Thermodynamic Study On Capture Of CO₂ From Flue Gas By Hydrate Crystallization

The global warming issue has attracted the world attention. Excessive emission of greenhouse gas has caused disastrous effect, such as: global warming, climate anomaly, extreme bad weather increases, and so on. Recognizing the problem of potential global climate change, the WMO and UNEP established the Intergovernmental Panel on Climate Change(IPCC). The IPCC have organized five global climate researches, found that the updated 100-year linear trend(1906 to 2005)of 0.74°C is therefore larger than the corresponding trend for 1901 to 2000 given in the Third Assessment Report of 0.6°C.According to the different CO2 capture time and recycle methods, there are 3methods for CO2 capture: Post-combustion capture, Pre-combustion capture and Oxy-fuel Combustion capture. Traditional CO2 capture methods include absorption,adsorption, membrane separation technology, electrochemical and low temperature distillation separation. Hydrate-based capture of CO2 is a new separation technology,it offered several advantages including simple process, mild reaction conditions and without producing secondary pollution. The development of hydrate technology will lead to the great progress for new energy technology, new cold storage technology,new separation technology and new gas pollutant treatment technology.In this investigation, the mixed gases include CO2 and N2 are being used as the experimental object. Tetra-n-butyl ammonium bromide(TBAB), Cyclopentane(CP)and glucoamylase were chosen as additives for hydrate formation. Based on the predecessor investigations, the orthogonal optimization experiment was carried out with the separation factor, the CO2 capture rate, the gas consumption and the CO2 concentration of hydrate phase. Through detailed study, the following conclusions can be made:(1) The thermodynamic study of CO2 hydrate formation in glucoamylase solution.The effects of water-soluble glucoamylase at four concentrations of 2.79, 5.58,13.95 and 27.9 wt % on CO2 hydrate formation phase equilibrium have been investigated. When the concentration of glucoamylase was 5.58 wt %, it has positive effect on lowering CO2 hydrate formation. Compared with TBAB, the promotion effect of glucoamylase is a little bit milder. When the concentrations of glucoamylase were 27.9 and 13.95 wt %, the glucoamylase have inhibiting effect on hydrate formation. Compared with methanol, the inhibiting effect of glucoamylase system is a little bit milder. When the concentration of solutions was 2.79 wt %, the effect was almost the same as pure water. It is the critical concentration of glucoamylase solution which appeared positive effect or inhibition effect on CO2 hydrate formation.(2) The thermodynamic study of CO2/N2 hydrate formation in TBAB/CP/glucoamylase solution. The influences of TBAB/CP solution and TBAB/CP/glucoamylase solution on flue gas hydrate formation characteristic have been analyzed, respectively. The result shows that both the TBAB/CP solution and the TBAB/CP/glucoamylase solution can help the hydrate to be formed. When the concentration of glucoamylase was 5.58 wt%, the function of TBAB/CP/glucoamylase solution is much better than TBAB/CP solution.(3) The kinetic study of CO2/N2 hydrate formation in TBAB/CP/glucoamylase solution. The TBAB/CP/glucoamylase solution could improve the gas consumption of flue gas hydrate. When the concentrations of glucoamylase is5.58 wt%, the gas consumption of flue gas hydrate in TBAB/CP/glucoamylase solution has improved 37.6% than TBAB/CP solution.(4) The study of capture process. In this investigation, we use TBAB/CP/glucoamylase solution to be additives to help the capture process. The effect of different operating conditions on separation process is analyzed.Orthogonal optimization method is being used to select the optimum separation conditions. The result shows that when the liquid gas ratio is 100:200, the concentration of glucoamylase was 5.58 wt%, the initial temperature is 280.15 K and the initial pressure is 4MPa, the investigation shows the optimal result. The optimal experimental results are the separation factor is 16.77, the CO2 capture rate can reach 79.29%, the gas consumption can reach 0.132 mol and the CO2 concentration of hydrate phase is 51%.