A newly created material has the ability to separate closely related components of natural gas from one another, a task that currently demands a great deal of energy to accomplish. The material could end up improving the efficiency of the distillation process.
The material is a new type of metal-organic framework (MOF), a class of materials whose high surface area and tunable properties make them promising for applications as varied as gas storage, catalysis and drug delivery.
This iron-based MOF, which the research team refers to as Fe-MOF-74, started in the lab of Jeffrey Long, a professor of chemistry at the University of California Berkeley, and analyzed by the team at the National Institute of Standards and Technology (NIST) and the Australian Nuclear Science and Technology Organization’s Bragg Institute.
Natural gas taken straight from the ground consists of a complex mixture of molecules called hydrocarbons, only some of which see use in any specific product such as fuel or plastic.
Separating the lighter types of hydrocarbon from one another — propane and ethylene, for example — is difficult because their weights are so similar. Currently, the most effective separation method involves chilling light hydrocarbons down to the point where they all liquefy, sometimes as low as 100 degrees below zero Celsius, and waiting until the heavier liquids settle below the lighter ones.
“A good percentage of the energy needed for separation goes to the cooling process,” said Wendy Queen, a postdoctoral fellow at the NIST Center for Neutron Research. “A material that can selectively adsorb light hydrocarbons could offer significant energy savings, making separation more economical.”
Through a microscope, Fe-MOF-74 looks like a collection of narrow tubes packed together like drinking straws in a box. Each tube consists of organic materials and six long strips of iron, which run lengthwise along the tube.
The team’s analysis shows different light hydrocarbons have varied levels of attraction to the tubes’ iron, a finding that researchers can exploit for separation. By passing a mixed-hydrocarbon gas through a series of filters made of the tubes, researchers can remove the hydrocarbon with the strongest affinity in the first filter layer, the next strongest in the second layer, and so on.
“It works well at 45 degrees Celsius, which is closer to the temperature of hydrocarbons at some points in the distillation process,” Queen said. “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.”