Over the last century the fluid mixing process in stirred vessels has become one of the most common operations in the chemical, petrochemical, pharmaceutical and food industries. Despite its wide use, the process is often poorly understood and empirically handled. Numerous efforts. both experimental and theoretic, have been undertaken to understand mixing inside stirred vessels. Because of its complexities in terms of flow regimes and multiphase operations, the process design and optimisation is not well defined and often evolved by empirical scale-up from laboratory results (at around 1dm³ scale) to plant-scale manuiacturing involving vessels up to 100m³ in size. Over the last two decades with the development of more powerful computers the theoretical study of the fluid mixing has become more tractable and attractive for use in engineering practice. This has led to the emergence of software packages based on detailed mathematical models. This work is an attempt to develop a mathematical model based on a simplified networks-of-zones concept, capable of providing detailed predictions in 3-D of the gas-liquid mixing and reaction behaviour in a triple impeller industrial bioreactor. Two major improvements of the previous gas-liquid model (Hristov et al, 2001) have been achieved: i. The effect of baffles, which was previously ignored, has been introduced to the model for the Rushton turbine (RT), using a simple pattern for the turbulent flow around them (Rahimi, 2000). ii. A 3-D networks-of-zones model has been developed to describe the performance of a novel geometry Narcissus (NS) impeller. The treatment in this case was simply adapted from the earlier version based upon the Rushton turbine, by a geometric stretching of the zone volumes. This accommodates the complex up-down liquid flow pattern by a simple adjustment of the zone volumes matrix, whilst the underlying mass and component balances on every zone stay unchanged.

This study investigated the influence of the attainable region technique to ball milling as applied in reactor technology. Flow rate, ball filling, mill speed, ball size and mill density were varied. When each was varied, the rest of the parameters were kept constant in-order to determine the influence of each parameter on the process of milling. Selection function and breakage function parameters were selected for the mill model. These were kept constant for all four circuit configurations: open milling circuit, normal closed circuit, reverse closed circuit, and combined closed circuit. Flow rate was varied from 10 tph to 150 tph. It was observed that in all circuit configurations the optimum results were obtained from 90 tph upwards. When ball filling was varied, the optimum results were obtained between 30 % and 40 % of ball filling. At this range the mill is neither experiencing under-filling nor over-filling. When the mill speed was varied, at 60 – 80 % of critical speed the product specification was achieved and for grinding balls, sizes of between 60 mm and 90 mm yielded the optimum results. Varying the mill density resulted in insignificant changes. From the results, the combined closed circuit produced more of the product specification. / School of Engineering / M. Tech. (Engineering: Chemical)