| Pressure swing adsorption(PSA)oxygen production has been widely used in the field of oxygen production at the plateau.At present,there are few theoretical studies on PSA oxygen production at the plateau.The influence of altitude on the performance of oxygen production device is not systematic enough,and absorbents for oxygen production that are adaptive to the plateau environment are lacking.In order to solve these problems,this paper carried out the experiment of high altitude PSA oxygen production process and molecular simulation research of oxygen production materials under high altitude environment conditions.Based on the improved Skarstrom two-bed cycle PSA oxygen production process,a PSA oxygen production experimental device with flexible and adjustable process parameters was established.The effects of process time parameters,purge gas volume,purge flow rate and product gas flow rate on oxygen production performance in plateau area were studied experimentally and theoretically analyzed,and the adjustment strategy of process parameters under different altitude conditions in plateau environment was discussed.The results show that the high altitude PSA oxygen generating performance is significantly lower in plain area,adjust the oxygen producing process parameters can effectively improve oxygen effect.The appropriate extension of adsorption time to improve the adsorption gas pressure to improve product performance,but time is too long can lead to zeolite adsorption bed through making product gas oxygen concentration fell sharply;The oxygen production effect is better under the condition of low purge time.Excess purgeing time will not further improve the product gas purity,but lead to a large amount of high purity gas waste.In the same amount of purge gas,different purge flow rate will also have an impact on the product gas flow and purity;Prolonging the pressure equalization time increases the adsorption pressure to some extent,and then improves the gas performance of the product.However,a long time of pressure equalization would lead to a large number of low-purity gas with pressure equalization entering the adsorption tower which has just completed the desorption process,which will affect the gas performance of the product.High-power air compressors should be equipped to improve the oxygen production performance of the oxygen generator under high altitude conditions.The higher the altitude.the lower the oxygen production efficiency.Appropriately increasing the product gas flow rate is helpful to eliminate the concentration of high purity product gas at the top of the adsorption tower and prevent oxygen from returning to the mass zone.Adsorption thermodynamics of N2 and O2 on two most commonly used zeolites(5A and Li-LSX)under plateau special conditions of wide ranges of temperature(238-298 K)and pressure(0-300 kPa)for pressure swing adsorption(PSA)oxygen production were studied based on molecular simulations and theoretical model fitting.Adsorption isotherms for each adsorbate-adsorbent pair at different temperatures were fitted by Langmuir,Freundlich and Sips models,obtaining equilibrium parameters including adsorption capacity,strength,heterogeneity and N2/O2 selectivity.Thermodynamic parameters such as Gibb’s free energy,enthalpy,and entropy changes were numerically derived,and isosteric adsorption heats and distributions of potential energy were simulated.The results suggest that the adsorption capacity and affinity follow the order of N2-Li-I.SX>N2-5A>O2-5A>O2-Li-LSX,leading to higher N2/O2 selectivity on Li-LSX compared to 5A.For Li-LSX,N2/O2 selectivity increases significantly with temperature above 273 K due to the difference in adsorption thermodynamics of N2 and O2 varying with temperature in terms of different adsorption heterogeneities.For 5A,the homogeneous distributions of N2 and O2 as well as the wide scopes of affinities from Na+ and Ca2+ exhibit N2/O2 selectivity with negligible temperature dependence,which is beneficial for stable oxygen production with fixed PSA processes under large day-and-night temperature difference.This study provides theoretical basis for obtaining an optimal balance between efficiency and stability of oxygen production by using a proper mixture of Li-LSX and 5A.After a long time of operation,the oxygen production efficiency may be reduced due to the absorption of water in the air.In the feed air with very low dew point(213 K),the adsorption capacity of FAU zeolite to water can still reach 10 wt%.The adsorption and separation characteristics of N2 and O2 in the process of oxygen production by PSA with water content varying from 0 to 10 wt%were studied under the two possible extreme temperature conditions of 238 K and 298K in plateau region.Binary adsorption isotherms and N2/O2 selectivity on each adsorbent were obtained with the ideal adsorbed solution theory method.The moisture content of FAU-1 zeolite is 0 wt%and the temperature changes from 238 to 298 K,the N2/O2 selectivity decreases by 18.9%.The selectivity of N2/O2 decreases by 58.3%when the moisture content changes from 0 to 10 wt%at 298 K,and the effect of the moisture content of the molecular sieve on the separation performance of N2/O2 is greater than the temperature change.Strong interaction between water and zeolite through hydrogen bond reduces the pore volume of zeolite by 34.56%and occupies the cationic position,thus forming competitive adsorption of N2 and O2 in thermodynamics.The increase in Si/Al molar ratio suppressed the negative impact of water and temperature change but sacrificed the N2/O2 selectivity.To reach an optimal balance between the air separation performance and the adsorbent robustness.The zeolite model of FAU was constructed,and the zeolite surface structure of the model FAU zeolite Si ’or Si position was screened by adding Cr3+,CO3+,Zn2+,Cu2+,Fe3+,Ag+,Pt2+,Sr2+,Ba2+,Ca2+,K+,Li+,La3+.Ce3+and Eu3+to the zeolite SI’ or SI position.Finally,the optimal Zn-FAU object was determined.The adsorption behaviors of N2 and O2 on Li-FAU and Zn-FAU zeolite models were studied.The results show that the adsorption energies of N2 at SⅡ and SⅢ sites on Zn-FAU are increased by 34.4%and 25.8%,respectively,and the adsorption energies of O2 are increased by 18.3%and 10.4%,respectively,after the addition of mixed cation.By adsorbent adsorption mass transfer model of the electronic properties,the study found by SⅠ’ position of Zn2+ to join,changed the atomic charge distribution of zeolite framework,making the oxygen atoms on the zeolite framework in zeolite adsorption role of Li+super cage shielding effect is abate,and Li+have to super cage center moving trend,under the condition of two kinds of function coupling,the Zn-FAU of N2 and O2 to enhance the adsorption effect.Based on DFT calculations Morse force field parameters of each other can curve fitting in Li-FAU and Zn-FAU GCMC simulation model,was obtained under the condition of different moisture content and temperature of N2 and O2 on the two kinds of zeolite adsorption isotherm and separation factor,combined with the plateau environment conditions of two kinds of zeolite adsorption separation performance was evaluated.The research results of this paper will provide theoretical and technical reference for the optimization of PSA oxygen production process in high altitude region and the development of oxygen production zeolite with good adaptability to the plateau environment,which has important scientific significance and application value. |
PDF Full Text Request