Mass transfer for dissolving solids in supercritical carbon dioxide Part II: Resistance in the porous layer

The internal mass transfer resistance was measured by dissolving naphthalene from a porous insoluble rod in a stream of supercritical carbon dioxide. Initially the pores of the porous medium (sintered bronze) were filled with naphthalene. During dissolution the naphthalene surface retreats into the porous rod and the length of the diffusion path increases. The tortuosity factor for the porous material was determined from experimental data. The temperature was varied from 35 to 55 °C, the pressure from 118 to 226 bar and the mean pore diameter was either 8 or 20 μm. The results show that the tortuosity factor does not depend on the pressure and pore diameter. This result was to be expected since the mean free path of the molecules is much smaller than the pore diameter. The average tortuosity factor, however, decreased with increase of temperature. From this temperature dependence surface diffusion of the solute absorbed on the pore wall is concluded to take place and thereby to contribute to the observed overall rate of mass transfer.