A process plant the size of Pearl GTL has huge input requirements. Gas, air, power, water and thousands of tonnes of the catalyst trigger the various chemical reactions required to produce the end-products.
The project at Ras Laffan Industrial City is integrated with the development of an offshore block in Qatar’s giant North Field. The block produces 1.6 billion cubic feet a day of gas from two platforms 60 kilometres out to sea, each with 11 wells. Over the lifetime of the project the Pearl GTL will process about 3 billion barrels of oil equivalent of gas. Two 30-inch diameter pipelines link the platforms to the Pearl GTL plant onshore. The first sour gas began flowing from the wells to the plant on 23 March 2011.
Methane conversion at Pearl GTL
Once at the plant, the gas enters the feed gas processing area where it undergoes gravitational separation, cleaning, cooling and distillation. This separation process removes all the naturally occurring gas liquids, such as liquefied petroleum gas, condensate and ethane, and contaminants like metals and sulphur, leaving just methane. Some 120,000 barrels a day (b/d) of associated products are produced at this stage of the process.
The Pearl GTL plant has 24 reactors, each containing 29,000 tubes of Shell’s cobalt synthesis catalyst
In the first step of the three-stage GTL process, methane is converted into synthesis gas – a mixture of carbon monoxide and hydrogen. This is done through the partial oxidisation of the methane using 99.8 per cent pure oxygen. The reaction is conducted at temperatures of about 1,300 degrees centigrade. The oxygen for the gasification process is produced in the air separation plant, which is the largest in the world in terms of high purity oxygen. The $800m plant, built by Germany’s Linde, comprises eight units, which produce a combined total of 28,000 tonnes of oxygen a day. The units take air, compress it and then chill it to -190 degrees centigrade, which separates the liquid nitrogen from the oxygen. The compressors are powered by steam.
The syngas then passes through the GTL reactors – run at about 1,350 degrees centigrade – where it is converted into long-chained waxy hydrocarbon molecules using Shell’s proprietary cobalt synthesis catalyst.
The Pearl GTL plant has 24 reactors, weighing 1,200 tonnes a piece. They each contain 29,000 tubes full of the catalyst, which speeds up the chemical reaction. Shell says if placed end to end, the tubes would stretch from Doha to Tokyo, while the combined surface area of the catalyst is almost 18 times the size of Qatar.
The catalyst comes in the form of pellets that are as small as grains of rice. The vast surface area is due to the catalyst’s many nano-sized inner channels, which make it highly porous so that huge volumes of gas can be exposed to the catalysts’ chemically treated surface, accelerating the speed of reaction.
Catalyst production for Pearl GTL
Shell’s catalyst company CRI/Criterion spent four years in full-time production to provide the 5,000 tonnes of catalysts needed for the start of production at the Pearl GTL project. Samples of each shipment were tested by Shell at its research and technology centre in the Qatar Science & Technology Park.
The Pearl GTL plant does not use any fresh water during processing and does not discharge wastewater into the sea
In the third step of the GTL process, the molten wax is refined, cracked and distilled. In the hydrocracker, the long-chained waxy hydrocarbon molecules are brought into contact with pure hydrogen at high temperature and pressure in the presence of a catalyst, which facilitates the breakdown into smaller molecules. The distillation process is adjusted to achieve the desired product range and properties. The Pearl GTL can produce up to 30,000 b/d of naphtha and normal paraffins, 25,000 b/d of kerosene, 50,000 b/d of gas oil and 30,000 b/d of base oils.
The products are then sent to the main tank farm for storage. The farm comprises 32 tanks and has a capacity of just under 1 million cubic metres. It was built by the US’ Chicago Bridge & Iron.
The utilities requirements at the Pearl GTL are huge. The plant is fitted with six 40MW gas turbines and four steam turbines. The gas turbines were supplied by the US’ GE, while the contract for the plant’s switchgear and electrical control system was executed by Switzerland’s ABB.
Some 8,000 tonnes an hour of steam is distributed around the plant. But the plant is also highly efficient. The gas turbines are fitted with dual fuel IGCC (integrated gasification combined-cycle) type combustion systems, so they can burn a range of fuel gases, including a process off-gas derived from the Pearl reactors themselves. The heat is recovered to generate steam to power the compressors. Some 99 per cent of the steam produced is also recovered and the Pearl GTL does not use any fresh water nor does it discharge wastewater into the sea.
When the syngas is passed over catalysts in the GTL process, huge volumes of water are produced along with the waxy hydrocarbons molecules – about 1,400 cubic metres an hour. The water is taken to an effluent treatment plant for cleaning to remove trace metals, hydrocarbons and any particles, so that it can be reused in the steam systems and for cooling. The treatment plant has the capacity to treat 280,000 barrels of water a day and is the world’s largest industrial water processing plant. The design-build contract for the effluent treatment plant was awarded to a joint venture of the UK’s Veolia Water, Italy’s Saipem and the local construction company Al-Jaber.
Water produced in the feed gas processing stage is reinjected into the ground via two wells. Even the parts of the catalyst can be reused. The catalyst will be replaced every three or so years, one reactor at a time, to minimise disruption, and CRI/Criterion will reclaim the cobalt inside used catalysts to make new ones.
It is the seamless integration of the various processes and inputs developed over several decades that makes the Pearl GTL work. It is a complex plant that combines, breaks up and then rearranges chains of hydrocarbon atoms. But what makes this project stand out is its sheer scale. With a capacity of 140,000 b/d of GTL products, it is the largest GTL plant ever built.