Experimental and theoretical study of oxygen diffusion within packed beds of carbon black

Modelling the performance of regenerative soot traps for Diesel exhausts re quires a knowledge of oxygen diffusivity within beds of Diesel soot. For co nvenience carbon black, a commercial product with similar properties to Die sel soot, is often used for combustion experiments. Both of these carbonace ous materials are composed of spherules of approximately 20 nm in diameter assembled within aggregates. The oxygen diffusivity within beds of carbon b lack with different densities was measured at 22 degrees C. An increase in the porosity of the bed from 0.746 to 0.820 increased the diffusion coeffic ient of oxygen from 0.7x10(-6) to 2.0x10(-6) m(2) s(-1). Two simulations of molecular movement in aggregates of spheres or 'cannonball' solids were pe rformed. The kinetic theory (KT) model assumes that the carbon spherules ar e regularly placed at the summits of cubes and a mean travel distance repla ces the mean free path. The Monte Carlo (MC) model is based on a random wal k among spherules placed randomly but homogeneously. The diffusivity values returned by the two models are strongly dependent on the tortuosity of the bed. The KT model returned oxygen diffusivities that were similar to those measured while the MC model gave values which were higher. When the MC val ues were modified to allow for non-homogeneous packing, i.e., uneven distri bution of density, the results were improved. The concept of tortuosity, wh ich is based on interconnected cylindrical pores, seems unrealistic in the case of beds of highly porous cannonball materials. Beds of Diesel soot are so open that Fickian diffusion of oxygen should occur, whereas combined Fi ckian and Knudsen diffusion seems to operate within the beds of carbon blac k. The mechanisms are uncertain since Knudsen diffusion within aggregates o f carbon black or Diesel soot probably controls the overall diffusion proce ss. (C) 2000 Elsevier Science Ltd. All rights reserved.