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A few plants burning only hydrogen sulfide or hydrogen sulfide
plus elemental sulfur use a simplified version of the above pro-
cess. Wet gases from the combustion chamber and waste heat
boiler are charged directly to the converter with no intermediate
treatment. Gases from the converter flow to the absorber, through
which 70 to 93 percent sulfuric acid is circulating. In a plant
burning only hydrogen sulfide, all of the sulfur trioxide from
the converter is in the form of acid mist, much of which is not
absorbed in the absorption tower. High efficiency mist collectors
both recover product acid and prevent excessive air pollution.
3.1.3 Dual Absorption Plants
In the dual absorption process, Figure 3.3, a greater fraction of
the sulfur 1n the feedstock 1s converted to sulfuric add than
1n the single absorption process. The SO. formed 1n the primary
conversion stages 1s removed 1n a primary absorption tower and
the remainder of the gas 1s returned to the final conversion
stage(s). Removal of a product of a reversible reaction such
as:
S09 + 1/2 0, - SO, (3.1)
£ £ O
drives the oxidation further toward completion* approaching
the reaction equilibrium expressed by:
(so3)
(3,2)
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where K 1s the reaction equilibrium constant peculiar to the
temperature of the reaction and to the units of the parenthetical
entitles, which are usually taken as the molar concentrations of the
gases involved.
The resulting SO, Is absorbed In a secondary absorption tower
obtaining at least 99.7 percent overall conversion of the sulfur
to sulfurlc acfd.
Figure 3.3 depicts primary absorption after the third conversion
stage with one final conversion stage. Dual absorption plants are
also designed with primary absorption after the second conversion
stage and two final conversion stages.
The dual absorption process penults higher Inlet StL
concentrations than normally used in single absorption plants
since the final conversion stage(s) effectively handles the
residual S02 from the first conversion stages. Higher inlet SCL
concentrations permit a reduction in equipment size which
partially offsets the cost of the additional equipment required
for a dual absorption plant. The dual absorption equipment
occupies little more space than a conventional plant even though
an additional absorber is required.
As shown in Table 2.1, the dual absorption process has been
applied to sulfuric acid plants burning sulfur, spent acid and
hydrogen sulfide; to metallurgical plants; and to plants producing
acid and oleum. However, most applications have been for sul^ur-
burning and metallurgical plants producing acid only.
3-8
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• The 99.7 percent overall conversion efficiency of the dual
• absorption process corresponds to a stack emission of 4.0 pounds
of SCL per ton of acid produced. This same low Sf^ emission level
I can be achieved in a single absorption plant by the use of a tail
gas recovery system. In the United States, three such systems which
I have been commercially demonstrated to achieve this level or below
. are sodium sulfite scrubbing, ammonia solution scrubbing, and
molecular sieve separation.
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The sodium sulfite scrubbing system scrubs Sn« from the tail
gas yielding various percentages of sodium sulfite, bisulfite
and sulfate in the spent scrubbing liquor (2). The bisulfite is
• then thermally decomposed to yield sodium sulfite crystals, $0
and water vapor. Most of the water vapor is condensed and the
| wet S0? is sent back to the acid plant. The crystals are separated
• from the mother liquor and are either dissolved in recovered con-
densate and recycled to the absorber or are consumed in the manu-
Ifacture of other products. The mother liquor or spent scrubbing
liquor must be purged to prevent sulfate buildup, and this purge
I
stream is usually treated to reduce water pollution or may be dried
— for sale.
I
The ammonia solution scrubbing system scrubs S^2 from the tail
| gas yielding various percentages of ammonium sulfite, bisulfite
_ and sulfate (5). The spent scrubbing liquor can be converted
* to ammonium sulfate, if a market exists, or can be thermally
•decomposed to produce S0?, nitrogen, and water vapor, the S0?
beino sent back to the acid plant. In the TVA "ABS" process, the
f
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ammonium sulfate is melted and decomposed to form ammonium
bisulfate and ammonia gas which are both recycled.
The molecular sieve separation system removes SO^ from the tail
gas on an adsorbent bed (6). Just before the bed becomes
completely saturated with Sf^. the feed gas is switched to an
alternate bed and the saturated bed is regenerated with a purge
of hot, dry air. The effluent purge stream, rich in SOp, is
fed back to the acid plant. The entire adsorption/regeneration
cycle operates continuously on an automatically timed basis.
Relative acid mist production in dual vs. single absorption
plants and the location of mist eliminators in the dual absorption
process and in tail gas recovery systems is discussed in Section
4.3.
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I
3.2 REFERENCES FOR SECTION 3.
1. Establishment of National Emission Standards for
I Stationary Sources. Vol. IV. Sulf uric Add Plants.
_ Research Triangle Institute and PEDCO - Environmental
• Specialists, Inc. Final Report. Contract CPA 70 - 164,
• Task Order No. 3, National Air Pollution Control
Administration. Research Triangle Park, North Carolina.
| September 30, 1970. 43 p.
• 2. Engineering Analysis of Emissions Control Technology
for Sulf uric Add Manufacturing Processes. Chemical
I Construction Corporation. Final Report. Contract 22-69-81 ,
_ National Air Pollution Control Administration, PHS, U. S.
" DHEW. New York, N.Y. Publication No. PB-190-393. March
•
1970. Vol. 1.
3. Shreve, R. N. Chemical Process Industries. 3rd
Edition. New York, McGraw-Hill Book Company, 1967. pp.
I
322-342.
• 4. Cuffe, S. T., and C. M. Dean. Atmospheric Emissions
from Sulfuric Acid Manufacturing Processes. National Air
M Pollution Control Administration. Durham, North Carolina.
Publication No. 999-AP-13. 1965. 127 p.
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5. Horlacher, W.R. et. al. Four SOp Removal Systems.
Chemical Engineering Progress. 68.: 43 - 50, August 1972.
o
6. The Pura Siv Process for SOp Removal and Recovery:
A New Molecular Sieve Process for SCL Removal and Recovery
from Sulfuric Acid Plant Vent Gas. Union Carbide Corporation,
Materials Systems Division, New York, N.Y.
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• 4. ACID MIST EMISSIONS (1,2,3)
• 4.1 POINTS OF EMISSION.
_ The principal point of acid mist emission in a sulfuric acid
™ plant is the exit gas from the final absorber, more commonlv
• referred to as "stack gas" or "tail gas".
m 4.2 FORMATION AND CHARACTERISTICS.
Hexavalent sulfur is present in the stack gas as sulfuric acid
• vapor, gaseous sulfur trioxide (S03), and particulate acid mist.
• -1.2.1 Sulfuric /^cid Vapor
• The stack gas leaving the final absorber always contains sulfuric
acid vapor. This vapor is in equilibrium with the acid in the
• absorber at its operating acid concentration and temperature ,
• and on cooling may condense in long ducts leading to the stack
or in the stack itself. If no mist controls are employed or
• if the cooling occurs after the mist eliminator, the condensed
vapor can be carried out of the stack as relatively large droplets
• which fall in the vicinity of the plant. Acid vapor may be reduced
M by operating the absorber at lower temperatures where H-SO. vapor
pressure is lower; however, this may increase acid mist formation.
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Table 4.1 shows that the H^SO, vapor pressure is dependent upon
temperature and, to some extent, upon acid concentration (4). To
reduce acid vapor emissions from a specific absorber, the lowest
operating temperature consistent with good operation must be
found. This generally lies in the range 170 - 185°F.
4.2.2 Sulfur Trioxide Vapor
If significant gaseous S03 is present in the stack gas as a
result of poor absorber operation, it will combine with water
vapor in the atmosphere to produce a visible acid mist. The
only way to prevent this mist formation is through proper
absorber operation and design (5).
Table 4.1 shows that the vapor pressure of S(L increases
rapidly above 99 percent acid concentration (4). Since SCL
absorption efficiency drops off below 98 percent acid concentration,
control of concentration in the range 98 - 99 percent is generally
good practice.
Concentrations of SO., in the absorber exit aas vill of necessity
exceed the equilibrium concentrations given in Table 4.1. This
is because no absorber of finite heiqht can achieve S03 equilibrium
between the acid entering the top of the tower and the tower top
exit gas.
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4-3
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4.2.3 Particulate Add H1st
Acid mist is formed anywhere in a sulfuric acid plant where
sulfuric acid vapor is cooled below the dewpoint corresponding
to that particular acid vapor concentration; the original
H-SO. vapor can arise from acid vapor pressure, or from reaction
of SCL with water vapor in the carrier gas stream. Once formed,
this mist is extremely stable, is not readily separated or
absorbed, and much of it passes through the absorber. The
quantity and particle size distribution of acid mist are functions
of the sulfur feedstock and the strength of the acid produced.
For a bright elemental sulfur plant, the only sources of water
vapor are moisture in the sulfur and in the inlet air to the
drying tower. The drying towers in most contact plants are
able to dry the process gas to a moisture content of about
3 milligrams per standard cubic foot (mg/scf) (6). Theoretically,
the 3 mg/scf of water vapor will form 16 mg/scf of sulfuric
acid mist. Part of the mist is probably removed in the absorber,
however.
