When selecting high-purity tubing for extraction systems, most engineers follow a simplistic rule: the higher the surface finish and the lower the Ra value, the better the pipe quality.

Electropolished (EP) tubing achieves a mirror-like surface with an Ra value below 0.05 μm, while mechanically polished (MP) tubing typically has an Ra value of ≤ 0.4 μm. On paper, EP tubing appears to be the undisputed superior option.

However, our years of field operation and pipe disassembly verification prove that the selection logic is far more complex under supercritical CO₂ working conditions.

We have reached a counter-intuitive conclusion: supercritical CO₂ has powerful solvation and carrying capacity, especially with the addition of co-solvents such as ethanol, which enables it to dissolve and carry pipe surface contaminants downstream. MP tubing has microscopic polishing textures that seem rough macroscopically but act as tiny storage micro-pockets at the micro level. These structures can stably trap trace amounts of sealing oil and particulate impurities, preventing them from being washed away by high-speed CO₂ flow.

In contrast, EP tubing has an ultra-smooth surface with no micro-pocket structures. Once contaminants adhere to the surface, high-speed CO₂ flow will spread them into a uniform thin film, making impurities more difficult to completely remove and clean.

This does not mean EP tubing is inferior. EP tubing boasts excellent corrosion resistance and dead-zone-free cleaning performance. Electropolishing removes surface stress layers and inclusions, forming a complete and stable passive film, making it the gold standard for aseptic biopharmaceutical production processes.

For supercritical extraction systems characterised by high pressure, high flow velocity and occasional solid particle entrainment, tubing selection should be based on installation position rather than simple surface finish parameters:

CO₂ feed pipelines before the extractor adopt EP tubing. The medium is clean at this stage, and the ultra-smooth surface of EP tubing effectively reduces pipeline pressure drop.

Pipelines from the extractor to separators adopt MP tubing. These pipelines carry extracts and fine solid particles, and the microscopic textures of MP tubing can buffer flow erosion and inhibit local pitting corrosion for better durability.

Equipment material selection should never rely solely on theoretical parameters. The core principle is to match materials with actual working conditions – this is the valuable experience we have summarised from numerous practical projects.