The Fundamental Study On Enhancing Adsorption Performances Of Zeolites Y And X For Deep Purification Of Sour Natural Gas

It is well known that with the growing natural gas consumption in the domestic market,China's natural gas industry is facing serious challenges.Chuanxi region has rich sour natural gas resources,however,the implement of traditional solvent absorption process for purifying Chuanxi natural gas is limited by the dense human population in this district.In this work,we focused on the adsorption removal of sulfides from natural gas and revealed the adsorption mechanisms by density functional theory?DFT?calculation.A series of ion-exchanged zeolites Y were prepared by ion-exchange method.Multiple characterization methods and DFT calculation were used to analyze and simulate the adsorption behaviors of H₂S,MeSH,CS₂,COS and CO₂ on adsorbents.The calculation results show the H₂S adsorption energy on Ag-Y is-92.3kJ/mol,which is 3.78 times than Ag-Y.Among all the samples,the MeSH adsorption energies of Cu-Y?i?,Cu-Y?ii?and Ag-Y are-119.9 kJ/mol,-136.3 kJ/mol and-131.8 kJ/mol,respectively,which were increased by 47.6%,67.8%and 62.3%from Na-Y.Moreover,the Mayer bond and Mulliken population analysis indicate the MeSH adsorption process is dominated by S-M interaction.Ag-Y has the strongest adsorption energy of-78.5 kJ/mol for CS₂,which was increased by 82.1%from Na-Y.The Mayer bond and Mulliken population analysis indicate that the adsorption process of CS₂ on Na-Y is dominated by ?-complexation,but the mechanism of CS₂ adsorption on Ag-Y is S-M interaction.The adsorption process of COS is also dominated by the S-M interaction,and Ag-Y shows the strongest COS adsorption energy of-62.7 kJ/mol.The adsorption energies of CO₂ on Cu-Y?i?,Cu-Y?ii?and Ag-Y are-29.8 kJ/mol,-35.3 kJ/mol and-24.0 kJ/mol respectively,which are much weaker than the adsorption energies of sulfides.On this basis,the competitive absorption behaviors between CO₂ and sulfides were simulated by DFT calculation,indicating CO₂ can weaken the S-M interaction,as a result,the adsorption performances of zeolites would be declined.The MeSH,CS₂ and COS adsorption performances of ion-exchanged zeolites Y were examined by using a static adsorption set-up.The MeSH equilibrium adsorption capacities of Cu-Y and Ag-Y were 73.0 mg/g and 81.0 mg/g,equivalent to 4.1 and 4.5 times to Na-Y,respectively.The CS₂ and COS equilibrium capacities of Ag-Y were 50.0mg/g and 55.2mg/g,equivalent to 6.0 and 5.0 times to Na-Y,respectively.The experimental results were consisted with DFT calculations.Then,the effects of temperature,pressure and CO₂ concentration on MeSH,CS₂ and COS adsorption performances were studied.The results Indicate that the increasing of adsorption temperature and CO₂ concentration could lead to decrease equilibrium capacity.Refer to the operation conditions in Chuanxi natural gas,the adsorption temperatures of MeSH,CS₂ and COS were set to 25?,the adsorption pressures were set to 5.0MPa and the CO₂ concentration of feed gas was set to 5.5v%.Moreover,the effects of pressure on MeSH/CO₂,CS₂/CO₂ and COS/CO₂ mixed gas were also studied.With the pressures increased from normal pressure to 5.0MPa,the MeSH equilibrium adsorption capacities of Cu-Y and Ag-Y were increased from 66.5mg/g and 74.3mg/g to 70.1mg/g and 79.6mg/g,increased by 5.4%and 7.1%,respectively.The CS₂ and COS equilibrium capacities of Ag-Y were increased from 42.3 mg/g and 47.4mg/g to 44.7mg/g and 50.3mg/g,increased by 5.7%and 6.1%,respectively.Indicating the increasing of pressure still can strengthen the sulfides adsorption performances of Cu-Y and Ag-Y,but the competitive adsorption between CO₂ and sulfides would weaken the desulfurization performances of Cu-Y and Ag-Y.In