Physical adsorption relies on the affinity of CO2 to the surface of a material under certain conditions without forming a chemical bond. Adsorbents can separate CO2 from a stream by preferentially attracting it to the material surface at high pressures through weak interactions such as van der Waals forces. During capture, the chemical potential of the adsorbed CO2 is lower than the chemical potential of CO2 in the gas mixture.
Regenerable Physical Adsorbents
Regenerable adsorbents must have the ability to reverse the chemical potential of the adsorbed phase upon changing the conditions to remove the CO2. This is done primarily through changes in pressure or stripping with an easily separable gas such as steam. Limited temperature changes can improve efficiency, but take time cycle due to the heat capacity of the adsorbent material. Since adsorption is a surface phenomenon, a successful adsorbent will have a high surface area to volume ratio. The central advantage of physical adsorption methods is the possibility for low energy requirement to regenerate the sorbent material and the quick regeneration time associated with changing the pressure.
Proposed absorbents include activated carbon, zeolites (molecular sieves), and promoted hydrotalcites. Current zeolite systems can produce nearly pure streams of CO2, but have high energy penalties due to vacuum pumps and dehumidification equipment. Hydrotalcites are most effective at high temperatures (450-600 K), enabling capture inside or near combustion or gasification chambers. Research is required to decrease the pressure difference requirement and increase the capacity of current adsorbents.
Membrane based Separation
Membrane systems include thin barriers that allow selective permeation of certain gases, allowing one component in a gas stream to pass through faster than the others. Membrane separation can be considered a steady-state combination of adsorption and absorption. A successful membrane allows the desired gas molecule to adsorb to the surface on one side, often at higher pressure. The molecule then absorbs into the membrane interior, eventually reaching the other side of the membrane where it can desorb under different conditions, such as low pressure.
Membrane gas separation processes have been widely used for hydrogen recovery in ammonia synthesis, removal of CO2 from natural gas, and nitrogen separation from air. Each of the membranes used in these capacities could be applied to carbon capture. Commonly used membrane types for CO2 and H2 separation include polymeric membranes, inorganic microporous membranes, and palladium membranes.
Polymeric membranes, including cellulose acetate, polysulfone, and polyimide are the most commonly used for separation of CO2 from nitrogen, but have relatively low selectivity to other separation methods. Inorganic membranes, able to withstand high temperatures, are capable of operating inside combustion or gasification chambers. Membrane reactors based on inorganic membranes with palladium catalyst can reform hydrocarbon fuels to mixture of H2 and CO2 and at the same time separating the high-value H2. Combining membranes with chemical solvents has also been proposed. Despite an extra energy requirement, this arrangement may eliminate problems associated with direct contact between the liquid solvent and gas mixture.
Most membranes have inherent difficulty achieving high degrees of gas separation due to varying rates of gas transport. Stream recycling or multiple stages of membranes may be necessary to achieve CO2 streams amenable to geologic storage, increasing energy consumption. However, the potential for high surface area could reduce the chemical potential difference required to drive gas separation.
There are a variety of options for using membranes to recover CO2 from flue gas. In one concept, flue gas would be passed through a bundle of membrane tubes, while an amine solution flowed through the shell side of the bundle. CO2 would pass through the membrane and be absorbed in the amine, while impurities would be blocked from the amine, thus decreasing the loss of amine as a result of stable salt formation. Also, it should be possible to achieve a higher loading differential between rich amine and lean amine. After leaving the membrane bundle, the amine would be regenerated before being recycled. R&D pathways to an improved system include increased membrane selectivity and permeability and decreased cost (Falk Pederson et al., 2000).
Another concept under development is the use of an inorganic membrane. The University of New Mexico researchers have previously shown the ability to prepare a silica membrane that can selectively separate CO2 from CH4 and are developing amicroporous inorganic silica membrane containing amine functional groups for the separation of CO2 from flue gas. The membrane is produced by sol–gel dip processing. By modifying the membrane, the strong interactions between the permeating CO2 molecules and the amine functional membrane pores should enhance selective diffusion of CO2 along the pore wall of the membrane with subsequent blocking of the transport of other gases, such as O2, N2, and SO2. Thus, this novel membrane should have better CO2 selectivity than a pure siliceous membrane, if the illusive balance between permeance and selectivity can be achieved. New Mexico Institute of Mining and Technology is looking at zeolite membranes. Zeolites are crystalline aluminosilicate materials with well-defined subnanometer pores and unique surface properties appropriate for molecular separations, such as CO2 from flue gas. The current work is focusing on the separation of CO2 from N2 at high temperatures. The target operational temperature for membrane development is 400 C (Zhang, 2006).
Innovative use of membranes for CO2 capture from flue gas is also being investigated. Membrane Technology and Research (MTR) is investigating novel thin-film composite polymer membranes and capture configurations to increase the flux of CO2 across the membrane, thereby reducing required membrane area. These membranes will be developed based upon thin-film membranes previously developed by MTR utilizing Pebax1 polyether-polyamide copolymers. This research effort includes studying placement of the membrane modules in the power plant in an optimal configuration so that the driving force across the membrane is maximized.
Polymer-based membranes, in comparison to other separation techniques, such as pressure swing absorption, are less energy intensive, require no phase change in the process, and typically provide low-maintenance operations. A polybenzimidazole (PBI) membrane under development at DOE’s Los Alamos National Laboratory (LANL) has demonstrated long-term hydrothermal stability up to 400 C, sulfur tolerance, and overall durability while operating in simulated industrial coal-derived syngas environments for over 400 days at 250 C. Membrane thickness has been decreased to less than 3 mm while operating at simulated industrial syngas conditions.
NETL researchers have recently fabricated and tested a supported liquid membrane that is CO2 selective and stable at temperatures exceeding 300 C. The membrane consists of an advanced polymer substrate and an ionic liquid developed in a collaborative effort with the University of Notre Dame. Supported liquid membranes are of interest because transport takes place through the liquid within the pores rather than through a solid phase. This feature allows the membranes to take advantage of higher liquid phase diffusivities while maintaining the selectivity of the solution diffusion mechanism. NETL researchers were able to fabricate membranes operational at elevated temperatures due to negligible volatility of the ionic liquid and the exceptional resistance to plasticization of the substrate (Ilconich et al., 2007).