A mechanistic communicating channel bed model is developed to describe the miscible displacement of a model black liquor (MBL) from a model channel bed with either water or a polymer solution. The channel bed consists of a 1.7 cm diameter central channel of coarse beads (638 mu m) surrounded by an annulus of fine beads (121 mu m), contained in a 5.2 cm diameter cylindrical column. The communicating channel bed model accounts for the communication between the channel and annulus in the model channel bed using a technique called network of zones. The model invokes two adjustable parameters to describe the displacement with water and three adjustable parameters for displacement with the polymer solution in the channel bed. The experimental breakthrough curves and profiles of interstitial velocities in the channel bed during displacement of the MBL with either water or the polymer solution are fitted well by the model. Although the model is capable of fitting the pressure drop profiles during displacement of the MBL with water, the pressure drop calculated by the model is higher than that obtained experimentally during displacement of the MBL with the polymer solution in the channel bed. The mismatch between the model and experimental pressure drops for displacement of the MBL with the polymer is explained in terms of the limitations of the model. The communicating channel bed model is capable of fitting the improved displacement of M BL in the channel bed when the polymer solution is used as the displacing phase instead of water. The improvement in the displacement of MBL with the polymer solution is due to plugging of pore throats with precipitate formed by the reaction of lignin and the polymer and the reduction in the permeability of the center channel, resulting in the suppression of channeling in the channel bed. In the absence of communication between the channel and annulus in the channel bed, a model is not capable of fitting the results of displacement of MBL with water or the polymer solution.