Lori Ann Bacca

Quantifying Mixing using Magnetic Resonance Imaging

Mixing is a unit operation that combines two or more components into a homogeneous mixture. This work involves mixing two viscous liquid streams using an in-line static mixer. The mixer is a split-and-recombine design that employs shear and extensional flow to increase the interfacial contact between the components. A prototype split-and-recombine (SAR) mixer was constructed by aligning a series of thin laser-cut Poly (methyl methacrylate) (PMMA) plates held in place in a PVC pipe. Mixing in this device is illustrated in the photograph in Fig. 1. Red dye was added to a portion of the test fluid and used as the minor component being mixed into the major (undyed) component. At the inlet of the mixer, the injected layer of tracer fluid is split into two layers as it flows through the mixing section. On each subsequent mixing section, the number of horizontal layers is duplicated. Ultimately, the single stream of dye is uniformly dispersed throughout the cross section of the device. Using a non-Newtonian test fluid of 0.2% Carbopol and a doped tracer fluid of similar composition, mixing in the unit is visualized using magnetic resonance imaging (MRI). MRI is a very powerful experimental probe of molecular chemical and physical environment as well as sample structure on the length scales from microns to centimeters. This sensitivity has resulted in broad application of these techniques to characterize physical, chemical and/or biological properties of materials ranging from humans to foods to porous media 1, 2. The equipment and conditions used here are suitable for imaging liquids containing substantial amounts of NMR mobile 1H such as ordinary water and organic liquids including oils. Traditionally MRI has utilized super conducting magnets which are not suitable for industrial environments and not portable within a laboratory (Fig. 2). Recent advances in magnet technology have permitted the construction of large volume industrially compatible magnets suitable for imaging process flows. Here, MRI provides spatially resolved component concentrations at different axial locations during the mixing process. This work documents real-time mixing of highly viscous fluids via distributive mixing with an application to personal care products.

Chapter 14:Process Flow of Wormlike Micelle Solutions in Simple and Complex Geometries

This chapter summarizes recent efforts to evaluate and enhance the predictability of process flows involving wormlike micelle (WLM) solutions. We focus on two extremes of laminar flows. First we explore the predictability of pressure drop in laminar pipe flow, arguably the simplest process flow. Second, we explore the laminar flow in static mixers, which are a complex geometry analogous to porous media. We utilize a model WLM system that is a mixture of commercially relevant surfactants. Rheological properties of this model system are measured as a function of salt content, or at various compositions along the salt curve. We conduct this exploration using NMR-velocimetry for laminar pipe flow and investigate the applicability of static mixer flow models. Our first conclusion is that models using apparent wall slip enable pragmatic modelling and analysis of laminar pipe flow. Our second conclusion is that the linear viscoelastic complex viscosity is the appropriate effective viscosity for modelling static mixer flows, where shear banding is unlikely to occur.