Determination of Unsaturates in Hydrocarbon Gases

Abstract

MOSTmethods currently used to determine the concentration of unsaturated compounds in gaseous hydrocarbon mixtures were introduced years ago. The procedures have been modified occasionally to increase accuracy, but some methods give results which are inherently inaccurate and are retained either because of convenience or because errors have not been recognized or investigated. Recent improvements in older refining processes and the introduction of new processes, such as polymerization, alkylation, and the production of synthetic rubber raw materials, which involve olefinic compounds, have demanded higher accuracy in the analysis for these substances.A method for determining unsaturates in gaseous hydrocarbons by catalytic hydrogenation has been described by McMillan, Cole, and Ritchie (5). It is claimed to give results accurate within a few tenths of 1 per cent•and to be as rapid as others in general use at present. The method is apparently satisfactory from the chemical standpoint: the hydrogenation is complete, free of side reactions, and sufficiently rapid for routine use, and the catalyst is easily saturated with gas and produces no interfering effects. However, the method, as originally described by McMillan, Cole and Ritchie, is based on the assumption that the lower hydrocarbons are ideal gases. Since these compounds are known to deviate considerably from the ideal gas laws, the nature of the errors introduced by the assumption was studied.In order to formulate a convenient procedure for correcting the analytical results for such deviations, it is necessary to consider the nature and sequence of the gas volume measurements made during an analysis. These are:

Determination of Ethylacetylene and Vinylacetylene in C4 Hydrocarbon Gases

“…The experimental work described in this paper is divided into four phases according tc the type of measurement being made, and the data obtained are used to calculate the percentages of ethylacetylene and vinylacetylene. The samples were synthetically blended by measuring the pressure of each gas separately in a calibrated pipet and correcting the pressures for compressibility of the gases (8). Mercury was used to transfer the gases to a receiver where they wrere blended.…”

“…The factor 1.031 corrects for the deviation of C4 hydrocarbons from the ideal gas law at standard temperature and pressure (8).…”

“…McMillan's catalytic hydrogenation method ( 5) is used to determine total unsaturation. The correction given by Robey and Morrell (8) for the nonidealistic behavior of C4 hydrocarbon gases is used with the catalytic hydrogenation method. Table III shows the results of the hydrogenation of blends containing acetylenes along with 2-butene and 1,3-butadiene.…”

Chemical Analysis of Refinery C5 Hydrocarbon Fractions

“…The blending technique chosen and the gas law cor ections involved have been described by Robey and Morrell (19).…”

“…To obtain the true mole fraction of hydrocarbon in the blend from the pressure readings it is necessary to take into account deviation from the gas laws. From approximate data (19) on pentane-hydrogen mixtures the compressibility of C6 hydrocarbon vapor is about 1.024 at 250 mm. and that of a 33% mixture of C6 in hydrogen, 1.011.…”