Vacuum Desorption Behavior And Flame Retardants Modification Of Activated Carbon For VOCs Adsorption

Treatment of industrial waste gases containing volatile organic compounds（VOCs）have become key issues in the field of environmental protection.Adsorption is regarded as one of the key methods for industrial treatment of VOCs pollution emissions due to the advantages of its convenient operation and lower cost.In practical application,the adsorbent needs to be regenerated through high temperature desorption to increase operating frequency and reduce cost.It has become the focus on how to perform the desorption and regeneration for the adsorbents safely and efficiently.Here,a high-temperature vacuum desorption system was designed and built.The influence of temperature and vacuum degree on vacuum desorption efficiency of activated carbon（AC）and other porous VOCs adsorbents for different vacuum desorption characteristics were studied systematically.The changes of ignition temperature and adsorption-desorption characteristics of modified activated carbon by Tetraethyl orthosilicate（TEOS）were investigated.Furthermore,the modified activated carbon adsorbent with excellent thermal stability and high VOCs adsorption capacity was prepared,which effectively reduced the cost and potential safety hazards of the adsorption-desorption for VOCs.Taking toluene as the model VOCs,the optimal test model of activated carbon and other adsorbents were selected through static adsorption.Then the vacuum desorption characteristics and main influencing factors of VOCs adsorbents were explored by multi-factor orthogonal test.The results showed that the vacuum desorption rate of activated carbon was slower,and all the experimental groups completed the desorption process within 130 min basically.The desorption rate on activated carbon was positively correlated with temperature and vacuum degree,it was great promotion of toluene on activated carbon to increase temperature.Besides,the"pumping three constants one"mode was conducive to the vacuum desorption of toluene.Reusability tests showed that the desorption rate of the modified AC remained at nearly 80%even after five cycles of repeated adsorption-desorption.The desorption cost of the adsorbent was analyzed and compared,it was found that the cost of activated carbon was lower and the advantage was obvious in long service life.Based on the above desorption results,a series of modified activated carbons with different Si-C-mass ratio（Si/C-0.1,0.3,0.5,0.7 and 0.9）were prepared,and the research object was to study the ignition temperature and VOCs adsorption-desorption characteristics of the modified carbon.The results showed that not only the ignition temperature of Si/C-0.1 increased by 56℃,but also the saturation adsorption capacity increased by 31.31%.Furthermore,increase of ignition temperature by 79℃was achieved over the Si/C-0.9 sample.The characterization results of XPS,FTIR,SEM,HRTEM,BET and XRD of the modified activated carbon materials revealed that TEOS continuously hydrolyzed to form amorphous Si O₂ nanoparticles in the alkaline high-temperature environment.Moreover,the"conversion threshold"appears,that was,Si-O bond continuously transforms to Si-C bond when the Si-C mass ratio is higher than 0.5 in the physical phase system.Because of the formation of amorphous silicon carbide dense layer structure blocked the adsorption hole of modified carbon,reducing the original adsorption site of activated carbon,and resulting in the continuous decrease of the adsorption capacity.Taking toluene as the model VOCs,the static,dynamic adsorption kinetic characteristics and repeated adsorption-desorption properties of modified activated carbon were further studied.The results showed that,comparing to Si/C-0.9,the adsorption potential of Si/C-0.1was higher and diffusion rate was faster,the adsorption diffusion rate of both were mainly determined by Knudsen diffusion.The dynamic adsorption of toluene by modified ACs fitted well with the Yoon-Nelson model,adsorption isotherm model and pseudo-first-order adsorption kinetics model.Reusability tests showed that the desorption rate of the modified AC remained at nearly 80%even after five cycles of repeated adsorption-desorption,indicating that the modified AC should have a great potential for industrial applications.