User:Milton Beychok/Sandbox

Liquefied natural gas or LNG is natural gas (predominantly methane, CH₄) that has been converted into liquid form for ease of transport and storage. More simply put, it is the liquid form of the natural gas that people use in their homes for cooking and for heating,

A typical raw natural gas contains only about 80% methane and a number of higher boiling hydrocarbons as well as a number of impurities. Before it is liquefied, it is typically purified so as to remove the higher-boiling hydrocarbons and the impurities. The resultant liquefied natural gas contains about 95% or more methane and it is a clear, colorless and essentially odorless liquid which is neither corrosive nor toxic.

LNG occupies only a very small fraction (1/600th) of the volume of natural gas and is therefore more economical to transport across large distances. It can also be stored in large quantities that would be impractical for storage as a gas.

Process description for the production of LNG

(CC) Diagram: Milton Beychok
Fig.1: Block flow diagram of the LNG liquefaction process. See Natural gas processing for more details.

The liquefaction process involves separating the raw natural gas from any associated water and high-boiling hydrocarbon liquids (referred to as natural gas condensate) that may be associated with the raw gas. The raw gas is then further purified in a natural gas processing plant to remove remove impurities such as the acid gases hydrogen sulfide (H₂S) and carbon dioxide (CO₂), any residual water liquid or vapor, mercury, nitrogen and helium which could cause difficulty downstream. ( See the block flow diagram of the liquefaction process in Fig.1)

The purified natural gas is next refrigerated and distilled to recover ethane (C₂H₆), propane (C₃H₈), butanes (C₄H₁₀) and any higher boiling hydrocarbons, collectively referred to as natural gas liquids (NGL). The natural gas is then condensed into a liquid at essentially atmospheric pressure by using further refrigeration to cool it to approximately -162 °C (260 °F).

As mentioned above, the reduction in volume makes LNG much more cost efficient to transport over long distances where pipelines do not exist. Where transporting natural gas by pipelines is not possible or economical, it can be transported by specially designed cryogenic sea-going vessels called LNG carriers or by either cryogenic rail or road tankers.

History

Natural gas liquefaction dates back to the 1820s when British physicist Michael Faraday experimented with liquefying different types of gases. German engineer Carl Von Linde built the first practical vapor-compression refrigeration system in the 1870s.

The first commercial LNG liquefaction plant was built in Cleveland, Ohio, in 1941 and the LNG was stored in tanks at atmospheric pressure, which raised the possibility that LNG could be transported in sea-going vessels. In January 1959, the world's first LNG carrier, a converted freighter named The Methane Pioneer, containing five small, insulated aluminum tanks transported 5,000 m³ (about 2,250 metric tons) of LNG from Lake Charles, Louisiana in the United States to Canvey Island in England's Thames river. That voyage demonstrated that LNG could be transported safely across the oceans. During the next 14 months, that same freighter delivered seven additional cargoes with only a few small problems.

The demonstrated ability to transport LNG in sea-going vessels spurred the building of large-scale LNG liquefaction plants at major gas fields world-wide. The first large-scale LNG plant began operating in 1964 at Arzew, Algeria and initially produced about 2,360 metric tons/day (MT/day) of LNG. In 1969, another LNG plant began operating near Kenai, Alaska and initially produced LNG at a rate of about 3,400 MT/day.

By mid-2008, there were 19 LNG liquefaction plants operating in 15 countries worldwide. There were also 65 LNG receiving terminals (often referred to regasification terminals) operating in 19 countries world-wide.

LNG transportation

(CC) Photo: Xoán Porto
LNG tanker with 5 spherical LNG tanks. Total length is 285 metres (311 yards).

As of 2008, a typical sea-going LNG carrier could transport about 150,000 m³ (70,000 MT) of LNG, which will become about 92,000,000 standard m³ of natural gas when regasified in a receiving terminal. LNG carriers are similar in its size to an aircraft carrier and are very expensive to build and to operate. Therefore, they cannot afford to have idle time. They travel fast, at an average speed of 18 to 20 knots, as compared to 14 knots for a sea-going crude oil carrier. Also, loading at the LNG liquefaction plants and unloading at the receiving terminals usually requires only 15 hours as an average.

In case of an accident LNG tankers have a double-hulled structure specially designed to prevent leakage or rupture. The cargo (LNG) is carried at atmospheric pressure and -162 ºC in specially insulated tanks (referred to as the containment structure) inside the inner hull. The cargo containment structure consists of: a primary liquid container or tank; a layer of insulation; a secondary liquid barrier; and a secondary layer of insulation. Should there be any damage to the primary liquid tank, a secondary barrier will prevent the leakage. All surfaces in contact with LNG are constructed of materials resistant to the extreme low temperatures. Therefore, the material is, typically, stainless steel or aluminum or invar.

The “Moss” vessels, currently accounting for 41% of the world LNG fleet, have distinctive, self-supporting spherical cargo tanks. Vessels using membrane tank systems represent 57% of LNG vessels. The membrane designs utilize a much thinner membrane that is supported by the walls of the hull.

LNG vessels use the gas that boils off from the cargo as fuel gas for propulsion and therefore LNG carriers do not arrive at the destination port with the same LNG amounts as were loaded at the liquefaction plant. The accepted maximum figure for boil off is about 0.15% of cargo volume a day of natural gas when regasified at the receiving terminal.

LNG Terminals

(PD) Photo: Yo-sei Shosi
LNG storage tanks in LNG terminal at Yokohama, Japan.