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Energy Procedia 37 (2013) 219 – 224
GHGT-11
Encapsulated Solvents for Carbon Dioxide Capture Roger D. Ainesa*, Christopher M. Spaddaccinia, Eric B. Duossa, Joshuah K Stolaroffa, John Vericellaa, Jennifer A. Lewisb, George Farthingc a
Lawrence Livermore National Laboratory, Livermore, CA, USA b University of Illinois, Urbana-Champaign, Illinois, USA c The Babcock and Wilcox Company, Barberton, Ohio, USA
Abstract Many attractive options for carbon capture solvents suffer from high viscosity, making it difficult to generate large surface areas for fast absorption, and amine-based aqueous liquids suffer from potential environmental impacts from solvent release. As part of a US-DOE ARPA-E program, a team from the University of Illinois Urbana-Champaign, Babcock and Wilcox, and Lawrence Livermore National Laboratory have created a new encapsulated form of carbon capture solvents in which the operating fluid, amines or carbonates in our tests to date, is enclosed in a thin polymer shell forming 200-400 μm beads. While mass transport across the polymer shell is reduced compared to the neat liquid, the large surface area of the beads lessens this disadvantage. The liquid, as well as any degradation products or precipitates, remains encapsulated within the beads, which can be thermally regenerated repeatedly. Encapsulated solvents have the capacity of liquids and the physical behavior of solid sorbents. We imagine them to be useful in fairly conventional-style capture applications, as well as exotic new approaches facilitated by their high surface area. The beads appear to be both chemically and mechanically stable under typical industrial conditions. Examples of the engineering constraints that the beads must satisfy for several application strategies, including their use in fluidized beds, will be presented. To date we have encapsulated MEA, piperazine, and a variety of carbonate solutions, which appear to be optimal for this application. We have demonstrated rapid CO2 uptake and desorption using colorimetric methods, which permit rapid spectroscopic determination of the extent of CO2 uptake and release (shown to the left, loaded form is yellow). Carbonate capsules are created using a silicone polymer shell which is both rugged and permeable to CO2. Results of mechanical/thermal cycling tests demonstrate long-term stability of siliconeencapsulated carbonate. © 2013 2013 The Authors. © Authors.Published PublishedbybyElsevier ElsevierLtd. Ltd. Selection and/or and/or peer-review of of GHGT Selection peer-reviewunder underresponsibility responsibility GHGT
Carbon capture; solvents; sorbents
* Corresponding author. Tel.: +0 925 423 7184; fax: +0 925 422 6434. E-mail address:
[email protected].
1876-6102 © 2013 The Authors. Published by Elsevier Ltd.
Selection and/or peer-review under responsibility of GHGT doi:10.1016/j.egypro.2013.05.105
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1. Introduction Many attractive options for carbon capture solvents suffer from high viscosity, making it difficult to generate large surface areas for fast absorption. Amine-based aqueous liquids suffer from potential environmental impacts from solvent release. As part of a US-DOE ARPA-E program, a team from the University of Illinois Urbana-Champaign, Babcock and Wilcox, and Lawrence Livermore National Laboratory have created a new encapsulated form of carbon capture solvents in which the operating fluid, amines or carbonates in our tests to date, is enclosed in a thin polymer shell forming 200-400 μm beads. These beads are intended to dramatically increase the surface area of solvent in contact with flue gas as alveoli do in mammalian lungs (Figure 1).
Figure 1. Schematic view of alveoli in human lungs.
silicones and NOA (Norton Adhesive) as the polymer material.
In order to provide this functionality, the polymer shell must be highly permeable to carbon dioxide, permitting the inner solvent to perform the selectivity role, but it must also be strong enough to survive an industrial regime in which capture, and presumably release of pure CO2 via heating, occur over thousands of cycles. We have now encapsulated several carbon dioxide capture solvents of interest, including MEA, piperazine, sodium carbonate, and potassium carbonate, at concentrations up to 30 wt. %. (Figure 2). We have used
Optical
In all these systems the liquid remains immobilized within the polymer capsule while gas is absorbed through the shell. The capsules may then be heated to release the capture CO2. Encapsulated solvents have the capacity of liquids and the physical behavior of solid sorbents. We imagine them to be useful in fairly conventional-style capture applications, as well as exotic new approaches facilitated by their high surface area.
2. Application to Carbon Dioxide Capture
Figure 2. Encapsulated MEA displayed on a finger tip.
