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Sulfur dioxide is one of the more important chemical compounds formed from the combination of sulfur and oxygen. The chemical formula for sulfur dioxide is “SO2”, and as the formula indicates, two atoms of oxygen are chemically combined with one atom of sulfur.
Although in many applications, sulfur dioxide is a bleach, reducing agent, solvent, or a raw material, it is also listed as one of the primary chemical air pollutions in the United States today.
In the areas of water and waste treatment, sulfur dioxide is
one of the most popular compounds used for dechlorination of water and reduction
of chromates in wastewater (see Table III for reactions).
Sulfur dioxide is commercially produced by heating of sulfur, sulfur-bearing ores, or by the recovering of stack gases to meet clean air requirements.
Sulfur dioxide is stored and transported in tank cars 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.
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 at atmospheric pressure boils at 14°F (-10°C) and freezer at –103.9°F (-75.5°C).
Liquid sulfur dioxide is heavier than water, having a specific gravity of 1.436 at 32°F (0°C). As a vapor, sulfur dioxide is heavier than air, with a relative density of 2.2636 when compared to air at atmospheric pressure and a temperature of 32°F.
When heated about its critical temperature, 314.82°F (157.12°C), sulfur dioxide can only exist as a vapor regardless of pressure.
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, which, when plotted, yields a smooth curve as shown in Figure 1.
Sulfur dioxide is somewhat soluble in water (18.59% by weight at 32°F/0°C) and forms a weak solution of sulfurous acid (H2SO3). The degree of solubility is directly dependent on temperature (see Figure 2).
Generally, undiluted (dry) sulfur dioxide is not corrosive to ordinary metals; however, when small amounts of moisture are present, sulfur dioxide will attack most metals.
Although sulfur dioxide is normally shipped and stored in liquid form, many applications require sulfur dioxide to be supplied as a vapor. Due to the inherently low vapor pressure of sulfur dioxide, vaporization of the liquid requires heat, which must be supplied to the cylinders from an external source. Electric strip heaters or steam coils equipped with thermostatic control are generally used for this purpose.
Because fusible safety devices in the cylinders melt at 165°F (74°C), great care must be taken not to allow cylinders to reach exceedingly (125°F (51.7°C).
The withdrawal weight of sulfur dioxide vapor from 150 lb. (68 kg) and 1 ton (907 kg) cylinders is less than that of chlorine, 40-50 PPD (0.76-0.94 kg/hr) from a 150 lb. (68 kg) cylinder and 400-450 PPD (7.6-9.4 kg/hr) from a ton cylinder at 70°F (21.1°C). To maintain relatively high withdrawal rates without excessive frosting of cylinders or reliquefaction in the supply header, the ambient temperature in the vicinity of the cylinders should be set at 80-85°F (26.7-29.4°C).
When the application requires large amounts of sulfur dioxide, vaporizer systems are usually specified. Generally, liquid sulfur dioxide and liquid chlorine are handled with identical vaporizer systems. Therefore, the same precautions apply for both, such as filters and drip legs in the supply and header systems.
Due to the fact that the latent heat of vaporization for sulfur dioxide is 150 Btu/lb. (83.3 g-cal/g) at 70°F (21.1°C) when compared to 123 Btu/lb. (68.3 g-cal/g) for chlorine; as would be expected, a standard vaporizer has slightly less capacity (approximately 80%) when used for sulfur dioxide.
In liquid systems, the sulfur dioxide is withdrawn from the
bottom of the storage tank, either by a connection located at the bottom of the
tank or by a connection at the top of the tank joined to a dip tube located
within the tank.
In certain instances, it may be necessary to apply heat to the tanks to facilitate the flow of sulfur dioxide. Again, precautions should be taken not to allow containers to reach temperatures exceeding 125°F (51.7°C).
A major concern, which is inherent to liquid systems, is the possibility of excessive pressure developing between closed valves. Sulfur dioxide’s large coefficient of expansion with temperature can develop pressures capable of causing piping to rupture. To remedy potential problems caused by excessive pressure, a liquid expansion system should always be provided.
MATERIALS OF CONSTRUCTION
Schedule 80 Steel pipe with socket welded fittings is normally used to transport undiluted (dry) sulfur dioxide. If the piping is to conduct diluted sulfur dioxide (containing over 1,000 PPM water), 316 stainless steel or Alloy 20 would be appropriate. Zinc-coated or galvanized pipe should never be used for service with sulfur dioxide.
Flanges can be slip-on and flat-face, and should be fitted carefully to prevent leaks. Flanges should allow for easy disassembly of pipe and provisions should also be made to protect pipe from the effects of expansion, contraction, jarring, vibration, and settling. After assembly, piping should be inspected for leaks or other unusual conditions. The majority of applications using sulfur dioxide will require flexible piping, tubing, or hose in the system. Pipes can be made flexible by providing a series of reverse bends in the line. Heavy-duty copper tubing may also be used to provide flexibility when transporting dry sulfur dioxide. Flexible metal hose for service with corrosive acids under pressure makes the best all purpose flexible line for carrying dry sulfur dioxide.
Normally, 316 stainless steel is satisfactory for valves, gauges, and pressure regulators in service with sulfur dioxide. Alloy 20 is better for ‘wet’ sulfur dioxide (see Table III).
Metallic and non-metallic gasket and packing material such as Teflon, graphite asbestos, and lead perform well in sulfur dioxide. If a packed valve is used for wet sulfur dioxide, Teflon packing is recommended.
STORAGE AND SHIPMENT
In the United States, sulfur dioxide is shipped in specially designed steel railroad tank cars and tank trucks that conform to DOT guidelines. Single unit railroad tank cars and generally range 15-55 ton capacity. The capacity for large tank trucks ranges between 15 and 20 tons, while capacity of smaller tank trucks may very depending on particular needs. Sulfur dioxide is also available in 150 lb. cylinders, 2000 lb. steel drums, and 1lb. containers for laboratory use.
Sulfur dioxide bulk storage tanks are normally constructed of carbon steel with provision made to prohibit process gases and solutions from entering the tank. Again, the size of the storage tanks will depend on particular requirements.
Sulfur dioxide storage tanks are 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 the use of an ammonia swab. When the ammonic comes in contact with the sulfur dioxide vapor, dense white fumes of ammonium sulfate form near the leak.
If a leak does occur, only authorized personnel should attempt to stop the leak. If there is any question as to the size of the leak, a suitable gas mask should be worn.
Normally, leaks that do develop are not serious and can be readily controlled. Where leaks do occur, the supply of sulfur dioxide should be shut off immediately by closing appropriate valves.
If the leak is large and continuous, all personnel in the immediate area should be removed and qualified help should be summoned. If possible, the leaking container should be moved to an open area where the danger of escaping sulfur dioxide is minimized.
While, at low concentrations, sulfur dioxide vapor is extremely irritating to the eyes and mucousal membranes of the upper respiratory tract, it is easily detectable at 3 to 5 PPM in the air. Exposure to high concentration produces a suffocating effect due to the closing of the glottis to shut out the gas.
For the physiological response to various concentration of sulfur dioxide see Table IV.
Exposure to escaping sulfur dioxide liquid will result in freezing action of the skin. This freezing action is a natural result of the escape of a liquefied refrigerant under pressure.
Persons having chronic lung diseases, heart disease or persons having shown evidence of hypersensitivity to sulfur dioxide should not be employed in areas where sulfur dioxide is being used. The odor makes it impossible for a person to voluntarily remain in a seriously contaminated area for a dangerously 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.
Persons responsible for first aid services should be familiar with special procedures required in cases of sulfur dioxide exposure.