Читать книгу Encyclopedia of Renewable Energy - James Speight G., James G. Speight - Страница 76

Alicyclic hydrocarbons contain cyclic structures in all or part of the chemical skeleton. The saturated alicyclic hydrocarbons have the general formula C_nH_2n. When the molecular formula of a saturated hydrocarbon corresponds to the general formula C_nH_2n-2, then the compound contains two rings; if the formula corresponds to C_nH_2n-4, it contains three rings, etc. Their boiling points and densities are higher than alkanes having the same number of carbon atoms. In crude oil, the most frequently found rings are those having five or six carbon atoms. In these rings, each hydrogen atom can substituted by a paraffinic alkyl chain that is either a straight chain or branched. Monocyclic naphthenes with one ring are major constituents of the light fraction. Monocyclic naphthene of carbon numbers C₂₀, to C₃₀, with long side chains can be isolated from paraffin waxes.

Cycloalkanes (naphthenes) catalytically crack by both ring and chain rapture and yield olefins and paraffin. In the case of hydrocracking process, polycyclic hydrocarbons aromatic are partially hydrogenated. Naphthene rings in polycyclic compounds are readily removed by ring–opening followed by cracking. Single–ring naphthenes and paraffin are more resistant to cracking. It was also found that single-ring naphthenes appear to react more slowly at high conversion levels. In the catalytic reforming process, the fundamental reaction occurring is (i) dehydrogenation of six members rings naphthenes by platinum catalyst, and (ii) isomerization of alkyl cyclopentane derivatives to cyclohexane derivatives.

The stability of the cycloalkane derivatives increases up to the six member ring, then decreases from seven to eleven member ring, and from the twelve member onwards attains the stability of two six member rings (Table A-12).

Table A-12 Properties of selected alicyclic hydrocarbon derivatives.

Alicyclic hydrocarbon (naphthene)	Melting point, °C	Boiling point, °C	Density, @20 °C
Cyclopropane	-127	-33
Cyclobutane	-80	13
Cyclopentane	-94	49	0.746
Cyclohexane	6.5	81	0.778
Cycloheptane	-12	118	0.810
Cyclooctane	14	149	0.830
Methylcyclopentane	-142	72	0.749
Cis-1,2- dimethylcyclopentane	-62	99	0.772
Trans-1,2- dimethylcyclopentane	-120	92	0.750
Methylcyclohexane	-126	100	0.769