Why Combine Membrane Separation with Cryogenic Purification for SF₆ Recovery?

Introduction

Recovering SF₆ from GIS, GIL, circuit breakers, and other high-voltage equipment is increasingly important for both gas reuse and emission reduction. However, recovering SF₆ is not simply a matter of collecting the gas. When SF₆ has been mixed with nitrogen, air, or other gases during equipment maintenance, transportation, or gas replacement, the recovered gas may require further separation and purification before it can be reused.

Different separation technologies can be applied to SF₆-containing gas mixtures, including membrane separation and low-temperature separation. Each technology has its own advantages and limitations.

For this reason, KSTONE has developed a hybrid approach that combines membrane-assisted SF₆ recovery with ultra-low-temperature purification.

Rather than relying on one separation principle alone, the two technologies perform complementary functions: cryogenic purification provides the primary high-purity separation, while membrane separation helps recover residual SF₆ from the remaining gas.

Why Is SF₆ Separation More Challenging Than Simple Gas Recovery?

The composition of recovered gas can vary significantly depending on the application.

During GIS or GIL maintenance, for example, SF₆ may be recovered together with nitrogen or air. In some applications, the gas mixture may contain a relatively high concentration of SF₆, while in others, the SF₆ concentration can be much lower.

This creates two different requirements.

First, the process needs to concentrate and purify SF₆ to a level suitable for further use.

Second, the process needs to minimize the amount of SF₆ remaining in the non-SF₆ gas stream.

These two objectives are related, but they are not exactly the same.

A technology that is effective at enriching SF₆ is not necessarily the most effective technology for achieving final high purity. Conversely, a process designed for high-purity separation may not be the most efficient way to recover trace amounts of SF₆ from a large volume of residual gas.

This is where a combined process becomes valuable.

What Can Membrane Separation Do?

Membrane separation uses differences in gas permeability to selectively transport certain gas components through a membrane.

For SF₆-containing mixtures, membrane technology can be used to enrich and recover SF₆, particularly when the gas stream contains a relatively low concentration of SF₆.

One of the advantages of membrane separation is that it can operate continuously without requiring a conventional phase-change separation process. It can therefore serve as an effective recovery stage for residual SF₆.

However, membrane separation has practical limitations when it is expected to perform the entire purification process by itself.

Achieving both extremely high SF₆ purity and very low SF₆ concentration in the remaining gas can require multiple membrane stages, additional compression, and more complex process control.

Therefore, KSTONE does not treat membrane separation as the sole purification mechanism.

Instead, it is used where it provides the greatest value: recovering SF₆ that remains in the gas stream after the primary purification stage.

Why Use Ultra-Low-Temperature Purification?

The physical properties of SF₆ are significantly different from those of gases such as nitrogen and oxygen.

At sufficiently low temperatures and under controlled pressure conditions, SF₆ can undergo a phase change while other components remain in a different physical state.

This difference in phase behavior provides another mechanism for separating SF₆ from other gases.

KSTONE utilizes this principle through ultra-low-temperature purification.

Instead of depending entirely on differences in membrane permeability, the process takes advantage of the different phase-change characteristics of the gas components.

The result is a separation mechanism particularly suitable for obtaining a high concentration of SF₆.

In other words:

Membrane separation uses differences in permeability.
Cryogenic purification uses differences in physical phase behavior.

These are fundamentally different separation principles, which is why they can complement each other.

Why Combine the Two Technologies?

The key idea behind KSTONE’s hybrid technology is not to make membrane separation and cryogenic purification perform the same job.

Instead, each technology is assigned a specific role.

1. Cryogenic purification for primary separation

The ultra-low-temperature stage performs the primary purification of the SF₆-containing gas.

By utilizing the different phase-change characteristics of SF₆ and other gas components, SF₆ can be separated and concentrated to a high purity level.

This stage is therefore responsible for the main purification performance.

2. Membrane separation for residual SF₆ recovery

Even after the primary purification process, the remaining gas may still contain a small amount of SF₆.

