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Gas carrier guide - The principle products - Properties and hazards of various liquefied gases
Most liquefied gases are hydrocarbons and the key property that makes hydrocarbons the world’s primary energy source – combustibility – also makes them inherently hazardous. Because these gases are handled in large quantities, it is imperative that all practical steps are taken to minimize leakage and to limit all sources of ignition.
The Principal Products described are as below :-
Liquefied Natural Gas (LNG)
Natural gas is transported either by pipeline as a gas or by sea in its liquefied form as LNG. Its composition varies according to where it is found but methane is by far the predominant constituent, ranging from 70% to 99%. Natural gas Density - 26.5 LB./Cu Ft. Boiling point - (-259 degree F ). They are lighter than air (Gas Density - 0.47) & (Air - 1.0). So natural gas rises under normal atmospheric conditions.
Fig: Spherical or moss type lng carrier
LNG comes from natural gas that's been cooled to
below -256 degrees F, with some impurities removed.
Natural gas comes from underground gas fields by
itself or in oil fields, along with crude oil. There's very
little difference between natural gas and vaporized
LNG; mostly LNG is a little purer; before liquefying
the natural gas engineers remove the pollutants, like
LNG, as mentioned, is very cold natural gas that is in
a liquid form rather than gas. Chemically, it's mostly
methane, with small amounts of ethane, propane,
and butane. LPG (liquefied petroleum gas), some
times referred to as bottled gas, is a heavier gas that
can be liquefied under pressure or by refrigeration. It
is mostly propane and butane. Gasoline is heavier
still and is a liquid at room temperature. Heating oil
is even heavier and doesn't boil unless heated. And
asphalt is so heavy that it's a solid. But in a way they
are all pretty similar, because they all burn.
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 steam and to load raw NGLs onto semi-pressurized 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 degrees at atmospheric pressure or at -45 degree 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 utilized 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 metal cutting and as a petrochemical feedstock.
With increased pressure on the world’s food resources, the demand for nitrogen-containing fertilizers, 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.
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 fibers. It is obtained by cracking either naphtha, ethane or LPG.
Propylene is a petrochemical intermediate used to make polypropylene and polyurethane plastics, acrylic fibers and industrial solvents.
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.
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 degrees, but is also, with a specific gravity of 0.97, much denser than the other common gas carrier cargoes.
- Design characteristics of liquefied gas carriers
How liquefied gas carriers are built ?
Gas carriers are high-tech ships, designed with safety and security in mind & using special materials and designs to safely handle the very cold LNG. They are robustly designed and constructed and the majority of hazards are minimised by strict compliance with the requirements of the International Maritime Organization (IMO) and other organisations during the design process. ....
- Liquefied gas carrier -applicable regulations
The Master, Chief Officer, Chief Engineer, and personnel engaged in liquefied gas handling shall comply with the following rules and regulations :
(1) The Gas Carrier Codes
- Liquefied gases - Flammability
All liquefied gases presently transported in bulk by sea, with the exception of chlorine and nitrogen, are flammable. The vapours of liquefied gases are generally as easily ignited as those of oil cargoes. ....
- Dispersal of Vented Cargo Vapours
Behaviour of LNG in the cargo tanks - Dispersal of vented cargo vapours
When loaded in the cargo tanks, the pressure of the vapour phase is maintained substantially constant, slightly above atmospheric pressure....
- Hazards with carrying propylene oxide mixtures
Propylene oxide (PO) is an eserine with wide flammable limits of 2.8 - 37% by volume. The TLV of this product is 100 ppm whilst its odour threshold is 10 ppm. Exposure to the liquid or high concentrations of the vapour can lead to eye burns, skin irritation and blistering, vomiting, lack of co-ordination and depression
- Liquefied gas cargo reactivity
Liquefied gas cargo corrosion
- Liquefied gas cargo vapour characteristics
- Liquefied gas cargo - low temperature effects
External links :
International maritime organization
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