Packing Efficiency Data

Currently, interpolation of experimental HETP data is the most reli able means of obtaining packed-tower design HETPs. Due to our poor understanding of packing hydraulics and mass tiansfer, rules of thumb outperform theoretical models, while data interpolation outperforms both (Secs. 9.1.4 to 9.1.6). 

Eckert (6) was first to tabulate HETP data in a format suitable for interpolation. Vital, Grossel, and Olsen (65) recommended such data over predictive methods, subject to availability of data fw a system similar to that considered. The author concurs with this recommendation. In this chapter, k ert 8 table is updated and largely expanded, and new tables and charts axe added for structured packh.  

Examine Sec. 9.1.3, and drtermine idiat constitutes a system similar to the system under oonsideratioo. Hien scan Table 11.1, marking all the data for similar systems. 

Check if sufficient mariked data are awulable for the paddng under consideration. H so, use tiiese data and go direct to step 6 it miy p to use steps 3 to 5 as a dieck. If not, proceed to step 3. 

Compare HETPs for the marked systems to HETPs of other systems with the same packing. If significant hi fosr, a system ef  

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Efficiency data for a representative structured packing at two column diameters are shown in Fig. 14-74. The Max-Pak packing has a surface area of 246 m /m (7.5 ft /fE). The same test mixture (cyclo-hexane//i-heptane) and operating pressure was used for both tests. It would appear that column diameter does not have an influence in this range of values (0.43 to 1.2 m). 

FIG. 14-60 Effect of structured packing surface areas, loads, and inclination angle on packing efficiency. Efficiency expressed as number of theoretical stages per meter, the reciprocal of HETP. Sulzer data, chlorobenzene-ethylbenzene, 100 mbar, at total reflux 250-mm-diameter test column. (Reprinted courtesy of Sulzer... 

Comparison of Various Packing Efficiencies for Absorption and Stripping In past editions of this handbook, extensive data on absorption/stripping systems were given. Emphasis was given to the following systems ... 

Some tests by Koshy and Rukovena (110,111) with aqueous, high-relative-volatility systems (a >2) gave much lower random packing efficiencies at high and low L/V than close to total reflux. They interpreted the results as an LfV ratio effect however, underwetting (below) can also explain these data. 

At deep vacuum (< 1 to 2 psia) there are data to suggest that efficiency decreases as pressure is lowered for random packings (117,118), but most of these data can also be explained by poor wetting or maldistribution. For distillation at high pressures (>200 to 300 psia), there is evidence to suggest that structured-packing efficiency diminishes as pressure is raised (24,31a). 

Harrison and France (52) presented the only published rule of thumb for structured packings efficiency. This rule states that HETP is 9 in for Winch crimp height, 18 in for 1/2-inch crimp, and 33 in for 1-in crimp. The author found this rule to do well when the crimp angle is 45°, but to be less satisfactory for other angles. Basad on data in Chap. 11, the author proposes an alternative rule of thumb. 

Efficiency check. HETP data will be compared with the calculated HETP (Sec. 9.4.4). Table 11.1 is searched for efficiency data on columns in similar service that contain random packings. The following data, extracted from Table 11.1 are relevant ... 

Table 11.1 contains published efficiency data for random packings. Section 11.1.1 presents a procedure recommended by the author for applying these data. Section 11.1.2 is a legend for the comments in the right-hand column of Table 11.1. 

TABLE 11.1 Published Efficiency Data ter Random Packings (Conf/m/eri)... 

TABLE 11.2 Published Efficiency Data tor Structured Packings (Continued)... 

In general, performance data published in the literature are usually obtained from small laboratory equipment. As a result, efficiency and flooding correlations should be used with caution. The limits of models should be checked. For example, since most published packed extractor data are based on small diameter columns, the packings studied are usually small with a high specific surface area. The extrapolation of models primarily based on such high specific surface areas to larger packings with much lower areas... 

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