Impeller helical ribbon-type

Some fermentation broths are highly viscous, and many are non-Newtonian liquids that follow Equation 2.6. For liquids with viscosities up to approximately 50 Pa s, impellers (Figure 7.7a-c) can be used, but for more viscous liquids special types of impeller, such as the helical ribbon-type and anchor-type, are often used. 

FIGURE 10.1 Types of stirred tank reactor, (a) Multiphase stirred reactor. I impeller, 2 baffles, 3 cooling coils, 4 gas sparger, (b) Stirred reactor with gas-inducing impeller (dead-end type), (c) Stirred reactor with helical ribbon impeller (used with or without a draft tube). 

At transitional flow conditions, larger diameter impellers are used to improve mixing times, but they require more power than equivalent mixing rates under turbulent conditions. Baffles improve mixing rates for Re > 300, but have the opposite effect for Re below this point. Helical ribbon-, anchor-, and gate-type impellers are commonly used at the low end of the transitional flow range. 

Other types of impellers may be preferred for mixing certain liquids. A helical ribbon agitator gives much shorter mixing times for the same power input with very viscous liquids but is slower than the turbine with thin... 

Cabaret et al. (2008) and Gagnon et al. (1998) concluded that better mixing and higher product conversion can be achieved if a close clearance impeller, such as the helical ribbon, is used in conjunction with a radial flow impeller such as the RT in a highly viscous system. The Rushton-type turbine provides proper gas dispersion, while the close clearance impeller attempts to contact most of the reactor volume and provides proper bulk mixing, shear distribution, lower apparent viscosity, and minimal stagnant zones (Tecante and Choplin, 1993). These effects also lead to higher reactor utilization and can decrease power requirements. 

The most commonly used impeller for laminar blending applications is the helical ribbon. Other impeller types have been studied, including anchors and helical screws, but the helical ribbon is most effective. A helical ribbon impeller will have a large diameter, typically 90 to 95% of the vessel diameter. This ensures that the fluid is positively displaced by the ribbons. This is important because there is no mixing due to entrainment by eddies in the laminar regime. 

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