Batch crystallisation is a common operation in the pharmaceutical and fine chemical
industries for purification and separation. With the advent of industrially robust
instruments to monitor crystallisation, it is now possible to develop more sophisticated
control systems, to better control the crystal size and shape. However, with batch
cooling crystallisation, the number of control handles that can be used in any control
system is limited; a stirrer speed and heating/ cooling rates are the two that are readily
available.
The work carried out in this PhD project aims to demonstrate that an online video
imaging technique can be used to both monitor and control batch cooling crystallisation.
The measuring technique has been developed using low-cost readily available camera
and has been utilised to perform measurements of meta-stable zone width (MSZW) at
different operating conditions and other key properties of L-glutamic acid and glycine
solutions. Nucleation kinetics parameters for both the polythermal and isothermal
experiments were calculated according to KBHR method. The outcomes of these
experiments exhibited a good agreement with previous workers using more
sophisticated measuring methods.
Traditional laboratory scale batch cooling systems use one hot/cold source in order to
study crystallisation. When information from laboratory is applied to industrial scale,
there is inherent issue with heat transfer related to the time constant which industrial
systems can respond to. Therefore, in this thesis, a system which has the advantage of
introducing a method to rapidly heat and cool a batch crystalliser has been developed.
This was achieved by switching the water flow through the crystalliser jacket between
hot and cold water baths using six solenoid valves. Different variables were examined;
those included the switching frequency and duration as well as the temperature set point
of the two baths. It was found that the switching duration had a little effect on the
nucleation time and temperature. In contrast, the switching frequency had impact which
was more obvious when the ratio became higher either for the cold bath duration to the
hot one or vice versa. Moreover, the temperature set point of the hot and cold baths
showed to be of great potential for switching effectiveness. For the first time, a comparison of the switching technique, crash cooling and constant
linear cooling rate effects on the nucleation point of glycine was presented. It was found
that the switching mechanism gives controllable profile by selecting the hot and cold
bath temperatures set point and the switching frequency. Therefore, switching method
adds an additional level of control not possible with one water bath which is used in
traditional cooling profiles (crash and linear).
Understanding the heat transfer phenomena in processes that are temperature limited,
for instance cooling crystallisation, is of great importance for the overall process
efficiency. Consequently, a simple heat transfer model of agitated vessel was developed
in this work and showed its ability to predict the vessel temperature in the case of
switching between hot and cold baths, programmed heating/cooling rates and crash
heating/cooling. The evaluation of the different heat transfer resistances was also
considered by using Wilson method.
There has recently been increasing emphasis on the control of crystallisation process to
obtain particular physical properties for the produced crystals as this has a major effect
on the effectiveness of the downstream processes. Accordingly, a control approach that
integrated the process video imaging system with the switching technique was
developed in this thesis. The experimental findings showed that the developed PVIswitching
control system was able to control the crystallisation of LGA and glycine as it
improved the overall quality of the crystals produced in terms of size and presence of
fines over conventional methods. This was proved by analysing the images captured of
the crystals at the end of experiment. In addition, the sensitivity and robustness of the
developed control approach were also verified
