Non-Newtonian Effects in Bubble Columns

This relation indicates that the apparent viscosity varies depending on the shear rate y. unlike the Newtonian viscosity. Therefore, the definition of the appropriate shear rate characterizing hydrodynamics is required to estimate the effective apparent viscosity for non-Newtonian fluids in bubble columns. In bubble columns, the shear rate is not uniform and unknown. The motion of bubbles results in the wide variation of the shear rate, which is hopelessly complicated and cannot be analyzed. Therefore, the characteristic shear rate should be evaluated on the basis of a simplified physical picture of hydrodynamics in bubble columns and aided by experimental observations. 

Correlations for k a in bubble columns such as Equation 7.41 should hold for non-Newtonian fluids with use of apparent viscosity To estimate the effective shear rate (s" ), which is necessary to calculate jMj, by Equation 2.6, the... 

It is customary to account for the non-Newtonian fluid behavior by introducing the so called effective viscosity to define various dimensionless groups. Unlike its constant value for Newtonian liquids, the effective viscosity of non-Newtonian pseudoplastic type fluids depends upon the operating conditions (e.g., gas and liquid velocities) as well as on the geometrical details of the system. Indeed, the lack of a rationed definition of the apparent viscosity or characteristic shear rate appears to be the main impediment in extending the well established predictive correlations for Newtonian media to non-Newtonian media. When we develop correlations for design parameters in bubble columns with non-Newtonian media in an analogous manner to the case of Newtonian media, Newtonian viscosity p is simply replaced by an apparent viscosity for non-Newtonian media. 

The heat transfer rates in bubble columns are much higher than that anticipated from single phase flow considerations. This enhancement is ascribed solely to the bubble-induced turbulence and liquid circulation. Little work has been reported on heat transfer, both at wall and to/from immersed surfaces, in bubble columns employing non-Newtonian media. Nishikawa et al. reported the first set of data on the effect of shearthinning viscosity of CMC solutions on jacket and coil heat transfer coefficients [7]. They reconciled their results for Newtonian and power law liquids by introducing the notion of an effective viscosity estimated via Equation 3, provided the gas velocity was greater than 40 mm/s. For superficial gas velocity lower than this value, the effective shear rate varies as for coil heat transfer... 

A cursory inspection of Table 1 shows that in most instances polymer solutions (carboxymethyl cellulose, polyacrylamide, xanthan) have been used to mimic the non-Newtonian features of biological systems, encompassing wide ranges of shear thinning conditions (though viscoelastic effect have been studied only scantily) in bubble columns up to as large as 760mm in diameter. 

The effects of broth viscosity on k a in aerated stirred tanks and bubble columns is apparent from Equations 7.37 and 7.41, respectively. These equations can be applied to ordinary non-Newtonian liquids with the use of apparent viscosity as defined by Equation 2.6. Although liquid-phase diffusivity generally decreases with increasing viscosity, it should be noted that at equal temperatures, the gas diffusivities in aqueous polymer solutions are almost equal to those in water. 

In a bubble column of 14 cm I.D. gas holdups, volumetric mass transfer coefficients and specific interfacial areas have been measured in CMC solutions of highly viscous pseudoplastic flow behavior. In the slug flow regime q, kLa and both a and kL follow simple relations (eqs. 8,10-12) which consider the influence of the gas velocity and the effective viscosity. The predictions of eq. (10) are in close agreement with the kLa values observed in a tower fermenter with fermentation broths of pronounced non-Newtonian flow behavior. 

Non-Newtonian fluids are frequently encountered in the process industries, especially in biochemical and biotechnological applications. It has been known that the rheological properties of the liquids have profound effects on the performance of bubble columns. The non-Newtonian anomalies create uncertainties in the design, scale-up, and operation of bubble columns. Most analyses and correlations are based on the following two premises, which are not necessarily always applicable ... 

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