Figure 2.1 Molecular structure of some saturated hydrocarbons (single bonds)
Figure 2.2 Molecular structure of some unsaturated hydrocarbons (double bonds)
this reason they are called unsaturated. These links between carbon atoms are weaker than single bonds, with the result that such compounds are chemically more reactive than the single-bonded compounds.
Figure 2.2 illustrates the molecular structure of two such unsaturated hydrocarbons, propylene (C3H6), and butadiene (C4H6). Ethylene (C2H4) is a further example of an unsaturated hydrocarbon.
The third group of liquefied gases consists of the chemical gases. These are characterised by additional atoms other than carbon and hydrogen. Figure 2.3 illustrates the molecular structure of two such compounds, propylene oxide (C3H6O) and vinyl chloride (C2H3CI). Most compounds in this grouping are chemically reactive.
Figure 2.3 Molecular structure of some chemical gases
2.2 SATURATED AND UNSATURATED HYDROCARBONS
Saturated hydrocarbons
The saturated hydrocarbons, methane, ethane, propane and butane are all colourless and odourless liquids.
They are all flammable gases and will burn in air or oxygen to produce carbon dioxide and water vapour. They do not present chemical compatibility problems when in
contact with the construction materials commonly encountered in gas handling. In the presence of moisture, however, the saturated hydrocarbons may form hydrates (see 2.7).
Unsaturated hydrocarbons
The unsaturated hydrocarbons, ethylene, propylene, butylene, butadiene and isoprene are colourless liquids with a faint, sweetish odour. Like the saturated hydrocarbons they are all flammable in air or oxygen, producing carbon dioxide and water vapour. They are more reactive, from a chemical viewpoint, than the saturated hydrocarbons and may react dangerously with chlorine. Ethylene, propylene and butylene do not present chemical compatibility problems with materials of construction, whereas butadiene and isoprene, each having two pairs of double bonds, are by far the most reactive within this family. They may react with air to form unstable peroxides which tend to induce polymerisation (see 2.6). Butadiene is incompatible in the chemical sense with copper, silver, mercury, magnesium, aluminium and monel. During production, butadiene streams often contain traces of acetylene which can react with brass and copper to form explosive acetylides.
Water is soluble in butadiene, particularly at high temperatures and Figure 2.4 illustrates this effect. In this diagram the figures quoted are for the purpose of illustration only. As can be seen, on cooling water-saturated butadiene, the solubility of the water decreases and water will separate out as droplets which settle as a layer in the bottom of the tank. For instance, on cooling water-saturated butadiene from +15°C to +5°C approximately 100 parts per million of free water separates out. On this basis, for a 1,000 m3 tank, 0.1 m3 of free water would require to be drained from the bottom of the tank. On further cooling to below zero, this layer of water would increase in depth and freeze.
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