Getting gas ready to go to market

Posted: Sunday, April 23, 2000

Natural gas needs more than a pipeline to be successfully exported from Alaska. It needs to be converted into something that can be shipped efficiently -- liquefied natural gas.

But the cost of equipment to produce and transport what is known as LNG can double its delivered price.

``The whole reason you liquefy natural gas is to get it to a market,'' said R.J. ``Jed'' Watkins, senior production engineering specialist for Phillips Petroleum Co. in Nikiski, on the shore of Cook Inlet.

Condensing natural gas to a liquid reduces its volume by more than 600 times, making it feasible to ship long distances.

Watkins said the first choice for any producer is to sell natural gas through a pipeline to buyers near the field. If that isn't possible, the second choice is usually a gas-fired electrical power plant near the field.

``It's a lot easier to lay a power line across rough terrain than it is to lay a pipeline,'' he said.

In Alaska, though, demand for natural gas and electricity is limited.

To sell gas overseas, the industry must convert it to products that can be economically shipped. In Cook Inlet, Unocal combines natural gas with nitrogen from the air to produce fertilizers.

Phillips converts gas to LNG, but that adds significantly to its delivered price. Cook Inlet gas sells for about $1.65 per thousand cubic feet, Watkins said. Phillips and Marathon sell LNG in Japan for the equivalent of about $3.25 per thousand cubic feet.

The Nikiski plant used to liquefy Cook Inlet natural gas is little more than a turbo-charged version of your kitchen refrigerator.

The Phillips plant turns the gas into a liquid by cooling it to extremely low temperatures so the gas is easier to ship.

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

``In any refrigeration cycle, you have a compressor. Ours are big,'' said Watkins of Phillips' Nikiski plant.

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 the gases 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 into a beaker.

Every nine days, Phillips delivers 550,000 barrels of LNG to a ship bound for Japan.

LNG plants are expensive to build because they require costly metals to withstand the extremely cold temperatures used in processing, Watkins said. Natural gas, composed mostly of methane, has to be cooled to minus 258 degrees Fahrenheit to turn into the desired product. LNG is so cold that a steel pipe dipped in it shatters.

A crucial part of the process is removing contaminants from the gas, Watkins said. The main contaminants - carbon dioxide and water - freeze at temperatures far above that at which gas turns to liquid, so if Phillips did not remove the contaminants, they would clog the plant.

To achieve the extreme cold required to liquefy natural gas, Phillips uses propane to cool the natural gas down to minus 32 degrees and ethylene to bring it down 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.

Next, the pressurized LNG passes through a third stage using methane to cool 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.

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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