to drive the hydraulic ram was a gear pump powered by a three
phase 1.5 kW electric motor. The pump and motor size meant that
a maximum flow rate of 7.5 l min1 and a maximum pressure of
14 MPa could be generated. The flow rate through the Venturi was
determined from measurements of the ram position as pressed
into the lactose filled cylinder. A ruler was placed alongside the
ram and a video camera placed above it. At the end of each
experiment, frames were grabbed from the video camera footage
at particular time intervals and the ram speed was calculated.
For each experiment, 200 ml of lactose solution with a
concentration of 90 g lactose/100 g water was heated to 95 1C to
dissolve all the lactose. USP grade lactose was used and to ensure
the absence of insoluble foreign matter, the solution passed
through a Whatman No.1 filter, then reheated it to 95 1C for
30 min to dissolve any nuclei. The hot solution was then gently
poured into the pre-heated cylinder where it was left to cool to
40 1C. At 40 1C the lactose solution had an absolute alpha lactose
supersaturation of 26.17 g/100 g water; this is equal to a relative
supersaturation (RSS) of 3.31. This was above the supersaturation
identified in the work of McLeod et al. [32] required for
homogenous nucleation to occur. A constant supersaturation
allowed any effect of the beta lactose isomer on lactose nucleation
to be ignored. Heating tape wrapped around the second cylinder
regulated the temperature. The heating tape was controlled using
a CAL 3200 Autotune temperature controller. Feedback to the
controller came from a T type thermocouple connected to the
cylinder. The cooling process took 60–70 min and was monitored
using a T type thermocouple immersed in the solution. Once the
solution had cooled, the cylinder was attached to the Venturi and
the solution forced though the Venturi at the desired rate.
The supersaturated solution from the Venturi was collected in
an agitated vessel, Fig. 3, containing 2000 ml of lactose solution.
The solution had a concentration of 45 g lactose per 100 g water,
maintained at a temperature of 40 1C and was therefore slightly
supersaturated. This diluted the highly supersaturated Venturi
solution. The final concentration of the blended solutions supplied
sufficient supersaturation for crystal growth, but minimised
additional nucleation. A two blade pitched turbine operated at
300 r min1
, dispersed the Venturi solution into the diluting
solution as quickly as possible. The impeller was positioned just
above the entry point of the solution coming from the Venturi.
The mixed solution was agitated for 3 h allowing any crystals
formed to grow up to a size detectable under a microscope.
After the 3 h, five, 3 ml samples were withdrawn from the bulk
solution. A small sub-sample of each solution was placed into a
Neubauer counting chamber and polarised light was used to
illuminate the crystals. Digital images of the samples, viewed
under a microscope, were taken and the number of crystals was
determined by counting the crystals in the chamber. From the five
samples the average number of crystals per millilitre of solution
was determined.