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Study finds more effective way to dry ethanol

Wed, 09/14/2011 - 4:56am
Brian Wallheimer, Purdue University

Purdue University researchers have found an alternative environmentally friendly and energy-efficient way to dry corn ethanol, and their proof is in the pudding.

Michael Ladisch, a distinguished professor of agricultural and biological engineering; Youngmi Kim, a Purdue research scientist; and Ahmad Hilaly, director of process research at Archer Daniels Midland, found that the shape and structure of tapioca pearls are ideal for removing water from ethanol. Their findings were reported in Industrial & Engineering Chemistry Research.

After fermentation, ethanol contains between 6 and 12% water, which must be removed to make it fuel-grade. Many ethanol plants use corn grits, which absorb water, or molecular sieves, silica-based particles with tiny pores that only retain water molecules. Ladisch and Kim found that tapioca pearls work better than the conventional corn grit adsorbents.

"Any starch will absorb water. That's how you cook rice or pasta," Kim says. "The tapioca pearl is made of aggregated cassava starch granules that can adsorb more water."

Ladisch says tests found tapioca collected about 34% more water than corn. Molecular sieves, while effective, eventually wear out and create waste that must be disposed of. The tapioca can be dried and reused, and when they wear out, they can be used to make more ethanol.

"Tapioca is very efficient, and it's all-natural," Ladisch says. "There are no disposal issues. It's much more environmentally friendly."

Tapioca pearls, essentially spherical, are structured differently than corn grits, Ladisch says. While corn grits are solid, irregularly shaped particles, tapioca pearls contain a gelatin starch core upon which dry starch granules are aggregated, significantly increasing surface area.

While tapioca pearls are 100% starch, corn grits also contain fiber, protein, and other substances that are not efficient for absorbing water.

Starch-based adsorbents like tapioca pearls also take up the heat created during drying, allowing that heat to be reused to evaporate water during regeneration of the drying bed.

"This combines fundamental biochemistry, biology, and engineering with thermodynamics to obtain an efficient separation system," Ladisch says.

After trying several options to maximize water absorption, including corncobs and wood chips, inspiration struck Ladisch while watching his mother-in-law fix Thanksgiving dinner. As she started mixing up homemade tapioca pudding, Ladisch noticed that the tapioca pearls looked similar to the beads used in molecular sieves.

"I started thinking, 'It's a starch. Might this work?'" Ladisch says.

Ladisch says tapioca pearls may be used effectively in U.S. ethanol facilities, but he believes they could be more significant in facilities in South America and Africa where the plant used to create tapioca—cassava—is grown.

Ladisch and Kim say they would continue to test uses for tapioca pearls, including drying other alcohols. They also plan to create synthetic, starch-based adsorbents from other cheaper materials to lower the cost.

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