Simple and effective method to measure the diffusion coefficient of organic vapors in porous media

Abstract

A quick and reliable method to measure a vapor concentration within porous substrate was developed. The technique consists of two steps, where a modified head space single drop microextraction (HS-SDME) sampling is used to entrap the vapor phase. In the second step, the entrapped vapor concentration is measured by gas chromatography (GC-FID). The technique is used to measure an effective diffusion coefficient of n-pentane in dry medium grain sand, with the sand partially saturated with water as an inert liquid. The measurements are carried out in a cylindrical sand holder on which the HS-SDME sampling ports are mounted. A linear vapor concentration profile along the bed thickness is found. From known concentration gradient and measuring the mass flux gravimetrically, the effective diffusion coefficient is determined. It turns out that the diffusion coefficient decreases from 8.49 × 10⁻⁶ for dry sand to 7.13 × 10⁻⁶ m² s⁻¹ as a function of water saturation. Additional hindrance to the vapor transport is observed from both the simple volumetric effect due to the porous medium void space reduction caused by the presence of water and increase of the tortuosity.