column outlet over the course of several months。 Because   the

inlet gas flow and its composition varied from day to day, three representative design cases were examined for the retro- fit。 Some of the temporal data collected for the selectivity after the retrofit are shown in Figure 15。 Also included are selectivity results for the three reported design cases (labeled in the figure as conditions A, B, and C) calculated with Aspen Rate Based Distillation v7。2 using the Aspen v7。0 MDEA model。72 Only the correlations developed in this article yielded selectivity results in line with those measured。 The selectivities calculated with the BRF85,6 BRF92,7 and Billet and Schultes5 correlations were found to be orders of magnitude greater than the experi- ment values。 Also shown in Figure 15 are the calculated CO2 and H2S composition profiles for the BRF85 correlation and the new correlation reported here。 Note that the CO2 composition profiles for the two correlations are different by up to a factor of

3。 The H2S profiles differ by a factor of up to 10,000。 deMontigny et al。52  reported on the absorption of CO2  by

MEA using the metal gauze structured packing DX。 We used the correlation developed from BX data, along with the Aspen v7。0 MEA model,69 to predict the performance of deMon- tigny’s absorber equipped with DX。 Figure 16 is a comparison of the measured CO2 composition profile for run DX-1 with simulated results for the BRF85 correlation and the BX corre- lation  described  above。  Also  included  in  the  figure  are the

Figure 15。 Top: Comparison of the data of Shiveler et al。51

(-l-) for the CO /H S selectivity of the    outlet

authors’ predictions based on their own mass-transfer correla- tions。 The BX correlation of this work successfully reproduces the reported CO2 composition profile。 Even though no other size of gauze packing was included in our BX fit, the BX cor- relation appears to be applicable to DX and EX also。

Implications: Mass-Transfer Area

The data analysis methodology described in this article results in inpidual correlating expressions  for  kx,  ky,  and am。 The fractional mass-transfer area, am/ad, is the subject of considerable controversy。 The Delft correlation73 and that of Onda et al。3 predict that the available area for mass transfer approaches the geometrical surface area of the packing expo- nentially as the liquid rate is increased。 The BRF85 correla- tion6 for metal gauze structured packings assumes that am ¼ ad under all  operating  conditions。  The  correlation  of Rocha et al。42 for am is a power law in the liquid rate; am/ad can be greater or less than unity depending upon the other parame- ters in the correlation。 The correlation of Billet and  Schultes5 is also a power law in the  liquid  rate  below  the  loading point。 However, if the surface tension of the liquid is below 30 dyn/cm, then the mass-transfer area is taken to be the ge- ometrical surface area regardless of the nature of the pack- ing’s material of construction。 For the sake of comparison, effective mass-transfer areas for MELLAPAK 250Y sheet metal structured packing are displayed in Figure 17 for  the total reflux distillations of chlorobenzene/ethylbenzene (CB/ EB) at 75 and at 760 torr。 Clearly, there are noticeable dis- agreements among the correlations。 It might also appear sur- prising that some correlations, including the new structured packing correlation developed in this article, predict mass- transfer areas well in excess of the dry geometrical surface area of the packing itself。 It is important to stress that the interfacial area participating in mass transfer is only loosely

correlated to the extent of wetting of the dry packing’s geo- metrical surface area。 Additional  mass-transfer  area  can come from the presence of waves on the  surface  of liquid films flowing over the packing surface, from liquid droplets, or from liquid filaments between packing elements, for example。 Incomplete wetting of the packing surface would tend to reduce the mass-transfer area relative to  the  pack- ing’s geometrical surface area。 It is more surprising, there- fore, that some correlations identify the geometrical surface area of the packing as the maximum  amount  of interfacial area available for mass  transfer。