Sulfur Dioxides
And Its Properties
General
Sulfur Dioxide is one of the most important basic chemicals. The chemical formula for sulfur dioxide is SO2.
Sulfur dioxide is used as a bleaching agent, reducing agent, solvent or a chemical building block.
In the areas of water and waste treatment, sulfur dioxide is one of the most popular compounds used for
dechlorination of water and wastewater and reduction of chromate in wastewater (Table III).
Sulfur dioxide is commercially produced by the heating of sulfur, sulfur bearing ores, or by the recovery of stack
gases to meet clean air requirements.
Sulfur dioxide is stored and transported in tank cars, ton containers, and cylinders as a liquid under pressure, and
is classified by the Department of Transportation (DOT) as a non-flammable compressed gas that must be stored or
shipped in DOT specification containers.
Definitions of Sulfur Dioxide
Sulfur Dioxide - The chemical element in whatever state or condition it may exist under the conditions being
considered
Liquid Sulfur Dioxide - The element, sulfur dioxide, in the liquid state
Sulfur Dioxide Gas - The element, sulfur dioxide, in the gaseous state
Dry Sulfur Dioxide - Liquid or gaseous sulfur dioxide that contains less than (approximately) 100 ppm of water
Wet Sulfur Dioxide - Liquid or gaseous sulfur dioxide that contains more than (approximately) 100 ppm of water
Moist Sulfur Dioxide - Synonymous with wet sulfur dioxide
Saturated Sulfur Dioxide Gas - Sulfur dioxide gas in such condition that the removal of any heat or an increase in
pressure will cause some portion of it to condense to a liquid. This term should not be confused with wet or moist
sulfur dioxide.
Saturated Sulfur Dioxide Liquid - Sulfur dioxide liquid in such condition that the addition of any heat or a decrease
in pressure will cause some portion of the sulfur dioxide to vaporize to a gas. This term should not be confused with
wet or moist sulfur dioxide.
Sulfur Dioxide Solution (Sulfur Dioxide Water) - A solution of sulfur dioxide in water, see Figure 2 for solubility of
sulfur dioxide in water.
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Figure 1 - Vapor Pressure of Liquid Sulfur Dioxide
Figure 2 - Solubility of Sulfur Dioxide in Water
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Properties
At atmospheric temperatures and pressure, sulfur dioxide is a colorless vapor with a characteristic, pungent odor.
When compressed and cooled, sulfur dioxide forms a colorless liquid which boils at 14°F (-10°C) and freezes at
-103.9°F (-75.5°C) (1 atmosphere).
Liquid sulfur dioxide is heavier than water and gaseous sulfur dioxide is heavier than air.
Liquid sulfur dioxide exists in equilibrium with its vapor when stored in a closed container. The vapor pressure within
the container is directly proportional to the temperature. (Figure 1)
Sulfur dioxide is somewhat soluble in water and forms a weak solution of sulfurous acid (H2SO3). The degree of
solubility is directly dependent upon temperature (Figure 2).
Generally, dry sulfur dioxide is not corrosive to ordinary metals; however, when small amounts of moisture are
present, it will attack many metals.
Table I - General Properties
Molecular symbol
SO2
Molecular weight
64.06
Boiling point (1ATM)
14°F (-10°C)
Freezing point (1 ATM)
-103.9°F (-75.5°C)
Critical temperature
314.82°F (157.12°C)
Latent heat of vaporization at 32°F (0°C)
