The importance of technologies for the separation of carbon dioxide (CO₂) from gaseous process streams has gained in significance, not the least because of concerns over greenhouse gas emissions. As of yet there has been no known high-temperature membrane technologies that solely separate carbon dioxide. The goal of this research is to extend CO₂ separation techniques to a highly selective, high-temperature membrane technology that operates in the temperature range from 500°C to 900°C.

The membrane being investigated is a composite material derived from a combination of molten carbonate and solid oxide electrolyte technologies. CO₂ is transported across the membrane as a carbonate ion in a molten carbonate phase. CO₂ permeation occurs through a facilitated transport mechanism, driven by passive chemical potential gradients. On the surface of the membrane, conversion between gaseous CO₂ and CO₃^2- occurs through the donation of an oxide ion, O^2-, from a conductive solid oxide phase. The reverse is true where the partial pressure of CO₂ is low. Carbonate ions convert back into CO₂, releasing the oxide ion back into the solid oxide phase to complete the circuit (fig. 1). Thus the driving force is the partial pressure gradient of CO₂ established across the membrane.

Figure 1. Depiction of the transport occurring across the membrane thickness. In the upstream gas mixture (feed), the partial pressure of CO₂ is high and combines with an oxide anion to become CO₃^2-. At the opposite face where P_CO2 is low (permeate), CO₃^2- decomposes releasing gas phase CO₂ and an oxide back into the solid oxide phase.

Material compatibility experiments between various alkali carbonate and solid oxide phases have been undertaken with TGA measurements and x-ray diffraction analysis. Further, the materials have been combined together into a composite membrane structure whereby the carbonate is infiltrated and immobilized into the pore space of a solid oxide material. SEM images of the membrane microstructure showing the porous solid oxide matrix and the cooled solid oxide / carbonate composite are given in figure 2. Permeability and selectivity experiments along with the mechanical integrity of the composite membrane are currently underway.

Figure 2. SEM images of membrane cross sections within the bulk. The image on the left is of the porous solid oxide phase. On the right is the solid oxide infiltrated with a tertiary alkali carbonate mixture.

Researchers

Jennifer Wade, Ph.D. Candidate, Chemical Engineering, jlw2103@columbia.edu
Klaus Lackner, Ewing-Worzel Professor of Geophysics, klaus.lackner@columbia.edu
Alan West, Professor and Chair of Chemical Engineering, acw17@columbia.edu

Publications

Wade, J.L., Lackner, K.S., and West, A.C. (2007). Transport model for a high temperature, mixed conducting CO2 separation membrane. Solid State Ionics. 178(27-28): p. 1530-1540.

Lackner, K.S., West, A.C., and Wade, J.L. (2006). Ion Conducting Membranes for Separation of Molecules. Publication Number: WO2006113674

Wade, J., and K.S. Lackner (2005). Development of a coal-based solid-oxide fuel cell system, in The 30th International Technical Conference on Coal Utilization and Fuel Systems, Clearwater, Florida.