Incorporation and deagglomeration of fine particles using an in-line rotor-stator

The research presented in this thesis is concerned with a processing device that can be used for both the incorporation and deagglomeration stages of creating a nanoscale dispersion, namely the Ytron ZC1. The objectives of this project fall into two themes: Firstly, establishing the power characteristics and suction performance of the Ytron ZC1 using simulant liquids, and secondly using powder trials to establish the performance of the device for incorporation and deagglomeration processes. Two similar rotor-stator heads were used, that differed in the gap width between teeth, gaps of 1.5 and 3.0 mm were used.

Power characteristics and suction performance were studied with a range of glycerol-water mixtures and rotor speeds to achieve a Reynolds number range of 2 x 103 – 9.6 x 105. Liquid flow rates through the Ytron ZC1 were varied between 0.15 to 3.06 L s-1. Suction performance was assessed by making measurements of air velocity at the powder inlet without powder present and was found to initially increase with increasing liquid flow rate, then found to be constant with regards to flow rate, after which air suction decreased sharply. Air suction velocities increased with increasing Reynolds number and were found to be higher with the 3.0 mm head.

Between Reynolds numbers of 1 x 104 and 9.6 x 105 the power from the Ytron ZC1 could be expressed with a single expression for each head:

P=0.20 ρN^3 D^5+13.02 ρN^2 D^2 Q_2 for the 1.5 mm head

and

P=0.18 ρ N^3 〖 D〗^5+14.08 ρ〖 N〗^2 〖 D〗^2 Q_2 for the 3.0 mm head.

The incorporation rate of Aerosil® 200V into water was found to be proportional to air suction, thus conditions that result in higher air suction velocities result in faster incorporation rates. Incorporation rates were constant with respect to solids concentration until a solids concentration of 10% w:w was reached with the specific particle liquid pair used in this study. This corresponded to a change in the rheology of dispersions. Based on the results of the incorporation trials an empirical relationship between air flow rate and incorporation rate was developed, allowing for the reduction in powder trials necessary to include the Ytron ZC1 Into processes. Although this relationship is specific to this design of rotor stator, a similar approach can be taken for any device that operates by drawing in powder at lower than atmospheric pressure.

Deagglomeration was studied at solids concentrations up to 10% w:w over a flow rate range of 1 – 3 L s-1 and rotor speeds between 5100 and 6380 RPM. These conditions allow a Reynolds number range of 2.3 – 9.65 x105 and a power per unit volume range of 28.62 – 51.86 kW m-3. Erosion was found to be the dominant mechanism of breakup, and the smallest attainable size was found to have a Sauter mean diameter [d32] of ~200 nm. This mechanism of deagglomeration and smallest attainable size was also found with bench scale processing devices. Deagglomeration kinetics were found to increase with power input and were found to be unaffected by the gap widths studied, and solids concentration in the range of 1 – 10% w:w, therefore when assessed on the basis of power input per unit mass of solids it was found to be more energy efficient to operate at higher powder concentrations. At constant power input as residence time in the head increases more fines are generated per pass, in this study this resulted in an increase in deagglomeration kinetics. in previous studies, at constant power input, higher flow rates have shown an increase in deagglomeration kinetics. The results of this study can be used to design dispersion processes without the need for expensive and lengthy powder trials. The mechanism of deagglomeration and smallest attainable size can be determined in smaller scale devices and the kinetics of deagglomeration can be predicted by running targeted powder trials to determine optimum conditions.