Rate-based calculations for trayed and packed columns offer process engineers a more rigorous and reliable basis for assessing column performance than the traditional equilibrium-stage approach, especially for multicomponent separations. Although the mathematics, thermodynamics, and transport-related physics upon which nonequilibrium separations theory is founded are generally true, it is also true that rate-based simulations today suffer from a serious weakness-they are ultimately tied to underlying equipment performance correlations with questionable predictive capability. In the case of packed columns operated countercurrently, correlations are required for the mass-transfer coefficients, k(x) and k(y), for the specific area participating in mass transfer, a(m), for the two-phase pressure drop, (Delta p/Z)(2 phi), and for the flood capacity of the column. In particular, it is generally well known that packing mass-transfer correlations available in the public domain are unreliable when they are applied to chemical systems and column operating conditions outside of those used to develop the correlations in the first place. For that reason, we undertake the development of dependable, dimensionally consistent, correlating expressions for the mass-transfer-related quantities k(x), k(y), and a(m) for metal Pall rings, metal IMTP, sheet metal structured packings of the MELLAPAK type, and metal gauze structured packings in the X configuration, using a new data fitting procedure. We demonstrate the superior performance of these correlations for a wide range of chemical systems and column operating conditions, including distillations as well as acid gas capture with amines. Further, we show that these new correlations lead to predictions for the relative interfacial area participating in mass transfer that can be greatly in excess of the geometrical surface area of the packing itself. (C) 2011 American Institute of Chemical Engineers AIChE J, 58: 132-152, 2012