Unit operations overview: Which of the following is NOT categorized as a mechanical operation in chemical engineering?

Introduction / Context:
Chemical engineering groups practical steps into unit operations such as mechanical handling (crushing, screening), fluid flow, heat transfer, and mass transfer. Knowing which category a process belongs to helps in choosing the right design equations and scale-up rules.

Given Data / Assumptions:

• Mechanical operations manipulate size, shape, and phase dispersion using mechanical energy.
• Mass transfer operations involve species transfer across phases driven by concentration differences.
• Heat/momentum transfer may accompany either category but do not define it.

Concept / Approach:

Filtration, agitation, and size enlargement rely primarily on mechanical work and equipment geometry. Humidification is a gas–vapor mass transfer operation where moisture content changes by diffusion and interfacial transfer, typically analyzed with psychrometrics and HTU/NTU methods rather than purely mechanical balances.

Step-by-Step Solution:

Verification / Alternative check:

Textbook classifications place humidification in gas–vapor contacting, alongside dehumidification and drying (with mass transfer cores), not in mechanical operations.

Why Other Options Are Wrong:

Size enlargement / Filtration / Agitation: Classic mechanical operations driven by equipment-imparted mechanical energy and force balances.

Common Pitfalls:

Equating “moving equipment” with “mechanical operation.” Some mass transfer equipment also moves, but the governing mechanism is species transfer.

Final Answer:

Humidification (gas–vapor mass transfer)