Use of absorbents in natural gas analysis

• Main Author: Campbell, John Stuart
• Format: Others
• Published: 1927
• Online Access: http://thesis.library.caltech.edu/229/1/Pomeroy_r_1927.pdf; Campbell, John Stuart (1927) Use of absorbents in natural gas analysis. Master's thesis, California Institute of Technology. http://resolver.caltech.edu/CaltechETD:etd-01192005-154120 <http://resolver.caltech.edu/CaltechETD:etd-01192005-154120>

The separation of the members of the paraffin series is the analysis of natural gas presents many difficulties. Chemical methods are of negligible value because of the close series relationship of the compounds. Combustion methods are fairly satisfactory for determining a mixture of two hydrocarbons, and theoretically can be used to determine a ternary mixture, but in the latter case it is impossible to attain an accuracy of any useful significance. Fractional combustion is of limited value: it permits the determination of H2, CH4, and C2H6, but cannot separate the higher hydrocarbons. The use of physical properties, such as density, viscosity, and optical constants, is also limited to the determination of a mixture of two gases. The only methods of analysis in this field capable of making comprehensive and accurate determinations have depended upon some physical method of isolating the constituents and measuring them separately. Fractional distillations making use of the differences in boiling point between the members of the paraffin series require the use of temperatures low enough to liquefy the compounds to be separated. This suggests the advantageous procedure of employing temperatures at which the vapor pressures in question are very small, since the ratio between the vapor pressures of the successive paraffins increases as the pressures are reduced, increasing the degree of fractionation possible. In this manner Lebeau and Damiens fractionated natural gas into groups of two compounds, using single distillations. Later Burrell employed low temperatures obtained with liquid air to make analytical separations of natural gas into constituents as high as butane, making multiple distillations with recombination of fractions. His method was very slow, requiring as much as a week of pumping with a Toepler pump. Shepherd and Porter improved Burrell’s procedure in point of speed, reducing the pumping necessary by making distillations between three bulbs in series. Through the use of an evacuated apparatus entirely of glass with mercury filled gas lines they were able to separate natural gas with three distillations into (CH4, N2, H), C2H6, C3H6, C3H8, C4H10 and higher fraction to an accuracy of approximately 0.1%. Shepherd and Porter’s procedure would leave little to be desired had it not three separate defects from the standpoint of practical application. It requires an accurate control of rather widely separated temperatures in the neighborhood of the boiling point of liquid air. It moreover requires extreme care in manipulation and in making cuts to secure the accuracy stated above. Finally, the separation extends only to butane, whereas in the very important application of such analysis to testing natural gas for recoverable gasoline the percentages of the still heavier compounds are of great interest. A.J.L. Hutchinson, working in this laboratory in 1925, applied the effect of selective adsorption to the problem of natural gas analysis. One of the most noticeable characteristics of the adsorption of gases on solid adsorbents is the fact that different gases are adsorbed to different extents. In general, and with the major exception of hydrogen, a gas is the more strongly bound to the adsorbent the higher its boiling point. The relation between degree of adsorption and critical temperature is even more exact and of deeper theoretical significance. Thus the paraffin hydrocarbons are adsorbed in the order of their boiling points, which increase with the molecular weight as the series is ascended; in other words, at a given equilibrium pressure and temperature a greater quantity will be retained in the adsorbed state in the case of a heavier paraffin compound than of a lighter one. Hutchinson proceeded to test the separations attainable by distillation from activated charcoal at each of an ascending series of temperatures. A sample of natural gas was adsorbed on charcoals and put into communication with a bulb surrounded with liquid air for a period of several hours. The distillate corresponding to each temperature of the charcoal was separately measured and analysed by combustion. On the basis of these experiments Hutchinson found a series of temperatures at each of which the lightest compound adsorbed on the charcoal, the still lighter compounds having been already removed, came over completely, leaving the heavier compounds in their entirety on the charcoal. At least this was the indication of his combustion analysis of the fractions. The conclusion drawn was that at the selected temperatures the ratio between the partial pressure of the lightest member present and that of the next higher is so large that the pressure of the former can become high enough to produce flow away from the charcoal without danger of appreciable contamination with the more strongly adsorbed compounds. The analytic procedure employing these empirically determined temperatures was briefly as follows: O2 and CO2 were removed and measured by means of KOH and pyrogallol absorption pipettes. The sample was then liquefied at the temperature of liquid air and methane pumped off with a Toepler pump. The remainder was adsorbed on charcoal at liquid air temperature and the succeeding cuts made as described above, starting with the ethane distillation at 45° and ending with pentane at 230°. Higher temperatures caused cracking, so that compounds heavier than pentane could not be drawn off intact but were estimated by subtraction. The accuracy of the method of combustion analysis to determine the purity of the fractions taken off is rather questionable, and inasmuch as Hutchinson did not work with pure hydrocarbons or known mixtures, there is some uncertainty as to just how accurate the method is. The research on the Standard Oil Co. Fellowship at the California Institute of Technology during the past year has been directed toward a more thorough investigation of the method of analysis and toward an improvement of the apparatus and procedure. In particular, a study has been made of the properties of silica gel with a view to substituting it for charcoal as the adsorbent.