a) The Bayer process, used to produce alumina from bauxite, has a final step that calcines solid hydrated alumina or gibbsite (Al(OH)3) to form solid anhydrous alumina or aluminium oxide (Al2O3) via an endothermic reaction, shown below as reaction (1), at 1100C. 2Al(OH)3 Al2O3 + 3H20 AH = +185.2 kJ.mol-1 (1) In modern plants, alumina calcination occurs in a flash calciner with product residence times in the order of a few seconds. The flash calciner is heated via combustion with pre-heated secondary air. Consider an alumina calciner producing 10 tonne/hr of alumina fired using natural gas (LHV = 45 MJ/kg) supplied at 15C with 5% excess air. Pure Al(OH)3 is fed to the calciner at 50C and is fully converted to Al2O3, which exits the calciner at 1100C. i. ii. Calculate the feed rate of Al(OH)3 in kg/s (3 marks) Calculate the useful energy that must be transferred to the solid product in the calciner. Note that the heat of reaction for reaction (1) is given on the basis of the energy required per mole of reactant (4 marks) If the overall efficiency of the calciner is 60%, calculate the mass flow rate of natural gas and air to the calciner. To determine the combustion stoichiometry, assume that the composition of the fuel is comprised of methane only. (4 marks) As a means of reducing greenhouse gas emissions, it has been proposed to replace natural gas as the fuel in the flash calciner with solid fuel (particle size = 3 mm) manufactured from wood and demolition waste. Given what you know about residence time in the flash calciner, comment on the suitability of this particular fuel substitution. (4 marks) iv. Data: MW of Al(OH)3 = 78 g/mol MW of Al2O3 = 103 g/mol Cp of Al(OH)3 = 1.75 kJ.kg-1.K-1 (assume constant for T = 0 - 1100C) Now see the Chemied reaction envahion. 2 AM) -> AlO + 3 HO. So we can jee required 2 mole. 1 male Al2O produced Al(OHz to be 30 required 78 x2 = 156 g ALLO6/ 1039 Ala 03 Thus we get 1 kg Adels required 156 103 X1000 Albt); = 1514.563 g Al(OH)3 = 1.514563 kg Ad6H3. de Now Production rate of the = 10 ton Al2O conr. = 2.77777 kr/s YOXOOO 3600 in order to continue this production, we need. 2.77777 X 1.514563 4.20712 kg/s of Alm i rate of A16H), 4.20712 kg/s. - Wrong solution Now see the energy required per mole of