Internal Compression Vs. External Compression – Choosing The Right ASU Configuration

Internal Compression vs. External Compression – Choosing the Right ASU Configuration

When selecting an air separation unit for an industrial application, one of the fundamental decisions is whether to specify an external compression or internal compression configuration. Each approach has distinct advantages and trade-offs that must be evaluated based on the specific requirements of the application.

 

External Compression: The Traditional Approach

In an external compression ASU, the distillation column produces gaseous oxygen and nitrogen at relatively low pressure-typically near atmospheric. These products are then compressed to the customer's required delivery pressure using external compressors. The oxygen compressor handles gaseous oxygen, raising its pressure to the specified level for pipeline delivery or further processing.

The external compression configuration has been the industry standard for decades. Its primary advantage is operational flexibility: the compression ratio can be adjusted to meet varying delivery pressures without impacting the distillation process. Additionally, external compression plants tend to have lower initial capital costs for smaller capacities, as the compression equipment is separate from the cold box.

However, external compression presents significant safety considerations. Gaseous oxygen at high pressure is a potent oxidizer; any contamination-particularly oil or grease-can lead to combustion or explosion. The oxygen compressor must be meticulously maintained with strict oil-free operation protocols. Furthermore, external compression requires the main condenser to operate with high oxygen concentrations in the liquid phase, potentially increasing the risk of hydrocarbon concentration if proper purging procedures are not followed.

 

Internal Compression: The Modern Alternative

Internal compression represents a fundamentally different approach-. Instead of compressing gaseous oxygen, the ASU produces liquid oxygen directly from the distillation column. This liquid oxygen is pumped to high pressure using cryogenic liquid pumps-a relatively safe operation, as liquid oxygen at cryogenic temperatures poses minimal combustion risk. The high-pressure liquid oxygen is then vaporized and warmed in the high-pressure heat exchanger, emerging as high-pressure gaseous oxygen ready for delivery.

The internal compression flow can be further categorized based on the air boost configuration:

 

Single-pump internal compression: A single liquid oxygen pump provides the required pressure.

 

Dual-pump (or multi-pump) internal compression: Multiple pumps in series achieve very high delivery pressures.

 

Air-boosted internal compression: A portion of the feed air is boosted to high pressure to provide the heat for vaporizing the liquid oxygen.

 

Nitrogen-boosted internal compression: Nitrogen is used as the heat source for oxygen vaporization.

The expander air can be fed either to the upper column or the lower column, each configuration offering different operational characteristics.

 

Advantages of Internal Compression

The internal compression configuration offers several compelling advantages:

 

Enhanced Safety: By eliminating the high-pressure gaseous oxygen compressor, internal compression removes the single greatest fire and explosion hazard in the ASU-. The oxygen remains in liquid form until it reaches the required pressure, and the vaporization occurs within the cold box where oxygen concentrations are carefully controlled.

 

Reduced Hydrocarbon Risk: Internal compression requires significant liquid oxygen extraction from the main condenser, which continuously removes potential hydrocarbon contaminants that might otherwise accumulate. This dilution effect substantially reduces the risk of hydrocarbon concentration reaching dangerous levels.

 

Lower Maintenance: Cryogenic liquid pumps have fewer moving parts and operate at lower temperatures than gaseous oxygen compressors, resulting in reduced maintenance requirements and longer service intervals.

 

Higher Efficiency for High-Pressure Applications: For applications requiring oxygen at pressures above approximately 3.0 MPa, internal compression can achieve higher overall efficiency than external compression, as the energy required for liquid pumping is significantly less than the energy required for gaseous compression.

 

Considerations for Internal Compression

Internal compression is not without its challenges. The high-pressure heat exchanger must withstand substantial pressure differentials while maintaining the required heat transfer performance. The air booster compressor adds complexity and capital cost. Additionally, internal compression plants typically have longer startup times and are more sensitive to variations in product demand.

 

Making the Choice

The selection between internal and external compression depends on several factors:

Delivery pressure: For low-pressure applications (below 1.5 MPa), external compression may be more economical. For high-pressure applications (above 3.0 MPa), internal compression often provides superior efficiency and safety.

 

Product mix: Plants producing significant liquid products benefit from internal compression, as liquid extraction is already part of the process.

 

Safety requirements: Facilities with stringent safety regulations or limited experience with high-pressure oxygen handling may prefer internal compression.

 

Capital vs. operating cost: External compression typically has lower capital cost but higher maintenance and energy costs; internal compression has higher capital cost but lower operating costs.

 

Understanding these trade-offs enables informed decision-making when specifying ASU equipment for new projects or expansions.