Interfaces and Mass Transfer at Elevated Pressure

In the context of the energy transition, processes related to storage and utilization of CO2 and H2 as well as fluid mixtures containing natural gas are gaining relevance. These processes take place at elevated pressure for which determining the respective phase equilibria as well as system properties like the interfacial tension and mass transfer is challenging. Therefore, it is advisable to apply physically based theoretical models that allow systematic calculation of material properties in a wide range of parameters making use of a relatively small number of experimental data. The starting point is a theoretical model, recently, established for mass transfer in liquid-liquid systems that will be further developed for calculating the mass transfer across vapor-liquid-liquid boundaries. As model systems, water n-dodecane n-butanol CH4 as well as water n-dodecane n-butanol CO2 are selected, representing systems that are of high scientific interest since from literature it is known that two of the transferring compounds, n-butanol and CO2, are enriched at the interface. For the first time, two quaternary systems will be investigated systematically in this project, i.e. all relevant thermodynamic properties will be determined, also of all binary and ternary subsystems. Further, new experimental procedures will be employed to systematically investigate the mass transfer that include the implementation of results from the thermodynamic modelling that on its turn will deliver a thermodynamically consistent approach based on PC-SAFT. PC-SAFT has already be applied successfully in literature for describing high pressure phase equilibria. In combination with the density gradient theory, interfacial properties can be calculated. From an expression of the Helmholtz energy of an inhomogeneous system, the chemical potential can be derived that on its turn is the driving force of mass transfer. The thermodynamic as well as mass transfer model are parametrized with help of the binary subsystems and validated with help of the ternary and quaternary systems. Properties that are not experimentally accessible like the local non-equilibrium density can be determined and used for the experimental evaluation of transient drop profiles in terms of the dynamic interfacial tension. In case of a successful project, a thermodynamically consistent model is provided for determining the mass transfer in multiphase systems at elevated pressures. Further, the data base on fluid mixture properties is extended and new experimental methods are provided.

Liquid fuels, water and gases such as natural gas, carbon dioxide and hydrogen come together in many energy supply processes: in pipelines, tanks, underground storage facilities and in the recovery of crude oil and natural gas. It is precisely where two fluids meet - at the so-called interface - that determines how quickly substances migrate and whether a process works safely and efficiently. Until now, these processes have been understood primarily at normal pressure, but hardly at the significantly higher pressures that are common in practice. This project therefore investigated systems consisting of water, a typical oil component, an alcohol (n-butanol) and the gases methane or carbon dioxide at high pressures. In special high-pressure cells, cameras and sensitive scales were used to observe how droplets grow or shrink when gas enters or exits. Laser light (Raman spectroscopy) was used to measure, without contact, how the concentrations of the substances inside the liquids change over time. At the same time, a detailed computer model was developed that replicates and extrapolates the measured data. The aim is to describe how composition, density and interfacial properties (interfacial tension) influence each other and how they control the exchange of substances. Even complex mixtures containing several liquids and gases can be virtually simulated in this way, without having to test every possible combination in the laboratory.