Supersonic gas separation technology, which was first used offshore Malaysia, is being developed for a subsea field application.

Petronas and Shell began operation of the patented Twister supersonic separation technology on the B11, a normally unmanned facility offshore Sarawak which started up in January 2004. It is located 106 miles (170 km) north of Bintulu.

Two 300 MMcf/d Twister units are in use dehydrating gas with a high - up to 20 mol% - concentration of CO2, and containing up to 3,500 parts per million of H2S. Dried gas from the platform is exported to an LNG plant at Bintulu.

Technology is provided by Twister BV, a Dutch-based company in Rijswijk affiliated to Shell, which provides supersonic separation of production gases with a residence time inside the separation chamber measured in milliseconds - avoiding hydrate formation and eliminating hydrate inhibition chemicals. The device is static, operates without glycol and has no moving parts, hence offering low maintenance and normally unmanned operation.

In October 2000, Twister commissioned a demonstration skid at Shell Petroleum Development Co's Utorogu gas plant in Nigeria to dehydrate up to 140 MMcf/d(4 MMcm) of gas using supersonic separation. Four other demonstration plants have also been installed, including in Holland since 1998 and in Norway since 2002. The technology has been under development since 1997.

In May 2001, Twister announced plans to collaborate with FMC Kongsberg Subsea on developing a subsea application of the gas dehydration technology.

Earlier this year moves were made towards further development of the technology for a subsea version. "We have been working very hard on this," explained Twister BV Sales Director Hugh Epsom.
In 2001, Twister formed a new joint venture with FMC Kongsberg with support from Norske Shell to work on a subsea application of the technology. A grant of US $1.83 million was obtained through the Norwegian Demo2000 to support a feasibility study for a subsea Twister design. That was completed early 2003. "The conclusion was that it was feasible and that there are no technology blockers," Epsom said. However, several technologies need further development including a maintenance free sea water cooler. "We know how to do it," he said, and added that first a prototype has to be built and thoroughly tested in a surface application. "We are now finalizing a partnership agreement to develop and test it."
Further European Union (EU) funding for $8.29 million was approved at the end of 2003 for prototype development. Twister and FMC Kongsberg signed another agreement January 2004 to move ahead with the project. "Our schedule is to have a subsea Twister up and working by 2007 or 2008." Epsom said.
An improved performance Twister is another element of the project's goals - requiring a prototype to be built and tested onshore and then extensive marinization carried out. "That is a very realistic program but the EU subsidies cover only 35% of project costs," Epsom pointed out. Consequently the partners are looking for further support to fund more work. "There is a lot of industry interest," Epsom reported. One company has provided a "positive response" to the project proposals and three others are also closely interested.

There is no upper limit to the design capacity for Twister, Epsom said. Gas inflow rates of between 15 to 100 MMcf/d are possible with a single tube he said, and a multiple tubing array is possible. "The only limit is at the bottom end, not much below 15 MMcf/d," he said.

The Twister process works from a supply of saturated feed gas at a typical feed pressure of around 100 bar (1,450 psi) and a typical feed temperature of around 68° F (20° C). From the inlet, the saturated gas is fed into a Laval expander nozzle - a highly efficient way of expanding gas to supersonic velocity, typically Mach 1.3 -1,650 ft (500 m) per second. Typical mid Twister conditions are 435 psi and minus 40° F (30 bar and -40° C), causing a mist of water and hydrocarbon droplets to form. The mixture then passes through a cyclone separator. As it travels over a wing, the saturated gas is coerced into a high-speed swirl generating 300,000 g, forcing droplets to the wall for drainage. This section is followed by a diffuser, recompressing gas to 65% to 80% of feed pressure. Liquids and slip-gas (gas slipping with the liquid stream) exits via the drainage chamber at the same pressure to a secondary liquid de-gassing vessel. Slip-gas is separated and recombined with the primary gas stream exiting the Twister process.
For a 35 MMcf/d (1 MMcm) gas throughput, Twister is very compact with the necessary separator measuring just 6 ft (2 m) long in a 6-in. casing normally vertically placed, providing a very small footprint. "The simplicity and reliability of a static device, with no rotating parts and operating without chemicals, ensures a simple facility with a high availability, suitable for unmanned operation even in an offshore environment," the company has argued.

"You have to have a pressure drop to drive it," explained Epsom. "It does not matter what the inlet pressure or temperature is. For current designs you need a 25% to 35% pressure drop to drive the process." Work is underway to reduce this pressure drop requirement to 20% to 25%.

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