161.77 Btu/lb.
(89.87 g-cal/g)
0.1827 lbs/ft 3
(2.927 kg/m 3)
89.58 lbs/ft 3
(1435 kg/m 3)
Vapor density at 32°F (0°C) and 1 ATM
Liquid density at 32°F (0°C)
Specific volume of vapor at 32°F (0°C) and 1 ATM
5.47 ft 3/lb
(0.34 m 3/kg)
* 1 Atmosphere (Atm) = 760 mm of mercury = 1.01325 bar = 14.74 lbs/sq. in
Table II Suitable Handling Materials
Materials proven suitable for service with sulfur dioxide are:
Material
Wet SO2
Dry SO2
Steel, Mild
X
Stainless Steel Type, 316
X
X
Stainless Steel Type, Alloy 20
X
X
Brass
X
Copper
X
Lead
X
X
Aluminum
X
Teflon
X
Schedule 80 PVC (vacuum only)
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X
X
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Table III - Chemical Reactions
Sulfur dioxide can function as either
an oxidizing agent or as a reducing
agent. However, sulfur dioxide is
used most frequently as a reducing
agent. Some examples of the
reduction reactions are:
SO2 + Cl2 + 2H2O → 2HCl + H2SO4*
3SO2 + 2H2CrO4 → Cr2(SO4) + 2H2O*
*The quantity of sulfur dioxide required for
complete reduction may vary depending upon
the environment in which the reaction is taking
place. The standard amount of sulfur dioxide
required for each reaction is as follows:
Reacting
Species
Pounds of SO2 required to treat one
pound of reacting
species
Cl2
H2CrO4
0.9
2.0
Vapor Withdrawal
Although sulfur dioxide is normally shipped and stored in liquid form, many applications require sulfur dioxide to be
supplied as a vapor. Vaporization of the liquid may require external heat, which must be supplied to the cylinders
from an external source. For some situations at low feed rates, electric strip heaters for piping or container blanket
type heaters equipped with thermostatic control (125°F [51.7°C] maximum) may be used.
Because fusible safety devices in the cylinders melt at 165°F (74°C), great care must be taken to prevent cylinders
reaching temperatures exceeding 125°F (51.7°C).
The withdrawal rate at 70°F (21.1°C) of sulfur dioxide vapor is 50 PPD (1 kg/h) from a 150 lb. (68 kg) cylinder and
300 PPD (5.7 kg/h) from a ton container . To maintain relatively high withdrawal rates without excessive frosting of
cylinders or liquefaction in the piping, the ambient temperature around the containers should be maintained at
80-85°F (27-29°C). Manifolding of containers may be required for continuous feed.
Liquid Withdrawal
When an application requires large amounts of gaseous sulfur dioxide, vaporizer systems are used. Liquid sulfur
dioxide systems are handled similarly to liquid chlorine systems. Therefore, the same design requirements apply.
Since the latent heat of vaporization for sulfur dioxide is 150 Btu/lb (83.3 g-cal/g) at 70°F (21°C) compared to 123
Btu/lb (68.3 g-cal/g) for chlorine, a chlorine vaporizer has slightly less capacity (approximately 80%) when used for
sulfur dioxide.
In liquid systems, sulfur dioxide is withdrawn from the bottom of the chemical container, either by a connection
located at the bottom of the chemical container or by a connection at the top of the container with a dip tube.
A major concern in liquid systems is the possibility of excessive pressure developing between closed valves. Sulfur
dioxide has a high coefficient of expansion and a liquid expansion system must be provided to protect piping
systems that can trap liquid between closed valves.
Materials of Construction
Schedule 80 Steel threaded piping and fittings, 3/4" or 1" may be used for gaseous or liquid sulfur dioxide. Zinccoated or galvanized pipe must never be used for service with sulfur dioxide.
If the use of sulfur dioxide requires flexible piping, tubing or hose in the system, heavy-duty copper tubing may be
used for dry sulfur dioxide.
Normally, 316 stainless steel is satisfactory for valves, gauges, and pressure regulators in service with sulfur dioxide.
Alloy 20 stainless steel and schedule 80 PVC (see Table II) are better for wet sulfur dioxide with the clarification that
schedule 80 PVC is suitable for vacuum service only with temperature limitations and has major cost of material
advantages over Alloy 20 stainless steel for this application.
Schedule 80 PVC is also suitable for sulfur dioxide solution pipe with temperature and pressure limitations.
Metallic and non-metallic gasket and packing material such as teflon, graphite asbestos, and lead perform well in
sulfur dioxide. Teflon packing is recommended for wet sulfur dioxide.
