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Carbon-Based Energy Systems > CO2 Capture

Advanced CO2/H2 Separation Materials Incorporating Active Functional Agents

Start Date: September 2008
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Investigators

Yuichi Fujioka, Shingo Kazama, and Katsunori Yogo, Research Institute for Innovative Technology for the Earth (RITE)

Objective

The objective of this research is to develop an excellent CO2 separation membrane that greatly reduces the energy consumption and costs in gas separation associated with carbon capture and storage (CCS). Innovative composite materials that have both polymeric and inorganic functional agents can create interactive molecular forces that selectively extract CO2 from a gas mixture. This research will use nano-composite technology to control the composition of the pores, surfaces, and functional compounds, and study the molecular dynamics.

Background

CCS has been proposed as a strategy to assist in reducing the levels of carbon dioxide that would otherwise be released to the atmosphere as a result of fossil fuel combustion. However, a major barrier for CO2 separation membranes has been scaling the technology so that it can be cost effective in applications beyond current market use.

There are several approaches for CO2 separation which include chemical absorption, physical absorption, cryogenic separation, and membrane technologies. 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.

This research will develop polymer and inorganic membranes for carbon separation. Inorganic membranes have much better chemical and thermal stability and are used at high temperatures of 300°C or more. Polymer membranes are economical and can be produced in volume, even though they need to be operated at lower temperatures of 150°C or less. Additionally, the polymer morphology and mobility can determine the gas permeability and selectivity.

Figure 1

Figure 1:
Concept of the CO2 molecular gate membrane

From previously funded GCEP work, it was found that the surface of carbon membranes treated with CO2 affinity agents such as alkali metal carbonate, amino acid, and dendrimers, could enhance CO2 selectivity. The study indicated that further active incorporation of CO2 affinity agents in nanopores would lead to greater CO2 selectivity and allow them to function like a CO2 molecular gate.

It was also found that the CO2 molecular gate needs a strict morphological arrangement. If the distance between two amine moieties is too small, it allowed strong hydrogen bonding of the amine moieties, and the membrane would not have sufficient CO2 permeability. On the other hand, if the distance is too large, there would not be enough carbamate ion pairs for the gate.

Approach

To achieve these goals, the researchers will first synthesize a porous substrate, optimize the pore size and select a CO2 affinity agent. Then the team will establish techniques for preparing densely-packed active functional agents in the nanoporous substrate using the process described in Figure 2.

Figure 2

Figure 2:
Process for incorporating CO2 affinity molecules into the nanopore.

In order to include a CO2 affinity molecule in a nanopore, an appropriate medium must be selected to deliver the CO2 affinity agent into the nanopores. The medium should be able to penetrate the nanopores freely and have sufficient solubility for the CO2 affinity molecules. Initial experiments will use super-critical CO2 (sc-CO2) as the transport medium. Additionally methods to enhance solubility will be tested. The reaction in the nanopores will be closely evaluated to determine if the interactions are favorable, transfer of the CO2 affinity molecule from the sc-CO2 to the nanopore substrate is optimal, and a stable CO2 molecular gate is formed. A stable, optimal molecular gate will contribute greatly to membrane permeance.

Simulation will guide the development process. Molecular dynamics, the Monte Carlo method, and other molecular simulation techniques will be used to predict the optimum morphology. Meanwhile process simulation will estimate the impact of an effective membrane.

The performance target for this research is to provide an excellent CO2 separation membrane for which the CO2 selectivity and CO2 permeance will be equal to or greater than 300 and 5 × 10-9 m3 m-2 s-1 Pa-1 respectively, and the CO2 capture cost is reduced to 10 US$/ton-CO2 or less for processing and distribution of CCS. These improvements in performance and cost reduction will make CO2 separation membranes a viable option for use in power generation.