When dark or contaminated sulfur is burned, hydrocarbon impurities
present in the sulfur burn to produce water vapor. This in turn
combines with SCL to form acid mist as the gas cools in the
4-4
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economizer or absorption tower. This mist formation may be
M accentuated by sudden chilling of the gas on cold surfaces,
an effect sometimes produced by rain on the gas duct. Existing
• information indicates that this mist consists of 1 to 5 micron
particles (7).
Another cause of mist formation is the presence of nitrogen
• oxides in the converted gas. Although the nitrogen oxides may
result from fixation of atmospheric nitrogen in high temperature
• furnaces or from arcing of electrostatic precipitators in the
« purification section upstream of the drying tower in non-sulfur
burning plants, they more commonly result from nitrogen compounds
I in the raw material used. Spent acids recycled from organic
reactions are most likely to produce nitrogen oxides. Part of
| the mist formation undoubtedly results from oxidation of S0? by
— these nitrogen oxides (the chamber plant reactions) (8). It is
* also believed that nitric oxide (NO) reacts with S03 to form
• nitrosyl pyrosulfate, (N0)2 S-j07; and that nitrosyl pyrosulfate
reacts with atmospheric moisture to form nitrosyl bisulfate,
p NOHSO,. Nitrosyl pyrosulfate will pass through the final absorber
_ and any mist eliminator in gaseous form at normal exit gas
• temperatures, and both nitrosyl pyrosulfate and nitrosyl bisulfate
• can exist in the stack gas as very fine mist. (9). These mist
emissions can be minimized through use of high efficiency mist
• eliminators and/or electrostatic precipitators in the purification
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section and in the stack gas, and by proper furnace operation. |
As an example of the latter, one report claims that furnace g
operation below 2000°F with low oxygen content will insure that
the decomposition products contain no more than 100 ppm of I
nitrogen oxides (8).
In ''wet gas" plants burning hydrogen sulfide, no attempt is made
to remove water vapor either from the combustion air or from the W
gas resulting from combustion of the hydrogen sulfide. Hence,
the amount of water vapor in the gas entering the converter is •
more than enough to combine with all of the sulfur trioxide M
produced, and the entire output of the plant initially is in the
form of acid mist. Most of this mist is recovered as product I
acid with gas cooling equipment and high efficiency mist eliminators.
In oleum producing plants, a greater quantity of mist and a much
finer mist is produced. In these plants, oleum (i.e. sulfuric •
acid containing excess SO- in solution) is produced in a pre-
•
liminary absorption step before the final absorption tower. Only •
part of the S03 is absorbed and the gas leaving the oleum tower m
still contains SOs which is absorbed in the final absorption
tower. In spite of this preliminary absorption, the stack gas •
always contains more mist than when the oleum tower is bypassed.
The quantity of mist appears to be proportional to the oleum/ •
acid production ratio and to the strength of the oleum produced (10). m
The mechanism is not clearly understood but it has been established
4-6
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1
1
1
1
that the mist is formed i
the oleum tower (7).
Table 4.2 gives a partial
mist emissions at plants
and 32 percent oleum (11
using a cascade impactor
Table 4.2 indicates that
n the final absorption tower, not in
e size distribution for sulfuric acid
producing strong acid, 20 percent oleum
»12)- These distributions were obtained
(See Section 6.2.1.2 for a description).
oleum production results in a finer
particle size distribution than acid production alone, and that
the distribution becomes
finer with increasing oleum concentration.
TABLE 4.2
PARTICLE SIZE DISTRIBUTIONS IN SELECTED SULFURIC ACID PLANT ABSORBER
1
1
1
1
1
t
1
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EFFLUENTS
Cumulative weight percent smaller than
Particle diameter Acid production 20% oleum 32% oleum
(microns)
0.2
0.4
0.6
0.8
1.0
1.5
2.0
only production production
0.4 3.6
2 16
1 4.8 30
7 8 42
12 11.6 53
21 48 86.5
40 84.5 97
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4.3 TYPICAL PLANT MIST EMISSIONS. "
Figure 4.1 shows the relationship between mist concentration ™
and pounds of mist per ton of acid produced. For a given mass •
emission rate, acid mist concentration (in milligrams of H2SO.
per standard cubic foot) is a function of the volume of air I
fed to the process. The air volume in turn depends on the SCL _
concentration in the gas stream fed to the converters. The •
curves can be used for any gas stream before or after mist elimina- •
torss provided there is no air dilution.
Stack gas acid mist emissions range from 2 to 20 milligrams ™
per standard cubic foot (mg/scf) for a plant producing no fl
oleum to 5 to 50 mg/scf for an oleum plant (13). For a typical
sulfur-burning system feeding 8 percent S02 to the converter, £
these stack gas emissions are equivalent to 0.4 to 4 pounds per _
ton (Ib/ton) of 100 percent H^SO. produced for an acid plant *
and 1 to 10 Ib/ton of 100 percent HpSO* produced for an •
oleum plant (refer to Figure 4.1). The lower mist limit in each
range requires some form of mist control device while the upper £
limit is typical of no control. _
Generally speaking, the dual absorption process does not reduce
the acid mist emission, and a dual absorption plant will require f
the same type of mist control device as a conventional plant
I
An additional mist eliminator is required on the primary absorption ™
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0.10
0.01
0.02 0.03 0.04 0.10 0.20 0.30 0.50
ACID MIST EMISSIONS, Ib H2S04/T OF 100 PERCENT H2S04 PRODUCED
Figure 4.1 Sulfuric acid plant concentrations of mist for mass stack
emissions per unit of production at inlet SO,, volume concentrations.
4-9
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tower (see Section 3,1.3) to protect the downstream heat exchangers I
from corrosion. This additional mist eliminator will often allow
. ...... .. ,. ._
6.2.1.3) to do an adequate final cleanup on the secondary •
absorption tower, whereas a high efficiency mist eliminator
(Section 6.2.1.1) might otherwise have been required. •
The use of a tail gas scrubbing system, for removal of S0?, g
such as sodium sulfite or ammonia scrubbing, does not reduce ^
the need for a mist eliminator since - as mentioned in Section
4.2 - acid mist is not readily absorbed.
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With the sodium sulfite system, it is best to locate the •
mist eliminator upstream of the scrubber to minimize the
formation of sulfates which must be purged from the system. |
It may even be desirable to have two high-efficiency mist —
eliminators: one installed in the absorber and the other *
"at grade", downstream of the absorber and upstream of the •
scrubber. ("At grade" and "piggyback" installations are discussed
in Section 6.2.2). The scrubber exit gas does not normally f
require mist removal.
The ammonia solution scrubbing process requires a pH of
6.0 or greater for effective S(L control. However, as the pH £
of the liquor increases, ammonia losses increase and the ammonia
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combines with the SCL to form a highly visible plume of ammonium
sulfite, bisulfite, and sulfate. A high efficiency mist •
eliminator must be installed downstream of the scrubber to
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- control these emissions. Further information on the use of mist
1
eliminators in ammonia solution scrubbing systems can be found
in reference (15).
J The use of a molecular sieve requires removal of all H^SCL mist,
H2S04 vapor» and 9aseous S03 upstream of the sieve, as H2SO.
•
• cannot be regenerated from the sieve. This, and the fact that
A the sieve has a higher capacity at lower temperatures, requires
that the absorber tail gas first be cooled by passage through a
J refrigeration system before passage through a high efficiency mist
eliminator. This cooling cannot be achieved by water Injection
• since the sieve absorbs water vapor. It cannot be achieved by
H lowering the acid temperature to the absorber as this may
increase acid mist formation. A plant may choose to install
fl another mist eliminator.upstream of the cooler to reduce the
load on the second mist eliminator. This mist eliminator would
8 usually be installed in the absorber in new plants and "piggyback"
£ or "at grade" in existing plants. Due to the extensive preliminary
treatment, sieve stack gas should contain virtually no acid
|
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4.4 REFERENCES FOR SECTION 4. •
1. Establishment of National Emission Standards for Stationary •
Sources. Vol. IV. Sulfuric Acid Plants. Research *
Triangle Institute and PEDCO - Environmental Specialists, ft
Inc. Final Report. Contract CPA 70 - 164, Task Order No.
3, National Air Pollution Control Administration. Research p
Triangle Park, North Carolina. September 30, 1970. 43 p. ,_
2. Engineering Analysis of Emissions Control Technology for
Sulfuric Acid Manufacturing Processes. Chemical Construction |
Corporation. Final Report. Contract 22-69-81, National .
Air Pollution Control Administration, PHS, U. S. OHEW. *
New York, N.Y. Publication No. PB - 190-393. March 1970. •
Vol. 1.
