Abby L. Harvey
GHG Monitor
10/17/2014
A new hybrid form of absorption discovered by a team of researchers from California, Switzerland and China may address issues present with two promising but flawed methods of capturing carbon, according to a paper published by the team this week. Currently, liquid absorption is the leading method used for carbon capture, though using a solid material in the form of a metal organic framework (MOF) has shown promise, the researchers wrote in a paper published in Nature Communications. While MOF absorption is more efficient, implementation of the process can be tricky, and for this reason liquid absorption, which is less efficient, is still the most commonly used process. By combining these two methods into a hybrid substance—a “slurry”—the researchers have potentially addressed the issues with both methods presenting a “best of both worlds” alternative. The slurries are made “by suspending solid adsorbents in a liquid absorbent,” wrote the team, made up of researchers from the China University of Petroleum in Beijing, the Beijing University of Chemical Technology, the University of California, Berkeley and the Institute of Chemical Sciences and Engineering in Switzerland.
Recent advancements in MOFs, which are “three-dimensional networks of metal clusters that are connected with organic linkers,” have been largely dismissed, according to the team, because the use of an MOF absorber is not particularly user-friendly. “To understand why not, consider a simple solid adsorption separation process for carbon capture from flue gas. The first step involves an adsorber, containing the nanoporous material, which selectively adsorbs CO2 from the flue gas. Once the adsorber is saturated, regeneration is required, which is typically done by supplying heat (temperature swing adsorption) or applying vacuum (pressure swing adsorption) to the adsorbent. This process needs at least two columns, which alternate between the adsorption and the regeneration mode. Liquid absorption uses a similar process, replacing the nanoporous material with, for example, an amine solution,” the paper explains.
Liquid absorbers, while easier to implement, are less efficient, according to the study. “If we now compare one of the most promising MOFs with commercially available amine solutions, the energy required to regenerate the amine solutions is about one order of magnitude larger than the energy required to regenerate this MOF. This is because in amine solutions, the CO2 is so strongly bound that one needs to boil the amine solution to reverse the chemical bonding; as the amine solution contains 70 [percent] water, most of this energy is actually used for boiling water,” according to the paper.
In developing a slurry, and combining the best attributes of solid and liquid absorbers, the team created an effective and efficient means of capturing carbon. “From a process engineering point of view, slurries are very similar to liquids. Hence, with slurries we can develop a continuous process and use heat integration. For conventional adsorbents, slurries would be a terrible idea, as the liquid would fill the pores. The beauty of MOFs is, however, that we can use their tunability to select a material with pores that are sufficiently small to prevent liquid-absorbent molecules from entering the materials, but sufficiently large for the gas molecules to be adsorbed,” the team concluded.
