TU Berlin

Chair of Water Resources Management and Modeling of HydrosystemsAbstract

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Interphase mass transfer between fluids in porous media is an important part of transport phenomenon in nature and found distinct applications in different fields of science and technology such as oil extraction, Non-aqueous phase liquid (NAPL) remediation and CO2 injection into deep saline aquifers. In this study different aspects of this process are investigated in terms of both experimental and computational work. The investigations incorporated also different couples of fluids to examine the impact of fundamental fluid properties on this process. Influence of characteristic fluid/porous medium properties and phase formations on this process constitutes a major point of focus. Interphase mass transfer in porous media involves a number of complex processes mostly in relevance with the complexity of the flow in porous media. The interfacial domains being formed by multiphase flow processes cause further complications. The alteration of the fluid properties due to interactions occurring between different types of fluid couples is also a formidable subject as each fluid pair may entail unique interactions e.g., the different interactions between cosolvent containing aqueous solutions and organic phase compared to surfactant containing aqueous solutions and organic phase. Despite the association of diverse fields of science and the complexity appearing in this process, there is not an integral review of this topic in the literature. To address this absence part of this thesis is dedicated to review the numerous scientific articles committed to investigate interphase mass transfer. In this review, characteristic length scale of the existing literature and the flow configurations of the fluids in porous media served as a foundation to group studies with similar conditions. Apart from the experimental work also both analytical and numerical models were reviewed. This work provides a unified comprehension of interphase mass transfer in porous media where the cross interactions between different system parameters due to multiphase flow conditions are revealed. The impact of pore scale mechanisms on interphase mass transfer is not a subject that is investigated in detail as the most of the investigations focusing on interphase mass transfer are conducted at meso-scale. To investigate the impact of pore scale mechanisms a pore scale numerical model is developed. The results of the numerical model demonstrated that the distribution of NAPL at the pore scale has a large impact on the developed meso-scale interphase mass transfer correlations. Especially the geometric allocations of interfacial area which makes up the actual domain for interphase mass transfer were shown to have considerable impact. The reason for that was observed to be the alteration of the concentration profile at pore scale. The results also showed that the interfacial area is not linearly correlated to interphase mass transfer rate due to this alteration. A particular result was that the same interfacial area can lead to an order of magnitude lower interphase mass transfer rate if the orientation with respect to flow direction is changed from orthogonal to parallel. The explicit mass transfer coefficients, which are derived by considering the impact of interfacial area separately, are supposed to consider this effect. It was pointed out that the existing correlations of mass transfer coefficient are mostly dependent on flow velocity of solvent phase and do not consider the impact of the spatial distribution of the interfacial area. Several chemical agents have found significant use in different fields to enhance interphase mass transfer e.g., cosolvents and surfactants in petroleum and environmental engineering. Accordingly, the combination of ethanol as a chemical agent with water was examined with respect to its impact on the recovery of NAPL toluene from water saturated porous medium. The two recovery mechanisms, enhanced solubilization and mobilization, were inspected through column experiments. It was demonstrated that there is an interaction between these two mechanisms such that the sufficient reduction of interfacial tension (with use of intermediate ethanol contents in the flushing solution) leads to creation of preferential flow paths inducing a steep decrease in the interphase mass transfer rates. This was attributed to the existence of lower capillary pressure present at larger pore throats. It was demonstrated that solvent solutions applied at slow velocities may prevent the creation of such preferential flow paths and the reduced interphase mass transfer rates. Along with the column experiments, an REV (Representative elementary volume) based multiphase multicomponent numerical model was also used to investigate if this condition is captured by the model. The only available numerical model that can simulate cosolvent use, UTCHEM, is employed for this purpose. It was shown that this pore scale condition cannot be simulated. In such numerical models these pore scale conditions are considered within the interphase mass transfer formulations (i.e., Sherwood formulations) adopted in the model. However in literature there is not a Sherwood formulation that is developed for the cases where such chemical agents are present. Recently a long known Sherwood formulation found significant use in several multiphase numerical models. However no analysis of this Sherwood formulation exists and therefore its validity is dubious. To address this absence in literature a detailed analysis of this formulation is conducted. It was demonstrated that this particular Sherwood formulation involves a fundamental mathematical error that renders it unusable. Moreover, it was also demonstrated that the original data used by this Sherwood formulation incorporates a high level of uncertainty meaning that even the corrected version of this formulation is not eligible to be used in a numerical model. The implication of this error is the invalidity of the results presented in some recent publications that used this Sherwood formulation in a certain type of REV based numerical model. The thesis in hand attests to the cross interactions between various parameters involved in interphase mass transfer in porous medium. As the predictive accuracy of interphase mass transfer dictates the consideration of these parameters and interactions fairly well, further research must set sight on the clear characterization of the system at various length scales. With that perspective potential areas for future research are also highlighted.


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