3. Cuffe, S. T. and C. M. Dean. Atmospheric Emissions from
Sulfuric Acid Manufacturing Processes. National Air Pollution •
Control Administration. Durham, North Carolina. Publication
No. 999-AP-13. 1965. 127 p. 1
4. Gmitro, J. I. and T. Vermeulen. Vapor-Liquid Equilibria p
for Aqueous Sulfuric Acid. Lawrence Radiation Laboratory,
Berkeley, Calif. Contract No. W-7405-eng-48. June 24, 1963. ™
5. Reference 2, above, p. 111-11. •
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• 6. Reference 3, above, p. 30.
m 7. Reference 2, above, p. I I 1-9.
8. Reference 2, above, p. 111-10.
•
9. Kurek, R.W. Special Report on EPA Guidelines for State
• Emission Standards for Sulfuric Acid Plant Mist dated
m June 1974. E.I. du Pont de Nemours & Co., Inc., Industrial
Chemicals Department. Wilmington, Delaware. Prepared for
I U.S. Environmental Protection Agency , Office of Air
Quality Planning & Standards. October 4, 1974. Exhibit 15.
t
10. Reference 3, above, p. 32.
11. Brink, J.A., Jr. Cascade Impactor for Adiabatic fteasure-
H ments. Industrial & Engineering Chemistry. 50: 647,
April 1958.
12. Reference 9, above, Exhibit 12.
13, Reference 2, above, pp. 111-14, 15 and 22.
* 14. Reference 2, above, p. IV-15.
j[ 15. Brink, J.A., Jr. and C.N. Dougald. Particulate Removal
— from Process Exhaust Gases. Proceedings of International
* Sulfite Conference, TAPPI and CPPA, Boston, Mass., October
• 30 - November 1, 1972. October 30, 1972. pp. 377-389.
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5. HEALTH ANN WELFARE EFFECTS OF ACIP MIST
I
5.1 INTRODUCTION
I
In accordance with 40 CFR 60.22(b), promulgated on November 17, 1975
• (40 FR 53340), this chapter presents a summary of the available
information on the potential health and welfare effects of sulfuric
£ acid mist and the rationale for the Administrator's determination
— . that it is a health-related pollutant for purposes of section lll(d)
™ of the Clean Air Act.
m The Administrator first considers potential health and welfare
« effects of a designated pollutant in connection with the establishment
of standards of performance for new sources of that pollutant under
fl section lll(b) of the Act. Before such standards may be established,
the Administrator must find that the pollutant in question "may contribute
jjf significantly to air pollution which causes or contributes to the
— endangerment of public health or welfare"(see section 111 (b)(l )(A)).
* Because this finding is, in effect, a prerequisite to the same
flj pollutant being identified as a desiqnated pollutant under section lll(d),
all designated pollutants wtll have been found to have potential adverse
| effects on public health, public welfare, or both.
W As discussed in section 1.1 above, Subpart B of Part 60 distinguishes >
between designated pollutants that may cause or contribute to endangerment
| of public health (referred to as "health-related pollutants") and those
for which adverse effects on public health have not been demonstrated
—
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("welfare-related pollutants"). In general, the sionificance £
of the distinction is that States have more flexibility in
establishing plans for the control of welfare-related pollutants w
than is provided for plans involving health-related pollutants. •
In determining whether a designated pollutant is health-related —
or welfare-related for purposes of section lll(d), the Administrator *
considers such factors as: (1) Knov/n and suspected effects of the •
pollutant on nublic health and welfare; (2) potential ambient
concentrations of the pollutant; (3) generation of any secondary ||
pollutants for which the designated pollutant may be a precursor;
(4) any synergistic effect with other pollutants; and (5) potential <•
effects from accumulation in the environment (e.g., soil, water •
and food chains).
It should be noted that the Administrator's determination whether *
a designated pollutant is health-related or welfare-related for ff
purposes of section lll(d) does not affect the degree of control
represented by EPA's emission guidelines. For reasons discussed J[
in the preamble to Subpart B, EPA's emission guidelines (like ^
standards of performance for new sources under section lll(b) are *
based on the degree of control achievable with the best adequately •
demonstrated control systems (considering costs), rather than on
direct protection of public health or welfare. This is true whether p
a particular designated pollutant has been found to be health-related
or welfare-related. Thus, the only consequence of that finding is •
the degree of flexibility that will be available to the States in •
establishing plans for control of the pollutant, as indicated above.
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5.2 HEALTH EFFECTS
I Short-term human exposure to sulfuric acid mist can cause temporary
and permanent damage to the lungs and bronchial tubes. Long-term
w exposure can cause skin damage, inflamation of the eyes, mouth and
m stomach, and permanent tooth damage, the latter being the most
serious (1 ,2).
* One hour exposure to a. concentration of 39,000 micrograms per cubic
• meter (|ig/m3) of dry mist has produced persistent wheezing for up
to 4 days after exposure, an increase in airway flow resistance of
• 35.5 to 100 percent above normal, and long-lastinq bronchial
irritation (3,4). A deep breath at a concentration of 5000 yg/m3
•
will usually produce coughing. A concentration of 3000
A produces a noticeable odor, although concentrations below 600 yg/m3
usually cannot be detected(l). Occupational exposure to 1000 ya/m3
• is unlikely to result in lung injury (2,5).
A Workers exposed to long-term concentrations of 3000 to 16000 yg/m
evidenced severe corrosion of dental enamel (2,6), but no damage
£ was noted after occupational exposure to 1000 yg/m3 £2»«5).
w A threshold limit value of 1000 yg/m3 for 8-hour workday exposure
has been set by the American Conference of Governmental Industrial
• Hygienists, a level which should not cause irritation of respiratory
» passages and tooth injury (2). This same level was recommended by
the National Institute for Occupational Safety and Health for
• occupational exposure to sulfuric acid mist as a time-weighted
average exposure for up to 10 hours per day, 40 hour work week (10).
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The effects of sulfuric acid mist on the lungs are aggravated by *
high humidity. In terms of sulfur equivalent, sulfuric acid is •
considerably more of an irritant to humans than is sulfur dioxide (2,4,39).
Emissions of the acids and oxides of sulfur contribute to the total
sulfate concentration in the air. A method for measuring atmospheric V
H2S04 is not available. A comparison of measured atmospheric sulfate
with atmospheric metals and ammonium ions indicates that about half •
of the atmospheric sulfate could be in the form of ^804 (38). In 1970, ^
the national average sulfate concentration at urban locations was 10.1
The nonurban average was 6.3 yg/m^ (37). Acid mist emissions add to the ft
total background, but reliable no-effect threshold levels have not been
established. ft
i"
A recent investigation in guinea pigs demonstrated that the total ft
respiratory deposition rate of inhaled particles and the pattern of
regional respiratory deposition of these particles was altered by ft
sulfuric acid mist inhalation. These effects were noted at acid mist ^
•
concentrations as low as 30 yg/nr*, particle size < 1 ym, for 1 hour.
This response was probably associated with increased pulmonary airflow- ft
resistance. Increased pulmonary airflow resistance is the principal
physiologic response in uncomplicated asthma. It has been hypothesized, p
therefore, that sulfuric acid mist inhalation may act to increase the . ^
incidence of asthma attacks through increased deposition of inhaled ' *
particles and/or a shift in the principal site of desposition of ' •
inhaled particles to airway reaions where asthma can be triggered (8).
Another recent animal study examined respiratory physiologic responses
to a variety of sul fates of similar aerosol size and mass concentrations (9). ft
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Sulfuric acid was found to be the greatest respiratory irritant. The
• differences in the inhalation response of some of these sul fates was
• small. Although these data do not constitute an adequate basis for a
determination of the comparative toxicity for specific inorganic sul fates,
J the data do suggest that the toxicological evaluation of parti cul ate
_ sulfur oxides must consider the cation as well as the an ion of the molecule,
* and that aerosol acidity is of great importance. These studies were
jtt based upon a sensitive respiratory physiologic response, primarily
increased pulmonary airflow resistance in guinea pigs. This response
g results from narrowing of the airways within the respiratory system.
A similar response has been observed in men exposed to sulfur dioxide
• and H2S04 aerosol. This physiological response is a generally accepted,
• sensitive measure of airway irritation.
^ Data on sulfuric acid mist toxicity in humans are limited, but there
* is some information on short-term exposures. One study reported
• an increase in pulmonary flow resistance in humans of 18 percent at
H2S04 aerosol levels (particle size 1.8 iom, count median diameter)
p as low as 10 - 100 ug/m^ (40), although the experimental techniques
used in this study have been faulted by independent reviews.
