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BENZENE / TOULENE

DISTILLATION

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.