Shrinking core model and experimental study of the hydrolysis reaction in the Cu-Cl cycle for hydrogen production

Radwan, Huda M. (2024) Shrinking core model and experimental study of the hydrolysis reaction in the Cu-Cl cycle for hydrogen production. Masters thesis, Memorial University of Newfoundland.

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Abstract

Renewable and more environmentally friendly energy carriers, such as hydrogen, are increasingly crucial in achieving net-zero goals. Hydrogen, recognized as an efficient energy carrier, is gaining prominence. However, existing methods of hydrogen production from fossil fuels lead to carbon dioxide emissions. Consequently, emission free processes like water splitting are more environmentally sustainable. The copper-chlorine (Cu-Cl) cycle has promising potential as a thermochemical process for producing hydrogen through water-splitting, especially when combined with solar and nuclear energy technologies. One of the challenging aspects of the cycle is the heterogeneous hydrolysis reaction, where it is essential to ensure high conversion of the reaction with minimal steam consumption. Therefore, understanding the reaction and mechanisms through experimental analysis and reaction modelling is crucial. This thesis aims to comprehensively investigate the kinetics of the hydrolysis reaction, taking into account various methods of pre-processing the solid reactant with different particle sizes. As this is a cyclical process, the solid reactant of the hydrolysis reaction is the product of the preceding electrolysis step. Consequently, the process to retrieve the material from the previous step and prepare for hydrolysis should be examined, along with its impact on the material's particle size and reaction kinetics. Three processes were analyzed in this thesis: drying, crushing, and crystallisation. Initially, the particle size and morphology were examined through the utilization of scanning electron microscopy (SEM). Subsequently, the material was introduced into a vertical semi-batch fixed bed reactor, and the reaction conversion was monitored over the reaction time interval up to 30 minutes. The analysis was repeated for temperatures ranging from 350 - 400°C and for different steam-to-copper ratios. Furthermore, the time conversion data were analyzed using a shrinking core model (SCM) to identify the predominant step and its associated coefficient. The study revealed that the morphology of CuCl₂ tends to exhibit a stick-like shape, approximating a cylindrical form. When CuCl₂ is retrieved from a water solution using HCl as an anti-solvent, it can generate particles and flakes within a wide range of sizes, ranging from 65 to 1300 μm. However, approximately 75% of the crystallised reactant had a 230 μm average particle size, while the dried material particles were 95 μm and the crushed had 27 μm. The hydrolysis reaction was investigated for dried, crushed, and three different particle sizes of crystallised materials. All samples revealed that the rate of conversion increases with an increase in temperature. The dried sample achieved the lowest rate of conversion, while the crushed sample achieved the highest rate of conversion. The crystallised materials achieved a rate of conversion higher than the dried material and lower than the crushed samples. When comparing the three different particle sizes of the crystallised material (crystallised 230 μm, crystallised 615 μm, and crystallised 1100 μm), the rate of conversion increased as the particle size decreased. A model was developed for spherical and cylindrical particle shape assumptions, with four equations for each shape, and the models were compared with experimental data. Experiments showed that the reaction control model with a cylindrical shape assumption exhibited the best fit for dried and crystallised 615 μm and crystallised 1100 μm materials. The crushed material and crystallised 95 μm samples indicated that gas film diffusion controls the conversion. Dried and crystallised 95 μm material with a reduced S/Cu ratio showed that gas film diffusion was the controlling step. X-ray diffraction (XRD) was utilised to analyze the solid product following the reaction, with the desired product Cu2OCl2 being identified for temperatures between 370 - 400°C. In contrast, the reaction at 350°C resulted in the side product of CuCl. The activation energy was 32 to 54 kJ/mol, varying for each particle size based on the linearization of the controlling step coefficient with temperature. Furthermore, the research emphasized the alterations in surface area and porosity resulting from modifications in solid processing, as well as the impact of crystallised particle size distribution on the conversion. All experiments were conducted at a S/Cu ratio of 10, while two experiments were conducted at 390°C and a S/Cu ratio of 5, resulting in low conversion rates. Specifically, the dried material of 95 μm achieved only 36% conversion, while the crystallised material of 230 μm had 50% conversion within 30 minutes. Ensuring a high conversion rate, while minimizing the steam consumption, during the hydrolysis reaction are important for improving the overall efficiency of the Cu-Cl cycle and therefore the overall rate of hydrogen production.

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