In another study, respiratory rate has been reported to increase by
g about 30 percent, tidal volume to increase by about 28 percent, and
_. maximum inspiratory-expiratory flow rates to decrease by about 20
* percent at exposure levels of 350-500 ng/m3, concentrations below
• subjectively detectable levels (5). These changes occurred during
the first three minutes of exposure, were maintained throughout the
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15 minute exposure period, and returned to pre-exposure levels within
15 minutes after the exposure ended. At higher levels, bronchospasm,
5-5
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increased upper respiratory tract secretions, increased flow I
resistance and increased respiratory rate have been consistently
found. It thus appears that as hUSO, concentration increases, Q
so do respiratory rate and pulmonary air flow resistance. All —
of the subjects involved in the clinical studies were healthy, *
young adults who could easily compensate for the increased •
resistance imposed upon their breathing. Effects on persons
with pre-existing disease have not been determined. |
Visibility decreases with increasing acid mist concentration and
increasing relative humidity, and is particularly important in
1
5.3 WELFARE EFFECTS
In addition to its effect on the bronchial tubes, another annoying |
property of sulfuric acid mist is the ability of the aerosol particles _
to reduce visibility. They do this by scattering and absorbing the ™
light passing from object to observer thus reducing the eye's ability A
to distinguish objects from their background, and by scattering light
from the sky and sun into the line of sight of an observer (12). |
The most serious sulfuric mist visibility reduction is caused by I
small particles from 0.2 to 2 microns in diameter. About 5 to 20
percent of the particles in urban air are sulfuric acid and other |
sulfates, and 80 weight percent or more of these sulfate particles ^
are smaller than 2 microns in diameter (13).
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aircraft operations. At a visual range of less than 5 miles,
operations are slowed at airports because of the need to maintain •
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5-6
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• larger distances between aircraft (13). Sulfuric acid mist can
I
limit visibility to 5 miles at 98 percent relative humidity and
3
an acid mist concentration of 200 yg/m , at 90 percent relative
3
m humidity, and 60 yg/m and at 50 percent relative humidity and
* 200 yg/m3 (14).
*
In atmospheres containing sulfur dioxide 'and sulfuric acid, an
|B increase in humidity increases the ratio of sulfuric acid to sulfur
dioxide and this results in an increase of sulfuric acid concentration
• in the size range characteristic of acid fogs (15).
| Sulfuric acid mist exerts a negative economic effect by damaging
— materials and vegetation. Acid mist accelerates the corrosion of
* most metals, in particular iron, steel, and zinc. The damage increases
• with increasing relative humidity and temperature. In addition,
atmospheric sulfuric acid can react with some suspended parti culates
£ to form sulfate salts which further accelerate the corrosion (16,17,
18,19,20).
Sulfuric acid will attack building materials and deface monuments.
I The attack is very severe if the building material contains calcium
JB carbonate, as do limestone, marble, roofing slate, and mortar. The
carbonate is converted to relatively soluble sulfates and then leached
V away by rainwater (21,22,23,24). Dolomites, which contain both calcium
and magnesium carbonates, are particularly vulnerable as magnesium
• carbonate is readily soluble in an acid environment (21,25,26,27).
• Granite, gneiss, and many sandstones, which do not contain carbonates,
and well-baked bricks, glazed bricks and glazed tile are less readily
• attacked by sulfuric acid. Sulfuric acid can also disintegrate
stone structures by corroding iron tie rods (21,25).
•
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Textile fabrics made of cellulosic vegetable fibers, such as cotton,
linen, hemp, jute, rayons and synthetic nylons, are particularly
vulnerable to sulfuric acid. After exposure, these fibers lose
tensile strength (21,28). Animal fibers, such as wool and furs,
are more resistant to acid damage (21,27). Certain classes of fabric
dyes are attacked by sulfuric acid which is often absorbed or adsorbed
on atmospheric particles. The dye coloring is reduced or sometimes
destroyed entirely (21,29,30).
Sulfuric acid also causes discoloration, embrittlement, and a
decrease in folding resistance of both book and writing paper (25,31,32)
Sulfuric acid droplets have settled on dry leaves without causing
injury but when the leaf surface was wet, as may occur during
polluted fogs, a spotted injury has developed. The injury consists
of progressive cellular collapse from the exposed Surface through
the leaf leaving scorched areas (33,34,35). Injury may occur at
concentrations of 100 yg/m (36). Injury has occured on Swiss chard,
beets, alfalfa and spinach, the latter showing a more diffuse type of
injury (33).
5.4 RATIONALE
Based on the information in sections 5.2 and 5.3, it is clear that
sulfuric acid mist has significant health and welfare effects. To
be classified as a health related pollutant, the health effects
of acid mist must be present at reasonably expected ambient con-
centrations. Results of diffusion modeling presented 1n section
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• 8.1.1.2 indicate that expected maximum ground-level concentrations
m from uncontrolled acid and oleum plants are in the range of 0.6 to
12 vg/m3 on an annual average, 3 to 60 yg/m3 on a 24-hour average,
I 40 to 300 -pg/m3 on a one hour average, and 640 to 4700 yg/m3 on a
ten second average. (See Table 8.1 for complete results.)
I
The predicted short-term concentrations are in the range where
• health effects have been observed in healthy, young subjects (see
• section 5.2). It is a reasonable conclusion that potentially more
sensitive individuals (e.g., infants and others of great susceptability
• such as persons whose health is already compromised by pre-existing
disease conditions and whose physiologic reserves are, therefore,
• reduced) would exhibit adverse effects at even lower concentrations
I
than the clinical studies indicated, or more serious adverse effects
at the levels studied.
" Therefore, the Administrator concludes that sulfuric acid mist
• contributes to endangerment of public health and may in fact cause
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that endangerment. Thus, sulfuric acid mist will be considered a
health-related pollutant for purposes of section lll(d) and Subpart B
of Part 60.
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5.5 REFERENCES FOR SECTION 5.
1. Air Quality Criteria for Sulfur Oxides. National Air *
Pollution Control Administration. Washington, D. C. •
Publication No. AP - 50. April 1970.
2. Documentation of the Threshold Limit Values for Substances
in Workroom Air. Cincinnati, Ohio, American Conference of w
Governmental Industrial Hygienists, 1971. pp. 239 - 240.
I
3. Reference 1, above, p. 95.
4. Sim, V. M. and R. E. Pattle. Effect of Possible Smog *
Irritants on Human Subjects. Journal of the American I
Medical Association. 165; 1908 - 1913, December 14, 1957.
5. Amdur, M. 0., L. Silverman, and P. Drinker. Inhalation of H2S04 Mist •
by Human Subjects. Archives of Industrial Hygiene and Occupational •
Medicine. 6: 305, October 1952.
6. Malcolm. D. and E. Paul. British Journal of Industrial •
Medicine. 1_8: 63, 1961.
7. Paule, A. Med. d. Lavoro 45; 59, 1954. Abstract in American
Industrial Hygiene Association Quarterly. 1_6_: 153, 1955 (abstract). ff
8. Fairchild, G. A., S. Stultz, and D. L. Coffin. Sulfuric Acid Effect on
Deoosition of Radioactive Aerosol in Respiratory Tract of Guinea Pigs. •
J. Am. Ind. Hyg. Assoc. (In Press) 1975. m
9. Amdur, M. 0., J. Bayles, V. Aqro, M. Dubriel, and D. W. Underbill,
Respiratory Response of Guinea Pigs to Sulfuric Acid and Sulfate Salts, *
presented at Symposium: Sulfur Pollution and Research Approaches, Duke •
University, May 27-29, 1975.
5-10 •
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10. Criteria for a Recommended Standard . . . Occupational Exposure to
M Sulfuric Acid, U. S. Department of Health, Education and Welfare,
m National Institute for Occupational Safety and Health, 1974.
11. Reference 1, above, p. 111.
' 12. Reference 1, above, p. 10.
m 13. Reference 1, above, p. 14.
| 14. Reference 1, above p. 13.
• 15. Reference 1, above, p. 15.
• 16. Reference 1, above, p. 51.
• 17. Serada, P. J. Atmospheric Factors Affecting the Corrosion
of Steel. Industrial and Engineering Chemistry. 52;
• 157-160, February 1960.
• 18. Greenblatt, J. H. and R. Pearlman. The Influence of
Atmospheric Contaminants on the Corrosion of Steel.
fl Chemistry in Canada. J£: 21 - 23, November 1962.
19. Vernon, W. H. J. The Corrosion of Metals, Lecture I.
I Journal of the Royal Society of the Arts. pp. 578 - 610,
m July 1, 1949.
20. Sanyal , B. and D. V. Bhardwar, The Corrosion of Metals
• in Synthetic Atmospheres Containing Sulphur Dioxide.
m J. Sci. Ind. Res. (New Delhi). 18A; 69-74, February 1959.
. 21. Reference 1, above, p. 54.
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22, Yocum, J. E, The Deterioration of Materials in Polluted
Atmospheres. Journal of the Air Pollution Control Association,
8_ (4): 203 - 208, November 1958.
23. Benner, R. C. The Effect of Smoke on Stone, In: Papers
on the Effect of Smoke on Building Materials, University
of Pittsburgh, Mellon Institute cf Industrial Research
and School of Specific Industries, Bulletin 6, 1913.
24. Turner, T. H. Damage to Structures by Atmospheric
Pollution. Smokeless Air. 23: 22 - 26, Autumn 1952.
25. Parker, A. The Destructive Effects of Air Pollution on
Materials. In: Proceedings, 1955 Annual Conference,
National Smoke Abatement Society, London, 1955. pp. 3 - 15.
(Presented at Sixth Des Voeux Memorial Lecture).
