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Crude Benzol and its major Components

Crude Benzol and its major Components

Crude benzol is a product which is produced during carbonization of coking coal. Benzol (also called as benzole) is the name normally applied in the chemical industry to a mixture of hydrocarbons of the benzene series, in which benzene itself predominates, in association with certain of its homologues and various impurities. The term is not applied to any particular mixture or quality of the liquids. The recovery of benzol from the coke oven gas implies the removal of vapours of the benzene series, and their subsequent conversion by condensation into different liquid products. The CAS number of crude benzol is 65996-78-3.

Benzol fraction produced during the high temperature carbonization of the coal is around 0.7 % to 1.1 % of dry coal. It is present in the coke oven gas in the range of 25 grams per normal cubic meters (g/N cum) to 40 g/N cum of coke oven gas.

The benzene series is the most important group of substances in the class of aromatic hydrocarbons.  It is the series of carbon-hydrogen compounds based on the benzene ring, with the general formula CnH2n-6, where ‘n’ is 6 or more. Examples are benzene (C6H6), toluene (C7H8), and xylene (C8H10). The members of particular interest of this series are the first three namely benzene, toluene and xylene, which at normal temperatures are clear colourless liquids having very similar properties. Crude benzol also contains some amount of naphthalene (C10H8).

Crude benzol is also known as light oil and contains small quantities of a large number of impurities which consists of the unsaturated and sulphur compounds. These impurities have negative effects on the organic processes. Even insignificant sulphur impurities in benzene and toluene used in organic processes cause fast poisoning of the catalyst, and resinous substances formed as a result of polymerization of unsaturated compounds coat the catalyst surface and deactivate it. Some of these impurities have the proximity of the boiling points of the pure products. To produce pure products from crude benzol, it is necessary to have its preliminary treatment to remove these impurities. Hence, any processing scheme includes preliminary preparation stages which ensure the removal to the required extent the components of sulphurous unsaturated and saturated hydrocarbons.

Crude benzol is a complex mixture of the volatile aromatic hydrocarbons which includes mainly benzene, toluene, and xylene. It contains benzene (around 65 % to 70 %), toluene (around 10 % to 13 %), and xylene (around 4 % to 6 %) and several other minor constituent such as di methyl benzene, tri methyl benzene, ethyl toluene, and unsaturated compounds (cyclopentadiene, styrene, and indene). Sulphur compounds (carbon bi-sulphide, and thiophene), saturated hydrocarbons, phenol, and pyridine are also part of the composition of crude benzol.

Crude benzol is a transparent yellow brown liquid with its colour ranging from light colour to dark brown. It does not contain suspended particles and has the smell of hydrocarbons. The boiling point is 150 deg C to 200 deg C, and the freezing point is in the range of -20 deg C to -25 deg C. Its density at 20 deg C is in the range of 0.871 g/cc (grams per cubic centimeters) to 0.9 g/cc. Water solubility of crude benzol at 20 deg C is in the range of around 0.1 mg/l (milligrams per litre) to 100 mg/l. Its flash point is less than 2 deg C. The auto flammability of crude benzol is 485 deg C. The vapour pressure of crude benzol at 20 deg C is 0.054 kg/sq cm.

Crude benzol is stable under normal conditions but reacts with oxidizing agents often explosively.

Crude benzol is used in the production of aromatic chemicals and as carbon black feedstock. It is also used for its complete conversion into carbon black. The aromatic carbons normally produced from crude benzol are benzene, toluene, and xylene.

BTX fraction

BTX refers to mixtures of benzene, toluene, and the three xylene isomers, all of which are aromatic hydrocarbons. Xylene in BTX is present in the form of its three isometrics polymers (ortho-xylene, para-xylene, and meta-xylene) with meta- xylene dominating. BTX fraction is the intermediate product produced during the production aromatic chemicals from crude benzol. Structural diagrams of the BTX hydrocarbons are given in Fig 1.

