AN EXPERIMENTAL INVESTIGATION OF AIR VENTING OF VOLATILE LIQUID-HYDROCARBON MIXTURES FROM HOMOGENEOUS AND HETEROGENEOUS POROUS-MEDIA

Experimental results are reported which delineate the vaporization modes of single and binary hydrocarbon liquids held in homogeneous and heterogeneous porous media during air venting. The experiments were performed in a two-dimensional apparatus which allowed visual observation of the evolving geometry of the liquid contaminant pool under various conditions. In homogeneous sand packings, the contaminant pool receded primarily from the upstream edge to the downstream edge during venting. In heterogeneous (dual permeability) sand packs, the permeability ratio of the high- to low-permeability zones was found to be an important factor on the geometric evolution of the contaminant pool. Different permeability ratios were investigated for a high-permeability layer above a contaminant pool trapped in a low-permeability layer. For permeability ratios below 10:1, the pool geometry evolved in a similar manner as in the homogeneous sand pack because a significant amount of airflow passed through the contaminant in the low-permeability zone. For ratios greater than 100:1 the pool receded downward from the interface of the two layers into the low-permeability zone, and the venting became limited by diffusion of the contaminant vapors from the low-permeability zone to the air flowing in the high-permeability zone. During venting of binary mixtures from dual-permeability sand packs with large permeability ratios, the mass fractions of the two components in the vapor-phase exhibited three distinct stages: (1) an early, rapid decrease in the mass fraction ratio of the more volatile to less volatile compound due to a depletion of the more volatile component at the evaporating surface in the liquid; (2) a period of constant mass fraction ratio resulting from a quasi-steady-state evaporation of the liquid pool which receded into the low-permeability zone; and (3) a long-term gradual decrease in the mass fraction ratio of the more volatile to less volatile compound caused by the diminishing size of the contaminant pool.