Average Content of Light Oils in Various Gases. The amount of benzol and toluol formed in any one of these processes is by no means definite. It depends upon the operating conditions and the quality of the raw materials (coal or oil). It would therefore be impossible to predict exactly what the yield of products in a given case would be, but an extensive inquiry into the operation of a number of typical plants has given the following tabulation as the usual range of figures for the various processes. Individual results may vary widely from them in a particular case. TABLE 1.—Approximate Yields of Crude Light Oil and Pure Products and Approximate Composition of Crude Light Oil. APPROXIMATE YIELD OF CRUDE LIGHT OIL. Coal gas- .3.0-4.0 gallons per short ton coal carbonized APPROXIMATE COMPOSITION OF CRUDE LIGHT oil. The coke resulting from cannel coal is not of satisfactory quality for ordinary purposes. However, it is satisfactory for making producer gas or burning as a domestic fuel in hard coal burners, provided a small amount of bituminous matter remains in it. Refining of Oil for Road Building and Paving Purposes. The various methods of refining which yield residues adaptable or used for road building and paving purposes are as follows: Sedimentation. Dehydration. Fractional distillation by direct fire. Forced fire distillation with direct fire. Steam distillation. Inert gas distillation. Air blowing. In the types of oil which are ordinarily used for making asphalt or road binders, water is one of the most common impurities. The water is ordinarily salt water and may contain more or less other mineral matter than the salt. These impurities are insoluble in the bitumen proper and as they differ from the bitumen in specific gravity, they may be removed wholly or in part by the process of sedimentation or separation by gravity. In the more fluid petroleums, sedimentation occurs during storage in the large tanks and the water is ordinarily automatically drawn off from the bottom of the tank by reason of the different heads produced by the salt water and by the oil. However, a small amount of emulsified water nearly always remains in all petroleums, so that there will always be a small amount of sediment. If the petroleum is very heavy and viscous, approximately equal in gravity to water, then the water will remain emulsified and will not separate by gravity. This type of oil happens to be the most suitable in quality for producing asphalt and special means of removing this water is necessary before the oil can be reduced to the desired consistency. The dehydration processes are designed primarily for removal of the water in the bituminous material which will not completely separate by sedimentation. It is desirable to do this before distillation because of the fact that the presence of the water will cause foaming when the mixture is heated to the temperature of boiling water. Dehydrating plants vary considerably in design, but those more commonly used for petroleum in California are spoken of as topping plants. In this sort of plant the oil is pumped with or without pressure through a length of pipe containing many bends and turns, so that the oil is considerably stirred. The pipe coils are set in furnaces, so that they may be suitably heated to a temperature above that of boiling water. This pipe discharges the foam into a large expansion chamber, where the water and more volatile constituents separate in the form of vapor which is condensed in an ordinary condenser for the recovery of the light products. This sort of plant is commonly spoken of as a pipe still. From the pipe still, the oil passes through another line, direct to a large batch still, where it is subjected to the ordinary fractional distillation. The essential principle in the distillation of an oil for road purposes is that it shall distill at a temperature sufficiently low to pre vent the decomposition of the hydrocarbons. Since asphalt hydrocarbons begin to decompose at a temperature of 600°F or slightly below, it is desirable that the fire distillation be carried only to that temperature. After this temperature has been reached, the usual method is to blow superheated steam, which mechanically carries over the more volatile hydrocarbons at a temperature much below the actual boiling point. This distillation has a special action in removing the paraffin compounds which are particularly undesirable in that they have very little ductility and cementation value. The distillate will contain any light oils such as are used as spindle oils and for general lubrication, as well as any paraffin wax. It is particularly desirable in this distillation to prevent the formation of free carbon or coke. The distillation with steam may be carried down until the residue shows a penetration of about 10 millimeters. A method of distillation which gives very great yields of solid or semi-solid asphalt even from semi-paraffin base oils is that of blowing the oil at moderately high temperature with air. The amount of air and rate in blowing is usually about 300 cubic feet per barrel of oil per hour (see p. 375). For delivering air to an asphalt blowing still with the oil at a temperature of 400°F and producing about 250 bbls. per day, 100 H. P. is required. Air blowing in many MidContinent oils gives much more asphalt than naturally exists in the oil. The action of the air is to produce a more viscous product which is very much less susceptible to temperature changes than the natural asphalt. It is strictly a chemical transformation process formed from the hydrocarbons in the oil which are ordinarily not useful for asphalt making purposes. It has been found from practical experience that this type of asphalt is not sufficiently cementitious and ductile to be used for ordinary paving purposes in producing first class asphalt pavement. It can, however, be successfully used and is in great demand for water-proofing purposes, for filler in brick and wood block pavement, for roofing purposes and for fluxing ductile asphalt. The best types of petroleum for asphalt paving purposes are those from California, Mexico, Trinidad and Texas. ASPHALT PAVEMENT. Asphalt is a black non-oxidized bituminous hydrocarbon, semifluid to hard in consistency, the heavy residuum from petroleum or occurring naturally. The residua from petroleum are known as oil asphalts and come most largely from California, Mexican, Texas and Mid-Continent petroleums. The most commonly used natural asphalts are Trinidad, Bermudez, Cuban and Gilsonite. The term asphalt is commonly applied to bituminous pavements, being mixtures usually of oil asphalt with dust, sand, gravel or rock in varying proportions from 6% to 20%. The terms "bitumen" or "asphaltic cement" are commonly applied to the pure asphalt material. The types of asphalt construction now commonly used are: 1. Asphaltic concrete. This mixture is very common in localities where Joplin chats are available. It is known also as "Topeka Specifi cation Pavement" and "Bituminous Concrete," but it might be called bituminous gravel. The stone it carries is of 1⁄2" and 4" size. (Fig. 76.) 2. Sheet asphalt is the original type of asphalt pavement laid in two courses, the bottom one with coarse stone, the top with sand mixed with the bitumen. (Fig. 77.) 3. Bituminous concrete (Warren) is laid with coarse stone in the wearing surface. (Fig. 78.) 4. Bituminous earth is laid without an appreciable amount of sand or rock. (Fig. 79.) There are two different basic principles involved in proportioning the mineral matter of an asphalt pavement. One is to so grade the coarse mineral particles that they support each other and interlock. The other is to produce a mastic of bitumen and finely divided earthy material that is rigid and self-supporting because of surface tension action. This mastic fills the voids in the coarse material and has a much higher melting point than the pure bitumen and does not so readily allow softening or movement of the pavement. |