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Following Flow by Mapping Heat

Photonics Spectra
May 2007
Infrared imaging provides insight into distillation process.

Hank Hogan, Contributing Editor

Researchers at Zhejiang University of Technology in Hangzhou, China, have found that a little bit of warmth can go a long way when it comes to understanding the distillation process. The group used a thermal infrared camera to track the flow of warm and ambient-temperature water across a distillation tray.

The scientists also determined how long the water spent at given locations and, thus, its residence time distribution. Their technique could provide a simple, direct and accurate indication of residence time distribution in complicated setups, which could benefit distillation processes in a variety of industries.

Distillation has been used for thousands of years to separate chemical substances based upon their evaporation characteristics, or volatility. Today, industrial distillation is performed in large columns, using trays to hold the liquid and to function as vapor-liquid contact devices. Within the columns, the trays are placed over one another, and components called downcomers direct the liquid from an upper tray to the one below. The downcomer can be a simple opening, a downspout perforated with holes or a more complicated device.

Trays can measure meters across, and that large size allows for far from uniform flow. These nonuniformities can rob trays, and distillers, of their efficiency. Although techniques such as the injection of a fluorescence tracer have been developed to enable flow tracking, they have their drawbacks. Some, for example, can disturb the flow; others introduce contaminants into the system.

As they reported in the March 30 online edition of Industrial & Engineering Chemistry Research, the investigators used warm water as a tracer because it would neither disturb the flow nor pollute it. After first heating water up to 90 °C, they mixed it with an ambient stream until the result was water at a temperature 2 °C higher than that of the ambient water. They injected the tracer into the flow, creating a boundary between ambient and warmer water that moved outward from the injection point. They tracked this moving border using a quantum well infrared photodetector-based thermal infrared camera from Flir Systems Inc. of Wilsonville, Ore.

The scientists built two test trays, each in a 1.2-m-diameter column. One tray had a single downcomer located in the middle of the tray; the other, two downcomers at the edges. They injected the warmer water and captured the results in a movie by using the infrared camera. They analyzed the resulting video, extracting residence time distribution from the data.

They found that the residence time changed with increasing flow and that it was altered when they introduced air bubbles into the water. They also discovered a stagnant region in the multiple-downcomer configuration. The size of the region could be decreased by cutting the injection rate of the liquid or by bubbling gas into the water.


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