Gas separation is one of the great success stories in industry. However, separating carbon dioxide from other gases at the scale proposed, especially other combustion gases, presents a monumental challenge. One technical solution that is needed is a method of separating gases with minimal energy penalty. If one or more of the gases exhibits properties that are substantially different from the others, it greatly simplifies the problem. Nitrogen and carbon dioxide are relatively similar in their properties and are both relatively inert. As a result, a significant amount of energy must be expended to separate them. (Hydrogen is somewhat different because its relatively small molecular diameter allows it to diffuse rapidly through membranes). Areas of research need include:
• Enhanced Solvents: Improve the efficiency of CO2 uptake, reduce the parasitic energy losses found in the current stable of chemical agents
• Membranes: Develop permeable or semi-permeable membranes to selectively diffuse CO2 across the membrane.
• Oxy-combustion: Reduce the pollutants emitted when burning low quality fuels. Produce an exhaust gas rich in CO2 to permit direct sequestration.
• Solid Sorbents: Develop solids that can absorb CO2, comparable to that found in liquid solvents. Carbonates are a likely choice. Like the enhanced solvents, reduce the energy penalty for solvent regeneration.
These studies have already begun, but they are quite a long way from commercialization.
The current stable of solvents is liquid amines, such as monoethanol amine (MEA) or Methyl-Diethanol Amine (MDEA). They are effective in recovering CO2, but their use results in a substantial decrease in operating efficiency. These solvents may be considered first generation solvents and can degrade plant performance by up to 12% points. Continued investigation of the traditional stable of solvents shows that there is still some room for improvement. One approach is to use the solvent as a multi-pollutant control; it doesn't reduce the energy burden to zero, but it's another incremental step in the research process .
Improving the performance (i.e., maximizing the facilities final overall efficiency) could be achieved by using solvents that require less energy to release the CO2 from the solvent. These are considered second-generation solvents; chemicals that bind the CO2 less tightly than current solvents or these results may be obtained with chemical additives that can accelerate the rate of absorption of CO2 by the solvent. One of the more intriguing materials to come along is a unique matrix that has the capacity to absorb CO2 readily, even at ambient conditions. These are called Metal Organic Frameworks, or MOF's.
Like catalysts, it is the surface and its morphology that affect the performance of Metal Organic Frameworks (MOF). These compounds are porous crystalline materials composed of metal clusters that are organically linked together. First noted in 1999, they exhibit unusually high surface areas. While activated carbon can reach surface area of 300-1,000 m2/gm, MOF materials have been found with three to five times larger. They have unique properties that can be exploited in gas separation applications such as extraction of CO2 from exhaust gases. One framework, MOF-177 reportedly could contain 320 volumes of CO2 per unit volume of solid sorbent . Beyond CO2, these organic structures can also be used to increase the energy density of hydrogen carriers. A team at UCLA led by chemistry Professor Omar M. Yaghi identified an organic framework that can store up to 7.5 wt % of hydrogen with a volumetric capacity of 32 kg/m3 at 77 K , with a unique framework that exhibits a surface area of over 5,000 m2/gm. The chemical manufacturer BASF demonstrated that MOF's can also be used to increase the energy density of methane stored in canisters at high pressure. Methane has greater market options if the fuel energy density is increased beyond what is available from compressed natural gas, and it creates new opportunities for utilization of this fuel with the lowest carbon content.
Some of the properties noted in MOF's are also found in membranes. Membranes preferentially diffuse one species over others. For molecules with small molecular diameters and high diffusivity, like hydrogen or helium, simple materials such as latex and rubber can be used to effectively separate the lighter gas from air. However, the molecular size of CO2 is large relative to the gases normally found with it. Membranes with high selectivity for CO2 could reduce both the energy penalty and the capital costs for carbon capture.
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