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.
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