, Washington, D.C., 1967. p. 59.
48. Synthetic Organic Chemicals, United States Production and
Sales, 1966. TC Publication 248, United States Tariff Com-
mission, Washington, D.C., 1968. p. 58.
49. Synthetic Organic Chemicals, United States Production and
Sales, 1967. TC Publication 295, United States Tariff Com-
mission, Washington, D.C., 1969. p. 56.
50. Synthetic Organic Chemicals, United States Production and
Sales, 1968. TC Publication 327, United States Tariff Com-
mission, Washington, D.C., 1970. p. 213.
72
-------
51. Synthetic Organic Chemicals, United States Production and
Sales, 1969. TC Publication 412, United States Tariff Com-
mission, Washington, D.C., 1971. p. 203.
52. Synthetic Organic Chemicals, United States Production and
Sales, 1970. TC Publication 479, United States Tariff Com-
mission, Washington, D.C., 1972. p. 211.
53. Synthetic Organic Chemicals, United States Production and
Sales, 1971. TC Publication 614, United States Tariff Com-
mission, Washington, B.C., 1973. p. 204.
54. Synthetic Organic Chemicals, United States Production and
Sales, 1972. TC Publication 681, United States Tariff Com-
mission, Washington, D.C., 1974. p. 203.
55. Synthetic Organic Chemicals, United States Production and
Sales, 1973. ITC Publication 728, United States Inter-
national Trade Commission, Washington, D.C., 1975. p. 201.
56. Synthetic Organic Chemicals, United States Production and
Sales, 1974. USITC Publication 776, United States Inter-
national Trade Commission, Washington, D.C., 1976. p. 199.
57. Standard for Metric Practice. ANSI/ASTM Designation
E 380-76e, IEEE Std 268-1976, American Society for Testing
and Materials, Philadelphia, Pennsylvania, February 1976.
37 pp.
73
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APPENDIX A
LOCATION, POPULATION DENSITY, AND CAPACITY DATA
FOR UREA PLANTS IN THE U.S.
74
-------
TABLE A-l. LOCATION, POPULATION DENSITY, AND CAPACITY DATA FOR UREA PLANTS IN THE U.S. (7)
ui
County, parish,
or borough
Company
Air Products and Chemicals, Inc.
Allied Chemical Corp. , Union
Texas Petroleum Division
Agricultural Department
Specialty Chemicals Division
American Cyanamid Co.
Beker Industries Corp.
Borden, Inc. - Borden Chemical
Division
California Oil Purification Co.
CF Industries, Inc.
Cherokee Nitrogen Co.
Coastal States Gas Corp.,
Colorado Interstate Corp. ,
subsidiary; Wycon Chem. Co. ,
subsidiary
Columbia Nitrogen Corp.
City/state
Pace, Fla.
Geismar, La.
Omaha , Nebr .
South Point, Ohio
Fortier, La.
Carlsbad, N. Mex.
Geismar, La.
Ventura , Calif.
Donaldsonville, La.
Fremont , Nebr .
Olean, N.Y.
Tunis, N.C.
Tyner , Tenn .
Pryor, Okla.
Cheyenne , Wyo .
Augusta, Ga.
Population
density, Annual capacity,
Name persons/km 10 ^ metric tons
Escambia
Ascension
Douglas
Lawrence
Jefferson
Eddy
Ascension
Ventura
Ascension
Dodge
Cattaraugus
Hertford
Hamilton
Mayes
Laramie
Richmond
116.6
46.7
446.3
47.0
393.6
3.7
46.7
77.6
46.7
25.3
23.4
24.5
170.3
12.8
7.9
192.3
21
222
127
63
132
145
200
120
329
16
68
150
30
16
54
35
(continued)
-------
TABLE A-1 (continued).
County, parish,
or borough
Company
Enserch Corp. , Nipak, subsidiary
Esmark, Inc.
Farmland Industries, Inc.,
Cooperative Farm Chemicals
Assoc. (CFCA)
Gardiner Big River, Inc.
General American Oil of Texas
Goodpasture, Inc.
W. R. Grace & Co.
Hercules, Inc.
Kaiser Aluminum and Chemical
Corporation
Mississippi Chemical Corporation
City/state
Kerens, Tex.
Pryor, Okla.
Pryor, Okla.
Beaumont, Tex.
Dodge City, Kans.
Lawrence , Kans .
Lawrence , Kans .
Helena, Ark.
Pasadena, Tex.
Pasadena, Tex.
