May 4 2018
In the Journal of Renewable and Sustainable Energy, from AIP Publishing, fundamental researchers at the Colorado School of Mines have suggested a unique two-part system for separating impurities from natural gas.
Natural gas mainly comprises methane, but impurities in the gaseous mixture have to be eliminated before the methane can be sent into the pipeline. The newly suggested purification system brings together two separation approaches and, in principle, promises to enhance performance, lower costs, and reduce ecological side effects compared to standard technologies.
Natural gas processing usually depends on high-temperature methods that suffer high-operating costs. "We propose an integrated process consisting of gas hydrates and membranes, to make the overall process of purifying natural gas potentially more economical without high-temperature driven processes," said co-author Moises Carreon, an expert in membrane separating technologies.
Membrane technology uses various types of material to strain out carbon dioxide and nitrogen from raw natural gas. Hydrogen sulfide and propane are also frequently present and can negatively impact membrane performance. A probable solution to this issue emerged when Carolyn Koh, an expert in hydrates, Pramod Warrier, postdoctoral researcher, and Carreon started talking.
"Hydrates form hydrogen-bonded water cages that trap the gases you want to separate," Koh said. "It's a potentially very selective way of trapping those gases." Different temperature and pressure conditions are essential for impurity hydrates to develop compared to methane hydrates. The researchers decided to add selective hydrate formation as an initial step to the membrane separation process.
In the paper, the team used proven algorithms to establish the feasibility of specific hydrates' formation. These conditions have a comparatively low-energy demand that could make the integrated system inexpensive.
"In this integrated system, we first remove all of the nasty chemicals present in natural gas using gas hydrates to produce a purer mixture," Cameron said. He then described that by using membrane technology it would be easier to separate the purer mixture of gases.
Hydrate formation is energy-efficient as well as environmentally friendly. Water is the only extra material needed for gas hydrate formation, and it effectively sequesters dangerous gases like hydrogen sulfide into the solid hydrate form, which prevents its discharge into the environment. There are certain other contaminants that have to be removed separately; however, this recently proposed system decreases environmental impact from the existing industrial processes.
The integrated hydrate-membrane system can also be used as a model for other molecular gas separations, such as hydrogen gas purification.
The researchers are currently focused on demonstrating their theoretical concept. "We're trying to demonstrate that this integrated system can effectively separate natural gas impurities in laboratory experiments," said Carreon. Part of that work will require optimization to confirm that the hydrate structures form rapidly and easily, and can preserve their stability.