Natural
gas extracted from the nation’s coal beds and methane-rich geologic
features must first be purged of hydrogen sulfide before it can be used
as fuel. Until now, processing methods have often proved to be
inefficient, requiring large amounts of heat.
But
a team of Battelle researchers at the Department of Energy’s Pacific
Northwest National Laboratory has discovered a method that could
dramatically cut the amount of heat needed during processing, reducing
the amount of energy needed during a key processing step by at least 10
percent. The research team believes the discovery could ultimately lead
to a more cost-effective way of tapping into extremely “sour” natural
gas reserves – those reserves that contain significant amounts of
hydrogen sulfide and that may not have been economically viable to tap
up to this point. Battelle operates the Pacific Northwest National
Laboratory for DOE.
The
researchers lay out the more efficient process and suggest how it could
be applied to processing raw natural gas in the March 11 online issue
of the journal Energy and Environmental Science.
Raw
natural gas is purified in a process called “sweetening” before it can
safely be used as a fuel. Thermal Swing Regeneration is a common
industry process used for sweetening natural gas. In that process,
chemical sponges called sorbents remove toxic and flammable gases, such
as rotten-egg smelling hydrogen sulfide from natural gas.
The
gas must first be treated with a solution of chemical sorbents that are
dissolved in water. That solution must then be heated up and boiled to
remove the hydrogen sulfide, in order to prepare the sorbent for future
use. Once the hydrogen sulfide is boiled off, the sorbent is then cooled
and ready for use again. The repeated heating and cooling requires a
lot of energy and markedly reduces the efficiency of the process,
scientists say.
The
new, Battelle-created process called Antisolvent Swing Regeneration
takes advantage of hydrogen sulfide’s ability to dissolve better in some
liquids than others at room temperatures. In this process, the hydrogen
sulfide “swings” between different liquids during the processing at
nearly room temperature, resulting in its removal, in just a few steps,
from liquids that can be reused again and again.
“Because
hydrogen sulfide is such a common contaminant in methane, natural gas
processors could potentially use this method in the sweetening process,
reducing their energy use and saving money on the cost of sorbent
materials,” said Phillip Koech, lead author and senior research
scientist.
In
the new work, Koech and colleagues tested how well they could swing
hydrogen sulfide through a series of processing liquids without using
water or heat. They began with a substance known as a recyclable binding
organic liquid that could hold onto hydrogen sulfide without the
addition of water.
First,
they dissolved hydrogen sulfide in several different recyclable binding
organic liquids and found that nearly all of them could hold the
chemical without added water. They found one — DMEA — that could hold
the most hydrogen sulfide. A chemical analysis suggested that hydrogen
sulfide forms a salt with DMEA, turning the DMEA from an oily liquid
into something more like salty water, but not water at all.
Based
on the chemical characteristics of the salty DMEA, the team thought the
salt could be easily disrupted and turned back into the gas hydrogen
sulfide by adding a liquid hydrocarbon called an alkane. First, they
mixed the hydrogen sulfide-containing DMEA with the alkane known as
hexane and shook it like a bottle of salad dressing. Most of the
hydrogen sulfide returned to its gaseous nature and bubbled out of the
mix, leaving a soup of DMEA and hexane.
Having
successfully removed the hydrogen sulfide from the DMEA, the team
needed to find an alkane that would separate the hexane and the DMEA,
and found one in hexadecane, which separates from DMEA in the same way
that oil and vinegar drift apart in salad dressing. The team suggested
the components separated due to a bit of salt left in the DMEA.
However,
unlike hexane’s ability to perform at room temperature, the team had to
warm the DMEA-hexadecane just a little — to about 40 degrees Celsius
(104 degrees Fahrenheit), the temperature of a hot summer day — to get
the liquids to release the hydrogen sulfide. After the gas bubbled off
and the two liquids separated, the team could pour off the hexadecane
and re-use the left over DMEA.
Lastly,
the researchers tested how well the chemicals could be re-used by
recycling the hydrogen sulfide through the DMEA and hexadecane five
times. The liquids retained their ability to remove the hydrogen sulfide
and recover the DMEA in its initial form. The team expects DMEA will be
able to pull hydrogen sulfide from natural gas using this process and
they expect to scale up the process with future research.
This
chemical process, called a polarity swing, occurs naturally at nearly
room temperature, drastically reducing the need for heat during
sweetening. Scientists estimate this method could cut the amount of
energy needed to complete the sweetening process by at least 10 percent.
In addition to energy savings, scientists say there are other potential benefits of using Antisolvent Swing Regeneration.
“Applying
ASSR to natural gas sweetening could result in a more environmentally
friendly process because hexadecane is non-toxic,” said David
Heldebrant, corresponding author and project manager.
“We
also anticipate chemical sorbents could last longer because they are
not subjected to repeated heating and cooling, which degrade the
sorbent.”
Battelle’s
Independent Research and Development fund supported this work. Patents
are pending on this technology and it is now available for licensing
worldwide.
Citation:
Phillip K. Koech, James E. Rainbolt, Mark D. Bearden, Feng Zheng, David
J. Heldebrant, Chemically Selective Gas Sweetening Without
Thermal-Swing Regeneration, Energy Environ. Sci., doi:
10.1039/c0ee00839g
