% ky for such systems)。 Calculated HETPs more than 20% different from the measured value range are bolded and underlined。 Table 2 is a similar comparison for structured packings。8,10–12 Once again,  calculated  HETPs  more  than 20% different than the measured value range are bolded and underlined。  In  Table  1,  two-thirds  of  the  calculated HETPs

for random packings are more than 20% above or below the experimental values。 In Table 2, about 40% of the predicted HETPs for structured packings are outside the  T20%  enve- lope。 These results clearly demonstrate that results generated during a rate-based simulation will often be limited by the reli- ability of the auxiliary  mass/heat-transfer  correlations  needed to complete the mathematical description of the packed   tower。

Mathematical Preliminaries

Before we turn to the development of specific mass-transfer correlations for random and structured packings, here we will first discuss several preliminary topics of interest primarily related to the proper interpretation of binary mass-transfer experiments using standard test mixtures。

The point HETP

The defining expression for the point HETP in a packed tower is developed by treating the packed tower as an equi- librium-stage contactor and then doing a mass balance over a single equilibrium stage (see, for example, Ref。 13)。 Nor- mally, it is assumed that the major resistance to mass  trans- fer resides in the vapor phase。 Further, the development assumes that the equilibrium curve for the stage in question is straight for the composition changes encountered on the stage。 For the sake of simplicity, we will assume here that constant molal overflow is true throughout the column。 The assumption of constant molal overflow is almost always ac- ceptable for analyzing binary distillation experiments with standard test mixtures that have small relative volatilities, a。 The development below is strictly valid for binary systems。 The equilibrium-stage concept is incompatible with the rate theory for mass transfer in multicomponent   systems。14

Table 2。 Experimental Results from Kister,8  Fitz et al。,10  Agrawal et al。,11  and Kean et al。12

System Pressure (torr) Packing HETP (Billet)5 HETP (BRF85)6 HETP (BF92)7 HETP (Aspen) HETP (Expt)

Ar/O2 1550 Flexipac 500Y 15。6 19。1 22。1 17。6 17。511

p/o-Xylene 16 Mellapak 250Y 38。1 35。6 50。8 43。2 33。010

p/o-Xylene 100 Flexipac 700Y 23。6 9。1 14。7 17。3 20。310

CB/EB 76 Mellapak 350Y 29。2 24。8 33。0 27。4