Fundamental Study On The Effect Of Nanoparticles On Ultrafine Powder Fluidization

A good particle fluidization behavior is very important in fluidization technology.The major challenge in fluidization of Geldart Group C fine powder is its cohesive nature because of the strong interparticle forces,which normally results in agglomeration and channeling.Nanoparticles as fluidization aids can improve the flowability of Group C powder and promote its fluidization behavior.This thesis has studied the effect of nanoparticles on Group C and A powder flowability,including FT4,angle of repose.Fundamental studies on fluidization behavior of Group C powder with nanoparticles have been carried out,including minimum fluidization velocity and bed expansion rate etc.,and a comparison between Group C and A powder fluidization behaviors has been made.In addition,dense phase properties of Group C and A powder have been analyzed through bed collapse test.In this study,the results show that nanoparticles can significantly improve the flowability of Group C powder.The FT4 cohesion and angle of repose of Group C powder with nanoparticles decrease indicating a better flowability,and an optimum nanoparticle concentration exists in improving the flowability.Fundamental studies on fluidization behaviors of Group C and A powder with respect to minimum fluidization velocity and bed expansion rate were conducted in a small fluidization column with 5.08 cm I.D.and 45.72 cm in height.It is concluded that nanoparticles can dramatically decrease the minimum fluidization velocities of Group C powder.Group C powder without nanoparticles cannot be fluidized,forming channeling.However,Group C particles after adding nanoparticles can be fluidized well at a suitable gas velocity.In this study,two methods have beed adopted to evaluate bed expansion rate,observed directly and calculated from pressure drop.Group C powder with nanoparticles can achieve a very high bed expansion rate,and the optimum nanparticle concentration is 0.8 wt%,at which the bed expansion rate reaches the highest.And for Group C powder,bed expansion rates of two methods are almost the same.For Group A powder,it can be fluidized well without nanoparticles and the effect of nanoparticles on its fluidization behavior can be ignored.The bed expansion rate of Group A powder is much lower compared to Group C powder.Moreover,the bed expansion rates obtained by two methods have a big difference,meaning that there are many big bubbles in Group A powder fluidization and bed pressure drop fluctuates fiercely.While there are few bubbles in Group C powder fluidization,the bubbles are smaller,resuling in a better gas-solid contact.Last but not least,dense phase properties of Group C and A powder have been investigated by conducting bed collapse test.The results show that the dense phase bed height and dense phase voidage of Group C powder are much higher than that of Group A powder,which means that there are more gas in the dense phase of Group C powder fluidization,resulting in a larger gas-solid contact area.This phenomenon has a significant influence on gas-solid reactions in industry.What is more,a new parameter RG,called bed height growth factor,has been proposed to characterize particle expansion ability.It is defined as the ratio of bed expansion rates to gas velocities after minimum fluidization.An increasing bed height growth factor indicates a better ability in expansion and a larger gas-solid conatct area,which has a profound meaning for improving reactivity in industry.