New material cuts energy costs of separating gas

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A
new type of hybrid material developed at the University of California,
Berkeley, could help oil and chemical companies save energy and money — and
lower their environmental impacts — by eliminating an energy-intensive
gas-separation process.

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 Co. 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, the National
Institute of Standards and Technology (NIST) in Gaithersburg, Md., and the
University of Amsterdam in the Netherlands 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
coauthor 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.

Long’s other colleagues are UC Berkeley graduate students
Eric D. Bloch and Joseph M. Zadrozny; Rajamani Krishna of the Van’t Hoff
Institute for Molecular Sciences at the University of Amsterdam; and Craig M.
Brown of NIST and The Bragg Institute at the Australian Nuclear Science and
Technology Organisation in Menai, New South Wales.

The research is part of the Center for Gas Separations
Relevant to Clean Energy Technologies, an Energy Frontier Research Center
funded by the Department of Energy that focuses primarily on creating novel
materials for capturing and storing carbon dioxide.