Controlled-Freeze-Zone Technology for the Distillation of High-CO2 Natural Gas
- Chris Carpenter (JPT Technology Editor)
- Document ID
- Society of Petroleum Engineers
- Journal of Petroleum Technology
- Publication Date
- November 2015
- Document Type
- Journal Paper
- 100 - 101
- 2015. Society of Petroleum Engineers
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- 54 since 2007
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This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 171508, “Controlled-Freeze-Zone Technology for the Commercialization of Australian High-CO2 Natural Gas,” by Jaime A. Valencia, Robert D. Denton, P. Scott Northrop, Charles J. Mart, and Ransdall K. Smith, ExxonMobil, prepared for the 2014 SPE Asia Pacific Oil and Gas Conference and Exhibition, Adelaide, Australia, 14–16 October. The paper has not been peer reviewed.
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.
The tower normally incorporates three zones: the specially designed section that addresses the solidification region and two conventional distillation sections, one for rectifying and one for stripping, which cover the vapor/liquid areas above and below the CO2-solidification region. The relationship between the three sections of the column and the methane/ CO2 phase diagram is illustrated in Fig. 1.
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