Investigation Of Effects Of Rotating Liquid Film Reactor On Precipitation Processes

Compared with other reactions,precipitation is a fast reaction process and hard to control,which restricts its development on preparing inorganic functional nano-materials.Although there are many researchers using various reactors to control the precipitation,the limited reaction area and high mixing velocity still cannot be integrated well.Considering this aspects,the present work aims to use rotating liquid film reactor(RLFR) to prepare nano-sized BaSO₄ and MgAl-CO₃-LDHs precipitates in a controllable manner,investigate its effects and mechanism on precipitation and explain the flow field of RLFR,mass fraction of species as well as crystal size and crystal size distribution(CSD) of precipitates by simulation.RLFR was improved in the structure and put forward critical volume flow rate,which was successfully applied to precipitation reactions of BaSO₄.Nano-sized BaSO₄ with mean diameter of 45.5 nm was precipitated in RLFR,and the results of XRD and TEM suggested that BaSO₄ prepared in RLFR have small mean diameter and narrow CSD compared with those came from conventional precipitation,and the crystal morphology were also different,which means BaSO₄ precipitation was greatly influenced by RLFR.The Crystal size and CSD of BaSO₄ decreased with the increasing of reactant concentration from 0.1 M to 1.3 M,rotor speed from 1000 rpm to 5000 rpm and rotor-stator gap from 0.1 mm to 0.5 mm,while kept constant in various solution adding speed in further experiments with different operation conditions.Nano-sized MgAl-CO₃-LDHs with mean diameter of 55.2 nm was prepared in RLFR and compared with slow double drop method.XRD pattems showed that the crystallinity of MgAl-CO₃-LDHs was increased by RLFR.TEM and laser particle size analyzer displayed that MgAl-CO₃-LDHs prepared in RLFR have smaller crystal size and narrower CSD.Further investigation was carded out and the results showed that there are no obvious differences when characterized by XRD and laser particle size analyzer in rotor speed of 1000～5000 rpm and rotor-stator gap of 0.1～0.5 mm.Slow and fast double drop method were used to prepare MgAl-CO₃-LDHs,and the results suggested that MgAl-CO₃-LDHs prepared with fast double drop method has smaller crystal size and narrower CSD,which proved that precipitation can be intensified by increasing reactant mixing rates.RLFR can enhance this intensification effects for its good micro-mixing effect in all operating conditions.After confirming the flow type,selecting the flow models,setting the boundary conditions and calculating parameters,the flow field of RLFR was simulated by FLUENT.The simulated results demonstrated that the flow field is laminar and the species transport effect is weak below the rotor speed of 3000 rpm and rotor-stator gap of 0.3 mm.The flow field is inclined to form Taylor vortices by increasing the rotor speed and rotor-stator gap,which are beneficial to the species transport effect.The precipitation of BaSO₄ was simulated using FLUENT and user defined function(UDF) coupled with Navier-Stokes equations,species transport equations and population balance equations.The mass fraction of BaSO₄,crystal size and CSD was obtained and which were approximately identical with experimental results.The calculated results showed that Kolmogorov scale was small,mixing time was shorter than induction time and particles had big angle velocity in rotor-stator gap,which explained the effects of RLFR on precipitation.The calculated nucleation rate of MgAl-CO₃-LDHs was 3.13×10²¹ P·(m³s)^-1.According to population balance model,the precipitation process of MgAl-CO₃-LDHs was simulated in RLFR.Crystal size and CSD of MgAl-CO₃-LDHs were in good accordance with laser particle size analyzer and TEM respectively with and without considering aggregation effect.