(399d) Nano-TiO2 Sol-Gel Synthesis in the Spinning Disc Reactor

Nano-TiO₂ sol-gel synthesis in the
spinning disc reactor

Somaieh
Mohammadi* and Kamelia V.K. Boodhoo

School
of Chemical Engineering & Advanced Materials, Newcastle University, Merz
Court, Newcastle Upon Tyne, NE1 7RU, UK

Abstract

The
purpose of the experimental study is to determine the feasibility of producing
nanoparticles of TiO2 in a continuous spinning disc reactor (SDR) which
represents an example of a process intensification technology. The primary
characteristics of the SDR relevant to this study are the thin film flow of the
liquid on its surface, the very vigorous mixing within the film and the
extremely short and controllable residence times of the liquid. A liquid-liquid
sol gel technique would be chosen to investigate experimental parameters of
rotational speed, flowrate and concentration and disc surface configuration. The
nanopowders of TiO₂ synthesized by sol gel method in the spinning
disc reactor will be compared with the nanopowders produced in a conventional stirred
tank reactor.

Preliminary
results indicate that the higher the liquid flowrate on the disc and the higher
the rotational speed of the disc, the smaller the particle size and the narrower
particle size distribution (PSD) in the SDR. In the relative conditions the
nanopowders produced by Stirred tank reactor had wider PSD.

Keywords:
TiO₂, Spinning disc reactor, particle size distribution

Introduction

Nowadays
there is an increasing number of industries that rely on nano titanium dioxide
(TiO₂) which is used in a variety of products such as white pigment and
white food colouring, cosmetic and skin care products, photocatalysts for use under
ultraviolet light and surface coatings amongst others. Although the processes
for making titanium dioxide are well-established and developed, new processing
methods which are more economical and have the potential to offer better
product characteristics are important in driving the TiO₂ industry
forward to continue to be profitable. In recent years, the spinning disc
reactor (SDR) has been developed as process intensification equipment where
rapid mass and heat transfer rates can be obtained from the thin film of liquid
produced due to centrifugal acceleration of rotating disc. In developing these
characteristics, the SDR is considered a tool of process intensification due to
its compactness, flexibility as an inherently safe and continuous reactor
technology and its capability to deliver better product quality. The spinning
disc reactor would appear to be an excellent candidate for industrial
applications due to its very short and controllable residence time. SDR is also
a highly intensified reactor when dealing with rapid exothermic reactions and
viscous liquids such as bulk polymerization [1], solution[2], condensation[3]
and also cationic polymerization [4] of styrene. The SDR thin films, on the
other hand, offer higher mixing intensity and high heat and mass transfer rates
[5] which results in tighter molecular distribution and higher quality of
polymer product [6, 7]. Spinning disc reactors were
previously investigated in the precipitation of barium sulphate [8] and calcium
carbonate [9]. It was found that high rapid mixing coupled with high levels of
supersaturation lead to very small crystals with a tighter size distribution
being produced. This was considered to produce better quality than conventional
process techniques used in the pharmaceutical industry [10]. It was shown that the
precipitation of barium sulphate on a spinning disc yielded significantly
smaller crystals than the batch technique. The main factor controlling the
crystal size was the very high rates of mixing experienced on the spinning
disc, which lead to the rapid depletion of supersaturation, and much higher
nucleation rates. Cafiero et al [12] 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 attaining
better control of crystal size. With the understanding that the rapid mixing
and high supersaturation leads to very small crystals being produced along with
very short residence times and tighter CSD, it was hypothesised that a similar circumstance
could exist for titanium dioxide precipitation routes. The morphology of
titanium dioxide is more complicated than that of barium sulphate, capable of
forming three different crystal forms, and therefore emphasis on making the
right size shape and form is important in the final product. Understanding the
morphology from this rapid precipitation method is a factor in the present
study.

This
work aims to develop and demonstrate a novel processing method on the basis of
the spinning disc reactor (SDR) for the production of titanium dioxide
nanoparticles in order to enhance the efficiency and the higher quality
products. The spinning reactor seems more desirable due to its considerably
lower specific energy consumption. Additionally this technique is utilized to give
high flow rate and very short residence time in continuous mode of operation [11].

Materials
and Methods

The
production process of the titanium dioxide nanoparticles consisted of the
reaction between distilled water and titanium tetra isopropoxide (TTIP) at 50^oC under nitrogen
and the subsequent precipitation of the produced titanium dioxide. The
adopted apparatus is equipped with a disc of 20 mm of diameter rotating at 300-
1200 rpm. The two reagent solutions were separately fed at the centre of the disc
as indicated in Fig. 1. The total feed flow rate of reagent solutions was between
1.66- 5 ml/s. A pH value of the distilled water was equal to 1.5 was achieved
by the addition of nitric acid to the distilled water reservoir. The ratio of
water to TTIP  was between 2 and 8.  For sake of comparison the same procedure
was applied by using a baffled stirred vessel, 250 ml in capacity.

Figure1. Schematic of TiO₂ experiments set up

Results
highlights

Exploratory
works and primary studies show that the SDR technology gives increased number
of particles with controlled size, shape and size distribution to reach quality
requirements. It has been observed that under comparable
operating conditions, the SDR provides more uniform and smaller
nanoparticles with respect to a mechanically stirred vessel. The nanoparticles
produced by the SDR also have a narrower size distribution than the STR.

References

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styrene polymerisation. Applied Thermal Engineering, 2000. 20(12): p.
1127-1146.

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