On-line monitoring and controlling of batch crystallisation using rapid heating and cooling

Abstract

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.