New method could improve economics of sweetening natural gas

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.

Battelle researchers have found a way to drastically reduce the heat needed to sweeten natural gas. The new process could make tapping extremely sour gas reserves more economically friendly. Above, a natural gas rig in Euless, Tex. Photo courtesy of Steve Reisman.

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.