United States
Environmental Protection
Agency
Office of Pollution
Prevention and Toxics
Washington, DC 20460
EPA-745-R-97-007
November 1997
Updated March 3, 1998
vvEPA
EMERGENCY PLANNING AND
COMMUNITY RIGHT-TO-KNOW ACT
- SECTION 313
Guidance for Reporting Sulfuric Acid (acid aerosols
including mists, vapors, gas, fog, and other airborne
forms of any particle size)
Section 313 of the Emergency Planning and Community Right-to-Know Act of 1986 (EPCRA) requir
certain facilities manufacturing, processing, or otherwise using listed toxic chemicals to report
their environmental releases of such chemicals annually. Beginning with the 1991 reporting year, such
facilities also must report pollution prevention and recycling data for such chemicals, pursuant to
section 6607 of the Pollution Prevention Act, 42 U.S.C. 13106. When enacted, EPCRA section 313
established an initial list of toxic chemicals that was comprised of more than 300 chemicals and 20
chemical categories. EPCRA section 313(d) authorizes EPA to add chemicals to or delete chemicals from
the list, and sets forth criteria for these actions.
Section 1.0
1.1
1.2
1.3
Section 2.0
2.1
2.2
2.3
Section 3.0
3.1
CONTENTS
Introduction
Who Must Report
Thresholds
What Constitutes Aerosol Forms of Sulfuric Acid and Their
Manufacture, Processing, or Otherwise Use
3
3
4
Guidance On Sulfuric Acid Aerosols For Certain Specific Activities That
Generate Aerosols Forms 5
Sulfuric Acid Aerosols Generated In Acid Reuse Systems 5
Sulfuric Acid Aerosols Removed By Scrubbers 6
Sulfuric Acid Aerosols Generated In Storage Tanks 6
Sulfuric Acid And Its Formation in Air
Industrial Sources of Sulfuric Acid Aerosols
CONTENTS cont.
7
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3.1.1 Pulp and Paper Mills 9
3.1.2 Acid Aerosols From Sulfuric Acid Manufacture 11
3.1.3 Smelters 14
3.1.4 Petroleum Refining 14
3.1.5 Sulfuric Acid Aerosol Formation In Stacks From Combustion
Processes 14
3.1.6 Coal Combustion 16
3.1.7 Fuel Oil Combustion 16
Section 4.0 Measurement Methods 17
References 18
Appendix 1 20
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Section 1.0. Introduction
On June 30, 1995 (60 FR 34182), EPA modified the listing for sulfuric acid (Chemical Abstracts
Service Number 7664-93-9) on the list of toxic chemicals subject to the reporting requirements under
section 313 of the Emergency Planning and Community Right-to-Know Act of 1986 (EPCRA) (5). EPA
modified the listing by deleting non-aerosol forms of sulfuric acid from the section 313 list based on
the conclusion that they cannot reasonably be anticipated to cause adverse effects on human health or
the environment. EPA added a modifier to the listing for sulfuric acid to exclude the non-aerosol
forms. The listing now reads "Sulfuric acid (acid aerosols including mists, vapors, gas, fog, and
other airborne forms of any particle size)." Therefore, beginning with the 1994 reporting year,
facilities are no longer required to include non-aerosol forms of sulfuric acid in threshold and
release determinations. In this document we will use the term "sulfuric acid aerosols" to indicate
airborne forms of sulfuric acid as listed in section 313 of EPCRA.
The purpose of this document is to assist facilities in determining the sources and amounts of
sulfuric acid aerosols that are to be included in threshold and release determinations under EPCRA
section 313. This document is not meant to be exhaustive, but rather provide some guidance to help
facilities in their determination of threshold and release quantities. Threshold and release
determinations for sulfuric acid aerosols are highly dependent on site specific conditions and
equipment. Therefore, this document can only provide general information concerning the possible
formation and release of sulfuric acid aerosols.
Section 1.1. Who Must Report
A plant, factory, or other facility is subject to the provisions of EPCRA section 313, if it
meets all three of the following criteria:
• It is included in the primary Standard Industrial Classification (SIC) codes 20
through 39 and beginning January 1, 1998, it is in one of the following industries:
Metal Mining, SIC code 10 (except SIC codes 1011, 1081, and 1094); Coal
Mining, SIC code 12 (except SIC code 1241); Electric Utilities, SIC codes 4911,
4931, or 4939 (each limited to facilities that combust coal and/or oil for the purpose
of generating power for distribution in commerce); Commercial Hazardous Waste
Treatment, SIC code 4953 (limited to facilities regulated under the Resource
Conservation and Recovery Act, subtitle C, 42 U.S.C. section 6921 et seq.); Chemicals
and Allied Products-Wholesale, SIC code 5169; Petroleum Bulk Terminals and
Plants, SIC code 5171; and, Solvent Recovery Services, SIC code 7389 (limited to
facilities primarily engaged in solvent recovery services on a contract or fee basis);
and
• It has 10 or more full-time employees (or the equivalent of 20,000 hours per year); and
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• It manufactures (includes imports), processes or otherwise uses any of the toxic
chemicals listed on the EPCRA section 313 list in amounts greater than the threshold
quantities specified below.
In addition, pursuant to Executive Order 12856 entitled "Federal Compliance with Right-to-
Know Laws and Pollutant Prevention Requirements," federal facilities are required to comply with the
reporting requirements of EPCRA Section 313 beginning with calendar year 1994. This requirement is
mandated regardless of the facility's SIC code.
Section 1.2. Thresholds
Thresholds are specified amounts of toxic chemicals manufactured, processed, or otherwise
used during the calendar year that trigger reporting requirements. Reporting is required for sulfuric
acid aerosols if the following thresholds are exceeded.
• If a facility manufactures or imports 25,000 pounds of sulfuric acid aerosols over the
calendar year.
• If a facility processes 25,000 pounds of sulfuric acid aerosols over the calendar year.
• If a facility otherwise uses 10,000 pounds of sulfuric acid aerosols over the calendar
year.
The quantities of sulfuric acid aerosols included in threshold determinations are not limited
to the amounts of sulfuric acid aerosols released to the environment. All sulfuric acid aerosols
manufactured, processed, or otherwise used are to be counted toward threshold determinations. This
includes any amount of sulfuric acid aerosols that may be generated in closed systems or that are
generated in stacks prior to or after being treated by scrubbers.
