New Way of Making Membranes Could Reduce Energy Needs of Biofuel Production
By Scott Doggett July 31, 2009
Engineers have developed a method for creating high-performance membranes from crystal sieves that could increase the energy efficiency of chemical separations up to 50 times over conventional methods and enable higher production rates.
So say a team of researchers led by chemical engineer Michael Tsapatsis of the University of Minnesota, in an article that appeared in today's issue of Science.
The ability to separate and purify specific molecules in a chemical mixture is essential to chemical manufacturing. Many industrial separations rely on distillation, a process that is easy to design and implement but consumes a lot of energy.
Tsapatsis's team developed a rapid heating treatment to remove structural defects in zeolite membranes that limit their performance, a problem that has plagued the technology for decades.
Rosemarie Wesson of the National Science Foundation said that using membranes rather than energy-intensive processes could increase the energy efficiency of producing renewable biofuels such as ethanol and butanol.
Creating Zeolite Membranes
Researchers create zeolite membranes by growing a film of crystals with small organic ions added to direct the crystal structure and pore size - two zeolite properties that help determine which molecules can pass through the material. Then they slowly heat the zeolite film in a process called calcination to decompose the ions and open the pores.
However, Tsapatsis explained, this method for creating zeolite films often leaves cracks at the boundaries between grains of zeolite crystals. These defects have prevented zeolite films from being used effectively as membranes, because molecules of unwelcome chemicals that are rejected by the zeolite pores can still penetrate through the membrane defects.
"While it may be possible to correct some of these defects, the repair process is difficult and expensive," Wesson said. Currently zeolite membranes have found use only in specialized, smaller-scale applications, such as the removal of water from alcohols or other solvents.
In an effort to minimize the formation of cracks and other defects, the heating rate during calcination is very gentle, and the process can take as long as 40 hours - typically a material is heated at a rate of 1 degree Celsius per minute up to a temperature between 400 and 500 degrees Celsius, where it is held steadily for several hours before being allowed to slowly cool.
Because conventional calcination is time-consuming and energy-intensive, it has been difficult and expensive to produce zeolite membranes on a large scale.
Tsapatsis's team developed a treatment in which zeolite film is heated to 700 degrees Celsius within one minute and kept at that temperature for no more than two minutes. Acting as an annealing method, the treatment refines the granular structure of the zeolite crystal film.
When the researchers examined the treated films, they found no evidence of cracks at grain boundaries. Although they found other types of defects, these don't seem to affect the membrane properties or performance.
In a comparison to conventionally-made zeolite membranes, Tsapatsis said, he observed a dramatic improvement in the separation performance of the treated membranes. A second round of treatment improved separation performance even further, to a level on par with current industry separation methods.
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