Bestemmeklse af gasdispersion i porøse medier - Metode og anvendelse

Studenteropgave: Kandidatspeciale og HD afgangsprojekt

  • Maren Hostrup
A simple model for determining the gas diffusion-dispersion coefficient in porous media based on oxygentransportation was tested. The method is an experimental setup consisting of a column containing a porous media, with a gas inlet in one end and a gas outlet in the other end. The oxygen concentration in the exit gas is measured with an oxygen electrode and from the measurements, a breakthrough curve is drawn. From the breakthrough curves, the diffusion-dispersion coefficient can be determined by fitting data to an analytical model for the convection-dispersion equation. Atmospheric air and nitrogen were used as breakthrough gas. By plotting the diffusion-dispersion coefficient against the porevelocity the dispersivity the diffusion was found. The columns used for the experiment had a lenght of 103 cm and a diameter of 14 cm. The measurements carried out in this project were performed on a homogenious small grained sand with a particle size between 0,40 and 0,80 mm. The dry bulk density was between 1,06 and 1,62 g/cm3 and the gravemetric water content was between 0,005 and 0,100 g water/g dry matter. A good consistency between the measured data and the analytical model of the convection-dispersion equation was found. By using the convection-dispersion equation, it was found that it was important to be precise in deciding the starting time, because small changes in the starting time can lead to comparatively large changes in the estimation of the porevelocity and the diffusion-dispersion coefficient. There were observed no distinction between results obtained by using oxygen and results obtained by using nitrogen as the send through gas. During the experiment, the water level in the test sampels changed due to the equilibrium between the dry gas and the porewater. This caused the sand to lose water as the experiment was conducted. The loss of water in the samples was between 5 and 37 %. The dispersivities for the samples in this project are between 0 and 0,445 cm. Generally, the dispersivities are rising in proportion with the water content of the samples, whereas they fall invert proportional with the dry bulk density. At the lower dry bulk densities, the dispersivity showed a greater dependence on the water content than at the higher dry bulk densities. At the lower dry bulk densities, the dispersion was 2 – 3 times larger than they were at the higher dry bulk densities. This might be due to the possibility of more free macropores in the media at the looser packings of the sand. This might cause a larger difference in the porevelocities in the sand and thereby enlarge the mechanical mixing. This has to be investigated by more measurements. The most important parameters in estimating the dispersivity for sand are dry bulk density and the water content. A model for the dispersivity in sand has been set up using these two parameters. When comparing with former results for usaturated sand samples, the dispersivity estimated in this project is below the others. One exception is the model by Levenspiel from 1972. This can be due to the homogenity of the samples and the small scattering on the particle sizes. The diffusion for the samples are estimated to be between 5,44 og 8,65 cm2/min. The diffusion is lessened with falling porevolumn, this beeing when the water content or the dry bulk density is rising. This fits well with observations of diffusion in earlier literature. The diffusion coefficients measured in this project are larger than coefficients measured earlier on the same sand by Pindstofte 2007 and larger than what can be estimated by the WLR model as presentet by Moldrup et al., 2000. This leads to the assumption that the estimated diffusion contains more than just the diffusion and more likely is a token for everything else than the dispersion in the sand. It is concluded that the experimental setup is suitable for estimating dispersivities, while the diffusion becomes a combination of more things and therefore, the equipment is not well suited for estimating the size of this. For conducting measurements the atmospheric air and the nitrogen are suitable, and this helps to bring down the costs in using this form of measurements compared to other setups. Furthermore, the equipment is easy to use and is presumed to be usable for measuring on different porous mediums.
Udgivende institutionaau
ID: 13792912