Tobias Fieback

Characterization of solid and liquid sorbent materials for biogas purification by using a new volumetric screening instrument.

With the increasing utilization of biogas as an energy source the need for new materials and methods to purify and clean the corresponding gas mixtures is rising. In this regard, the application of ad- or absorptive gas purification methods has increased significantly over the last years. For fast and economic evaluation of the potential of different sorbent materials, a new volumetric screening instrument has been developed. First the measuring method and the new instrument design will be described. This instrument allows ad- and absorption, as well as desorption measurements in a technically relevant, wide pressure, and temperature range. It was used for the characterization of common sorbent materials such as activated carbons and zeolite molecular sieves. Additionally, new substances like metal-organic frameworks and ionic liquids were analyzed. Thereby the sorption of CO(2), CH(4), N(2), and H(2) was measured. The obtained data allow the direct comparison of the sorption properties of the different materials, the results of which will be presented in the second part of the paper.

In situ gas analysis for high pressure applications using property measurements

As the production, distribution, and storage of renewable energy based fuels usually are performed under high pressures and as there is a lack of in situ high pressure gas analysis instruments on the market, the aim of this work was to develop a method for in situ high pressure gas analysis of biogas and hydrogen containing gas mixtures. The analysis is based on in situ measurements of optical, thermo physical, and electromagnetic properties in gas mixtures with newly developed high pressure sensors. This article depicts the calculation of compositions from the measured properties, which is carried out iteratively by using highly accurate equations of state for gas mixtures. The validation of the method consisted of the generation and measurement of several mixtures, of which three are presented herein: a first mixture of 64.9 mol. % methane, 17.1 mol. % carbon dioxide, 9 mol. % helium, and 9 mol. % ethane at 323 K and 423 K in a pressure range from 2.5 MPa to 17 MPa; a second mixture of 93.0 mol. % methane, 4.0 mol. % propane, 2.0 mol. % carbon dioxide, and 1.0 mol. % nitrogen at 303 K, 313 K, and 323 K in a pressure range from 1.2 MPa to 3 MPa; and a third mixture of 64.9 mol. % methane, 30.1 mol. % carbon dioxide, and 5.0 mol. % nitrogen at 303 K, 313 K, and 323 K in a pressure range from 2.5 MPa to 4 MPa. The analysis of the tested gas mixtures showed that with measured density, velocity of sound, and relative permittivity the composition can be determined with deviations below 1.9 mol. %, in most cases even below 1 mol. %. Comparing the calculated compositions with the generated gas mixture, the deviations were in the range of the combined uncertainty of measurement and property models.

New sample carrier systems for thermogravimetric analysis under forced flow conditions and their influence on microkinetic results

For thermogravimetric analysis, it has been shown that, depending on the type of sample container, different kinetic results could be obtained despite regarding the same reaction under constant conditions. This is due to limiting macrokinetic effects which are strongly dependant on the type of sample carrying system. This prompted the need for sample containers which deliver results minimally limited by diffusive mass transport. In this way, two container systems were developed, both characterized by a forced flow stream through a solid, porous bed: one from bottom to top (counter-current flow) and one from top to bottom (co-current flow). Optical test measurements were performed, the results indicating that reaction proceedings are almost fully independent of the geometrical shape of the sample containers. The Boudouard reaction was investigated with a standard crucible and the new developed systems; the reaction rates determined differed significantly, up to a factor of 6.2 at 1373 K.

Enhancement of gravimetric forced flow through system to determine sorption, swelling, and mass transfer characteristics of liquid sorbents

An existing apparatus for forced flow through of liquid sorbents has been enhanced with an optically accessible system including a transparent crucible, high pressure viewing cell, and camera. With this optical system, the active surface area between gas and liquid can be determined in situ for the first time under industrial process conditions while maintaining the accuracy of a magnetic suspension balance. Additionally, occurring swelling and the resulting buoyancy changes can now be corrected, further improving the quality of the data. Validation measurements focusing on the sorption isotherms, swelling, and bubble geometry of 1-butyl-3-methylimidazolium tetrafluoroborate with nitrogen at 303 K and up to 17 MPa, as well as with carbon dioxide at 303 K, 323 K, and 373 K at up to 3.5 MPa were completed. Absorption of nitrogen resulted in no observable volume change, whereas absorption of carbon dioxide resulted in temperature independent swelling of up to 9.8%. The gas bubbles structure and behavior during its ascend through the liquid was optically tracked in situ. Combining these two data sets with the absorption kinetics forms the basis to determine the measuring system independent mass transfer coefficients, which are applicable in other laboratory scale and industrial processes.

A Reduced Numerical Model for the Thermit Rail Welding Process

ABSTRACT In this article, a reduced numerical model for the heat transfer in a commonly used Thermit rail welding procedure is presented. A geometrically reduced calculation domain was deduced from the welding system consisting of rails, weld material and mold. The geometrical domain is restricted to heat transfer in the rail web. Unsteady heat conduction in base rail and weld regions undergoing melting and solidification are modeled using the finite difference method. Therefor the consecutive periods of the process are described by specified initial and boundary conditions: preheating, tapping time, pouring and the final cooling. The solid-liquid phase change occurring during pouring and cooling is described using the enthalpy method. Thermal radiation between rail and mold surfaces is considered. Validation is carried out against results of models using computational fluid dynamics and solidus temperature isothermal positions in micrographs of longitudinal weld cuts from experiments. A sensitivity analysis was performed for the reduced model. The temperature of the liquid steel melt and the specific heat of the rail steel have the largest impact on the fusion zone width whereas mold material properties show negligible influences. The calculated width of the final fusion zone agrees within a deviation of 16% to experimental results.