This article, written by Assistant Technology Editor Karen Bybee, contains highlights of paper IPTC 12708, "Controlled Freeze ZoneTM Technology for Enabling Processing of High CO2 and H2S Gas Reserves," by J.A. Valencia, P.S. Northrop, and C.J. Mart, ExxonMobil, originally prepared for the 2008 International Petroleum Technology Conference, Kuala Lumpur, 3-5 December. The paper has not been peer reviewed. Global natural-gas reserves increasingly are reported with significant CO2 and/or H2S content. Concentrations of 20 to 40% are not uncommon. Commercialization of such resources poses significant economic and technical challenges. Approximately one-sixth of ExxonMobil's hydrocarbon resources are categorized as sour gas. With these volumes, ExxonMobil has the industry's largest sour-gas resource base and has more than 50 years experience in managing sour gas. Introduction Monetization of lesser amounts of saleable methane/hydrocarbons while at the same time producing, removing, and disposing of major quantities of acid-gas contaminants calls for technologies that can reduce capital and operating costs. The proper disposal of these contaminants adds to the challenge. Release of CO2 to the environment is not desirable, its geosequestration is a much better alternative, and it would be better yet if it can be turned into a product stream to be sold or used for enhanced-oil-recovery (EOR) purposes. Similarly, alternatives for the disposal of H2S are needed because the conventional conversion to elemental sulfur led to the saturation of many markets. While there has been a recent resurgence in the demand for sulfur, uncertainties about wide-spread demand sustainability encourages the development of alternative H2S-disposal options. The cryogenic technology, developed at ExxonMobil Upstream Research Company, is being advanced to the commercially ready stage in response to these needs. The first choice for many separations in the gas industry are those that take advantage of the simplicity provided by differing relative volatilities and phase behavior and are implemented by flash drums, separators, and distillation and fractionation towers. In the presence of methane, and to yield sales-quality gas, such separations imply very low, cryogenic temperatures of −120°F and lower, depending upon the operating pressure. CO2 solidifies at these low temperatures. The experimental measurement and analysis of CO2-solidification conditions in the presence of methane were pioneered in 1954. Shortly thereafter, a new process was proposed for freezing CO2 from natural gas by flashing a cooled gas stream into solidification conditions and using a cyclonic device to separate the resulting slurry into solid CO2 and liquid methane.