Surface-Area Measurement of Geologic Materials
Authors: Donaldson E.C. , Kendall R.F. , Baker B.A. , Manning F.S. 
Published: December 2007

The Bartlesville Energy Research Center of the U. S. Bureau of Mines bas developed a method for determining the surface area of geologic materials. A gas chromatograph is used to measure the amount of nitrogen adsorbed on the samples from which the surface areas are computed. The results are compared to surface areas calculated from the Kozeny-Carman equation using Carman's textural factor of 5.0. Excellent agreement is obtained for spherical glass beads and crushed sand. However, the ratio of the surface areas of live homogeneous sandstones obtained by nitrogen adsorption to that determined by the Kozeny-Carman equation ranged from 26 to 43. Hence, gas adsorption should be used for surface area measurements of geologic materials.

Introduction

The U. S. Bureau of Mines is investigating the adsorption properties of organic compounds on geologic materials at subsurface conditions to determine the migration patterns of waste chemical compounds injected into deep wells. The surface area of the adsorbent is one of the important parameters in the study of adsorption. A search of parameters in the study of adsorption. A search of the literature revealed that very little work on the surface areas of geologic materials has been reported. Brooks and Purcell and Tignor et al. reported surface areas of a few sandstones. They also compared their nitrogen-adsorption surface area measurements to results calculated from the Kozeny-Caman equation and noted a wide discrepancy. These data were insufficient to satisfy the needs of this research problem; therefore, several methods for determining surface area by nitrogen adsorption were studied to selected or devise by modification, a method suited for routine measurement of surface areas.

Brooks and Purcell used a static, or pressure-volume, system to measure the surface area pressure-volume, system to measure the surface area of geologic materials. More recently, Nelsen and Eggertsen discussed a continuous-flow apparatus that utilized the principles of gas-liquid chromatography for the surface-area measurement of catalysts. The latter method seemed best suited for routine analysis because it does not require the long periods of evacuation, with attendant experimental error, that are inherent in the static system. A Perkin-Elmer Model 154-D gas-liquid chromatograph was modified to provide the continuous-flow system used in this research.

APPARATUS AND PROCEDURE

The determination of the surface area of catalysts is explained in detail in the literature. The principles are reiterated here to clarify the modifications of Nelson and Eggertsen's apparatus that were made to simplify and adapt the process to the determination of surface areas of porous geologic materials.

The procedure involves measuring the nitrogen physically adsorbed as a monolayer on the surface physically adsorbed as a monolayer on the surface of the rock at the liquefaction temperature of nitrogen. Using the theory developed by Brunauer et al., the adsorbed nitrogen is related to the concentration (or partial pressure) of nitrogen.(1)p 1 (C-1)p---------- = ------- + ---------V(po-p) VmC VmCpo

A plot of p/V (po-p) vs p/po yields a straight line having intercept 1/VmC and slope C-I/VmC, from which Vm can be determined. The surface area is then given by(2)VmpgN A N2 10-20A = ------------------s M

The only data that must be determined experimentally are the volumes of gas (V), adsorbed on the interstitial surface of the rock at several pressures (p).

A schematic diagram of the converted Model 154 chromatograph is shown in Fig. 1.

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