Spinning Disc Reactor for Green Processing and Synthesis - Process Intensification for Green Chemistry - page 74

Turbulent
eddies in wavy
film flow

Parabolic velocity
profile in smooth
laminar flow

Uniform velocity
profile in wave-
induced
turbulent flow

r

r

Radial flow liquid
film on rotating disc

Radial flow of liquid
film on rotating disc Smooth liquid

film

Wavy
film

Disc surface

Disc surface

(a) (b)

Figure 3.17 Illustration of differences in velocity profiles in (a) smooth, laminar film and
(b) wavy film with induced turbulence. Reproduced from [ref 38] Poster presented at ISCRE22,
2-5 Sept. Maastricht, Netherlands (2012).

Achieving plug flow behaviour is dependent on a uniform velocity profile in a
direction perpendicular to the flow direction and on negligible dispersion in the direction
of flow. In the context of film flow on the rotating disc where bulk flow is in the radial
direction, the film-surface instabilities or waves are likely to induce turbulence within the
layer underneath via the formation of turbulent 'eddies' (as illustrated in Figure 3.17),
giving a more uniform velocity profile in the transverse direction, as opposed to the
parabolic velocity profile that characterizes waveless laminar flow. Therefore, the more
waves are formed, the greater the transverse mixing and the more uniform the radial
velocity across a given cross-section in the film. Furthermore, the higher the centrifugal
force exerted on the film, the lower the tendency for dispersion in the radial direction
away from the direction of flow, which contributes to suppressing deviations from plug
flow behaviour.

3.3.5 SDR Applications

The niche applications for SDR technology are kinetically fast, heat- and mass-transfer
limited processes, which require good mixing. The thin, wavy films allow high levels of
heat transfer across the liquid-solid interface and mass transfer across the liquid film or
across the gas-liquid interface. The thin films, which represent a very small inventory of
material in the reactor at any instant, also provide a high surface area to volume ratio,
typically in the range 5000-15 000m 2 /m 3 , to promote high rates of heat transfer from
the disc to the liquid or vice versa. High shear rates enable intense local mixing in the
film so that inherently fast reactions can proceed at rapid rates dictated only by the
chemical kinetics of the process, rather than by mixing intensities. When mixing
limitations are thus removed, short residence times can be applied to exploit the
very rapid reaction rates. Considerable research has been conducted into the application
of SDR technology to numerous reactions conforming to these criteria, as shown in
Table 3.3. A selection of these processes pertaining to green chemistry applications will
be reviewed in Section 3.4.