Gassed power measurement

Fig. 2.8 shows a paddle stirrer, in which six paddles are arranged underneath the disk running conically downwards from the disk perimeter to the stirrer shaft (paddle height = disk diameter). Power measurements [255, 672] in gassed liquids gave Ne(Q) characteristics, which were similar in form to those of classical turbine stirrers, but were a factor of 7 to 9 lower ... 

Stirred-tank bioreactors mechanically agitate the gas-liquid dispersion, and the resulting power draw is an important parameter in these bioreactors. The measured power draw is used to quantify two dimensionless numbers in air-sparged stirred-tank bioreactors, the ungassed and gassed power numbers. 

The gassed power draw should be measured using the techniques described in Section 4-3. 

There are numerous papers in which the question of stirrer power in gassed liquids has been treated. Although Calderbank [64] as early as 1958 correlated his measurements on stirrer power in gassed liquids (water, ethanol, glycol) in the dimensionless space P/Pq = the first important contribution in this area... 

The mixing power of a Rushton turbine with 6 flat blades and with 6 blades concavely curved in the direction of rotation was measured in pure and in gassed liquid of Newtonian and non-Newtonian (pseudoplastic and viscoelastic) behaviour [660]. The results were plotted as P/Pq = /(Q) and proved that the concave turbine used only 1/5 to 1/3 of the power at the same Q. In [709] power characteristics of turbine stirrers wirh 6 different shapes of concave blades are presented in the same form P/Pq = f Q). 

As already discussed in context with the single-phase simulations, boundary conditions at the impeller are predicted from measured data of the averaged velocities. The gassed linear liquid velocities differ from the ungassed velocities because impeller power consumption and pumping capacity of the impeller decrease due to gassing. 

The transport properties of the supercritical fluids fall somewhat in between the gas and the liquid and also depend on how removed one is from the critical point. Dense gasses have the solubilizing power of liquids and the mobility of gasses as depicted in Table 20.1.3. There are quite a few empirical correlations and theoretical models, which are primarily extensions of corresponding low-pressure liquid and gas counter parts. Similarly, some of the classical experimental methods can be used for measurement of transport properties of supercritical fluids. A rather brief overview of the methods applicable for supercritical fluids will be presented since specialized reviews in the area give a good account of the state of the art. " " For engineering purposes, one can use applicable property estimation methods available in flowsheet simulators such as ASPEN PLUS, PROll, G-PROMS, and CHEMCAD. These methods are discussed in a text classical in the field." ... 

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