Tar, Distillation

Other Organic Processes. Solvent extraction has found appHcation in the coal-tar industry for many years, as for example in the recovery of phenols from coal-tar distillates by washing with caustic soda solution. Solvent extraction of fatty and resimic acid from tall oil has been reported (250). Dissociation extraction is used to separate y -cresol fromT -cresol (251) and 2,4-x5lenol from 2,5-x5lenol (252). Solvent extraction can play a role in the direct manufacture of chemicals from coal (253) (see Eeedstocks, coal chemicals). 

The industrial source of indole has been isolation from coal-tar distillate (7). Several patents for the manufacture of indole have been issued with aniline and ethylene glycol (8), aniline and ethylene oxide (9), 2-ethy1ani1ine (10), and /V-ethy1ani1ine (11) as the starting materials. 

The coal tar first is processed through a tar-distillation step where ca the first 20 wt % of distillate, ie, chemical oil, is removed. The chemical oil, which contains practically all the naphthalene present in the tar, is reserved for further processing, and the remainder of the tar is distilled further to remove additional creosote oil fractions until a coal-tar pitch of desirable consistency and properties is obtained. 

There are two serious problems associated with continuous tar distillation. Coal tar contains two types of components highly corrosive to ferrous metals. The ammonium salts, mainly ammonium chloride, associated with the entrained Hquor remain in the tar after dehydration, tend to dissociate with the production of hydrochloric acid and cause rapid deterioration of any part of the plant in which these vapors and steam are present above 240°C. Condensers on the dehydration column and fractionation columns are also attacked. This form of corrosion is controlled by the addition of alkaU (10% sodium carbonate solution or 40% caustic soda) to the cmde tar in an amount equivalent to the fixed ammonia content. 

Carbolic Oils and Tow Temperature Tar Middle Oil, TarMcids. The fractions of some coke-oven tars, distilling in the range of 180—240°C, and the middle oil fraction (180—310°C) from low temperature tars are treated for the recovery of tar acids (19). 

In the case of low temperature tar, the aqueous Hquor that accompanies the cmde tar contains between 1 and 1.5% by weight of soluble tar acids, eg, phenol, cresols, and dihydroxybenzenes. Both for the sake of economics and effluent purification, it is necessary to recover these, usually by the Lurgi Phenosolvan process based on the selective extraction of the tar acids with butyl or isobutyl acetate. The recovered phenols are separated by fractional distillation into monohydroxybenzenes, mainly phenol and cresols, and dihydroxybenzenes, mainly (9-dihydroxybenzene (catechol), methyl (9-dihydtoxybenzene, (methyl catechol), and y -dihydroxybenzene (resorcinol). The monohydric phenol fraction is added to the cmde tar acids extracted from the tar for further refining, whereas the dihydric phenol fraction is incorporated in wood-preservation creosote or sold to adhesive manufacturers. Naphthalene Oils. Naphthalene is the principal component of coke-oven tats and the only component that can be concentrated to a reasonably high content on primary distillation. Naphthalene oils from coke-oven tars distilled in a modem pipe stiU generally contain 60—65% of naphthalene. They are further upgraded by a number of methods. 

Crude Tar. Current world output of crude tar is estimated at between 11 and 12 x 10 t/yr. Table 8 gives such details as are available. The amount distilled is about 10 x 10 t/yr. Production of low temperature tar included in the U.K. total for 1994 was 35,000 t at its 2enith 203,000 t were produced in 1975. World production of all coal tar in 1975 was estimated at 17.3 x 10 t. The quantity of tar distilled at five yearly intervals since 1984 is shown in Table 9. Some comparative figures for the prices of coal-tar bulk products for 1982 and 1994 are given in Table 10. 

Tar. Before the development of gas chromatography (gc) and high pressure Hquid chromatography (hplc), the quantitative analyses of tar distillate oils involved tedious high efficiency fractionation and refractionation, followed by identification or estimation of individual components by ir or uv spectroscopy. In the 1990s, the main components of the distillate fractions of coal tars are deterrnined by gc and hplc (54). The analytical procedures included in the specifications for tar bulk products are given in the relevant Standardi2ation of Tar Products Tests Committee (STPTC) (33), ISO (55), and ASTM (35) standards. 

