Particle size measurements, modelling and control strategies of jet mill plants

Abstract

Jet milling is a size reduction method that is used to produce fine powder with a narrow size distribution, such as toners, pharmaceuticals, and plastics. The main disadvantage is the large energy consumption of the jet mill compared to other grinding processes. The aim of the Jet Mill Project is to reduce the energy consumption. This will be achieved by a controlled grinding system, minimising the production of off-spec material, and operating closer to the product specifications. The research within the Jet Mill Project therefore focusses on increasing the controllability of the PSD. In industrial practice the operating conditions of a jet mill are often determined by trial and error. To avoid off-spec material the mill has often to be tuned to lower risk. This results in a relatively large amount of off-spec material. A measurement and control system has to be designed in order to allow a better tuning that will reduce off-spec material. Main feature of this system will be control, based on in-line particle size measurements using light diffraction. As an alternative of the presently used laser diffraction instrument a particle size measurement instrument based on the scattering of incoherent light has to be tested. Data acquisition software has been customised in order to implement and test control strategies and to design automatic start-up and shut-down procedures. This allows to repeat experiments without interference of an operator. Data transfer from the particle sizer computer to the process computer was established. This data can be displayed on a trending screen and used for control routines. Experiments have shown that particle sizing can in principle be done with a low-cost instrument, the Jena Optik instrument, using a conventional white light source, a photomultiplier and a liquid crystal display. Recommendations for a redesign have been made to improve the accuracy of particle size measurements. A non-model-based controller is implemented on the 10" jet mill plant. This controller can bring the mill during start-up to the desired median particle size. The median particle size is used for control of the feedrate, which reduces the hold-up in the mill chamber until the desired median is reached. The robustness of this controller seems limited due to the contamination of the lenses of the in-line cell. From experiments can be derived that the first principal component of the Malvern Mastersizer X can be used for feedback control. The first principal component shows similar behaviour as the median particle size. A dynamic jet mill model was available within the Jet Mill Project. In order to obtain a more physically based model description the model has been extended with zones in a radial direction. Computational fluid dynamics and pilot plant experiments should provide data to verily and obtain parameters of this model.