Date

2023-1-24

Author

Karakaş Helvacı, Zeynep

Metadata

Show full item record

This work is licensed under a Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License.

Item Usage Stats

496
views

125
downloads

Cite This

The reactive systems between ammoniated CaCl2 and NH3 were investigated in terms of heat and mass transport phenomena coupled with reaction kinetics. The reversible reaction rate based on the Langmuir-Hinshelwood model was suggested to express the net reaction rate, including both the adsorption and desorption terms. Experiments at three temperatures were conducted to evaluate the kinetic parameters, pre-exponential factors, and activation energies. The parameters were evaluated by Non-Linear Regression (NLR) analysis using the Polymath software. Experiments with 0.3 grams of initial metal salt results were used for the reaction between two- and four- ammoniated CaCl2 in NLR analysis. Calculated reaction rates were also compared with the experimental result obtained from the 0.5 grams of initial CaCl2. Pre-exponential factors were obtained as 7.06∙10^(-22)min-1 bar-3 and 102 min-1 for adsorption and desorption terms, respectively. Also, apparent adsorption and desorption activation energies were obtained as -115600 J mol-1 and 23310 J mol-1, respectively. The calculated and experimental results showed well agreement. The parameters for the reaction between four- and eight-ammoniated CaCl2 were obtained by the 0.5 grams of initial CaCl2 experiments. Pre-exponential factors for adsorption and desorption reactions were obtained as 2.64∙10^(-18) min-1bar-3 and 3.44 min-1, respectively. Also, apparent adsorption and desorption activation energies were obtained as -88130 J mol-1 and 16500 J mol-1 for the adsorption and desorption terms. The negative activation energies in both reactions indicated the complex reactions in the reaction mechanism. Since CaCl2 is used in the pellet forms in the Chemical Heat Pump and hydrogen storage applications, a theoretical model was developed taking into account heat and mass transport phenomena with the consideration of chemical reaction rate. As a contribution to the literature, volume increase was integrated into the model, and the developed reversible reaction rate expression was used. In order to compare the simulation results, slab pellet experiments were also conducted in the same experimental setup. The simulation change in bulk ammonia pressure was compared with the experimental results. The results were in agreement for the reaction between two- and four-ammoniated CaCl2 in the first 700 seconds. The deviation beyond this time may be the constant effective diffusivity assumption in the model. The effective diffusivities of the reactive systems were assumed according to the literature and assumed as constant during the reaction, while it was thought that it was changing during the experiment. Moreover, the experimental and simulation results showed well agreement for the reaction between four- and eight-ammoniated CaCl2. The deviation observed in the former case was not seen in the latter case. This was attributed to the fact that at the end of the experiment, the slab pellet structure integrity disappeared, and the pellet dispersed. Therefore, the effective diffusivity might not decrease during the experiments, which was compatible with the simulation assumption.

Subject Keywords

Chemical heat pump, Hydrogen storage, Reaction kinetics, Solid-gas reactions, CaCl2-NH3

URI

https://hdl.handle.net/11511/102116

Collections

Graduate School of Natural and Applied Sciences, Thesis