Section 1.3. What Constitutes Aerosol Forms of Sulfuric Acid and Their Manufacture,
Processing, or Otherwise Use
For the purposes of the reporting requirements under EPCRA section 313, sulfuric acid aerosols
include mists, vapors, gas, fog, and other airborne forms of any particle size. Note that there is no
size limit for particles that must be included under the EPCRA section 313 sulfuric acid aerosols
listing. Although the qualifier includes the terms mists, vapors, gas, and fog these terms are not
specifically defined for EPCRA section 313 since the last part of the qualifier "other airborne forms
of any particle size" makes it clear that any airborne form is covered by the listing. The specific
terms mists, vapors, gas, and fog are included to make it clear that sulfuric acid that is identified
as being in one of these forms would be covered by the sulfuric acid aerosols listing.
If sulfuric acid is present in the form of a gas, fog, vapor, or mist or any other airborne form
then sulfuric acid is considered to be in the aerosol form and is covered by the EPCRA section 313
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sulfuric acid aerosols listing. Solutions of sulfuric acid which do not become airborne are not
covered by the EPCRA section 313 sulfuric acid aerosols listing but such solutions may generate
sulfuric acid aerosols during their manufacture, processing or otherwise use. In general, sulfuric
acid aerosols are manufactured any time a solution of sulfuric acid is made to become airborne such as
when it is sprayed or distilled. If the generation of sulfuric acid aerosols through spraying or other
means is intentional (i.e., it is intended that the sulfuric acid aerosol be generated for a particular
use activity) then, in addition to manufacturing the sulfuric acid aerosol, such aerosols are also
being otherwise used. Thus, spraying of sulfuric acid aerosols on to an item for cleaning, etching, or
other purposes constitutes the manufacture and otherwise use of sulfuric acid aerosols. If sulfuric
acid aerosols are used in a process in which any part of the sulfuric acid becomes incorporated into a
product which is then distributed in commerce then, under EPCRA section 313, the sulfuric acid
aerosols are considered to have been processed.
Section 2.0. Guidance On Sulfuric Acid Aerosols For Certain Specific Activities That Generate
Aerosols Forms
EPA has provided the following guidance for specific activities that generated sulfuric acid
aerosols. The guidance in sections 2.1, 2.2, and 2.3 is intended to apply only to the specific
situations discussed in these sections. If you are not sure whether this guidance applies to the
situation at your facility, then EPA should be consulted before using this guidance.
Section 2.1. Sulfuric Acid Aerosols Generated In Acid Reuse Systems
When solutions of sulfuric acid are aerosolized the "manufacture" of a listed chemical
(sulfuric acid aerosols) has occurred. This is a result of the qualifier to the sulfuric acid listing,
which excludes non-aerosol forms and limits the reporting to aerosol forms only. The addition of the
acid aerosol qualifier has an impact on certain processes that, prior to the addition of the qualifier,
would not have been considered to be "manufacturing" a listed chemical. Acid reuse systems that use
aqueous solutions of sulfuric acid to generate acid aerosols, use the acid aerosols, condense them
back into solution, and then reuse the acid solution again and again are impacted by the addition of
the acid aerosol qualifier. In such processes, the continuous reuse of the acid solutions generates
very large quantities of acid aerosols that technically should be counted towards the "manufacture"
[the generation of the acid aerosol is the "manufacture" of sulfuric acid (acid aerosol)] and
"otherwise use" thresholds. This may result in many facilities greatly exceeding the "manufacture"
and "otherwise use" reporting thresholds that, prior to the addition of the qualifier, would not have
exceeded thresholds.
While it is technically correct to apply all of the quantities of acid aerosols generated in
such systems towards the "manufacture" and "otherwise use" reporting thresholds, EPA did not intend to
increase the reporting burden as a result of addition of the sulfuric acid aerosol qualifier. In
addition, under EPA's general approach to reuse systems, a toxic chemical is not counted toward
thresholds each time it is reused but only once per reporting period, and that approach would apply to
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sulfuric acid reuse systems were it not for the aerosol qualifier. Therefore, EPA is providing the
following guidance to reduce the reporting burden for facilities that operate such processes and to
bring the treatment of such systems into alignment with EPA's general approach to reuse.
Rather than having facilities count all quantities of acid aerosol generated in such systems
towards the "manufacture" and "otherwise use" thresholds, EPA will allow facilities to apply the total
volume of acid in these systems only once to these thresholds. For example, if an acid reuse system
starts the year with 2000 pounds of acid and 500 pounds is added during the year then the total amount
applied towards acid aerosol thresholds would be 2500 pounds. This reflects a one time per year
counting of all of the acid molecules as being in the acid aerosol form rather than counting them over
and over again each time the acid aerosol form is generated and subsequently used. Since in these acid
reuse systems the acid aerosols are "manufactured" and then "otherwise used" the 10,000 pound
"otherwise use" threshold would be the threshold that would first trigger reporting from such systems.
This guidance applies only to acid reuse systems and the reporting of sulfuric acid aerosols
under EPCRA section 313. This guidance does not apply to any other types of processes or to any other
listed chemical.
2.2. Sulfuric Acid Aerosols Removed By Scrubbers
When a scrubber is used to remove sulfuric acid aerosols prior to or in a stack, the acid
aerosols are usually converted to the non-aerosol form. The non-aerosol forms of sulfuric acid are not
reportable under EPCRA section 313 because the qualifier to the sulfuric acid listing includes only
acid aerosol forms. Sulfuric acid as a discrete chemical has not actually been destroyed by the
scrubber, but the form of sulfuric acid reportable under EPCRA section 313 has been destroyed.
Therefore, since sulfuric acid aerosols removed by scrubbers are converted to a non-reportable form,
the quantity removed by the scrubber can be reported as having been treated for destruction.
2.3. Sulfuric Acid Aerosols Generated In Storage Tanks
Sulfuric acid aerosols are generated in the empty space (head space) above sulfuric acid
solutions contained in storage tanks. The amounts of acid aerosols generated in such storage tanks are
to be applied towards the "manufacture" threshold for sulfuric acid aerosols. In such storage tanks
the sulfuric acid molecules are constantly moving between the atmosphere and the solution. EPA does
not intend for facilities to count such movement of the acid molecules in and out of the stored acid
solution as continuous "manufacture" of sulfuric acid aerosols. For such storage tanks the amount of
acid aerosol to be applied towards the "manufacture" threshold is the average amount that existed in
the atmosphere above the acid solution during the year.
Each facility should determine the average conditions for their specific storage tank (i.e.,
the capacity of the tank, the average amount in the tank, the average head space in the tank, the
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concentration of the acid solution stored, the temperature, and other information that may have an
impact on aerosol calculations) and make the appropriate calculation of the amount of acid aerosol to
apply towards the "manufacture" threshold. Any amounts of sulfuric acid aerosols that may be released
from the storage tank through venting or fugitive releases must also be included in the threshold
determination. If the storage tank is refilled and drawn down several times during the year then the
calculations should be based on all of the acid that was stored in the tank. For example, if a 10,000
pound capacity tank is refilled and drawn down 6 times during the year (such that 60,000 pounds of acid
were stored in the tank during the year) then the tank calculations, based on the average condition for
one 10,000 pound tank of acid, should be multiplied by 6.