26. Regan, C. J. A Chadwick Lecture on Air Pollution. Smokeless
Air. 88_: 67 - 76, 1953.
27. Petrie, T. C. Smoke and the Curtains. Smokeless
Air. J8_: 62 - 64, Summer 1948.
28. Waller, R. E. Acid Droplets in Urban Air. International
Journal of Air and Mater Pollution. 7_: 773 - 778, 1963.
29. Salvin, V. S. Effect of Air Pollutants on Dyed Fabrics.
Journal of the Air Pollution Control Association. 13;
416 - 422, September 1963.
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30. Salvin, V. S. Relation of Atmospheric Contaminants and
I Ozone to Lightfastness. American Dyestuff Reporter, 53;
33-41, January 6, 1964.
31. Reference 1, above, pp. 55.
* 32. Burdick, L. R. and 0. F. Barkley. Effect of Sulfur
A Compounds in the Air on Various Materials. U. S. Bureau
of Mines, Information Circular 7064, April 1939.
33. Reference 1, above, pp. 66 - 67.
• 34. Thomas, M. D., R. H. Hendricks, and G. R. Hill.
• Some Impurities in the Air and Their Effects on Plants.
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In; Air Pollution, McCabe, L. C. (ed). New York, McGraw
Hill, 1952. pp. 41 - 47.
35. Middleton, J. T. , E. F. Darley, and R. F. Brewer.
Damage to Vegetation from Polluted Atmospheres. Journal
of the Air Pollution Control Association. 8; 9 - 15, 1958.
" 36. Reference 1, above, p. 68.
9 37. Summary Report on Suspended Sul fates and Sulfuric Acid
g Aerosols. U.S. Environmental Protection Agency, NERC,
* Research Triangle Park, North Carolina. EPA 650/3-74-000.
• March 1974.
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I 5"13
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38. Altshuller, A.P. Atmospheric Sulfur Dioxide and Sulfate: _
Distribution of Concentration at Urban and Nonurban Sites *
in United States. Environ. Sci. Techno!. 7: 709-712, •
August 1973.
39. Amdur, M. 0. Toxicologic Appraisal of Particulate Matter,
Oxides of Sulfur, and Sulfuric Acid, Journal of the Air tt
Pollution Control Association. 19_: 642, September 1969.
40. Toyama, T. and K. Nakamura, Synergistic Response of Hydrogen Perioxide
Aerosols and S02 to Pulmonary Airway Resistance, Ind. Health 2:34-45, I
March 1964.
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5-14 •
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* 6. CONTROL TECHNIQUES FOR ACID MIST
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As mentioned in Section 1, the intent of the acid mist new source
performance standard and these guidelines for existing facilities
is to limit the ^$04 concentrations in the atmosphere resulting
• from particulate acid mist, ^$04 vapor, and gaseous $03. Acid mist
is defined by EPA Method 8 which measures virtually all of the parti -
• culate acid mist, but only a fraction of the 503 and ^$04 vapor.
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Effective control of acid mist as defined in the standard thus
requires more than control of particulate acid mist alone. As
mentioned in Section 4.2, it also requires control of ^$04 vapor
I and 503 through proper absorber operation. Consequently, Section 6.1
deals with absorber operating parameters that can affect the emission
| of H2S04 vapor and $03; and Section 6.2 deals with control techniques
• for particulate acid mist.
Section 6.3 presents the results of EPA source tests to support the
• standard of performance for new stationary sources (SPNSS) for acid
• mist, EPA source tests to support this lll(d) document dealing with
retrofit plants, and miscellaneous company-run source tests. All of
• the plants for which data are given were tested using EPA Method 8.
• Section 6.4 presents EPA's emission guideline for existing sources
based on applying the best system of emission reduction—considering
I cost—that is available to existing plants. This guideline reflects
_ the application of the vertical panel or horizontal dual pad mist
* eliminators, as a minimum, to sulfur burning plants producing acid
• or low strength oleum, and generally require the application of
vertical tube mist eliminators to other non-metallurgical sulfuric
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acid plants.
Section 6.5 deals with good plant operating practices that can •
reduce the generation of particulate acid mist upstream of the
absorber.
6-2
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. 6.1 ABSORBER OPERATION. (1,2)
In an absorption process, a soluble component of a gas
w mixture is dissolved into a relatively nonvolatile liquid.
• As the component is dissolved, it may react chemically with the
liquid, with evolution or absorption of heat. Furthermore, if
I the gas and liquid enter the absorber at different temperatures,
ordinary heat transfer will also occur from one stream to the
• other.
| The final operation in a contact process sulfuric acid unit
is the absorption of gaseous SO, into a liquid stream of strong
I
• HpSO.. ^e ^3 ^s absorbec* fforo a gas stream which also
• contains nitrogen, oxygen, SO^, and particulate acid mist.
Absorption is carried out by passing the liquid H^SO^ and the
• gas streams countercurrent to each other in a vertical packed
cylindrical tower known as an absorber. The liquid JLSO. drains
• down the packing by gravity and the gas flows upward through the
tower, coming into intimate contact with the liquid on the surface
of the packing. The gaseous SO., diffuses out of the gas stream
• into the liquid H2$0., reacts with the water in the acid stream
to form more H-SO^, and releases heat. Water make-up is necessary
• to maintain constant acid concentration to the absorber. The
m operation of the absorber also involves the physical transfer of
heat from the gas to the liquid. In a typical absorber, acid
• enters the tower at 180°F and cools the gas stream nearly to its
own inlet temperature, from about 450°F. The heat generated
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in the absorber leaves with the acid stream, thus requiring acid _
cooling external to the absorber. ™
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In a well-designed and operated absorber, sufficient contact
time is provided between the gas and the liquid streams so that
the gas leaving the absorber contains equilibrium vapor con-
centrations of the liquid and of the component being dissolved. •
These equilibrium concentrations are characterized by the vapor
pressures of the liquid and the component at the liquid concentration |
and temperature entering the absorber. _
Table 4.1 (Section 4.2) gives H0SO. and SO- vapor pressures at
C. "t J •
selected acid temperatures and concentrations. The table shows I
a distinct increase in H-SO. vapor pressure (acid volatility) with an M
increase in temperature, and emphasizes the importance of control!inn
the acid temperature to the absorber. As mentioned in Section 4.2, I
a good operating range is generally 170 - 185°F. The table
also shows a rapid increase in the vapor pressure of SO, as jj
the acid concentration exceeds 99 percent, and emphasizes the g
importance of controlling the acid strength so that the concentration
does not approach that of an oleum (greater than 100 percent acid). ft
Although not shown in the table, the vapor pressure of SO-
over oleum is even higher than its vapor pressure over 100 percent p
HpSO.. Since SO., absorption efficiency drops off below 98 percent —
acid concentration, a good operating range is generally 98-99 *
percent. •
6-4 •
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Proper absorber operation requires limiting the liquid temperature
• and concentration rises across the tower, and this requires that
the liquid flow be maintained above a minimum level. Ideas on
V proper absorber acid flowrate have changed over the years, but
m it appears that the minimum flow required is about 2 gallons
per minute of acid per ton per day of 109 percent H^SO^
ft produced. Installation of a flowmeter indicating acid flowrate
to the absorber is good operating practice.
I
Proper absorber operation also requires even cross-sectional
• distribution of the liquid from the top to the bottom of the
tower packing so that the gas receives maximum contact time
I on the surface of the packing and does not channel past the
m liquid. This even distribution requires proper arrangement of
the packing and proper liquid distribution at the top of the
• packing. A detailed discussion of tower internals can be found in
references (3,4).
I
The condition of the distributor and the packing should be
• checked during scheduled downtimes. The acid distribution can
be checked by running acid over the tower with no gas flow.
• Also, if the packing is dirty, the tower should be washed out
M with clean eicid during the downtime.
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| 6-5
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6.2 FIBER MIST ELIMINATORS
Effective control of stack gas acid mist emissions can be1 achieved
by fiber mist eliminators and by electrostatic precipitators.
Although electrostatic precipitators are frequently used in
the purification section of spent acid plants, there is no
evidence that any have been installed to treat the stack gas of
sulfuric acid plants in the last two years (5,6). This disuse
is probably due primarily to their relatively large size and
resultant high installation cost compared to fiber mist eliminators
and to the high maintenance cost required to keep the units
operating within properrtolerances in the acid environment which
is corrosive to the mild steel equipment. Hence, although electro-
static precipitators do have the advantage of operating with a lower
pressure drop than fiber mist eliminators (normally less than 1 inch
of H20), attention in this document is concentrated on fiber mist
eliminators.
Fiber mist eliminators utilize the mechanisms of impaction and
interception to capture large to intermediate size acid mist
particles and of Brownian movement to effectively collect low
to submicron size particles. Fibers used may be chemically
resistant glass or fluorocarbon. Fiber mist eliminators are
available in three different configurations covering a range of
efficiencies required for various plants having low to high acid
mist loadings and coarse to fine mist particle sizes respectively.
