Pitch distillation characteristics

Pitches can be transformed to a mesophase state by further chemical and physical operations. Heat treatment of conventional pitches results in additional aromatic polymeriza tion and the distillation of low molecular weight components. This results in an increase in size and concentration of large planar aromatic molecular species whereupon the precursor pitch is transformed to a mesophase state exhibiting the characteristics of nematic Hquid crystals (1). Additional heat treatment converts the mesophase pitch to an infusible aromatic hydrocarbon polymer designated as coke. 

It is my contention that the optical and physical properties and the optical structure produced during the destructive distillation or thermal decomposition of vitrinite is closely related to mode of carbonization and, in the case of pitch, is intimately related to the method of pitch preparation. For instance, a pitch may be produced from a high or low temperature tar, from a primary cooler tar, or from a flushing liquor tar. In addition, it may be air blown, thermally or chemically treated, straight distilled, or cut back, just to mention a few. Under similar carbonization conditions almost any one of these pitches will produce a coke which has certain characteristics that are related to the parent pitch. Even pitches similarly processed from the tar can differ in the content of quinoline- and benzene-insoluble material and P-resin, and can contain more than one distinct liquid phase. None of these points of difference has been discussed by Dr. Taylor or even recognized in the preparation. To interpret the structure of pitch coke divorced from a knowledge of the pitch source and/or carbonization conditions can lead to erroneous conclusions. These are pertinent data omitted by the authors. 

This chapter describes the preparations and characteristics of highly aromatic and highly anisotropic pitches from the distillate fraction of catalytic cracker bottoms (CCB). CCB is the aromatic residue from a catalytic cracking process. 

Both CCB fractions (distillate and residue) were transformed into aromatic pitches by a high temperature thermal process at atmospheric pressure followed by vacuum stripping. A number of reaction parameters effecting pitch yield and characteristics were investigated. 

SCT can be converted into highly aromatic pitches by physical, thermal and chemical processes such as vacuum or steam stripping, thermal or catalytic oxidative-polymerization at 229-260°C, or by a thermal process at 370-450°C at atmospheric nitrogen or hydrogen pressure. The physical or chemical characteristics of the pitches produced from SCT depend on the type of process and conditions used. Table II gives the characteristics of SCT pitches produced by distillation, catalytic air-oxidation and thermal process. 

Table II. Characteristics of SCT, SCT-Pitches Produced by Distillation, Polymerization and Thermal Processes...

This phase is selective for aromatics and its effectiveness is demonstrated in Figure 11.5. Normal coal-tar pitch (residue from commercial high-temperature coal tar distillation) was analysed and around 90 components could be distinguished, including polycyclic aromatic hydrocarbons and related heterocyclic and oligomer systems. Twenty-four of these were clearly identified by their characteristic UV spectra and comparison with known reference spectra. 

This profile addresses the toxicological and toxicokinetics database for several substances, wood creosote, coal tar creosote, coal tar, coal tar pitch, and coal tar pitch volatiles, whose production stems from the incomplete combustion or pyrolysis of carbon-containing materials. Creosotes, coal tar, coal tar pitch, and coal tar pitch volatiles are composed of many individual compounds of varying physical and chemical characteristics. In addition, the composition of each, although referred to by specific name (e.g., wood creosote or coal tar creosote) is not consistent. For instance, the components and properties of the mixture depend on the temperature of the destructive distillation (carbonization) and on the nature of the carbon-containing material used as a feedstock for combustion. 

PHYSICAL PROPERTIES mixture of coal tar, coal tar pitch, and creosote condensed coke oven emissions consist of a brownish, thick liquid or semisolid naphthalene-like odor is characteristic of condensed coke oven emissions uncondensed coke oven emissions contain vapors that escape when the ovens are changed and emptied contain chemicals such as benzo(a)pyrene, benzanthracene, chrysene, and phenanthrene OSHA defines coke oven emissions as the benzene-soluble fraction of total particulate matter present during destructive distillation or carbonization of coal to produce coke. 

Tar is a product of fractional distillation of primary tar produced by carbonation of natural organic matters, such as coal or wood. When the primary tar is derived from coal, the product is called pitch tar. Similarly, when it is derived from wood, it is called wood tar. The pitch tar is further mixed with oil distillates, to produce the processed tar, commonly known as tar. The tar differs in chemical composition and odour to bitumen. Tars consist of variable mixtures of phenols, polycyclic aromatic hydrocarbons (PAHs) and heterocyclic compounds, and their odour is characteristically more aromatic than the odour of the asphalt. 

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