To model the behavior of a three-phase stream involving a change of state, an energy balance is derived and then reduced into a practical design equation for a hammer mill. A combination of experimental data, known physical properties, and classical thermodynamic relationships are used to relate processing rate to energy input to the mill. The design equation is applicable to the processing of three-phase streams containing a low-melting organic compound and an evaporating cryogenic liquid, such as nitrogen, which is used as a coolant to preserve the state of the organic material.