Liquified Gas Transport

The gas charge in a conventional refrigeration system, is first compressed to raise its pressure and relative boiling or saturation temperature, before being liquefied by cooling in a condenser. A gas which is to be transported by sea or land, can be liquefied in the same way; being first compressed and then cooled.

Many gases, once liquefied, will remain in that state at atmospheric temperature provided that the necessary pressure is maintained. The storage pressures for these gases is lower if cooling is introduced.

There is a limiting feature with respect to pressure, in that there is an upper (critical) pressure for each gas, above which it cannot be liquefied.

Critical pressure is the ultimate for liquefaction; a gas higher than critical pressure cannot be liquefied.

Whilst in general, gases can be liquefied by compressing and cooling, there are some such as methane, which cannot remain liquid at ordinary temperatures regardless of the pressure. Methane, must be cooled to temperature of - 82°C to become a liquid and the liquid state can only be maintained if the gas is held at a pressure of at least 47 bar. The pressure required to maintain liquidity, reduces with temperature however, and if methane is stored at -162°C it will remain liquid at atmospheric pressure.

Critical temperature is the limiting temperature for liquefaction of a gas. Every gas has a critical temperature above which it cannot be condensed regardless of the pressure.

Properties of some gases
Gas	Critical pressure bar abs.	Critical temperature °C
methane	46.4	-82.1
ethylene	50.7	9.25
ethane	48.9	32.3
propylene	46.2	91.8
propane	42.6	96.8
butadiene	43.3	152
i-butane	36.7	135.2
n-butane	38	152
ammonia	112.8	132.9
vinyl chloride	53.4	158.4

Liquefied gas carriers may be categorized as suitable for the transport of LPG and ammonia or LNG or both if equipped appropriately.

LPG (liquefied petroleum gas) is the general term for gases such as propane, butane, propylene, butylene and C,-isomers. These products can be liquefied at modest pressures.

LNG (liquefied natural gas) distinguishes methane and mixtures containing mainly methane (with small amounts of ethane or traces of other gases) from the gases mentioned above.

Liquefied chemical gases carried by gas tankers, include ammonia, vinyl chloride and chlorine.

Types of liquefied gas carrier

The carriage of gas in bulk, necessitates that It be maintained in a liquid condition by keeping it

(1) Under pressure;

(2) At moderate pressure and moderately low temperature or

(3) At low temperature.

Fully pressurized ships used for the carriage of LPG and ammonia, have tanks in which the cargo is kept liquid, solely by pressure. At low ambient temperature the pressure is moderate but tanks are designed for pressures of 18 bar or more, relating to maximum possible temperatures which may be taken as plus 45 °C This pressure requires containment of great Strength and fully pressurized tanks having great thickness, are consequently of small size and tend to be of cylindrical shape with rounded ends. Large tanks require disproportionately large wall thickness for the same design pressure, compared with small tanks. To avoid the excessive weight penalty, fully pressurized tankers have been built with a large number of small tanks. This makes poor use of hull space.

Part pressurized ships, suitable for LPG, have larger tanks, of cylindrical or near cylindrical shape with convex ends and insulation. Tanks are shaped to make better use of hull space with overall, a larger cargo quantity for the equivalent ship size, compared with the fully pressurized type. Tank strength must be adequate for pressures of perhaps 6 bar relating to a maximum tank temperature of 10 °C. Tank material must be suitable for temperatures possibly

Low temperature cargoes create difficulties because steel and other metals tend to become brittle at low temperature. Cracking (brittle fracture) can result if undue stress Occurs from localized expansion or contraction, or from impact particularly if there is a flaw in the material. There is a range of nickel steels for very low temperatures. The nickel gives toughness and reduces the coefficient of expansion and thus the stresses due to expansion and contraction.

Aluminium is used as an alternative to nickel steel where appropriate. Non-pressurized ships, have the largest tanks for the transport of LPG. Ammonia and vinyl chloride. The tanks are dimensioned for a maximum.

Working pressure of only 0.25 bar above atmospheric, because boil-off enables carrying temperature to be maintained at the saturation temperature of the gases for atmospheric pressure. Self-cooling occurs as evaporation removes latent heat from the remaining cargo. Liquefaction of the boil-off is usual with direct or indirect refrigeration equipment being fitted for the purpose. The saving in weight and cost plus the large size of tanks possible for pressures at just above ambient, makes refrigeration viable.

The problem of metal brittleness due to low carrying temperatures, is overcome by using special nickel steels or aluminium alloys, for tank construction. Liquefied gases are carried in tanks which are insulated but heat leakage is inevitable, the capacity of liquefaction equipment is equated to heat leakage. Insulated ships for carriage of LNG, are designed so that the boil-off amounting to between 0.25 and 0.3% of cargo daily, can be used as fuel in boilers supplying steam to main propulsion turbines. In addition to providing fuel, the gas boil off removes latent heat and keeps the rest of the liquefied gas cargo at a temperature of about -162 C The boil-off is delivered to the machinery space by compressor.

Diesel engines have also been designed for operation on boil-off, with Ignition of the gas and air charge in the cylinder by a pilot injection of distillate fuel or a spark ignition was introduced to save liquid fuel in shore installation diesels operating on gas. The use of liquefied natural gas boil-off to provide steam for main propulsion, continues to be preferred to re liquefaction for LNG carriers. Plant for re liquefaction and partial re liquefaction of LNG has been developed.

However, operating costs are apparently still too high to achieve a saving by using conventional fuel. In the future, price fluctuations could make it more economical to re liquefy the gas boil-off and operate the ship on conventional fuel.

Liquefied gas cargo tanks

The non-pressurized tanks of larger LPG ships and those for LNG cargo, may be of spherical, prismatic or membrane construction, with heavy insulation.