Study On Microwave Regeneration Mechanism And Key Technology Of Waste Activated Carbon

With the improvement of the standard of living,people pay more and more attention to the air quality of the living environment.As a common indoor pollutant,formaldehyde poses a serious threat to people’s health.Therefore,it is very important to study the degradation technology of formaldehyde.Activated carbon pore structure and specific surface area is very developed,adsorption performance is superior,often used to adsorb pollutants.However,the service life of activated carbon is short,and after saturated adsorption,it will lose activity and become waste,resulting in environmental pollution and waste of resources,so the regeneration research of waste activated carbon is of great significance.Microwave regeneration is a superior waste activated carbon regeneration technology with high efficiency,uniformity and low consumption,which has attracted more and more attention.In this thesis,formaldehyde was selected as the target pollutant,coconut shell granular activated carbon was used as adsorbent,and then the regeneration experiment of waste activated carbon loaded with formaldehyde was carried out by microwave heating method.The influence of different microwave regeneration conditions on the regeneration effect of activated carbon was investigated.The feasibility of microwave regeneration of waste activated carbon was verified through experiments,and an efficient and energy saving regeneration scheme was explored.In addition,the experiment also discussed the influence of HNO₃modified activated carbon on formaldehyde adsorption behavior in the way of converting physical adsorption into physical and chemical combined adsorption,and then compared and analyzed the formaldehyde removal ability of waste activated carbon before and after modification under the action of microwave heating and the recovery of adsorption performance of reclaimed activated carbon.In this thesis,the physical structure and surface chemical properties of activated carbon before and after regeneration were characterized by BET pore structure analysis,N₂isothermal adsorption,FT-IR spectroscopy,Boehm titration and XRD phase analysis,and the regeneration mechanism of waste activated carbon by microwave heating was analyzed.The results show that:（1）The experiments verified that microwave regeneration of waste activated carbon is feasible,and the adsorption performance of the regenerated activated carbon recovered well.This study has shown that 400°C is the ideal temperature for regeneration and formaldehyde can be desorbed in a short time.And comprehensive consideration of microwave energy utilization rate and regeneration effect,the carrier gas flow rate of 4 g/min is the optimal choice.If the influence of diffusion resistance is ignored,the larger the thickness of activated carbon bed is,the better the regeneration effect is.When the thickness of activated carbon bed is 4 cm,the waste activated carbon can fully absorb microwave energy,and the regeneration effect is significant.Comprehensive analysis shows that the ideal regeneration conditions of formaldehyde-loaded waste activated carbon are as follows:the thickness of activated carbon bed is 4 cm,the regeneration temperature is 400°C,and the carrier gas flow is 4 g/min.After microwave regeneration of waste activated carbon containing formaldehyde,compared with the iodine adsorption value of the original carbon（804.16 mg/g）,the iodine adsorption value of the regenerated activated carbon reached 1073.25 mg/g.We can see that the adsorption performance of activated carbon after microwave regeneration has been recovered,even higher than that of the original carbon.At the same time,the microporous structure and specific surface area of activated carbon were positively affected by microwave,and microwave also has a physical modification effect on activated carbon.Therefore,microwave irradiation can not only achieve the pollutant desorption of waste activated carbon,but also play the role of pore formation and pore expansion,so that the microporous structure of the regenerated activated carbon is richer,and its adsorption performance is better than that of the original carbon.（2）The modification of activated carbon by nitric acid can effectively remove the carbon debris in the pores of activated carbon and facilitate the formation of new pores in the activated carbon.However,the strong oxidation of nitric acid will also destroy the surface of activated carbon,resulting in the collapse of part of the carbon hole wall,and the destruction of the internal tubular pores to form a transition hole to mesoporous development.In the process of formaldehyde adsorption,this kind of pore structure is more of a channel.At the same time,nitric acid can significantly increase the content of acidic oxygen-containing groups on the surface of activated carbon,and the effect ofπ-πbond is enhanced,which is conducive to the absorption of polar substances in the gas phase.The adsorption behavior of formaldehyde on the modified activated carbon was highly correlated with the fitting degree of Langmuir equation,Freundlich equation and quasi-second-order adsorption rate kinetic model,indicating that the modification of nitric acid was beneficial to the adsorption of formaldehyde on the activated carbon,and the physical-chemical combined adsorption was the main action.（3）Microwave regeneration experiment of waste activated carbon modified by nitric acid was carried out.It was found that some formaldehyde molecules remained in the pore structure of the modified waste activated carbon after microwave heating,which had not been desorbed completely.The adsorption performance of regenerated activated carbon was only partially recovered,and the iodine adsorption value was 915.35 mg/g,which was far lower than that of modified carbon and unmodified regenerated carbon.This is because chemical adsorption is more difficult than physical adsorption to achieve the adsorption of activated carbon adsorption adsorbent desorption.The physical adsorption of formaldehyde under the action of van der Waals force can be easily desorbed gradually with the increase of temperature.However,the chemisorption of modified activated carbon is more stable,and has irreversibility,and basically does not occur desorption,so the regeneration is difficult.Although the surface of the activated carbon modified by nitric acid is rich in acidic oxygen-containing functional groups,most of them are converted to alkaline functional groups after microwave heating.This phenomenon greatly weakens the adsorption capacity of the regenerated activated carbon to polar substances,which is also the reason why the adsorption performance of the modified activated carbon after microwave regeneration is inferior to that of the unmodified regenerated activated carbon.