ATTENTiox has been turned, in recent years, to various possible means of utilisation of the gaseous olefines present in certain industrial gases. Gases produced simultaneously with petrol in oil-cracking operations constitute a rich source of these olefines, especially in the case of the vapour-phase processes. The gases produced in coal carbonisation also contain olefines, and methods of separation involving liquefaction and fractionation have been suggested.
The formation of liquid fuels by polymerisation is the means of utilisation of gaseous olefines under review in this paper. The olefines have a marked tendency to polymerise especially under such influences as: ultra-violet light, silent electric discharge, heat, pressure, catalysis, or combinations of these.
From free energy calculations it has been shown that olefine hydrocarbons tend to polymerise below about 425 C., above which temperature cracking takes place, whilst the isomerisation of the higher olefines into the corresponding naphthenes is possible below that temperature. The polymerisation processes are usually exothermic and are favoured by low temperatures and hence catalysts.
In general, at lower temperatures the polymerisation products of olefines include higher olefines and naphthenes, at medium temperatures higher olefines, paraffns, naphthenes and polynaphthenes, and at higher temperatures secondary reactions cause the formation of aromatic hydrocarbons.
Research upon the formation of liquid fuels by polymerisation of gaseous olefines can be divided under three headings (1) polymerisation under pressure and at moderate temperatures; (2) polymerisation under atmospheric pressure at high temperatures; (3) polymerisation under pressure in the presence of anhydrous aluminium chloride.' The conversion of gaseous olefines into liquid polymers at moderate temperatures and under pressure, without catalysts, is nearly complete, and a high proportion of the crude oil boils in the spirit range. The liquids formed under these conditions are not aromatic and they contain paraffns, naphthenes and higher olefines, such as the hexenes, in considerable quantity. The spirit produced under these conditions is about five-sixths as good as benzene in anti-knock value when rated on a volume basis.
At high temperatures, in the absence of catalysts, gaseous olefines are readily converted into aromatic hydrocarbons in quite good yields. The liquid products are similar to those obtained in the pyrolysis of paraffin gases at high temperatures, but the temperature necessary for the conversion to aromatic liquids is lower in the case of the olefine than the paraffin gases. The spirit fraction, containing benzene and other aromatic hydrocarbons including toluene, would thus be of good antiknock value.