Benzene

 Benzene is prepared in the laboratory as well as in the chemical industry. Benzene is a starting material for many other aromatic hydrocarbons.

Industrial production of Benzene

Benzene is firstly discovered in1825 by Michael Faraday who isolate it from oily condensate that was deposited in a compressed gas that was used at that time for the purpose of illumination. This gas was produced by the hydrolysis of whale oil. Benzene and some other simple aromatic compounds are the main starting materials for nearly all aromatic compounds.

Benzene is prepared in a laboratory or in an industry. Benzene in addition to as a starting material for other hydrocarbons, it is used as a solvent. There was therefore high demand for benzene and other simple aromatic compounds. Their demand is mainly met by two natural sources namely Coal and petroleum.

There are the following methods for the production of benzene on industrial scales :

1. Fractional distillation of coal tar (coal tar is obtained from steel industries)

2. Catalytic reforming of naphtha in petroleum refineries

3 . From pyrolysis of gasoline via steam cracking

4. Toluene hydrodealkylation and disproportionation

Explanation:

1. Fractional distillation of coal tar :

This method requires coal tar that is obtained by steel industries. But the proportion of benzene obtained from this method is very low to full fill the demands of benzene. 

This method requires the following steps :

1. Firstly coal tar is taken from steel industries.

 In steel industries when the coal is burnt. when the coal is burned it gives solid residue that is a coke and gaseous vapors, that consist of coal gas and condensate. 

The above condensate has a black viscous liquid known as coal tar and aqueous ammonia. The coal tar is then separated.

2. Fractional distillation of coal tar is done to get the benzene. On fractional distillation following fractions are obtained;

• light oil fraction: It is obtained at about 2.25%  at170 degrees centigrade.it consists of Benzene, Toluene, and Xylene. This trio fraction is also known as BTX.
• Middle oil fraction: It is obtained at about 7.5% at 170-230 degrees centigrade. It consists of phenol, cresol, and naphthalene.
• Heavy oil fraction: It is obtained at about 16.5% at 230-270 degrees centigrade. It consists of cresol, naphtha, and naphthalene.
• Anthracene: It is obtained at about 12% at 270-360 degrees centigrade. It consists of anthracene.

Carbon still remains as residue with 56% content. 

3. Separation of the benzene by the light oil fraction. This light oil fraction also contains some basic content like pyridine and acidic content like phenol and cresol. The basic content is removed by pre-treatment with H2SO4 and after this neutralization is done by treating NaOH to remove the acidic content like phenol and cresol. This way Benzene, Toluene, and Xylene are obtained that can be separated on the basis of differences in boiling points or by further fractional distillation of BTU.

B.P of benzene is 80.1 degrees centigrade

B.P of toluene is 110.6 degrees centigrade and

B.P of xylene is 139 degrees centigrade respectively.

This way the benzene is obtained by fractional distillation of coal tar in chemical industries.

2. Catalytic reforming of naphtha in petroleum industries

Naphtha is a general term given to a mixture of any type of hydrocarbons. In this process catalytic hydrogenation of naphtha takes place and benzene is obtained. It is a well-known method these days for the production of benzene and 50% demand for benzene is fully filled by this method. Hydrogenation of benzene takes place by following steps.

• In the first step crud's oil naphtha is mixed with hydrogen gas. 

Then it is heated at 500-525 degrees centigrade. 
It is sent into a catalytic reactor. Catalysts for this is platinum or rhenium. This way hydrocarbons firstly form rings and then lose their hydrogen atoms to become aromatic hydrocarbons. This way other aromatic hydrocarbons are also obtained along with benzene that is separated from them by applying purifying techniques.

In this step separation of benzene takes place by extractive distillation or azeotropic distillation, or by solid adsorption using molecular sieves.

3. From pyrolysis of gasoline-steam cracking

• Pyrolysis gasoline is obtained by steam cracking of natural gas concentrates, heavy naphtha, and gas oils. This pyrolysis fraction contains 40-45% benzene and other hydrocarbons.
• Then pyrolysis gasoline is sent into a separating chamber where extraction of benzene takes place. The Extraction may be liquid-liquid extraction, or extraction distillation /fractional distillation takes place. This way benzene is separated from aliphatic and other aromatics.

4. Toluene hydrodealkylation and disproportionation :