Liquefied Natural Gas (LNG)
Natural gas is transported either by pipeline as a gas or by sea in its liquefied form as LNG. Natural gas comes from underground deposits. Its composition varies according to where it is found but methane is by far the predominant constituent, ranging from 70 per cent to 99 per cent. Natural gas is now a major commodity in the world energy market and approximately 73 million tonnes are carried by sea each year. This is expected to increase to 100 million tonnes per year by the end of the millennium.
Natural Gas Liquids (NGLs)
Associated gas, found in combination with crude oil, comprises mainly methane and NGLs. The NGLs are made up of ethane, LPGs and gasoline.
A small number of terminals, including several facilities in Europe, have the ability to strip methane from the gas stream and to load raw NGLs onto semi-pressurised gas carriers. These ships are modified with additional compressor capacity for shipment to customers able to accept such ethane-rich cargoes. These NGLs are carried at -80°C at atmospheric pressure or at -45°C at a vapour pressure of 5 bar.
The Liquefied Petroleum Gases (LPG)
The liquefied petroleum gases comprise propane, butane and mixtures of the two. Butane stored in cylinders and thus known as bottled gas, has widespread use as a fuel for heating and cooking in remote locations. However, it is also an important octane enhancer for motor gasoline and a key petrochemical feedstock. Propane, too, is utilised as a bottled gas, especially in cold climates (to which its vapour pressure is more suited). However, LPG is mainly used in power generation, for industrial purposes such as metal cutting and as a petrochemical feedstock. About 169 million tonnes of LPG are produced each year worldwide and, of this, about 43.7 million tonnes are transported by sea.
With increased pressure on the world’s food resources, the demand for nitrogen-containing fertilisers, based on ammonia, expanded strongly during the 1970s and 1980s. Large-scale ammonia plants continue to be built in locations rich in natural gas which is the raw material most commonly used to make this product. Ammonia is also used as an on-shore industrial refrigerant, in the production of explosives and for numerous industrial chemicals such as urea. Worldwide consumption of this major inorganic base chemical in 1996 was 120 million tonnes. About 12 million tonnes of ammonia are shipped by sea each year in large parcels on fully refrigerated carriers and this account for the third largest seaborne trade in liquefied gases — after LNG and LPG.
Ethylene is one of the primary petrochemical building blocks. It is used in the manufacture of polyethylene plastics, ethyl alcohol, polyvinyl chloride (PVC), antifreeze, polystyrene and polyester fibres. It is obtained by cracking either naphtha, ethane or LPG. About 85 million tonnes of ethylene is produced worldwide each year but, because most of this output is utilised close to the point of manufacture, only some 2.5 million tonnes is moved long distances by sea on semi-pressurised carriers.
Propylene is a petrochemical intermediate used to make polypropylene and poly-urethane plastics, acrylic fibres and industrial solvents. As of mid-1996, annual worldwide production of propylene was 42 million tonnes, with about 1.5 million tonnes of this total being carried by semi-pressurised ships on deep-sea routes.
Butadiene is a highly reactive petrochemical intermediate. It is used to produce styrene, acrylonitrile and polybutadiene synthetic rubbers. Butadiene is also used in paints and binders for non-woven fabrics and, as an intermediate, in plastic and nylon production. Most butadiene output stems from the cracking of naphtha to produce ethylene. Worldwide total production of Butadiene in 1996 was 6.9 million tonnes. About 800,000 tonnes of butadiene is traded by sea each year.
Vinyl chloride is an easily liquefiable, chlorinated gas used in the manufacture of PVC, the second most important thermoplastic in the world in terms of output. Vinyl chloride not only has a relatively high boiling point, at -14°C, but is also, with a specific gravity of 0.97, much denser than the other common gas carrier cargoes. Worldwide production of vinyl chloride in 1996 was 22.3 million tonnes. Some 2 million tonnes of vinyl chloride is carried by sea each year.
Sources : Liquefied Gas Handling Principles On Ships and in Terminals – McGuire and White