The influence of agitator type on fluid dynamics and oxygen mass transfer in a pilot scale mixing vessel

The power consumption (gassed and ungassed), bulk and local impeller fluid dynamics, gassed hold-up and oxygen mass transfer characteristics of a variety of industrially significant agitator types have been investigated using a pilot-scale mixing vessel. The purpose built perspex vessel of 0.72 m diameter was operated at both H/T=l (V=0.293 m\(^3\) ) and 3 R H/T=2 (V=0.586 m\(^3\) ). Agitators tested were the Ekato InterMIG\(^R\) , Lightnin’ A315 hydrofoil. Rushton disc turbine, pipe-section disc turbine and mixed flow impeller, with the latter in both upward and downward pumping modes. Test fluids were deionised water and solutions of non-Newtonian, shear-thinning sodium carboxymethylcellulose, giving a viscosity range of from 0.001 Pa. s to 0.4 Pa. s. Gas flowrates of up to 1.5 vvm and gassed -3 power inputs of up to 5 kWm\(^-3\) were used. A novel steady-state oxygen mass transfer measurement technique was developed and used to determine the volumetric oxygen mass transfer coefficient (k\(_L\)a). The technique utilizes the continuous generation of oxygen in the liquid phase by the degradation of hydrogen peroxide in the presence of the enzyme catalase. The oxygen produced in this manner is stripped from the liquid phase by the continuously flowing gas phase until steady-state conditions are achieved. The technique proved to be easy to use, provided reproducible results and was independent of peroxide concentration over a wide range and catalase concentration over a more restricted range. It avoids potential problems associated with tiny bubble hold-up and gas phase mixing encountered when using dynamic techniques in viscous liquids. At constant P/V and v\(_S\) in water, inter-impeller k\(_L\)a and hold-up variation was low. The k\(_L\)a and \(\varepsilon_G\) dependence on P/V was similar irrespective of impeller, but dependence on v\(_S\) was not. Low viscosity CMC solutions provided equal or greater k\(_L\)a and hold-up than water due to coalescence suppression. Higher viscosity solutions provided greatly reduced k\(_L\)a and hold-up. Inter-impeller k\(_L\)a and hold-up variation rose with increased shear-thinning viscosity. Low power number agitators (InterMIG, A315) consistently provided enhanced k\(_L\)a at constant power input and gas velocity in CMC. Correlation of k\(_L\)a data for various impellers in CMC was achieved with the inclusion of an apparent viscosity term. Low power number impellers ran at higher speed and hence lower apparent viscosity at a given power input, supporting the use of the Metzner and Otto\(^4\) shear-rate determination method for viscosity dependent k\(_L\)a correlation