Why Is the Full Low-Pressure Process Ideal for Large & Medium-Sized ASU?

Why do large and medium-sized Air Separation Unit (ASU) overwhelmingly favor the full low-pressure process? Here’s a concise breakdown for industry peers:

1. Core Driver: Reduced Specific Energy Consumption (SEC)

Lowering operating pressure directly cuts oxygen /nitrogen /argon SEC. Unlike small ASU (operating pressure far higher than the lower column), the full low-pressure process aligns system pressure closely with the lower column’s working pressure. While lower pressure

 reduces expansion refrigeration capacity, large ASU offsets this via:

- Minimized cold loss (decreases with capacity scaling)

- Optimized heat exchange (smaller hot-end temperature differences reduce incomplete heat loss)

2. Turboexpander Efficiency Synergy

Low-pressure operation requires high-efficiency expanders for sufficient refrigeration under limited pressure differentials. For large ASU:

- Turboexpander efficiency improves with capacity (lower optimal rotational speed = better stability/energy recovery)

- Small ASU faces tradeoffs: Even 100,000+ rpm turboexpanders deliver low efficiency and high maintenance costs

3. Scale-Driven Process Suitability

Viability hinges on expansion air dynamics:

- Large/medium ASU: Expansion air accounts for a small share of total processed air. Post-refrigeration, most air re-enters distillation, maintaining high oxygen extraction rates.

- Small ASU: Low-pressure operation demands massive expansion air volumes (unable to fully participate in distillation), leading to poor oxygen recovery and higher SEC.

4. Technological Advancements Expand Applicability

Molecular sieve adsorption, booster-turbine integration, and advanced plate-fin heat exchangers have extended the process to smaller scales. Example: KDON-340/800 ASU (340 m³/h O₂, 800 m³/h N₂) with just 0.59 MPa compressor discharge pressure.

Why It Matters

The full low-pressure process is the gold standard for large/medium #ASU, balancing energy efficiency, stability, and cost-effectiveness. Critical for industrial gas production, coal chemical plants, and green hydrogen projects—where SEC and reliability directly impact ROI.