Chapter02
2 ACTUAL GAS CARGOES
In the late 1920th
transportation of liquefied gases in bulk started. In the very beginning it was
transportation of propane and butane in fully pressurised tanks. When the steel quality became better and the
knowledge about propane and butane was better
they started to carry those liquefied gases under temperature control. From the mid-1960th we have
carried fully refrigerated liquefied gases and now the biggest gas carriers are
more than 125 000 m3.
Liquefied gas is
divided into different groups based on boiling point, chemical bindings,
toxicity and flammability. The different groups of gases have led to different
types of gas carriers and cargo containment system for gas carriers.
IMO
divides liquefied gases into the following groups:
LPG -
Liquefied Petroleum Gas
LNG -
Liquefied Natural Gas
LEG - Liquefied Ethylene Gas
NH3 - Ammonia
Cl2
- Chlorine
Chemical
gases
The IMO gas carrier code
define liquefied gases as gases with vapour pressure higher than 2,8 bar with
temperature of 37,8oC.
IMO gas code chapter 19
defines which products that are liquefied gases and have to be transported with
gas carriers. Some products have vapour pressure less than 2,8 bar at 37,8oC,
but are defined as liquefied gases and have to be transported according to
chapter 19 in IMO gas code. Propylene oxide and ethylene oxides are defined as
liquefied gases. Ethylene oxide has a vapour pressure at 37,8oC on
2,7 bar. To control temperature on ethylene oxide we must utilise indirect
cargo cooling plants.
Products not calculated as
condensed gas, but still must be transported on gas carriers, are specified in
IMOÅ‚s gas code and IMOÅ‚s chemical code. The reason for transportation of
non-condensed gases on gas carriers is that the products must have temperature control
during transport because reactions from too high temperature can occur.
Condensed gases are
transported on gas carriers either by atmospheric pressure (fully cooled) less
than 0,7 bars, intermediate pressure (temperature controlled) 0,5 bars to 11
bars, or by full pressure (surrounding temperature) larger than 11 bars. It is the strength and construction of the
cargo tank that is conclusive to what over pressure the gas can be transported.
Examples of
some gas pressure at 37,8oC and boiling point at atmospheric
pressure:
Condensed gas
Gas pressure at 37,8oC
bars absolute
Boiling
point at atmospheric pressure
in oC
Methane CH4
Gas
- 161
Propane C3H8
12,9
- 43
n - Butane C4H10
3,6
- 0,5
Ammonia NH3
14,7
- 33
Vinyl Chloride C2H3Cl
5,7
- 14
Butadiene
C4H6
4,0
- 5
Ethylene
oxide C2H4O
2,7
10,7
2.1
LPG
LPG - Liquefied
Petroleum Gas is a definition of gases produced by wet gas or raw oil. The LPG
gases are taken out of the raw oil during refining, or from natural gas
separation. LPG gases are defined as propane, butane and a mixture of these.
Large atmospheric pressure gas carriers carry most of the LPG transported at
sea. However, some LPG is transported with intermediate pressure gas carriers.
Fully pressurised gas carriers mainly handle coastal trade. LPG can be cooled with water, and most LPG
carriers have direct cargo cooling plants that condenses the gas against water.
The sea transport of LPG is
mainly from The Persian Gulf to Japan and Korea. It is also from the north- west Europe to USA, and from the
western Mediterranean to USA and Northwest Europe.
LPG is utilised for energy
purposes and in the petro-chemical industry
2.2
LNG
LNG - Liquefied Natural Gas
is a gas that is naturally in the earth. Mainly LNG contains Methane, but also contains
Ethane, Propane, Butane etc. About 95%
of all LNG are transported in pipelines from the gas fields to shore, for
example, gas pipes from the oil fields in the North Sea and down to Italy and
Spain. Gas carriers transport the remaining 5%. When LNG is transported on gas
carriers, the ROB and boil off from the cargo is utilised as fuel for
propulsion of the vessel. Cargo cooling plants for large LNG carriers are very
large and expensive, and they will use a lot of energy. Small LNG carriers have
cargo-cooling plants, and can also be utilised for LPG transportation.
