In this series we have learnt about types of gas carrier ships and also about the basics of natural gas. Ships are a reliable mode of gas transport but do you know how gases are transported by ships? Gases are always transported in liquefied state. Being 600 times lighter than when in liquid state, it does not make economic sense to transport them in gaseous state. There are several methods used to liquefy a gas depending on its physical properties.
Methods to Liquefy Gases
Liquefaction of a gas occurs when its molecules are pushed closer together. The molecules of any gas are relatively far apart from each other, while the molecules of a liquid are relatively close together. Gas molecules can be squeezed together by one of two methods: by increasing the pressure on the gas or by lowering the temperature of the gas or by both.
Two key properties of gases are important to decide on methods for their liquefaction: Critical Temperature and Critical Pressure. The Critical Temperature of a gas is the temperature at or above which no amount of pressure, however great, will cause the gas to liquefy. The minimum pressure required to liquefy the gas at the Critical Temperature is called the Critical Pressure. A difference in Critical Temperatures among gases means that some gases are easier to liquefy than are other.
Reliable Gas Transport by Sea
Gases are generally liquefied for transportation by sea in four ways namely;
- Compressing and liquefying by cooling at ambient temperature.
- Compressing and liquefying at low temperatures.
- Liquefying at low temperatures.
- Liquefying at very low (Cryogenic) temperatures
Two kinds of liquefied gases are widely used commercially and transported by sea: Liquefied Natural Gas (LNG) and Liquefied Petroleum Gas (LPG). LPG is a mixture of gases obtained from Natural Gas or petroleum that has been converted to the liquid state. The mixture is stored in strong containers that can withstand high pressures. Liquefied Petroleum Gas (LPG) is generally transported in ships in liquefied state by compression at low pressure and partially refrigerated to low temperatures. This has two advantages;
- It does not require heavy tanks, low temperature prevents BOIL OFF during transport and does not require expensive heavy insulation.
- It does not require special expensive materials for storage tanks on ship.
Liquefied Natural Gas (LNG) is generally transported in ships in liquefied state at atmospheric pressure but at very low temperatures. Since the temperature is low, following problems may be faced during transport; Heavy insulation is required which is expensive and there is always a problem of cargo vaporization (BOIL OFF) due to very low temperatures of the gas. BOIL OFF creates increase in pressure in the storage tanks which are not designed for pressure. Gas compressors are required onboard the ships to maintain low pressure in the storage tanks.
Vaporized cargo creates disposal problems which have to be wasted by flaring as it can not be released to atmosphere. Now a day boiled off gas is burned as fuel in the boilers or in the diesel engines on the ship. Ship machinery is modified accordingly. The cargo being at very low temperature, special expensive materials for storage tanks are required as ordinary steel can not withstand low temperatures. Specially designed insulated tanks are used for storage of cargo on ships.