| The gas hydrate equilibrium data were measured for oxygen, nitrogen, and air in the presence of additives, respectively. A novel method of oxygen enrichment via air hydrate crystallization(OEAHC) was proposed in this work, which based on the difference in the phase equilibrium temperature and pressure for oxygen and nitrogen when air hydrate formed. The regularity of oxygen enrichment in the hydrate phase was studied respectively for the air and the oxygen enriched air(OEA) system in the process of hydrate crystallization. The process was established by Aspen Plus11.1 software of OEAHC and the separation energy ratio(SER) of OEA production was calculted by process simulation at a variety conditons of air feed pressure and oxygen concentration, and the direction and measure was pointed out to reduce the energy consumption of OEAHC.Three phase equilibrium data(liquid water-hydrate-vapor) were determined for oxygen, nitrogen, and air in the presence of the promoters, respectively. The oxygen + SDS + water system was measured in a high pressure stainess vessel by the method of pressure search at constant volume and temperature, and the temperature and pressure range is 276.4 K~284.2 K and 17.39 MPa ~ 41.994 MPa, respectively. The air + CP + water system was measured in a stainess vessel by the method of temperature-pressure search with continuous heating, and the temperature and pressure range is 284.0 K ~ 296.2 K and 1.09 MPa ~ 9.45 MPa, respectively. The oxygen + THF + water system, the nitrogen + THF + water system, the air + THF + water system and the air + TBAB + water system was determined in a stainess vessel by the method of temperature-pressure search with stepwise heating, and the temperature and pressure range is 279.7 K~299.4 K and 0.46 MPa ~16.68 MPa, respectively. To the four kinds of additives used in this work, the results show that SDS could hardly affect the phase equilibrium conditions of oxygen hydrate, but introduction of THF, CP and TBAB can significantly change the hydrate equilibrium data and make them shift to the higher temperature and lower pressure areas. For the air + water system, when the system pressure is less than 1.5 MPa, three kinds of additive effect from strong to weak are TBAB,CP and THF, when the system pressure is in the range of 1.5 to 2.5MPa, the order is CP, TBAB and THF, and when the system pressure is higher than 2.5MPa, the order is CP, THF and TBAB.The rule of oxygen concentration change with time in the hydrate phase was studied in the air hydrate formation process under the synergistic effect of THF and SDS. The 5.56 mol% of THF and 250 ppm of SDS was introduction into the system, and the residual gas in the vessel was samped every 10 minutes in the process of air hydrate formation to analysis its compositon by gas chromatography. The oxygen mole concentration in the hydrate pahse was calculated based on the mass conservation of air in the vessel. The results show that the rules of oxygen concentration change with time in the air hydrate formation process are similar to each other for the experiments with different initial pressure in the range of 3.12 MPa ~ 6.98 MPa. That is a large value in the beginning, and then it gradually decreases with the hydrate formation process until it reaches a minimum value, and then start to increase graduallyand finally reached to a constant value. For the system with different initial pressure of 3.12 MPa and 4.50 MPa and with the same supercoiling temperature difference of 9.7℃, the oxygen concentration in the hydrate phase and oxygen recovery ratio for the former is 23.5 mol% and 51.2 mol%, respectivly, and the latter is 25.5 mol% and 41.4 mol%, respectively. Which indicates that the higher initial pressure is beneficial to improve the oxygen concentration, but the lower initial pressure is favour of oxygen recovery.The oxygen enrichment regulation was studied of the oxygen enriched air of 38.9 mol% via hydrate crystallization. The first scheme is to maintain the gas hydrate formation pressure in a certain range through change the effective volume of the vessel. The results show that the lower hydrate formation pressure is favor of improve the oxygen concentration, but is adverse to the oxygen recovery. A maximum oxygen concentration of 43.2 mol% and a minimum recovery of 40.0 mol% was got in the experiment with the hydrate formation pressure is in the range of 3.0 MPa ~3.50 MPa. The second scheme is that the air hydrate was formed in the high pressure range of 27.86 MPa ~34.72 MPa with introduction of SDS into the system, and the results show that the rules of oxygen concentration and recovey ratio change with hydrate formation time is similar to the system introduction of THF in the first scheme, but the maximum oxygen concentration is only can reach to 41.4 mol%. This incidates that the conditions of the high pressure without the thermodynamic promoter is not suitable for the oxygen enrichment of oxygen enriched via hydrate crysatllization.A process was established with Aspen Plus11.1 for oxygen enrichment via air hydrate crystallization. In the condition of the feed air is of 15.786 kg· h-1, the simulation results show that the oxygen enriched air output is 5.468 kg· h-1, and the energy consumption is 5.31 k W·h·(Nm3·O2)-1. The energy consumption could be decreased to 2.33 k W·h·(Nm3·O2)-1 if the static hydrate formation enhencetment method was used to instead of the mechnical stirring and the minimum recycling solution in theory was used. In this case, the energy ratio of oxygen enrichment is 403, which is far greater than the traditional air enrichment methods. The SER of oxygen enrichment was simulated in multi-operation conditions for two kinds optimization process, respectively. To the procss of integrated with the heat pump system, the SER of oxygen enrichment via air crystallization is equal to cryogenic distillation in the condition both of the hydrate phase equilibrium is reduced to 0.11 MPa and the oxygen enrichment concentration is not less than 50 mol%. And to the process integrated with waste heat utilization, The SER of oxygen enrichment via air crystallization via air crystallization is equal to or less than that of cryogenic distillation if the feed air pressure could be reduced to 0.11 MPa and the product concentration of oxygen could be enriched to not less than 30 mol%. |
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