Determination of adsorption kinetics and equilibria of gases and vapors

The mass transport of gases in porous solids plays a decisive role in many energy and chemical engineering processes, such as the purification of exhaust gas streams, storage of CO2 in geological formations, or the combustion of solid fuels. When gas molecules attach to the surface of porous solids, this process is called adsorption.

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As part of the Collaborative Research Center SFB/TRR 129 Oxyflame, funded by the German Research Foundation (DFG), the Chair of Thermodynamics investigates mass transfer and pore structure evolution during pyrolysis and combustion of biomass. In close cooperation with chairs of the RWTH Aachen, the TU Darmstadt, and other chairs of RUB, the combustion of biomass in oxyfuel atmosphere is investigated. In a previous funding phase of the Oxyflame project, the combustion of coal was initially investigated, but in the meantime, research is being conducted exclusively on biogenic fuels. The project also investigates how findings from coal combustion can be transferred to biomass combustion. For this purpose, gravimetric investigations of the adsorption kinetics of oxyfuel relevant gases (e.g. O2, CO2, H2O, CO, and CH4) are performed on different biomass chars at the Chair of Thermodynamics. This data is used for a self-developed mass transfer model, which is able to calculate and predict mass transfer quantities such as effective diffusion coefficients, mass transfer coefficients and adsorption flows [1]. In an international cooperation with researchers from the University of Western Australia in Perth (Australia), volumetric adsorption investigations are also carried out to determine the adsorption properties [2,3].

Important information for the mass transfer modelling is the porous structure of the investigated fuels. Based on volumetric adsorption studies, the pore structure of solid fuels can be analyzed [1,2,4,5]. For this purpose, we cooperate not only with research groups within the Collaborative Research Center Oxyflame, but also with a group from the STEMS-CNR in Naples (Italy) [6]. In addition to classical methods such as the analysis of an N2 adsorption isotherm at 77.36 K (Brunauer, Emmett and Teller) or a CO2 adsorption isotherm at 273.15 K (Dubinin-Astakhov or Dubinin-Radushkevich), modern analytical methods based on non-local density functional theory (NLDFT) can also be applied.

For the gravimetric determination of adsorption kinetics and equilibria, three different measuring devices based on magnetic suspension balances are available. The first device is designed for kinetic adsorption measurements of gases such as CO2, O2, CH4, or H2 at temperatures up to 150 °C and pressures up to 6 MPa. In order to determine the adsorption of low adsorbing gases like H2, this device is equipped with a highly-sensitive 1 µg balance. The second device (10 µg balance) is optimized for adsorption measurements in the low-pressure range from (0.1 to 200) kPa at temperatures up to 80 °C. With this device, adsorption measurements of the gas phase of a VLE equilibrium of fluids as methanol, toluene, or water can also be performed. These fluids can be filled in an inert gas atmosphere using an existing glove box. The third device (10 µg balance) is originally designed for thermogravimetric measurements at temperatures up to 1200 °C and pressures up to 2 MPa, but is also adapted for adsorption measurements at elevated temperatures. The temperature range of the adsorption measurements is limited due to possible reactions taking place and therefore depends on the measuring system to be investigated.


[1] C. Wedler, R. Span, A pore-structure dependent kinetic adsorption model for consideration in char conversion – Adsorption kinetics of CO2 on biomass chars, Chemical Engineering Science (2020) 116281. 

[2] C. Wedler, K. Lotz, A. Arami-Niya, G. Xiao, R. Span, M. Muhler, E.F. May, M. Richter, Influence of Mineral Composition of Chars Derived by Hydrothermal Carbonization on Sorption Behavior of CO2, CH4, and O2, ACS Omega 5 (2020) 10704–10714. 

[3] C. Wedler, A. Arami-Niya, G. Xiao, R. Span, E.F. May, M. Richter, Gas Diffusion and Sorption in Carbon Conversion, Energy Procedia 158 (2019) 1792–1797. 

[4] C. Wedler, R. Span, M. Richter, Comparison of micro- and macropore evolution of coal char during pyrolysis, Fuel 275 (2020) 117845. 

[5] S. Heuer, C. Wedler, C. Ontyd, M. Schiemann, R. Span, M. Richter, V. Scherer, Evolution of coal char porosity from CO2-pyrolysis experiments, Fuel 253 (2019) 1457–1464. 

[6] O. Senneca, N. Vorobiev, A. Wütscher, F. Cerciello, S. Heuer, C. Wedler, R. Span, M. Schiemann, M. Muhler, V. Scherer, Assessment of combustion rates of coal chars for oxy-combustion applications, Fuel 238 (2019) 173–185.​