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Building the world’s largest gas to liquids plant

17 February 2009

The drive is on to make tomorrow's transport fuels and lubricants on a large scale from clean-burning natural gas. Construction of the world's largest gas to liquids (GTL) plant, Pearl GTL in Qatar, is a major step towards meeting the world’s growing demand for cleaner energy.

Building the world’s largest gas to liquids plant
Building the world’s largest gas to liquids plant

The plant, a joint development by Qatar Petroleum and Shell, will process about three billion barrels-of-oil-equivalent over its lifetime from the world’s largest single non-associated gas field, the North Field, which stretches from Qatar’s coast out into the Gulf. The North Field contains more than 900 trillion cubic feet of gas, about 15% of worldwide gas resources.

Pearl GTL will produce cleaner-burning diesel and kerosene, base oils for top-tier lubricants, naphtha, which is used to make plastics, and normal paraffin, which is used to produce detergents.

The construction of Pearl continues apace at Ras Laffan, a vast industrial zone on Qatar’s coast, some 90km north of Doha.

A gigantic crane will lower steel GTL reactors onto concrete bases in the heart of the plant.. Barges ferry the giant reactors, each containing hundreds of kilometres of pipes along the Rhine to the Dutch port of Rotterdam. From there, they are shipped to Qatar.

60km offshore, preparations to produce the raw gas from the North Field are under way. Two platforms sitting in water up to 40m deep will feed gas to the plant. The steel structures, or jackets, to support the platform are already in place on the seabed. The upper sections of the production platforms, or topsides, are being built in shipyards in Dubai. When complete, barges will take them out into Gulf where a crane will lift them on to the platform legs. Raw gas will flow to Pearl GTL from these two platforms.

Two underwater 30-inch-diameter pipelines will carry the raw gas to Ras Laffan. Once it arrives onshore, a traditional gas separation plant will extract ethane, LPG and condensates. The process will also remove contaminants like metals and sulphur. The sulphur will be extracted from the gas, turned into pellets, and shipped to the nearest market where it can be used to make hydrosulphuric acid, fertiliser, or turned into other valuable products.

What remains is a clean gas (methane) which will then flow to the GTL section of the plant, where it will be converted in a three-stage process into a range of gas to liquids products using Shell proprietary technology. First, the methane reacts with oxygen to create synthesis gas in reactors operating at up to 1,300 degrees Celsius. Then, the synthesis gas is converted into liquid waxy hydrocarbons in the Fischer-Tropsch process.

Finally, the liquid waxy hydrocarbons are upgraded using specially developed technology involving novel catalysts into a wide range of products, including transport fuels, base oils and feedstocks for the chemical industry.

The oxygen for the process will be made in the world’s largest industrial oxygen separation plant, consisting of eight oxygen separators. They will suck in air, liquefy it by chilling it to -180 degrees Celsius, then separate oxygen from nitrogen and generate over 20,000 tonnes of oxygen a day.

"It will be an engineering marvel, and will provide cleaner fuels and products for customers right across the world for many decades," said Shell’s Country Chairman in Qatar, Andy Brown. "The North Field has about 15% of the world’s gas resources, so this plant has a very long term future, and our children and our children’s children will benefit from it."

Pearl GTL sits in the desert, where summer temperatures exceed 40 degrees Celsius and rainfall is slight. Conserving water is therefore critical. Pearl was designed to be self-sufficient in terms of water use. The gas to liquids conversion process produces one barrel of water for every barrel-of-oil-equivalent produced when the hydrogen in the natural gas reacts with oxygen. All the water produced will be treated in a plant that has a capacity to treat 300,000 barrels of water a day, enough to supply a town of one million people. Most of the water will be used and re-used in the GTL plant, and some for irrigation on the building site.

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