The sea transport of LNG is
from the Persian Gulf and Indonesia to Japan, Korea and from the Mediterranean
to Northwest Europe and the East Coast of USA and from Alaska to the Far East.
LNG is used for energy
purposes and in the petro-chemical industry.
2.3
NGL
NGL - Natural Gas
Liquid or wet gas is dissolved gas that exists in raw oil. The gas separates by
refining raw oil. The composition of wet gas varies from oil field to oil
filed. The wet gas consists of Ethane, LPG, Pentane and heavier fractions of
hydrocarbons or a mixture of these. Atmospheric pressure gas carriers and
semi-pressurised gas carriers carry the most of the wet gas.
Ethane can only be
transported by semi-pressurised gas carriers, which have direct cascade cooling
plants and are allowed to carry cargo down to
104oC. This is because Ethane has a boiling point
at atmospheric pressure of
89oC.
This will create too high condense pressure if using water as cooling
medium. The cargo is condensed against
Freon R22 or another cooling medium with boiling point at atmospheric pressure
lower than
20oC.
Wet gas is transported from
the Persian Gulf to the East, Europe to USA and some within Europe. There is also some transport of wet gas in
the Caribbean to South America.
NGL is utilised for energy
purposes and in the petro-chemical industry.
2.4
COMPOSITION OF NATURAL GAS
2.5
LEG
LEG - Liquefied Ethylene Gas. This gas is not a natural
product, but is produced by cracked wet gas, such as, Ethane, Propane, and
Butane or from Naphtha. Ethylene has a
boiling point at atmospheric pressure of -103,8oC, and therefore has
be transported in gas carriers equipped with cargo compartment that can bear
such a low temperature. Cascade plants
are used to condense Ethylene. As
critical temperature of Ethylene is 9,7oC one can not utilise water
to condense Ethylene. The definition of
Ethylene tankers is LPG/LEG carrier.
Ethylene is very flammable
and has a flammable limit from 2,5% to 34% by volume mixed with air. There are
stringent demands regarding the oxygen content in Ethylene. The volume of
ethylene must be less than 2% in the gas mixture to keep the mixture below the
LEL “lower explosion limit". Normally, there are demands for less than 0,2%
oxygen in the gas mixture in order to prevent pollution of the cargo.
Ethylene is utilised as raw
material for plastic and synthetic fibres.
Ethylene is transported from
the Persian Gulf to the East, the Mediterranean to the East and Europe, the
Caribbean to South America. There is also transport of Ethylene between the
countries Malaysia, Indonesia and Korea
2.6
AMMONIA NH3
The next gas we
will focus on is Ammonia, which is produced by combustion of hydrogen and
nitrogen under large pressure. Ammonia is a poisonous and irritating gas, it
has TLV of 25 ppm and the odour threshold is on 20 ppm. It responds to water
and there are special rules for vessels that transport Ammonia. We can locate the rules in the IMO Gas Code,
chapters 14, 17 and 19.
When ammonia gas is mixed
with water, a decreased pressure is formed by 1 volume part water absorbing 200
volume parts ammonia vapour. A decreased tank pressure will occur if there is
water in the tank when commence loading ammonia and the tank hatch is closed.
With an open hatch, we can replace the volume, originally taken up by the
ammonia gas, with air.
One must not mix ammonia
with alloys: copper, aluminium, zinc, nor galvanised surfaces. Inert gas that
contains carbon dioxide must not be used to purge ammonia, as these results in
an carbamate formation with the ammonia. Ammonium carbamate is a powder and can
blockage lines, valves and other equipment.
The boiling point for
ammonia at atmospheric pressure is
33oC, and must be transported at a temperature
colder than
20oC. One can cool ammonia with all types of cargo cooling plants.
