Table 2.4 Inert gas compositions

Component	Inert Gas by combustion	Nitrogen Membrane Separating Process
Nitrogen	85 to 89%	up to 99.5%
Carbon dioxide	14%	—
Carbon monoxide	0.1% (max)	—
Oxygen	1 to 3%	>0.5%
Sulphur oxides	0.1%	—
Oxides of Nitrogen	Traces	—
Dew point	-45°C	-65°C
Ash & Soot	Present	—
Density (Air = 1.00)	1.035	0.9672

 

nitrogen can mean inert gas without carbon dioxide but with some oxygen present (as for shipboard production systems) or it can relate to the pure nitrogen used for special inerting prior to loading an oxygen critical cargo.

As mentioned above, in the past LNG ships were fitted with storage tanks for liquid nitrogen. Where the production of nitrogen is an on board feature using a membrane-type system, (see 4.7.2) storage tanks are fitted to cope with the large demand during cooling down periods. Alternatively, nitrogen can be generated on board ships by the fractional distillation of air or by pressure swing adsorption but these methods are rare.

Only nitrogen of high purity is fully compatible, in the chemical sense, with all the liquefied gases. Many components of combustion-generated inert gas can put the liquefied chemical gases off specification. In particular, as far as personal safety and chemical reactivity are concerned, the following points regarding the constituents of inert gas should be noted:

Carbon particles in the form of ash and soot can put many chemical gases off specification.

Carbon dioxide will freeze at temperatures below -55°C thus contaminating the cargo if carriage temperatures are particularly low, such as in the case of ethylene or LNG. Carbon dioxide will also contaminate ammonia cargoes by reacting to produce carbamates. Both solid carbon dioxide and carbamate formation result in cargo contamination and operational difficulties, such as clogging of pumps, filters and valves. Carbon dioxide can also act as a catalyst in complicated chemical reactions with sulphur compounds in some LPG cargoes.

Carbon monoxide, if generated in sufficient quantities, can cause difficulties during any subsequent aeration operation. When aeration is thought complete, the levels of toxic carbon monoxide may still be unacceptable from the aspect of personal safety. (It should be noted that carbon monoxide has a TLV-TWA of 50 parts per million.)

Moisture in inert gas can condense and in so doing hydrates can form in cargoes and inerted spaces can suffer from severe corrosion. When cold cargo is to be loaded, it is therefore important that the inert gas in cargo tanks has a sufficiently low dew point to avoid any water vapour freezing out and other operational difficulties. Furthermore, moisture can create difficulties particularly with butadiene, isoprene, ammonia and chlorine cargoes.

Oxygen even in the small percentages found in shipboard produced inert gas is incompatible with butadiene, isoprene, vinyl chloride and ethylene oxide. In contact with oxygen, these cargoes may combine to form peroxides and polymers.

For the foregoing reasons, only pure nitrogen taken from the shore can be considered to be fully inert, in the chemical sense, for all the liquefied gases. Nevertheless, for the inerting of hold spaces and cargo tanks on ships carrying LPG cargoes at temperatures down to about -48°C, inert gas generation by good quality fuel burning under carefully controlled combustion or by the air separation process can provide an inert gas of acceptable quality.

2.6 POLYMERISATION

While many of the liquefied gases are polymerisable (as indicated by a double bond in their molecular structure), cargo polymerisation difficulties only arise in practice in the case of butadiene, isoprene, ethylene oxide and vinyl chloride. Polymerisation may be dangerous under some circumstances, but can be delayed or controlled by the addition of inhibitors.

Polymerisation takes place when a single molecule (a monomer) reacts with another molecule of the same substance to form a dimer. This process can continue until a long-chain molecule is formed, possibly having many thousands of individual mole­cules (a polymer). The mechanism is illustrated for vinyl chloride in Figure 2.5. The process can be very rapid and involves the generation of a great deal of heat. It may be initiated spontaneously or may be catalysed by the presence of oxygen (or other impurities) or by heat transfer during cargo operations (see also 7.6). During polymerisation, the cargo becomes more viscous until, finally, a solid and unpumpable polymer may be formed.

Polymerisation may be prevented, or at least the rate of polymerisation may be re­duced, by adding a suitable inhibitor to the cargo. However, if polymerisation starts, the inhibitor will be consumed gradually until a point is reached when polymerisation may continue unchecked. In the case of butadiene, tertiary butyl catechol (TBC) is added primarily as an anti-oxidant but, in the absence of oxygen, it can also act, to a limited extent, as an inhibitor.

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