Spinning Disc Reactor for Green Processing and Synthesis - Process Intensification for Green Chemistry - page 83

improved mechanical strength of materials (e.g. in polymer nanocomposites, bone
cement), thermal stability, catalytic activity and so on. To achieve nanosized crystals
from precipitation, high supersaturation levels that are uniform throughout the precipita-
tion system are key processing requirements for high nucleation rates and small particles.
The SDR, by virtue of its rapid micromixing in the thin films, has the capability to meet
these requirements. By providing the opportunity for a homogeneous reaction environment
in terms of concentration, temperature and mass transfer, the SDR should not only enable
small particle sizes to be formed but allow the size distribution to be tightly controlled.
Several studies of precipitation processes in the SDR have indeed validated these expect-
ations. One involved the liquid-liquid precipitation of barium sulfate crystals of 0.7 m min
size [42]; more recently, Tai and coworkers have demonstrated the synthesis of nanoparticles
of magnesium hydroxide [63] and of silver [44]. In the latter work, silver nanoparticles
less than 10 nm in diameter were produced by using a benign rawmaterial such as glucose as
the reducing agent and were stabilized on formation by polyvinyl pyrrolidone (PVP),
commonly employed as a protecting agent to prevent particle agglomeration.
A greener method of silver nanoparticle synthesis involves the use of a more environ-
mentally friendly protecting agent such as starch combined with glucose as the reducing
agent [64]. Silver particles with a mean size between 13 and 16 nmwere formed in the SDR
after 10 minutes of processing, which was much less than the 20 hours needed in an STR
for the same result [65].
Another 'green' approach adopted in the synthesis of superparamagnetic magnetite
nanoparticles has been reported by Chin et al. [39]. Alginic acid, a natural biopolymer
originating from algae, was employed as a surfactant to stabilize the Fe 3 O 4 particles during
processing in the SDR. The ability to inject the alginic acid into the magnetite particle
suspension and achieve uniform mixing of the two streams on the rotating disc constituted
a considerable processing advantage in producing highly stable particles of about 10 nm
and of very narrow distributions.
3.5 Hurdles to Industry Implementation
In spite of the bountiful processing benefits offered by the SDR, as discussed in this chapter
in the context of green chemical processing, industrial take up of this technology has been
rather slow. This is quite surprising given the great deal of interest expressed in SDR by
certain industrial sectors, with some of the multinational companies representing these
sectors having even participated in evaluation projects that have produced extremely
encouraging performance data, including GlaxoSmithKline, Rhodia and so on. To date,
there are no applications of the SDR in the chemical and processing industry, to the best of
the author's knowledge. There are clearly important technological barriers that need to be
addressed in order to facilitate industrial implementation. These are reviewed briefly in this
section, highlighting progress made to date towards tackling some of them.
3.5.1 Control, Monitoring and Modelling of SDR Processes
One of the most important development issues concerns the monitoring and control of
processes in the SDR. In fact, the general lack of control-related studies, in particular, is a