PETRoLEum gases consist essentially of mixtures of gaseous hydrocarbons together with varying proportions of non-hydrocarbon gases, including carbon dioxide, nitrogen, etc. Gases from cracking operations differ from natural gas in their content of unsaturated hydrocarbons and hydrogen. Accurate and expeditious methods for the quantitative examination of hydrocarbon gas mixtures have only become available during the last few years. Ordinary methods of gas analysis, such as those used for. coal gas, etc., are inadequate for the complete examination of petroleum gases, though they are often valuable when used in conjunction with some physical method of partial or complete segregation of the various individual hydrocarbons present. Methods of separation of hydrocarbon gases by a series of simple distillations or fractional condensations at low temperatures have been described, but the procedure is too tedious for most purposes. The greatest advance in the technique of the analysis of hydrocarbon gases is due to the successful development of efficient fractionating columns, capable of being used at low temperatures. Several fractionating columns are described, notably those of Podbielniak and MacGillivray, and the details of their application briefly outlined. The main' disadvantage of the low-temperature fractionation method of analysis is that considerable quantities of liquid air, or liquid nitrogen, are required for each analysis, while the technique is. somewhat complicated. Various attempts to simplify the procedure and to employ solid carbon dioxide as refrigerant instead of the more expensive liquid air are discussed.
The analytical segregation of hydrocarbons in gas mixtures has been accomplished by Van Dyck by taking advantage of the varying solubilities of the hydrocarbon constituents in a suitable solvent. This method possesses the advantage that no refrigerant is required and the construction of the apparatus is simple, though rigid adherence to specified dimensions is said to be essential.
The analysis of the unsaturated hydrocarbons of cracking gas is of special interest in view of the increasing importance of these materials as chemical raw materials. Methods of analysis based on preferential absorption in sulphuric acid of graded concentrations allow of the separation of the unsaturated gases into three main groups, namely, (i) tertiary olefmes such as isobutylene, (ii) propylene, n-butenes, butadiene and similarly reactive ole fines and (iii) ethylene. The determination of bromine addition values, or the quantitative examination of bromine addition compounds of unsaturated hydrocarbons, yields valuable information as to the nature of such hydrocarbons present. Special methods for the quantitative determination of individual Cy unsaturated hydrocarbons in mixtures of these materials are briefly summarised.
Natural gas, which is found in very large qu