Residual pitch

Physical Properties. The physical properties of cmde tars vary over a wide range. Investigation has been mainly concerned with estabHshing correlations between the more readily deterrnined chemical and physical properties of the distillate oils and residual pitch, and other properties. Based on the correlations, other properties can be predicted with an accuracy sufficient for such purposes as plant design (Table 4). 

So far as coal tar is concerned, it was formerly the custom to use the straight residual pitch, but nowadays shock resistance is improved by a so-called plasticising process. 

Figure 40 shows an apparatus for the distillation of 3-naphthol. It is heated by gas (three ring burners) and is designed for about 1000 kilograms. A distillation requires about 4 hours. The residual pitch amounts to about 5 per cent of the crude naphthol. (Most distillation apparatus for naphthol have stirring mechanisms so that no charring occurs at the bottom of the kettles.) The distilled naphthol is allowed to solidify in lumps and is centrifuged, after the product has been pulverized. 

The temperature to which the distillation of the heavy oil fraction is taken depends on the type of residue pitch (Nair, 1978) that is desired but usually lies within the range 450 C-550°C (840°F-1020°F). In all cases, the distillate is an excellent source of hydrocarbons such as anthracene, phenanthrene, acenaphthene, fluorene, and chrysene. The residual coal tar pitches are complex mixtures that contain several thousand compounds (mostly condensed aromatic compounds)... 

The feedstock available (natural gas, residual gases from refineries, LPG (Liquefied Petroleum Gas), naphtha, heavy oils, distillation residues, pitch, coal, carbon dioxide, oxygen) and process utilities (steam, cooling water,...). 

Vacuum distillation. In this process, the residue from the atmospheric distillation tower is separated under vacuum into one or more heavy gas oil streams and heavy residual pitch. 

Crude tar is normally distilled in continuous plant into distillate fractions which can vary in boiling range and in name, leaving pitch as a residue. 

Naphthalene, anthracene, carbazole [86-74-8] phenol [108-95-2] and cresyUc acids are found in the tar. Phenol and cresyUc acids are useful as chemical and resin intermediates. The aromatic chemicals are useful in the manufacture of pharmaceuticals, dyes, fragrances, and pesticides. Various grades of pitch are made from residues of tar refining. Coal-tar pitch is used for roofing and road tar, and as a binder mixed with petroleum coke to produce anodes for the aluminum industry. 

A residuum, often shortened to resid, is the residue obtained from petroleum after nondestmctive distillation has removed all the volatile materials. The temperature of the distillation is usually below 345°C because the rate of thermal decomposition of petroleum constituents is substantial above 350°C. Temperatures as high as 425°C can be employed in vacuum distillation. When such temperatures are employed and thermal decomposition occurs, the residuum is usually referred to as pitch. By inference, the name is used in the same manner as when it refers to the nonvolatile residue from the thermal decomposition of coal tar (3). 

Large amounts of tar or pitch by-products are produced by industrial processes. The distillation of cmde petroleum (qv) yields a pitch-like residue termed bitumen or asphalt (qv). In the United States, these terms are interchangeable, but in Europe the term asphalt is generally restricted to naturally occurring rock or lake asphalt, whereas the residual product of cmde-od distillation is termed bitumen. Although these are important industrial materials produced in millions of metric tons annually, they are not included herein (see Asphalt Petroleum, products). 

Coal tar is the condensation product obtained by cooling to approximately ambient temperature, the gas evolved in the destmctive distillation of coal. It is a black viscous Hquid denser than water and composed primarily of a complex mixture of condensed ring aromatic hydrocarbons. It may contain phenoHc compounds, aromatic nitrogen bases and their alkyl derivatives, and paraffinic and olefinic hydrocarbons. Coal-tar pitch is the residue from the distillation of coal tar. It is a black soHd having a softening point of 30—180°C (86—359°F). 

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