Section 3.0. Sulfuric Acid And Its Formation In Air
Sulfuric acid (H2SO4) is miscible in water in all proportions and has a strong attraction for
water. The anhydrous chemical boils at 279.6°C (1). Commercial sulfuric acid normally contains 93 to
98% sulfuric acid with the remainder being water. A boiling point-composition diagram (Figure 1,
Appendix 1) for aqueous sulfuric acid, indicates that below 75% H2SO4, the vapor evaporating from a
solution of the acid is essentially water. This fact is illustrated in Table la of Appendix 1, which
contains the partial pressure of sulfuric acid and total vapor pressure of the solution over aqueous
sulfuric acid solutions at various concentrations (1). Since the partial pressure of concentrated
sulfuric acid is very low, little sulfuric acid is expected to volatilize from sulfuric acid solutions
such as may be present in storage tanks. However, as discussed above, the amount of acid aerosols
generated is dependent on the quantity of acid in the tank(s) during the year, the concentration,
temperature, and other factors. The information in Appendix 1 and the guidance in section 2.3 can be
used to assist in determining if significant amounts of sulfuric acid aerosols are present in storage
tanks.
Sulfuric acid containing dissolved sulfur trioxide (SO3) is known as oleum, fuming sulfuric
acid or disulfuric acid. The vapor pressure of sulfuric acid over oleum containing 10% to 30% of free
SO3 by weight is shown in Table 2a of Appendix 1 (2). Since the vapor pressure of sulfur trioxide over
oleum is high, sulfuric acid aerosols also form when oleum is exposed to air containing moisture.
Sulfuric acid is generally formed by the oxidation of sulfur dioxide (SO2) and the reaction of
the resulting sulfur trioxide (SO3) with water.
so2 + y2o2 -» so3
SO3 + H2O -^H2SO4
While thermodynamically, SO2 has a strong tendency to react with oxygen to form SO3; under normal
tropospheric (lower atmosphere) conditions the reaction rate is very slow in the gas phase (3, 17, 18,
19). Other than within a reaction chamber, SO2 is unlikely to generate SO3 and then sulfuric acid.
However, once SO3 is formed, it is converted to H2SO4 so rapidly (within milliseconds) at normal
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humidities, that any reaction in which SO3 is formed in moist air is equivalent to forming H2SO4 (3,18).
In stacks from combustion processes, moisture in the stack would be expected to convert any SO3 present
into sulfuric acid aerosols. This being the case, the quantity of SO3 generated in such stacks
(multiplied by 98.08/80.07, the molecular weight of sulfuric acid divided by the molecular weight of
sulfur trioxide) should be included with those of H2SO4. If SO3 is produced within a stack or unit of
the plant without moisture present, sulfuric acid aerosols would not be produced. Although the SO3
releases may be converted to H2SO4 in the environment, facilities are not responsible for conversions
that may take place in the environment after a chemical that is not listed under EPCRA section 313,
such as SO3, has been released. Therefore, if SO3 is the chemical that is released from the facility,
the facility is not required to include it, or any H2SO4 produced in the environment from the released
SO3, in any EPCRA section 313 calculations of thresholds or releases.
Section 3.1. Industrial Sources Of Sulfuric Acid Aerosols
It is clear that industries required to report sulfuric acid aerosol releases will be among
those that had previously reported sulfuric acid releases to air under EPCRA section 313. Facilities
that previously reported over 25,000 pounds of sulfuric acid releases to air have exceeded the
manufacturing threshold quantity of sulfuric acid aerosols necessary for reporting under the new
listing. Additional facilities may be required to report because releases of sulfuric acid to air
would not have included, for example, amounts of sulfuric acid aerosols that were produced in the stack
and subsequently removed by scrubbers or produced internally during sulfuric acid manufacturing,
processing, or use and that were removed by scrubbers prior to the stack. Since these amounts of
sulfuric acid aerosols do count towards the EPCRA section 313 threshold determinations, facilities
with less than 25,000 pounds of air releases are not excluded from reporting. In addition, some
facilities may be using sulfuric acid aerosols in excess of 10,000 pounds and would also be required to
report. According to the 1993 Toxics Release Inventory (TRI), there were 191 facilities reporting
releases of 25,000 pounds or more of sulfuric acid to air. The number of these facilities in each of 2-
digit standard industrial categories (SIC) is shown in Table 1, as well as the prominent types of
industries within the category that have reported sulfuric acid emissions to air. The industries
shown in italics include 34 facilities that are in the 80th percentile (over 178,000 pounds per year)
for sulfuric acid releases to air. These 34 facilities' air emissions are almost entirely from point
sources, suggesting that either sulfuric acid is formed in the stack from sulfur trioxide or sulfuric
acid is aerosolized in a process that leads to its release in the stack. Thirty two of these sites
reported producing the sulfuric acid; the copper smelters and phosphate fertilizer plants produced
sulfuric acid for sale or distribution.
Table 1. Industrial Categories of Facilities Emitting over 25,000 Ibs/yr of Sulfuric Acid Aerosols in 1993
Category (SIC Code)
Paper and Allied Products (26)
Chemicals and Allied Products (28)
No. Sites*
185
53
Major Industries"
Pulp, paper and paperboard mills.
Phosphate fertilizers; Industrial inorganic chemicals.
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Category (SIC Code)
Primary metal industries (33)
Food and Kindred Products (20)
Lumber, Wood Products (24)
Stone, clay, glass, concrete (32)
Petroleum refining (29)
Metal Products, except machinery (34)
Tobacco Manufacturers (21)
No. Sites*
23
11
8
6
4
4
1
Major Industries"
Copper smelting; Blastfurnaces and steel mills
Wet corn milling; fats and oils; liquors', malt beverages.
Sawmills; Reconstituted wood products
Glass
Petroleum refining
Metal coatings
Tobacco stemming and redrying
*A site may list more that one SIC code.
"The industries shown in italics include facilities that are in the 80th percentile for sulfuric acid releases to air.
