Solid adsorbents, including metal-organic frameworks (MOFs), have shown great promise in carbon-capture applications as an alternative to traditional amine-based processes. Typically, adsorbent materials are manufactured as powders and converted into beads or pellets to make them more suitable for large-scale processes, but this also limits their mass-transfer capabilities. An industry-first fabrication technology, combined with an ultra-low-cost MOF, both developed by Decarbontek, Inc. (Woburn, Mass.; www.decarbon.tech), enable much faster CO2-adsorption kinetics than competing materials.
Source: Decarbontek
MOF powders are usually compressed into pellets or beads to help reduce system pressure drop and to make the materials easier for large-scale transport. “These bead-shaping processes require high pressure, which is not really suitable for MOFs, because the materials will compress together, creating high resistance for gas to diffuse inside the MOF. Shaping MOFs for practical application is a problem that has not really been solved yet,” explains Yong Ding, CEO of Decarbontek. Furthermore, adds Ding, in order to sufficiently decrease pressure drop, the bead diameter is limited to a minimum of around 6 mm, which significantly diminishes adsorption capacity.
Decarbontek has developed a platform technology that can shape any powder into solid fibers with very precise diameter and spacing control. “We can make the diameters relatively small and then wind the fibers into a specific structure. We use computers to control the spaces between the fibers to ensure uniform packing, forming a helical, structured adsorbent system,” says Ding. Because the fiber diameters are very small, and the fibers are built around a porous polymer matrix that does not contribute mass-transfer resistance, both gas and heat can transfer very easily into and out of the structured adsorbent via what the company calls “electrified rapid temperature-swing adsorption.”
“Basically we put an electrified heating element inside the fiber structure, and because all of the fibers are in intimate contact with the heating element, we can heat and cool the system in seconds,” says Ding.
In parallel, Decarbontek has also developed a low-cost MOF to use in its fibrous structures. “We identified a MOF that has fantastic CO2-capture performance that is made from zinc oxide and citric acid, both of which have no supply chain issues and are very inexpensive,” says Ding, noting that the MOF synthesis process operates at ambient conditions, uses only water as a solvent and boasts over 90% yield. So far, the company has carried out field demonstrations of its adsorbent system with two companies, including a 500-kg/day unit.