Mixing: The Hidden Unit Operation
Mixing is arguably the most ubiquitous operation in chemical processing, yet it remains one of the least understood. Nearly every chemical process requires blending of raw materials, suspension of solids, dispersion of gases, or homogenization of products. Poor mixing causes hot spots in reactors, inconsistent product quality, and wasted energy. The stirred tank, with its rotating impeller and optional baffles, is the most common mixing vessel in industry.
Flow Regimes and the Reynolds Number
The impeller Reynolds number Re = rho*N*D^2/mu determines whether mixing occurs by orderly laminar flow or chaotic turbulent eddies. In laminar flow (Re < 10, typical of viscous polymers), mixing relies on stretching and folding fluid elements. In turbulent flow (Re > 10,000), energy cascades from large eddies to small ones, achieving rapid micromixing. The transition regime requires careful impeller selection.
Power and Scale-Up
The power number Np relates the power drawn by the impeller to the fluid properties and operating conditions. In turbulent flow, Np is constant for a given impeller geometry, meaning power scales as N^3*D^5. This has profound scale-up implications: maintaining the same tip speed when scaling up from lab to plant requires dramatically different RPM, and the power per unit volume changes non-linearly.
Visualizing the Flow
This simulation shows tracer particles moving through the flow field generated by a radial-flow impeller with baffles. Watch how particles near the impeller experience high shear and rapid mixing, while regions far from the impeller mix more slowly. The color gradient from red (unmixed) to cyan (fully mixed) reveals the spatial non-uniformity that is the central challenge of mixing design.