Sulfur Dioxides And Its Properties General Sulfur Dioxide is one of the most important basic chemicals. The chemical formula for sulfur dioxide is SO 2. 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 classifi ed by the Department of Transportation (DOT) as a non-fl ammable compressed gas that must be stored or shipped in DOT specifi cation 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. - 1-010.3005.6
Figure 1 - Vapor Pressure of Liquid Sulfur Dioxide Figure 2 - Solubility of Sulfur Dioxide in Water 010.3005.6-2 -
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 (H 2 SO 3 ). 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. Molecular symbol SO 2 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) Vapor density at 32 F (0 C) and 1 ATM Liquid density at 32 F (0 C) Table I - General Properties Specifi c volume of vapor at 32 F (0 C) and 1 ATM 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 ) 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: 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: SO 2 + Cl 2 + 2H 2 O 2HCl + H2SO 4 * 3SO 2 + 2H 2 CrO 4 Cr 2 (SO 4 ) + 2H 2 O* *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 Cl 2 0.9 H 2 CrO 4 2.0 Pounds of SO 2 required to treat one pound of reacting species Material Wet SO 2 Dry SO 2 Steel, Mild Stainless Steel Type, 316 Stainless Steel Type, Alloy 20 Brass Copper Lead Aluminum Tefl on Schedule 80 PVC (vacuum only) 010.3005.6-3 -
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 coeffi cient 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 fi ttings, 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 fl exible 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 clarifi cation 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 tefl on, graphite asbestos, and lead perform well in sulfur dioxide. Tefl on 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 certifi ed 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. 010.3005.6-4 -
All other personnel in the immediate area should be alerted and qualifi ed 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 Physical Response Parts Per Million of Sulfur Dioxide in Air 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 fi rst aid services should be familiar with special procedures required for sulfur dioxide exposure. Figure 3 - Sulfur Dioxide Liquid Expansion - 5-010.3005.6
Safety and First Aid Equipment The following safety and fi rst 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: info.dnwt@denora.com SEP 2015 010.3005.6-6 - Registered Trademark. 2015. All Rights Reserved.