High energy density liquid fuels are required in many applications where chemical energy is converted into controlled motion, such as rockets, gas turbines, boilers, and certain vehicle engines. Aside from their combustion characteristics and performance, it is also critical to ensure the safety and stability of these fuels during use, transport, and storage.
When dealing with liquid fuels, one common hazard is that they evaporate quickly if given enough space, producing clouds of highly flammable gases. As one might expect, this can result in disastrous explosions or fires. To address this issue, researchers have considered the use of gelled fuels, or fuels that have been transformed into thick gel-like substances due to cold temperatures. Unfortunately, there are many aspects to optimize and hurdles to overcome before gelled fuels can go beyond the research phase.
Luckily, a team of researchers led by Prof. Naoki Hosoya from Shibaura Institute of Technology (SIT) and Prof. Shingo Maeda from Tokyo Institute of Technology (Tokyo Tech), Japan, recently investigated a more compelling solution to the safety problem of liquid fuels, namely storing them inside polymeric gel networks. In their study, the team analyzed the performance, advantages, and limitations of storing ethanol, a common liquid fuel, within a chemically cross-linked poly(N-isopropylacrylamide) (PNIPPAm) gel. This paper was made available online on and published in Volume 444 of the Chemical Engineering Journal.
Polymeric gel storage could prevent explosions and fires by drastically reducing fuel evaporation and, as a result, the formation of flammable gaseous mixtures, which can easily happen after a leak in a storage facility.
Prof. Hosoya
First, they investigated whether trapping ethanol molecules within the long and chemically intertwined PNIPAAm polymer chains aided in lowering the rate of evaporation. To put this theory to the test, the researchers loaded small spheres of PNIPAAm gel with ethanol and placed them on an electronic scale to record how the mass changed as the ethanol vaporized. This experiment was also carried out with an equivalent puddle of ethanol, which had roughly the same surface area and mass as the gel sphere.
They discovered that storing ethanol within the polymer gel completely prevented the fuel from rapidly vaporizing. This is most likely due to the way ethanol molecules are constructed. The polymeric gel contains innumerable three-dimensional polymer chains that are chemically cross-linked in a strong way,” Prof. Hosoya explains. Through various physical interactions, these chains bind the ethanol molecules, limiting its evaporation.” Interestingly, the loaded gel behaves differently than a wet towel. Unlike a wet towel, which releases its liquid when wrung, the polymeric gel did not easily release ethanol when subjected to external forces.
With the evaporation issue resolved, the team investigated the actual combustion characteristics of the ethanol in the polymeric gel network to see if it burned efficiently. They ignited ethanol-loaded gel spheres of varying sizes and monitored the changes in their mass and shape profiles in real time. Based on this, they determined that the burning of the loaded PNIPAAm gel spheres consisted of two phases: one dominated by pure ethanol burning, followed by a second phase dominated by the burning of the PNIPAAm polymer itself.
Following a theoretical analysis of these results, the team came to an important conclusion: the first and main combustion phase of the loaded PNIPAAm gel spheres follows a constant droplet temperature model, also known as the “d2 law.” This means that the burning of the ethanol-loaded gel can be described by the same model used for liquid fuel droplets, implying that their combustion performances should be comparable.
Overall, this research is an important step toward developing new methods for transporting and storing liquid fuels inside polymer gels, which could save many lives. “Polymeric gel storage could prevent explosions and fires by drastically reducing fuel evaporation and, as a result, the formation of flammable gaseous mixtures, which can easily happen after a leak in a storage facility,” Prof. Hosoya explains. “There is still much work to be done on this front, such as testing the stability and performance of polymeric gels at different temperature, pressure, and humidity conditions, as well as developing simpler fabrication procedures and better ways to use these fuel-loaded gels in real engines.”
