Process fluids in real life rarely match the idealised conditions of basic research. Therefore, in this project solutions simulating fermentation broth and other process fluids are employed to investigate the power draw, the gas hold-up and the oxygen mass transfer in a agitated gas-liquid reactor. Through dissolving small quantities of a salt and a typical surfactant the coalescence of gas bubbless is gradually retarded. A standard Rushton turbine is compared with a novel hydrofoil impeller, which is used more and more in the process industries. The book also contains exceptional images of 'in situ' coalescence events of gas bubbles in the mixing tank.