Controlled-Freeze Technology for Processing Sour-Gas Resources
Authors: Denney Dennis 
Published: February 2015

This article, written by Senior Technology Editor Dennis Denney, contains highlights of paper IPTC 16848, ’The Controlled-Freeze-Zone Technology for the Development of Sour-Gas Resources,’ by J.A. Valencia, SPE, and S.D. Kelman, SPE, ExxonMobil Upstream Research, prepared for the 2013 International Petroleum Technology Conference, Beijing, 26-28 March. The paper has not been peer reviewed.

A single-step cryogenic-distillation process that removes carbon dioxide (CO2), hydrogen sulfide (H2S), and other impurities from natural gas has been developed by ExxonMobil. Rather than avoiding the freezing of CO2 at cryogenic temperatures, solidification is allowed to take place in a very controlled fashion. The technology has potential to separate CO2 and other impurities from natural gas and to discharge these contaminants as a high-pressure liquid stream.

Introduction

Meeting the demand for natural gas will require new resources. Many of those new resources will contain substantial amounts of CO2 and H2S that must be managed properly at the surface along with the produced hydrocarbons. Returning the CO2 and H2S to the subsurface for geosequestration or for use in enhanced oil recovery is emerging as a preferred option for management and disposal. A controlled-freeze technology can help meet that goal.

Acid-Gas Injection

When acid-gas injection is required for processing sour-gas resources, the treatment-process options shift toward releasing the acid gases from the natural gas at a relatively high pressure and as a liquid. Natural-gas fractionation can be designed to recover acid gas at high pressure and, at least partially in some cases, as a liquid. This method minimizes the power required for compression or pumping for reinjection of the waste acid gases. Fractionation-based processes rely on the relative volatility of the components in the sour-natural-gas stream. Bulk fractionation uses a single refrigerated tower for the bulk removal of acid-gas compounds, but the overhead sweet product still has a high residual-CO2 content of 15% or more. Higher purities would require lower temperatures, which in turn would lead to CO2 solidification occurring in the distillation column. Therefore, further treatment, usually with a solvent process, is needed to achieve the quality specifications normally required for natural-gas sales to transmission systems.

The Ryan-Holmes process can achieve the required high-purity natural gas by use of a multiple-tower fractionation operation. This process uses a heavier-hydrocarbon liquid additive to suppress CO2 freezing during the distillation process. This hydrocarbon additive lowers the freezing point of CO2 in the first column and is recovered in later columns and recycled. While effective, this process requires that the recovered acid gas be vaporized during recovery of the hydrocarbon-liquid additive.

Like the Ryan-Holmes process, the controlled-freeze process (CFP) is a single-step cryogenic-distillation process that is capable of achieving a high-quality sales-gas product. However, no hydrocarbon additive is required for the desired separation, so the second distillation step to recover the additive is not required. Also, the acid gas can be recovered as a high-pressure liquid, so pumping can be used for reinjection.