Diffusion of Ethylene Into and Through Wood

The passage of water and other liquids through wood has been shown to be the result of at least three different processes: 1) water may move through wood as a result of liquid flow in the coarser capillary structure; 2) bound water may move by diffusion through the cell walls; and 3) water may move by vapor diffusion through the coarser capillary voids. Since the magnitude of the individual contributions of the different types of water movement to the total passage of water has not yet been definitely established, a project was initiated to study the nature of the diffusion of ethylene, slightly water-soluble gas, through the coarse capillary structure of wood under steady-state conditions. Measurements were made of the diffusion of ethylene gas in nitrogen through thin specimens of eastern hemlock in the tangential and radial directions and through thicker-than-fiber-length specimens in the fiber direction at temperatures ranging from 5? to 70?C under conditions of constant total pressure across the specimens. This diffusion was confined to passage through fiber cavities, ray cells, pit chambers, and pit membrane pores in series and parallel combinations. Longitudinal diffusion coefficients were about 100 times the transverse value. Within the range of accuracy of the measurements, the diffusion coefficients are independent of the thickness of the specimens. The amount of summerwood in a specimen, especially that on the entrance faces of radial diffusion specimens, seems to decrease the diffusion coefficient more than would be expected from the increase in specific gravity. The experimental diffusion coefficients are appreciably less than the theoretically calculated values on the basis of diffusion through various structures in series and in parallel being combined as in electrical conduction. This agrees with previous findings for the diffusion of carbon dioxide through Sitka spruce. The temperature coefficients for the diffusion through wood are quite constant between 0? and 50?C. They decrease appreciably, however, with a further increase in temperature.

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