Coal tar is a dark, viscous liquid obtained as a by-product during the destructive distillation (carbonization) of coal in coke ovens or gas plants. It is a complex mixture of hundreds of organic compounds, mainly aromatic hydrocarbons.
Why Fractionation is Done?
Fractionation (also called fractional distillation) of coal tar is carried out to separate it into various fractions based on their boiling points. Each fraction contains specific groups of chemicals that are further processed to recover valuable industrial products such as benzene, toluene, naphthalene, phenols, creosote oil, pitch, etc.
Principle of Fractionation
The separation is based on the difference in boiling points of the components present in coal tar. The process is carried out in a series of fractionating columns under controlled temperature and pressure (sometimes under vacuum to prevent decomposition).
Industrial Fractionation Process
Main Fractions of Coal Tar and Their Composition
| Fraction | Boiling Range (°C) | Percentage (approx.) | Major Components | Important Products Recovered |
|---|---|---|---|---|
| Light Oil / Crude Naphtha | Up to 170 | 2–8% | Benzene, Toluene, Xylenes, Pyridine, Cyclopentadiene | Benzene, Toluene, Xylene (BTX), Pyridine bases |
| Middle Oil / Carbolic Oil | 170–230 | 8–12% | Phenols, Naphthalene, Cresols | Phenol, Cresols, Naphthalene |
| Heavy Oil / Creosote Oil | 230–270 | 8–10% | Naphthalene, Methylnaphthalenes, Higher phenols | Creosote (wood preservative), Naphthalene |
| Anthracene Oil (Green Oil) | 270–360 | 15–25% | Anthracene, Phenanthrene, Carbazole, Fluorene | Anthracene, Phenanthrene, Carbazole |
| Pitch (Residue) | >360 | 50–60% | High molecular weight polycyclic aromatic hydrocarbons | Electrode pitch, Road tar, Roofing materials, Carbon black feedstock |
Step-by-Step Fractionation Process
- Dehydration: Crude coal tar is first heated to remove water and light volatile impurities.
- Primary Distillation: Tar is heated in a pipe still furnace and fed into a fractionating column.
- Fraction Collection: Different fractions are collected at different heights of the column according to their boiling points.
- Secondary Processing: Each fraction undergoes further refining:
- Acid washing and alkali washing for removal of impurities
- Crystallization (for naphthalene and anthracene)
- Rectification / Redistillation
- Residue Handling: The remaining pitch is collected at the bottom and used directly or processed further.
Important Chemicals Obtained from Coal Tar
- Aromatic Hydrocarbons: Benzene, Toluene, Xylenes, Naphthalene, Anthracene, Phenanthrene
- Phenolic Compounds: Phenol, Cresols, Xylenols
- Heterocyclic Compounds: Pyridine, Quinoline, Carbazole
- Others: Creosote oil, Pitch, Carbon black
Applications of Coal Tar Fractions
- Production of dyes, drugs, plastics, and synthetic fibers
- Wood preservation (Creosote oil)
- Manufacture of electrodes for aluminum and steel industry (Pitch)
- Road construction and waterproofing
- Explosives and perfumes (from specific aromatics)
🧠 Which is not obtained by fractionation of coal tar ?
(a) Light oil
(b) Heavy oil
(c) Middle oil
(d) Vegetable oil
Answer
The correct answer is (d) Vegetable oil.
Why Vegetable Oil is the Odd One?
Coal tar fractionation is an industrial process used to separate the various chemical components of coal tar based on their boiling points. Vegetable oil, however, is a biological product extracted from seeds, nuts, or fruits (like sunflowers, olives, or soybeans) and has no chemical relation to coal or its by-products.