| During the last decades,haze caused a lot of serious problems,not only had negative effects on transportation such as traffic jams,delayed air travel and serious traffic accidents,but also was a warning instrument of high concentrations of PM2.5 which can influence people’s health.Research has shown that haze caused serious economic loss as well.The latest survey of the American chamber of commerce in China showed that air quality have become the biggest obstacle to top talents when they choose cities for getting a job.Lately,the size of particulates in exhaust fumes from factories has reduced.Many European cities and Asian cities have conducted a number of experiments and actual investigations to explore the emission and formation of ultrafine particles.So recent interests have been attracted to particles with much smaller diameter even ultrafine particles(diameter less than 0.1 μm).However,due to the limitations of the developed technologies,most of the current gas filtration systems can only remove particulates after condensing or quenching the raw gas,this can result in high costs and low efficiency.The demand for hot gas filtration,therefore,is urgently driven by serious air pollution.Currently,various materials have been employed to fabricate filter media for hot gas filtration,such as polytetrafluoroethylene(PTEF)fibers,polyphenylene sulphide(PPS),glass,ceramic and metal.However,some of these filter medias have short working lives in corrosive and high temperature atmosphere leading to frequent filter replacement.At the same time,others are too expensive to be used in a large quantity for this application.As to the filter structure,most of the current hot gas filter media are composed of regular fibers with diameters around tens to hundreds micrometers.These filters have low filtration efficiency especially for particulates at the PM 2.5 level.Filters with smaller pore sizes are usually thought to have higher filtration efficiency.This observation seems to be logical,since a smaller mean pore size would account for the improved filtration efficiency that is associated with increased fiber coverage density.Due to the high surface to volume ratio and small pores,ultrafine fibrous filters displayed much higher filtration efficiency.Gas slip effect,which occurs on ultrafine fibers,reduces air resistance.Thus,ultrafine fibrous filters,though have a compact structure,do not cause much pressure drop.A number of ultrafine fibrous filters have been developed,such as Ultra-Web?,Fibra-Web?,Finetex Mats TM and AMSOIL Ea Air Filters.However,fine fibers do not possess sufficient mechanical properties to withstand the high pressure of gas flow for industrial applications,especially when heated at a high temperature.Efficient,economic and reliable filter media for hot gas particulate removal are still needed to be developed.Herein,polyimide nanofiber membrane,carbon woven fabrics,CNT sheet were introduced into this research work of hot gas filtration to manufacture the filters with good filtration performance.This work can been divided into four parts as follows:(1)In order to fabricate a hot gas filter with excellent filtration performance,polyimide nanofiber membrane/ woven fabric composite hot gas filter media was designed to efficiently capture fine particles.Polyimide(PI)nanofibers membranes were synthesized on supporting layers served as filtration layers.In order to compensate the low pressure resistance and poor mechanical properties of PI nanofiber membrane,the supporting layers were made from carbon woven fabrics,glass filament woven fabric or Kevlar woven fabric.The result showed that the carbon woven fabrics have the best mechanical property among this three materials.The filtration performance of this novel filter showed that the maximum filtration efficiency of PI/carbon woven fabric composite filter could reach 99.99%,while maximum pressure drop was only about 230 Pa after continuously testing for 17 minutes under a constant filtration velocity of 2 m/min.The composite filters exhibited good gas filtration property not only because of the excellent thermal and chemical stability of PI and carbon fibers,but also owing to the excellent stable structure maintained during filtration test.