Air Permeability Specific Surface - Methods

Methods

Measurement consists of packing the powder into a cylindrical "bed" having a known porosity (i.e. volume of air-space between particles divided by total bed volume). A pressure drop is set up along the length of the bed cylinder. The resulting flow-rate of air through the bed yields the specific surface by the Kozeny–Carman equation:

where:

S is specific surface, m2·kg-1
d is the cylinder diameter, m
ρ is the sample particle density, kg·m-3
ε is the volume porosity of the bed (dimensionless)
δP is the pressure drop across the bed, Pa
l is the cylinder length, m
η is the air dynamic viscosity, Pa·s
Q is the flowrate, m3·s-1

It can be seen that the specific surface is proportional to the square root of the ratio of pressure to flow. Various standard methods have been proposed:

  • Maintain a constant flowrate, and measure the pressure drop
  • Maintain a constant pressure drop, and measure the flowrate
  • Allow both to vary, deriving the ratio from the characteristics of the apparatus.

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