The industrial breakdown does not necessarily indicate that emissions result from processes
unique to the industry. For example, phosphate fertilizer manufacturers, which use sulfuric acid to
make phosphoric acid and normal superphosphate, may produce their own sulfuric acid from elemental
sulfur and may also use it captively. Sulfuric acid emissions from phosphate fertilizer manufacturing
may therefore be primarily from sulfuric acid manufacturing. Similarly, sulfuric acid is a known
component in flue gas from fossil fuel combustion and waste incineration. The SO3/H2SO4 (SO3, as stated
previously is immediately transformed into sulfuric acid in the presence of water) produced from
combustion sources is between 1 and 3% of the SOX emitted by these sources (the rest being SO2). It is
not clear why sulfuric acid is emitted from combustion sources. A possible explanation is that there
are substances in the flue gas or on the stack walls that catalyze the oxidation of sulfur dioxide to
sulfur trioxide, similar to the heterogeneous reactions that can occur in the atmosphere (17-19).
Sulfuric acid is also formed in some flue desulfurization processes (7).
Section 3.1.1 Pulp and Paper Mills
The kraft pulping process involves the digesting of wood chips at elevated temperature in
"white liquor", an aqueous solution of sodium sulfide and sodium hydroxide, to dissolve the lignin
that binds the cellulose fibers of the wood together. The spent liquor used to digest wood chips,
called "black liquor", is combusted in recovery furnaces to recover heat and cooking chemicals.
Sulfuric acid is present in flue gas from kraft recovery furnaces and has been cited as being one of the
five most prevalent air toxics released from recovery furnaces of the direct contact evaporator (DCE)
and non-direct contact evaporator (NDCE) types (21). In a DCE, the flue gas comes in contact with the
black liquor, whereas in a NDCE, it does not. Field tests on five Kraft recovery furnaces showed
SO3/H2SO4 levels ranging from 0 to 3 ppm in the flue gas, with an average level of 0.81 ppm, or about 10%
of that found in fossil fuel plants burning fuel containing 1-3% sulfur (4). In these tests, no
correlation was found between SO2 and SO3/H2SO4 levels. Therefore, one cannot estimate emission factors
for SO3/H2SO4 based on those for SO2. While EPA has compiled emission factors for pollutants from
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kraft, acid sulfite, and neutral sulfite semichemical (NSSC) pulping (9), no emissions factors have
been presented for SO3/H2SO4. However, industry tests of Kraft recovery furnaces have been performed
between 1989 and 1993 and the results are presented in Table 2 (20).
Table 2. Emission Factors from Kraft Recovery Furnaces*
Mill Code (date built/rebuilt)**:
Control device/ Type evaporator
A: WB, Cascade
B:ESP
RFI (1973): WB ESP, Cascade
RFRIG1 (1991):WB ESP, Cyclone
RFRIG2 (1991): WB ESP, Cyclone
SUMMARY
C:DB
D: DB ESP
E:ESP
RFO(1986):DBESP
SUMMARY
Type
DCE
DCE
DCE
DCE
DCE
DCE
NDCE
NDCE
NDCE
NDCE
NDCE
BLS
MPPD
3.12
16.56
4.60
0.85
2.63
9.84
18.60
12.00
1.90
H2SO4 Emissions in Ib/ton BLS
Range
ND to 4.7E-02
NDto 1.5E-02
NDto2.5E-02
NDto 1.9E-02
NDtol.9E-02
0.21 to 0.91 ppm
0.17 to 2.98 ppm
0.49 to 1.71 ppm
NDto 1.6E-01
Average
1.6E-02
8.4E-03
1.4E-02
ND(2.0E-02)
8.2E-03
8.4E-03 (Median)
3.3E-02
7.1E-02
5.1E-02
ND(1.3E-02)
4.2E-02 (Median)
'Abbreviations: BLS = black liquor solids; ND = not detected; DCE = direct contact evaporator; NDCE = non-direct contact evaporator; ESP =
electrostatic precipitator; WB = wet bottom; DB = dry bottom; MPPD = million pounds per day. Type evaporator given for DCE types only.
"Mills with codes not preceded by "RE " are from a 1980 study (4). Dates when these mills were built or rebuilt are not available.
The median sulfuric acid emissions from the direct contact (DCE) and non-direct contact (NDCE)
evaporator recovery furnaces, 8.4E-03 pounds per ton black liquor solids (BLS) and 4.2E-02 pounds per
ton BLS, respectively, can be used to estimate sulfuric acid emissions. For example, if a kraft mill
using 1100 air dry tons of unbleached pulp per day (ADTUBPD) generates 3300 pounds BLS per ADTUBPE
operates two DCE furnaces 365 days per year, the pounds of H2SO4, H, emitted during the year will be:
H = 1100 ADTUBPD x 365 days/year x (3300 Ibs. BLS/ADTUBPD x ton BLS/2000 Ibs ffi/S$*03
Ibs. H2SO4/ton BLS = 5,565 Ibs. of H2SO4
The pounds of sulfuric acid aerosols produced in recovery furnaces on site should be combined with that
produced from fuel oil and coal combustion. Should the total equal or exceed 25,000 pounds per year,
reporting would be required under EPCRA Section 313. It should be noted that sulfuric acid used at the
site for such purposes as C1O2 generation, pH control, and ion exchange generation no longer must be
reported since aerosol forms of H2SO4 are not involved.
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Section 3.1.2 Acid Aerosols from Sulfuric Acid Manufacture
Sulfuric acid may be manufactured commercially by either the lead chamber process or the
contact process. However, sulfuric acid is usually produced by the contact process (1, 2, 10, 22). In
the contact process, sulfur is oxidized to SO2 which is subsequently fed into a converter where it is
catalytically oxidized to SO3. Finally, the sulfur trioxide is absorbed in a strong sulfuric acid
solution or oleum. Sulfuric acid plants are further classified by feedstock: elemental sulfur
burning, spent sulfuric acid and hydrogen sulfide burning, and metal sulfide ores and smelter gas
burning. Contact sulfuric acid plants vary in design depending on the raw material used to produce
SO2. Oleum is also produced in contact plants, where SO3-containing gases are passed through a special
oleum tower. Regeneration of spent sulfuric acid is another form of sulfuric acid manufacture, often
performed in order to comply with antipollution regulations (1).
Sulfuric Acid Manufacture
The amount of sulfuric acid aerosols produced in sulfuric acid manufacture is a function of the
type of sulfur feedstock, the concentration of the absorbing acid, and the conditions in the absorber
(1, 2, 22). Elemental sulfur produces little acid mist when burned because there is little water
present. However the hydrocarbons in other feedstock (such as spent acid) produce water vapor during
combustion. The affect of acid strength on mist production is illustrated by results showing 0.5 to
5.0 kilograms (kg) of uncontrolled acid aerosol emissions per Megagram (Mg) of acid produced (1.0 to
10.0 pounds (Ib) of acid emissions per ton of acid produced) from oleum plants burning spent acid
compared with 0.2 to 2.0 kg/Mg (0.4 to 4.0 Ib/ton) of emissions from 98% sulfuric acid plants burning
elemental sulfur. In addition, the aerosol particle size from oleum plants is finer than that from the
98% sulfuric acid plants. The operating temperature of the absorption tower affects SO3 absorption
and, accordingly, acid mist formed in the exit gas.