(2)A series of Polyimide nanofiber membrane/carbon woven fabric composite filters were successfully fabricated by controlling the mass of polyimide membrane.Compared with the commercial filters(P84/glass),all these prepared filters showed an excellent filtration efficiency(above 99.99%)and lower pressure drop even double the productivity.As the areal density of the PI nanofiber membrane increases,the filtration efficiency increases first and then levels off when the areal density reaches 11.64 g/m2.In regeneration performance test with back air flush at a pressure of 500 k Pa,the composite filter maintains a filtration efficiency of 99.99% and a pressure drop of about 410 Pa.After heat treatment at 260 and 300oC,the composite filter shows a relatively high filtration efficiency while the tensile strength of the carbon fabric does not change significantly.SEM images showed that PI membrane played a dominant role in the filtration process,while carbon woven fabric with large pore size reduced the pressure drop and in favor of dust cleaning.The excellent structural and high filtration performance endowed the Polyimide nanofiber membrane/carbon woven fabric composite filters with potential use in hot gas filtration.(3)Carbon woven fabrics supplied with high voltage were served as hot gas filters.This study investigates the effects of high voltages on the filtration properties of carbon woven fabric filters by varying parameters of the areal density of fabric,face velocity,the number of fabric layers,and the relative humidity.The filtration performance tests show that the corona initiation voltage applied in carbon woven fabric should be higher than 20 k V,and filtration efficiency can reach 90% while the pressure drop is just 6 Pa when the supplied voltage is 40 k V.Meanwhile,filtration efficiency increases with the increasing of fabric areal density and supplied voltage.As the face velocity increases from 2 m/min to 4 m/min,the filtration efficiency under high voltages between 30 k V and 40 k V decreases approximately 13%.While when the face velocity increases form 4 m/min to 8 m/min,the filtration efficiency keeps almost unchanged.Results also show that the increased layers of filter only have little influence on the filtration efficiency,not very prominent,while the filtration efficiency increased dramatically with the increasing of relative humidity.The maximum filtration efficiency reaches 99.8% for PM 1.0.Meanwhile,the corona inception voltage decreased to 10 k V as the relative humidity increased from approximately 40% to 95%.In addition,the results of antibacterial tests show that high voltage filters can reduce the concentration of microorganism in ambient air.(4)Filters composed of aligned carbon nanotube(CNT)sheets with average diameter of 30 nm sandwiched between polyimide nanofiber membranes were fabricated to capture fine aerosol particles.Electrospun polyimide(PI)nanofibers with average diameter of 260 nm served as the supporting layers.Electrospun polyetherimide(PEI)nanofibers with average diameter of 600 nm were used as a bonding layer.The CNT sheets and PI nanofiber membranes were bonded together by melting PEI nanofibers.Two different kinds of filter structures were prepared where the multiple layers of CNTs were either stacked together on top of each other or separated from each other by a PEI layer.The filtration performance tests showed that the filtration efficiency increased as the number of CNT sheet embedded in the filters increased.The maximum filtration efficiency of composite filters with 4-layers of CNTs reached 99.99% for particles with diameters of 0.3 um,while the pressure drop only reached 120 Pa.In mechanical testing,the CNT sheets also reinforced the composite filters,even though the PI nanofiber membrane has relatively high mechanical properties.The tensile strength of 4-layer CNT composite filters showed the highest tensile strength of 9 MPa which is amoung the highest strengths for a nanofiber membrane.The composite filters could be used for hot gas filtration applications as the filters performed well up to 200 oC.In summary,three different kinds of hot gas filters were fabricated.Firstly,polyimide nanofiber membrane/carbon woven fabric sandwiched hot gas filters were manufactured for removal of PM 2.5 particles.Their structural properties,tensile properties,bending properties and filtration performance before and after heat treatment were tested.Secondly,carbon woven fabric filters supplied with high voltage for removal of PM 1.0 particles were fabricated by combining the filtration mechanisms of bag filter with mechanisms of electrostatic precipitator.To obtain the filters with best filtration performance,the effect of fabric structure,supplied voltage,the number of carbon woven fabric layers and relative humidity on the filtration performance were studied.In addition,high performance carbon nanotube/ PI nanofiber membrane composite filters with two different kinds of structures were fabricated.The structural properties,thermal stability of filters and its effect on filtration performance were also evaluated.The results showed that all the three different kinds of filters manufactured in this work possessed the potential to serve as filtration media in bag filters for hot gas filtration to remove particles at PM 2.5 level. |
