Wednesday, April 11, 2012

Some flame retardants make fires more deadly

Anna A. Stec, Ph.D., led the research, which focused on the most widely-used category of flame retardants, which contain the chemical element bromine. Scientists term these "halogen-based" flame retardants because bromine is in a group of elements called halogens.

"Halogen-based flame retardants are effective in reducing the ignitability of materials," Stec said. "We found, however, that flame retardants have the undesirable effect of increasing the amounts of carbon monoxide and hydrogen cyanide released during combustion. These gases, not the thermal effects of burns on the body, are the No. 1 cause of fire deaths." Stec, who is with the University of Central Lancashire, Centre for Fire and Hazards Science, Lancashire, U.K., spoke at an ACS symposium on "Fire and Polymers," which included 60 presentations.

Almost 10,000 deaths from fires occur in industrialized countries worldwide each year, including about 3,500 in the U.S. Contrary to popular belief, inhalation of toxic gases released by burning materials –– not burns –– causes the most deaths and most of the serious injuries. Stec's team set out to determine the effects of flame retardants on the production of those gases. The scientists tested brominated flame retardants with antimony synergists, mineral-based and so-called intumescent agents, which swell when heated, forming a barrier that cannot penetrate.

Unlike the halogen-based retardants, mineral-based fire retardants have little effect on fire toxicity. Most intumescent fire retardants reduce the amount of potentially toxic gases released in a .

Provided by American Chemical Society (news : web)

Images capture split personality of dense suspensions

 Stir lots of small particles into water, and the resulting thick mixture appears highly viscous. When this dense suspension slips through a nozzle and forms a droplet, however, its behavior momentarily reveals a decidedly non-viscous side. University of Chicago physicists recorded this surprising behavior in laboratory experiments using high-speed photography, which can capture action taking place in one hundred-thousandths of a second or less.

UChicago graduate student Marc Miskin and Heinrich Jaeger, the William J. Friedman and Alicia Townsend Friedman Professor in Physics, expected that the dense suspensions in their experiments would behave strictly like viscous liquids, which tend to flow less freely than non-viscous liquids. Viscosity certainly does matter as the particle-laden liquid begins to exit the nozzle, but not at the moment where the drop's thinning neck breaks in two.

New behavior appears to arise from feedback between the tendencies of the liquid and what the particles within the liquid can allow. "While the liquid deforms and becomes thinner and thinner at a certain spot, the particles also have to move with that liquid. They are trapped inside the liquid," Jaeger explained. As deformation continues, the particles get in each other's way.

"Oil, honey, also would form a long thread, and this thread would become thinner and break in a way characteristic of a viscous liquid," Jaeger said. "The particles in a dense suspension conspire to interact with the liquid in a way that, when it's all said and done, a neck forms that shows signs of a split personality: It thins in a non-viscous fashion, like water, all the while exhibiting a shape more resembling that of its viscous cousins."

It took Miskin and Jaeger six months to become convinced that the viscosity of the suspending liquid was a minor player in their experiments. "It is a somewhat heretical view that this viscosity should not matter," Jaeger said. "Who would have thought that?"

Miskin and Jaeger presented their results in the March 5 online early edition and the March 20 print edition of the Proceedings of the National Academy of Sciences.

In their experiments, Miskin and Jaeger compared a variety of pure liquids to mixtures in which particles occupy more than half the volume.

"The results indicate that what we know about drop breakup from pure liquids does not allow us to predict phenomena observed in their experiments," said Jeffrey Morris, professor of chemical engineering at City College of New York. "The most striking and interesting result is the fact that, despite these being very viscous mixtures, the viscosity plays little role in the way a drop forms."

Few studies have examined droplet formation in dense suspensions. As Morris noted, such work could greatly impact applications such as inkjet printing, combustion of slurries involving coal in oil, and the drop-by-drop deposition of cells in DNA microarrays.

Scientific defiance

In these applications particles often are so densely packed that their behavior defies a simple scientific description, one that might only take into account average particle size and the fraction of the liquid that the particles occupy, Morris explained. The UChicago study showed that particles cause deformations and often protrude through the liquid, rendering any such description incomplete until fundamental questions about the interface between a liquid mixture and its surroundings are properly addressed.

"Miskin and Jaeger provide arguments for the importance of these protrusions in their work and suggest that the issue is of broader relevance to any flow where a particle-laden liquid has an interface with another fluid," Morris said.

