The simple separation of carbon dioxide (CO2) from natural gas by distillation would involve cryogenic temperatures at which CO2 solidifies. Most CO2-separation processes instead use solvents that bind to CO2 molecules. For solvent regeneration, the binding process is reversed. The technology described by the authors provides the simplicity of a single-step distillation process for the separation of CO2 from natural gas. The technology has shown the potential to separate CO2 and other impurities from natural gas more efficiently and more cost-effectively.
In the gas industry, separations based on differing relative volatility and phase behavior are relatively simple, easy to implement, and widely used. The significant difference in relative volatility of methane and CO2 makes this system an ideal candidate for separation by distillation, were it not for some cold-temperature conditions leading to solidification of CO2. At 600 psig, conventional distillation is interfered with, for liquid-methane concentrations between approximately 25 and 85%, by the presence of this solid phase. Raising the pressure to 800 psig shifts the operating conditions away from the solidification boundary but into a new limitation: critical conditions. The critical pressure of pure methane is 667 psia; thus, methane-rich mixtures become supercritical. A high-purity methane-product stream cannot be obtained by distillation at 800 psig, and the overhead product purity is limited to 80 to 85% methane with a residual CO2 content of 15 to 20%. This is the basis for CO2 bulk fractionation.
The technology described in this paper, rather than avoiding the solidification of CO2, allows CO2 to freeze, though under carefully controlled conditions and in a specially designed section of an otherwise conventional distillation tower....
Controlled-Freeze-Zone Technology for the Distillation of High-CO2 Natural Gas
15 October 2015