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injects them at about 200 psig into the absorber. Countercurrent flow of stripped gasoline from the saturator or of fresh gasoline from storage is used to absorb the hydrocarbon vapors. Gasoline from the absorber bottoms is returned to storage while the tail gases, essentially air, are released to the atmosphere through a backpressure regulator. Some difficulty has been experienced with air entrained or dissolved in the sponge gasoline returning to storage. Any air released in the storage tank is discharged to the atmosphere saturated with hydrocarbon vapors. A considerable portion of the air can be removed by flashing the liquid gasoline from the absorber in one or more additional vessels operating at successively lower pressures.
Another type of package unit adsorbs the hydrocarbon vapors on activated carbon.
The vapors displaced during bottom filling are minimal. A volume displacement ratio of vapor to liquid of nearly 1 : 1 is usually achieved. A closed system can then be employed by returning all the displaced vapors to a storage tank. The storage tank should be connected to a vapor recovery system.
or concrete for transmitting waste water from processing units to large basins or ponds used as oil-water separators. These basins are sized to receive all effluent water, sometimes even including rain runoff, and may be earthen pits, concrete- lined basins, or steel tanks. Liquid wastes discharging to these systems originate at a wide variety of sources such as pump glands, accumulators, spills, cleanouts, sampling lines, relief valves, and many others The types of liquid wastes may be classified as waste water with: oil present as free oil, emulsified oil, or as oil coating on suspended matter; chemicals present as suspensoids, emulsoids, or solutes. These chemicals include acids, alkalies, phenols, sulfur compounds, clay, and others. Emissions from these varied liquid wastes can best be controlled by properly maintaining, isolating, and treating the wastes at their source; by using efficient oil-water separators; and by minimizing the formation of emulsions.
The waste water from the process facilities and treating units just discussed flows to the oil water separator for recovery of free oil and settleable solids. Factors affecting the efficiency of separation include temperature of water, particle size, density, and amounts and characteristics of suspended matter. Stable emulsions are not affected by gravity-type separators and must be treated separately. The oil-water separator design must provide for efficient inlet and outlet construction, sediment collection mechanisms, and oil skimmers. Reinforced concrete construction has been found most desirable for reasons of economy, maintenance, and efficiency.
The effluent water from the oil-water separator may require further treatment before final discharge to municipal sewer systems, channels, rivers, or streams.
The
type and extentof
treatment depend upon the nature of the contaminants present, and on the local water pollution ordinances governing the concentration and amounts of contaminants to be discharged in refinery effluent waters. The methods of final-effluent clarification to be briefly discussed here include (1) filtration, (2) chemical flocculation, and (3) biological treatment. Several different types of filters may be used to clarify the separator effluent. Hay-type filters, sand filters, and vacuum precoat filters are the most common. The selection of any one type depends upon the properties of the effluent stream and upon economic considerations. Methods of treatment are either by sedimentation or flotation. In sedimentation processes, chemicals such as copper sulfate, activated silica, alum, and lime are added to the waste-water stream before it is fed to the clarifiers. The chemicals cause the suspended particles to agglomerate and settle out. Sediment is removed from the bottom of the clarifiers by mechanical scrapers. Effectiveness of the sedimentation techniques in the treatment of separator effluents is limited by the small oil particles contained in the waste water. These particles, being lighter than water, do not settle out easily. They may also become attached to particles of suspended solids and thereby increase in buoyancy. In the flotation process a colloidal floc and air under pressure are injected into the waste water. The streamINHALATION HAZARDS IN IUIFINERIES 141
is then fed to a clarifier through a backpressure valve that reduces the pressure to atmospheric. The dissolved air is suddenly released in the form of tiny bubbles that carry the particles of oil and coalesced solids to the surface where they are skimmed off by mechanical flight scrapers. Of the two, the flotation process has the potential to become the more efficient and economical. Biological treating units such as trickling filters activated sludge basins, and stabilization basins have been incorporated into modern refinery waste disposal systems. By combining adsorption and oxidation, these units are capable of reducing oil, biological oxygen demand, and phenolic content from effluent water streams. To prevent the release of air pollutants to the atmosphere, certain pieces of equipment, such as clarifiers, digesters, and filters, used in biological treatment should be covered and vented to recovery facilities or incinerated.