Miskin and Jaeger verified their results by systematically evaluating different viscosities, particle sizes and suspending liquids, and developed a mathematical model to explain how the droplet necks evolve over time until they break apart.

One initially counter-intuitive prediction of this model was that larger particles should produce behavior resembling that in pure water without any particles. "If you want to make it behave more like a pure non-viscous liquid, you want to make the particles large," said Jaeger, who finds himself intrigued by nature's seemingly endless store of surprises.

Miskin and Jaeger indeed observed this when the particle size approached a significant fraction of the nozzle diameter, making the particles visible to the naked eye.

"You think you have a pretty good idea of what should happen, and instead there's a surprise at every corner. Honestly, finding surprises is what I love about this work," Jaeger said.

Story Source:

The above story is reprinted from materials provided by University of Chicago, via Newswise.

Note: Materials may be edited for content and length. For further information, please contact the source cited above.

Journal Reference:

M. Z. Miskin, H. M. Jaeger. Droplet formation and scaling in dense suspensions. Proceedings of the National Academy of Sciences, 2012; 109 (12): 4389 DOI: 10.1073/pnas.1111060109

Novel filter metal-organic framework material could cut natural gas refining costs

Today, to separate hydrocarbon gas mixtures into the pure chemicals needed to make plastics, refineries "crack" crude oil at high temperatures – 500 to 600 degrees Celsius – to break complex hydrocarbons into lighter, short-chain molecules. They then chill the gaseous mixture to 100 degrees below zero Celsius to liquefy and divide the gases into those destined for plastics and those used as fuel for home heating and cooking.

"Cryogenic distillation at low temperatures and high pressures is among the most energy-intensive separations carried out at large scale in the chemical industry, and an environmental problem because of its contributions to global climate change," said Jeffrey Long, a professor of chemistry at the UC Berkeley and a faculty researcher at Lawrence Berkeley National Laboratory.

Long and his UC Berkeley colleagues now have created an iron-based material – a metal-organic framework, or MOF – that can be used at high temperatures to efficiently separate these gases while eliminating the chilling.

"You need a very pure feedstock of propylene and ethylene for making some of the most important polymers, such as polypropylene, for consumer products, but refineries dump a lot of energy into bringing the high temperature gases down to cryogenic temperatures," Long said. "If you can do the separation at higher temperatures, you can save that energy. This material is really good at doing these particular separations."

"The research conducted by the Long group exemplifies the potential of MOF-based materials relative to olefin/paraffin separations," said chemist Peter Nickias, a Dow Fellow at Dow Chemical Company in Michigan who was not involved in the research. "More specifically, the ability of the reported iron-based MOF to separate a variety of unsaturated hydrocarbons from saturated species not only shows the versatility of the iron-MOF system, but also clearly reveals the potential of MOFs as alternative adsorbents."

In the chemical industry, ethylene and propylene are called olefins, while methane, ethane and propane are called paraffins.

Long and his colleagues at UC Berkeley and the National Institute of Standards and Technology (NIST) in Gaithersburg, Md., report their findings in the March 30 issue of Science.

MOFs for natural gas purification

The iron-MOF is also good at purifying natural gas, which is a mixture of methane and various types of hydrocarbon impurities that have to be removed before the gas can be used by consumers. These impurities can then be sold for other uses, Long said.

"MOF compounds have a very high surface area, which provides lots of area a gas mixture can interact with, and that surface contains iron atoms that can bind the unsaturated hydrocarbons," Long said. "Acetylene, ethylene and propylene will stick to those iron sites much more strongly than will ethane, propane or methane. That is the basis for the separation."

Nickias noted that increased supplies of natural gas from shale have provided more opportunity to extract and use ethylene and propylene from natural gas, and a variety of materials and approaches are being examined to cut energy use during the refining and purification of olefins.

"Significant energy savings could be achieved if a non-distillation separation could be implemented, or more realistically, the load on a cryogenic distillation unit can be reduced via upstream modifications to the process," Nickias said.

Petroleum refined for the chemical industry is typically a mix of hydrocarbons, primarily two-carbon molecules – ethane, ethylene and acetylene – and three-carbon chains – propane and propylene. Cryogenic distillation separates these compounds – all of them gases at room temperature – by liquefying them at low temperatures and high pressure, which causes them to separate by density. Ethylene and propylene go into plastic polymers, while ethane and propane are typically used for fuel.

