Gas liquefaction — The Linde (Joule–Thomson) process primarily achieves cooling by which mechanism in its core cycle?

Introduction:
Large-scale cryogenic air separation and natural gas liquefaction often begin with the classical Linde cycle. Engineers must differentiate between Joule–Thomson (throttling) cooling and work-producing expansion (Claude cycle) to select appropriate equipment and predict temperatures.

Given Data / Assumptions:

• Linde process uses high-pressure gas, regenerative heat exchange, and throttling valve(s).
• No external shaft work is extracted during the core expansion.
• Gas has a positive Joule–Thomson coefficient at the conditions considered.

Concept / Approach:
In the Linde process, the gas is compressed, precooled in a countercurrent heat exchanger by cold return stream, and then expanded through a throttling valve. Throttling is an isenthalpic process (h ≈ constant) that leads to a temperature drop when the Joule–Thomson coefficient is positive. Recirculating the cold stream regeneratively cools the incoming high-pressure gas, progressively lowering the temperature until liquefaction occurs.

Step-by-Step Solution:
Pressurize gas → cool in recuperative exchanger.Throttle through valve: isenthalpic drop, temperature decreases.Return cold low-pressure stream to precool the high-pressure feed; repeat to reach cryogenic temperatures.

Verification / Alternative check:
Claude cycle employs an expander (work-producing, isentropic-ish) in addition to or instead of pure throttling. Linde’s defining feature is throttling with strong regenerative cooling.

Why Other Options Are Wrong:

• (b) Describes Claude or expander-based processes.
• (c) Isobaric cooling alone cannot reach very low temperatures without expansion.
• (e) Flashing is a consequence after throttling, not the sole cooling mechanism.

Common Pitfalls:
Assuming “expansion” always means work extraction; mixing up Linde and Claude cycles.

Final Answer:
Throttling expansion through a valve (Joule–Thomson expansion)