Fluorescence Technique Analyzes Fluid Mixing

Hank Hogan

When chemical engineers talk about a good mixer, they probably are describing a reactor in which liquids are brought together to manufacture a product. Reactant mixing has a large impact on the quality of the products and yields of the process. In making fine-quality polymers, for example, the mixing efficiency greatly affects the distribution of molecular weight and characteristics of the product. Engineers, therefore, want to make a good mixer, and for that, t hey need a basic understanding of what goes on inside such devices.

Planar laser-induced fluorescence enables the high-resolution study of mixing processes. Courtesy of Dantec Dynamics A/S.

That’s where Yi Cheng, an associate professor of chemical engineering at Tsinghua University in Beijing, comes in. He is investigating liquid mixing using planar laser-induced fluorescence. In the Jan. 18 issue of Industrial & Engineering Chemistry Research, Cheng and his colleagues from the university and from Yantai Wanhua Polyurethane Co. Ltd. of Yantai, China, report the results of a series of experiments in which they used the approach to examine thin liquid sheets impinging upon one another at various angles and conditions in Plexiglas models of reactors.

In planar laser-induced fluorescence, which is similar to particle image velocimetry, researchers inject a fluorescent dye into a moving liquid stream. They illuminate the stream with a laser and detect the resulting fluorescence. From the intensity of the signal and a calibration curve of intensity versus tracer concentration, they extract concentration contours and mixing in the liquid.

The technique enables the study of mixing processes at a spatial scale of about 50 µm, Cheng said, a far higher resolution than other methods offer. For instance, he said, one alternative involves bringing competitive reactants together and evaluating the resulting byproducts. Although it has the benefit of being inexpensive, this offers only a global view and cannot pinpoint problem areas for improvement.

In the work, the scientists used a FlowMap planar laser-induced fluorescence system and a HiSense 1280 × 1024-pixel CCD camera, both from Dantec Dynamics A/S of Skovlunde, Denmark. For a tracer, they used rhodamine B, which they injected into water. They pumped the solution into the mixer models so that two liquid sheets with cross sections of a few millimeters ran into each other at 30° or 45°. The system’s double-pulsed Nd:YAG laser illuminated the fluid jets with 532-nm light in a 0.5-mm-thick plane across the mixing area. That caused the dye in the path to fluoresce with an emission peak at 625 nm.

“The measured plane is representative for our research purpose,” Cheng said. “You can, of course, get the data from different views.”

To eliminate the effects of the illumination on imaging, they used a high-pass optical filter that cut out light below 550 nm. They captured images at 400 ms between frames to create two-dimensional image pairs and thereby to derive a snapshot of fluid movement within the reactor.

The group found that a larger impingement angle and a higher velocity ratio between the two liquid sheets improved mixing performance, which was expected. What was not expected was that restricting the impingement zone resulted in much more efficient mixing than a configuration without a restriction. For example, the 95 percent mixing point — the time that it takes two liquids to mix to a uniformity of 95 percent — was 3.4 ms for the restricted zone, but it was 8.4 ms for the unrestricted configuration under equal velocity conditions.

Cheng noted that the models for these tests were relatively simple but still offered worthwhile information. “It is good enough to let the engineers understand how to tune their design and operation in the real unit,” he said.

He added that research in the area is ongoing, although the cost of planar laser-induced fluorescence keeps it from being used as widely as some other techniques.