chapter  9
94 Pages

Industrial Mixing Technology

References ......................................................................................................................................705

It has been estimated that well over 70% of all mixing is done under turbulent conditions. Turbulent mixing occurs in pipelines, liquid jets, impingement devices, mixing vessels, tee-junctions, static mixers, pumps and orifices, and anywhere turbulence can be created. Eddy interactions lead to turbulent mixing in low-viscosity fluids. Large, high-energy eddies become smaller as a result of viscous-energy dissipation. During this process, regions of segregation are reduced in both size and intensity by the eddy motion. This process is normally fast. In turbine-equipped baffle-stirred vessels, turbulent mixing develops at impeller Reynolds numbers (Re), (

D

N

ρ

/

μ

)

. Turbulent mixing is faster than transitional-flow mixing and is orders of magnitude faster than laminar mixing. Virtually all gas-gas mixing occurs under turbulent conditions. Commonly used units of power intensity are kW/m

or hp per 1000 gal. 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.