The three fiber mist eliminator configurations are:
6-6
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™ (a) Vertical tubes
• (b) Vertical panels
(c) Horizontal dual pads
6.2.1 Description
• 6.2.1.1 Vertical tubes (7,8,9,10)
| Tubular mist eliminators consist of a number of vertically oriented
. tubular fiber elements installed in parallel in the top of the
absorber on new acid plants and usually installed in a separate
• tank above or beside the absorber on existing plants. Each element
(see Figure 6.1) consists of glass fibers packed between two
jj concentric 316 stainless steel screens. In an absorber installation,
the bottom end cover of the element is equipped with a liquid seal
• pot to prevent gas bypassing. A pool of acid provides the seal in
• the separate tank design. Mist particles collected on the surface
of the fibers become a part of the liquid film which wets the fibers.
• The liquid film is moved horizontally through the fiber beds by the
gas drag and is moved downward by gravity. The liquid overflows the
• seal pot continuously, returning to the process.
g Tubular mist eliminators use inertia! impaction to collect larger
_ particles (normally greater than 3 microns) and use direct inter-
• ception and Brownian movement to collect smaller particles. The
• low superficial velocity of gas passing through the fiber bed--
20 to 40 feet per minute—provides sufficient residence time for
I nearly all of the small particles with random Brownian movement
to contact the wet fibers, effecting removal from the gas stream.
I
The probability that such a particle could pass through the bed
6-7
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CLEAN GAS TO
ATMOSPHERE
MIST-LADE
GAS IN
(COURTESY OF MONSANTO
ENVIRO-CHEM SYSTEMS, INC.)
Figure 6.1. Vertical tube mist eliminator element.
6-8
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• following the resultant greatly lengthened travel path is
• very low.
« Design volumetric flow rate through an element is about 1000
standard cubic feet per minute (scfm) (11) and the number of
fl elements required for a given plant size can be determined
from the scfm handled at capacity. Depending on the size
| of the sulfuric acid plant, anywhere from 10 to 100 elements
— may be used; each element is normally 2 feet in diameter and
• 10 feet high (11).
| Pressure drop across the element varies from 5 to 15 inches of
• HpO with a higher pressure drop required for a higher removal
efficiency on particles smaller than 3 microns. The manufacturer
I of these elements guarantees a mist removal efficiency of 100
percent on particles larger than 3 microns and 90 to 99.8 percent
| on particles smaller than 3 microns with 99.3 percent being
— most common (11). These efficiencies can be achieved on the stack
• gas of sulfuric acid plants burning elemental sulfur or bound-
• sulfur feedstocks (spent acid, wet gas, etc.}, and producing
acid or oleum.
™ Because the vertical tube mist eliminator does not depend only
• upon impaction for mist removal, it can be turned down (operated
at a volumetric flow rate considerably below design) with no loss
• in efficiency.
• 6.2.1.2 Vertical panels (7,8,9,10,12)
polygon framework closed at the bottom by a slightly conical
6-9
Panel mist eliminators use fiber panel elements mounted in a
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drain pan equipped with an acid seal pot to prevtnt gas bypassing. I
The polygon top is surmounted by a circular ring which is usually
installed in the absorption tower and welded to the inside of the m
absorption tower head. Each panel element consists of glass fibers •
packed between two flat parallel 316 stainless steel screens (see
Figure 6.2). In large high velocity towers, recent designs have •
incorporated double polygons, one inside the other, to obtain more
bed area in a given tower cross section. |
As in the high efficiency tubular mist eliminator above, the gas I
flows horizontally through the bed, but at a much higher superficial
velocity (400 to 500 feet per minute) using the impaction mechanism •
for collection of the mist particles. Gas leaving the bed flows •
upward to the exit port while the collected liquid drains down-
ward across the pan and out through the seal pot back into the •
tower or to a separate drain system.
I
The polygon may contain 10 to 48 vertical sides, each side normally
consisting of an 18 1/2" x 53" panel. A smaller 18 1/2" x 26" panel I
is available for small plants, e.g., 35 tons per day (11).
Pressure drop across the panel is usually about 8 inches of H^O.
•
The manufacturer of panel mist eliminators will usutlly guarantee |
an emission no higher than 2 milligrams per cubic foot (equivalent
to 0.375 pounds per ton of 100 percent HgSO. produced -- see I
Figure 4.1) for a sulfur-burning plant producing oleum up to 20 •
percent in strength and/or acid (9,11). For an inlet loading of 20
milligrams per cubic foot which is typical of a plant not producing I
oleum (refer to Section 4.3), 2 milligrams per cubic foot outlet
loading corresponds to a 90 percent removal efficiency. I
6-10
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CLEAN GAS TO
ATMOSPHERE
MIST-LADEN
GAS IN
(COURTESY OF MONSANTO
ENVIRO-CHEM SYSTEMS, INC.)
Figure 6.2. Vertical panel mist eliminator.
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Before guaranteeing that the above emission level will be met,
it is necessary to obtain an acid mist particle size distribution
curve on the absorber tail gas. This is done by sampling with
a cascade impactor. Use of one such impactor available on the
market is described in Industrial and Engineering Chemistry (13).
The impactor separates the mist particles into several size
fractions by passage in series through several impaction jets
designed to collect progressively smaller particles. From these
fractions, a particle size distribution curve can be constructed.
The collection efficiency of the panel mist eliminator falls off
below 1 micron. From the particle size distribution curve, the
mist eliminator removal efficiency curve, and the acid mist loading,
the expected acid mist emission from the panel mist eliminator can
be calculated. Sampling with an impactor and calculating the particle
size distributions can be time-consuming operations. However, the
problem of guaranteeing an emission level is of more concern to a
vendor than it is to EPA or to a State agency.
Because of the large percentage of submicron (below 1 micron) mist
present in the stack gas of a spent acid plant and of a plant producing
oleum stronger than 20 percent, the vertical panel mist eliminator
will usually give unsatisfactory performance for these plants.
(See Table 4.2 for oleum plant particle size distributions.)
Removal efficiency decreases as the gas velocity through the vertical
panel mist eliminator drops below the lower design limit. This limit
varies from unit to unit, the design limit being dependent upon many
factors including local ordinances. As the velocity is lowered below
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this limit, acid mist emissions and the stack opacity increase. Hence,
to properly enforce a standard, a State agency should measure the
stack gas acid mist loading with the unit running at or near rated
• capacity, and not during unit startup or shutdown.
• Vertical panel mist eliminators normally operate with a liquid
level in the acid seal pot below the conical drain pan. Although
I the velocity through the panels could be increased at lower
throughputs by raising the liquid level to cover the lower part
• of each panel, this would not be good practice since it would cause
• re-entrainment of spray by the gas passing over the liquid level in
the basket.
• Vertical panel mist eliminators also have an upper velocity design
• limit above which acid spray re-entrains from the inner surface
of the polygon. This spray may or may not reach the atmosphere,
| depending upon the configuration of the ductwork. If it does, it
normally will not cause an increase in stack opacity and will fall
• out on the plant equipment. Hence, operating above the upper limit
• should be of more concern to the plant operator than to EPA or a
State agency. Further information on removal efficiency is contained
• in references (10,12).
I
6.2.1.3 Horizontal dual pads (7,14)
Two circular fluorocarbon fiber beds held by stainless steel
screens are oriented horizontally in a vertical cylindrical vessel
I one above the other, so that the coarse fraction of the acid mist
is removed by the first pad (bottom contactor—see Figure 6.3) and
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\ [CLEAN GAS
1 TO ATMOSPHERE
DRAIN:
TOP CDWTACTOR.
f BOTTOM CONTACTOR-
i
DRAIN
MIST-LADEN
GAS IN
(COURTESY OF YORK SEPARATORS, INC.)
Figure 6.3. Horizontal dual pad mist eliminator.
6-14
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_ the fine fraction by the other (top contactor). The bottom
contactor consists of two plane segmented sections installed at an angle
I to the horizontal to facilitate drainage and give additional area
for gas contact. The assembly may be located adjacent to—or
0 positioned on--an absorption tower.
• This unit uses the high velocity impaction mist collection mechanism,
as does the panel mist eliminator; however, the collected acid
I drains downward through the pads countercurrent to the gas flow
•j producing a scrubbing action as well. Collected acid may be
drained from external connections or returned directly to the
• absorber through liquid seal traps.
• Total pressure drop across both pads is usually about 9 inches of
HpO. The superficial velocity through the unit is 9 to 10
• feet per second. Hence, the diameter of the cylindrical shell
and the pads is determined from the volume of gas handled. In
• one application, a 9-foot diameter unit was installed to handle
• 34,000 actual cubic feet per minute (acfm) at 160°F, and in
another application an 11-foot diameter unit was installed to
I handle 51,000 acfm at 175°F. Height requirements for the unit
depend upon whether it is located adjacent to or positioned on the
I absorber, but are roughly 1 1/2 to 2 times the diameter nf the unit.
