Ungassed Liquids

The ratio of the power requirement of gas-sparged (aerated) liquid in a stirred tank, Pq, to the power requirement of ungassed liquid in the same stirred tank, Pq, can be estimated using Equation 7.34 [7]. This is an empirical, dimensionless equation based on data for six-flat blade turbines, with a blade width that is one-fifth of the impeller diameter d, while the liquid depth Hp is equal to the tank diameter. Although these data were for tank diameters up to 0.6 m. Equation 7.34 would apply to larger tanks where the liquid depth-to-diameter ratio is typically... 

When gas is bubbled below the impeller, the power drawn by the agitator decreases relative to the non-gassed liquid. This is because the gas/liquid mixture creates a zone of lower density below the stirrer, that strongly reduces the power consumption. From the point of view of design, however, it should be noted that the drive-motor- should be designed for the most demanding case, that is, the ungassed condition. 

Figure 10.11. Power consumption, (a) Ratio of power consumptions of aerated and unaerated liquids. Q is the volumetric rate of the gas (O) glycol ( X ) ethanol ( ) water. [After Calderbank, Trans. Inst. Chem. Eng. 36, 443 (1958)]. (b) Ratio of power consumptions of aerated and unaerated liquids at low values otQ/Nd3. Six-bladed disk turbine ( ) water ( ) methanol (10%) (A) ethylene glycol (8%) (A) glycerol (40%) Pg = gassed power input P = ungassed power input Q = gas flow rate IV = agitator speed d = agitator-impeller diameter. [Luong and Volesky, AIChE J. 25, 893 (1979)].

For gas-liquid systems, the power dissipated by the stirrer at the same stirrer speed N is lower than the corresponding power input for liquid systems due to reduced drag on the impeller. The power of the gassed system PG is related to that of the ungassed system P0 by using the power number NP correlation with the aeration number Y, ... 

For large-scale fermenters with multiple impellers, there are very few heat transfer correlations available. Most of the experimental investigations have been done with Newtonian liquids with a single, standard impeller. The following ungassed liquid phase Nusselt number correlation for heat transfer at the tank wall, in different vessel geometries with four standard baffles and a Rushton turbine, was developed by Strek.t ... 

If higher power numbers are required, flat-blade turbines with more than six blades (preferably 12 or 16) can be used, for which Kq/o eventually drops to about 0.4, but not until much higher flow numbers than for six flat blades, g/u foi up-pumping wide-blade hydrofoils remains close to 1.0. Axial-flow impellers (e.g., Lightnin A345, Prochem Maxflo W, APV B6), if used for gas-liquid duty, should be preferentially operated in the upflow direction, when they are stable and maintain more than 70% of their ungassed power draw... 

Height of liquid in vessel (m) Gassing factor, i.e., ratio of gassed to ungassed power dissipations... 

The Prochem Hydrofoil stirrer shown in Fig. 1.13 exhibited a much improved pumping effect over conventional turbine stirrers, which makes them more efficient particularly for gas/liquid mass transfer in viscous material systems (e.g. fermentation with micelles). Pilot plant experiments (D = 0.8 m) in ungassed liquids at Re > 2 X 10 yielded Ne = 6.5 for the classical turbine stirrer and Ne = 1.1 for the Hydrofoil stirrer [150]. The power loss due to gassing, the Ne(Q) dependence, was found, however, to be identical for the two stirrer types. 

Re > 900 Viscoelasticity also reduced the Ne values in ungassed liquids. The Ne values were at Re > 10 not similar to those for water. Higher stirrer speeds were necessary to realize a complete dispersion. 

This is due to the formation of gas cushions and gas trails behind the flowed against stirrer blades. They reduced the flow resistance with the result that the power number was much lower than that with ungassed liquids. 

If gas-liquid contacting takes place in the liquid volume, e.g. in stirred tanks and in bubble columns, a quasi-uniform material system is usually assumed (same bubble density in each volume element) and the mass flow G is related to the (ungassed) volume of liquid V ... 

The interfacial area per unit volume being the sum of all the bubble surfaces, is related to the volume of the ungassed liquid and is defined as ... 

The gas hold-up e in a gassed liquid can be related both to the ungassed and to the gassed (g) liquid. In the first case ... 

The absorption process is also measured here in the volume and thereby k a determined. Through multiplying this value by the volume V of the ungassed liquid k A = k aV is then obtained. The liquid is also fully back-mixed in the surface aeration, so that here Ac is given by Ac = Cj — c. 

The curves show the horsepower range at the air orifice from zero to sonic velocity which can be obtained by knowing the ungassed liquid height (differential pressure cell), the air pressure upstream of the orifice, and the scfin of air used. 

---