Using Water, Not Air for Spray Drying Simulations
The process allows experiments to be performed at a laboratory level that otherwise would require full-scale equipment of several feet in all dimensions. The data gathered helps GEA Niro, headquartered in Soeborg, Denmark, to refine the development of its spray drying equipment and ensures that its technology keeps pace with, and sometimes drives, its customers’ production needs.
The laboratory equipment is financed by GEA Niro. Inside the scaled-down, clear polycarbonate spray dryer, swirling water contains thousands of microscopic particles that go “live” by two green flashes from a laser. Meanwhile, a powerful computer indicates particle speed and direction of flow visually, displaying an image full of arrows in different shapes and sizes.
“We let the water run through the container and then create vertically aligned laser light,” says Knud Erik Meyer, associate professor of mechanics at the university. “We shoot twice in a row and record each laser pulse. As the particles in the water move from the first to the second picture, we can see how fast and in which direction the particles move.”
Thorvald Ullum, GEA Niro's fluid mechanics manager, has been working on the development of spray dryers primarily used for powder production in food, chemicals and pharmaceuticals for many years.
“This research collaboration makes it possible for GEA Niro to optimize our systems to be as compact and energy efficient as possible,” he says. “At the same time, it makes it possible to improve the drying process, allowing the systems to produce better products by controlling the temperature of the particles during the drying process.
"We can now make computer simulations of the spray drying process very accurately,” Ullum says. “By using CFD [computational fluid dynamics], we can predict how the air moves and thereby how the particles dry. It is essential to know how close the simulations are to reality. How much can we trust our computer simulations? In truth the simulations are only part of the evaluation process. Although the simulation results are very accurate, every assumption has to be tested and validated on production equipment.”
Using water rather than air allows the experiments to be performed on a smaller scale.
Turbulence is a known problem in fluid dynamics and has to be taken into account when performing CFD calculations. Turbulence makes the flow pattern complicated, and it would take too much computing power to predict the flow accurately without some simplifications. Therefore, the system uses commercially available turbulence models, and different models are used depending on the flow conditions.
The aim is to get as complete a picture as possible of the turbulent flow pattern in a spray dryer for as many different case scenarios as GEA Niro finds relevant. The recorded experimental flow patterns then are compared with theoretical flow patterns from computer simulations. The results from the experiments will help the university researchers choose the right turbulence model for their simulations to further refine the drying processes in newly developed spray dryers.