Determination of mixing time in mechanically stirred tank using electrochemical limiting current diffusion technique

Mixing processes have vital importance and are commonly used in a lot of industrial fields such as, chemical, petrochemical, biochemical, pharmacuitical, and food. Since the mixing of miscible liquids by diffusion or natural convection takes a long time, forced flow is employed for rapid mixing to reach a desired homogenization degree faster. In the determination of mixing performance of single phase mixing processes, one of the most important parameters, which are used commonly, is mixing time. Mixing time is not only to characterize quantitavely liquid homogenization, but also it is very important parameter in tank design, in the selection of stirrer type and geometry, in scaling up. It is important to develop a technique for mixing time measurement, which does not disturb the real hydrodynamic in the vessel, does not need complex and expensive measurement system, is practicle, does not react chemically with original reactor content, and does not contain radiactive material. When taking into consideration of measurement techniques, electrochemical limiting diffusion current technique (ELDCT) provides a quite sensitive and reliable measurement especialy in mass transfer operation. This technique is employed to measure mass transfer rate between a solid surface and electrolyte. The mesured quantity is the current by tranferring some ions reacted on the electrode surface. In this study, ferri-ferro cyanide solution was used as electrolyte, while potassium carbonate solution was used as a supported electrolyte. In a stirred tank system and in transition region, the applicability of electrochemical limiting diffusion current technique was tested by conducting simultaneous experiments with the conductivity measurement technique previously used to determine mixing time. Since the measurements made with the conductivity measurement probe were local, the mixing times were measured by using the local cathode for electrochemical limiting diffusion current technique. The findings of two methods were found to be in 95% similarity. In the mixing system with electrochemical limiting diffusion current technique, four nickel panels were used as a flow deflector (buffle). One of these panels (1B) was cathode, while the other three ones (3B) were anodes. The value of mixing time for measurement of "1B+3B" electrode system was observed to be the same with the avarage mixing time for five different zones in the tank. Additionaly, it has been determined how the number of Re in solutions with different viscosities, the number of Rushton Turbine wings and wing angle influence the mixing time. Correlation showing the dependency of these parameters on mixing time was found as t_M=7363750 〖Re〗^(-1,3)*θ^(-0,42)*n^(-0,59). Finally, mixing time was calculated for solutions with different volumes and viscosities.