The comparative analysis of adsorbents suitable for thermal desalination system

Abstract

Adsorption desalination is a new type of desalination that uses an adsorption-desorption cycle. The adsorbent and adsorbate are the two main components of this cycle. The adsorbent is a substance with both hydrophilic qualities and hydrophobic qualities that adsorb (attracts) and desorbs (releases) adsorbate that comes into contact with its surface area. When the adsorbent is exposed to low temperatures, the hydrophilic property is activated, whereas when the adsorbent is exposed to high temperatures, the hydrophobic property is stimulated. Hydrophilic happens during the adsorption process and hydrophobic takes place during desorption. Each process is conducted for a period of time and that time is called the half cycle. By including the adsorption desalination (AD) cycle, the water vapour uptake rate is increased, resulting in more desalination water being produced in a shorter amount of time. Silica gel (SiO2), activated alumina oxide (Al2O3), and molecular sieve zeolite are the three specific adsorbents used. These adsorbents used are determined by their natural sorption properties, such as porosity and surface area. These adsorbents were compared to find which one is suitable for the thermal desalination process. To examine the performance of adsorbent material, the impact of cycle time, hot, cold and chilled water temperature, adsorption isotherms, cycle performance (COP) as well as adsorption energy are also required. Adsorption isotherms speak to the extreme measure of adsorbate adsorbed per unit mass of dry material at a particular pressure. A weighing apparatus capable of operating in the range of 5 g–10 kg of sample mass with an accuracy of 0.1 g and a time response less than 0.1 s used to directly monitor the dynamic development of the uptake during the isobaric adsorption and desorption stage. The innovative lab work was performed on an AD-heat exchanger made up of granules of silica gel, zeolite and activated alumina using an aluminium designed heat exchanger. The process operated at different thermodynamic cycles (Th =70 °C, Teva =10 °C, Tc = 30 and 35 °C), which are actually for the adsorption/desorption process. The half cycle time was found to be 1200s, because all three adsorbents were at equilibrium phase even though silica gel adsorbed 12% water vapour higher than zeolite, and 18% higher than activated alumina. The heat of adsorption of this process conducted using Origin software and it operates in the range of 12 kJ/mol – 15 kJ/mol.