An unsteady 2D model of mass transport in the powder charge accounting for evolution of the powder source during long-term growth of bulk SiC crystals is suggested. The model operates with such powder parameters as the local porosity, granule size and graphitization degree. In accordance with observations, intensive SiC evaporation accompanied by graphitization in the hot zones along crucible walls is predicted, as well as recrystallization on the top of the powder charge, resulting in formation of a dense material. The effect of the powder parameters on the crystal growth rate is discussed. The growth rate computed using the developed model is found to be higher than that predicted by the conventional approach assuming surface SiC sublimation to occur on the powder top only.