Fig 1 Structural diagrams of the BTX hydrocarbons

BTX is a yellowish liquid with an aromatic odour. Sulphur in BTX fraction is normally less than 0.10 %. Its freezing point is less than -100 deg C and boiling temperature range is 75 deg C to 95 deg C. The flash point of BTX fraction is -16 deg C. The density of BTX fraction is in the range of 0.85 g/cc to 0.88 g/cc. the water solubility is in the range of 0.035 g/l to0.16 g/l. The auto ignition temperature is 440 deg C.

BTX fraction is flammable liquid with CAS number 91723-50-1.

Benzene

Benzene is an organic chemical compound with the chemical formula C6H6. The benzene molecule is composed of six carbon atoms joined in a ring with one hydrogen atom attached to each of carbon item. As it contains only carbon and hydrogen atoms, benzene is classed as a hydrocarbon. The CAS number of benzene is 71-43-2.

X-ray diffraction shows that all six carbon-carbon bonds in benzene are of the same length, at 140 picometres (pm). The C–C bond lengths are greater than a double bond (135 pm) but shorter than a single bond (147 pm). This intermediate distance is consistent with electron delocalization. The electrons for C–C bonding are distributed equally between each of the six carbon atoms. Benzene has 6 hydrogen atoms. The molecule is planar. The molecular orbital description involves the formation of three delocalized ? orbitals spanning all six carbon atoms, while the valence bond description involves a superposition of resonance structures. It is likely that this stability contributes to the peculiar molecular and chemical properties known as aromaticity. To accurately reflect the nature of the bonding, benzene is often depicted with a circle inside a hexagonal arrangement of carbon atoms. Some of the representations of benzene are given in Fig 2.

Fig 2 Some of the representations of benzene

Due to the cyclic continuous pi bond between the carbon atoms, benzene is classed as an aromatic hydrocarbon. Benzene is a colourless and highly flammable liquid with a sweet smell. It is used primarily as a precursor to the manufacture of chemicals with more complex structure, such as ethyl-benzene and cumene.

Benzene has a molar mass of 78.114 g/mol. It is a colourless liquid and has an aromatic odour. The density of benzene is 0.88 g/cc. Its freezing point is 5.5 deg C and boiling point is 80.1 deg C. The solubility of benzene in water at 30 deg C is 1.84 g/l. Its vapour pressure at 25 deg C is 0.13 kg/sq cm. Its viscosity at 25 deg C is 0.61 cP (centipoise). Flash point of benzene is -11.6 deg C and its auto ignition temperature is 498 deg C. Its explosive limit is in the range of 1.2 % to 7.8 %.

Benzene is used mainly as an intermediate to make other chemicals, such as ethyl-benzene, cumene, cyclohexane, nitrobenzene, and alkyl-benzene. More than half of the entire benzene production is processed into ethylbenzene, a precursor to styrene, which is used to make polymers and plastics like polystyrene and EPS (expanded polystyrene).  Around 20 % of the benzene production is used to manufacture cumene, which is needed to produce phenol and acetone for resins and adhesives. Cyclohexane consumes around 10 % of the benzene production; it is primarily used in the manufacture of nylon fibers, which are processed into textiles and engineering plastics. Smaller amounts of benzene are used to make some types of rubbers, lubricants, dyes, detergents, drugs, explosives, and pesticides.

Toluene

Toluene is also known as toluol, is an aromatic hydrocarbon with the chemical formula of C7H8. It is a colourless, water-insoluble liquid with a sweet pungent smell associated with paint thinners. It is a mono-substituted benzene derivative, consisting of a CH3 group attached to a phenyl group. As such, its IUPAC systematic name is methylbenzene. The CAS number of toluene is 108-88-3.

Toluene reacts as a normal aromatic hydrocarbon in electrophilic aromatic substitution. Because the methyl group has greater electron-releasing properties than a hydrogen atom in the same position, toluene is more reactive than benzene toward electrophiles. It undergoes sulphonation to give p-toluenesulfonic acid, and chlorination by chlorine (Cl2) in the presence of ferric chloride (FeCl3) to give ortho and para isomers of chlorotoluene.