The researchers found that when pumping a gas mixture through the iron-based MOF (Fe-MOF-74), the propylene and ethylene bind to the iron embedded in the matrix, letting pure propane and ethane through. In their trials, the ethane coming out was 99.0 to 99.5 percent pure. The propane output was close to 100 percent pure, since no propylene could be detected.

After the ethane and propane emerge, the MOF can be heated or depressurized to release ethylene and propylene pure enough for making polymers.

"Once you saturate the material with ethylene, for example – you shut off the valve, stop the feed gas, warm up the absorber unit and the ethylene would come out in pure form as a gas," Long said.

MOFs like packed soda straws

Through a microscope, Fe-MOF-74 looks like a collection of narrow tubes packed together like drinking straws in a box. Each tube is made of organic materials and six long strips of iron, which run lengthwise along the tube. Analysis by Long's colleagues at the NIST Center for Neutron Research showed that different light hydrocarbons have varied levels of attraction to the tubes' iron. By passing a mixed-hydrocarbon gas through a series of filters made of the tubes, the hydrocarbon with the strongest affinity can be removed in the first filter layer, the next strongest in the second layer, and so forth.

"It works well at 45 degrees Celsius, which is closer to the temperature of hydrocarbons at some points in the distillation process," said Wendy Queen, a postdoctoral fellow at NIST who worked for six months in Long's UC Berkeley lab. "The upshot is that if we can bring the MOF to market as a filtration device, the energy-intensive cooling step potentially can be eliminated. We are now trying out metals other than iron in the MOF in case we can find one that works even better."

Long and his laboratory colleagues are developing iron-based MOFs to capture carbon from smokestack emissions and sequester it to prevent its release into the atmosphere as a greenhouse gas. Similar MOFs, which can be made with different pore sizes and metals, turn out to be ideal for separating different types of hydrocarbons and for storing hydrogen and methane for use as fuel.

More information: E.D. Bloch, W.L. Queen, R.Krishna, J.M. Zadrozny, C.M. Brown and J.R. Long. Hydrocarbon separations in a metal-organic framework with open Iron(II) coordination sites. Science, March 30, 2012.

Provided by National Institute of Standards and Technology (news : web)

New method for cleaning up nuclear waste

While the costs associated with storing nuclear waste and the possibility of it leaching into the environment remain legitimate concerns, they may no longer be obstacles on the road to cleaner energy.

A new paper by researchers at the University of Notre Dame, led by Thomas E. Albrecht-Schmitt, professor of civil engineering and geological sciences and concurrent professor of chemistry and biochemistry, showcases Notre Dame Thorium Borate-1 (NDTB-1) as a crystalline compound that can be tailored to safely absorb radioactive ions from nuclear waste streams. Once captured, the radioactive ions can then be exchanged for higher-charged species of a similar size, recycling the material for re-use.

If one considers that the radionuclide technetium (99Tc) is present in the nuclear waste at most storage sites around the world, the math becomes simple. There are more than 436 nuclear power plants operating in 30 countries; that is a lot of nuclear waste. In fact, approximately 305 metric tons of 99Tc were generated from nuclear reactors and weapons testing from 1943 through 2010. Its safe storage has been an issue for decades.

"The framework of the NDTB-1 is key," says Albrecht-Schmitt. "Each crystal contains a framework of channels and cages featuring billions of tiny pores, which allow for the interchange of anions with a variety of environmental contaminants, especially those used in the nuclear industry, such as chromate and pertechnetate."

Albrecht-Schmitt's team has concluded successful laboratory studies using the NDTB-1 crystals, during which they removed approximately 96 percent of 99Tc. Additional field tests conducted at the Savannah River National Laboratory in Aiken, S.C., and discussed in the paper have shown that the Notre Dame compound successfully removes 99Tc from nuclear waste and also exhibits positive exchange selectivity for greater efficiency.

Story Source:

The above story is reprinted from materials provided by University of Notre Dame. The original article was written by William G. Gilroy.

Note: Materials may be edited for content and length. For further information, please contact the source cited above.

Journal Reference:

Shuao Wang, Ping Yu, Bryant A. Purse, Matthew J. Orta, Juan Diwu, William H. Casey, Brian L. Phillips, Evgeny V. Alekseev, Wulf Depmeier, David T. Hobbs, Thomas E. Albrecht-Schmitt. Selectivity, Kinetics, and Efficiency of Reversible Anion Exchange with TcO4- in a Supertetrahedral Cationic Framework. Advanced Functional Materials, 2012; DOI: 10.1002/adfm.201103081