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Figure 3. Microfluidic creation of encapsulated solvents by the method of Utada et al. 2005, superimposed over a photo of silicone-encapsulated carbonate solutions being created at approximately 50 Hz. The final capsules are approximately 400 μm in diameter, as seen in Figure 4. Successful operation of the microfluidics requires the accurate matching of viscosity and flow rate; shown are the appropriate ranges for our application using silicone shells and carbonate solutions.
We use the method pioneered by Utada et al. (2005) to encapsulate solvents in silicone polymers for carbon capture purposes (Figure 3). The capsules tested for this paper are made with Semicosil, a commercial silicone polymer. We are currently testing a variety of carbonate solutions as the working fluid in order to take advantage of the lower energy demands upon recovery. Figure 4 shows a set of capsules made with 3% potassium carbonate as the working fluid. We incorporate thymol blue as a pH indicator to demonstrate when the capsules have absorbed about half of Figure 4. Carbonate-filled capsules with thymol blue as a pH their working capacity (about half of indicator (inset shows loaded capsules). the carbonate converted to bicarbonate). The inset in Figure 4 shows the change from blue to yellow upon this transition, after passing carbon dioxide over the originally blue beads. In order to examine the industrial application of these capsules we have additionally added a catalyst for the conversion of carbon dioxide to carbonic acid, as described by Satcher et al. (2011). 50 mM of
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Zn-cyclen is used in the capsules in Figures 4-6. This catalyst approximately doubles the rate of mass transfer in carbonate systems (Kozoil et al. 2012). Figure 5 shows the 3% potassium carbonate-filled microcapsules placed in a gas flow apparatus to test the absorption and desorption of carbon dioxide. Gas composed of 12% CO2 and 88% N2 is allowed to flow over the capsules from the bottom to the top, such that both the capsules without catalysts (to the right in each frame) and those with catalyst see the same gas conditions. Approximately one minute is required to cause the carbon dioxide loading at room temperature, with concomitant color change due to thymol blue indicator.
Figure 5. Catalysts increase rate of absorption in microcapsule configuration. This time series of images shows microcapsules which both contain 3 wt% K2CO3 and thymol blue pH indicator, but only the capsules on the left side of the divider contain the cyclen catalyst. Faster pH swings are seen repeatabled and reproducibly in the cyclen-containing capsules. Gas flows in the direction perpendicular to the page.
The device shown schematically in Figure 6 was used to test the cycling of the absorption and desorption using heated nitrogen gas to desorb the carbon dioxide. Over the course of two hours, eighty
Figure 6. Schematic of temperature and gas cycling device used to test the ability of the capsules to undergo multiple absorption/desorption cycles at up to 100ºC.
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loading and unloading cycles were conducted (Figure 7). The capsules were observed to dimple slightly over this time as they slowly dehydrated due to the low water vapor pressure in the dry gas being used to desorb them at approximately 100ºC, but did not break or loose working fluid (Figure 8). Additional testing has shown that the capsules can loose as much as half their internal working water and recover fully by immersion in distilled water (or any solution with an osmotic pressure less than that of the interior fluid), but above about half water loss, it is possible for the two inner walls of the capsules to touch and become stuck internally in a collapsed state. We are examining better curing methods to eliminate this problem, which we deem to be related to incomplete curing in the interior of the capsules. A new apparatus for testing is being constructed which incorporates humid gas to desorb the carbon dioxide, as would be the case in a power plant. New tests also being conducted to quantitatively measure the rate of uptake of CO2 in carbonate-filled capsules compared to the same un-encapsulated fluid. Initial results indicate a significant increase in mass transfer (approximately a factor of five) using 400 μm capsules compared to neat fluid.
Figure 7. Temperature recorded at the capsule location during 80 cycles of sorption and desorption.
Figure 8. Before testing, and after 80 cycles (Figure 7), 3% carbonate solution in Semicosil, observed via the camera shown schematically in Figure 6.
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Acknowledgements This work was performed under the auspices of the U.S. Department of Energy by University of California, Lawrence Livermore National Laboratory under Contract W-7405-Eng-48. This document was prepared as an account of work sponsored by an agency of the United States government. Neither the United States government nor Lawrence Livermore National Security, LLC, nor any of their employees makes any warranty, expressed or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States government or Lawrence Livermore National Security, LLC. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States government or Lawrence Livermore National Security, LLC, and shall not be used for advertising or product endorsement purposes.
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