Separation methods for liquid solutions: Which of the following unit operations cannot be used to separate the components of an existing liquid–liquid or liquid solution mixture?\r (Select the operation that is not a solution-separation method.)

Introduction:
This question tests your understanding of which mass-transfer or phase-change operations are suitable for separating components already present in a liquid solution. While several unit operations are routinely used to split solutes or solvent from a liquid mixture, others are designed for gas–liquid systems and therefore are not appropriate for separating a liquid solution into its constituents.

Given Data / Assumptions:

• The feed is a liquid solution or a liquid–liquid mixture whose components need to be separated.
• We compare common unit operations used in chemical engineering.
• We select the operation that does not achieve separation for a liquid solution.

Concept / Approach:
Separation of liquid solutions typically uses differences in volatility (distillation or evaporation), solubility at different temperatures (fractional crystallisation), or distribution of a solute between two immiscible liquids (liquid–liquid extraction). Gas absorption, by contrast, removes a gaseous solute from a gas stream into a liquid; it is not a method for splitting the components of an existing liquid solution.

Step-by-Step Solution:
List solution-separation methods: distillation/evaporation, crystallisation, liquid extraction.Identify absorption as a gas–liquid contact process, not a liquid-solution separation.Select the non-applicable method: absorption.

Verification / Alternative check:
Process design textbooks classify absorption under gas cleaning operations, whereas crystallisation, extraction (L–L), and evaporation belong to liquid-solution separations.

Why Other Options Are Wrong:

• Fractional crystallisation: Separates based on solubility differences.
• Liquid extraction: Transfers a solute between immiscible liquids.
• Evaporation: Removes solvent to concentrate or precipitate solute.
• Fractional distillation: Splits volatile components by relative volatility.

Common Pitfalls:
Confusing absorption (gas into liquid) with extraction (solute between two liquids). They are distinct operations with different applications.

Final Answer:
absorption