in order to understand the increase in pressure drop in hydrotreating reactors due to deposition of fine solids, experiments. were conducted with a model suspension of kaolin clay in kerosene. The suspension was circulated through packed beds of catalyst pellets in the trickle-flow and pulse-flow regimes, and the increase in pressure drop measured as a, function of particle concentration in the bed. The increase in pressure drop was linear with particle concentrations over the range 0-60 kg(.)m(-3). A consistent approach to modeling the pressure drop behavior was to determine an effective porosity of the packed bed a's a function of the concentration of fine particles, then. use this porosity in the Ergun equation as a basis for calculating the two-phase pressure drop.