Research On Preparation Of Biomass-based Activated Carbons And Their Performance On VOCs Removal

Volatile organic compounds（VOCs）have attracted much attention due to their harm to the environment and the human body.In their treatment methods,the adsorption method using activated carbons as adsorbents has become the most widely used method in the industry due to its low cost and convenient operation.However,the method now still has the problems,such as limited raw materials,lower adsorption capacity and difficulty of desorption.In this paper,corncob,corn straw,millet straw,pepper straw and cotton straw were used as raw materials to prepare the activated carbons.Experimental conditions of impregnation ratio,carbonization temperature and carbonization time were controled to change the structure of activated carbons.The adsorption capacity was determined by dynamic adsorption method with ethyl acetate as adsorbent.The influence of experimental conditions on the pore structure of activated carbon and the effect of pore structure of activated carbon on the saturated adsorption capacity of toluene were studied.The saturated activated carbon was regenerated by VSA-TSA（vacuum-temperature swing adsorption）technology.The effects of desorption temperature,desorption pressure and purge gas flow rate on the desorption ratio of activated carbon and VOC concentration of desorbed gas were investigated.The results of N₂ dsorption-desorption,SEM and FT-IR characterization of corncob-based activated carbons showed that with the increase of carbonization temperature,the surface functional groups of activated carbons decreased and the specific surface area increased first and then decreased.The activated carbon prepared at 550℃ have the largest specific surface area.With the increase of impregnation ratio,the micropore volume increased first and then decreased,and the mesoporous pore volume increased continuously.When the impregnation ratio was higher than 1.5,the activated carbon was mainly mesoporous structure.The effect of carbonization time on the pore structure was smaller.The results of dynamic adsorption experiments showed that the activated carbon prepared under the conditions of carbonization temperature of 550℃,impregnation ratio of 1.0 and carbonization time of 1.0 h had the highest adsorption capacity.When the toluene concentration was 3000 mg/m³,the saturated adsorption capacity was 414.6±13.0 mg/g.Micropores play a dominant role in the toluene adsorption,the presence of mesopores promotes the diffusion of molecules which facilitates adsorption behavior.The VSA-TSA experimental results show that when the purge gas flow rate was 0.2 L/min,the desorption temperature was 65℃,and the desorption pressure was 11 k Pa,the desorption rate was 74±3% after 40 minutes.After five adsorption-desorption cycle,the saturated adsorption amount was stable.This paper also explores the structure of activated carbons with different biomass（core of corn straw,peel of corn straw,millet straw,cotton straw,pepper straw）as raw materials.Under the condition of considering the yield of activated carbon,the yield of raw materials and the adsorption capacity of toluene and ethyl acetate,the millet straw-based activated carbon was selected as the main research object.The preparation conditions of the activated carbon were optimized by the central combination design（CCD）method.The response surface analysis showed that when the carbonization temperature was 572℃,the carbonization time was 1.56 h,and the impregnation ratio was 1.60,the expected function of the toluene adsorption capacity and the ethyl acetate adsorption capacity reached the maximum value.It was 321.9 mg/g of toluene and 240.4 mg/g of ethyl acetate.The VSA-TSA technology was used to regenerate the saturated activated carbon.The experiment showed that the desorption rate of ethyl acetate was higher than that of toluene under the same operating conditions,which was related to the less desorption resistance of ethyl acetate.After several cycles of experiments,the adsorption capacity of toluene and ethyl acetate on the millet straw-based activated carbon was stable,indicating that it can be used as a potential adsorbent in industry.