Fluid and Solid Particle Dynamics in Stirred Vessels

Doctoral thesis, 1999

The present thesis deals with the fluid dynamics of stirred vessels and particularly those that are to be used in conjunction with solids suspensions. Stirred vessels are very common in the process industry and large amounts of energy can be saved by optimising their operation. Several types of unit operations such as chemical reaction, leaching, dissolution and crystallisation involve solids suspensions. Axial flow impellers are best suited for suspension of solids and therefore, pitched blade turbines were used in the present work. A standard type of stirred vessel with a single impeller and an industrial type of tank with two parallel impellers, were studied. The single-phase flow and the simultaneous flow of particles and liquid were investigated. Measurements were made on the single-phase flow using Laser-Doppler anemometry and on the two-phase flow using Phase-Doppler anemometry. Numerical computations were performed using the commercially available CFD-code CFX4. The two-phase calculations were done using the multi-fluid approach. Good agreement between the experiments and calculations was obtained for the fluid phase but the particle phase predictions were less satisfactory. The axial component of the slip velocity was generally slightly overestimated and the radial and tangential components severely underestimated by the computational model. The single-phase flow in a completely filled vessel equipped with a lid was investigated in detail and compared with results for a vessel lacking a lid. It was found that the influence of the lid was small and changes in the impeller discharge flow were observed only after lowering the lid to half of the vessel height. The simultaneous measurement of fluid and particle velocities revealed that generally, particles lag when the fluid is moving upwards and vice versa but also, that exceptions to this are common. The largest differences between the velocities of the two phases where found in regions with large mean velocity gradients such as the impeller discharge. For the industrial type of tank, it was found that horizontal displacement of the two impellers in opposite directions could break-up the segregated mixing zones that tended to form in the tank.