Dimmitt, Tex.
Memphis, Tenn.
Memphis, Tenn.
Hercules, Calif.
Hercules, Calif.
Louisiana, Mo.
Louisiana, Mo.
Savannah, Ga.
Yazoo City, Miss.
Yazoo City, Miss.
Name
Navarro
Mayes
Mayes
Jefferson
Ford
Douglas
Douglas
Phillips
Harris
Harris
Castro
Shelby
Shelby
Alameda
Alameda
Pike
Pike
Chatham
Yazoo
Yazoo
Population
density, Annual capacity,
persons/km2 103 metric tons
10.8 63
12.8 93!
12.8 77/ 17°
98.5 45
7.8 58
38-6 244
38.6
22.3 61
385'9 85
385.9 HD
4.5 21
367.6 125\
367.6 317J
557.8 181
557.8 18)
9.4 32\
9.4 54 j85
158.7 73
11.2
, , - 127
11.2
(continued)
-------
TABLE A-l (continued).
County, parish,
or borough
Company
N-Ren Corp., (see also Cherokee
Nitrogen Co. , and St. Paul
Ammonia Products, Inc.)
Olin Corp. , Agricultural
Chemicals Division
Phillips Pacific Chemical Co.
Phillips Petroleum Co.
Reichold Chemicals, Inc.
St. Paul Ammonia Products, Inc.
J. R. Simplot Co.
Skelly Oil Co. , Hawkeye Chemical
Co., subsidiary
Standard Oil Co. , (Ohio) -
Vistron Corp. , - subsidiary
Tennessee Valley Authority
Terra Chemicals International,
Inc.
Triad Chemical
City/state
Plainview, Tex.
Lake Charles, La.
Finley, Wash.
Finley, Wash.
Beatrice, Nebr.
Beatrice, Nebr.
St. Helen, Oreg.
East Dubuque, 111.
Pocatello, Idaho
Clinton, Iowa
Lima , Ohio
Muscle Shoals, Ala.
Port Neal, Iowa
Port Neal, Iowa
Donaldsonville, La.
Donaldsonville, La.
Population
density, Annual capacity,
Name persons/km2 10 3 metric tons
Hale
Calcasieu
Benton
Benton
Gage
Gage
Columbia
Jo Daviess
Bannock
Clinton
Allen
Colbert
Woodbury
Woodbury
Ascension
Ascension
13.2
49.7
14.91
14.9)
11. 6\
11. 6/
17.2
13.5
17.5
31.3
104.4
31.8
44.61
44.6 f
46.7
46.7
41
150
34
45
62
77
14
55
200
55
145
426
(continued)
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TABLE A-l (continued).
oo
County, parish,
or borough
Company
Tyler Corporation -Atlas Powder Co.,
subsidiary
Union Oil Company of California -
Collier Carbon and Chemical
Corp. , subsidiary
United States Steel Corporation,
USS Agri-Chemicals, Division
Valley Nitrogen Producers, Inc.
The Williams Companies Agrico
Chem. Co., subsidiary
City/state
Joplin, Mo.
Brea, Calif.
Kenai, Alaska
Cherokee, Ala.
El Centre, Calif.
Helm, Calif.
Helm, Calif.
Blytheville, Ark.
Donaldsonville, La.
Verdigris, Okla.
Name
Jasper
Orange
Kenai Pen.
Colbert
Imperial
Fresno
Fresno
Mississippi
Ascension
Rogers
Population
density,
persons/km2
47.3
695.8
0.5
31.8
6.7
26.3
26.3
26.3
46.7
14.9
Annual capacity,
10 3 metric tons
67
109
308
23
135
19 I 41
22 J
299
181
190
TOTAL
5,895
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APPENDIX B
DERIVATION OF AVERAGING EQUATIONS
When the average emission factor for a source must be determined
from a collection of average values, an averaging procedure must
be devised since previously used concepts are not applicable.
The following rationale was used to determine Equations 7 and 8
in Section IV.B.3 of the text.
Let m be the number of aggregate samples, with each sample having
a_certain number of points (njj . An average for all of the points
(xi) within a given sample can be calculated by the following
equation:
ni
n.
(B-l)
where x. = the value of a point in the i-th sample
The standard deviation can be calculated according to the follow-
ing equation:
^ _ 2
(X. - X.)
si~ = J"V - i (B-2)
If each data point, xj, for every sample were identifiable, the
average for the entire group of samples could be found by summing
the entire set of points and dividing by the total number of
points as follows:
m n-[
y y x
L-J L-J D
*T = ^r^ (B-3>
79
-------
However, since the exact data points are not identifiable, only
the averages and total number of points for each sample, Equa-
tion B-l, can be rearranged and substituted into B-3, giving:
m
n. x.