Distillation System. The cmde condensate consists of the desired product, some low boiling constituents, and a smaller quantity of high boiling tar. Distillation separates the low boiling components, which are invariably incinerated, followed by the product fraction. Tar accumulates in the stiU ketdes, from which it is periodically removed, again to incineration. Stills work at atmospheric pressure and are vented to the incinerator. 

Since the thermal dehydrocondensation proceeds by a free-radical mechanism (37), various radical-forrning promoters like acetone, ethanol, or methanol have been found useful in improving conversion of ben2ene to condensed polyphenyls. In the commercial dehydrocondensation process, ben2ene and some biphenyl are separated by distillation and recycled back to the dehydrocondensation step. Pure biphenyl is then collected leaving a polyphenyl residue consisting of approximately 4% o-terphenyl, 44% y -terphenyl, 25% -terphenyl, 1.5% triphenylene, and 22—27% higher polyphenyl and tars. Distillation of this residue at reduced pressure affords the mixed terphenyl isomers accompanied by a portion of the quaterphenyls present. 

Vinyl carbazole is obtained by reacting carbazole, readily available as a byproduct of coal tar distillation, with acetylene in the presence of a catalyst and solvent under pressure Figure 17.7). 

A number of other phenols obtained from coal tar distillates are used in the manufacture of phenolic resins. Of these the cresols are the most important... 

The cresols occur in cresylic acid, a mixture of the three cresols together with some xylenols and neutral oils, obtained from coal tar distillates. Only the /n-cresol has the three reactive positions necessary to give cross-linked resins and so this is normally the desired material. The o-isomer is easily removed by distillation but separation of the close-boiling m- and p-isomers is difficult and so mixtures of these two isomers are used in practice. 

Other higher boiling phenolic bodies obtainable from coal tar distillates are sometimes used in the manufacture of oil-soluble resins. Mention may also be made of cashew nut shell liquid which contains phenolic bodies and which is used in certain specialised applications. 

Coal tar distillation Vehicle exhausts Petrochemicals Pesticides Herbicides Pigment and dye Pesticide and herbicide production... 

An electrostatic precipitator is used to remove more tar from coke oven gas. The tar is then sent to storage. Ammonia liquor is also separated from the tar decanter and sent to wastewater treatment after ammonia recovery. Coke oven gas is further cooled in a final cooler. Naphthalene is removed in a separator on the final cooler. Light oil is then removed from the coke oven gas and is fractionated to recover benzene, toluene, and xylene. Some facilities may include an onsite tar distillation unit. The Claus process is normally used to recover sulfur from coke oven gas. During the coke quenching, handling, and screening operation, coke breeze is produced. The breeze is either reused on site (e.g., in the sinter plant) or sold offsite as a by-product. 

The technical method for obtaining phenol is by shaking out with caustic soda the middle oil of the coal-tar distill ate,. after some of the naphthalene has crystallised out. The phenol dis-soKes m the alkali, and is then lemoved fiom insoluble oils. The alkaline liquid is acidified, the phenol separated, distilled, and finally purified by freezing. 

When coal is heated to a high temperature in the absence of air, it undergoes decomposition volatile products (coal gas and coal tar) distill away and a residue called coke remains. Coke is a valuable industrial material which finds its chief use in the reduction of iron ore (iron oxide) to iron for the manufacture of steel. Coke is essentially carbon that still contains the mineral substances that are present in all coals (and form the ash that results when coal or coke is burned). 

Aromatic hydrocarbons G L Benzene Toluene Xylene Coal tar distillation Vehicle exhauks Petrochemicals Pesticides Herbicides... 

Figure 1.30 Chemical products from coal tar distillation.

Coal tar is keratolytic and may have anti-proliferative and anti-inflammatory effects.2 Coal tar products include crude coal tar and tar distillates (liquor carbonis detergens) available as ointments, creams, and shampoos in various strengths. Preparations containing coal tar may precipitate folliculitis,10 will stain clothing, and have an unpleasant odor. They are also photosensitizing and can be combined with UVB phototherapy (Goekermann s regimen) to increase treatment response.19,20... 

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