Phillip Ng and Dr. Anton Bowden Mechanical Engineering Department
INTRODUCTION
Pyrolytic Carbon Infiltration Carbon Nanotubes (CI-CNTs) can isolate water and oil molecules due to its superhydrophobic and oleophilic properties, unique cylindrical nanostructure, and functional groups1. Because the waste produced from fracking is nonreusable due to the molecular oil droplets contained in it, the waste is typically injected back underground, which pollutes safe drinking water2. This can be avoided with an inexpensive and efficient oil and water filter. The mentored research project revolved around a CI-CNT filter used to salvage waste created from fracking.
The design of a CI-CNT device was approached by three main considerations: a geometry that would encourage the fluid to have optimal contact with the CI-CNT surface, a method to reuse the CI-CNT surface once saturated with oil, and a fabrication method that could make large, robust, and cost efficient CI-CNT devices.
METHODOLOGY
The fabrication of CI-CNTs with the desired properties require a robust substrate, optimal growth conditions, and correct surface properties. CI-CNTs are grown in a furnace by flowing ethylene and an inert gas, such as argon, at high temperatures and with a controlled mass flow rate.
In the beginning, silicone wafers layered in alumina and iron were used as the main substrate. However, wafer preparation was time consuming, and the resulting CI-CNT’s were extremely fragile. Due to its low tensile strength and single crystal structure, which propagated cracks easily, silicone wafers wouldn’t be the ideal substrate to withstand the mechanical forces of fluid flow. Key parameters for a good material are the iron content of the substrate, cost, fabrication steps, and mechanical strength. Based on this criterion, 316L stainless steel sheet with a mirror finish was selected3. Growth on this substrate produced robust CI-CNT’s and gave us the flexibility of producing shapes with sheet metal deformation processes. However, shape complexity was limited to channel-like device geometries.
The CI-CNT superhydrophobic and oleophilic properties are dependent on its surface chemical composition. To realize this chemical composition, the CI-CNT device is placed in a vacuum oven at over 250 degrees Celsius for 24 hours to remove oxidized functional group4. Once taken out of the oven, a series of oil and water experiments to determine the saturation point, as well as its maximum usage before losing superhydrophobic and oleophilic properties, were performed.
For the first experiment, the net weight of the CI-CNT before and after oil saturation was measured. Several one centimeter squared CI-CNT tiles (see figure 1) have had oil dropped on its surface until it was evident that the entire surface was wetted. We tried to control for oil amounts exclusively absorbed by the CICNT surface and not due to surface tension, which was done by observation. It is estimated that the saturation point is roughly 0.1 grams of oil per centimeter squared of CI-CNT area. This infers that the capacity of a potential CI-CNT oil and water separation device is significantly limited in volume of fracking waste, and might be difficult to implement on a large scale.
For the second experiment, distilled water was ran through the CI-CNT channel and the total amount of water that passed through was measured. Hypothetically, in the absence of oil compounds or compounds with functional oxygen groups, there should be continuous superhydrophobic properties. Nevertheless, it was observed that the water began sticking to the CI-CNT surface after dripping a small amount of distilled water, signaling that its properties are no longer superhydrophobic (see figure 2). This may be due to an error in experimental methods that have yet to be determined. Another method to eliminate this problem is to improve the wetting properties of CNTs, giving it stronger hydrophobic characteristics4.
CONCLUSION
Growing CI-CNT surfaces on a system of channels is a viable option for the fabrication of a passive oil-water CI-CNT filtration device, but presents unique challenges. Challenges include maximizing the oil absorption amount in CI-CNT’s, expelling oil from saturated CICNT’s, and increasing the efficiency in oil-water separation.
DISCUSSION
For future research, it is desirable to explore growing CI-CNT’s on stainless steel which has been machined to complex geometries, as well as determine the minimum surface finish quality required. These procedures can not only reduce material cost, but leverage a more efficient fluid flow path that will increase filtration capabilities. In addition, in the above stated experimentations, since few samples were done and that the lipid composition of Canola oil, which was used in the test, isn’t representative of fracking waste oil, validation requires further testing of additional samples under more exact conditions.
REFERNCES
1. Fazio, Walter C. 2012. “Mechanical Properties and MEMS Applications of Carbon-Infiltrated Carbon Nanotube Forests”. All Theses and Dissertations. 3224.
2. Ferrer, I., & Thurman, E. M. 2015. Analysis of hydraulic fracturing additives by LC/Q-TOF-MS. Anal Bioanal Chem Analytical and Bioanalytical Chemistry, 407(21), 6417-6428. doi:10.1007/s00216-015-8780-5
3. Chuanwei Zhuo, XinWang, Welville Nowak, Yiannis A. Levendis. 15 September 2014. Oxidative heat treatment of 316L stainless steel for effective catalytic growth of carbon nanotubes, Applied Surface Science Volume 313. Pages 227-236
4. Aria, A.I., Gharib, M. 2013. Dry Oxidation and Vacuum Annealing Treatments for Tuning the Wetting Properties of Carbon Nanotube Arrays. J. Vis. Exp. (74), e50378, doi:10.3791/50378.