Macropore-Size Distributions in Some Typical Porous Substances’ L. C. DRAKE
H. L. RITTER N. J.
AND
Socony-Vacuum Oil, Co., Paulsboro,
density of a porous filter plate may be 70% of its real density, and its surface area is only 1000 times its geometric area. The pores within a granule of porous material may also vary widely in abso1:te dimensions, from those readily seen by the naked eye (106 A. in diameter) t o those with diameters of only 4 or 5 A. It is obvious that the large pores contribute very little to the total surface area of the material. Wicke (If) estimated that the surface area contributed by pores averaging 20 A. in OROUS materials are characterized by two related quantidiameter in a sample of charcoal is 1500 sq. meters per gram, ties: a particle density which is appreciably lower than the while the surface area contributed by a practically equal volume real or true density of the material, and a surface area which is of pores averaging 20,000 A. in diameter is only 1.7 sq. meters. greater than the observable geometric surface area. Porous maThis ratio of surface area contribution by large and small pores terials differ widely in each of these characteristics; for example, will vary from one material to another, depending upon the porethe particle density of some diatomaceous earths is only 20% of size distribution. the true density, and some activated carbons have surface areas Although these large or macropores contribute little to the 500,000 times the external geometric area, while the particle total surface area, they may serve an important function as distributing arteries to the smaller or micropores (2,4, IO). T h e ‘Second part of paper on “Pore-Sine Distribution in Porous hlaterials.” First part is found on page 782. rate of adsorption or catalytic action is intimately tied up with the diffusion velocity of the reactants into the porous material and this is governed in part by the quantity and size distribution of the macropores. Table I. Variation of Macroporosity Measured at Low Pressures A rough value for the average diameter of the pores Volume of Mercury Removed from ( - ) and Forced into (+) Pores. CC. per Gram X 10-4 a t Mm. of Hg Pressure Indicated Of a may be from pore Material 550 660 860 1070 1300a 1460 1560 1660 volume and surface area measurements (6). Thus, as2 Coors Dorous Dhte -2.4 -2.2 - 1 2 -0 3 0 +0.9 + i . 6 f i . 8 suming uniform cylindrical pores we have: Macropore-size distributions in typical porous substances have been measured, using the pressure porosimeter described in the previous paper. Substances investigated include fuller’s earth, diatomaceous earths, silica-alumina gels, flint quartz, porous iron, activated clay pellets and porous desiccants.
,P
6 Activaied clay -3.3 -2.7 -1.3 -0;3 0 5 Fuller’s earth -10 -3.4 -1.8 0 -56 -37 -22 3 Diatomaceousearth -10 0 -150 -66 16 TypeMfrittedglass -300 -22 0 Atmospheric pressure plus mercury height in dilatometer.
...
Table Material
11.
Mesh Size (Tyler)
Surface Areas
Density Particleb Real Gram/cc. Clay Type 0.68 1.111 2.614 0.81 1.242 2.495 0.55 0 . 8 6 0 2.660 0.32 0.631 2.265 0.29 0.470 2.327 Bulk
4-6 4-6 5-7 4-6 4-8
223 147 129 4.2
