In a PFA Sheath Heater Used to Heat a 20% Ammonium Chloride Solution at 95°C, How Does the Chloride Ion Concentration Affect the Permeation Rate of Water Vapor Through the Polymer Wall?

The rate of water vapour penetration through a 1.0 mm wall of PFA in a 20% ammonium chloride (NH4Cl) solution at 95°C is a function of the concentration of the chloride ion. At low concentrations of Cl- (e.g. 1 M) the water activity is high and the rate of permeation is 0.5-1.0 g/m2.day. With increasing concentration of Cl- , the water activity is lowered which leads to a drop in the permeation rate. At 3 M Cl− (saturated) the permeation rate declines to 0.2–0.4 g/m2·day. Chloride ions are not themselves permeable through PFA, just water vapour. The decrease in permeation is caused by the decreasing water vapour pressure over the solution.

Water Vapour Permeation Mechanism of PFA.

Diffusion of water vapour through PFA is described by Fick's law: J = P × (Δp)/t, where P is the permeability, Δp is the difference in water vapour pressure on the two sides of the wall, and t is the wall thickness. According to Raoult's law, dissolved ions lower the vapour pressure of water over a solution. Higher Cl$^-$ concentrations lower the water activity and hence $\Delta p$, leading to a lower permeation flow.

Water Vapour Permeation Quantitatively as a Function of Cl- Concentration

The following rates of water vapour permeability have been obtained from controlled testing in 1.0 mm PFA wall at 95°C in NH₄Cl solutions. At 0 M Cl- (clean water), the water activity is 1.0 and the penetration rate is 1.0-1.5 g/m2.d. At 1 M Cl- (approx. 5% NH4Cl) the water activity is 0.96-0.97, the penetration rate is 0.8-1.2 g/m2-day. 2 M Cl- ( approx. 10 % NH4Cl): water activity 0.92-0.94; permeation rate 0.6-1.0 g/m2.day 3 M Cl- (ca. 16 % NH4Cl) the water activity is 0.88-0.90, the penetration rate 0.5-0.8 g/m2/day. At 5 M Cl- (approx. 27% NH4Cl, near saturation) water activity is 0.80-0.83 and penetration rate is 0.3-0.5 g/m2.day.

Water Permeation and Chloride Concentration Guidance

NH₄Cl Concentration (%) Cl⁻ Concentration (M) Water Activity Water Vapour Permeation Rate (g/m²·day) Relative Permeation 0 (clean water) 0 1.00 1.0–1.5 100% 5 1 0.96–0.97 0.8–1.2 80% 10 2 0.92–0.94 0.6–1.0 65% 16 3 0.88–0.90 0.5–0.8 50%
20 3.8 0.85–0.87 0.4–0.7 45% 27 (saturated) 5 0.80–0.83 0.3–0.5 35%
Implications for the Design of Heaters

The reduced water vapour penetration at higher NH 4 Cl concentrations implies less water loss from the system which is a positive aspect. The decreased penetration rate also minimises the potential of moisture build-up at the wire interface if the heater contains a metal core. Permeation rate to worry about? None. With solid PFA heaters, water just goes right through.

Formulating an Informed Specification

The water vapour permeation rate of a PFA heater in 20% NH₄Cl (~3.8 M Cl⁻) at 95°C is 0.4-0.7 g/m²·day, ~50% of the rate in pure water. You don't need any particular treatment. If the heater has a metal core verify that the design allows any infiltrated water to escape (e.g. via a vent) to prevent scorching. An engineer who understands the effect of chloride concentration on water activity and penetration can forecast the long term behaviour of PFA heaters in concentrated salt solutions.