innovative technology could commercialize challenging gas resources

Process may make carbon capture and storage more affordable while significantly reducing greenhouse gas emissions.

Brilliant ideas sometimes strike the mind with the force of a lightning bolt. But the idea that an ExxonMobil scientist had for a revolutionary natural-gas processing technology drifted into his thoughts like gently falling snowflakes.

Reminiscing about his college days at the Massachusetts Institute of Technology during a wintertime walk by the Charles River, Jaime Valencia recalls, “In the coldest winters in Boston, there were times when the river would freeze completely and snow would accumulate on top of it. More often though, the river was only partially frozen, and the snow that landed on the water simply melted and flowed downstream. That observation became a key element in the development of a novel idea Robby Denton [co-inventor of the technology] and I were pursuing,” says Valencia, gas and facilities technology division, ExxonMobil Upstream Research Company.

The idea led to the CFZ technology that is being brought to commercial readiness at a new demonstration plant near LaBarge, Wyoming, beginning next year.

Simpler process, lower costs

“The idea of snow falling into a flowing river helped crystallize the design of the CFZ process for Robby and me,” Valencia says. “The technology simplifies the process for separating carbon dioxide [CO₂] and other impurities from natural gas, lowering costs. Just as important, it facilitates carbon capture and storage.”

“Today we have a large resource base, which includes some challenging gas reservoirs where CFZ technology could be used,” says Randy Howard, project executive, CFZ commercialization, ExxonMobil Development Company. “As much as one-third of world gas reserves have a high level of CO₂. If CFZ is commercialized, it could help us bring important new energy supplies to consumers.”

This is possible because the CFZ technology reduces capital costs, as the process involves fewer steps and less equipment. For offshore applications, this also means less size and weight.

The innovative, single-step separation process, invented and patented by ExxonMobil in the 1980s, was proven at a pilot plant in Texas, and has now led to the May 2008 announcement by ExxonMobil that it has committed more than $100 million to build a commercial demonstration plant near LaBarge. The plant, located at the company’s Shute Creek Treating Facility, will advance CFZ to the commercial-application stage.

“The goal of the plant is to demonstrate that we can meet or exceed the specifications for sales gas over a wide range of field compositions, while capturing design data that will allow us to scale CFZ facilities up to world-class-size applications,” Howard says. “As is customary at ExxonMobil, the first priority will be the safety of all the people involved, from plant design through operation.”

Construction of the plant began this summer, and startup is scheduled for late 2009. The testing of CFZ will take place over a one- to two-year period. During that time, the plant will process about 14 million cubic feet of gas per day of various gas compositions to confirm how the CFZ technology will work on the world’s challenging gas resources.

Over the last several years, various patented improvements to the process were developed by the original inventors as well as by company researchers Don Victory and Gene Thomas, both of whom played key roles in the pilot plant project.

Business and environmental benefits

CFZ lowers the cost of carbon capture and storage because the process separates CO₂, and other contaminants, as a high-pressure liquid stream that can be reinjected underground. Conventional processes require expensive recompression of the CO₂ for reinjection. The CFZ technology has additional benefits: there’s no need to use chemical agents in the process, and it also eliminates sulfur production from hydrogen sulfide often found in gas streams.

An extensive team of scientists, engineers and designers from three ExxonMobil companies and the engineering and construction contractor, URS, is working on the project. The team is led by Jeff Daly, CFZ commercial demonstration plant project manager, ExxonMobil Development Company, in close cooperation with Tim Mullen, Shute Creek Treating Facility senior plant superintendent, ExxonMobil Production Company.

ExxonMobil has researched and developed carbon-management technologies for more than 30 years. In addition to its research programs, the company has supported carbon-capture and storage research conducted by the International Energy Agency, MIT, Georgia Tech University, the University of Texas and Stanford University.

The CFZ technology could bring additional needed energy supplies to world markets from challenging natural-gas resources while benefiting the environment. Its genesis was more than two decades ago when an innovative ExxonMobil scientist recalled snowflakes falling onto a river. 

A look at the technology

Typically, the natural-gas industry uses distillation processes to separate hydrocarbon natural-gas components such as methane, ethane, propane and butane. In doing so, very cold temperatures are used to condition the gas for sale. If appreciable amounts of carbon dioxide (CO₂) are present it will freeze, forming solids inside a processing tower which normally handles only liquids and gases.

Existing commercial technologies avoid the troublesome solidification of CO₂ by using entirely different separation processes that involve multiple steps. They also require chemical agents that must be subsequently recovered. These alternate technologies primarily are aimed at removing low impurity levels of contaminants, and the processing costs rise considerably when high concentrations of undesirable components are present. For example, some of the most challenging natural-gas fields in the world have a hydrocarbon content of only 20 to 40 percent, with the rest being contaminants.

Rather than avoiding the solidification of CO₂, Valencia and Denton’s concept involved the freezing and remelting of CO₂ under carefully controlled conditions in a specially designed section of a conventional distillation tower. That section, an open chamber with an “unfrozen flowing river” at its bottom, came to be known as the controlled freeze zone. Not only did it allow the single-step separation of CO₂ from methane, but it provided the added benefit of discharging CO₂ as a liquid, which could then be pumped and injected underground, making carbon capture and storage more efficient and affordable in reducing greenhouse-gas emissions.