Ammonia is transported with atmospheric pressure gas carriers or
semi-pressurised gas carriers. Gas carriers carrying Ammonia must be
constructed and certified in accordance with IMOÅ‚s IGC code for transportation
of liquefied gases. The definition for ammonia tanker is LPG/NH, carrier.
Ammonia is utilised as raw
material for the fertiliser industry, plastic, explosives, colours and
detergents.
There is a lot of
transportation from the Black Sea to USA, from USA to South Africa and from
Venezuela to Chile.
2.7 CHLORINE CI2
Chlorine is a very toxic gas
that can be produced by the dissolution of sodium chloride in electrolysis.
Because of the toxicity of Chlorine it is therefore transported in small
quantities, and must not be transported in a larger quantity than 1200m3.
The gas carrier carrying chlorine must be type 1G with independent type C
tanks. That means the cargo tank must, at the least, lie B/5 “Breadth/5" up to
11,5 meter from the ships side. To transport Chlorine, the requirements of IMO
IGC code, chapters 14, 17 and 19 must be fulfilled. Cooling of Chlorine requires indirect cargo cooling plants.
The difference of Chlorine
and other gases transported is that Chlorine is not flammable.
Chlorine is utilised in
producing chemicals and as bleaching agent in the cellulose industry.
2.8
CHEMICAL GASES
The chemical gases mentioned
here are the gases produced chemically and are defined in IMOÅ‚s rules as
condensed gases. Because of the gasesł boiling point at atmospheric pressure
and special requirements for temperature control, these gases must be carried
on gas carriers as specified by the IMO gas code. Condensed gases are liquids with a vapour pressure above 2,8 bars
at 37,8oC. Chemical gases
that are mostly transported are Ethylene, Propylene, butadiene and VCM. Chemical gases that have to be transported
by gas carriers are those mentioned in chapter 19 in IMO IGC code. There are,
at all times, stringent demands for low oxygen content in the cargo tank
atmosphere, often below 0,2% by volume. This involves that we have to use
nitrogen to purge out air from the cargo compartment before loading those
products.
In addition, even though the
vapour pressure does not exceed 2,8 bars at 37,8oC such as, ethylene
oxide and propylene oxide or a mixture of these, they are still in the IMO gas
code as condensed gases. Gas carriers that are allowed to transport ethylene
oxide or propylene oxide must be specially certified for this. Ethylene oxide and propylene oxide have a
boiling point at atmospheric pressure of respectively 11oC and 34oC
and are therefore difficult to transport on tankers without indirect cargo
cooling plants. Ethylene oxide and propylene oxide can not be exposed to high
temperature and can therefore not be compressed in a direct cargo cooling
plant. Ethylene oxide must be transported on gas tanker type 1G.
Chemical gases like
propylene, butadiene and VCM are transported with medium-sized atmospheric
pressure tankers from 12000 m3 to 56000 m3.
Semi-pressurised gas carriers are also used in chemical gas trade and then in
smaller quantity as from 2500 m3 to 15000 m3.
Chemical gases are
transported all over the world, and especially to the Far East where there is a
large growth in the petro-chemical industry. Chemical gases are mainly utilised
in the petro-chemical industry and rubber production.
2.9
LNG CONDSATION PLANT FLOW DIAGRAM
2.10
OIL/GAS FLOW DIAGRAM
2.11
PRODUCTION OF CHEMICAL GASES
2.12
CONNECTION TABLE
Table showing connection between cargo temperature and type
of compartment and secondary barrier requirement
Cargo
temperature at atmospheric pressure
- 10oC
and above
Below -10oC
down to
55oC
Below -55oC
Basic
tank type
No
secondary barrier required
Hull
may act as secondary barrier
Separate
secondary barrier where required
Intergral
Tank type not normally allowed
Membrane
Complete secondary barrier
Semi-membrane
Complete secondary barrier
Independent
Type A
Complete secondary barrier
Type B
Partial secondary barrier
Type C
No secondary barrier required
Internal insulation
Type 1
Complete secondary barrier
Type 2
Complete secondary barrier is incorporated
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