Process Intensification: Spinning Disc Reactor Technology for TiO2 Nanoparticles Production

Process intensification: Spinning disc reactor technology for TiO₂ nanoparticles production

Somaieh Mohammadi*, Kamelia V.K. Boodhoo, Adam P. Harvey

School of Chemical Engineering & Advanced Materials,

Newcastle University, Merz Court,

Newcastle Upon Tyne NE1 7RU, UK

Corresponding author: s.mohammadi@ncl.ac.uk

Tel: +44 191 222 7169

Fax: +44 191 222 5292

Abstract

An in-depth study of TiO₂ precipitation has been performed on a spinning disc reactor (SDR). Effects of physical parameters such as rotational speed, surface texture and size, and operating parameters such as flowrate, ratio of precursor to water and location of feed introduction points were studied.

Samples collected from the disc were subjected to dynamic light scattering (DLS) to quantify the particle sizes and particle size distribution. At higher disc rotational speeds, particles with sizes close to 1 nm and with a narrow particle size distribution (PSD) were formed due to the high disc speeds, the thin films formed on the surface of SDRs experience high mixing intensity and a high degree of plug flow. Smaller particles with a narrower PSD were obtained using the grooved disc texture because the grooved disc is more effective at ensuring plug flow behaviour compared to a smooth disc under the same operating conditions.

Smaller particles and narrower particle size distributions were achieved by introducing the titanium tetra isopropoxide (TTIP) precursor into the water film away from the centre which allows better mixing between the two streams.  

1.       Introduction

Most pharmaceuticals and fine chemicals are produced in stirred tank reactors. The degree of heat/mass transfer and mixing of stirred vessels is reduced at larger scales because the surface to volume ratio decreases and the distribution time of contents increases, whilst the corresponding mixing intensity decreases at invariable stirrer speed. Accordingly, large vessels tend to inhibit reactions, which may be inherently rapid and highly exothermic.

The SDR is regarded as a key process intensification (PI) development in chemical production. Such PI significantly decreases the scale of the production process, which can lead to a smaller and simpler plant, lower material costs, lower waste, improved temperature control, excellent heat transfer and a safer process [1].

The fluid residence times in a SDR are in the range of a few seconds compared with a few hours in a stirred tank. The SDR generates higher mixing intensity than a stirred vessel and should consequently be capable of retaining uniform concentration profiles within a fast reacting fluid. Therefore, better control would be applied over the reaction path than that possible in a conventional stirred vessel. Considering the pharmaceutical industry's drive to cut down production times, the SDR, with its high potential productive capacity, may allow laboratory-scale or pilot-scale vessels to achieve the same production levels as full scale vessels, thereby preventing delays in validating various levels of scale-up. Small reactor holdup (<100 mL) and tighter fluid temperature control make the SDR suitable for highly hazardous reactions [2].

The high mixing intensity within the liquid film, enhanced by waves and ripples on the interface, means that it is attractive for homogeneous reactions such as crystallisation and precipitation. An important aspect of employing SDR as the apparatus of choice, for precipitation production of nanoparticles, is the micromixing time, which needs to be very short to ensure that homogeneous nucleation dominates [3]. The micromixing [4] and macromixing [5] in SDR films have been studied recently. The SDR behaves as a plug flow reactor at a wide range of rotational speeds and flowrates [5]. A plug flow crystalliser provides excellent productive crystallisation methodology and a consistent product quality (narrow particle size distribution). PI techniques might facilitate better methods of precipitation of TiO₂ and other products, as demonstrated by the precipitation of barium sulphate on a spinning disc, which yielded significantly smaller crystals than the batch technique [6]. The main factor controlling this was the very high rates of mixing on the spinning disc, which led to the rapid depletion of supersaturation, and much higher nucleation rates. Cafiero et al. [6] also demonstrated that the energy input in the spinning disc process was much lower than the use of a T-mixer arrangement, suggesting that operating costs would also be reduced along with better control of crystal size.

Nanosized TiO₂ represents a promising research subject for various modern fields of science and technology, including nanobiotechnology and fundamental medicine [7]. Nevertheless, the synthesis and stabilisation of nanodispersed forms of TiO₂ are challenges for industry and better methods of production are required. Therefore, the intention of this study is to determine the experimental conditions for which consistent product quality for TiO2 particle formation prevails in the spinning disc reactor.

