Synthesisation, characterisation and water adsorption on novel functional metal organic frameworks for heat transformation process

• Main Author: Han, Bo
• Other Authors: Anutosh Chakraborty
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2021
• Subjects: Engineering::Mechanical engineering
• Online Access: https://hdl.handle.net/10356/146505
• Institution: Nanyang Technological University

Referring to the present situation of the global climate condition, the reduction of the energy consumption is always of great importance in combatting the grave situation of enhanced global warming potential (GWP). Adsorption-assisted heat transformation (AHT) systems such as chillers, heat pumps and desalination can predominantly help us to reduce the global warming. Therefore, the adsorption devices should be improved with the use of highly efficient adsorbents. The main drawback of AHT system is its large thermal compressor size, which depends on the quality of the porous adsorbents, ranging from surface characteristic to uptake-offtake difference (Δq) per adsorption cycle. Therefore, novel adsorbents with desirable adsorption performances need to be designed and synthesised. Due to some striking properties such as high microporosity with structure diversity, promising isotherms and kinetics characteristics and outstanding thermal stability, metal organic frameworks (MOFs) have been identified as the potential adsorbent in recent years. This thesis aspires to develop the thermodynamic frameworks of AHT from entropy flow and generation points of view employing the experimentally confirmed isotherms and kinetics of novel functionalised metal-organic-frameworks (MOFs). The MOFs are synthesised by hydrothermal techniques with alkali ions doping, conventional zeolite mixing and functional group implanting methods. Prior to fabrication of the proposed MOFs, a Grand Canonical Monte Carlo (GCMC) simulation is performed to predict the water adsorption performances on various structural-configuration of MOFs in terms of shorter hydrophobic length and uptake-offtake difference. It is found that the addition of alkali ions or zeolite-MOFs composites increases the hydrophilicity in Henry’s regions, which thereby shows “S-shaped” isotherms. Secondly, the hydrophilicity of the parent UiO-66 (Zr) MOFs in Henry’s region are enhanced by the functionalisation of -NH2, -N or -OH. Furthermore, it is also observed from GCMC results that the implantation of -CH3 additives on MOF-801 (Zr) shows longer hydrophobic length as compared to that of the parent MOF-801 (Zr), which delivers higher uptake-offtake difference (Δq) with fast kinetics per adsorption desorption cycle. The GCMC results show here the benchmark for the synthesisation and post modification of MOFs and composite adsorbents. The MOFs such as Al-Fum, UiO-66(Zr), MOF-801 (Zr) and AL-Fum/zeolite composites are at first synthesised and later the MOFs are post synthesised with various alkali dopants and the additives (-NH2, -N, -OH and -CH3). After synthesisation, the parent and modified adsorbents are characterised by X-Ray diffraction (XRD), scanning electron microscopy (SEM), thermogravimetric analysis (TGA) and N2 adsorption approaches. The water adsorption isotherms and kinetics are studied by a thermo-gravimetric analyser. It is found that 5% Li-Al-Fum, 30% AFI-Al-Fum and 30% CHA-Al-Fum adsorbents provide faster water-adsorption kinetics and higher uptake-offtake difference (Δq) as compared with the original MOFs. Secondly, it is also found that the amino (-NH2) and hydroxyl (-OH) functionalised UiO-66 (Zr) MOFs improve water transfer from 0.05 kg/kg to 0.32 kg/kg. On the other hand, N-UiO-66 (Zr) shows the faster uptake/offtake rates as compared with the parent UiO-66 (Zr) MOFs. Furthermore, the methyl (-CH3) functional group also improves Δq with two-times faster adsorption kinetics as compared to the parent MOF-801 (Zr) adsorbent. Employing experimentally confirmed isotherms, kinetics and data related to bed dimensions and overall heat transfer coefficient, the energy and entropy balances for each component of heat transformation system (for cooling, heat pumping and desalination) are developed to calculate the system-performance and overall entropy generation from transient to cyclic steady state. The present results show that the smaller entropy generation ( ̇ ) of an AHT system is obtained under lower regeneration temperature and shorter half cycle time, which leads to the reduction of system irreversibility and the improvement of efficiency. By analysing temperature-entropy maps for each component of the adsorption heat transformation process, the functionalised (CH3)2-MOF-801 (Zr) is shown to be a the promising adsorbent for adsorption cooling purpose whereas N-UiO-66 (Zr) is found suitable for adsorption desalination application. The temperature-entropy maps could be used to analyse ̇ , which could be used as a tool to enhance the performances of the AHT system. These contributions provide a benchmark for new MOFs design, fabrication and the optimisation of adsorption assisted cooling/heat pump and desalination system. The simulation results show that as compared with the original Al-Fum, the modified Li doped Al-Fum achieves 42.3% higher COP and 70% higher SCP. By functionalisation on the parent UiO-66 (Zr) MOF, the SCP (specific cooling capacity) improves from 0.36 kW/kg to 0.84 kW/kg, the SHP (specific heating power) enhances from 0.9 kW/kg to 1.48 kW/kg and the SDWP (specific daily water production) increases from 24 m3 to 40 m3 of desalinated water per tonne of functionalised UiO-66 (Zr) adsorbent per day. Furthermore, the methyl functionalised MOF-801 (Zr) provides relatively higher SCP, SHP and SDWP as compared with the parent MOF-801 (Zr). By analysing temperature-entropy generation diagrams with respect to the best AHT-performance-parameters, it is found that (CH3)2-MOF-801 (Zr) is the best candidate for adsorption cooling application and N-UiO-66 (Zr) is the most promising adsorbent for adsorption desalination applications.