In an elemental sulfur burning plant, after the sulfur is burned to SO2 and catalytically
converted to SO3, the gas enters one or multiple absorption towers (packed columns), usually operated
in countercurrent, in which the sulfur trioxide is absorbed in sulfuric acid of 98-99% concentration
to form more sulfuric acid (1, 2, 22). The optimal operating temperature of the absorption tower
depends on the strength of the acid produced, throughput rate, inlet sulfur trioxide concentrations
and other factors peculiar to a particular plant. The optimal concentration of the absorbing acid is
the azeotrope (see Appendix 1), 98.3%, where the combined vapor pressures of H2SO4, SO3, and water are
at a minimum. At lower concentrations, the water vapor partial pressure is higher and there is a
greater risk of sulfuric acid mist formation. At higher concentrations the tail gas will contain
increased amounts of H2SO4 and SO3 because of their higher partial pressures. Both sulfuric acid mist
formed within the system and gaseous sulfuric acid vaporized from the concentrated acid in the
absorption towers and carried along with the predominant sulfur trioxide gas, constitute sulfuric acid
aerosols that are being manufactured and therefore contributing to the manufacturing threshold of
sulfuric acid aerosols for reporting under section 313 of EPCRA. Sulfuric acid mists entrained in tail
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gas are separated by special filters and determined by measurement of the acid content.
Regeneration of Spent Sulfuric Acid
The regeneration of spent sulfuric acid normally comprises two major steps, concentration to
the highest feasible level and decomposition of the spent acid (1). Water is essentially the only
substance evaporated (other than volatile organic impurities) in concentrating the acid to <75% H2SO4.
Vapors evolved during the concentration of spent sulfuric acid to a more highly concentrated state
(93-98% H2SO4) contain significant quantities of gaseous sulfuric acid (1). The formation of this
gaseous sulfuric acid contributes to the manufacturing threshold of sulfuric acid aerosols for
reporting under section 313 of EPCRA. Spent sulfuric acid may be concentrated in either vacuum or drum
concentrators. While vacuum concentrators yield negligible emissions, those from drum concentrators
contain acid mist. Exit gas is passed through scrubbers before being vented to the atmosphere.
Emissions from acid drum concentrators operating at 55, 73, and 100% of capacity are reported to be
7034, 2401, and 2334 metric ton/day (12).
Acid Aerosol Emissions
Nearly all the sulfuric acid aerosols emitted from sulfuric acid manufacturing plants come
from the absorber exit gases. The exit gas contains small amounts of SO2, even smaller amounts of SO3,
and sulfuric acid vapor and mist. Even with efficient gas drying, mist formation is impossible to
eliminate completely. Once formed, these aerosols are of such a fine particle size and so stable that
only a small amount can be removed in the absorber. Sulfuric acid is normally combined with SO3 in
determining an emission factor because SO3 reacts so rapidly with water vapor. The emission factor for
SO3 is calculated as 100% H2SO4 and added to the H2SO4 value.
Sulfuric acid mists are always formed when sulfur trioxide combines with water vapor at
temperatures below the dew point of sulfur trioxide. The dew point is a function of gas composition
and pressure and is generally around 140-170°C. Equations are available that predict the dewpoint for
different concentrations of H2O and H2SO4 (4). Examples are given in Section 3.1.5 in Tables 6 and 8
for coal and fuel oil combustion.
Use of Sulfuric Acid Emission Monitoring Data
Some sulfuric acid manufacturing facilities may have sulfuric acid emission monitoring data
available that can be used to estimate emissions for sulfuric acid mist under the Clean Air Act New
Source Performance Standards (NSPS). Sulfuric acid plants constructed or modified after August 17,
1971, are subject to a sulfuric acid mist emissions limit of 0.15 pounds of sulfuric acid per ton of
100% sulfuric acid produced (see Part 60 Subpart H of Title 40 of the Code of Federal Regulations). If
such information is available, it is preferable to use such data for estimating uncontrolled emissions
of sulfuric acid, rather than published emission factors since monitoring data should be the best
available data. If the measured data available is for controlled emissions, then the amount of
sulfuric acid generated prior to emission controls should be calculated based on the average actual
12
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control efficiency for the acid mist.
Uncontrolled H2SO4 emissions = actual emissions to air/(l-efficiency)
(Efficiency expressed as a fraction)
Emission factors for sulfuric acid plants have been compiled by EPA (10). Uncontrolled
emission factors for various sulfuric acid plants are shown in Table 3. Table 4 contains emission
factors for plants using three of the most commonly used fiber mist eliminator control devices,
vertical tube, vertical panel, and horizontal duel pad types.
Table 3. Uncontrolled Emission Factors for Sulfuric Acid Plants
Raw material
Recovered sulfur
Bright virgin sulfur
Dark virgin sulfur
Spent acid
Oleum* produced
% Total output
0-43
0
0-100
0-77
Emissions of H2SO4 aerosol per unit product
kg/Mg
0.174-0.4
0.85
0.16-3.14
1.1-1.2
Ib/ton
0.348-0.8
1.7
0.32-6.28
2.2-2.4
'Sulfuric acid containing dissolved sulfur trioxide. Also known as fuming sulfuric acid or disulfuric acid.
Table 4. Controlled Emission Factors for Sulfuric Acid Plants
Raw material
Elemental sulfur
Dark virgin sulfur
Spent acid
Oleum* produced
% Total output
—
0-13
0-56
Emissions of H2SO4 aerosol per unit product
kg/Mg
0.064
0.26-1.8
0.014-0.20
Ib/ton
0.128
0.52-3.6
0.28-0.40
'Sulfuric acid containing dissolved sulfur trioxide. Also known as fuming sulfuric acid or disulfuric acid.
Section 3.1.3. Smelters
Sulfuric acid is a byproduct of metals production, notably copper, and is accordingly
sometimes referred to as smelter acid. Smelters produce sulfuric acid by the contact process with the
raw material being classified as 'metal sulfide ores and smelter gas burning' (see Section 3.1.2). The
smelter gas (SO2 from the smelter furnace) is passed through cyclone dust collectors, electrostatic
dust and mist precipitators, and scrubbing and gas cooling towers to remove dust, acid, mist and other
impurities. The gas is then converted to SO3 and then H2SO4 in processes similar to those used in
sulfuric acid plants using elemental sulfur as a raw material (Section 3.1.2). Therefore, the
potential for sulfuric acid aerosols formation is similar to the described above in the third
paragraph of Section 3.1.2.