(B-4)
To determine the standard deviation for "i" samples we must first
rearrange and expand Equation B-2.
ni JH ni 2
x.
x. - 2x. > ' x. + ^ * - {B~5)
D
j=l
By reeirranging Equation B-l and substituting into Equation B-5:
_ 2
Si
_2
Rearranging,
ni
(B-7)
80
-------
If each data point, xj, for every sample were identifiable, the
standard deviation for the entire group of samples, s«r, could be
found by the following equation:
m n-;
V V (x - x )2
„ 2L, L~I (xj v
Since
Then
n. - 1
m n-\ m / n.; n-; n-:
- 2 ^ ^ 2 - ^ - 2
j=l
m n
2 o- 2 - 2
X " " + X
Z
j=l
m nj_ m
(B-9)
m n. m
*" \
E»J
"1
By substituting Equation B-7 into B-10 the following equation is
determined:
81
-------
m m
2 v~^ — 2 —2
(n, -1) +
m
(B-ll)
Or
ST '
m
m
m
E2 v— * _ 2 _ 2 T— -\
s. (n. - 1) + > n.x. - xm > n
11 / ^ 11 T / ^
m
IXH
(B-12)
Equation B-12 is the same as Equation 8 shown in Section IV.B.3
of the text.
82
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GLOSSARY
additive: Any material added to the concentrated urea before
solids formation which changes the natural mechanical
characteristics of the solid urea particle.
affected population: Number of persons living in the area near
an average process where the source severity is greater
than 1.0.
biuret ((NH2CO)2NH): Impurity formed when solid urea is heated
above 130°C at atmospheric pressure.
coating: Any material placed externally on a prill or granule
designed to enhance shelf-life and reduce hygroscopicity.
crystallizer: Equipment in which a crystalline solid is formed
from the liquid urea by using the sensible heat of the
solution and the heat of crystallization.
emission factor: Mass of an emission per unit weight of final
product.
emission growth factor: Ratio of emissions for 1978 versus 1972.
emission rate: Mass of emissions per unit time.
evaporator: Equipment in which the liquid urea is concentrated
by passing it countercurrent to an air stream.
granulator: Equipment in which concentrated urea solution is
solidified by spraying the concentrated solution on a
falling curtain or rolling bed of seed particles to build
a larger particle.
granules: Solid urea particles formed by applying molten urea
to fine urea seed particles to increase the particle size
in a layered fashion.
national emission burden: Mass of particulates emitted from urea
manufacturing divided by the total national particulate
emissions expressed in percent.
once-through process: Urea solution process in which there are
no recycle streams of unreacted reactants.
83
-------
partietl recycle process: Urea solution process in which only
excess ammonia is recovered and recycled to the reactor.
phytotoxic: Poisonous to plant life.
prill,, micro: Small size prills or prill tower screenings used
for blending with livestock feed.
prill, standard: Solid urea particle formed when a drop of
liquid urea is solidified.
prilling: Process in which concentrated urea solution is
solidified by spraying the concentrated solution in a
tower so that the drops formed fall countercurrent to a
stream of cooling air.
source: severity: Ratio of the ground level concentration of each
emission species to its corresponding ambient air quality
standard (for criteria pollutants) or to a reduced TLV (for
noncriteria emission species).
Sphercdizer®: Specific make of drum granulator.
state emission burden: Mass of particulates emitted from the
ammonium nitrate industry in a particular state divided by
the total state particulate emissions expressed in percent.
threshold limit value (TLV): Refers to the airborne concentra-
tion of a substance and represents conditions under which it
is believed that nearly all workers may be repeatedly
exposed day after day without adverse effect for a 7- or
8-hour workday and 40-hour workweek.
total recycle process: Process utilizing excess ammonia recycle
plus recycle of unreacted reactants as a gas or in a
liquid carbamate form.
