The petroleum industry may be grouped into three divisions for convenience of the study of waste water and its control methods.
1. Production
2. Refining,
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3. Transportation and marketing.
Production includes all probable operations, from drilling oil fields to transporting the crude to refineries. During the refining operation, the crude is converted to saleable products and marketing involves the distribution and sale of the finished petroleum products. All these activities are responsible to some extent for emission of undesirable materials into the atmosphere.
Pollutants from Refining Processes:
Pollutants from the petroleum refinery are the by-products of various refining processes. These different pollutants emitted from different types of units, Refineries manufacture motor fuels, burning oils, lubricating oils and greases, waxes, asphalts and other by-products.
Therefore, it is apparent that these wastes are produced from all these units. The wastes may be classified as free oil, emulsified oil, condensate water, acid wastes, waste caustics, alkaline water, special chemicals, waste gases, sludges and solids sanitary waste.
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Free Oil. About 0.1 to 2 percent of crude oil flows into sewers in the form of free oil from large refineries. This free oil comes from sampling taps, pump gland leaks, valve and pipeline leaks. The characteristics of oil, reaching a sewer, depend on the processing operations.
The presence of light ends may even create a potential explosion hazard. Oil exists in the waste water in two fractions: one fraction is carried in suspension and the other remains in the form of oil-water emulsion.
The world consumption of oil (excluding planned economic countries) in 1975 was about 6 million tonnes per day. Depending upon prices and rate of growth of demand, the consumption could be between 10 & 13 million tonnes per day, during the period 1985 to 2000. The proven reserves of oil in 1975 were estimated to be:
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Emulsified Oil:
This type of emulsion is not removed in the gravity type oil separator. When it is discharged into a large stream of water, it usually breaks, as a result of dilution, and oil rises to the surface of the water. Emulsion may be formed in a sewerage system as a result of intimate contact between oil, water and emulsifying agents.
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The non separable oil content of waste water increases in the presence of coke clay, sanitary sewage, water treating plant sludges, flocculent and fibrous solids, tars, asphalts, petroleum sludges, soaps and other numerous solvent and treating chemicals.
The chemical treatment of lubricating oils, waxes and burning oils, barometric condensers and distillate separators, desalting operations, pump gland leakage, acid sludge recovery processing, barrel and truck washing result in the direct formation of emulsions.
The removal of emulsified oil, from a petrochemical refinery, is accomplished by means of coagulation and sedimentation processes. The sludge from these processes are thickened by means of vacuum their salts, mercaptans, amines, amides, phenoles, etc.
The other contaminants may be ammonia, caustic soda, calcium hydroxide, etc. Refineries, processing sour crudes, usually produce condensate wastes, high in sulphides, e.g., hydrogen sulphide, ammonium sulphides and some sodium and potassium sulphides.
Condensate waste, from catalytic cracking units, contains phenolic materials which may be removed by means of biological filters.
The other methods for removing hydrogen sulphide are steam stripping, use of fine gases, oxidation and burning the waste.
Acid Wastes In the petroleum industry, sulphuric acid is used as both a treating agent and catalyst. Other chemicals used as catalysts are hydrofluoric acid, phosphoric acid, aluminium chloride and zinc chloride. The source of acid bearing wastes is acid treatment of gasoline, white oils, lubricating oils and waxes.
Waste Caustics Waste of this type originates from the caustic washing of light oils to remove mercaptans, hydrogen sulphide, and other acidic material that may occur naturally in crude oil. The constituents of waste causing pollutional problems are mercaptans, thiophenol, thiocresols, phenols, cresols, disulphides, alkylsulphides.
Alkanline Waters This is generated from the washings of neutralized acid treated oils, the washing of caustic treated oils and the dehydrates of treated light oils.
Special Chemicals:
This includes the solvents and extraction solutions, utilized in selective solvent refining, gas purification and light oil treating.
Waste Gases The waste gases from petroleum refining are stack gases from furnaces and reactors, hydrogen sulphide and sulphur dioxide. These gases also cause water pollution problems. Hydrogen sulphide is a contaminant to other gases, i. e., methane and ethane. In removing the hydrogen sulphide from liquid and gaseous hydrocarbon streams, waste waters of highly undesirable characteristics are produced.
Sludges and Solids The sludges are fundamentally accumulated at the bottom of various crude or product storage tanks. Solids include coke, waste catalysts, filtering clays, slag and tank bottoms. It is highly objectionable to dispose of these sludges and solids into plant sewers. About 119 to 710 pounds of dry solids, per million gallons of waste, are produced in a typical refinery.