13
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Section 3.1.4. Petroleum Refining
Crude oil contains a small amount of sulfur as an impurity. As a result, sulfur oxides are
emitted from petroleum refineries. EPA has compiled emission factors of sulfur oxides, SOX, for
petroleum refining, but factors for sulfuric acid are not provided (11). However, according to
information provided by the American Petroleum Institute (API), the staff in API's Health and
Environmental Sciences Department uses the following EPA reference for sulfuric acid emission
factors: EPA. 1995. Compilation of Air Pollutant Emission Factors. Vol. 1: Stationary Point and Area
Sources. Section 8.10, AP-42, 5th ed. (January 1995). Research Triangle Park, NC: U.S. EPA, OAQPS.
Section 3.1.5. Sulfuric Acid Aerosol Formation In Stacks From Combustion Processes
Sulfuric acid aerosols are often formed in flue gas in a stack during combustion of fuel oil,
coal, or other sulfur-containing fuels. Both water and sulfur trioxide are combustion products and
they have great affinity for each other; as discussed they react quickly to form sulfuric acid. When
flue gas is cooled to temperatures at or below the dew point, a sulfuric acid mist will form from any
sulfuric acid gas present (16). The dew point is the temperature at which the air becomes saturated
and produces dew; sulfuric acid mists are always formed when sulfur trioxide combines with water vapor
at temperatures below the dew point of sulfur trioxide. Because of the enormous attraction between
sulfur trioxide and water "only a very small amount of sulfur trioxide in combustion gas is required to
draw water from the gas and form a fairly concentrated acid" (16). In fact, flue gas containing 1%
sulfuric acid has the corrosive properties of 85% sulfuric acid solution. All sulfuric acid produced
within the stack, including the gas not just the mist, falls under the EPCRA section 313 definition of
a sulfuric acid aerosol. The information on dew points can be used to determine if any of the sulfuric
acid present as gas will form a mist that could potentially condense inside the stack.
Tables 5 and 7 below contain expected sulfur trioxide levels in flue gas resulting from the
combustion of fuel oil and coal, respectively, as a function of the sulfur content of the fuel and the
percent of excess air available (16). If water is present in the stack, Tables 5 and 7 can be used to
estimate the amount of sulfuric acid gas that can be formed. Tables 6 and 8 contain empirically-
derived dew points of SO3 for different concentrations of SO3 in stack gas of oil- and coal-fired units,
respectively. These tables can be used to determine whether the stack temperature is below the dew
point of SO3 and sulfuric acid mists are being formed in the stack in flue gas. Examples of how to use
these tables to determine the dew point of SO3 are given below.
Assume a typical oil-fired unit is burning fuel oil containing 2% sulfur and that 17% excess
air is present. From Table 5, we see that 15 ppm of SO3 will be present in the flue gas and available to
form sulfuric acid gas. From Table 6, we find that the dew point of SO3 should be 139°C. Therefore, if
the temperature in the stack is at or below 139°C, sulfuric acid mists will very likely form in the
stack.
14
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As an example dealing with coal combustion, assume a typical unit is burning coal containing 3%
sulfur in the presence of 25% excess air. From Table 7, we see that between 20 and 40 ppm of SO3 will be
present in the flue gas and available to form sulfuric acid gas. From Table 8, we find that the dew
point should be between 136°C and 143 °C . Therefore, if the temperature in the stack is at or below
136°C to 143 °C, sulfuric acid mists will very likely form in the stack.
Table 5. SO3 Production in Oil Fired Units
Excess air (%)
Sulfur in fuel (%)
5
11
17
25
SO3 Concentration in Flue Gas (ppm)
0.5
2
6
10
12
1.0
o
J
1
13
15
2.0
3
8
15
18
3.0
4
10
19
22
4.0
5
12
22
26
5.0
6
14
25
30
Table 6. Dew Point of SO, in Stacks of Oil Fired Units*
SO3 in gas (ppm)
Dew point (°C)
5
130
10
135
15
139
20
141
25
143
30
145
35
147
40
148
45
149
50
150
55
151
60
152
65
153
70
154
'Using typical value of 10% water in oil
Table 7. SO3 Production in Coal Fired Units
Excess air (%)
Sulfur in fuel (%)
25
SO3 Concentration in Flue Gas (ppm)
0.5
3-7
1.0
1 - 14
2.0
14-28
3.0
20-40
4.0
27-54
5.0
33-66
Table 8. Dew Point of SO, in Stacks of Coal Fired Units
SO3 in gas (ppm)
Dew point (°C)
5
125
10
130
15
134
20
136
25
138
30
140
35
142
40
143
45
144
50
145
55
146
60
147
65
148
70
149
'Using typical value of 6% water in coal
Steel stacks are generally designed and operated so that a temperature between 135°C (275°F) and 149°C
(300°F) is maintained throughout the stack (16). These stack temperatures are such that they may be
below the dew point for SO3 in the flue gas, leading to the formation of sulfuric acid mists in the
stacks.
Section 3.1.6. Coal Combustion
15
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Sulfuric acid aerosols are produced as a byproduct from boilers during coal combustion. U.S.
coals contain from 0.2% to 7% sulfur by weight (13). On average, about 95% of sulfur present in
bituminous coal will be emitted as gaseous sulfur oxides (SOX) when burned, whereas somewhat less will
be emitted when subbituminous coal is burned (15). In general, boiler size, firing configuration, and
boiler operations have little effect on the percent conversion of sulfur in fuel to sulfur oxides.
About 0.7% of fuel sulfur is emitted as SO3/H2SO4 (15). This information can be expressed as an
uncontrolled emission factor (EF) of 0.43 x S pounds H2SO4 per ton of coal burned, where S is the weight
percent sulfur in coal. The uncontrolled emission factor also represents the amount of sulfuric acid
produced in the stack, as well as that released to the atmosphere in the absence of scrubbers or other
emission control devices. The emission factor for sulfur oxides (SOX) for bituminous coal combustion
should not be used to estimate sulfuric acid emissions since the factor includes sulfur dioxide. If C
is the tons of coal burned, the pounds of H2SO4 generated (H), would be:
H = 0.43 x S x C
For example, if 9,000 tons of coal were burned and the coal contained 3% sulfur, then:
H = 0.43 x 3 x 9,000 =11,610 pounds of H2SO4
Note that the values for the variables C and S have been chosen as an illustration. Values must be
chosen that are appropriate for the particular operations at each facility.
