Break-up of nano-particle agglomerates by hydrodynamically limited processes

When dry nano-particulate powders are first added into a liquid, clusters as large as hundreds of microns can be formed. In this study, high shear impellers, such as the sawtooth Ekatomizer and rotor-stator impellers were used to suspend and break-up these agglomerates in a stirred vessel. The high local energy dissipation rates generated by these impeller could slowly break up clusters to sub-micron sizes by an erosional mechanism. In comparison, single and multiple passes through a valve homogeniser could quickly break the nano-particle clusters to sub-micron sizes; single pass operation had the highest breakage efficiency for a given specific energy input. For both equipment types, the rate of fines generation was found to be controlled by the maximum energy dissipation rate. However, the size of the fine aggregates produced was a constant and was not a function of the energy dissipation rate

Break up of silica nanoparticle clusters using ultrasonication

This study is concerned with the deagglomeration of hydrophilic silica nanoparticle clusters (Aerosil® 200V) in water using an ultrasonicator operated in batch mode. An impeller was also present in the tank to ensure homogeneity. The effect of power input was studied in the range of 18 to 77 W (9 to 39 kW m-3) on the kinetics and mechanisms of deagglomeration and the dispersion fineness. The effect of particle concentration was also studied in the range of 1 to 15% wt. The process was monitored through the evolution of particle size distribution (PSD), which indicated erosion as the dominant mechanism of breakup. The smallest attainable particle size was found to be independent of power input and solid concentration. Faster break up kinetics were noted as the power input was increased whereas increasing the solids concentration to 15% wt. slowed the process. It could also be shown that processing concentrated dispersions can be beneficial as the break up rate assessed on the basis of energy per unit mass of solids was faster for increased particle concentration

Comparative performance of in-line rotor-stators for deagglomeration processes

Comparative performance of in-line rotor-stators for deagglomeration processe

Gas-liquid mass transfer : a comparison of down-and up-pumping axial flow impellers with radial impellers

The performance of a down- and up-pumping pitched blade turbine and A315 for gas-liquid dispersion and mass transfer was evaluated and then compared with that of Rushton and Scaba turbines in a small laboratory scale vessel. The results show that when the axial flow impellers are operated in the up-pumping mode, the overall performance is largely improved compared with the down-pumping configuration. Compared with the radial turbines, the up-pumping A315 has a high gas handling capacity, equivalent to the Scaba turbine and is economically much more efficient in terms of mass transfer than both turbines. On the other hand, the uppumping pitched blade turbine is not as well adapted to such applications. Finally, the axial flow impellers in the down-pumping mode have the lowest performance of all the impellers studied, although the A315 is preferred of the pitched blade turbine

Dispersion of clusters of nanoscale silica particles using batch rotor-stators

Nanoparticle powders added into a liquid medium form structures which are much larger than the primary particle size (aggregates and agglomerates)-typically of the order of 10’s of microns. An important process step is therefore the deagglomeration of these clusters to achieve as fine a dispersion as possible. This paper reports the findings of a study on the dispersion of hydrophilic fumed silica nanoparticle clusters, Aerosil 200 V, in water using two batch rotor-stators: MICCRA D-9 and VMI. The MICCRA D-9 head consists of a set of teeth for the stator and another for the rotor, whereas the VMI has a stator with slots and a rotor which consists of a 4-bladed impeller attached to an outer set of teeth. The dispersion process, studied at different power input values and over a range of concentrations (1, 5, 10 wt.%), was monitored through the evolution of PSD. Erosion was found to be the dominant breakage mechanism irrespective of operating conditions or rotor-stator type. The smallest attainable size was also found to be independent of the power input or the design of the rotor-stator. Break up kinetics increased upon the increase of power input, and this also depended on the rotor-stator design. With MICCRA D-9 which has smaller openings on both the stator and rotor, the break up rate was faster. Increasing the particle concentration decreased break up kinetics. It could also be shown that operating at high concentrations can still be beneficial as the break up rate is higher when assessed on the basis of specific power input per mass of solids

