Adsorptive Separation Of Xe/Kr Mixtures Using Nanoporous Carbons: A Molecular Simulation Study

With the development of modern industry and the improvement of human living standard,the consumption of fossil-fuel resources is increasing sharply,accompanied by a lot of greenhouse gas emissions,resulting in the greenhouse effect is one of the great challenges facing mankind.Nuclear energy has been recognized a promising alternative to fossil-fuels for its high energy density,minimal land requirement and extremely low greenhouse gases emission.There are a lot of radioactive gases（such as iodine（I₂）,methyl iodide（CH₃I）,krypton（Kr）and xenon（Xe））in the off-gas of used nuclear fuel（UNF）reprocessing,among which,due to the wide commercial value and application of Xe and Kr,it is very important to the adsorption and separation Xe/Kr mixture.A promising technology for separating Xe/Kr mixture is an adsorption-based process using a nanoporous material adsorbent that has the advantages of simple operation and low energy consumption.Nanoporous carbon has been widely used in gas mixture adsorption and separation because of its large specific surface area,adjustable pore size,and economy.In this work,we have adopted grand canonical Monte Carlo（GCMC）simulations to comprehensively investigate the adsorption of Xe/Kr,Xe/Kr/N₂,Xe/Kr/N₂/I₂,Xe/Kr/N₂/CH₃I and Xe/Kr/N₂/I₂/CH₃I mixtures of different compositions in well-shaped single-walled carbon nanotubes（CNT）,stratified activated carbon fibre（ACF-15）and completely disordered silicon carbide derived carbon（Si C-DC）,at different temperatures.The effects of CNT pore size,amorphous carbon structure,pressure,temperature,gas composition on the adsorptive separation performance of nanoporous carbons were thoroughly determined in this work.In consideration of extremely low partial pressures of Xe and Kr in the off-gas UNF reprocessing,the adsorption and separation performance of nanoporous carbon was studied.In addition,the impacts of impurity gases（I₂ and CH₃I）on the performance of nanoporous carbon separating Xe/Kr mixture was considered.The main research contents and results are as follows.By using GCMC simulations,we investigated the adsorption of Xe/Kr mixtures with the mole fractions of Xe ranging from 10%to 90%in CNTs having diameters of0.81 to 2.03 nm,ACF-15 and Si C-DC,at the temperatures of 248 to 348 K.For the（mole ratio,20/80）Xe/Kr mixture,the adsorption capacity of Xe and the Xe/Kr selectivity are related to the diameter of CNT.The increase of diameter of CNT will weaken the interaction of adsorbate-CNT,resulting in the decrease of Xe/Kr selectivity with the increase of diameter of CNT.At 0.1 MPa and 298 K,the adsorption of Xe and the Xe/Kr selectivity achieve their maxima,1.69 mol/kg and 36.3,in the（6,6）CNT among the CNTs considered,and this optimized selectivity is much superior compared to larger CNTs,ACF-15,Si C-DC and a large number of metal organic frameworks（MOF）including SBMOF-1 and Z11CBF-1000-2.We find that increasing temperature reduces the adsorption capacity and Xe/Kr selectivity in the all CNTs,ACF-15 and Si C-DC,due to the exothermal nature of adsorption.Furthermore,in the larger CNTs,the Xe/Kr selectivity is observed to be almost independent of the Xe concentration,as the adsorption of Xe/Kr mixtures approximately retains adjacent to Henry region.However,in the small CNTs,ACF-15 and Si C-DC,the Xe/Kr selectivity decreases with increase of mole fraction of Xe.To comprehensively measure the adsorptive separation performances of CNTs on separating Xe/Kr mixture,the performance coefficient,that evaluates both adsorption and selectivity is employed.It was found that the performance of the CNTs reduces with temperature,while demonstrating a complex dependency on the composition of Xe/Kr mixtures.Nevertheless,among the CNTs considered,the（6,6）CNT holds the best performance on the adsorptive separation of Xe/Kr in all the cases considered.This states the fact that the（6,6）CNT is a very promising material for the capture of Xe from the off-gas of UNF reprocessing.The effect of topology of nanoporous carbon on the adsorptive separation performance of nanoporous carbons was studied.It is found that when the average pore size is the same,the cylindrical structure of CNT strongly facilitates the overlap of the interaction energies from the opposite walls.Therefore,the adsorption of Xe and Xe/Kr selectivity of（8,8）CNT were higher than that of stratified ACF-15 and completely disordered Si C-DC.However,for ACF-15 and Si C-DC,the adsorption of Xe is basically the same as well as Xe/Kr selectivity.Therefore,the structure of amorphous carbon with the similar pore size distribution has little influence on the adsorption and separation performance of Xe/Kr mixture.To reveal the influence of volatile radionuclides,I₂ and CH₃I,on the adsorptive separation of Xe/Kr using nanoporous carbons,the adsorption of Xe/Kr/N₂ mixtures with and without I₂ and CH₃I in the CNTs,ACF-15 and Si C-DC were systematically investigated using GCMC and molecular dynamic（MD）simulations.In the absence of I₂ and CH₃I,the adsorption of Xe is adequately high（0.37 mol/kg）,and the corresponding Xe/Kr selectivity reaches up to 45.3 in the（6,6）CNT with a pore size of0.81 nm.Such a performance is significantly superior compared with the amorphous ACF-15,Si C-DC and CNTs with larger pore size.We find that albeit enhancing the amorphization of nanoporous carbons hardly impacts the sorption capacity in nanoporous carbons,it reduces the diffusion rate of sorbate by orders of magnitude due to introduction of the energy barriers to mass transport.Nevertheless,when either I₂ or CH₃I presents in the gas phase,the adsorptions of Xe and Kr are severely excluded in nanoporous carbons due to the dominant adsorption of I₂ and CH₃I.For the Xe/Kr/N₂/I₂/CH₃I mixture,the intense competitive adsorption occurs between I₂ and CH₃I in nanoporous carbons.Except the dominant adsorption of CH₃I in the（6,6）CNT,the adsorption of I₂ dominates in the rest carbon structures.It is found while the（6,6）CNT demonstrates the greatest potential on separating CH₃I from the Xe/Kr/N₂/I₂/CH₃I mixture,the（15,15）CNT achieves the best performance on separating I₂ among the carbon structures considered.Due to the combined effects from I₂ and CH₃I,the adsorptions of Xe and Kr are completely excluded in the nanoporous carbons investigated,emphasizing the necessity for the extraction of I₂ and CH₃I in prior to the adsorptive separation of Xe/Kr.