Rice University scientists have found a way to make their asphalt-based sorbents better at capturing carbon dioxide from gas wells by just adding water.
The Rice lab of Chemist James Tour discovered that treating grains of inexpensive Gilsonite asphalt with water allows the material to absorb more than two times its weight in greenhouse gases. The treated asphalt selects carbon dioxide over valuable methane at a ratio of more than 200-to-1.
The material performs well at ambient temperatures and under the pressures that are typically found at wellheads. When pressure abates, the material releases the carbon dioxide that can then be stored and sold for other industrial uses. It can also be pumped back downhole.
Natural gas at the wellhead usually contains between 3 to 7 percent carbon dioxide, but at some locations may contain up to 70 percent. Oil and gas producers traditionally use one of two methods to sequester carbon dioxide. One of the methods is to physically seize carbon dioxide through the use of membranes or solid sorbents, like zeolites or porous carbons. The other method is to chemically repossess carbon through filtering with liquid amine, a derivative of ammonia.
Both of these methods have their issues. Physical filters have a hard time differentiating between carbon dioxide and methane molecules, which are almost exactly the same in size and polarizability. Chemical approaches have better selectively but are more expensive and corrosive. They require a large input of energy and large equipment. Although they have high selectivity, amines capture only 13 percent by weight in carbon dioxide and need superheated steam to recycle the filtration system. The Rice team’s system is capturing more than 200 percent by weight and no thermal source is needed.
The new Rice material features the selectivity of amines with a much higher uptake of carbon dioxide and no thermal requirements, according to Tour. Coating pore surfaces with water adds weak chemical absorption and high selectivity while retaining the material’s strong physical adsorption.
"This is known as a pressure-swing adsorption system, which is easy to implement due to its small size, and there's no need for heating since it works with the inherent pressure in the gas well," Tour said.
Water in Gilsonite forms a hydrate within pore microstructures that greatly increases the binding selectively of carbon dioxide over methane. Grains’ micropores, at 23 angstroms, are far larger than the target molecules; the addition of water tightens the pores and decreases the pore volume that the molecules must travel through. The prepared Gilsonite has a surface area of 4,200 square meters per gram, so adding water still leaves plenty of room to capture carbon dioxide.
Over multiple testing cycles at various pressures and temperatures between freezing and 50 degrees Celsius, degradation of the material was insignificant. The researchers found that one percent of the weight of the water content was lost during cycling but determined the water content of natural gas itself would likely replace that.
The paper on this research was published in the journal Nature.