I As with the panel mist eliminator, the dual pad unit will reduce
_ acid mist emissions to 2 milligrams per cubic foot (0.375 pounds
• per ton of 100 percent HpSO^) or less, provided the plant burns
• sulfur and does not produce oleum stronner than 20 percent 0A)
I 6-15
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and provided that a particle size distribution curve shows
that this level can be met. (See Section 6.2.1.2 for a dis-
cussion of how a particle size distribution curve is obtained).
The removal efficiency of the horizontal dual pad mist eliminator
decreases below the lower velocity design limit as it does for
the vertical panel mist eliminator. When properly designed and
installed, no increase in visible emissions should result from
reducing the superficial velocity to 5 feet per second. However,
just as with the vertical panel mist eliminator, it would be
desirable for a State agency to measure the acid mist loading
with the unit running near rated capacity, and not during unit
startup or shutdown.
If a plant plans to run considerably below capacity for an extended
period of time, it is possible to blank off some of the segments
of the bottom contactor to maintain the desired removal efficiency.
Above a superficial velocity of 12 feet per second, the top
contactor will not drain properly and the result is the same as
for the vertical panel mist eliminator. Further information on
rental efficiency is contained in reference (14).
6-16
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• 6.2.2 Installation and Maintenance
| 6.2.2.1 Vertical tubes (11)
I Figure 6.4 shows the installation of vertical tube elements in
a separate tank ("at grade") which is the usual case for
• existing plants. The elements are bolted into a tube sheet
• supported by I-beam stiffeners and provided with a liquid seal
to prevent gas bypassing. The tube sheet is one-inch carbon
• steel, and the tank is carbon steel above the tube sheet and
carbon steel lined with acid-proof brick below the tube sheet.
| The vessel must have both sufficient space above the tube
m sheet and a large enough manway to allow positioning the ele'ments.
* Representative tank sizes are 10'9" diameter x 23'5" for a 250
• ton per day plant and 21'6" diameter x 25'3" for a 1000 ton per
day plant.
I
The weight of the internals is determined by calculating the number
• of elements required and using a factor of 850-900 pounds for the
unit weight of one element and its associated tube sheet when wetted
• with acid. The ducts leading to and from the tank are carbon steel,
m the inlet duct being sized for an average velocity of 1500-2000 fpm
and the stack for 2000-4000 fpm. A new sump and pump is usually
• required to transport the collected acid to a storage tank.
• If space is available, the elements can be installed in the final
absorber. It is more common to install them in a "piggyback" unit
• mounted above the absorber on separate footings (10). These
arrangements eliminate the sump and pump and minimize the ductwork.
6-17
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ACCESS
MANHOLE
MISTY
GAS IN
CLEAN GAS OUT
TUBE PLATE
CYLINDRICAL
SCREENS
, FIBER! ELEMENTS
RECOVERED
LIQUID (MIST)
•SEAL PIPES
LIQUID SEAL
(COURTESY OF MONSANTO ENVIRO-CHEM SYSTEMS, INC.) Lj QUID
Figure 6.4. Vertical tube mist eliminator
installation.
6-18
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•
•
According to the manufacturer, tubular mist eliminators have
been operating maintenance-free.
Acid plants' are usually designed with 20-30 inches of H20 unused
pressure drop out of a total of about 140 inches of HgO plant
pressure drop. However, as the unit becomes dirty this safety
I factor is used up. In order to insure no drop in production
in a controlled plant, an additional fan to pull 25 inches of
I hLO should be installed in series with the existing blower,
M unless a sufficient design allowance has been included in the
total plant drop.
I
I
6.2.2.2 Vertical panels (11)
Figure 6.5 shows the installation of a vertical panel polygon
I in the top of the absorber, which is the usual case for existing
• plants. The polygon is constructed of 316 stainless steel and
the top of the carbon steel tower is lined with acid-proof brick
I up to the dished head. About eight feet of vessel height are
required to install the polygon. It is normally installed by
| putting a new top on the existing absorber or by cutting slits
« in the top of the existing absorber, lowering the panels through
the slits, and assembling the cone inside the vessel. If the
fl vertical panel unit was installed in a separate vessel, representative
tank sizes would be 8'0" diameter x 10'7" for a 250 ton per day
| plant and 19' 0" diameter x 13' 7" for a 1000 ton per day plant.
• Comments on stack velocity and on pressure drop in Section 6.2.2.1
also apply to the vertical panel installation.
6-19
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CLEAN GASES OUT
ACCESS MANHOLE
DISTRIBUTOR PAN OF TOWER
FIELD WELD
STRUCTURAL
SUPPORT
CYLINDER
ELEMENTS
IN POLYGON
FRAME
RECOVERED
LIQUID
(COURTESY OF MONSANTO ENVIRO-CHEM SYSTEMS, INC.)
Figure 6.5. Vertical panel mist eliminator
installation.
6-20
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Vertical panel mist eliminators are subject to corrosion of the
wires holding the fibers in place in the panel by the high
velocity acid flow. The panels have to be rescreened every
• six to seven years at a cost of 15-20 percent of the original
equipment price of the unit. The corrosion is particularly
I severe on the bottom of the inside of the polygon. &-
I
6.2.2.3 Horizontal dual pads
I Figure 6.6 shows a specific retrofit installation of a horizontal
dual pad unit handling 34,000 acfm in the^tail gas of an existing
| plant producing about 400 tons per day of 100 percent H,,S04. In
_ this case (the 9-foot diameter unit discussed in Section 6.2.1.3)
™ the unit is offset from the stack on the final absorber to prevent
• sulfate fouling of the pads by corrosion products formed in the
stack (primarily iron sulfate). The unit is positioned on top of
| the adjacent drying tower (no process connection) and acid collected
_ on the pad is drained through two one-inch drains to the drying tower
The internal structural supports and ductwork for this installation
I are 304 low carbon stainless steel, as are the screens for the fiber
M beds. Stack and duct sizes for this installation are shown in
Figure 6.6.
I
I
_ As mentioned above, dual pad mist eliminators are vulnerable to
™ sulfate fouling. This fouling can be particularly severe when
• the plant is shutdown. When the process gas flow is turned off,
sulfate which has been held up in the stack can drain onto the
b-zl
The comment on pressure drop in Section 6.2.2.1 also generally applies
to a dual pad installation.
-------
60 INCHES
54
INCHES
STACK
DRAIN
BLANK
TOP CONTACTOR-
DUAL PAD-
UNIT
SCRUBBER
DRAIN
«—9 FEET-
iTTOSTcONtACTOR
£ 1
J
54 \
'"CHES BUNK^
\
PLATFORM
_£,'
I ri
SCRUBBER
DRAIN
ELEVATION 6ft
FEET
ABSORBING TOWER
48 (NCI
DRYING TOWER
*ES
Figure 6.6. Retrofit horizontal dual pad mist eliminator installation.
6-22
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* pads. Dual pad mist eliminators are also subject to corrosion
• of the wires holding the fibers in place.
• 6.2.3 Design, Installation and Start-up Times
_ Table 6.1 presents manufacturers' May 1974 estimates of the
• normal length of time required to design and install fiber mist
• eliminators and bring the retrofitted unit back to normal opera-
tion (15,16). It shows that the total lead time required can
I vary from seven or eight months up to a year and a half.
I The two items in Table 6.1 with the longest and most widely
varying lead times are "Initial Design and Approval" and
| "Fabrication". Initial Design and Approval includes (15):
_ 1) Engineering design of the overall layout including
• general specifications and drawings of the mist eliminator,
• tank and ductwork.
2) Project fund approval.
J 3) Control agency approval.
4) Order placement.
• The above are all items over which the mist eliminator manufacturer
• has little control.
_ The lead time for fabricating vertical tube and vertical panel
mist eliminators depends greatly upon the size of the order,
• the manufacturer's shop backlog, and the availability of steel for
tank fabrication. The fabrication lead times shown in Table 6.1
I are for tank fabrication; mist eliminator fabrication lead times
I 6-23
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TABLE 6.1
MIST ELIMINATOR LEAD
(weeks)
Initial Design and Approval
Preparation of Drawings
Plant Approval
Fabrication
Shipment
Installation
Startup
Totals
TIMES
Vertical
Vertical
8 -
4 -
13 -
1 -
32 -
Tube and
Panel
26
8
3
35
2
3
1
78
Horizontal
Dual Pad
4-20
2-6
3
30 - 45
2
1
1
43 - 78
6-24
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I
vary from 3-6 months (13-26 weeks). Tank fabrication lead times
I were no longer than 5 months (22 weeks) from 1960 to 1972, but
— increased dramatically from 1972 to 1974. Although 316 staln-
• less steel is the normal material of contruction for vertical
• tube and vertical panel mist eliminators, a plant may occasion-
ally require alloy 20 construction. In this case, the long
J delivery times on alloy 20 can make the mist eliminator fabrica-
tion lead time as long as a year (15). The long lead time for
• fabricating horizontal dual pad mist eliminators is due to long
• delivery times on steel. In 1973, fabrication took but 16-20
weeks (.16).
• The installation lead times in Table 6.1 assume that the mist
• eliminator can be tied-in as soon as it is delivered to the plant.
