Analysis and evaluation of permeability techniques for characterizing fine particles Part II. Measurement of specific surface area

Abstract

The performance of both steady-state and transient permeameters has been evaluated. A number of standard powders was chosen and their surface areas determined by gas adsorption techniques for comparison with surface areas obtained using permeameters. The powders were chosen to cover a wide range o density (0.92 – 18.7 g/cm 3) and surface area (0.06 – 350 m 2/g). The effect of particle shape, i.e. needles, spherical particles, flakes and irregularly shaped particles on surface area determined by permeametry has also been studied. A linear relationship between BET surface area and that obtained by using a simple U-tube transient flow permeameter has been obtained over a wide range of particle size and macroporosity. The Fisher Subsieve Sizer was chosen as a typical example of a steady-state permeameter, and a number of shortcomings in this instrument have been found. A simple steady-state permeameter was constructed to evaluate the theoretical model as developed in Part I for atmospheric pressure permeametry, and the model has been found to give good values of external surface areas for powders having an average particle size greater than 2 μm. The effect of slip flow for particles less than 1 μm in diameter has been established for atmospheric pressure permeametry. This paper also presents measurements of specific surface area determined by using a more recent permeameter, namely the Permaran. This instrument performed within a reproducibility of ±7% and produced data in agreement with that taken on similar devices and also with the theoretical analysis presented in Part I. The performance of a Knudsen-flow permeameter has been evaluated, and the instrument has been found to give accurate values of external surface area of fine as well as coarse powders.