Breakup of nanoparticle clusters using microfluidizerM110-P

A commercial design, bench scale microfluidic processor, Microfluidics M110-P, was used to study the deagglomeration of clusters of nanosized silica particles. Breakup kinetics, mechanisms and the smallest attainable size were determined over a range of particle concentrations of up to 17% wt. in water and liquid viscosities of up to 0.09 Pa s at 1% wt. particle concentration. The device was found to be effective in achieving complete breakup of agglomerates into submicron size aggregates of around 150 nm over the range covered. A single pass was sufficient to achieve this at a low particle concentration and liquid viscosity. As the particle concentration or continuous phase viscosity was increased, either a higher number of passes or a higher power input (for the same number of passes) was required to obtain a dispersion with a size distribution in the submicron range. Breakup took place through erosion resulting in a dispersion of a given mean diameter range regardless of the operating condition. This is in line with results obtained using rotor-stators. Breakup kinetics compared on the basis of energy density indicated that whilst Microfluidizer M110-P and an in-line rotor-stator equipped with the emulsor screen are of similar performance at a viscosity of 0.01 Pa s, fines volume fraction achieved with the Microfluidizer was much higher at a viscosity of 0.09 Pa s

Effect of Axial Agitator Configuration (Up-Pumping, Down-Pumping, Reverse Rotation) on Flow Patterns Generated in Stirred Vessels

Single phase turbulent flow in a tank stirred with two different axial impellers - a pitched blade turbine (PBT) and a Mixel TT (MTT)- has been studied using Laser Doppler Velocimetry. The effect of the agitator configuration, i.e. up-pumping, down-pumping and reverse rotation, on the turbulent flow field, as well as power, circulation and pumping numbers has been investigated. An agitation index for each configuration was also determined. In the down-pumping mode, the impellers induced one circulation loop and the upper part of the tank was poorly mixed. When up-pumping, two circulation loops are formed, the second in the upper vessel. The PBT pumping upwards was observed to have a lower flow number and to consume more power than when down-pumping, however the agitation index and circulation efficiencies were notably higher. The MTT has been shown to circulate liquid more efficiently in the up-pumping configuration than in the other two modes. Only small effects of the MTT configuration on the power number, flow number and pumping effectiveness have been observed

Characterisation of the delamination process of nanoclays

Nanocomposites comprising delaminated nanoclays result in superior properties of the final product. This study aimed at monitoring the processes of intercalation and exfoliation of different nanoclay-polyol pairs performed through different dispersion protocols. X-ray diffraction, rheology and particle sizing are discussed in relation to the evolving product properties

Mixing performance of viscoelastic fluids in a kenics km in-line static mixer

AbstractThe mixing of ideal viscoelastic (Boger) fluids within a Kenics KM static mixer has been assessed by the analysis of images obtained by Planar Laser Induced Fluorescence (PLIF). The effect of fluid elasticity and fluid superficial velocity has been investigated, with mixing performance quantified using the traditional measure of coefficient of variance CoV alongside the areal method developed by Alberini et al. (2013). As previously reported for non-Newtonian shear thinning fluids, trends in the coefficient of variance follow no set pattern, whilst areal analysis has shown that the >90% mixed fraction (i.e. portion of the flow that is within ±10% of the perfectly mixed concentration) decreases as fluid elasticity increases. Further, the >90% mixed fraction does not collapse onto a single curve with traditional dimensionless parameters such as Reynolds number Re and Weissenberg number Wi, and thus a generalised Reynolds number Reg=Re/(1+2Wi) has been implemented with data showing a good correlation to this parameter

Scalable production of large quantities of defect-free few-layer graphene by shear exfoliation in liquids

To progress from the laboratory to commercial applications, it will be necessary to develop industrially scalable methods to produce large quantities of defect-free graphene. Here we show that high-shear mixing of graphite in suitable stabilizing liquids results in large-scale exfoliation to give dispersions of graphene nanosheets. X-ray photoelectron spectroscopy and Raman spectroscopy show the exfoliated flakes to be unoxidized and free of basal-plane defects. We have developed a simple model that shows exfoliation to occur once the local shear rate exceeds 10(4) s(-1). By fully characterizing the scaling behaviour of the graphene production rate, we show that exfoliation can be achieved in liquid volumes from hundreds of millilitres up to hundreds of litres and beyond. The graphene produced by this method performs well in applications from composites to conductive coatings. This method can be applied to exfoliate BN, MoS2 and a range of other layered crystals