LNG production very chilling prospect

Posted: Sunday, April 23, 2000

The Nikiski plant used to liquefy Cook Inlet natural gas is little more than a turbo-charged version of your kitchen refrigerator. Natural gas is composed primarily of methane, the lightest of the hydrocarbon fuels. Cooling methane to minus 258 degrees Fahrenheit condenses it into a liquid. That is the principle behind the plant Phillips Petroleum Co. operates in Nikiski.

Like your refrigerator, the Nikiski plant relies on the fact that compressing gases heats them, and releasing the pressure cools them.

A kitchen refrigerator compresses freon gas, then blows off the heat, condensing it to a high-pressure liquid. It expands the liquid to a gas, cooling it again. Cooled freon circulates through the refrigerator plumbing to cool the food.

"In any refrigeration cycle, you have a compressor. Ours are big," said R.J. "Jed" Watkins, Phillips' senior production engineering specialist in Nikiski.

Phillips uses six turbine-driven compressors, a total of roughly 100,000 horsepower, to compress the gases it uses as refrigerants. Then, it dissipates the heat, condensing them into pressurized liquids. Finally, it allows the liquids to expand into gases, cooling them tremendously. Piping natural gas through several stages of cooled refrigerants reduces it to a liquid.

Each day, the Nikiski plant turns 240 million cubic feet of Cook Inlet natural gas into roughly 60,000 barrels of LNG -- a clear fluid that looks like water when poured in a beaker. Every nine days, Phillips delivers 550,000 barrels of LNG to a ship bound for Japan.

A steel pipe dipped in LNG shatters when struck with a hammer. Watkins said LNG plants are expensive to build because they require costly metals to withstand the cold. Piping at the Nikiski plant is stainless steel, and the LNG storage tanks are aluminum.

Phillips' partner in the enterprise, Marathon Oil Co., uses stainless steel pipes and aluminum tanks on the ships that carry LNG to their customers -- Tokyo natural gas and electric utility companies.

Before Cook Inlet gas runs through the LNG plant, it passes through beds of chemicals that absorb its primary contaminants, carbon dioxide and water. That is crucial, Watkins said. Carbon dioxide turns to dry ice at minus 70 degrees, long before natural gas turns to liquid. Water freezes at 32 degrees. If Phillips did not remove them, they would clog the plant.

To achieve the extreme cold required to liquefy natural gas, Phillips uses propane, ethylene and methane itself as refrigerants. The propane stage cools the natural gas to minus 32 degrees. The ethylene stage cools it to minus 132 degrees.

The natural gas already is under pressure, and pressurized gases liquefy at warmer temperatures. At pressures in the plant, minus 132 is cold enough to liquefy the natural gas.

 

Phillips Petroleum Company's liquefied natural gas plant sits on a bluff overlooking Cook Inlet.

Photo by M. Scott Moon

Next, the pressurized LNG passes through a third stage that uses methane as the refrigerant. That cools the LNG to minus 240 degrees. Then, Phillips releases the pressure, dropping the LNG to minus 258 degrees.

At the end of the process, about 11 percent of the LNG "flashes" back to the gaseous state. Phillips uses the flash gas to power its compressors and boilers.

That is only half the story, though. In what Phillips calls the "cascade system," each stage in the refrigeration process cools not just the natural gas passing through the plant, but also the refrigerant for the following stage.

First, Phillips compresses the propane, and uses water to cool and condense it. Then, it expands the liquid propane to a gas, cooling it tremendously. It uses the cooled propane gas to condense the compressed ethylene to a liquid.

Likewise, it expands the liquid ethylene to a gas and uses that to condense the compressed methane refrigerant to a liquid.

There are several other refrigeration processes used to produce LNG. Phillips began marketing its method only recently. The only other plant that uses it opened last year in Trinidad. That plant is about twice the size of the one in Nikiski, Watkins said.

Air Products and Chemicals Inc. based in Allentown, Pa., says it has supplied the technology for 15 of the 19 LNG plants either operating or under construction worldwide.

Air Products uses several processes, said spokeswoman Debbie Bauer. Its trademark process uses a mixed-gas refrigerant composed of nitrogen, methane, ethane and propane.

It takes considerable heat to boil LNG back to gaseous methane. Phillips stores the finished product at atmospheric pressure in heavily insulated tanks until it goes onto the ships. Even aboard the ships, there is no refrigeration. Insulated tanks are enough to keep most of the LNG in the liquid state.

What boils off as gas helps fuel the ships' engines.



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