Research Of Chemical Looping Air Separation Technology Based On Cu-Based Oxygen Carrier

Oxygen is an important chemical material and it has major applications in most industries such as metallurgy,chemistry,energy,aquaculture,and medical treatment etc.The yield of oxygen is about 1073 billion cubic meters in 2013.Air separation is the main approach for oxygen production including cryogenic distillation,swing adsorption and membrane separation.However,all of the three approaches have the disadvantages of energy-intensive,operation-complex and cost-expensive.Chemical looping air separation?CLAS?is a novel approach to produce oxygen.The features of CLAS are operation-simple,cost-effective,and energy-extensive.The oxygen carriers used in CLAS absorb oxygen from air in an oxidation reactor and then release oxygen in a reduction reactor.Based on the carrier gases in the reduction reactor,pure oxygen or oxygen-enriched gas will be produced.Thermodynamic analysis,investigation in preparation methods and preparation parameters,dynamic analysis,thermal experiment investigation,flow simulation and energy consumption analysis were done in this paper.The main content of the present thesis was focused on the items as follows:?1?Taking the reaction Gibbs free energy,equilibrium partial pressure of oxygen,and the reactivity with steam and carbon dioxide into consider,the active phases and binders of oxygen carriers suitable for CLAS technology were selected.Comparing to the other oxides,copper oxide has the advantages of high oxygen transport capacity,high reactivity,and low cost.Cu-based oxygen carrier was selected as the object in the research.The binders suitable for Cu-based oxygen carrier include:TiO₂,ZrO₂,SiO₂,MgAl₂O₄ etc.Based on the principle of the Gibb's energy minimization,the thermodynamic calculations on the CuO/Cu₂O in the steam and flue gas atmosphere were carried out.The redox reaction is not affected by the steam and the compositions such as CO?NO?CO₂?SO₂ in the flue gas.The optional reduction temperatures were determined.In air atmosphere,the optional oxidation temperatures were also determined.?2?Based on the characterization methods of thermogravimetric analysis,phase analysis,specific surface area test,mechanical strength test and surface morphology test,the effects of the preparation method,calcination temperature and time,binder type and binder adding ratio on the physical and chemical properties of oxygen carrier were investigated.The effect of preparation method on the physical and chemical properties of oxygen carrier is little.The effect of calcination temperature on the physical and chemical properties is great.And the mechanical strength of oxygen carrier increases as the calcination temperature increases.However,the specific surface area and the reduction rate of oxygen carrier decrease as the calcination temperature increases.The effect of calcination time on the physical and chemical properties of oxygen carrier is little.The oxygen carriers prepared by different methods and parameters have stable phases and low specific surface areas.The Cu-based oxygen carriers with MgAl₂O₄ and ZrO₂ as binders have high reduction and oxidation reactivity.?3?TG experiments were performed on STA409PC thermal analyzer.The effects of the heating rate,oxygen concentration,reaction temperature,and particle size on the reduction and oxidation properties of oxygen carrier were investigated.The starting temperature,ending temperature,and maximum mass loss/gain temperature all shift to high level as heating rate increases.However,the complete conversion time decreases with heating rate increasing.The starting temperatures of the reduction reactions shift to high level as oxygen concentrations increase.The effect of oxygen concentration on the oxidation reaction is little and the reduced oxygen carrier could be regenerated in low oxygen concentration.The reduction rate increases greatly with temperature increasing.The oxidation rate increases when temperatures are lower than 850? and then decreases when temperatures are higher than 850? with temperature increasing.The effect of particle size is little.The reduction rates of different types of oxygen carriers increase follows a sequence of Cu/Mg>Cu/Zr>Cu/Ti>Cu/Si.The differences of oxidation rates of the different oxygen carriers are little.The relativity of oxygen carrier keeps high and stable during the cycles.?4?Based on the TG data,the kinetic models of the reduction and oxidation reactions of Cu-based oxygen carriers were established.The activation energies were calculated by isoconversional method.The reduction and oxidation activation energies of oxygen carriers prepared by different binders and binder adding ratios are 145?163kJ/mol and 38?46kJ/mol respectively.The most possible mechanism functions were obtained by main function method.The most possible mechanism functions are not affected by the type of binders and binder adding ratios.The most possible mechanism functions of the reduction and oxidation reactions are the same and they can be described by nucleation and nuclei growth models.For reduction:f???= 3?1-??[-ln?1-??]^2/3 and for oxidation:f???=1.5?1-??[-ln?1-??]^1/3.?5?The contionous oxygen production process was simulated in a fixed-bed reactor and the effects of reaction temperature,gas flow,and oxygen concentration on the reduction and oxidation reactivity of oxygen carrier were investigated.The results showed that the outlet oxygen concentrations and reduction rates increase and the times for full conversion decrease with temperature increasing.The oxidation rates decrease with temperature increasing.The reduction and oxidation rates all increase with gas flow increasing.However,for reduction reaction,the highest oxygen concentration is restricted to equilibrium oxygen concentration.The highest oxygen concentrations are almost the same under different gas flows.The reduction and oxidation times for full conversion decrease as binder adding ratio increases.However,the reduction and oxidation rates also decrease as binder adding ratio increases.The reduction and oxidation rates of oxygen carriers with different binders increase in the order of Cu/Mg>Cu/Zr>Cu/Si.The reduction and oxidation reactions in the fixed-bed reactor all include the fast and slow stages.?6?Multi-cycles tests in the fixed-bed reactor were accomplished to investigate the durability and stability of oxygen carrier.Moreover,the reason of the deactivation of oxygen carrier was also analyzed.At the condition of no agglomeration,the optional adding ratios of SiO₂,ZrO₂ and MgAl₂O₄ in the oxygen carrier particles are 60wt.%,60wt.%and 50wt.%respectively under 1000? reduction and 900? oxidation temperatures.If the binder adding ratios are less than those,Cu-based oxygen carriers will deactivate.Based on XRD,BET and SEM-EDX characterization,the reasons of deactivation of Cu-based oxygen carriers were analyzed.Copper oxides could not react with binders and the active phase does not decrease in consecutive cycle reactions.The continuous oxygen diffusion process increases the number of pores inside the particles and the BET values of oxygen carrier increase.The element of Cu in the surface of oxygen carrier particle increases after cyclic experiment.The particles will stick together with CuO or Cu₂O as the bridge between particles.The sticking together of the particles prevents the oxygen diffusion among the particles and this is the main reason why the oxygen carrier deactivates.?7?Based on the heat and pressure analysis,chemical looping pure oxygen production system was designed.The effects of reduction temperature,oxidation temperature,reduction pressure,and oxidation pressure on the energy consumption of the system were analyzed.The energy consumption decreases with reduction temperature increasing and oxidation temperature decreasing.Moreover,the energy consumption is low when temperature difference of the two reactors is small.The energy consumption decreases with reduction pressure decreasing.The energy consumption decreases firstly and then increases with oxidation pressure increasing.