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%

Characterization of in-line mixing of pulp fibre suspensions based on electrical resistance tomography

In pulp bleaching processes, pre-distribution of chemicals in suspensions ahead of tower reactors is essential to ensure efficient lignin removal and optimal use of the chemicals. In-line mixers, combined with chemical injectors, are commonly used to achieve this goal. In spite of its importance, in-line mixing of pulp suspensions is not well understood. In this thesis, liquid distribution and gas dispersion were investigated downstream of in-line mixers, including jet and mechanical mixers, to provide better understanding and guidance for mixer design and process optimization. In the present work, non-intrusive electrical resistance tomography (ERT) was used to quantify mixing based on two novel mixing indices, derived from the standard deviation of image pixel values. This technique was also implemented as a real-time mixing assessment tool in industrial pulp bleaching, with success in monitoring mixing quality as a function of process operating conditions. Liquid jet mixing was found to depend strongly on the flow regime and jet penetration. For turbulent flow, the criteria for in-line jet mixing in water apply also to suspensions. When a suspension flows as a plug, mixing differs greatly from that in water, depending on the fibre network strength in the core of the pipe. With an impeller present, mixing improved substantially, primarily in the high-shear zone around the impeller, with rapid reflocculation downstream. Gas mixing depended on the flow regime and buoyancy in a complex manner. When buoyancy was not significant, impeller operation enhanced mixing since bubbles dispersed throughout the pipe cross-section, whereas without the impeller, the bubbles congregated near the wall due to robust fibre networks in the core of the pipe. For buoyancy-dominated flow, the impeller worsened mixing since it disrupted the fibre networks and delivered gas to the top of the pipe, whereas the networks caused liquid/pulp slugs to flow at the top for a tee alone.Applied Science, Faculty ofChemical and Biological Engineering, Department ofGraduat