air permeability specific surface
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The air permeability specific surface of a powder material is a single-parameter measurement of the fineness of the powder. The
specific surface Specific surface area (SSA) is a property of solids defined as the total surface area of a material per unit of mass, (with units of m2/kg or m2/g) or solid or bulk volume (units of m2/m3 or m−1). It is a physical value that can be used to dete ...
is derived from the resistance to flow of air (or some other gas) through a porous bed of the powder. The SI units are m2·kg−1 ("mass specific surface") or m2·m−3 ("volume specific surface").


Significance

The particle size, or fineness, of powder materials is very often critical to their performance. Measurement of air permeability can be performed very rapidly, and does not require the powder to be exposed to
vacuum A vacuum is a space devoid of matter. The word is derived from the Latin adjective ''vacuus'' for "vacant" or "void". An approximation to such vacuum is a region with a gaseous pressure much less than atmospheric pressure. Physicists often dis ...
or to gases or vapours, as is necessary for the BET method for determination of
specific surface area Specific surface area (SSA) is a property of solids defined as the total surface area of a material per unit of mass, (with units of m2/kg or m2/g) or solid or bulk volume (units of m2/m3 or m−1). It is a physical value that can be used to det ...
. This makes it both very cost-effective, and also allows it to be used for materials which may be unstable under vacuum. When a powder reacts chemically with a liquid or gas at the surface of its particles, the specific surface is directly related to its rate of reaction. The measurement is therefore important in the manufacture of many processed materials. In particular, air permeability is almost universally used in the
cement A cement is a binder, a chemical substance used for construction that sets, hardens, and adheres to other materials to bind them together. Cement is seldom used on its own, but rather to bind sand and gravel ( aggregate) together. Cement mix ...
industry as a gauge of product fineness which is directly related to such properties as speed of setting and rate of strength development. Other fields where air permeability has been used to determine specific surface area include: * Paint and pigments * Pharmaceuticals * Metallurgical powders, including sintered metal filters. In some fields, particularly powder metallurgy, the related Fisher number is the parameter of interest. This is the equivalent average particle diameter, assuming that the particles are spherical and have uniform size. Historically, the Fisher number was obtained by measurement using the ''Fisher Sub-sieve Sizer'', a commercial instrument containing an air pump and pressure regulator to establish a constant air flow, which is measured using a flowmeter. A number of manufacturers make equivalent instruments, and the Fisher number can be calculated from air permeability specific surface area values.


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 The Kozeny–Carman equation (or Carman–Kozeny equation or Kozeny equation) is a relation used in the field of fluid dynamics to calculate the pressure drop of a fluid flowing through a packed bed of solids. It is named after Josef Kozeny and ...
: :S=\cfrac\sqrt 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.


Lea and Nurse method

The second of these was developed by Lea and Nurse. The bed is 25 mm in diameter and 10 mm thick. The desired porosity (which may vary in the range 0.4 to 0.6) is obtained by using a calculated weight of sample, pressed to precisely these dimensions. The required weight is given by: :M=\tfrac\,d^2l\rho\,(1-\epsilon\,) A flowmeter consisting of a long capillary is connected in series with the powder bed. The pressure drop across the flowmeter (measured by a
manometer Pressure measurement is the measurement of an applied force by a fluid (liquid or gas) on a surface. Pressure is typically measured in units of force per unit of surface area. Many techniques have been developed for the measurement of pressur ...
) is proportional to the flowrate, and the proportionality constant can be measured by direct calibration. The pressure drop across the bed is measured by a similar manometer. Thus the required pressure/flow ratio can be obtained from the ratio of the two manometer readings, and when fed into the Carman equation, yields an "absolute" value of the air permeability surface area. The apparatus is maintained at a constant temperature, and dry air is used so that the air viscosity can be obtained from tables.


Rigden method

This was developed in the desire for a simpler method. The bed is connected to a wide-diameter u-tube containing a liquid such as
kerosene Kerosene, paraffin, or lamp oil is a combustible hydrocarbon liquid which is derived from petroleum. It is widely used as a fuel in aviation as well as households. Its name derives from el, κηρός (''keros'') meaning "wax", and was regi ...
. On pressurizing the space between the u-tube and the bed, the liquid is forced down. The level of liquid then acts as a measure of both pressure and volume flow. The liquid level rises as air leaks out through the bed. The time taken for the liquid level to pass between two pre-set marks on the tube is measured by stop-watch. The mean pressure and mean flowrate can be derived from the dimensions of the tube and the density of the liquid. A later development used
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in the u-tube: because of mercury's greater density, the apparatus could be more compact, and electrical contacts in the tube touching the conductive mercury could automatically start and stop a timer.


Blaine method

This was developed independently by R L Blaine of the American National Bureau of Standards, and uses a small glass kerosene manometer to apply suction to the powder bed. It differs from the above methods in that, because of uncertainty of the dimensions of the manometer tube, absolute results can't be calculated from the Carman equation. Instead, the apparatus must be calibrated, using a known standard material. The original standards, supplied by NBS, were certified using the Lea and Nurse method. Despite this shortcoming, the Blaine method is now by far the most commonly used for
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materials, mainly because of the ease of maintenance of the apparatus and simplicity of the procedure.e.g.
ASTM ASTM International, formerly known as American Society for Testing and Materials, is an international standards organization that develops and publishes voluntary consensus technical standards for a wide range of materials, products, systems, an ...
Standard Test Method C 204


See also

*
Klinkenberg correction In petroleum engineering, a Klinkenberg correction is a procedure for calibration of permeability data obtained from a minipermeameter device. A more accurate correction factor can be obtained using Knudsen correction. When using nitrogen gas fo ...


References

{{DEFAULTSORT:Air Permeability Specific Surface Chemical engineering Particle technology