Experimental investigation of agitation hydrodynamics and mixing-time of non-Newtonian solutions

Download
2011
Şen, Begüm
Mixing is a crucial process for many large scale and small scale applications from food industry to cosmetics, from drug industry to petrochemical processes, etc. Changes in parameters (temperature, viscosity, velocity distribution, etc.) during the mixing affect the production process and the end product quality and the cost. Thus, these parameters, mostly the hydrodynamic parameters, should be monitored closely during the process. In order to ensure good and efficient mixing in the solution, high degree of turbulence is maintained while dead zones in the tank should be avoided. In chemical industry, the mixing processes generally involve complex solutions that exhibit non-Newtonian flow behavior that merits a study on the agitation hydrodynamics and mixing time. Thus, in this study agitation of carboxymethyl cellulose (CMC) solution in a laboratory scale mixing tank is investigated. The effects of CMC concentration and agitation speed on the hydrodynamics of the solution and mixing time are studied in detail. CMC concentrations studied are 0.5 wt%, 1 wt% and 2 wt%. Impeller speeds, on the other hand, are set as 150 rpm, 300 rpm and 600 rpm. The hydrodynamics of mixing can be studied easily by Ultrasound Doppler Velocimetry (UDV) which is a fast, non-invasive measuring technique in fluid dynamics. Also, the mixing time measurements were carried out through electrical conductivity of the agitated solution. UDV results show that the flow field has a typical pattern produced by the Rushton turbine. The main characteristics of the flow are that, in the impeller region radial components of the flow dominate. Near the wall flow occurs mainly in the axial direction towards the top and bottom of the tank. Mixing time measurements reveal that mixing time increases with decreasing impeller speed and with increasing solution concentration (i.e. viscosity). Typical mixing time values are in the range of 250-2600 seconds for different impeller speeds and CMC concentrations.