Experimental Study On The Gas Absorption Performance Of Adsorbents Under Vacuum Conditions

High vacuum multilayer insulation technology is also known as"super insulation",which is widely used in cryogenic vessels such as liquid nitrogen,liquid hydrogen,liquid oxygen and corresponding vacuum insulation pipes.The deterioration of vacuum performance is one of the main reason for the decline of the life of cryogenic vessels.Therefore,improving the application level of vacuum technology in cryogenic vessels is one of the important tasks to ensure that low temperature liquids are stored and transported in an energy-saving,economical,safe and reliable manner.At present,two aspects of research have become hot issues:one is the analysis and research of the residual gas in the interspace of cryogenic vessels;the other is the research and promotion of adsorbents adsorption performance.Generally speaking,the source of gas leaking into the heat insulation interspace of the cryogenic vessels is air,and its main components are N₂ and O₂,while the metal materials and heat insulation materials in the cryogenic vessels emit H₂（70%）under vacuum.Therefore,it is extremely important to study adsorbents with excellent adsorption performance for N₂,O₂,and H₂ under low-temperature vacuum conditions.Molecular sieves have rich pore structure,large adsorption capacity,large specific surface area,strong polarity,and easy ion exchange modification,which makes it widely used in separation,drying,purification and dehydration.In order to investigate the adsorption characteristics of molecular sieve adsorbents used in the interspace of cryogenic vessels,the adsorption isotherms of 4A,5A and 13X molecular sieves for N₂、O₂ single components and air were obtained by static expansion method.In the range of 10^-310³Pa,the differences in adsorption capacity of different molecular sieves for gases were compared,and the adsorption mechanism of molecular sieves was explored.The results show that 5A and 13X molecular sieves have strong adsorption performance for N₂ and O₂ under vacuum conditions at liquid nitrogen temperature,the sorption capacity can reach 10⁴Pa·L/g magnitude.The sorption capacity of O₂ in 4A molecular sieve can also reach 10⁴Pa·L/g magnitude,while 4A molecular sieve has poor nitrogen sorption ability when the equilibrium pressure is higher,the saturated sorption capacity reaches only about 300Pa·L/g.The sorption capacity of three molecular sieves for air is:13X molecular sieve>5A molecular sieve>4A molecular sieve,and at liquid nitrogen temperature,the adsorption rate of 5A molecular sieve on air is higher than that of 13X molecular sieve.Studying the vacuum adsorption characteristics of molecular sieves at low temperatures helps to guide the application of molecular sieves in cryogenic vessels,which also provides a reference for the design of cryogenic molecular sieves.Molecular sieves are microporous materials.The adsorption and diffusion processes of adsorbent gases in micropores are related to microporous structure.Gas adsorption method is widely used to characterize the specific surface area,micropore volume and pore size distribution of porous materials.The microporous structures of4A,5A,and 13X molecular sieves were characterized by gas adsorption method using N₂、CO₂ as adsorbates.The BET specific surface areas of 4A,5A,and 13X molecular sieves are:484.4586cm²/g,573.9056cm²/g,and 673.8285cm²/g.The pore volumes of4A,5A,and 13X molecular sieves are 0.1519cm³/g,0.1949cm³/g,and 0.2523cm³/g,respectively.The three kinds of molecular sieves are rich in micropores,and the adsorption is mainly a process of filling the micropore volume.As catalytic hydrogenation reaction materials,palladium and platinum transition metal oxides have excellent hydrogen absorption performance.Due to poor H₂adsorption performance of molecular sieves at 77K,in order to investigate the adsorption characteristics of adsorbents used in the interspace of cryogenic vessels,adsorption isotherms of hydrogen adsorbents were obtained by static expansion method.The adsorption performance of adsorbents on hydrogen was compared by analyzing isotherms.The mechanism of hydrogen adsorption by platinum oxide was studied.The pore structure information of platinum oxide was analyzed by BET theory,t-Plot method and BJH method to guide the application of platinum oxide in cryogenic vessels.The results show that PdO has excellent hydrogen absorption performance in a wide pressure range,and the adsorption capacity can reach about1.7×10⁴Pa·L/g.After the equilibrium pressure is greater than 10Pa,PtO₂ has a strong adsorption effect on hydrogen.When the equilibrium pressure is in the range of1×10^-2Pa³×10^-2Pa,the adsorption performance of Ag₂O is better.While not reducing the adsorption performance as a whole,it is believed that the combination of PtO₂ and Ag₂O in the hydrogen absorbent PdO commonly used in cryogenic vessels to form combined adsorbents can greatly reduce the cost of the adsorbents and improve economics.The total pore volume,specific surface area,micropore volume,and micropore specific surface area of PtO₂ are all larger than PdO.The advantages of pore structure allow PtO₂ to provide a larger place for adsorption and reaction.