The sole reason for liquefying natural gas is to facilitate its shipment from source to point of use. Pipelines are not yet practical to move gas across oceans, except for relatively short distances between offshore terminals and land based infrastructure.
Liquefaction processes were developed as a means of reducing the space required to store and transport large quantities of gas.
The very first LNG tanker ‘Methane Princess’ was a converted liberty vessel, into which free-standing aluminium cargo tanks were fitted and insulated with balsa wood.
It was operated by British Gas starting in 1959 to transport loads of 5,000 cu.m (3,625 MT), of LNG from the Lake Charles LNG terminal in Louisiana, to Europe’s first commercial LNG import terminal at Canvey Island on the Thames Estuary.
In the quest for lower operating cost per cargo-ton kilometre, the size of LNG tankers grew rapidly to exceed 100,000 cu.m by 1970 and later to what is still the most popular class in the fleet; 125,000-140,000 cu.m.
The commissioning of deep-water LNG terminals allowed even larger carriers to be designed and the Q-Max, with a total capacity of 266,000 cu. m represents the state-of-the-art today.
Developed by joint venture partners Qatar Petroleum and ExxonMobil, Q-Max is equipped with Cryostar’s onboard re-liquefaction system that eliminates the loss of LNG as boil-off-gas while in transit.
This breakthrough allows the use of highly efficient diesel propulsion that, together with many other technical innovations, results in an energy requirement 40% lower than conventional carriers and correspondingly lower carbon emissions.
Over the past 40 years, four different containment systems have been developed and proven for use in the construction of LNG carriers.
1. The Moss Rosenberg system stores LNG in independent spherical tanks insulated with polyurethane foam and these comprise the majority of the existing fleet.
2. The IHI-SPB system uses self-supporting prismatic type B tanks similar to the original Methane Princess and there are only two of this type in the current fleet.
3. TGZ Mark III designed originally by Technigaz uses a welded stainless steel membrane with ‘waffles’ to absorb thermal contraction.
4. GT 96 is the second most popular design constructed with double membranes of Invar to minimise thermal contraction.
The development of LNG liquefaction technology is very closely linked with helium recovery, and Air Products built the first commercial gaseous helium extraction plant in 1962 at Navajo, Arizona.
“The most cost effective way to recover helium is from a liquefied natural gas base load facility,” explained Mark Modjeska, Director of LNG for Air Products. “Interestingly, it was the strategic nature of early helium recovery efforts that helped motivate the company to develop a more efficient liquefied natural gas technology.”
Currently there are four different processes in use for liquefying natural gas:
1. The Multi-Component Refrigerant (MCR ®) liquefaction process developed by Air Products, dominates the LNG industry – employed in 86 liquefaction trains out of the 100 trains that are either on-stream or under-construction with a total installed capacity of 243 million metric tonnes per annum (MMTPA).
2. The Philips Cascade process is the second most used, with 10 trains online and a total capacity of 36 MMTPA.
3. Shell’s DMR process is used in three trains with total capacity of 14 MMTPA.
4. The Linde/Statoil process is used only in the Snohvit 4.2 MMTPA single train.
New gas separation expertise
All liquefaction processes comprise of three stages:
1. In the pre-treatment stage, raw gas is treated to remove carbon dioxide, hydrogen sulphide, mercury, mercaptans and dehydrated to remove water.
2. In the pre-cooling stage, treated, dry gas is cooled to an intermediate temperature, 30~40°C causing the heavier hydrocarbon components, which can freeze at very low temperatures, to condense and separate from the gas.
3. Finally, in the liquefaction stage the temperature of the lean gas, mainly methane and ethane, is reduced to approximately 162°C.
AP-X technology, patented by Air Products in 2001, was successfully placed on-stream at the world’s largest LNG production plant in Qatar.
In the AP-X process cycle, LNG is sub-cooled by a simple, efficient, nitrogen expander loop that increases LNG production capacity over the current generation of LNG process trains, by approximately 50%.
AP-X uses a simplified version of the cycle employed in hundreds of Air Products’ air separation plants and nitrogen liquefiers worldwide. The Qatargas 2 Train 4 expansion project will also apply this new technology to three other LNG trains under construction for Qatargas.
Growing demand for helium and LNG
New applications for helium introduced since the 1960s have driven continuous growth in global demand for this inert gas with many unique properties.
Magnetic Resonance Imaging (MRI), which emerged in the 1980s and is now in constant use at most hospitals, was one of the first applications that significantly boosted world demand for liquid helium.
Research projects like the Large Hadron Collider demand major supplies of liquid helium to maintain the cryogenic temperature necessary for superconductivity.
The semiconductor manufacturing industry is a large and growing consumer of helium, as is the production of optical fibre. The aerospace industry and space exploration are critically dependant on helium supplies. Industrial uses include TIG welding, laser processing, leak detection, plasma arc coating and melting.
Undersea exploration and metal heat treatment require synthetic atmospheres containing helium. In the near term future, generation IV high-temperature gas-cooled nuclear reactors under development by the PBMR company in South Africa, could very significantly increase the global demand for helium.
Helium is continuously formed by the natural radioactive decay of uranium and thorium in the Earth’s crust, but unless trapped by certain geologic conditions most diffuses to the surface and floats up to the outer atmosphere.
Helium is found in varying concentrations as a constituent of natural gas in only a few places on Earth, including the US, Poland, Russia, Algeria and Qatar.
Recovery of helium where its concentration is as low as 0.3% has been proven viable, but the evaluation is complex and is influenced by the other products in the gas stream.
Attacks on the USS Cole and the oil tanker Limburg demonstrate the vulnerability of ships to terrorism, said MIT Mechanical Engineering Professor James A. Fay, an oft-cited expert on the safety of LNG and oil tankers.
A terrorist attack by an explosives-laden speedboat would surely provide an ignition source to the LNG as it spilled out the sides of a tanker.
The main concerns over an LNG tanker spill are the way in which the super-cooled liquid would quickly vaporize upon contact with the warmer water and the way in which the vapour, if ignited, would rapidly burn in a huge pool fire.
Studies in the 1970s and 1980s looked at spills of about 10,000 gallons, according to Jerry Havens, who directs the Chemical Hazards Research Centre at the University of Arkansas.
Those studies produced fires that measured about 15m across and 75 feet high, Havens recalled.
Scaling up to a multimillion-gallon spill scenario from that paltry burn requires a long cognitive leap. There’s some debate, of course, but most peer-reviewed predictions agree that such a spill could produce a fire of about a 400m radius.
Anyone within 800m of the fire’s edge would sustain second-degree burns on skin exposed for half a minute.