Koichi Yamada, Shingo Kazama, Katsunori Yogo, Research Institute for Innovative Technology for the Earth (RITE), Japan

Separating CO₂ from other gases present in flue or synthesis gas, in conjunction with a suitable means of storing the CO₂, could allow the utilization of abundant fossil fuel reserves with significantly decreased emissions of CO₂ to the atmosphere. This project intends to develop a variety of efficient, low-cost polymeric and inorganic membranes that separate CO₂. Material structure engineering at the scale of gas molecules will be used to increase permeability and selectivity.

Membrane separation of CO₂ from other gases is an active field, but the best membranes today are likely too energy intensive and expensive to be implemented on a large scale. Gas separation in membranes is driven by a pressure difference on either side of the membrane. Decreasing the required pressure difference by increasing the permeability of the membrane would reduce energy costs and required membrane area. However, in order to obtain a sufficiently pure stream of CO₂ , the selectivity for CO₂ must also be high. Many current systems require cascading the permeate through multiple membrane stages to achieve the desired purity.

Two areas of gas separation membrane research are polymer and inorganic membranes. Polymer membranes are relatively easy to manufacture and are suited for low temperature applications. The polymer morphology and mobility determine the gas permeability and selectivity. Figure 1 shows an asymmetric hollow fiber membrane. A thin layer of functional cardo polyimide material supported by a porous structure allows high permeability.

Inorganic membranes have much greater thermal and chemical stability. Appropriately sized pores in materials including zeolites and silicas can act as molecular sieves that separate gas molecules by effective size. Surface adsorption and diffusion inside the pores can also play a role. Figure 2 illustrates gas separation using an ordered array of pores in an inorganic material. Defects in the pore structure can have a large negative effect on selectivity.

Since the effective size of CO₂, N₂, H₂, and other gases present in fossil fuel conversion systems are very similar, membrane pore spaces must be controlled on a scale comparable to the size differences among these gas molecules. This will be achieved for a variety of membrane types using several different techniques.

Hollow fiber cardo polymer membranes will be optimized for CO₂ permeability and selectivity, for example, by carbonizing the outer surface of the membrane. Thermal motion of organic polymers can cause variations in the morphology and effective pore size of the membrane. Carbonization by UV, plasma, or ion beam treatment could serve to restrict the thermal motion of the polymer chain and enhance the molecular gate function of the polymer. Functionalizing the polymer may change its morphology at the sub-nanoscale level, allowing for fine tuning of the pore space.

While most zeolite membranes consist of randomly oriented crystals, a thin, mono-layer crystal with an ordered lattice of pores would demonstrate superior permeability and freedom from defects. As illustrated in Figure 3, this will be achieved by applying a coating of seed crystals on a substrate with perpendicularly oriented channels. After secondary crystal growth, the properties of the resulting pore structure will reflect the morphology of the seed crystal.