Question

Which solvent is used for removing benzene from coke-oven gas?

• A. Water
• B. Caustic soda
• C. Soda ash
• D. Hydrocarbon oil

Hint:

For gas absorption, follow the rule of "like dissolves like":
Non-polar organic vapors (like benzene, toluene, xylene) require non-polar organic solvents (such as hydrocarbon wash oils) for effective physical absorption.
Hydrophilic gases (like ammonia or hydrogen chloride) are absorbed using water.

Answer 1

The Correct Option is D

Step 1: Understanding the Question:
The question asks for the specific solvent employed in coke-oven plants to absorb and recover benzene (and other light aromatics) from coke-oven gas.
Coke-oven gas is a valuable by-product of coal carbonization, and recovering aromatic hydrocarbons (BTX: benzene, toluene, xylene) is economically and environmentally necessary.

Step 2: Key Formula or Approach:
The removal of benzene from gas streams is carried out using gas absorption.
According to the principle of "like dissolves like," a non-polar organic solute like benzene (\(\text{C}_6\text{H}_6\)) will dissolve preferentially in a non-polar organic solvent.
Polar solvents like water or aqueous alkaline solutions (caustic soda, soda ash) have virtually zero affinity for hydrocarbons and are ineffective.

Step 3: Detailed Explanation:
Let us evaluate the options and the industrial recovery process:
- Coke-oven gas contains about \(1\%\) to \(1.5\%\) light oil, which consists primarily of benzene (\(60\%\text{-}70\%\)), toluene, and xylene.
- To recover this light oil, the gas is passed upward through an absorption column (scrubber) counter-currently to a falling solvent spray.
- The solvent used is a high-boiling petroleum fraction or coal-tar fraction known commercially as wash oil or hydrocarbon oil.
- Because the wash oil is a mixture of liquid hydrocarbons, it dissolves the benzene vapor readily through physical absorption governed by Raoult's law.
- Polar or inorganic aqueous solutions like water (Option A), caustic soda (Option B), and soda ash (Option C) cannot dissolve benzene due to the massive difference in solvent polarity.
- After absorption, the benzolized oil is sent to a steam stripper where the volatile benzene is distilled off, and the lean hydrocarbon oil is cooled and recycled back to the absorber.

Step 4: Final Answer
Therefore, the solvent used is hydrocarbon oil, which corresponds to option (D).