Research On Preparation Of Coal-based Activated Carbon For Separating CO₂

The activated carbons for pressure swing CO₂ adsorption were prepared by the procedures of fracture,blending,extruding,carbonization and activation.On the basis of the existing research,the influences of different raw material coal,coal particle sizes,forming pressures,additives,activation of surface functional groups on the properties of activated carbon were studied.The properties of activated carbons were characterized by CO₂ adsorption capacity,iodine adsorption,crystallite size,intensity,pore structural parameters,pore size distribution etc;The central composite design of response surface method was used in the optimization of preparation process of activated carbon.The optimization of the preparation process of activated carbon was carried out according to the experimental results,and the rationality of the model was also analyzed.The optimization conditions of the model were verified and the correctness of the model was tested.The experimental results show that the CO₂ adsorption capacity of activated carbon prepared by Taixi anthracite is the highest and its comprehensive performance is the best among the five kind of coal.There is a high correlation between CO₂ adsorptive capacity of activated carbon and the coal powder which is of 61-75?m and <45?m grain size.But we should mix coal powder of above-mentioned grain size with the coal powder in other size range properly in preparation process to prepare the activated carbon of the best performance.Molding pressure can influence the CO₂ adsorption capacity of activated carbon,a higher molding pressure can increase the specific surface area of active carbon and micropore volume of active carbon,however,it is not good for increasing the CO₂ adsorption capacity,and has little effect on the total pore volume and hardness.The amount of NaNO₂ and Na2SO3 added as additives has a significant influence on CO₂ adsorption capacity.In experimental range,the more additive added,the higher CO₂ adsorption capacity becomes.Take CO₂ as activating agent,the maximum CO₂ adsorption capacity of activated carbons increases at different degree when adding Na NO₂?KNO3 and KNO3+NH4Cl as additive respectively,and their maximum CO₂ adsorption capacities are 54.9mL/g?56.1m L/g and 58.9m L/g.When taking KNO3+NH4Cl as additive,the micropore volume and total pore volume of activated carbon are greatly improved,pore volume in range of 1-1.5nm and the specific surface area of active carbon have increased and CO₂ adsorption capacity has significantly improved which is 5.1 m L/g higher than that without the additive,so KNO3+NH4Cl is an ideal additive in preparation of pressure swing adsorption activated carbon.In the activation process,if adopt water vapor as activating agent first to activate mildly and then adopt CO₂ gas to activate deeply,there is little difference on the specific surface area,micropore volume and total pore volume of the activated carbon compared with those who is activated by CO₂ gas only,and the adsorption performances of activated carbon prepared by these two activating methods are similar.So we can adopt the mix activating method in the actual production of pressure swing adsorption activated carbon to improve the activation efficiency and reduce production cost.There is no significant change of the oxygen-containing functional groups of activated carbon when taking different carbonization temperature,activation temperature and additive amount.There is a close relation between CO₂ adsorption capacity and mono substituted reaction or 1,3 substituted aromatics content on the crystallites surface,but different activating agents may not affect the surface functional groups of coal based activated carbon.However,the influence of specific functional groups on the adsorption properties of activated carbon still needed to be further studied.According to the quadratic polynomial stepwise regression method,the correlation coefficient of the acquired mathematical model is 0.7954 which shows a high degree of fitting and good repeatability A prediction of the optimum process parameters: carbonization temperature of 550?,activation temperature of 916?,the activation time of 120 min,additive?KNO3+NH4Cl?ratio of 3.73%,the prediction result of CO₂ adsorption capacity is 57.6m L/g?1atm,25??.Several experiments are carried out under this condition and the average results of activated carbon adsorption capacity for CO₂ is 57.2mL/g which is only 0.4m L/g lower than the predictive value.The experimental results indicate that the process model is reasonable and effective.