Internal Compression Air Separation Units: Economic Benefits & Selling Points

Internal Compression Air Separation Units: Economic Benefits & Selling Points

The internal compression air separation unit represents a fundamental rethinking of how cryogenic products are delivered. Conventional external compression plants draw low-pressure gas from the distillation column and compress it externally. Internal compression turns this sequence around: it pumps liquid oxygen from the bottom of the column to high pressure (using a liquid oxygen pump), then vaporises the pressurised liquid in the main heat exchanger using high-pressure air as the heat source. This small change in process sequence delivers profound advantages in safety, reliability, and product flexibility-making internal compression the process of choice for large-scale installations, high-pressure applications, and operations with stringent safety requirements.

How Internal Compression Works

The front end of an internal compression plant follows the familiar pattern: atmospheric air is compressed, purified, and cooled to cryogenic temperatures. However, while external compression splits the air flow in a straightforward manner, internal compression introduces an air booster compressor downstream of the purification system. A portion of the purified air is further compressed to high pressure (typically 70–110 bar, depending on product requirements), while the remainder continues to the cold box at moderate pressure. Inside the cold box, the distillation column separates air into its components as usual. But rather than drawing low-pressure oxygen gas from the column, internal compression extracts liquid oxygen from the bottom of the upper column. This liquid oxygen is pressurised to the required delivery pressure by a cryogenic liquid oxygen pump-typically a multi-stage centrifugal pump with variable frequency drive for precise flow control. The pressurised LOX then passes through the main heat exchanger, where it is vaporised by heat exchange with the high-pressure air stream. The resulting high-pressure oxygen gas is delivered directly to the end user without requiring an external oxygen compressor.

 

Economic Benefits

Lower Capital Investment in Certain Markets. While the internal compression process carries higher overall equipment costs in some markets due to the need for air boosters and specialised heat exchangers, its comparative economics shift dramatically depending on procurement location. In overseas markets where oxygen turbine compressors are extremely expensive-typically three times the cost of comparable domestic units-internal compression becomes significantly more cost-effective. Under these conditions, the combination of an air booster (priced at approximately 1.5 times a domestic turbine oxygen compressor) and a cryogenic liquid oxygen pump (negligible additional cost) often delivers substantial total capital savings over the external compression alternative. Internal compression plants also spend approximately 9% less on capital than externally compressed ones in some configurations, particularly for high-pressure oxygen filling applications.

Energy Efficiency at Larger Scales. While internal compression consumes 7–17% more power than external compression for the same oxygen output, the disparity narrows when accounting for total system optimisation. For large-scale air separation units-production scales of 3,000 Nm³/h and above-the energy penalty becomes acceptable in exchange for the operational benefits of internal compression. Optimisation studies indicate that when 27–29% of total air intake is compressed between 100 and 110 bar, the levelised cost of production for internal compression plants can be minimised, closing the gap with external compression economics at high volumes.

Higher Liquid Production Capacity. Internal compression configurations produce significantly more liquid product than their external compression counterparts. A conventional external compression ASU typically yields up to 10% of total output as liquid products, whereas internal compression can produce up to 20% liquid output, and specialised designs can achieve even higher liquid yields. When liquid oxygen production exceeds 8% of total output, the energy consumption advantage of the internal compression process becomes significant. For operators serving markets where liquid product pricing commands a premium, this additional liquid capacity directly enhances revenue generation.

 

Key Selling Points

Superior Safety for Oxygen Service. This is the defining advantage of internal compression. In external compression plants, the greatest hazard is accumulation of acetylene and other hydrocarbons in the main condenser of the distillation column. Internal compression largely eliminates this risk because liquid oxygen is continually drawn from the column and pumped away, preventing the localised concentration of hydrocarbons that can lead to explosive reactions. Additionally, pumping liquid oxygen remotely and vaporising it in the main heat exchanger-rather than compressing gaseous oxygen-removes the most dangerous element of the process: high-pressure gaseous oxygen handling. For applications in the chemical, petrochemical, and electronics industries, where safety requirements are exceptionally rigorous, internal compression is the preferred-and sometimes mandated-technology.

Flexible Pressure Capability. External compression processes are limited by the practical capabilities of oxygen compressors, which become increasingly difficult to manufacture and operate at pressures above approximately 3.0 MPa (30 bar). Internal compression has no such limitation. Liquid oxygen pumps can readily handle pressures up to 150 bar (3,000 psi) and beyond, making internal compression the only practical option for high-pressure oxygen requirements such as cylinder filling station applications.

High Reliability Through Redundancy. Internal compression plants typically install two or more liquid oxygen pumps in a duty/standby configuration. If the primary pump fails or requires maintenance, the standby pump can continue operations without shutting down the air separation unit [21†L17-L18]. This redundancy is difficult to achieve economically with external compression, where a single large oxygen turbine compressor often represents a single point of failure. The combination of pump redundancy and the inherent safety of the LOX pumping process delivers overall plant availability significantly higher than equivalently sized external compression plants.

Advanced Control and Smooth Operation. Modern internal compression plants incorporate frequency-controlled variable-speed drives on liquid oxygen pumps, enabling precise flow adjustment without the inefficiencies of throttling or venting. This capability is particularly valuable for users with variable oxygen demand profiles, as the plant can match production to consumption smoothly and continuously, rather than operating in a series of load/unload cycles that accelerate equipment wear and increase energy consumption.

Compact Footprint for High-Pressure Delivery. By eliminating the need for a large oxygen compressor and its associated building, cooling, and safety systems, internal compression plants can deliver high-pressure product from a significantly smaller footprint than external compression alternatives. The air booster can sometimes be integrated into the compressor train, replacing multiple machines and further reducing space requirements. For facilities with constrained real estate-urban industrial sites, offshore platforms, and certain petrochemical complexes-this space advantage alone can justify the selection of internal compression.

 

Where Internal Compression Excels

The internal compression air separation unit is not the universal answer. For small-scale applications or operations where oxygen pressure requirements are moderate (below 1.5 MPa or 15 bar), the lower capital cost and proven simplicity of external compression may be more attractive. However, for large-scale industrial gas production (3,000 Nm³/h and above), high-pressure oxygen delivery (above 3.0 MPa or 30 bar), high liquid output requirements (above 8% of total production), and applications where safety requirements are paramount, internal compression delivers unmatched value. In the coal chemical, petrochemical, electronics, and medical gas industries, where reliability, safety, and product flexibility are non-negotiable, the internal compression ASU has become the process standard-and for compelling economic and operational reasons.