Modulation and green synthesis of metal organic frameworks for higher water uptakes and kinetics : adsorption cooling application
Teo, Benjamin How Wei
Date of Issue2019-04-12
School of Mechanical and Aerospace Engineering
A central challenge of cooling science today is the development of clean energy assisted adsorption chillers, and also the reduction of carbon dioxide emissions, on the urgent agenda. The efficiency of an adsorption chiller is first and foremost governed by the micro porosity, hydrophilicity and hydrothermal stability of the adsorption materials. Traditionally, inorganic porous substances such as silica gel, aluminophosphates, or zeolites have been investigated for cooling purposes. The key problem with these adsorbents is that most of the water adsorption occurs at too high relative pressures, and the loading difference over the cycle is only a small part of the total adsorption capacity. So, novel adsorbents need to be designed and synthesised. The thesis focuses on the synthesis and modification of metal-organic frameworks (MOFs), an organic porous material, as the adsorbent for the adsorption cooling application. A Grand Canonical Monte Carlo (GCMC) simulation shows the molecular interaction between the adsorbents and water molecules during the adsorption process. A prediction of the water loading difference for modified MOF is achieved from GCMC simulation. The MOFs are then synthesised and characterised based on structural integrity, thermal stability, surface and porous characteristics, particle size and shape and water adsorption capabilities. The MOFs are then modified using alkali metal ions doping (MIL-101), modulation of formic acid and green synthesis (Al Fum MOF). The amount of water uptakes on the modified MOFs under static and dynamic conditions are measured experimentally and the data are fitted with isotherms and kinetics models to evaluate the limiting uptake, isosteric heat of adsorption, heterogeneity factor and activation energy, which are necessary to simulate the operating conditions of an adsorption chiller. Employing the isotherm and kinetics parameters, the performances of a two-bed adsorption chiller in terms of COP and cooling capacity are predicted. These results are presented for various cycle times and hot water inlet temperatures.
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