Molar mass of toluene is 92 g/mol. The density of toluene at 20 deg C is 0.87 g/cc. Its freezing point is -95 deg C and boiling point is 111 deg C. Its solubility in water at 20 deg C is 0.52 g//l. The vapour pressure of toluene is 0.029 kg/s cm. Its viscosity at 20 deg C is 0.59 cP. It is a highly flammable liquid. Its flash point is 6 deg C. The explosive limit of toluene is in the range of 1.1 % to 7.1 %.

Toluene is mainly used as a precursor to benzene via hydride-alkylation (C6H5CH3 + H2 = C6H6 + CH4). The second ranked application involves its disproportionation to a mixture of benzene and xylene. When oxidized it yields benzaldehyde and benzoic acid, two important intermediates. In addition to the synthesis of benzene and xylene, toluene is a feedstock for toluene diisocyanate (used in the manufacture of polyurethane foam), trinitrotoluene (the explosive, TNT), and a number of synthetic drugs. Toluene is a common solvent, e.g. for paints, paint thinners, silicone sealants, many chemical reactants, rubber, printing ink, adhesives (glues), lacquers, leather tanners, and disinfectants. Toluene can be used as an octane booster in gasoline fuels for internal combustion engines.

Xylene

Xylene is also known as xylol or di-methyl-benzene is any one of three isomers of di-methyl-benzene, or a combination thereof. With the formula C8H10, each of the three compounds has a central benzene ring with two methyl groups attached at substituents. They are all colourless, flammable liquids, some of which are of great industrial value. The mixture is referred to as both xylene and, more precisely, xylenes. The molar mass of xylene is 106.16 g/mol. The CAS number of xylene is 1330-20-7.

There are, in fact, three isomeric xylenes, or substances of the composition and molecular weight corresponding to C8H10. The isomers can be distinguished by the designations ortho-xylene (o-), meta- xylene (m-) and para-xylene (p-), which specify to which carbon atoms (of the benzene ring) the two methyl groups are attached. By counting the carbon atoms around the ring starting from one of the ring carbons bonded to a methyl group, and counting towards the second methyl group, the ortho- isomer has the IUPAC name of 1,2-di-methyl-benzene, the meta-isomer is 1,3-di-methyl-benzene and the para-isomer is 1,4-di-methyl-benzene. Of the three isomers, the p-isomer is the most industrially sought after since it can be oxidized to terephthalic acid. CAS number of ortho-xylene is 95-47-6, meta-xylene is 108-38-3, and para-xylene is 106-42-3.

Density of xylene is 0.864 g/cc, of ortho-xylene is 0.88 g/cc, meta-xylene is 0.86 g/cc, and para-xylene is 0.86 g/cc. Xylene is practically insoluble in water. Freezing points of xylene, ortho-xylene, meta-xylene, and para-xylene are -47.4 deg C, -25 deg C, -48 deg C, and 13 deg C respectively. Boiling points of xylene, ortho-xylene, meta-xylene, and para-xylene are 138.5 deg C, 144 deg C, 139 deg C, and 138 deg C respectively. Viscosities of ortho-xylene, meta-xylene, and para-xylene are 0.812 cP, 0.62 cP, and 0.34 cP respectively. Flash points xylene, ortho-xylene, meta-xylene, and para-xylene are 30 deg C, 17 deg C, 25 deg C, and 25 deg C respectively.

Xylene is used as a solvent. In this application, with a mixture of isomers, it is often referred to as xylenes or xylol. As solvent, xylene frequently contains a small percentage of ethyl-benzene. Like the individual isomers, the mixture is colourless, sweet-smelling, and highly flammable. Areas of application include the printing, rubber, and leather industries. It is a common component of ink, rubber, and adhesives. In thinning paints and varnishes, it can be substituted for toluene where slower drying is desired, and thus is used by conservators of art objects in solubility testing. Similarly it is a cleaning agent, e.g., for steel, silicon wafers, and integrated circuits. In dentistry, xylene can be used to dissolve gutta percha, a material used for endodontics (root canal treatments). In the petroleum industry, xylene is also a frequent component of paraffin solvents, used when the tubing becomes clogged with paraffin wax. For similar reasons, it is often the active ingredient in commercial products for ear wax (cerumen) removal.

 

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