Sound waves have been used for a wide range of applications from medical imaging to sonar and even to drug delivery. Now, one Iowa State University researcher believes that high-frequency sound waves could be the answer to improving biofuels production.

David Grewell, associate professor of agricultural and biosystems engineering and Center for Crops Utilization Research (CCUR) and BioCentury Research Farm affiliate, and his research team are using ultrasound to enhance chemical reactions of biomass from a raw feedstock to high-value fuels and chemicals.

High-powered ultrasonics are sound waves at a frequency above the normal human hearing range. The power generated from applying ultrasonic waves to liquid media breaks up solid materials and promotes mixing between the liquid and solids. It also exposes more of the surface area that can enhance chemical reactions.

In aiming to reduce energy usage in ethanol production, Grewell evaluated the use of ultrasonics to pretreat corn in preparation for fermentation. In a conventional dry-grind corn ethanol plant, ground corn is steamed in jet cookers, and with the addition of enzymes, the starch is converted to glucose for fermentation. In Grewell’s method, the ultrasonics break down corn into very small particles, giving the enzymes more surface to convert the starch into sugars that serve as food for the yeast. Their experiments have shown that when corn was pretreated with ultrasound waves, fermentation yields were comparable to jet cooking.

Grewell sees potential cost-savings by using this alternative method. “Using ultrasonics to pretreat corn in ethanol production would eliminate the need for large amounts of energy used for jet cooking in an ethanol plant,” he said.

Melissa Montalbo-Lomboy is treating corn to enhance ethanol production using the Branson 2000 series bench-scale ultrasonics unit. Montalbo-Lomboy was a post-doctoral research associate in David Grewell's research group. Photo courtesy of Iowa State University

Enhancing biodiesel production is also possible through the use of high-power ultrasound. By applying ultrasound to soybean oil, methanol and sodium hydroxide, Grewell’s research team has found they can reduce the time it takes to convert soybean oil to biodiesel from 45 minutes to less than two minutes, while maintaining the quality of the biodiesel. This greatly increases the efficiency of the process, which is key to making biofuels a viable source of energy.

Grewell’s researchers are also studying the use of ultrasound on microbes such as yeast and algae. They have found that it is an effective method to enhance the release of oils from these microbes for processing into biodiesel.

Grewell’s latest research is focused on using ultrasonics to process lignocellulosic biomass for ethanol production. Biomass feedstocks, such as switchgrass, are composed of cellulose and hemicellulose held together by lignin. Because of this complex structure, it is very difficult to break down biomass into sugars that can be used for fermentation. Grewell has discovered that biomass can be dissolved in ultrasonic-assisted ionic liquids in minutes compared to hours using standard mixing systems. These ionic liquids allow the removal of lignin so the cellulose can be converted into sugars. The sugars can then be used in ethanol production.

Grewell said that the power of sound does not stop with his research on improving biofuels production. He is also using ultrasonics to improve or create biobased plastics and composites.

“We have been using ultrasonics to enhance the mechanical properties of the corn stover used in our plastic composite materials,” he said. “Pretreating the corn stover makes the composite materials stronger by allowing the corn stover to better adhere to the plastic.”

They are also using sound waves to weld plastic materials. Plastic parts can be joined together through the heat produced from the high frequency sound waves. Grewell’s researchers have studied welding biobased plastics, such as polylactic acid, for food packaging applications. Commonly called PLA, polylactic acid is a plastic made from cornstarch and is used in a number of food packaging applications including water bottles, deli containers, salad bags, and even Frito-Lay SunChips bags. By developing a fundamental knowledge of ultrasonically welding PLA, his researchers are able to model welding processes giving them insight into process optimization.

Grewell’s research is made possible through a partnership with Branson Ultrasonics Corp. of Danbury, Conn., a business unit of Emerson Electric. They have donated a variety of systems ranging from 400 watts to 5,500 watts, with a typical frequency of 20 kilohertz. The equipment is in the Bioprocess Engineering Lab located at CCUR.

“These donations have allowed us to pioneer the use of ultrasonics in the fast growing biofuels industry and help the nation become energy independent,” Grewell said.