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Sweet solution to a salty situation
is also environmentally
friendly
by Jodi Genshaft
Each year, industry generates millions of gallons of toxic chemical waste while producing food, fuel, textiles, cosmetics and pharmaceuticals. Efforts to improve these expensive and dirty processes have been delayed by technical barriers – until now.
 WAFER STACK – Chemical engineer Paula Moon inspects the EDI wafer stack. |
Argonne researchers have developed a cleaner, cheaper salt-removal process that cuts chemical use by 90 percent while matching the existing technology's output. Researchers successfully demonstrated the process that removes salt, a natural byproduct of corn-syrup production, in a high-fructose corn syrup refinery in Lafayette, Ind.
The key to this process is an Argonne-designed porous wafer and gasket system coupled with a refinement to an existing purification process called electrodeionization (EDI). EDI combines electricity and ion exchange, which is a chemical process that separates charged molecules.
The improved EDI technology was named one of this year's top 100 inventions by R&D magazine. Argonne's EDI device could replace conventional ion-exchange technology and reduce energy costs and potential pollution by eliminating the need for chemical regeneration. This innovative device may have applications in cleaning radioactive and bio-based wastes.
Product purification
Every year, the United States pumps out more than 9 million tons of corn syrup to sweeten candy, soft drinks and ice cream. To make high-fructose corn syrup, salt-based enzymes are added to corn starch - a chain of glucose molecules - to convert it into a syrupy mixture of water, dextrose and fructose sugars. But the enzymatic process leaves the corn syrup full of salt that must be removed to preserve the syrup's sweetness.
Traditionally, the salt is removed by passing the syrup through 30-foot-tall columns packed with ion-exchange resin beads. The salt sticks to the resin beads - organic polymers about the size of sand grains. When the resin-filled columns are full of salt, the beads must be treated with strong acids and bases, such as hydrochloric acid and lye, which strip the salt from the beads. This process takes each column off-line for a period.
"For any amount of salt you put on the resin, you need an equal amount of chemicals to knock it off," said Seth Snyder, section manager for chemical and biological technology in Argonne's Energy Systems Division. "It takes more energy and more chemicals than it should."
These non-biodegradable chemicals are neutralized and sent to landfills, where they may eventually damage the ozone layer, pollute groundwater or threaten human health.
Argonne's Energy Systems Division scientists Snyder, Paula Moon, Michael Henry, Yupo Lin, James Frank and Carl Landahl joined forces with EDSep Inc., based in Mount Prospect, Ill., to develop a cleaner, more energy-efficient alternative. EDSep works with scientists to commercialize proprietary technology.
Separation solution
The researchers chose a cleaner alternative – EDI. "This process, used in water desalination, uses electricity rather than chemicals to drive salt separation," said Moon, a chemical engineer at Argonne.
 EDI CELL PAIR – This schematic diagram shows how the EDI cell pair removes unwanted salts from dextrose syrup. |
But EDI had not been used in chemical processing because it leaked and used electricity inefficiently.
"A corn syrup leak will contaminate all of your equipment and lower performance," Snyder said.
Researchers solved the leakage problem by developing sponge-like wafers made of the same type of ion-exchange resin beads used in the existing technology. Argonne scientists molded commercially available ion-exchange resins into thin, flexible wafers that perform rapid desalting with only 0.1 percent leakage.
Each wafer fits inside a compressible gasket that holds in liquids as the wafer expands and shrinks inside. The Argonne-refined EDI device consists of a series of "cell pairs" through which the corn syrup flows. A cell pair is made of a wafer and two ion-exchange membranes inside a gasket.
"The process solution flows through one chamber and the salt concentrate flows through an adjacent chamber," said Lin, a chemical engineer.
Commercial EDI devices would contain a series of 100 or more cell pairs sandwiched between two metal endplates.
Improving electric flow
Argonne also improved the electrical flow through the EDI wafers. A horizontal electrical current to the wafers splits the salt into its positively and negatively charged components. The salt-free corn syrup flows vertically through the wafers. For example, as sodium chloride travels across the wafer, the positive sodium ions are drawn toward the negative cation exchange membrane, and the negative chloride ions move toward the positive anion exchange membrane.
 EDI TEAM – Researchers in the EDI laboratory include (clockwise from the front) Paula Moon, Carl Landahl, Seth Snyder, Rathin Datta, James Frank, Yupo Lin and Michael Henry. Moon (front) holds an individual laboratory-scale resin wafer, while Lin (far right) displays a laboratory-scale resin wafer stack. |
At the same time, the electric current splits water into protons and hydroxide ions to regenerate the resin beads. These protons and hydroxide particles knock salt off the resin, regenerating the beads without using toxic chemicals. At any given time, about 50 percent of the resin is active in the EDI device compared to only 2 percent of the resin working at a time in conventional ion exchange.
The new EDI process was tested at Argonne, using a sugar solution similar to commercial high-fructose corn syrup, before researchers moved the pilot plant to Lafayette to work with real conditions.
Argonne tested various commercial resins to perfect the wafer performance and durability before moving to the pilot scale. In the lab, each 10-square-inch cell pair processed 60 milliliters of solution per minute.
In the pilot demonstration, the process flow was increased 170 percent. The 2-square-foot pilot-scale EDI device pumped about 2,000 gallons of dextrose syrup a day at a rate of 2 gallons per minute. To move up to the commercial scale, scientists would have to increase the processing rate by only 5 to 10 percent, Snyder said.
The operating cost for the pilot test was less than the cost of the chemicals needed to regenerate ion-exchange resin. In addition, the new EDI technology requires less energy and reduces environmental pollution by cutting nearly all toxic chemicals.
"Our process takes up about half of the space," said Henry, a process engineer. "When you're talking smaller size in industry, you're talking about lower cost."
Filters of the future
Researchers predict that industries will phase in this technology. The new EDI wafer systems would be installed when old ion-exchange columns need to be replaced or production capacity must be expanded.
"We're not targeting to change existing technology overnight but rather on a phased-in basis," said EDSep President Rathin Datta.
Argonne and EDSep recently were awarded a U.S. patent, and EDSep plans to license it.
Wafer-based EDI may have applications in industries that use high-performance desalination – including chemical, pharmaceutical, agriprocessing, water conditioning and environmental remediation.
Datta said a similar EDI device could capture radioactive wastes and purify chemicals such as ethylene glycol - a clear, odorless liquid commonly found in antifreeze and automotive cooling systems. The technology also could capture organic acids and other valuable bio-based products in future industrial bio-refineries.
"Now that we've got a process that works," Snyder said, "we're developing a more fundamental understanding of the process so we can apply it to new areas."
By 2020, EDI could save 5.3 trillion British thermal units (Btu) of energy per year.
This research was funded jointly by the Biomass and Chemicals Industries of the Future program in the Department of Energy's Office of Industrial Technologies.
For more information, please contact Evelyn Brown (630/252-5510 or eabrown@anl.gov) at Argonne.
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