Researchers at the University of Maine and the U.S. Department of Energy's (DOE) Oak Ridge National Laboratory (ORNL) are collaborating on a new way to dry non-aggregated cellulose nanofiber - a material that could replace plastics in a wide range of products.
The new technique uses counter-rotating vortices of heated compressed air to dry the nanofibers from a wet cellulose slurry mixture. The innovation of producing these mini tornadoes to dry cellulose nanofibers is more energy efficient, effective and scalable than the current freeze and spray drying methods.
From plants to products
All plants have cellulose - a carbohydrate made of linked glucose molecules. Cellulose is the fibrous string that makes eating celery a challenge; the rigidity of cellulose helps give trees their strength and stature. Cellulose has long been used in many products, including paper, clothing, and food. But scientists are just beginning to explore the unique qualities and potential applications of cellulose broken down to the nanoscale.
"Cellulose nanofibers are like branches on a tree," said Peter Wang, a research staff scientist in ORNL's Manufacturing Science Division. "You have a trunk that's smaller than the diameter of a hair, but then the ends continue to split until you have fuzzy ends that are nano-size."
When these tiny fibers grab onto each other, they form such an irreversible bond no glues or other chemicals are needed to hold them together. Cellulose nanofibers can be used to make products such as stronger concrete, bone replacements, and biodegradable packaging.
The nanofibers are made by grinding wood pulp mixed with water, much like how paper is made. The key difference between the two processes is the grinding time. To get nanofibers, the pulp is ground for so long that it produces a gel-like slurry made of up to 97 percent water and 3 percent cellulose nanofibers.
"You can use that form," said Wang, "but the problem is you'd be paying to ship water across the country." Instead, dehydrating the slurry into a powder greatly lowers the shipping costs and allows for rehydration of the nanofibers as needed.
The drying process needs to prevent fibers from aggregating or clinging to each other in a strong grip that can only be broken by going through the pulping process again. Freeze drying is effective but works best in small batches and cannot be easily scaled up to large-batch commercial production. Spray drying - expelling the slurry through a nozzle at high velocity - is scalable but less effective because it produces large amounts of aggregated fibers.
In 2018, University of Maine professor of chemical and biomedical engineering David J. Neivandt hypothesized that drying cellulose nanofiber slurries under high shear conditions would limit fiber aggregation, and that such a technique could be scaled in an energy efficient manner.
Since then, Neivandt and his graduate students at the Maine College of Engineering and Computing have developed a new, patent-pending process that rapidly dries nanocellulose using a unique nozzle and shear imparted by counter rotating vortices of heated compressed air. This technique significantly reduces energy consumption compared to the traditional freeze and spray drying methods while yielding a higher quantity and quality of dried cellulose nanofibers.
Computing provides air flow insights
Once a series of lab-scale prototype devices confirmed his hypothesis, Neivandt reached out to ORNL's Manufacturing Demonstration Facility to see if the technology could work on a larger scale. Wang and his team analyzed the fluid dynamics of the twin counter-rotating air vortices to better understand how the drying process worked.
Kevin Doetsch, a research scientist in ORNL's Computational Sciences and Engineering Division, determined that air enters the vortex generators at a speed of Mach 3 - three times the speed of sound. His model showed how the air imparted shear forces on the cellulose nanofiber slurry droplets, essentially tearing them apart.
Doetsch said simulating these tornado-like vortices within a small, complicated system can be difficult and time-consuming, but by using a high-performance computing cluster he was able to model the flow of the rapidly spinning air. "We can see why the nanomaterials are drying the way they are, and we can prove it computationally."
The next step for Wang's team is to design a means of generating the same droplet shear effect, but for larger flow rates of slurry which would produce hundreds of times more high-quality dried nanocellulose fibers than the current lab-scale system does (producing kilograms of powder per day versus grams).
"The collaboration between my research group and the team at ORNL has proven to be incredibly powerful," said Neivandt. "It is enabling the rapid transition of academic research into industrial application by pooling highly specialized skillsets focused on solving extremely complex problems."
This research is managed by the ORNL and UMaine's Advanced Structures and Composite Center's SM2ART Program, funded by DOE's Advanced Materials and Manufacturing Technologies Office, to address the growing demand for cellulose nanofiber across multiple industries seeking low cost, low energy intensive, and natural materials. These industries include packaging, building and construction, marine and automotive.
High shear drying of cellulose nanofiber is one of many platform technologies that are being researched and scaled for energy- and cost-efficient manufacturing processes, validated by economic analysis needed to enable the construction of a large-scale cellulose nanofiber production facility. SM2ART is seeking to increase industry adoption of refined cellulose functional additive products across industries.
UT-Battelle manages ORNL for DOE's Office of Science, the largest supporter of basic research in the physical sciences in the United States. DOE's Office of Science is working to address some of the most pressing challenges of our time. For more information, visit https://energy.gov/science . -Leslie Mullen
