Gas absorption with chemical reaction – identify the correct practical example Which of the following absorption operations is a classic example of gas absorption accompanied by a chemical reaction in the liquid phase?

Introduction / Context:
In gas–liquid operations, absorption may be purely physical (driven by solubility) or may be enhanced by a chemical reaction in the liquid phase. Correctly classifying examples helps in selecting packings, estimating heights of transfer units, and predicting rates via reaction–diffusion models.

Given Data / Assumptions:

• Sparingly soluble acid gas SO2 can react with alkaline absorbents.
• Ammonia is highly soluble in water without requiring reaction.
• Benzene (often called benzol in coke-oven gas practice) is absorbed physically into wash oil.

Concept / Approach:
Absorption with reaction occurs when the absorbed species participates in a rapid acid–base, complexation, or redox reaction in the liquid, increasing driving force and sometimes changing the controlling resistance. SO2 reacts with alkali (e.g., Na2CO3/NaOH) to form bisulfite/sulfite, which lowers the dissolved SO2 activity and promotes further uptake (enhancement). Ammonia–water absorption is mostly physical (though it forms hydrogen bonds), and benzene in wash oil is purely physical solubility-driven absorption.

Step-by-Step Solution:

Verification / Alternative check:
Design texts list SO2–alkali and CO2–alkali systems under chemically enhanced absorption with typical Hatta number analysis and enhancement factors > 1 compared to purely physical systems.

Why Other Options Are Wrong:

• Ammonia in water: Highly soluble but not a necessary chemical reaction system.
• Benzene in wash oil: Classic physical absorption into a nonpolar solvent.
• All of the above: Incorrect because only the alkaline SO2 system has obligatory chemical reaction.

Common Pitfalls:
Assuming “high solubility” implies reaction; solubility and chemical reaction are distinct mechanisms. Do not conflate absorption enhancement by reaction with mere heat of solution.

Final Answer:
SO2 in alkaline solution