Stofftransport und Adsorptionskinetik in porösen Adsorbenskompositen für Wärmetransformationsanwendungen

Aim of this work is the physical description of non-isothermal water adsorption and its dominating transport processes in novel adsorbent composites for heat transformation applications. The adsorbent composites studied in this work have been realised by direct crystallisation of SAPO-34 on a sintered aluminium fibre structure. Since the aluminium fibres potentially allow a very good heat transfer, a special focus is given to the influence of water vapor transport on the transient adsorption process. Adsorption technology is already successfully used for thermally driven heat transformation. Adsorption is an exothermal process. Heat is released when the gaseous adsorptive (e.g. water vapor) attaches to a microporous adsorbent (e.g. zeolite). The process is reversible, i.e. by supply of heat the adsorptive can be separated from the adsorbent. In a closed system, a thermodynamic cycle can be realised. During adsorption, heat is drawn from an evaporator that supplies the adsorptive. The adsorptive released during desorption is condensed in a condenser, releasing the heat of condensation. Water as adsorptive shows a few big advantages: It is not hazardous nor climate-damaging in its application, it is available ubiquitously and it has the largest enthalpy of evaporation of all known refrigerants. There are several adsorbents that show a large water uptake. One disadvantage is the low vapor pressure and the related low density. Gradients in density or pressure necessary for mass transfer thus limit the adsorption process much earlier than with other adsorptives. Goal of ongoing R&D work are compact adsorption devices with high transformation efficiency also in the low power region. Previous devices usually used an adsorbent pellet bed in a heat exchanger. Due to the typical point contacts between adsorbent grains and the heat exchanger in an adsorbent bed, usually heat transfer is limiting the dynamic adsorption process. A very promising opportunity to overcome this limitation is given by adsorbent coated porous metallic structures, which can enhance heat and mass transfer significantly. For the optimization of such adsorbers, a precise and experimentally validated description of the adsorption kinetics is necessary. Water vapor transport in the macropores of the hierarchical pore structure of such composites takes place in the transitional regime between viscous flow and Knudsen diffusion, in which the mean free path in pure water vapor atmosphere reaches the same order of magnitude as the characteristical geometric dimensions of the structure. In the micropores of the adsorbent crystallites, diffusion and adsorption are unseparably connected. Transport takes place by molecular diffusion and diffusion in the adsorbed phase. Starting from theory of adsorption thermodynamics as well as models of mass transfer, in this thesis a model of non-isothermal adsorption kinetics is developed that includes the influence of mass transfer on two scales (bidisperse model) on the transient adsorption process. Transport parameters needed in this model are derived experimentally by various methods. In particular, two experimental methods used in this thesis are described more in detail: On the one hand, a permeability measurement is presented, in which the porous structures are flowed through with gas in the transition area between viscous flow and Knudsen diffusion (low vacuum). By this method, macropore transport parameters (mean pore diameter, tortuosity) are identified. By comparison to results from porosimetry and microscopy, the results are checked for plausibility. On the other hand, the transient adsorption process of selected samples is measured with a volumetric pressure jump method. By fitting the physical bidispers model of coupled heat and mass transfer to the measurement data, the parameters not accessible by other measurements are identified. By simulation of adsorption cycles, the influence of the various transport processes on the overall adsorption process is quantified. With help of the developed model and the identified transport parameters, a novel adsorber geometry based on the aluminium fibre zeolite composites is studied. It is shown, that apart from heat transfer to the heat exchanger fluid, water diffusion in the micropores of the adsorbent is dominating the transient process, while macropore transport has only little influence for the considered composite thicknesses. A Pareto optimization for the two optimization criteria efficiency (COP) and volumetric power density finally show the huge potential of the novel adsorbent composites and give practical cues for optimal geometries and operating conditions of such a promising fibre composite adsorber