Section 3.1.7. Fuel Oil Combustion
Sulfuric acid aerosols are produced during fuel oil combustion from the oxidation of sulfur
contained in the fuel. There are various types of fuel oil combustion operations; the type of
operation depends on the type of fuel oil burned. There are mainly five types of fuel oil used for
commercial, industrial, and residential use in the U.S. The No. 1 and No. 2 fuel oils are known as
distillate oils. They have high volatility, low viscosity, and <0.3% sulfur by weight. They are
primarily used in domestic and small commercial operations. The No. 5 (also called low sulfur No. 6)
and No. 6 fuel oils are known as residual oils. They have low volatility, high viscosity, and high
sulfur content. They are mainly used in industrial operations. The No. 4 fuel oil is a mixture of
distillate and residual oils and can be used for both types of operations. Typical sulfur contents of
fuel oil are (13):
Fuel Oil Grade Sulfur Content (wt %)
No. 1 0.09
No. 2 0.22
No. 4 1.35
No. 5 0.84
No. 6 3.97
16
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Uncontrolled SOX emissions are almost entirely dependent on the sulfur content of the fuel and
are not affected by boiler size and design or the grade of fuel being burned. On the average, over 95%
of the sulfur in fuel oil is converted to SO2 on combustion; about 1 to 5 percent is further oxidized to
sulfur trioxide where it readily reacts with water vapor in flue gas to form sulfuric acid aerosols.
The emission factor (EF) for uncontrolled fuel oil combustion from industrial boilers is 0.002 x S
pounds SO3 per gallon of fuel oil burned (14) or 0.00245 x S pounds H2SO4 per gallon of fuel oil burned,
where S is the weight percent sulfur in the fuel oil. The uncontrolled emission factor also
represents the amount of sulfuric acid produced in the stack, as well as that released to the
atmosphere in the absence of scrubbers or other emission control devices. An example of the use of the
emission factor (EF) to calculate the pounds of sulfuric acid generated is shown below. If F is the
number of gallons of fuel oil burned, the pounds of H2SO4 generated (H), would be:
H = 0.00245 x S x F
For example, if 4,500,000 gallons of fuel oil were burned and the fuel oil contained 3.97% sulfur,
then:
H = 0.00245 x 3.97 x 4,500,000 = 43,769 pounds of H2SO4
Note that the values for the variables F and S have been chosen as an illustration. Values must be
chosen that are appropriate for the particular operations at each facility.
Section. 4.0. Measurement Methods
For source sampling, EPA has specified extractive sampling trains and analytical procedures
for SO3 and sulfuric acid aerosols (7, 8). Separation of particles containing Na2SO4 may present
problems in cases such as Kraft paper mills (4). If sodium sulfate is present, analytical results for
sulfuric acid would be high.
References
(1) Muller H. 1994. Sulfuric Acid and Sulfur Trioxide. Ullmonris Encyclopedia of Industrial
Chemistry, Vol A25, pp. 635-702.
(2) Donovan JR, Salamone JM. 1983. Sulfuric Acid and Sulfur Trioxide. Kirk Othmer Encyclopedia of
Chemical Technology, 3rd ed. Vol 22, pp. 190-232.
(3) Calvert JG. 1984. SO2, NO andNO2 Oxidation Mechanisms: Atmospheric Considerations. Butterswortl
Publishers, pp. 1-62.
(4) National Council for Air and Stream Improvement (NCASI). 1980. A Study of SOX measurement
procedures and their use at Kraft recovery furnaces. Atmospheric Quality Technical Bulletin No. 106,
National Council for Air and Stream Improvement, New York, NY. April 17, 1980.
17
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(5) EPA. 1995. Sulfuric acid: Toxic chemical release reporting: Community right-to-know. Final
rule. 60 FR 34182. June 30, 1995.
(6) Wolff GT. 1991. Air pollution. Kirk Othmer Encyclopedia of Chemical Technology, 4th ed. Vol Al,
pp. 725.
(7) Crocker BB. 1991. Air pollution control methods. Kirk Othmer Encyclopedia of Chemical Technology,
4th ed. Vol 1, pp. 749-825.
(8) EPA. 1975. Part 60 - Standards of performance for new stationary sources. Emission mon-
itoring requirements and revisions to performance testing methods. 40 FR 46250 October 1975.
(9) EPA. 1990. Compilation of Air Pollutant Emission Factors (AP-42). Wood Products Industry.
Chemical Wood Pulping. 10.2-1 to 10.2-20. September, 1990. Research Triangle Park, NC: U.S. EPA,
OAQPS.
(10) EPA. 1993. Compilation of Air Pollutant Emission Factors (AP-42). Inorganic Chemical Industry.
Sulfuric acid. 8.10-1 to 8.10-10. July, 1993. Research Triangle Park, NC: U.S. EPA, OAQPS.
(11) EPA. 1993. Compilation of Air Pollutant Emission Factors (AP-42). Petroleum industry.
Petroleum refining. 5.1-1 to 5.1-16. January, 1995. Research Triangle Park, NC: U.S. EPA, OAQPS.
(12) Gerstle RW, Katari VS. 1977. Industrial Process Profiles for Environmental Use: Chapter 23.
Sulfur, Sulfur Oxides and Sulfuric Acid. Prepared for Industrial Environmental Research Laboratory,
Cincinnati, OH, Austin, TX: Radian Corporation. [NTIS PB-281 490]
(13) Perry RH, Green DW. 1984. Perry's Chemical Engineers'Handbook, 6th Edition, pp. 3-68, 9-38 to
9-50. New York, NY: McGraw-Hill Book Company.
(14) EPA. 1995. Compilation of Air Pollutant Emission Factors (AP-42). Fuel Oil Combustion (AP-42).
pp. 1.3-1 to 1.3-34. January 1995. Research Triangle Park, NC: U.S. EPA, OAQPS.
(15) EPA. 1995. Compilation of Air Pollutant Emission Factors (AP-42). Bituminous and Subbituminous
Coal (AP-42). pp. 1.1-1 to 1.1-40. January 1995. Research Triangle Park, NC: U.S. EPA, OAQPS.
(16) Pierce R. 1977. Estimating acid dew points in stack gases. Chemical Engineering, April 11, 1997,
vol. 89, pp. 125-128.
(17) Eatough DJ, Caka FM, Farber RJ. 1994. The conversion of SO2 to sulfate in the atmosphere.
Israel Journal of Chemistry 34: 301-314.
(18) EPA. 1982. Air Quality Criteria for Particulate Matter and Sulfur Oxides, Volume II, EPA-600/8-
82-029b. Research Triangle Park, NC: U.S. EPA, Environmental Criteria and Assessment Office, pp. 2-1
18
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to 2-100.