addition,the dynamic adsorption performances on single sulfide and mixed gas were tested.Based on the optimized adsorption conditions,when added 5.5v%CO₂ into feed gas,the MeSH breakthrough capacities of Cu-Y and Ag-Y were decreased from 70.0mg/g and 78.0mg/g to 64.0mg/g and 72.0mg/g,decreased by 8.6%and 7.7%,respectively.The CS₂ and COS breakthrough capacities of Ag-Y were decreased from 44.8mg/g and 53.0mg/g to 39.0mg/g and 46.0mg/g,decreased by 15.2%and 13.2%,respectively.Before reaching the breakthrough point,the sulfur content in purified gas was close to 0,indicating the competitive adsorption between CO₂ and sulfides would weaken the desulfurization performances of Cu-Y and Ag-Y,but they still can deep remove sulfides from feed gas.Based on the static adsorption results of mixed gas,when the adsorption pressure is increased to 5.0 MPa,the MeSH breakthrough capacities of Cu-Y and Ag-Y are expected to increase by 5%and 7%,respectively,and the CS₂ and COS breakthrough capacities of Ag-Y are expected to increase by 5%and 6%,respectively.The main difficulties in purifying Chuanxi natural gas are how to deep remove MeSH and CS₂.Among all the samples,Ag-Y shows the best MesH and CS₂ adsorption performances.Therefore,Ag-Y was selected to purify the simulated natural gas.The increasing of temperatures were negative to purify the simulated natural gas,so the temperature was also set to 25?.With the pressure was increased to 5.0MPa,the breakthrough capacity of Ag-Y was increased from 67.8mg/g to 72.3mg/g,increased by 6.6%.Refer to the operation conditions in Chuanxi natural gas,the pressure was set to 5.0 MPa.Then,the dynamic adsorption performance of Ag-Y on simulated natural gas was also tested.Before reaching the breakthrough point,the sulfur content in purified natural gas was close to 0.At GHSV of 240h-1 and 25?,the breakthrough capacity of Ag-Y was 64.8mg/g.Based on the static adsorption results of simulated natural gas,when the adsorption pressure is increased to 5.0 MPa,the breakthrough capacity of Ag-Y is expected to increased by 5%.The desorption performances of used zeolites were also studied.After adsorbed simulated natural gas,the optimized desorption conditions for Ag-Y were air atmosphere,GHSV 400h-1 temperature 400?,duration 3h.After 7 adsorption-desorption cycles,The breakthrough capacity of Ag-Y was 58.4 mg/g,declined by 10.1%.Which is accounting for 93%of the sulfur capacity of UDS-2 solvent.Furthermore,the breakthrough capacity of Ag-Y can be expected to increase a little,when the adsorption pressure is at 5.0 MPa.The 240h aging test indicates that the desorption process will not change the framework of Ag-Y.The main difficulty in purifying Claus tail gas is how to deep remove COS.The dynamic adsorption performance of AgX on simulated Claus tail gas was also studied.Before reaching the breakthrough point,the sulfur content in purified tail gas was close to 0.At GHSV of 144h-1 and 25?,the breakthrough capacity of AgX was 52.6mg/g.The optimized desorption conditions for AgX were air atmosphere,GHSV 400h-1,temperature 400?,duration 3h.After 6 adsorption-desorption cycles,The breakthrough capacity of AgX was 49.2 mg/g,declined by 6.5%.Which is accounting for 79%of the sulfur capacity of UDS-2 solvent.The 240h aging test indicates that the desorption process will not change the framework of AgX.Based on the research results,the zeolites Y and X combination technology for deep purifying Chuanxi sour natural gas was initially completed.The research results of this work provide theoretical foundation and technical support for developing sour natural gas deep adsorption desulfurization process,and further reveal the selective adsorption mechanisms of sulfides on modified zeolites Y and X.