3.3. Modelling the emulsion separation process
For the water droplets at the top of the beaker of liquid to fall to
the interface layer means they must travel a set distance, depending
upon the water content used; for example water droplets in a
70/30 emulsion must travel a maximum distance of 5 cm. At room
temperature the minimum velocity of water droplet of a set diameter
can be calculated using Stokes’ Law and the physical properties
of the oil [17], and the separation time calculated based on
the time to travel 5 cm at that velocity. The separation times for
a range of droplet sizes are shown in Fig. 8.
From Fig. 1 the mean diameter of water droplets in the emulsion
is around 20 lm. The predicted separation time for droplets
of this size is over 1 106 s, compared to experimental data of
4000 s. To achieve the separation times for the untreated emulsion,
at room temperature, the average droplet diameter must be
345 lm, almost a two orders of magnitude increase from the starting
average of 20 lm. The difference between the predicted and actual
separation times shows that significant droplet coalescence
must occur during the droplet settling process, even without
microwave or conventional heating.
A second model was developed to assess the likely degree of
coalescence during the microwave heating process. It is assumed
that the average droplet size is 345 lm, i.e. the size of droplets
due to coalescence in the untreated emulsion. The separation time
of the 345 lm droplets can be predicted based on the assumed
temperature of the surrounding oil phase. It is known that the
water phase is heated selectively over the oil, however heat transfer
will occur during settling, which increases the temperature of
the oil and reduces its viscosity. For a constant droplet size, Stokes’
Law can be used to predict the separation time for a given oil viscosity,
which can be related to temperature according to the physical
property data [17]. Fig. 9 shows the predicted separation time
for microwave heated emulsions based on an oil temperature of
100 C, along with the minimum observed experimental settling
time.
The maximum temperature that can be achieved in the oil
phase during the microwave heating experiments is 100 C, the
boiling point of water at atmospheric pressure. Therefore the
separation times shown in Fig. 9 represent the lowest that can be