Storage and Shipment
Sulfur dioxide is shipped in specially designed steel railroad tank cars and tank trucks that conform to DOT
guidelines. Single unit railroad tank cars are generally 15 to 55 tons capacity. The capacity for large tank trucks
ranges between 15 and 20 tons, while the capacity of smaller tank trucks may vary depending upon particular
needs.
Sulfur dioxide is commercially available in 150 lb cylinders, 2000 lb. containers.
Sulfur dioxide bulk storage tanks are normally constructed of carbon steel and should be designed and fabricated in
accordance with the American Society of Mechanical Engineers code for certified pressure vessels.
Sulfur Dioxide Leaks
When a sulfur dioxide leak occurs, it is easily detected by the sharp, pungent odor of the vapor. The location of the
leak may be determined by means of ammonia vapor dispensed from a squeeze bottle, or by use of an ammonia
swab. When the ammonia comes into contact with the sulfur dioxide vapor, dense white fumes of ammonium sulfate
form at the leak. The use of a continuous sulfur dioxide gas detector will sense gas vapor and provide an alarm.
If a leak does occur, only authorized personnel should attempt to stop the leak. A suitable self-contained gas mask
should be worn.
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All other personnel in the immediate area should be alerted and qualified help summoned. If possible, the leaking
container should be moved to an open area where the danger of escaping sulfur dioxide is minimized.
Chlorine Institute Emergency Kits, adapted for sulfur dioxide use, can be used to stop leaks. Contact with the
supplier, local hazardous material personnel or the National Chemtrec emergency number is recommended (See
Safety and First Aid Equipment section).
While at low concentrations, sulfur dioxide vapor is extremely irritating to the eyes and mucous membranes of the
upper respiratory tract, it is easily detectable at 3 to 5 parts per million in the air. Exposure to high concentrations
(150 ppm) can produce a suffocating effect.
The following table shows the physical response to various concentrations of sulfur dioxide.
Table IV - Physical Response to Various Concentrations of Sulfur Dioxide
Parts Per Million of Sulfur
Dioxide in Air
Physical Response
Least detectable odor
3 to 5
Least amount causing immediate irritation to the eyes
20
Least amount causing immediate irritation to the throat
8 to 12
Least amount causing coughing
20
Maximum TWA 1 concentration allowable for 8 hours without a response
5
Maximum concentration allowable for short periods*
100
Dangerous for short exposure
400 to 500
Time Weighted Average (TWA)
Exposure to sulfur dioxide liquid must be avoided and suitable rubber suits, gloves and goggles must be available.
Persons having chronic lung diseases, heart disease or persons who have shown evidence of hypersensitivity to
sulfur dioxide should not be near areas where sulfur dioxide is being used. Sulfur dioxide odor makes it impossible for
a person to voluntarily remain in a contaminated area for a long period of time.
Any person who has been burned or overcome by sulfur dioxide vapors, should be placed under a physician’s care
immediately.
Personnel responsible for first aid services should be familiar with special procedures required for sulfur dioxide
exposure.
Figure 3 - Sulfur Dioxide Liquid Expansion
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Safety and First Aid Equipment
The following safety and first aid equipment should be provided where sulfur dioxide is used and stored:
A. Easily accessible shower and eye-wash and/or 50 gallons or more of clean water in a open-top container.
B. A self-contained air breathing apparatus.
C. Tight-fitting safety goggles and/or full face shield.
D. Protective gloves made of rubber or other material impervious to sulfur dioxide.
E. Slicker and/or pants and jacket made of rubber or other material impervious to sulfur dioxide.
F. Boots made of rubber or other material impervious to sulfur dioxide.
In case of accident or other incident beyond control, call:
Local supplier, or CHEMTREC EMERGENCY 24-HOUR NUMBER 800-424-9300
NOTE: Additional information is covered in more detail in Pamphlet G-3 “Sulfur Dioxide” published by the Compressed
Gas Association.
For a current MSDS, contact your supplier.
Design improvements may be made without notice.
Represented by:
De Nora Water Technologies
3000 Advance Lane Colmar, PA 18915
ph +1 215 997 4000 • fax +1 215 997 4062
web: www.denora.com
mail: [email protected]
®Registered Trademark. © 2015. All Rights Reserved.
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