Study of gas-liquid mixing in stirred vessel using electrical resistance tomography

This study presents a full operation and optimisation of a mixing unit; an innovative approach is developed to address the behaviour of gas-liquid mixing by using Electrical Resistance Tomography (ERT). The validity of the method is investigated by developing the tomographic images using different numbers of baffles in a mixing unit. This technique provided clear visual evidence of better mixing that took place inside the gasliquid system and the effect of a different number of baffles on mixing characteristics. For optimum gas flow rate (m3/s) and power input (kW), the oxygen absorption rate in water was measured. Dynamic gassingout method was applied for five different gas flow rates and four different power inputs to find out mass transfer coefficient (KLa). The rest of the experiments with one up to four baffles were carried out at these optimum values of power input (2.0 kW) and gas flow rate (8.5×10-4 m3/s). The experimental results and tomography visualisations showed that the gasliquid mixing with standard baffling provided near the optimal process performance and good mechanical stability, as higher mass transfer rates were obtained using a greater number of baffles. The addition of single baffle had a striking effect on mixing efficiency and additions of further baffles significantly decrease mixing time. The energy required for complete mixing was remarkably reduced in the case of four baffles as compared to without any baffle. The process economics study showed that the increased cost of baffles installation accounts for less cost of energy input for agitation. The process economics have also revealed that the optimum numbers of baffles are four in the present mixing unit and the use of an optimum number of baffles reduced the energy input cost by 54%

Solid distribution and mixing time in stirred tanks: The case of floating particles

This work concerns floating particle distribution and liquid mixing dynamics in a solid-liquid stirred tank. Measurements of local solids concentration distribution at steady-state conditions and of liquid homogenization in the presence of dispersed particles at transient conditions are collected with up-pumping and down-pumping pitched blade turbines. Electrical resistance tomography is the selected experimental technique for the data acquisition in the opaque solid-liquid mixture. For the time dependent conductivity data processing, the same method commonly applied to the mixing time determination from planar laser induced fluorescence measurements is considered. The method is successfully extended to the characterization of liquid mixing with floating particles. The local data obtained by the ERT technique over the whole vessel volume are fully exploited. The robustness of the method for tackling conductivity variations close to the liquid free surface and due to the particles movement is assessed