Methane is the most common organic compound on the planet. Not only is it good for fuel, but it absorbs more than 30x the amount of solar radiation as carbon dioxide, meaning that understanding the process by which methane turns into icy methane hydrates (or “clathrates”) can shed some light on climate change—or possibly even reduce the effects of it.
Much of the methane on planet Earth is locked below the ice in crystalized water molecules. The problem is that scientists are not exactly sure of the process by which gaseous methane becomes transformed into the icy clathrate variant.
Fortunately, a team of researchers at New York University Tandon School of Engineering are working to shed some light on this process with a new microreactor, which is designed to react to and monitor the process at discrete intervals in a way never before seen.
The team, led by Ryan Hartman, an assistant professor of chemical and biomolecular engineering, has opted to use microfluidic systems for the reactor, small fluid channels confined to submillimeter geometries, meaning that fluids can be metered out, monitored, thermally controlled and reacted at levels of resolution that were once thought impossible.The team hopes to learn more about the mechanisms of not only how the methane transforms into ice, but also how it is released from the ice as gas.
The new development will allow a stepwise examination of the process rather than a continuous one, meaning that the process can be reduced from days to minutes, or even seconds, which will accelerate the data accumulation and free up researcher time.
The team will be the first to measure the degree to which mass transfer affects crystal propagation rates.
It is believed that hydrate formation begins with nucleation, in which water molecules freeze into a structure, and foreign particles such as methane become trapped inside the structure. Crystallization then occurs and the structures grow outward into larger sheets at the liquid/gas boundary.
Current reactors are a little too clunky to be able to detect the minute and discrete changes that occur on the path to hydrate formation. These reactors tend to rely on high-pressure systems where the super-cooled methane is blasted at water, and where temperatures are varied at steps of 10 Kelvin. By using smaller-scale systems as permitted by the microfluidics, the temperature step can be reduced to just 1 Kelvin, meaning that the reactions occur more quickly and over smaller temperature increments.
"Nucleation is difficult to predict," explained Hartman. "It can take minutes or sometimes days in the formation of gas hydrates. But because we are able to cycle the temperature within seconds, we can form seed crystals and use the nuclei we form to reproducibly form larger crystals."
And for those of a more manufacturing engineering persuasion who are interested in how the reactor was made, the 2D masks for the channels were created in AutoCAD before being etched with traditional electronic etching methods. A 1mm thick Pyrex plate was then bonded to the etched substrate to form the 3D channels. A syringe pump system, a thermoelectric system for the stepwise control of the temperature, and a few other components (see Figure 2) were assembled to create the reactor. Once the reactor was completed, deionised water and methane were pumped into the system to create the hydrates.
If you’d like a more in-depth read of the project, you can get a free copy of the article (which was originally published in the Lab on a Chip journal) from this website (requires free sign-up).