QUALITY and eventually implode. When they burst, a shock wave sends cavitation bubbles out into the fluid. If those shock waves are left unchecked, the heat and mixing power of the bursting bubbles can create a lot of equipment damage. “Conventionally, engineers are taught to avoid cavitation,” he says. McGurk found a pathway that allows him to harness that energy, to control it, a technology that drives a reaction in a matter of seconds versus the minutes or hours typically seen in a continuous stir tank reactor. As an example of just how damaging some methods of cavitation can be, McGurk sites a retrofit project his team worked on in a chemical plant considering switching to biodiesel production. The plant was using a stir tank reactor set-up with cavitation. The system required the sparging of air into the tanks and when combined with the cavitation, the bubbles were so powerful they “ate through the walls at several points” and caused the two-story reactor tanks to lift off the floor. The ShockWave Power Biodiesel Reactor employs the “controlled” approach and uses a spinning rotor with depressions in it. As the device spins, cavitation bubbles are formed in the depressions, and as the fluid flows over the depressions the bubbles burst, sending out a massive wave of energy and heat that drives futher reaction of the materials. The amount of energy created by the bubbles breaks down the material into smaller pieces, creating more surface area for the reactants to take place, a situation McGurk says also helps to reduce side or reverse reactions. Already used in eight different biodiesel plants, including one of the largest production facilities in the country, the ShockWave
Power Biodiesel Reactor is an option for producers who, McGurk says, don’t want—or can’t afford—distillation. Some producers have sent McGurk samples of biodiesel that showed only a .19 monoglyceride content with a total glycerin content of .055, he says. “You are looking at a fifth of the total glycerin and that is with waste cooking oil, and many plants aren’t able to handle that in their transesterification process.” The idea of cavitation as a means to solve high mono levels in biodiesel produced from beef tallow or waste vegetable oil isn’t a new one though. Companies like Ultrasonic Power Corp. out of Freeport, Ill., employ a sound wave, or ultrasonic-based cavitation approach to biodiesel production. While cavitation approaches vary, the ultrasonic unit at Freeport, like the mechanical version, can reduce the one- to four-hour process to less than 30 seconds, while effectively dispersing nanoparticles and deconglomerating coalesced materials in the fluid. Mike Kass from Oak Ridge National Laboratory even recently investigated using ultrasonic acoustic energy (similar to that of Ultrasonic Power Corp.’s version) to remove precipitate formation in biodiesel. The rationale, Kass says, was that the application of ultrasound would concentrate heating at the precipitate-liquid interfacial region. “The advantage of this approach is that you can focus the heating on the precipitate itself rather than heating the bulk solution.” To test the theory that ultrasonic cavitation does, in fact, reduce precipitates, Kass and his team, which includes Sam Lewis and Maggie Connatser, performed a modified version of the ASTM D7501
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