Experimental Study On Reactive Crystallization In An Internal Airlift-loop Reactor

Airlift loop reactors(ALR)are widely used in biotechnology and wastewater treatment,because of their efficient mixing and good mass/heat transfer characteristics.Compared with the stirred crystallizer,ALR has no moving mechanical parts,and the shear rate is moderate and relatively uniform throughout the reactor volume.In view of these excellent properties,ALR has been considered as a new prospective crystallizer.However,few reports have been found in the literature for the study on reactive crystallization in an ALR.The objective of this work is to study the reactive crystallization in an ALR using the precipitation of Ni(OH)2 as a model reaction.Firstly,the spherical Ni(OH)2 particles were prepared by orthogonal experiments in an ALR.Then,the growth of Ni(OH)2 particles in an ALR and a stirred tank was quantified by scanning electronic microscope(SEM),X-ray diffraction(XRD),laser particle analyzer,tap densitometer and optical microscope,and the data on particle growth was analyzed.It is found that the tap density of Ni(OH)2 particles mainly depends on the size of crystallites:the bigger the size of crystallites is,the bigger the tap density is.Based on these,the growth process of Ni(OH)2 particles in an ALR is elaborated.Crystallites precipitated from solution aggregate to form large particles with much void.These constituting crystallites continue to grow up,which takes up the void inside particles and makes the tap density increase.Then,the influence of the volume of ammonium hydroxide(VN),pH,feeding methods,the height of draft tube(Hd)and gas flow rate(qg)on the growth of crystallites in an ALR is explored.The main conclusions are as follows:(1)The size of Ni(OH)2 crystallites increases firstly and then decreases with the increase of VN,pH and Hd.When VN=300 mL,pH=11.4 and Hd=250 mm,the size of Ni(OH)2 crystallites gets to the maximum.(2)The micro-mixing in the gas disengagement zone is the most efficient.The crystallites grow up bigger when the feeding position is located here.(3)Gas flow rate has an important effect on the growth of crystallites.The Ni(OH)2 crystallites grow up bigger with the increase of gas flow.