Diffusion Coefficients in Hydrocarbon Systems. Homogeneous Phases at Elevated Pressures

Abstract

The phenomenon of change in pressure accompanying gaseous diffusion in a closed isochoric, isothermal system has been studied experimentally and theoretically. Special equipment has been developed for measuring pressure differences with an accuracy better than 0.01 p.s.i. at total pressure levels of hundreds of pounds per square inch. The theoretical analysis shows that as the system approaches equilibrium, the pressure as a function of time assumes the simple exponential form p - p_(eq) = -ae^(-bD_(12)t, where p_(eq) is the equilibrium pressure, D_(12) is the binary diffusion coefficient, a is a thermodynamic quantity, and b is a constant depending on the size and shape of the diffusion cell. This expression can be used for the determination of D_(12) from pressure-time measurements. Measurements upon the system CH_4-C_3H_8 at T = 160° F. and p_(eq) = 570 p.s.i. have shown the applicability of this method, but more experimental work is required for ascertaining its accuracy.