2.       Materials and Methods

To characterize and predict experimentally the performance of the SDR, TiO₂ precipitation experiments were performed on a 30 cm stainless steel rotating disc. Smooth and grooved disc (8 concentric grooves) textures were studied.

The schematic set up of the rig is illustrated in Figure 1(a).

Titanium Tetra Iso Propoxide (TTIP) (the precursor) and acidified water (pH = 1.5) were introduced to the surface of the disc at 50 ^⁰C.  TTIP was injected into the water film at 3 different radial positions: centre of the disc; 5 cm distance to centre and 10 cm away from the centre as shown in Figure 1(b).

Figure 1. (a) Schematic setup of experiment, (b) top view of disc, showing TTIP stream injection positions

3.       Results and Discussion

Figure 2 shows the effect of rotational speed on particle size and particle size distribution on both the smooth and grooved discs. It can be seen that at higher disc speeds that narrower size distribution and smaller particles are achieved. Higher disc speeds cause strong shearing forces and produce a thin layer of the reagent solution, which results in uniform heat transfer and homogenous concentration fields throughout the entire reaction mixture. An increase in rotational speed also increases the intensity of surface waves and promotes transverse mixing across the film thickness, thus achieving a more uniform velocity profile at any given radial position. Consequently, after nucleation, all the particles have very similar growth conditions, resulting in narrower size distribution of the nanoparticles. Figure 2 also indicates that at identical operating conditions, a grooved disc leads to production of smaller and narrower particle sizes. A grooved disc is more effective at establishing plug flow than a smooth disc. This has been attributed to continuous film detachment and reattachment on the grooved disc which enhances turbulence in the film [5].

Figure 2. Effect of rotational speed and disc texture on particle size distribution

The effect of feed entry position on the particle size and distribution is shown in Figure 3. By injecting the TTIP at increasing distances from the centre, smaller particles and narrower particle sizes can be produced.  It is probable that there is better mixing within the outer section of the disc because of increased shear rate as the film thins out towards the edges. It is expected that nucleation rates were higher and more uniform throughout the film in this highly mixed environment, giving smaller particles with narrower PSDs. Moreover, with injection further way from the centre, the residence time of the mixed fluid stream is shorter. This is also likely to contribute to reducing the extent of growth of the particles, resulting in smaller particles.

Figure 3. Effect of feed location on particle size distribution on smooth disc

4.       Conclusion

In this work, it has been shown that the intensified processing of TiO₂ precipitation under high centrifugal fields on the spinning disc reactor offers advantages as, in the ciorrect conditions, production of very small (1-2 nm)particles with a very narrow PSD is possible.

At a rotational speed of 1200 rpm on a 30 cm diameter smooth disc with the feed pipe (feeding TTIP) 10cm from the centre, particles of  below 1 nm diameter were obtained.  Under the same conditions the grooved disc produced even smaller particle sizes than the smooth disc and with narrower distribution.

This intensified method of production of TiO₂ particles represents a viable way forward to continuous, industrial production of such an important chemical.

5.       References

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2.            Pask, S.D., O. Nuyken, and Z.Z. Cai, The spinning disk reactor: an example of a process intensification technology for polymers and particles. Polymer Chemistry, 2012. 3(10): p. 2698-2707.

3.            Nielson, A.E., Homogeneous Nucleation in Barium Sulfate Precipitation. Acta Chemica Scandinavica, 1961. 15: p. 441-442.

4.            Boodhoo, K.V.K. and S.R. Al-Hengari, Micromixing Characteristics in a Small-Scale Spinning Disk Reactor. Chemical Engineering & Technology, 2012. 35(7): p. 1229-1237.

5.            Mohammadi, S. and K.V.K. Boodhoo, Online conductivity measurement of residence time distribution of thin film flow in the spinning disc reactor. Chemical Engineering Journal, 2012. 207: p. 885-894.

6.            Cafiero, L.M., et al., Process intensification: Precipitation of barium sulfate using a spinning disk reactor. Industrial & Engineering Chemistry Research, 2002. 41(21): p. 5240-5246.

7.            Chen, X. and S.S. Mao, Titanium dioxide nanomaterials: Synthesis, properties, modifications, and applications. Chemical Reviews, 2007. 107(7): p. 2891-2959.