Static Mixers by Design

Shiping Liu, Andrew Hrymak, and Phil Wood, McMaster University, Hamilton, Ontario, Canada; and Rafiqul Khan, Fluent Inc.

SMX mixer geometry

Static mixers consist of an array of similar, stationary mixing elements, placed one behind the other in a pipe or channel. Liquids are pumped through the channel, and the elements act to accelerate the homogenization of material properties, such as concentration, temperature, and velocity. In some types of static mixers, the elements are rotated by some angle (say, 90°) relative to the previous element. The SMX mixer is one example of this type of mixer. The elements are complex networks of angled guide blades, positioned at an angle to the pipe axis, and mixing occurs through the continuous redirecting, splitting, stretching, and diffusion of the fluids as they pass through the available openings.

Since there are no moving parts involved, static mixing occurs with low shear, which is very important for some mixing processes where gentle treatment of the materials is required. Processes of this type are found in the food processing, pharmaceutical, and biotechnology industries. Static mixers are also widely used in a host of other industries, however, including oil and gas, chemical processing, polymer production and processing, and water and waste treatment. Some of the major manufacturers of static mixers are Sulzer Ltd., Koch-Glitsch Inc., and Chemineer Inc.

Using the DPM, the particle distribution through the mixer, using a central feeding of 20,000 tracers is shown

Using the species mixing approach, concentration contours on the center plane are shown

Researchers from the Department of Chemical Engineering at McMaster University have been investigating the laminar mixing characteristics of an SMX static mixer using the discrete phase model (DPM) in FLUENT. Typically a series of SMX elements is used to ensure adequate mixing. The mixing quality increases with the number of mixing elements, but so does the power required to pump the fluids through the channel. For this reason, the number of mixing elements used in any given mixer is a function of the required product quality and operating budget.

Mixing homogeneity is often rated using the coefficient of variation, or COV, which can be approximated using the fluid properties, operating parameters, and geometry of the mixing element. It can also be computed easily using CFD. Furthermore, CFD can be used to test the COV after the fluid has passed through different element designs, and to determine the minimum number of elements required to achieve the desired product quality. With CFD, these parameters can be established long before construction of an experimental apparatus begins, saving both time and money.

Using FLUENT, COV values, pressure drop, and power requirements have been computed for a series of test cases using four SMX elements in a pipe. Qualitative results from the DPM calculations have clearly shown the expected stretching and layering of the fluid during the mixing process. Simulations using a two species model to track the mixing of epoxy resins have also been performed, and the results, particularly the species distribution on several axial planes, are in close agreement with experimental data provided by Sulzer for the SMX mixer.