Tuesday, March 15, 2011

Sweeter Natural Gas

Natural gas extracted from the coal beds and methane-rich geologic features must first be purged of hydrogen sulfide before it can be used as fuel in a process called "sweetening".

Thermal Swing Regeneration, a common industry process used for sweetening natural gas, uses chemical sponges called sorbents to 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.

A new 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.

First hydrogen sulfide is dissolved in a substance known as a DMEA which is a recyclable binding organic liquid, a substance that can hold onto hydrogen sulfide without the addition of water. DMEA forms a salt with hydrogen sulfide. The salty DMEA is then mixed with hexane (or hexadecane and a small amount of heat) which returns most of the hydrogen sulfide back to the gaseous state which is then bubbled out of the mixture. Separating the hexane from the DMEA allows these substances to be re-used.

Scientists estimate that the Antisolvent Swing Regeneration method could reduce the amount of energy needed to complete the sweetening process by at least 10%.

Phillip K. Koech, James E. Rainbolt, Mark D. Bearden, Feng Zheng, David J. Heldebrant. Chemically selective gas sweetening without thermal-swing regeneration. Energy & Environmental Science, 2011; DOI: 10.1039/c0ee00839g

Further Reading:
Writing Ionic Formulae
Naming Straight Chain Alkanes
Intermolecular Forces

Study Questions:
  1. Write the chemical formula for each of the following:
    • methane
    • hydrogen sulfide
    • hexane
  2. In a sample of each of the following pure substances, what type of forces would you expect to attract molecules to each other?
    • methane
    • hydrogen sulfide
    • hexane
  3. Describe what would happen if each of these pure substances was mixed with water.
  4. Describe what would happen if each of these substances were mixed with a petroleum-based oil.
  5. Use the description of the Thermal Swing Regeneration process to draw a flow chart for this method of sweetening natural gas.
  6. Use the description of the Antisolvent Swing Regeneration process to draw a flow chart for this method of sweetening natural gas.

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