Paddle-type impellers

Mechanically agitated reactors have been used for cultivation of plant cells (Kato et al., 1972 Tanaka, 1981). Ajar fermenter with a six fiat-blade turbine and a modified paddle has been used by Tanaka (1981) and a similar jar fermenter with two disk turbine impellers has been used by Kato et al. (1972) at about 50 rpm with no significant shear damage to plant cells. Paddle-type impellers were found to be more appropriate (less shear damage) than flat-blade turbine type impellers (Kato et al., 1972). The only production-scale reactor used for shikonin production in Japan is also an agitated vessel. 

Paddle-type impellers are the simplest and lowest cost impellers, but they have small pumping capacity. They have very low axial flow, hence the pitched flat blade version is normally used for low-viscosity materials. The ratio of blade diameter to vessel diameter is usually Vs to... 

Stirred bioreactors have also been used for differentiating mESCs. Mouse embryoid bodies (EBs) can be formed directly from enzymatically dissociated mESCs m rotary cell culture systems (E et al., 2006). A similar approach was performed in 250-mL spinner Basks equipped with a paddle-type impeller (Zandstra et al., 2003) and a scaled-up version of the system with a fuUy automated 2-L bioreactor was reported later (Schroeder et al., 2005). Suspension cultures of mESCs were also expanded and subsequently differentiated Into cardiomyocytes in a single process, without an intermediate dissociation step (Fok and Zandstra, 2005). Differentiation of mouse ESCs into osteoblasts in spinner flasks has been described as well (Alfred et al., 2010). 

Figure 6 Various types of turbine (A, B and C) and paddle (D) impellers.

Use radial-flow impeller such as turbine or paddle types without pitched blades. 

As mentioned earlier, these impellers operate at relatively high speeds and are effective only in low to medium viscosity liquids. In most cases, the main flow in the vessel tends to be transitional and/or turbulent. For shearthinning polymer solutions and particulate suspensions agitated by paddle, turbine and propeller type impellers, many correlations of varying complexity and form are available for the estimation of the outside film coefficient. One such correlation, based on wide ranges of conditions (400 < Re < 10 4 < Pr < 1900 0.65 < /ieff < 283mPa s), is due to Edney and Edwards [1976] ... 

Additional power data for other impeller types such as anchors, cui ved-blade turbines, and paddles in baffled and unbaffled vessels are available in the following references Holland and Chapman, op. 

Impeller types usually used with mixing and listed in decreasing order of high volume ability (hence in increasing order of high head ability or requirement) are paddle, turbine, propeller, sawtooth impeller or propeller, cut-out impeller disc (no blades), colloid mill. 

Figures 5-3 and 5-5 illustrate a few of the types of impellers used for mixing. They may be basically classified as axial, radial and mixed. In general the most generally applicable are the. 3-bladed propeller, the flat-blade turbine, the curved blade turbine, and the paddle. The many other designs are either modifications of these or specially designed for a very special purpose with respect to a fluid system and/or its performance.

In Vermeulen s work, a paddle impeller stirred fixed amounts of gas and liquid in a closed vessel. When the impeller was brought to the proper speed (240-360 rpm), the liquid and the gas that had been above it were dispersed together and completely filled the vessel. It is impossible to extrapolate from this experimental set-up to the usual type of gas-liquid contacting operation. 

The impeller is the part of the agitator that impacts force to the material being mixed. Propellers, turbines, gates, anchors, and paddles are all types of impellers. Typically, the impeller is a single propeller or turbine blade connected to a shaft that is driven by an electric motor at a fixed speed. There are two classes of impeller agitators axial-flow and radial-flow, and the mixing characteristics are shown in Figure 3.14. 

Axial-flow impellers generate currents parallel to the axis of the impeller shaft. Radial-flow impellers generate currents in a direction tangential or radial to the axis of the impeller shaft. Within the two classes of impellers, there are three main types of impeller design. These are propeller, turbine, and paddle. The three main types are utilized in about 95% of most batch liquid agitation systems. Standard propellers have three blades, but two-bladed, four-bladed, or impellers encased by a circular guard can also be used. 

A large number of impeller types have been studied over the years, but interest has centered on three designs marine-type propellers, flat-or curve-bladed turbines, and flat paddles, with the first two of greater interest than the third. Propellers produce axial flow of the liquids and are turned in such fashion as to direct flow against the bottom of the tank. Turbines provide radial flow, but in any case the presence of baffles strongly influences the flow pattern in the tank. The effectiveness of these in liquid extraction has not been well established, but it appears that there... 

Figure 7.7 Typical impeller types (a) a six-flat blade turbine (b) a two-flat blade paddle and (c) a three-blade marine propeller. See the text for details of the abbreviations. 

Figure 7.7b shows a two-flat blade paddle. If the flat blades are pitched, then the liquid flow pattern becomes intermediate between axial and radial flows. Many other types of impellers are used in stirred tanks, but these are not described at this point. 

In Figure 7.8, the curves a, b, and c correlate data for three types of impellers, namely, the six-flat blade turbine, two-flat blade paddle, and three-blade marine propeller, respectively. It should be noted that, for the range of (Re) >10, Afp is independent of (Re). For this turbulent regime it is clear from Equation 7.30 that... 

The most useful impellers are ihe simple flat paddle, the marine-type propeller, and the turbine, If any of these are on a vertical shaft rotating on the center line of a cylindrical vessel, the fluid motion will be one of rotation. A vortex forms around which the liquid swirls. A minimum of turbulence and of vertical and lateral flow motion will result, very little power can be applied. 

Until now, bioreactors of various types have been developed. These include loop-fluidized bed [14], spin filter, continuously stirred turbine, hollow fiber, stirred tank, airlift, rotating drum, and photo bioreactors [1]. Bioreactor modifications include the substitution of a marine impeller in place of a flat-bladed turbine, and the use of a single, large, flat paddle or blade, and a newly designed membrane stirrer for bubble-free aeration [13, 15-18]. Kim et al. [19] developed a hybrid reactor with a cell-lift impeller and a sintered stainless steel sparger for Thalictrum rugosum cell cultures, and cell densities of up to 31 g L1 were obtained by perfusion without any problems with mixing or loss of cell viability the specific berberine productivity was comparable to that in shake flasks. Su and Humphrey [20] conducted a perfusion cultivation in a stirred tank bio-... 

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