Instead of allowing this SF₆ to remain in the residual gas stream, KSTONE uses membrane-assisted recovery to further capture and concentrate the remaining SF₆.

The recovered SF₆ can then be returned to the recovery process.

3. Reduced SF₆ loss

The combination of the two technologies creates a recovery loop.

The cryogenic stage focuses on high-purity SF₆ separation, while the membrane stage helps recover SF₆ that would otherwise remain in the residual gas.

This approach can reduce SF₆ loss and improve the overall gas recovery efficiency of the system.

A Different Way to Look at the Process

The hybrid technology can be understood through a simple division of responsibilities:

Mixed SF₆-containing gas

↓

Primary purification

Ultra-low-temperature phase-change separation

↓

High-purity SF₆

Residual gas containing a small amount of SF₆

↓

Membrane-assisted SF₆ recovery

↓

Recovered SF₆ returned to the process

The important point is that the membrane is not simply placed in front of the cryogenic system as an additional generic filter.

It is integrated into the overall recovery strategy to address a specific engineering challenge: recovering residual SF₆ after the primary purification stage.

Why Not Use Membrane Separation Alone?

A single technology rarely provides the optimum solution for every stage of gas separation.

For SF₆ recovery, membrane separation can provide useful enrichment and recovery performance, but depending on the gas composition and required product purity, achieving very high SF₆ purity through membrane separation alone may require additional stages and process equipment.

Cryogenic purification, on the other hand, provides a powerful physical mechanism for separating SF₆ based on its phase behavior, but the residual gas leaving the primary separation stage may still contain recoverable SF₆.

The hybrid approach addresses these two limitations from opposite directions.

Cryogenic purification focuses on product purity.

Membrane-assisted recovery focuses on minimizing residual SF₆ loss.

Together, they create a more comprehensive recovery strategy.

From SF₆ Recovery to SF₆ Reuse

The ultimate objective of SF₆ recovery is not simply to collect the gas.

The recovered gas should be processed, tested, and managed according to its actual composition and the requirements of the intended application.

With an appropriately designed recovery and purification system, recovered SF₆ can be concentrated to a high purity level. KSTONE’s system is designed to achieve up to 99.99% SF₆ purity under specified operating conditions.

After purification, the gas can be stored and subsequently evaluated for reuse according to the relevant gas quality requirements.

This changes the role of an SF₆ recovery system from a simple waste-gas collection device into a gas recovery and reuse system.

KSTONE’s Hybrid Purification Concept

KSTONE’s approach can therefore be summarized in three words:

Separate. Recover. Reuse.

• Separate: Use ultra-low-temperature phase-change purification to achieve the primary separation of SF₆ from other gases.
• Recover: Use membrane-assisted technology to recover residual SF₆ from the remaining gas stream.
• Reuse: Concentrate and purify the recovered SF₆ for subsequent storage, testing, and potential reuse.

This combination allows each technology to perform the task for which it is most suitable.

Rather than asking one technology to solve every separation problem, KSTONE integrates different physical separation principles into one engineered recovery system.

Conclusion

The challenge of SF₆ recovery is not simply how to recover gas from electrical equipment. The greater engineering challenge is how to separate SF₆ efficiently, obtain high-purity gas, and minimize SF₆ remaining in the residual gas stream.

Membrane separation and cryogenic purification address different parts of this challenge.

Ultra-low-temperature purification provides the primary high-purity separation by utilizing differences in phase-change behavior.

Membrane-assisted separation provides an additional recovery stage for residual SF₆, helping reduce gas loss after the primary purification process.

By combining these two technologies, KSTONE creates a hybrid approach designed around both SF₆ purity and overall recovery efficiency.

For large-scale GIS and GIL applications, this technology provides a pathway from conventional SF₆ gas recovery toward a more complete concept of SF₆ recovery, purification, and reuse.

KSTONE — Engineering Better SF₆ Recovery.