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Posted: January 8, 2008
New membranes improve carbon dioxide capture
(Nanowerk News) An EU-funded research is using a process known as facilitated transport in an effort to develop effective, inexpensive and eco-friendly membranes to remove carbon dioxide from other gases.
Approximately one third of the total carbon dioxide (CO2) emissions in the world come from energy production. CO2 free gas-powered plants are based on carbon dioxide being removed from the waste gases and deposited in the ground.
However, before CO2 can be stored, it must be separated from the waste gases. The current methods used for this type of filtration are expensive and require the use of chemicals. A new membrane technology is going to change that.
This new type of membrane has been internationally patented by researchers at The Norwegian University of Science and Technology (NTNU) in Trondheim. The membrane is made from a plastic material that has been structured by means of nano technology. It catches CO2 while other waste gases pass freely.
According to the scientists, the technology is effective, inexpensive and eco-friendly, and can be used for practically all types of CO2 removal from other gases. Its effectiveness increases proportionally to the concentration of CO2 in the gas.
This method, known as facilitated transport, is comparable to the way human lungs get rid of CO2 when we breathe: it is both a complex and an effective mechanism.
“The novelty is that instead of using a filter that separates directly between CO2 and other molecules, we use a so-called agent. It is a fixed carrier in the membrane that helps to convert the gas we want to remove,” says NTNU professor May-Britt Hägg. She is head of the research group Memfo that works on the new membrane technology.
The agent helps so that the CO2 molecules in combination with moisture form the chemical bicarbonate, which is then quickly transported through the membrane. In this manner, the CO2 is released while the other gases are retained by the membrane.
Various materials are used to make membranes, including plastic, carbon and/or ceramics. Membrane separation of gases is a highly complex process. The materials must be tailored in an advanced way to be adapted to separate specific gases. They must be long-lasting and stable.
The new membrane is made of plastic, structured by means of nanotechnology to function according to the intention. Membranes based on nano-structured materials are eco-friendly and will reduce the costs of CO2 capture.
“With this method, we can remove more CO2 and obtain a cleaner product for smaller plants. Thus, it becomes less expensive,” Hägg says.
“We also have membranes today that are used to separate CO2 and have been used for a couple of decades, but these membranes are used for natural gases at high pressures, and are not suited for CO2 from flue gas. If the membrane separated poorly, very large amounts of the material is needed, and that makes this separation expensive,“ Hägg explains.
To begin with, either single polyvinylamine (PVAm), or a blend of PVAm and additional polymer component solution is cast on a suitable support such as polysulfone (PSf). This composite membrane then undergoes drying and post-treatment, a process that promotes cross-linking (Fig. 1).
A number of important variables determine the final structure of the membrane. These include: the molecular weight of the PVAm; the porosity of support; temperature and time for drying and post-treatment; concentration of casting solution; and type of cross-linking agent used.
These variables can be modified depending on the feed gas composition and the place where the membrane will be used. For example, for natural gas sweetening applications the membrane would have to withstand higher pressures. The relatively low-pressure environment of flue gas carbon dioxide capture, on the other hand, would require different qualities from them.
The membranes rely on facilitated or carrier mediated transport for their function. This in turn involves a reversible chemical reaction in combination with a diffusion process.
The Memfo facilitated transport membrane uses amine groups as fixed-site-carriers (FSC) for CO2 transportation. A reversible reaction occurs at these amine fixed-site-carriers forming bicarbonates from CO2 and water molecules. The bicarbonates move to the other side of the membrane (permeate side) and release CO2.
The amine fixed-site-carriers together with water molecules give fast reversible reaction and high mobility of CO2 in the form of bicarbonate comparable to that of the mobile carrier membranes (liquid membranes) solving the degradation problem common to the liquid membranes at the same time.
The cross-linking agent ammonium fluoride makes water molecules more basic which will have an increased affinity for CO2. This leads to both increased concentration of bicarbonate in the membrane and increased transport of CO2. The transportation (diffusion) of non-polar gases such as methane and nitrogen is hindered due to the increased polarity of the membrane caused by fluoride ions. This should then lead to much enhanced CO2 permeance and high selectivity in favour of CO2 when this membrane is applied for natural gas sweetening or CO2 separation from flue gas.
The commercial view
Membranes have a major potential to become an inexpensive and eco-friendly alternative in the future. An international patent has been taken out for the new type. Manufacturers both in Europe and the USA have taken an interest in putting it into production, Hägg revealed.
Memfo recently joined a consortium of 26 European businesses and institutions within a project named Nanostructured Membranes against Global Warming. The consortium has received E13 million to develop such membranes and includes members such as EOn Engineering, KEMA Nederland, Endesa, Parker and Siemens.
Currently, five different types of nanomembranes are simultaneously being designed in the framework of the project: polymer membranes; diffusion transport membranes, block copolymers; fixed-site carrier-type membranes, cellulose acetate or polyamides; ionomeric high voltage membranes, electrically modified materials; and carbon membranes – carbon molecular sieve membranes; and ceramic membranes.
According to Hägg, the new technology ought to be very interesting for coal-powered plants. “Within a five-year period, the plan is to test the membrane technology in four large power plants in Europe. We believe this will result in an international breakthrough for energy-efficient CO2 membranes,” she says.
When it comes to gas-powered plants, the concentration of CO2 is so low that the pressure in the waste gas must be increased before the gas can be cleaned with this method. However, Hägg reveals that Statoil is currently developing a method for pressurised exhaust that could be combined with this membrane technology, and that would make it interesting for purification in gas-powered plants as well.
Besides CO2 purification in energy production, the method could be used for more or less any type of purification where carbon dioxide is removed from other gases.
“For instance, we are testing this method to purify CO2 from laughing gas in hospitals, and the results are promising,” she concluded.