From an air pollution standpoint the most objectionable contaminants emitted from liquid waste streams are hydrocarbons, sulfur compounds, and other malodorous materials. The effect of hydrocarbons in smog-producing reactions is well known, and sulfur compounds such as mercaptans and sulfides produce very objectionable odors, even in high dilution. These contaminants can escape to the atmosphere from openings in the sewer system, open channels, open vessels, and open oil-water separators. The large exposed surface, area of these separators requires that effective means of control be instituted to minimize hydrocarbon losses to the atmosphere from this source. The most effective means of control of hydrocarbon emissions from oil-water separators has been the covering of forebays or primary separator sections. Either fixed roofs or floating roofs are considered acceptable covers. Separation and skimming of over 80 percent of the floatable oil layer is effected in the covered sections. Thus, only a minimum of oil is contained in the effluent water, which flows under concrete curtains to the open afterbays or secondary separator sections. The explosion hazard associated with fixed roofs is not present in a floating-roof installation. These roofs are similar to those developed for storage tanks. The floating covers are built to fit into bays with about 1 inch of clearance around the perimeter. Fabric or rubber may be used to seal the gap between the roof edge and the container wall. The roofs are fitted with access manholes, skimmers, gage hatches, and supporting legs. In operation, skimmed oil flows through lines from the skimmers to a covered tank (floating roof or connected to vapor recovery) or sump and then is pumped to deemulsifying processing facilities. Effluent water from the oil-water separator is handled in the manner described previously.
In addition to covering the separator, open sewer lines that may carry volatile products are converted to closed, underground lines with waterseal-type vents.
Junction boxes are vented to vapor recovery facilities, and steam is used to blanket the sewer lines to inhibit formation of explosive mixtures. Accurate calculation of
the hydrocarbon losses from separators fitted with fixed roofs is difficult because of the many variables of weather and refinery operations involved.
Isolation of certain odor- and chemical-bearing liquid wastes at their source for
treatment before discharge of the water to the refinery waste water-gathering system has been found to be the most effective and economical means of minimizing odor
and chemicals problems. The unit that is the source of wastes must be studied for possible changes in the operating process to reduce wastes. In some cases the wastes from one process may be used to treat the wastes from another. Among the principal streams that are treated separately are oil-in-water emulsions, sulfur bearing waters, acid sludge, and spent caustic wastes.
Oil-in-water emulsions are wastes that can be treated at their source. An oil-in- water emulsion is a suspension of oil particles in water that cannot be divided effectively by means of gravity alone. Gravity-type oil-water separators are generally, ineffective in breaking the emulsions, and means are provided for separate treatment where the problem is serious. Oil-in-water emulsions are objectionable in the drainage system since the separation of otherwise recoverable oil may be impaired by their presence. Moreover, when emulsions of this type are discharged into large bodies of water, the oil is released by the effect of dilution, and serious pollution of the water may result. Formation of emulsions may be minimized by proper design of process equipment and piping. Both physical and chemical methods are available for use in breaking emulsions. Physical methods of separation include direct application of heat, distillation, centrifuging, filtration, and use of an electric field. The effectiveness of any one method depends upon the type of emulsion to be treated.
Sulfur-Bearing Waters: Sulfides and mercaptans are removed from wastewater streams by various methods. Some refineries strip the wastewater in a column with live steam. The overhead vapors from the column are condensed and collected in an accumulator from which the noncondensables flow to sulfur recovery facilities or are incinerated. Flue gas has also been used as the stripping medium. Bottoms water from steam stripping towers, being essentially sulfide free, can then be drained to the refinery's sewer system. Oxidation of sulfides in waste water is also an effective means of treatment. Air and heat are used to convert sulfides and mercaptans to thiosulfates, which are water soluble and not objectionable. Chlorine is also used as an oxidizing agent for sulfides. It is added in stoichiometric quantities proportional to the waste water. This method is limited by the high cost of chlorine. Water containing dissolved sulfur dioxide has been used to reduce sulfide concentration in waste waters. For removing small amounts of hydrogen sulfide, copper sulfate and zinc chloride have been used to react and precipitate the sulfur as copper and zinc sulfides. Hydrogen sulfide may be released, however, only if the water treated with these compounds contacts an acid stream.
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Acid Sludge: The acid sludge produced from treating operations varies with the stock treated and the conditions of treatment. The sludge may vary from a low- viscosity liquid to a solid. Methods of disposal of this sludge are many and varied.
Basically, they may be considered under the following general headings: Disposal by burning as fuel, or dumping in the ground; processing to produce byproducts such as ammonium sulfate, metallic sulfates, oils, tars, and other materials;
processing for recovery of acid. The burning of sludge results in discharge to the atmosphere of excessive amounts of sulfur dioxide and sulfur trioxide from furnace stacks. If sludge is solid or semisolid it may be buried in specially constructed pits.
This method of disposal, however, creates the problem of acid leaching out to adjacent waters. Recovery of sulfuric acid from sludge is accomplished essentially by either hydrolysis or thermal decomposition processes. Sulfuric acid sludge is hydrolyzed by heating it with live steam in the presence of water. The resulting product separates into two distinct phases. One phase consists of diluted sulfuric acid with a small amount of suspended carbonaceous material, and the second phase, of a viscous acid-oil layer. The dilute sulfuric acid may be (1) neutralized by alkaline wastes, (2) reacted chemically with ammonia-water solution to produce ammonium sulfate for fertilizer, or (3) concentrated by heating.