Encyclopedia of Renewable Energy. James G. Speight

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СКАЧАТЬ (sometimes referred to as paraffins or paraffin hydrocarbons) are aliphatic hydrocarbons (non-aromatic hydrocarbon derivatives) that contain carbon and hydrogen only in which all of the binding orbitals of the carbon atoms are satisfied by bonding to another carbon atom or to a hydrogen atom.

Normal alkanes (straight-chain paraffins) consist of a chain of carbon atoms. Each carbon atom is linked to four atoms, which can be either carbon or hydrogen, their general formula C_nH_2n+2 (Table A-16). The carbon skeleton can be structured as straight chains as are the normal paraffin. CH3(CH₂)_nCH₃. The boiling points increase with the number of carbon atoms. With the low carbon numbers, the addition of a carbon increases the boiling point by approximately 25°C (77°F). Further additions give smaller increase. At the same time, the density increases with the molecular weight 0.626 kg/L for pentane, and 0.791 kg/L for pentacosane; on the other hand, the density is always much lower than 1. The normal alkanes from C₁ to C₄ are colorless gases; C₅ to C₁₇ colorless liquids; and from C₁₈ onwards, colorless solids. Other physical properties, such as melting point, density, and viscosity, also increase in the same way as boiling point. There is a relationship between physical properties and chemical composition. The variation in the boiling point of compounds is due to different intermolecular forces such as hydrogen bonding. The alkanes are insoluble in water.

Table A-16 Physical properties of n-paraffins.

Alkane	Melting point, °C	Boiling point°C	Density, g/ml @20°C
Methane	-183	-162
Ethane	-172	-88.5
Propane	-167	-42
Butane	-138	0
Pentane	-130	36	0.626
Hexane	-95	69	0.659
Heptane	-90	98	0.684
Octane	-57	126	0.703
Nonane	-54	151	0.718
Decane	-30	174	0.730
Undecane	-26	196	0.740
Dodecane	-10	216	0.749
Tridecane	-6	234	0.757
Tetradecane	5.5	252	0.764
Pentadecane	10	266	0.769
Hexadecane	18	280	0.775
Heptadecane	22	292
Octadecane	28	308
Nonadecane	32	320

Isoparaffins are paraffins in which branching is present, usually at the number 2 carbon atom, although branching can take place at a different position in the chain, although such molecules are not strictly isoparaffins. Isoparaffins have a boiling point lower than normal paraffin with same number of carbon atoms, and generally, the greater branching has the lower boiling point (Table A-17).

Table A-17 Physical properties of selected branched paraffins.

Paraffins	Melting point, °C	Boiling point, °C	Density, g/ml @20 °C
Isobutane	-159	-12
Isopentane	-160	28	0.620
Neopentane	-17	9.5
Isohexane	-154	60	0.654
3-Methylbutane	-118	63	0.676
2,2-Dimethylbutane	-98	50	0.649
2,3-Dimethylbutane	-129	58	0.668

      The octane number is a measure of the ability of a fuel (gasoline) to avoid knocking. The test engine is adjusted to give knock from the fuel rated. Then, various mixtures of isooctane (2,2,4-trimethylpentane) and n-heptane are used to find the ratio of the two reference fuels that will give the same intensity knock as that from unknown fuel. Defining isooctane as 100 octane number and n-heptane as 0, the octane number is the volumetric percentage of isooctane in heptane that matches knock from the unknown fuel is reported СКАЧАТЬ