Experimental measurements of water content of methane gas in equilibrium with hydrate are presented at 1000 and 1500 psia for temperatures greater than -10 OF. A new chromatographic technique for determining water concentration of gas in equilibrium with hydrate was used to eliminate some of the errors inherent in previous investigations. A method is suggested for calculating the water content of methane in equilibrium with its hydrate above 32 O F at any pressure up to 1500 psia. The experimental data indicate that methane should be significantly drier than indicated by existing water content charts in order to prevent hydrate formation. The data reported herein should be strictly interpreted as applying only to methane, rather than natural gases, due to the fact that methane forms a different hydrate structure from that normally formed by natural gases.Empirical relationships from coexistence data form the basis for a description of the vapor-liquid critical region for pure fluids which is independent of but generally consistent with the scaling hypothesis. These relationships are: rectilinearity with temperature for mean density and for mean isochoric slope (dPld nP. issuing from the coexistence curve; and power law behavior for the vapor-liquid differences of density, enthalpy, and (d f / d 7&.The present description displays excellent agreement with data mapping divergences for various thermodynamic properties at the critical point. With one notable exception, this description also agrees with the theoretical predictions of the scaling hypothesis. The exception is: the present description produces 8 = 1 -2 p as a lower bound which is somewhat larger than the scaling hypothesis assertion that 8 = a. Unfortunately, the data cannot distinguish between these results. Another interesting result is that the present description correctly predicts maxima with temperature for both mean enthalpy and mean entropy near the critical point.

The self-diffusion coefficient of methane has been measured from 150" to 350°K. and from 200 to 5,000 tb./sq. in. abs. A t constant temperature, the density-diffusivity product is constant up to neighborhood of critical density and decreases sharply above that density. The temperature dependence of tke low density data agrees with Chapman-Enskog theory. The LennardJones (6-12) parameters determined from the low density data ( e / k = 130"K., u = 3.85A.) are in good agreement with those determined by other methods. A correlation for the sewdiffusion coefficient for methane has been developed which may provide predictions of other spherical nonpolar gores.The coefficient of self-diffusion was considered (9) to be the limiting case of the binary diffusion coefficient until Carr and Purcell (2) developed the pulsed (or spin echo) nuclear magnetic resonance (NMR) technique to measure self-diffusion. Directly, the study of this significant transport property, self-diffusion, has led to interesting results.The self-diff usion coefficient has considerable theoretical interest. I t is the transport coefficient most easily calculated by kinetic theory, and it provides a good check for the vnrious theories of the dense gas state. The possibility exists ( I ) that binary (mutual) diffusion coefficients may be calculated from the self-diffusion coefficients of the two pure components. The measurement of the self-diffusion coefficient of methane in the dilute and dense gas regions is reported here. T H E O R Y Theory o f Self-DiffusionThe molecules in a fluid are continuously in motion. In a uniform homogeneous fluid, this motion is random and represents the thermal energy present in all fluids. This random motion causes any particular molecule of the fluid to move from place to place in the fluid; this movement is called setf-digusion.The coefficient of self-diffusion is defined aswhere is the mean square net distance a molecule moves in time t. This definition is consistent with the Ficks law definition of a binary diffusion coefficient, 0 1 2 :On this basis D is defined as the limit of D12 as the molecules of 1 and 2 become identical.In the past, the closest approach to the limit of identical molecules was obtained by observing diffusion of isotopes.For example, C14 methane may be observed diffusing in C12 methane (20). The radioactivity of the former and the nuclear stability of the latter serves to distinguish the two and allows the motion of particular methane molecules to be observed. The change in the diffusion coefficient, caused by the difference in molecular weight of the two species, is small but not negligible. Spin echo NMR fol- Rapier Dawson is with Esso Production Research Company, Houston, Texas.lows the molecules by observing their nuclear spin states; this is a much more subtle "tag." Spin Echo N M RThe basic method of generating nuclear spin echos was discovered by Hahn ( 7 ) , who also suggested their use for measuring self-diffusion coefficients. His suggestions were clarified and the experimental me...

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Second virial coefficient data of ethane, hydrogen sulfide, and four mixtures of the two components at approximately equally spaced concentrations and at temperatures between 50 and 125°C were used for determining interaction second virial coefficients, evaluating the intermolecular force constants, and calculating the system second virial coefficients from the Lennard-Jones (6–12) and Stockmayer potentials. From second virial coefficient data between 25 and 200°C, the ethane force constants for the Lennard-Jones (6–12) potential were determined as ε / k = 219°K and σ = 4.59 Å, and the hydrogen sulfide force constants for the Stockmayer potential as ε / k = 194.5°K, σ = 5.15 Å, and t* = 0.1. Using the respective intermolecular potentials and force constants, it was found that the calculated second virial coefficients of ethane and hydrogen sulfide agreed with experimental data within 1% for the former and 2.5 to 4% for the latter. Because of the polarity of hydrogen sulfide, interaction second virial coefficients were calculated from the Lennard-Jones (6–12) potential using force constants obtained from modified mixing rules to account for the dipole–induced dipole interactions. The calculated interaction second virial coefficients differed from experimentally determined values by an average absolute deviation of 1.9% in the temperature range between 50 and 125°C. Experimental and calculated mixture second virial coefficients in the same region agreed within 2%.

Self-diffusion coefficients were measured by the pulsed nuclear magnetic resonance method for pure carbon tetrafluoride for the conditions −30 to 75°C and 20–440 atm. As predicted by the dilute gas theory, the density—diffusivity product (ρD) versus density (ρ) at constant temperature was found to be almost constant up to a reduced density of about 0.4, above which ρD started decreasing sharply. Based on the corresponding states principle, three semiempirical representations for the data were developed and compared. The low-density data were used, along with previously published second virial coefficient data, to evaluate intermolecular force constants for the Lennard-Jones (6–12) potential and the modified Buckingham (6-exp) potential.