Rate-based calculations for trayed and packed columns offer  process  engineers a more rigorous and reliable basis for assessing column performance than  the  tradi- tional equilibrium-stage approach, especially for multicomponent separations。 Although the mathematics, thermodynamics, and transport-related physics upon which nonequili- brium 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, kx and ky, for the specific area participating in mass transfer, am, for the two-phase pressure drop, (Dp/Z)2/, 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 col- umn operating conditions outside of those used to develop the correlations in the first place。 For that reason, we undertake the development of dependable, dimensionally con- sistent, correlating expressions for the mass-transfer-related quantities kx, ky, and am 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 pro- cedure。 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 pre- dictions for the relative interfacial area participating in mass transfer that can be greatly in excess of the geometrical surface area of the packing itself。 VVC 2011 American Institute of Chemical Engineers AIChE J, 58: 132–152, 2012  71922

Keywords: mass-transfer coefficients, mass-transfer area, packed column, HETP, rate- based simulation

Introduction Recently, the American  Institute  of  Chemical  Engineers, in conjunction with the US Department of Energy, published Vision 2020: 2000 Separations Roadmap。1 This document outlines technical barriers and research needs for the   foresee-

Correspondence concerning this article should be addressed to B。 Hanley at brian。hanley@aspentech。com。

VVC    2011 American Institute of Chemical Engineers

able future in the areas of adsorption, extraction, crystalliza- tion, membranes, bioseparations, and distillation。 Prominent among the most important research needs and barriers for distillation are as follows: (1) the need for a better under- standing of mass transfer and multiphase flow in both trayed and packed columns, (2) the lack of accurate real-stage effi- ciency models for these types of columns, and (3) the fact that nonequilibrium column models lack accuracy, general- ity, and ease of use。 In this article, we  attempt  to address some   of   the   issues   raised   in   the   Vision   2020     report

132 January 2012   Vol。 58, No。  1 AIChE  Journal

Table 1。 Experimental Results from Kister8 and Schultes9

System Pressure (torr) Packing HETP (Billet)5 HETP (BF82)4 HETP (Onda)3 HETP (Aspen) HETP (Expt)

C6/n-C7 760