Determination of Binary Diffusion Coefficients of Molecular Gas Mixtures by Using a Lochschmidt-Cell Combined with Holographic Iinterferometry

Summary and outlook: During this thesis the gaeous binary diffusion coefficients of the CH4-C3H2 system were investigated in a Lochschmidt cell combined with a holographic interferometry at 5 bar and 293.15 K using pure gases as well as corresponding mixtures resulting in different average propane mole fractions 1/6, 1/3, 1/2, 2/3 and 5/6. Preparation of the experimental set-up was carried out in order to be able to investigate the flammable gases and their mixtures, where in particular the sealing material had to be exchanged. In addition, the set-up peripheral gas piping system was modified to be able to fill, empty, and flush the cell in a convenient and safe way. The previous data evaluation procedure was implemented in MATLAB. This allows performing measurements and analyzing the measurement data at the same time, since the old software for data evaluation was not usable during the measurement runs. Furthermore, the new software allows for a standard evaluation of measurements where mixtures are used prior to the diffusion process.

The obtained results for the investigated system show that the overall average D12ρmix value does not depend significantly on the initial concentration gradients and the propane mode fraction. This is caused by a corresponding high estimated uncertainty, in particular for the overall average D12ρmix values for the high average propane mole fractions. This issue will need further analysis in upcoming work.

In the future, the system CH4-C3H8 will be investigated at pressures of 1 bar and 2 bar and at temperatures of 293.15 K and 313.15 K to access information on the temperature and pressure dependency, which is important in context with the ab initio calculation performed in a related project at Rostock university. For the same purpose, the investigation of the system CO2C3H8 is planned to performed at pressures of 1, 2, 5 and 10 bar and temperatures of 293.15 K and 313.15 K. Currently the data evaluation is based on the assumption that D12ρmix  is constant over whole investigating mole fraction. However, investigating the concentration dependency of the binary diffusion coefficient based on the mentioned assumption can be criticized. The newly implemented data evaluation can be used to examine any new proposed possible sources for the observed irregularities since it brings more freedom for data processing in comparison to the old software.