Title: Encapsulated liquid sorbents for carbon dioxide capture
Authors: John J. Vericella, Sarah E. Baker, Joshuah K. Stolaroff, Eric B. Duoss, James O. Hardin IV, James Lewicki, Elizabeth Glogowski, William C. Floyd, Carlos A. Valdez, William L. Smith,
Joe H. Satcher Jr., William L. Bourcier, Christopher M. Spadaccini, Jennifer A. Lewis & Roger D. Aines
By: Ehsan Moaseri, Behzad Changalvaie
Signs of global warming and climate change have resulted in worldwide efforts to reduce the concentration of atmospheric greenhouse gases, namely carbon dioxide. A common strategy to reduce atmospheric CO2 concentration is to capture, store, and utilize the CO2 emitted from prominent sources, such as coal plants. Among multiple methods that have been developed for CO2 capture, the most established technique is scrubbing with aqueous amine solutions. In this method, emitted flue gas is brought in contact with an amine solution, typically monoethanolamine (MEA); and MEA reacts with CO2 to form carbamates. This technique is used commonly because MEA has a fast CO2 absorption rate and high carrying capacity. However, it should be noted that MEA is highly corrosive, yields toxic products when degraded, and requires a high amount of energy during sorbent regeneration. Other traditional CO2 capture technologies suffer from similar problems, and are not generally feasible due to high capital, energy, and chemical costs. Therefore, a more efficient and cost-effective CO2 capture technology is needed.
Vericella and coworkers report a new class of sorbent materials that combines the advantages of liquid solvents (high capacity and high selectivity) and solid sorbents (high surface area and low volatility). These sorbent materials are microencapsulated structures. During absorption, CO2 diffuses through the thin capsule shells and then reacts with sorbents to form the desired products. The CO2-sorbent reaction can be reversed at high temperature, meaning that the sorbent materials can be regenerated. During the regeneration process, high purity CO2, which can be compressed for storage and utilization, is produced. Several criteria that must be considered when designing CO2 capture systems: A) the cores of these sorbent capsules must have a controlled composition and geometry, B) the polymeric shells (membranes) must be sufficiently permeable to CO2, so that the absorption rate is dominated by the encapsulated sorbent system, C) the sorbent system must be capable of multiple CO2 absorption-desorption cycles, and D) the mechanical integrity of these microcapsules must be retained during repeated cycling. These new class of microcapsules meet these criteria and offer several advantages over traditional CO2 capture systems.
One of the advantages of microcapsules is that the regeneration process can be completed at modest or high temperatures (80-150 °C). This relatively low regeneration temperature results in a more energy-efficient capture process compared to other processes. In addition, in other processes, CO2 absorption kinetics are slower and handling precipitated solids in liquid systems can be difficult. These drawbacks are overcome through encapsulation. With these microcapsules, it is easy to qualitatively monitor the level of CO2 saturation of the carbon sorbent; as a pH indicator dye can be injected in the capsules’ cores.
The CO2 capture microcapsules can be prepared and fabricated through a relatively simple process. Each capsule consists of three fluidic parts. The inner fluid, the core of the capsule, is composed of an aqueous potassium or sodium carbonate solution with a pH indicator dye and catalyst for enhanced CO2 absorption. The middle fluid is a photopolymerizable silicone to prevent release of the dye and the catalyst, and the outer fluid is composed of an aqueous solution with stabilizer and surfactant to maintain the microcapsule shape and prolong its half-life. This design provides capsules with enhanced mechanical stability, as they can withstand a fourfold increase in their diameter without rupture by osmotically swelling in pure water. This is a notable result because this amount of swelling is much higher than the capsules’ expected behavior during their use as carbon capture media.
Overall, these new silicone microcapsules filled with liquid carbonate sorbents are more efficient and cost-effective than other neat sorbents. Compared to MEA, these capsules have a lower environmental impact because of their improved mass transfer rates and their ability to contain precipitates and degradation products. Several improvements can be made to these capsules. For example, the capsule geometry can be optimized and a broad range of sorbent and catalyst chemistries can be studied. In conclusion, microencapsulated carbon sorbents offer a promising approach for large-scale carbon capture from power plants.