Properties of Liquefied Gases
This chapter deals with the basic physics and chemistry of liquefied gases. The text then discusses the theory of ideal gases and continues into a description of refrigeration and its application at sea. Certain sections explain particular problems encountered such as hydrate formation, polymerisation and stress corrosion cracking. Many of these particular issues are more fully appraised in other publications and reference can be made to Appendix 1 for further details.
2.1 CHEMICAL STRUCTURE OF GASES
Chemical compounds with the same chemical structure are often known by different names. An alternative name given to the same compound is called a synonym. Table 2.1 gives a list of the synonyms of the main liquefied gases against each common name and its simple formula. The more complex compounds tend to have a larger number of synonyms than the simple compounds.
The simple chemical formula, as shown in Table 2.1, gives the ratio of atoms of each element in the compound. Since a molecule is the smallest part of the compound which exhibits all the chemical properties of that specific material, this formula is often referred to as the molecular formula.
Hydrocarbons are substances whose molecules contain only hydrogen and carbon atoms. The molecules can be in various arrangements and the products may be gases, liquids or solids at ambient temperatures and pressures, depending upon the number of the carbon atoms in the molecular structure. Generally, those hydrocarbons with up to four carbon atoms are gaseous at ambient conditions and comprise the hydrocarbon liquefied gases. Hydrocarbons with five up to about twenty carbon atoms are liquid at ambient conditions and those with more carbon atoms are solid. The carbon atom has four bonds which can unite with other carbon atoms or with atoms of other elements. A hydrogen atom, however, has only one bond and can unite with only one other atom. Where the relative numbers of carbon and hydrogen atoms in a hydrocarbon molecule permit the carbon atoms to use their bonds singly to other carbon atoms, the molecule is said to be saturated. Figure 2.1 illustrates the saturated molecular structure of iso-butane (i-butane) and normal butane (n-butane). Examination of these examples shows that, for saturated hydrocarbons, the proportion of carbon and hydrogen atoms in the molecule is in accordance with the formula CnH2n+2. Thus, methane (CH4), ethane (C2H6), and propane (C3H8) are all saturated hydrocarbons.
Where there is less than the full complement of hydrogen atoms, as given by the above formula, two or more carbon atoms become inter-linked by double or triple bonds. For
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Table 2.1 Synonyms for the main liquefied gases
| Common Name | Simple Formula | Synonyms |
| Methane | ch4 | Fire damp; marsh gas; natural gas; LNG |
| Ethane | C2H6 | Bimethyl; dimethyl; methyl methane |
| Propane | C3H8 | - |
| n-Butane | C4H10 | Normal-butane |
| i-Butane | C4H10 | Iso-butane; 2-methlypropane |
| Ethylene | C2H4 | Ethene |
| Propylene | C3H6 | Propene |
| a-Butylene | C4H8 | But-1 -ene; ethyl ethylene |
| b-Butylene | C4H8 | But-2-ene; dimethyl ethylene; pseudo butylene |
| y-Butylene | C4H8 | Isobutene; 2-methylprop-2-ene |
| Butadiene | C4H6 | b.d.; bivinyl; 1,3 butadiene; butadiene 1-3; divinyl; biethylene; erythrene; vinyl ethylene |
| Isoprene | C5H8 | 3-methyl -1,3 butadiene; 2-methyl -1,3 butadiene; 2-methylbutadiene -1,3 |
| Vinyl chloride | c2h3ci | Chloroethene; chloroethylene; VCM; Vinyl chloride monomer |
| Ethylene oxide | c2h4o | Dimethylene oxide; EO; 1,2 epoxyethane; oxirane |
| Propylene oxide | C3H6O | 1,2 epoxy propane; methyl oxirane; propene oxide |
| Ammonia | NN3 | Anhydrous ammonia; ammonia gas; liquefied ammonia; liquid ammonia |
Note: Commercial propane contains some butane; similarly, commercial butane contains some propane. Both may contain impurities such as ethane and pentane, depending on their permitted commercial specification. Some further data on mixtures is given in 2.17 and 2.18.
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