Hydrates are akin to ice in which a molecule of gas is encaged by molecules of water. These cages group together and quickly form solid plugs. The hydrates form at fairly high pressures and low temperatures but free water must always be present to enable the hydrates to form.
Hydrates may create solid plugs in pipes, preventing the circulation of gas, which is why before gas is transported though a pipe it is dehydrated to ensure that in the worst case any water which may remain dissolved in the gas is not transformed into free water. The precautions to be taken (level of dehydration, P, T) can be obtained by calculations or from curves.
Hydrates also exist in nature and form pockets of gas in the form of ice which compose a wonderful source of energy, equivalent to twice the methane from identified reserves of coal, oil and gas combined.
7.13. DEW POINT
(for H2O and HC).All gases are products formed from a group of components, particularly different types of hydrocarbons and water (in gaseous form).
The dew point (for a given pressure) is the temperature above which the component passes from the gaseous state to the liquid state.
Therefore for water the dew point of a given gas at a given pressure is the temperature above which water in gaseous form liquefies. The same applies to gaseous hydrocarbons (HC dew point).
7.14. WATER CONTENT
All gases contain water in their gaseous form. The water content is the quantity of
water/value of gas generally expressed in Kg/Sm3. This value depends on the pressure of the gas concerned and its temperature.
The water content which is obtained from curves can be used to determine the quantity of water which must be removed from the gas by the drying process.
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It should be noted that:
For a given gas pressure, the lower the temperature of the gas, the lower the water content
For a given temperature, the higher the gas pressure, the higher the water content.
7.15. WOBBE INDEX
This is the highest heating value of a gas divided by the square root of its density; it is a parameter used to compare the combustion energy of a gas.
Two gases with the same index are interchangeable (from the point of view of the energy supplied) without altering the combustion system.
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8. EXERCISES
1. The 3 molecules below have the same empirical formula C6H14, which ones have
the same carbon chain?
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3. Name the following molecule:
4. Name the following molecule:
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10. LIST OF FIGURES
Figure 1: Propane ...5 Figure 2: Glycol...5 Figure 3: Ethylamine ...5 Figure 4: Methane CH4...9 Figure 5: Ethane C2H6...9 Figure 6: Stereoisomers...13 Figure 7: Iso-alkanes ...14Figure 8: Methane – CH4 (gas) ...16
Figure 9: Ethane – C2H6 (gas) ...16
Figure 10: Propane – C3H8 (gas) ...16
Figure 11: Butane – C4H10 (gas) ...16
Figure 12: Pentane – C5H12 (liquid) ...16
Figure 13: n-butane – C4H10...17
Figure 14: 2-methyl-propane(isobutane) – C4H10...17
Figure 15: Alkane: Ethane – C2H6...18
Figure 16: Alkene: Ethene – C2H4...18
Figure 17: Alkenes ...19
Figure 18: The alkynes ...20
Figure 19: Cyclanes ...21
Figure 20:Phenyl Radical C6H5...22
Figure 21:Benzene C6H6...22
Figure 22: Composition of a crude oil and raw natural gas ...23
Figure 23: The distillation cuts ...26
Figure 24: Characteristics of a number of natural gas deposits ...28
Figure 25: Gas processing...30
Figure 26: Gas treatment ...31
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11. LIST OF TABLES
Table 1: The most important hydrocarbons ...6
Table 2: Number of alkane isomers ...12
Table 3: The first 5 alkyl radicals ...13
Table 4: The linear alkanes...15
Table 5: Linear saturated hydrocarbons or Alkanes ...16
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12. EXERCISE SOLUTIONS
1. The 3 molecules below have the same empirical formula C6H14, which ones have
the same carbon chain?
There is no difference between molecules A and B; they both contain a (linear) chain of 5 carbon atoms with a CH3 branch on the 2nd carbon atom.
However, molecule C only contains one (linear) chain of 4 carbon atoms and 2 CH3
branches, therefore C is different from A and B.
2. Determine the number of constitutional isomers of hexane C6H14
There are 5 constitutional isomers: the simple chain which defines the “normal” isomer:
There are 2 isomers with a single carbon branch:
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3. Name the following molecule:
This molecule is derived from pentane as its longest chain contains 5 carbon atoms – there are 3 identical alkyl radicals (CH3 – methyl)- 2 of these radicals are in position 2 and one radical is on the 4th carbon.
This molecule is therefore 2,2,4-trimethylpentane. – but since it contains 8 carbon atoms and its formula is C8H18 it is isooctane.
4. Name the following molecule:
This molecule is derived from heptane as its longest chain contains 7 carbon atoms.- there are 2 alkyl radicals, one CH3 (methyl) on the carbon 2 and one CH2- CH3 (ethyl) on the carbon 4 This molecule is therefore 4-ethyl-2-methylheptane. – but since it contains 10 carbon atoms and its formula is C10H22 it is isodecane.
5. Name the following molecule:
This molecule is derived from hexane as its longest carbon chain contains six carbon atoms. It also contains a double carbon bond C=C, this molecule therefore belongs to the hexene family. It possesses 2 alkyl radicals(CH3 methyl) which are bonded to the carbons in positions 3 and 5 – this molecule is dimethyl-2-hexene. It contains 8 carbon atoms and its formula is C8H16. It is not the octane isomer as the empirical formula of octane is C8H18.