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You can only imagine how big a spray booth was needed to paint this baby. Controlling paint fumes is one aim of a new filtering device.
Courtesy Boeing.

Two black cylinders of activated carbon remove organic vapors from air. When the heat's turned on, the trapped vapors evaporate, condense inside the larger glass cylinder, and drain into the flask below, leaving fresh carbon ready to adsorb more vapors.
Courtesy Patrick Sullivan.

Holy fumes!
Courtesy Tom Smith Country.

POSTED 26 OCT 2000 Chances are, as you're reading this, you can reach out and touch an object that was coated using organic solvents. These handy hydrocarbon chemicals are used to carry coatings -- typically paint -- to objects. The solvent obediently evaporates, leaving a spiffy coat of paint.

Whether used on airplanes, cars, refrigerators or bicycles, spray coating is a huge industry. Organic compounds comprise the majority of roughly a billion kilograms of hazardous industrial air pollutants released annually in the United States. Many of those organics are used for coating.

Today, the standard way to deal with vapors from a spray-paint operation is to use destructive techniques, or to capture them with granular activated charcoal.

This extremely porous form of carbon traps gas molecules by the attraction of van der Waals forces. Van der Waals forces are caused by changes in the orbits of electrons in adjacent atoms or molecules; they are weaker than valence bonds, which involve a sharing of electrons between atoms.

While existing filters work, they are typically cleaned with water or steam, which leaves a messy residue of water mixed with solvent.

Now, after 10 years of work, a University of Illinois at Champaign environmental engineer thinks he's found a better solution -- a filter that is cleaned with heat, leaving a residue of pure solvent.

The trapped vapor is boiled off the filter when it is heated by an electric current. Then the vapor is condensed and captured in the filter housing. Ideally, says the inventor, Mark Rood, the solvent could be returned to a spray painter and reused.

"You go from a dilute gas to pure liquid," says Rood, who's applied for a patent on the system with co-inventors Patrick Sullivan and K. James Hay.

Classy carbon
Depending on the setup and the solvent, Rood says the system can remove 99.99 percent of vapor from air streams contaminated with a few hundred parts per million of vapor.

Like many other filters, including common household water filters, Rood's system depends on activated carbon, a form of carbon that's blasted with steam to create countless pores that attract small vapor molecules.

The pores are less than a billionth of a meter across, says Rood, and a gram of activated carbon can have a surface area of 1,000 to 2,500 square meters!

It's like, adsorbing
Surface area is important because the vapor binds to the surface in the process of adsorption. (In contrast, when a substance is absorbed, it is held within another material.) Rood says the system is approaching commercialization with a pilot-scale test unit now under construction. Within a year, he hopes to start a field test at an Army coating or cleaning plant.

As Rood envisions it, the system would use several filters. When filters got saturated with solvent, they would be heated for a couple of minutes to boil the solvent. The boiled solvent would condense on the cool filter housing, and the liquid would drain out. Then the filter could get back to work.

The primary advantage of the new system, says Rood, is the ability to deliver a pure solvent rather than a goop of water and solvent. That allows quick, on-site recycling.

The timing is right for such a system, Rood adds, since regulations under the 1990 Clean Air Act amendments are now being implemented. The law increased the number of regulated air pollutants from eight to 188, forcing manufacturers to find better ways to clean up their operations.

-- David Tenenbaum

Bibliography:
Adsorption and Electrothermal Desorption of Hazardous Organic Vapors, P. D. Sullivan, M. J. Rood, K. J. Hay, and S. Qi, Journal of Environmental Engineering, in press, September, 2000, pp. 30.