:: Petro Chemicals --- ID :: 10150
BENZENE / TOULENE
BENZENE/TOULENE

 

Benzene and Toluene are pure aromatic hydrocarbon compounds having fixed boiling points, unlike other petroleum products which are mixtures of hydrocarbon compounds. They are colorless and flammable liquids possessing characteristic aromatic odour.

The boiling points of Benzene and Toluene are 80.1 deg C and 110.6 deg C respectively. Benzene is slightly soluble in water whereas Toluene is virtually immiscible with water.

MANUFACTURING PROCESS

Benzene and Toluene are manufactured from stabilized reformat produced in the reformer. The feed to reformer unit is 60-90 deg C cut Naphtha which is produced in a feed preparation unit. Benzene and Toluene are extracted from the reformat with the help of Sulfolane Solvent in an extraction section. The extracted aromatics are separated by fractionation to produce Benzene and Toluene separately. The Sulfolane solvent is recovered and reused in the extraction plant.

BENZENE SPECIFICATION

Benzene marketed meets IS specifications 534 -1974 for both Benzene grades  1 & 2.

BENZENE END USE

It is primarily used as a chemical raw material in the synthesis of styrene (Polystyrene plastics and synthetic rubber), phenol, cyclohexane (nylon), aniline, maleic anhydride (Polyester resins), alkyl benzenes (detergents), chloro benzenes and other products used in the production of drugs, dyes, insecticides and plastics.

TOLUENE SPECIFICATIONS

Toluene is marketed meeting IS 537-1967 specification for Toluene, pure Nitration grade.

TOLUENE END USE

Toluene is primarily used in the manufacture of chemicals, drugs, explosives, food flavouring and sweetening agents, paints and resins.

IDENTIFICATION

Benzene and toluene being pure aromatic compounds, can easily be identified by measuring their physical properties like density, boiling point etc. In addition to this, Benzene can also be identified by checking its crystallizing point. In fact, the purity of Benzene or Toluene is assessed by measuring the above physical properties. Apart from the above, Gas chromatographic analysis is also carried out to exactly identify and determine the purity of Benzene or Toluene.

 

COLOUR

Colour is determined by visual comparison with standard aqueous solution of potassium dichromate or platinum cobalt in a Nessler's tube.

RELATIVE DENSITY /SPECIFIC GRAVITY

Being pure Aromatic Hydrocarbon compounds, Benzene and Toluene have their relative densities / specific gravities stipulated in their specifications unlike other petroleum products, where density is never specified. Measurement of density enables to check and keep a uniform quality of product during manufacture. It also gives a quick check for determining the contamination.

Benzene and Toluene have fixed boiling points. Presence of any impurities results in change in boiling points. The impurities may be praffinic, cyclo-paraffinic, and olefinic type of hydrocarbons etc. Hence distillation is carried out on Benzene and Toluene to determine their boiling points which in turn reflects their purity

CRYSTALLIZING POINT

Being a pure chemical compound Benzene exhibits characteristic crystallizing point. Trace amounts of impurities will result in lowering of crystallizing point. This is one of the ways to determine the purity of Benzene. Hence, there is minimum limit on this property in the specification. A knowledge of this also helps in determining suitability of its use as reagent chemical or for conversion to other chemical intermediates or finished products.

Crystallizing point is not specified in the specification of Toluene as it is below -90 deg C which is difficult to measure. Crystallizing point is also referred a solidification point.

ACID WASH COLOUR

The presence of olefinic compounds in Benzene or Toluene can be easily detected by carrying out "Acid wash colour" test. The principle of this test is the ability of olefines to react with concentrated sulphuric acid and discolour the acid.

SULPHUR AND SULPHUR COMPOUNDS

Sulphur can be present either as Thiophene sulphur or as Hydrogen Sulphide and Sulphurdioxide in Benzene or Toluene. The presence of Sulphur can adversely affect the chemical process where Benzene / Toluene are used as a raw material. Also presence of sulphur, beyond specified limit, causes corrosion of process equipment and storage tanks. They also emit objectionable odour. Hence, there is a limit on presence of sulphur.

 

 

COPPER CORROSION

The Benzene sample should not give any discoloration to copper strip. Discoloration of copper strip indicates presence of reactive sulphur or hydrogen sulphide as an impurity.

SAFETY AND HANDLING

Benzene and Toluene are classified as " Class A" petroleum products, and all safety precautions, as laid down in Petroleum Act and Rules, are to be observed in their handling. Because of their flammability, volatility and toxicity, they are handled under strict safety regulations. They are stored in steel containers. Adequate ventilation, labeling and electrical grounding are required. Fire must be extinguished by using CO2 or dry chemical fire extinguishers. They should be handled only in closed systems.

TOXICITY

Benzene, particularly, is a poisonous substance with chronic toxic effects. The permissible exposure limits in air is 10 ppm and shor term exposure limit is 25 ppm. The immediate dangerous to life or health is 2000 ppm. Exposure to liquid and vapour may produce primary irritation to skin, eyes and upper respiratory tract. Scaling dermatitis may also develop from defattening of skin.

Acute exposure to Benzene results in central nervous system depression, headache, dizziness, nausea, convulsions, coma and death. The short term effects of the inhalation, ingestion, or skin contact of benzene in high dosages, are immediately apparent but, the effects of chronic exposure to lower levels of Benzene are not. Periodic blood and unine tests are required for those who are regularly exposed to Benzene.

                       

SPECIFICATION FOR BENZENE

Sr. No.

CHARACTERISTICS

Test Method

GRADE 1

GRADE 2

1

Colour

5 of IS : 82-1973

Not darker than a freshly prepared solution of 0.8 ml of 0.1 N potassium dichromate and 12 ml of 0.1 N cobalt sulphate made up to 1000 ml with water

2

Relative Density at

 

 

 

 

15 deg /15 0C

6.3.3 of IS 82-1973

0.877 to 0.884

0.872 to 0.882

 

27 deg/27 0C

-         

0.866 to 0.873

0.861 to 0.871

3

Range on evaporation mg/100 ml max.

8 of IS : 82 - 1973

5

5

4

Total sulphur, percent by mass, max.

-

0.015

0.1

5

Hydrogen sulphide

-

To pass test

0.1

6

Mercaptans

P-19 of IS:1448

Shall give no positive reaction

Shall give no positive reaction

7

Thiophene (as sulphur), percent by mass, max.

-

0.005

-

8

Crystallizing point, min.

-

5.2 deg C

-

                                                                                                                                                                                                               

 

 

  

 

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