A prototype computer program SOLIDS has been developed for the simulation of processes with solids processing steps. In addition to heat and mass balances, the equipment units are modeled with discretized population balance equations. Any transformation mechanisms for the particle-size distribution (PSD) - nucleation, growth, dissolution, agglomeration, and breakage - can be represented With the sequential modular code, the PSD can be tracked from unit to unit in a complete plant. To provide maximum flexibility for further development, new or improved equipment models and discretization techniques can be included as additional subroutines. Case studies show that accounting for the PSD can lead to improved equipment design and a more complete description of the interactions among the units in the plant.