The degree of difficulty of separation (DDS), which is of critical importance in estimating the relative costs of alternative separation sequences or networks, has been hitherto unavailable for fluid flow-facilitated particle separation processes that are rate controlling. The expressions for DDSs are derived from the mechanisms and models of such processes as gravity sedimentation, centrifugation, and hydrocyclone separation. The resultant expressions differ substantially from those for equilibrium-sta,oed separations, e.g. distillation.