To minimize production downtime, this delivery is generally
I scheduled to coincide with a planned unit shutdown (16).
I Startup after a planned shutdown or after a shutdown specifically
for tie-in usually takes a week or less. This does not include
| the time to test for compliance which usually adds another week
_ to the total lead time. It is desirable to test for compliance
™ with the acid unit running at capacity.
I
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• 6-25
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I
6.2.4 Costs _
Table 6.2 summarizes the estimated costs for control of acid
mist from existing acid plants. For each control unit and each I
type of installation, the installed capital cost, the net annual •
cost, and the net annual cost per ton of production (unit cost)
are qiven for several sizes of acid plants as of November 1974. •
Depending on the physical considerations of a particular plant, •
the control unit may be installed on top of the existing absorber
or on the ground in an available space and connected by ducts to I
the absorber and the stack. The former is termed the "piggyback"
installation, and the latter the "at grade" installation. I
For cost estimation purposes, the piggyback horizontal dual pad J
installation is assumed to consist of dual pads pre-mounted inside
a stainless steel vessel, which is installed on top of the
existing absorber. The other two mist eliminator piggyback
installations involve an extension of the acid-resistant brick-
lined carbon steel absorber with the appropriate mist eliminator
mounted inside. It is assumed that the piggyback installations
require no additional supporting structure and that no additional
fan capacity is added in order to arrive at a least-cost case.
The at grade installation houses the same type of control
equipment mounted on a new foundation on the ground near the
absorber. The cost of these installations is based on a new
foundation, an acid return pump, additional ducting, and 25 inches
of H20 additional fan capacity.
6-26
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• The installed cost range between these two types of installations
• should be representative of the costs for most acid plants.
However, there may be certain plants which could experience
I costs outside the range due to the variability in factors
such as: additional structural support.requirements, fan
• requirements, congestion at the plant site with difficulties
m in ducting, and design allowances built into the existing
absorber for future installation of the control elements.
• The installed cost for the horizontal dual pad installation
• shown in Figure 6.6 was $57,000 in early 1970. Multiplying
by a cost index ratio of 1.5 gives an installed cost of $85,000
• for November 1974. The unit handles 34,000 acfm and by linear
interpolation of Table 6.2 would be expected to cost about $46,000
| (in November 1974 dollars). The additional cost is at least
_ partially due to the ductwork to and from the unit, the inclusion
I
™ of three access platforms, added structural support, and labor
•• costs above the national averige.
I The installed capital costs (Table 6.2) for the vertical tube
unit are based on element capital costs for 99.3 percent removal
• efficiency on particles 3 microns and smaller in diameter at 12
• inches of H20 pressure drop. Figure 6.7 shows the relative
element capital costs for designs at other combinations of removal
• efficiency and pressure drop.
I
- 6-2P
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1.0
0.9
0.8
0.7
cc
o
§0.6
s
o
<0.5
El
o
0.4
0.3
0.2
0.1
1
68 10 12 14
ELEMENT PRESSURE DROP, inches of water
(COURTESY OF MONSANTO
ENVIRO-CHEM SYSTEMS, INC.)
Figure 6.7. Relative changes in capital costs for vertical tube
mist eliminator elements at different removal effi-
ciencies and pressure drops.
6-30
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• The gross annual cost consists of: capital related charges such
• as depreciation, interest on borrowed capital, property tax,
(insurance, and overhead which add up to 26 percent of the installed
I capital cost; operating cost which is totally made up of power cost
for the pressure' drop caused by the control unit; and maintenance
• cost which is based on information supplied by the equipment
m manufacturers. In order to determine the net annual cost, the
credits for recovered acid are subtracted from the gross annual
I
•
I
cost.
The wide range of reported emission rates for acid mist results
in a range of cost credits and a range of net annual costs.
£ The higher the pre-control emission rate, the hiaher the credit
for recovered product would be. The value of the acid recovered
• is based on the production cost (see Table 6.4) rather than
• on the market sales price. The final figures shown in Table
6.2 are the net annual cost. per ton of production. An operating
I ratio (production/capacity) of 90 percent is assumed for this
calculation.
I
Table 6.3 shows the approximate installed capital cost_for
• control of acid mist in new acid plants as of November 1^74.
They were obtained by multiplying the costs in Table 15,
• reference (7), by a cost index ratio of 1.58. The cost for a
• new plant should always be less than the cost for retrofit since
the control unit can be designed in from the beginning (usually
• as an expanded section at the top of the absorber). The fact
6-31
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TABLE 6.3
INSTALLED CAPITAL COSTS FOR ACID MIST CONTROL IN NEW PLANTS
Horizontal Dual Pad ($)
Vertical Tube ($)
Vertical Panel ($)
Size (TPD of 100% H2S04)
50
15,800
87,400
22,000
250
31 ,600
111,000
36,400
750
44,200
166,000
87,000
1500
60,000 '
269,000
151,000
that this does not appear to be the case for the 50 TPD vertical
tube piggyback installation is probably due to differences in cost
estimating procedures.
To facilitate comparison of the above acid mist control costs to
the total costs of installing and operating a sulfuric acid unit,
Table 6.4 shows estimated capital cost and production cost for a
new sulfur burning dual absorption unit as of November 1974.
Accurate cost figures are not available to allow comparison with
an existing unit. Costs are given for a dual absorption unit
since all new units will have to be dual absorption or employ tail
gas scrubbing systems to control S02 emissions to the level re-
quired in the EPA standard of performance for new sulfuric acid
plants. Table 6.4 is based on information (17) used to support
this new source standard. This information source cites capital
6-32
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TABLE 6.4
ESTIMATED COSTS FOR NEW SULFUR BURNING DUAL ABSORPTION SULFURIC ACID UNIT
Capital Cost ($)
Production Cost ($/Ton)
Size
50
903,000
30.68
(TPD of 100% H2S04)
250
2,650,000
23.34
750
5,539,000
20.18
1500
8,810,000
18.7
and production costs for a 1000 TPD acid plant. The capital
costs for the sizes given in the table are extrapolated using an
exponent of 0.67 from the Chemical Engineering cost file (18).
The production costs for the sizes given were determined from
utility, raw material and labor requirements and capital charges
for the same 1000 TPD plant cited above (17).
6-33
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6.3 EMISSION REDUCTION
6.3.1 SPNSS Source Testing
Table 6.5 presents the results of testing performed during 1971 by EPA
in developing the acid mist standard of performance for new stationary
sources (SPNSS). All three plants tested employed vertical tube
mist eliminators. All runs were made using EPA Method 8. All of
the test results are equal to or less than the acid mist standard
of performance for new sulfuric acid plants of 0.15 pounds per
ton of 100 percent H^SO* produced.
Unit A had a capacity of 700 tons per day (TPD) of TOO percent
fLSO. and burned only dark sulfur at the time of the test. It
produced 35 percent oleum on February 17, and 30 percent oleum on
February 18, but the oleum/acid oroduction ratios are unknown.
The unit was less than a vear old at the time of this test, the
mist eliminator being installed when the unit was built.
Unit B had a capacity of 750 TPD at the time of the test. On
March 27, it burned 250 TPD of spent acid (on a 100 percent H^O^
basis) and the balance was elemental sulfur. On that same dav, it nro-
duced 70 TPD of 93 percent acid, 460 TPD of 98 percent acid, and
200 TPD of 20 percent oleum (all on a 100 percent H2S04 basis).
Unit B is an older unit whose mist eliminator had been retrofitted.
Subsequent to this retrofit but prior to the EPA tests, a sodium
sulfite scrubbing tower for SCL control was installed downstream
of the mist eliminator. The tests were conducted downstream of
this scrubbing tower.
6-34
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6-35
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I
Unit C had a capacity of 450 TPD at the time of the test. Durina
the period August 31 - September 2, 1t burned spent acid and sulfur
and produced 20 percent oleum and 99 percent acid. The spent
acid/sulfur feedstock ratios and the acid/oleum production ratios
are unknown. However, average consumption for the months of
August and September combined are 266 TPD of spent acid and 204
TPD of sulfur. For the same two-month period, average production
was 242 TPD of acid and 210 TPD of oleum (all grades). All of the
above rates are on a 100 percent H^SO. basis. The total production
and consumption rates of 452 and 470 TPD, respectively, indicate a
unit conversion efficiency of about 96 percent. Unit C is an
older unit with a very unusual design. The converter exit gas
is split in half and fed to two equivalently-sized final absorbers.
Each absorber is followed by a booster blower, a mist eliminator,
and a stack. Both mist eliminators are retrofits. EPA tested one
stack. The equivalent capacity for this one stack is thus 2?5TPD.
The production rates shown in Table 6.5 (185, 175, 193 TPD) also apolv
only to this stack and are one-half of the total unit production rates.
6.3.2 Section lll(d) Source Testing
Table 6.6 presents the results of additional testing performed under
EPA supervision by a contractor. The purpose of these two tests
was to define the performance of the vertical panel and the horizontal
dual pad mist eliminators covered in this document, and to compare
6-36
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