Diffusion in the gas phase: The effects of ambient pressure and gas composition

Abstract

Gas transport across the pores of a hen's egg shell occurs by a process of diffusion in the gas phase and for any particular gas depends upon its diffusion coefficient and the pore geometry. The egg shell is thus a convenient model for measuring the diffusive permeability of the shell to a given gas species when its diffusion coefficient is altered by either a change in ambient pressure or by changing the second gas in the diffusion pathway. In this study the permeability of the shell to water vapor and O 2 was inversely proportional to ambient pressures over the range of 06 to 8 atmospheres- absolute (ata). The permeability of the shell to water vapor in a He environment (K H 2 O , He) was 2.4 times K H 2 O , air. If K O 2 , N 2 ls taken as unity, the permeabilities of the shell to O 2 in He, Ar, CO, and Sf(, are 3.38, 0.95, 0.88, and 0.52, respectively. The results are interpreted in terms of the Chapman-Enskog equation, from which binary diffusion coefficients can be predicted for given gas pairs and ambient pressures. These results also provide possible explanations for the structural modification of egg shells in altitude-adapted chickens, and for the reduced insensible water loss in man at high ambient pressure.