I would like to clarify the difference between three distinctive terms which are often misunderstood and used instead of each other:
1. Pure Water Flux:
Pure water flux is the volume of water passes through the surface area of the membrane per time.
J=V/(A.Δt)
2. Permeance:
Permeance is the volume of water passes through the surface area of the membrane per time per transmembrane pressure.
PM=V/(A.Δt.Δp) or PM=J/Δp
3. Permeability:
Permeability is the volume of water passes through the thickness of membrane per time per transmembrane pressure.
P=V.l/(A.Δt.Δp) or P=PM.l or P=J.l/Δp
where V represents the volume, A is the membrane effective surface area, Δp and Δt are transmembrane pressure and sampling time, and l is the thickness of membrane selective toplayer.
Indeed they are. I have in fact seen permeability and permeance used as if they were the same concept, both in scientific papers and in industry.
I guess in liquid separations (i. e. Nanofiltration) we usually use permeance, while in gas separations many researchers use permeability in Barrers.
Yes Carlos,
It's also good to add that for liquid separations, might be better to report permeance as pressure effect is already included in the parameter compared to the flux.
Interesting discussion here for an entrant into membrane technology.
Just to be clear, it does seem that the value of permeability is relatively more representative in a spongy, relatively symmetric membrane than in an asymmetric membrane with macrovoids. Right?
Thanks Sithembela Zikalala
Basically, membranes can be divided into porous and non-porous membranes.
Non-porous membranes are then categorised into symmetric and asymmetric membranes.
The term "permeability" matters when membrane possesses a top-layer.
Unlike symmetric membranes, the structure changes across the thickness for asymmetric ones (i.e. a top-layer over macroporous sub-layer).
Thus, it can be quoted that the permeability is used for asymmetric membranes.
- Badges
- Science topic
- Similar topics
- Physical Sciences
- Materials Science
- Materials Chemistry
- Polymer Chemistry