Can a PFA Heater Be Used in a Supercritical CO₂ Extraction Vessel Without Premature Failure?

Supercritical CO₂ (scCO₂) extraction vessels are run at pressures between 100 and 500 bar and temperatures between 40 and 100 °C. This service does not work for PFA. The predominant failure mode is not chemical attack, scCO2 is generally benign to fluoropolymers, but pressure-induced collapse and creep. PFA is weak in mechanical strength. At 100 bar (10 MPa) the hoop stress on a 25 mm OD, 2 mm wall PFA tube is σ = P × D / (2 × t) = 10 × 25 / 4 = 62.5 MPa. The tensile strength of PFA is 25-30 MPa at 40°C. The stress is two times as great as the strength. stress = 31 MPa-above yield even at 50 bar. PFA heaters are not able to resist the pressures required for scCO₂ extraction. Vessels 20 bar PFA may be suitable, but scCO2 extraction required > 80 bar to achieve solvating power. ScCO₂ vessels can also be heated by exterior jackets, internal metal heaters (stainless steel, Inconel) or cartridge heaters in thermowells. PFA is not advised.

PFA Pressure Ratings for CO₂ Service
The PFA tube has a maximum permissible working pressure (MAWP) of: P_max = 2 x σ_max x t / D For max = 12 MPa (design stress with safety factor at 40°C) t = 2 mm D = 25 mm P max = 2 12 2 / 25 = 48 / 25 = 1.92 MPa (19.2 bar). This is the maximum absolute for short time use. For continuous service derate by 50% P_max_continuous = 0.5 x 1.9 = 0.95 MPa (9.5 bar) scCO2 extraction at 80-500 bar (8-50 MPa) is 8 to 50 times better than PFA.

Theoretically the PFA tube could handle the force, but the end seals (where the heater penetrates the vessel) would leak. PFA flanges and compression fittings are rated up to a maximum of 5–10 bar. No PFA fittings are currently available that are rated for 100 bar CO₂ duty.

Modes of Failure in scCO₂
Pressure (bar) Hoop Stress at 25×2 mm (MPa)PFA Strength (MPa) Failure mode Time to failure 10 6.3 25 None (safe) >10,000 hours
Marginal (creep) 1,000–5,000 hours 12.5 20 25
30 18.8 25 Creep rupture 100-500 h
50 31.3 25 Immediate yield Hours 80 50 25 Burst Minutes 100 62.5 25 Instantaneous failure Seconds
At 80 bar the PFA tube will bloat and burst. The failure is dramatic and sudden: the PFA sheath ruptures and scCO₂ leaks through the heater core (if hollow) or the core is exposed. The vessel is sealed. If the heater explodes it produces a hole for the gas to escape, but the vessel is still intact. However the extraction process is destroyed and the heater needs to be replaced.

scCO2 permeation and swelling
At pressures exceeding 80 bar scCO2 diffuses through PFA at substantial rates. Due to plasticization, the CO₂ permeability coefficient in PFA at 40 °C and 100 bar is around 10–50× higher than at 10 bar. CO2 molecules diffuse into the amorphous areas of the polymer expanding it to 2-5%. Swelling also affects mechanical strength (20–30% less). The swollen PFA softens and creep accelerates. Even if it were possible to construct a PFA heater to resist 100 bar, the penetration would pressurise the internal annulus with CO 2. When depressurised the contained CO2 would expand, scorching or delaminating the PFA (see Article #93). This cycle of pressurization/depressurization (typical of batch extractions) is extremely harmful.

Alternative heating techniques for scCO₂
PFA heaters are not a feasible choice for scCO2 extraction. These approaches include:

Heating Method Standard Pressure Rating Advantages Disadvantages
Jacketed external vesselFull vessel rating (300 bar)No penetration, homogeneous warmthSlow answer. Expensive.
Internal metal heater (Inconel 625) Full ship freight rateCompact, fast respondingReactions can be catalysed by metal.
Thermowell (metal) cartridge heaterFull vessel rating Replaceable without opening vessel Lower heat transfer efficiency
Band heaters (vessel outside)Full vessel rating Simple, no vessel change neededFor small craft only
Recirculating loop with external heat exchangerFull ship ratingGood Control; ScalableNeeds a pump; complicated.
For vessels requiring internal immersion heating (to prevent dead zones, for example) use a metal heater (Inconel 625 or Hastelloy C-276) rated for the entire vessel pressure. These alloys are resistant to scCO₂ and any co-solvents (ethanol, methanol) employed in extractions. The heater element is commonly a mineral insulated (MgO) cable in a metal jacket. The pressure load is carried by the metal sheath, not the insulation.

Conclusion: PFA Heaters Should Be Avoided in scCO2 Extraction
PFA heaters cannot be used for supercritical CO₂ extraction (80-500 bar). Ballooning, rupture, burst, or rapid creep occurs because the hoop stress is 2-50x greater than the tensile strength of PFA. Even at pressures as low as 20–30 bar, PFA is mediocre and will fail within hundreds of hours by creep. Metal heaters (Inconel, Hastelloy) or external jacket heating must be specified for every scCO2 vessel operating above 10 bar. PFA heaters are not approved for ScCO2 performance below their rated pressure (usually 5-10 bar). The material is not intended for it, the fittings are not rated for it and the risks of failure (process disruption, vessel contamination, safety risk) are not worth trying. PFA is for use in atmospheric or low pressure chemical service. scCO₂ is a high pressure process plant. Match the stuff to the pressure. PFA and scCO₂ are immiscible. No PFA, external heat, metal The boat will win. The heater is gonna lose. Deserve it.