Mixing Regime and Impeller Power
From Laminar to Turbulent Flow
In multiphase systems, the mass transfer between phases is key to scale-up and reactor performance, and the mechanical power transferred by the impeller interaction with the fluid is critical. It impacts the degree of dispersion of the various phases and the corresponding mass transfer between them. In a single liquid phase process, where a fast reaction can produce competing product and byproduct formation, the level of fluid turbulence can impact the selectivity and yield of the overall chemical process.
In such systems, the mechanical power per-unit-volume dictates the level of turbulence, and the process performance. For a given impeller and specific stirred tank reactor geometry, the universal impeller performance curve can be obtained. This characterizes the impeller power applied to fluid phase.
Typical power performance curves (Reynolds number (Re) vs. power number (Np), as shown on the right) are independent of the reactor volume, and therefore facilitate scale-up and system characterization. The dimensionless Reynolds number indicates whether the reactor fluid is flowing in laminar or turbulent regime. It represents the ratio of the inertial forces (forces of the impeller on the fluid), compared to the viscous forces intrinsic to the process fluid. When viscous forces dominate and Re < 10, then fluid flow is governed by fluid viscosity, and the flow pattern is laminar. When the impeller forces dominate and Re > 2,000-10,000, then the fluid flow is independent of the viscosity, proportional to density, and therefore turbulent. The exact transition point to the turbulent regime depends on reactor geometry, and the presence of "turbulence" promoting elements in the reactor, such as baffles. Laboratory reactors provide rapid means of testing each fluid and reactor-impeller setup.