Sunday, September 2, 2018

Carbon Capture: Cryogenic CO2 Separation

Sublimation is a phase change directly from a solid to a gas without transitioning through an intermedia liquid state. Desublimation is the opposite, where a gas crystalizes into a solid without becoming a liquid first. The most well-known example of desublimation is snow, where water vapor crystalizes into tiny bits of ice. When water vapor in the cloud first condenses into liquid and then freezes, the result is hail not snow.

Interestingly, and quite usefully for carbon capture, carbon dioxide will desublimate at -78 degrees Centigrade. This is a considerably higher temperature than the main components of the atmosphere like nitrogen and oxygen, and means that as air gets very cold that CO2 will be among the first components to turn into ice crystals. This allows the CO2 crystals to be harvested.

Several companies have working technology in this area:

  • Alliant Techsystems (now defunct) and ACENT Laboratories developed a supersonic wind tunnel which compresses incoming air, causing it to heat up, then expands the supersonic airflow causing it to rapidly freeze. CO2 crystals can be extracted via cyclonic separation, relying on the mass of the frozen particles.

  • Sustainable Energy Solutions in Utah uses a heat exchanger process to rapidly cool air, harvest the CO2 crystals, then reclaim the energy spent on cooling before exhausting the remaining gases.