84
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CONVERSION FACTORS AND METRIC PREFIXES (57)
CONVERSION FACTORS
To convert from
degree Celsius (°C)
degree Kelvin (K)
gram/second (g/s)
joule (J)
kilogram (kg)
kilogram/meter3 (kg/m3)
kilometer2 (km2)
meter (m)
meter2 (m2)
metric ton
metric ton
pascal (Pa)
pascal (Pa)
pascal (Pa)
radian (rad)
to
Prefix
mega
kilo
centi
milli
micro
Symbol
M
k
c
m
y
degree Fahrenheit
degree Celsius
pounds/hour
calorie
pound-mass (Ib mass
avoirdupois)
Q
pound/footJ
mile2
foot
foot2
pound-mass
ton (short, 2,000 Ib-mass)
atmosphere
torr (mm Hg, 0°C)
pound-force/inch2 (psi)
degree (°)
METRIC PREFIXES
Multiplication
factor
Multiply by
t0p = 1-8
t = t
+ 32
oK - 273.15
7.936
2.388 x 10- l
2.204
6.243 x 10"2
3.860 x 10~ l
3.281
1.076 x 101
2.205 x 103
1.102
9.869 x 10~6
7.501 x 10~3
1.450 x I0~k
5.730 x 101
Example
106
103
io-2
10~ 3
10~6
1 MPa = 1 x IO6 pascals
1 kJ = 1 x IO3 joules
1 cm = 1 x 10"2 meter
1 mg = 1 x 10"3 gram
1 ym = 1 x 10~6 meter
(57) Standard for Metric Practices. ANSI/ASTM Designation E 380-76 ,
IEEE Std 268-1976, American Society for Testing and Materials, Phila-
delphia, Pennsylvania, February 1976. 37 pp.
85
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TECHNICAL REPORT DATA
(Please read Instructions on the reverse before completing)
1. REPORT NO.
EPA-600/2 - 7 7-107 1
2.
4. TITLE ANDSUDTITLE
Source Assessment: Urea Manufacture
3. RECIPIENT'S ACCESSION NO.
5. REPORT DATE
November 1977
G. PERFORMING ORGANIZATION CODE
7. AUTHOR(S)
W.J. Search and R.B. Reznik
8. PERFORMING ORGANIZATION REPORT NO.
MRC-DA-728
9. PERFORMING ORGANIZATION NAME AND ADDRESS
Monsanto Research Corporation
1515 Nicholsis Road
Dayton, OMo 45407
10. PROGRAM ELEMENT NO.
1AB015; ROAP 21AXM-Q71
11. CONTRACT/GRANT NO.
68-02-1874
13. SPONSORING AGENCY NAME AND ADDRESS
EPA, Office of Research and Development
Industrial Environmental Research Laboratory
Research Triangle Park, NC 27711
13. TYPE OF REPORT AND PERIOD COVERED
Task Final: 3/76-9/77
14. SPONSORING AGENCY CODE
EPA/600/13
ic,. SUPPLEMENTARY NOTESJERL-RTP task officer for this report is Ronald A. Venezia, Mail
Drop 62, 919/541-2547.
s. ABSTRACT
repOrj- gjves results of an evaluation of the potential environmental
effects of air emissions from the production of urea. Urea production in the U.S.
was 3.45 million metric tons in 1975. Major products were urea solution (38%),
granulated solid material (53%), and prilled solid material (9%). Over 75% of the
urea produced is consumed in fertilizers. Both ammonia and particulates are relea-
sed to the atmosphere during its manufacture. Major emission points are the evapo-
rator, prilling tower, and granulator. The evaporator has the largest emission fac-
tor for ammonia, 1.73 g/kg; the prill tower has the largest for particulates, 3.2 g/
kg. Emissions from the evaporator and granulator are normally controlled by scrub-
bers; prill tower emissions are not controlled. Source severities were determined to
evaluate potential environmental effects: they were between 10 and 1 for ammonia
emissions from the prill tower and for particulate emissions from the evaporator,
granulator, and prill tower. (Source severity is the ratio of the average maximum
ground level concentration of an emission species to the ambient air quality standard
(particulates) or (for ammonia) to a reduced threshold limit value. )
17.
KEY WORDS AND DOCUMENT ANALYSIS
DESCRIPTORS
Air Pollution
Urea
Industrial Processes
Ammonia
Dust
1-f. DISTRIBUTION STATEMENT
Unlimited
b.lDF.NTIFIERS/OPEN ENDED TERMS
Air Pollution Control
Stationary Sources
Source Assessment
Emission Factors
Particulate
19. SECURITY CLASS (Tills Report)
Unclassified
20. SECURITY CLASS (This page)
Unclassified
c. COSATI l-H-lii/Uroup
13B
07C
13H
07B
11G
21. NO. OK I'
94
227PR7CE
Form 2220-1 (9-73)
86
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