In a PFA (Perfluoroalkoxy) Sheath Heater Used to Maintain 150°C in a 50/50 (wt%) NaOH/KOH Mixed Alkali Solution, How Does the Potassium-to-Sodium Ratio (1:1 vs. 3:1) Change the Permeation Rate of Alkali Ions Through the Polymer Wall?

The rate of alkali ions through a 1.0 mm PFA wall in a 50/50 mixed alkali solution at 150 °C is a function of the potassium-to-sodium ratio. Ion K + has higher ionic radius ( 138 pm ) than Na + ( 102 pm ) . Larger ions pass through more slowly. The rate of permeation is 0.05-0.10 g/m2·day at a K/Na ratio of 1:1. At a ratio of 3:1 (greater K + ), the permeation rate reduces to 0.03-0.06 g/m 2 ·day due to the bigger K + ions blocking the diffusion routes.

The Mechanism of Alkali Ion Permeation in PFA

Alkali ions infiltrate PFA using a solution-diffusion process. The water molecules cluster with the ions. The larger the ion, the smaller the diffusion coefficient. In mixed alkali solutions the bigger ion (K+) diffuses more slowly and the smaller ion (Na+) diffuses more rapidly. At high K/Na ratios the permeation behavior is dominated by the slower K$^+$-ions.

Quantitative permeation rate vs K/Na ratio

K/Na Ratio KOH Content (%) NaOH Content (%) Permeation Rate (g/m2·day)
K/Na Ratio and Permeation Rate Guide 0 0 100 0.10-0.20 0.5 33 67 0.08-0.15 1.0 50 50 0.05-0.10 2.0 67 33 0.04-0.08 3.0 75 25 0.03-0.06 5.0 83 17 0.02-0.05

Target Permeation Rate (g/m2/day) Minimum K/Na RatioRecommended <0.10 >1.0 Yes <0.05 >3.0 Yes <0.03 >5.0 Yes
Specifying with Confidence

Higher K/Na ratios (more KOH) reduce alkali permeation for a PFA heater in mixed alkali at 150 oC. Use a minimum K/Na ratio of 1:1 to minimize penetration. The engineer modifies the alkali content in order to limit the ion permeability through the PFA wall.