4 August 2009

Industrial separations feel the heat

Everything these days is about conserving energy, so thin-film zeolite membranes with tiny, molecule-sized pores are one step closer to replacing the energy-intensive processes now used in industrial separations.

The membranes’ ability to separate molecules in a mixture significantly improves by subjecting the zeolite to rapid thermal processing (RTP). By heating the membranes from room temperature to 700 degrees Centigrade in one minute, maintaining this temperature for up to two minutes and then quickly cooling it, researchers said. They also added they have been able to eliminate the formation of grain boundary defects that undermine the sieve-like quality of zeolite’s uniformly sized nanopores.

RTP shows promise in achieving greater yield and energy efficiency in zeolite membrane production, said Michael Tsapatsis, Amundson Chair Professor of chemical engineering and materials science at the University of Minnesota.

Zeolites are crystalline aluminosilicate materials whose compositions and nanoporous structures can go in applications in catalysis, adsorption, and ion exchange. They call them “molecular sieves” because their pores, being small and very uniform in size, can sort and separate molecules selectively according to the molecules’ size.

As microscopic particles, zeolites go in a variety of applications, including the creation of pure streams of oxygen and other gases, the catalytic cracking of petroleum into gasoline, water purification and softening, the dewatering of ethanol, and as additives in laundry detergents.

Zeolite membranes form by depositing zeolite crystals on a porous surface and inter-growing these crystals into a continuous film. Several challenges have kept zeolite membranes from achieving their full industrial potential. These include high processing costs; scalability, or the ability to make zeolite membranes in large area; and the difficulty in controlling grain boundary defects, or non-selective pathways at the crystal grain interfaces, which cause poor separation performance.

Meanwhile, an estimated 15% of the world’s energy consumption goes toward the industrial separations of molecules and mixtures, often in volatile, energy-hungry distillation towers. By contrast, zeolite membranes with optimal porosity consume much less energy when they perform separations.

When making zeolites, structure directing agents (SDAs) direct the formation of the porous crystalline structure. But the SDAs then become trapped inside the zeolite pores in what scientists call a “ship in the bottle” effect. These SDAs block the zeolite pores, and researchers must remove them so other molecules can pass through. Typically, they use high-temperature treatment to remove the pores, but the heat has little effect on the zeolite, which is stable. But the SDAs, being organic, break up and go away during heating.

This heat processing should occur after the formation of zeolite membranes. Scientists have long believed this must happen slowly to prevent cracks and other grain boundary defects from forming in the thin film. But the gradual heating promotes the formation of flexible grain boundary defects, or interfaces, between zeolite crystals. These defects can grow much larger than the zeolite pores to become “nonselective” pathways.

“The molecules that you are trying to separate with your zeolite film can now circumvent the highly selective pathways and pass through the grain boundaries,” said Mark A. Snyder, assistant professor of chemical engineering at Lehigh University in Bethlehem, Penn. “This has been a major issue for the commercial viability of zeolite membranes.”

Tsapatsis’ group uses a regimen of RTP to remove the SDA molecules from inside the zeolite pores and to promote what they believe could be chemical “gluing” of the zeolite crystal domains. After heating the zeolite to 700°C, the researchers hold it at that temperature for up to two minutes and then cool the material rapidly to room temperature.

The researchers said the rapid rise in temperature may cause bonding between the crystal domains. “This could possibly decrease the flexibility of the grain boundaries so that they no longer open up between the crystals during operation of the membrane,” said Snyder, who worked with Tsapatsis as a postdoctoral researcher. “In short, this type of heat treatment may chemically glue the crystal domains together.”

For related information, go to www.isa.org/manufacturing_automation.