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ePaper created 2014-04-30, 16:22:09 | version 1.30.01
FOCUS// The HZDR Research Magazine WWW.Hzdr.DE 12 13 Smaller bubbles for larger yield The gas, required for the reaction, is fed through holes of a perforated plate at the bottom of the cylinder. As the gas flows through these holes, small bubbles form that rise upwards within the column. That does not happen in a straight line at all, however, which a glance at the air bubbles produced by an exhaling scuba diver demonstrate. Also, it does not happen uniformly. While rising, a gas bubble is displacing the liquid at its front end, of course, thereby creating a kind of wake for the adjacent little bubbles below. "The rising bubbles resemble the movement of a flock of birds a bit, with the whole thing looking like ordered chaos," says Schubert in describing this behavior. "The rise of the gas bubbles also leads to backmixing of the liquids, of course, which influences the processes. The liquid flows primarily upwards in the middle of the tube, and circulates downwards again at the edges," the researcher explains further. Therefore, it is very important for a process engineer to carefully observe the structure of the two-phase flows. On top of that, he not only wants to know how large the individual bubbles are, but also how often two or more of them coalesce into a larger bubble. The more small bubbles exist in the liquid, the larger the surface area of the gas is. It is precisely this large surface area that is needed for the chemical reaction. Less big bubbles instead of many small bubbles, accordingly means lower yield and thus higher costs. To determine the optimum operating conditions for individual reactions, Schubert analyses the fluid dynamics using ultrafast computed tomography developed at HZDR. The tomography equipment normally employed for diagnostics in hospitals cannot be operated rapidly enough for the high-speed imaging necessary here. For that reason, HZDR researchers use X-rays produced from electron beams hitting a tungsten target. The electrons can be steered very rapidly using focussing coils; the position of the produced X-rays then changes correspondingly quickly. These X-rays are attenuated by water more than by gas. Detectors subsequently measure how intense the X-ray radiation is still after passing through the flow. Sophisticated software programs then reconstruct the images of the bubbles in the liquid, similar to the images known from medical diagnostics. This technique can produce thousands of images per second of the flow in a cylinder without problem. In parallel, Schubert is developing computer models to simulate the flows. Here, he is first considering water-air systems. In order to reliably simulate the flows of other liquids as well, a researcher would then just have to enter the commonly known properties of the liquid into these models. And surely, real experiments will follow later, using organic solvents for example. Eventually, the researcher wants to provide the industry with basic parameters and methods that enable them to reduce energy consumption for individual reactions. Contact _Institute of Fluid Dynamics at HZDR Dr. Markus Schubert m.schubert@hzdr.de BUBBLE FLOW: This image nicely documents how flow structures can arise inside a narrow 70 mm diameter bubble column. Measurements were obtained using ultrafast X-ray computed tomography developed at the HZDR. The flow's multiple individual "sectional images" that were taken at a 1,000 Hz frequency (i.e. 1,000 images per second) are ultimately compiled into three- dimensional structures. In this study, all liquids were aerated with the same amounts of air while their viscosity was increased from left to right. Viscous fluids, which are distinctly different from water, may arise during sewage treatment or inside bioreactors. Their viscosity affects their flow structure - from several smaller-sized bubbles to dense bubble flow all the way to regular large bubble formation. Glycerin (%) 0 20 38 50 70 100