Equilibrium Conditions And Kinetics Of Hydrates Forming With Quaternary Salts

Human civilization is increasingly depending on the energy sources since the industrialized society. The fossil fule as non-renewable energy resource will be the main supplement for human beings in a long period of time, and as the improvement of mankind energy demand year by year, people have to face with the issues about energy exhaustion and environmental pollution. The serious situation for pollution has been threat the survival of the creatures in Earth and the long-term development of human civilization. Thus, the scientific and effective utilization of energy has become the subject of the modern society. Gas hydrate as one kind of phase change material can be used as both cold storage material and transportation medium in the field of refrigeration and air-condition. Due to the phase change of gas hydrate, the cold storage capacity of gas hydrate is excellent, which leads to the reduction of the charging and leaking amount of the anti-environment primary refrigerant. Furthermore, the usage of gas hydrate can decrease the pumping power for primary refrigerant circulation. In addition, the CO2 gas(CO2/N2 mixture) in flue gas from power plants burning fossil fules can be separated by the formation and dissociation of gas hydrate. The separated CO2 gas will be collected and recycled to reduce the CO2 emission and the greenhouse effect. The formation of gas hydrate needs harsh conditions of high pressure and low temperature. To alleviate the formative condition and reduce energy consumption, some quaternary salts as phase transfer catalyst are adopted. The quaternary salts are compound where four hydrogen atoms in one cation are replaced by alkyls in which the cations include ammonium ion, phosphorus ion, etc. and the anions include fluorinion, chloridion, bromide ion,, etc. As a typical quaternary ammonium salt, tetra-n-butyl ammonium bromide(TBAB) draws lots of attentions, and the CO2+TBAB hydrate forming in CO2+TBAB+H2O system can be treated as the proper material for cold storage and gas separation for the effective utilization of energy and environmental protection.The phase equilibrium conditions of gas hydrate is very important for hydrate-based large-scale industrial application. However, the phase equilibrium data of gas hydrate forming from CO2+quaternary salt+H2O system such as CO2+TBAB hydrate is very deficient and the experimental measurements of the data is necessary. Different quaternary salts can form gas hydrate with CO2 with different characteristics of phase equilibrium conditions and enthalpy dissociation, etc. Likewise, the capacities of cold storage and gas separation may different. Thus, the TBAB, TBAC, TBPB and TBPC are adopted in the present paper. The equilibrium conditions of the above quaternary salt hydrates forming with CO2 are measured, and the morphologies of them in hydration processes are presented with visualization method. Meanwhile, the growth characteristics of TBAB hydrate and CO2+TBAB hydrate are studied. Moreover, the CO2 separation efficiencies in one-stage separation process from simulated flue gas are comparatively determined in the present paper. The research works are mainly as follows:With the method of observation and intermittently temperature increase, the phase equilibrium conditions of CO2+TBAB, CO2+TBAC, CO2+TBPB and CO2+TBPC hydrates are measurement in a wide range of pressure and mass fraction of solution. Besides, the phase equilibrium conditions of TBAB, TBAC, TBPB and TBPC hydrates forming at atmospheric pressure are measured. On the basis of the phase equilibrium conditions of hydrate, the dissociation enthalpies of the above hydrate forming with CO2 are obtained by the Clausius-Clapeyron equation.The morphologies of TBAB, CO2+TBAB, TBAC, CO2+TBAC, TBPB, CO2+TBPB, TBPC and CO2+TBPC hydrates in the growth processes are presented with visualization method, and the effects on the morphologies of hydrate from subcoolings and mass fractions of solution are concluded. The morphologies of TBAB hydrates forming at atmospheric pressure with and without agitation are presented and the mass fraction of w = 0.09 is favorable for the hydration with two exothermic processes, i.e., the type A and type B TBAB hydrates form in order. With the visualization method, the growth velocities of TBAB hydrate forming at w = 0.09 are measured at different subcoolings in the present paper.With the isochoric experimental method, the mole of CO2+TBAB hydrate forming at w = 0.05 and 0.10 are obtained at different subcoolings by mass conservation and the pressure variation of CO2 gas. The solubility of CO2 gas in pure water and TBAB aqueous solution at w = 0.05, 0.10 and 0.19 are experimentally measured. Meanwhile, the verification of the type B CO2+TBAB hydrate forming with agitation by the visualization method are also carried out. The results show that the number of CO2 gas molecules encaged into one CO2+TBAB hydrate molecule is in the range of 2.51 and 3. Moreover, the kinetics of CO2+TBAB hydrate forming at w = 0.05 and 0.10 at different subcoolings are determined in hydration process. Larger mass fraction of TBAB solution can increase the formation velocity of CO2+TBAB hydrate in a short time, but prolong the hydration process which is not favorable for the rapid growth of CO2+TBAB in hydration process.The phase equilibrium conditions of CO2+N2+TBPB hydrate with simulated flue gas are measured. On the basis of the equilibrium data, the experiments of CO2 separation from simulated flue gas are conducted at proper temperature. By two-stage separation process, the CO2 composition in initial gas mixture increases from 17.0 mol% to above 90.0 mol% with TBPB. In addition, TBAC, TBPB and TBPC are adopted for one-stage separation, and the results show that TBPB can achieve the best separation efficiency among them. Moreover, the dissociation enthalpies of the CO2+N2+TBPB hydrate forming in the two separation processes are obtained by the Clausius-Clapeyron equation and studied in comparison with that of CO2 +TBPB hydrate.On the basis of Chen-Guo model, one modified model is proposed in the present paper to calculate the phase equilibrium conditions of CO2+TBAB hydrate. The results from the model show better accordance to the experimental data and the trend of prediction by the model is the same as that of experimental data. The new model can provide theoretical basis for the phase equilibrium conditions of CO2+TBAB hydrate.