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Staff Accomplishments

To get good glass, you've got to do a lot of mixing...

April 2004
Can the planned Waste Treatment Plant pulse jet mixers combine radioactive slurries and glass-forming materials in several key tanks? This is the question that Battelle's researchers are helping the Waste Treatment Plant contractor answer. For most of the 300-plus tanks at the plant, mathematical modeling by the treatment plant contractor shows that the pulse jet mixers are satisfactory; however, seven tanks contain waste slurries that are particularly difficult to mix To mix these materials, the pulse jet mixers use air and gentle pressure. In 2003, concerns were raised about the ability of the mixers to combine slurries with high levels of solid particles and glass-forming materials. The combination, a non-Newtonian fluid, when sitting still acts more like a thick chocolate milkshake than a liquid. If the waste and glass formers are not mixed adequately, the resulting glass, that is, the vitrified waste, can be inhomogeneous and perform poorly. Battelle is verifying the performance of the mixers designs and recommending improvements for two of the three most problematic tanks, at the contractor's request. Perry Meyer, chief scientist on the pulse jet mixer task, said, "Many of the technical issues associated with pulse jet mixing in non-Newtonian slurries are at the very forefront of mixing science. The program is developing a basic understanding while performing many first-of-a-kind tests." Battelle's experts considered and rejected a number of simulants for use in testing the pulse jet mixers. After extensive evaluation, two simulants were selected: laponite, which is a standard thickening agent used in food processing, and kaolin/bentonite, which is an opaque clay. Both are inexpensive, nonhazardous, and closely mimic the tank waste. With the simulants selected, the Battelle scientists built 1/4-scale prototype tanks, ranging in volume from about 200 gallons to about 1100 gallons. Each of the tanks is made of clear acrylic plastic to allow scientists to observe mixing behavior. In addition, staff employed methods to prove that the mixing behavior in these smaller tanks would be representative of the real thing. This involved testing in a 12,000-gallon tank and comparing mixing results in a 200-gallon tank. To test the pulse jet mixers, the researchers fill the tanks with simulated waste, either laponite or kaolin/bentonite clay, and measure the amount of mixing with different pulse jet configurations and waste properties. According to Meyer, the researchers are moving into new scientific realms with these fluids, "there is very limited industrial or academic experience to draw from." So, the researchers develop completely new measurement methods, from the simple to the technologically sophisticated. One method involves adding dye to the tank, turning on the mixers, and watching the concentration of the dye change. Another involves adding neutrally buoyant Lexan beads and obtaining core samples of the mixed simulant in the tank. The number and distribution of the beads provides information on the extent of mixing. A third method involves adding tiny radiofrequency tags to the waste. The location of these tags is then monitored using antennas placed around or in the tank. The distribution and frequency of detection provides a measure of how well the simulant is mixed. The team has gathered valuable information about mixer configurations and effectiveness. The Waste Treatment Plant contractor will use the results and recommendations to determine if the pulse jet mixers will work in the problematic tanks or if they will need to make modifications, possibly adding several months and several million dollars to the project. These tests and the testing program have been "reviewed by an international panel of mixing experts, who have confirmed the leading-edge nature of our technical work, as well as validating our testing strategy," noted Meyer. In addition to the effectiveness of the pulse jet mixers on the waste, Battelle is studying how flammable gases behave during pulse jet mixer operation. A key function of the mixers is to safely manage the hydrogen that is generated in the waste. Current results show that oxygen introduced by the pulse jet mixers increases the rates of hydrogen or methane gas generation slightly, and oxygen substantially increases the generation of two-carbon gases. The data also show that pulse jet mixers are effective in removing hydrogen bubbles in a controllable fashion. This information will be used by the contractor to create and validate safety systems and procedures. How Pulse Jet Mixers Work Pulse jet mixers are long cylinders inserted into the tank so that the contents extend into the cylinder. First, compressed air is blown into the top of the cylinder, forcing the waste out through the nozzles, and then a vacuum is applied and the contents are drawn back into the cylinder. The contents move in and out of the cylinder with each pull and push, continuously mixing the wastes in the tank. Pulse jet mixers are advantageous because they have no moving parts, as other conventional mixing systems do. While pulse jet mixers have been used successfully in Sellafield, UK, to mix tank contents, they have never been used for mixing wastes with high solid content like those at the Waste Treatment Plant.

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