New material eliminates gas-separation energy cost plastics fuels

New material eliminates gas-separation energy cost for plastics and fuels

11:35 AM, 9th April 2012
New material eliminates gas-separation energy cost for plastics and fuels
Cross-section of the iron-based MOF bound to six ethylene molecules, as determined by neutron diffraction. The MOF consists of a carbon (gray) and oxygen (red-orange) framework with iron atoms (yellow-orange) at strategic sites to bind the ethylene carbon atoms.

CALIFORNIA, US: A new type of hybrid material developed at the University of California, Berkeley, could help oil and chemical companies save energy, money and lower their environmental impacts by eliminating an energy-intensive gas-separation process. To separate hydrocarbon gas mixtures into the pure chemicals needed to make plastics, refineries ‘crack’ crude oil at high temperatures 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.

Jeffrey Long, Professor, UC Berkeley and colleagues have created an iron-based material, metal-organic framework (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, but refineries dump a lot of energy into bringing the high temperature gases down to cryogenic temperatures. “If you can do the separation at higher temperatures, you can save that energy. This material is really good at doing these particular separations,” Long said.

The iron-MOF is also good at purifying natural gas, which is a mixture of methane and various types of hydrocarbon impurities. “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. 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,” said Long.

Peter Nickias, Scientist, Dow Chemicals 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. 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 per cent pure. The propane output was close to 100 per cent 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, 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,” said Long.

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.

© UC Berkeley News

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