(19) EPA. 1988. Acid Aerosols Issue Paper. EPA-600/8-88-005a. Washington, D.C.: Office of Heath and
Environmental Assessment, pp. 2-1 to 2-74.
(20) National Council for Air and Stream Improvement (NCASI). 1995. Compilation of 'Air Toxic' and
total hydrocarbon emission data for sources at chemical wood pulp mills. Volume 2. Technical Bulletin
No. 701, National Council for Air and Stream Improvement, Research Triangle Park, NC. October 1995.
(21) National Council for Air and Stream Improvement (NCASI). 1996. Proceedings of the 1995 NCASI
Southern Regional Meeting. Volume 2. Special Report No. 96-01, National Council for Air and Stream
Improvement, Research Triangle Park, NC. October 1995. P. 326.
(22) Muller TL. 1992. Air Pollution Engineering Manual. Buonicore, AJ and Davis WT, eds, Van Nostrand
Reinhold, New York., pp.469-476.
19
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APPENDIX 1
300 -
200
100
20 40 60
Concentration. wt°/o—
80
100
Figure la. Boiling curves for sulfuric acid at 1013 mbar a)
Vapor; b) Liquid (reference 1).
Information in the above figure shows that if the vapor above a solution of 85% sulfuric acid,
boiling at 223 °C, were to be completely condensed it would contain approximately 7 percent H2SO4. At
concentrations below approximately 75% H2SO4, the vapor that evaporates from the solution is
essentially water.
Table la. Sulfuric Acid Partial Pressure and Total Vapor Pressure (bar) over Aqueous Sulfuric Acid*
°F
32
68
104
140
176
212
302
392
482
572
Weight Percent Suliuric Acid
20.0
.843E-20
(.534E-02)
.769E-18
(.205E-01)
.389E-16
(.649E-01)
.121E-14
(.175)
.254E-13
(.417)
.381E-12
(.891)
.106E-09
(4.132)
.883E-08
(13.107)
.312E-06
(31.939)
.591E-05
(64.407)
40.0
.344E-17
(.326E-02)
.193E-15
(.130E-01)
.649E-14
(.427E-01)
.144E-12
(.119)
.225E-11
(.290)
.264E-10
(.634)
.460E-08
(3.090)
.278E-06
(10.245)
.793E-05
(26.056)
.130E-03
(54.869)
60.0
.438E-14
(.836E-03)
.149E-12
(.367E-02)
.317E-11
(.131E-01)
.462E-10
(.395E-01)
.492E-09
(.104)
.402E-08
(.244)
.316E-06
(1.392)
.975E-05
(5.312)
.156E-03
(15.351)
.156E-02
(36.361)
80.0
.161E-10
(.197E-04)
.305E-09
(.115E-03)
.379E-08
(.531E-03)
.334E-07
(.204E-02)
.222E-06
(.668E-02)
.117E-05
(.192E-01)
.343E-04
(.170)
.457E-03
(.913)
.358E-02
(3.439)
.266E-01
(9.916)
98.0
.187E-08
(.117E-07)
.224E-07
(.121E-06)
.191E-06
(.914E-06)
.122E-05
(.538E-05)
.622E-05
(.257E-04)
.261E-04
(.103E-03)
.493E-03
(.180E-02)
.470E-02
(.166E-01)
.278E-01
(.985E-01)
.117E-00
(.425)
100.0
.228E-08
(.323E-08)
.273E-07
(.435E-07)
.230E-06
(.425E-06)
.147E-05
(.319E-05)
.743E-05
(.193E-04)
.310E-04
(.966E-04)
.574E-03
(.287E-02)
.538E-02
(.427E-01)
.314E-01
(.389)
.130E-00
(2.476)
! Total pressure is in parentheses. Conversion Factors: 1 bar = 0.98677 atmospheres = 14.7 psia = 760 mm Hg = 0.1 MPa
20
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The above table contains the partial pressure of sulfuric acid and total vapor pressure of the
solution (in parentheses) over aqueous sulfuric acid solutions in the concentration range of 20 to 100
weight percent sulfuric acid (1). From Table la we see that the partial pressure of sulfuric acid
above a sulfuric acid solution is very low compared to the total vapor pressure for sulfuric acid
solutions below 80% sulfuric acid; the bulk of the vapor being composed of water. Consequently when a
solution of sulfuric acid boils, more water than sulfuric acid is volatilized, so that the
concentration of the remaining acid increases and the boiling point of the solution rises. This
process continues until the acid concentration reaches 98.3 weight % H2SO4, when an azeotrope (a
mixture of two liquids that boils at constant composition; i.e., the composition of the vapor is the
same as that of the liquid) is formed and the concentration of sulfuric acid in the vapor is the same as
that of the solution. The vapor above sulfuric acid solutions containing more than 98.3 weight % H2SO4
also includes considerable amounts of SO3, thus the difference between the partial pressure and total
pressure of 100% sulfuric acid is the partial pressure of sulfur trioxide. Since the partial pressure
of concentrated sulfuric acid is very low, little sulfuric acid is expected to volatilize from
sulfuric acid solutions such as may be present in storage tanks. No experimental data are available on
sulfuric acid aerosol emissions from storage tanks.
Above 340 °C, H2SO4 decomposes into sulfur tri oxide and water. The vapor-phase reaction of
sulfur trioxide and water results in aerosols of sulfuric acid. The H2SO4/H2O/SO3 system is important
in the production of sulfuric acid by the contact process as well as in the prevention of corrosion
from condensing sulfuric acid in stack emissions from the combustion of sulfur-containing fuels.
Sulfuric acid containing dissolved sulfur trioxide is known as oleum, fuming sulfuric acid or
disulfuric acid. The vapor pressure of sulfuric acid over oleum containing 10% to 30% of free SO3 by
weight is shown in Table 2a (2). Since the vapor pressure of sulfur trioxide over oleum is high,
sulfuric acid aerosols also form when oleum is exposed to air containing moisture.
Table 2a. Sulfuric Acid Partial Pressure (bar) over Oleum"
°c
20
40
60
80
100
Free SO3 in oleum, %
10
.227E-08
.1467E-07
.7333 E-07
.3066 E-06
.1067E-05
20
.120E-08
.667E-08
.400 E-07
.1600 E-06
.5333 E-06
30
.40 E-09
.267E-08
.1333 E-07
.600 E-07
.2133 E-06
Conversion Factors: 1 bar = 0.98677 atmospheres = 14.7 psia = 760 mm Hg = 0.1 Mpa;
°F= 1.8(°C) + 32.
21
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