Liquids Ease Acid-Gas Removal

Figure 1. Researchers can tailor liquids to absorb carbon dioxide, sulfur dioxide, carbonyl sulfide and carbon disulfide. (For a video showing carbon dioxide being released from the liquid, click here.) Source: Pacific Northwest National Laboratory.The liquids exhibit high gravimetric capacities for the gases. Current lab-scale results show, e.g., 25–27 wt. % for CO₂ versus 7 wt. % for a 30% MEA solution in water. "There's considerable room to improve results," adds Heldebrant. Changing the structure of the alcohol or the base can alter physical and chemical properties, he explains. The approach also may suit removing multiple gases but at this point researchers don't know whether an all-in-one system with a blend of liquids or a stepwise approach makes most sense. Researchers so far have run the liquids for CO₂ and SO₂ removal through five regeneration cycles. "There's been minimal evaporative loss and no absorption performance decline," says Heldebrant. The next step in the development is to run gas-specific trials in a wetted-wall column. PNNL engineers now are designing a unit, which probably will have a 100-ml to 1-L capacity, notes Heldebrant. Tests should begin early in 2010 and should allow pinning down economics by the end of the year. This hopefully will lead to piloting in early 2011, he says. PNNL already has received inquiries from several chemical and other companies, and would like to have an industrial partner involved in the piloting. Heldebrant says the key challenges are to cut costs of the liquids — the researchers plan to look at new ways to make them — and how to deal with any water that gets into the system — work on easing phase separation of water will start by the end of 2009. The economic analysis will factor in the need for an initial water-removal step, he adds. Researchers also are working on second-generation systems that use a single compound instead of a mixture. This won't pose any tradeoffs in tailoring, will make modeling easier, and may offer higher absorption capacity, explains Heldebrant. Such a liquid for CO₂ capture already has been developed and tested, he adds.