Efficiency, tray comparison

Hughmark [181] has proposed empirical correlations for better fit of experimental data to transfer units and thus tray efficiency comparison with the AIChE method [2]. 

Figure 9-18. Overall comparison of efficiency for "state-of-the-art random and structured packing with trays at 24-in. spacing. Reproduced with permission of the American Institute of Chemical Engineers Kister, H. Z., Larson, K. F., Yanagi, T. Chemical Engineering Progress, V. 90, No. 2 (1994) p. 23 all rights reserved.

A comparison has been made of the efficiencies of conventional and ultrasonically assisted pollutant extraction procedures using model soil samples (granular pieces of brick) which had been deliberately contaminated with copper oxide at 51 ppm [50]. Analysis of the brick particles after 30 min sonication on a Vibrating Tray [51] revealed an average reduction in copper content to 31 ppm, a reduction of about 40%. Using a conventional mechanically shaken tray for the same time period the residual contamination was 48 ppm representing a reduction of only 6% (Tab. 4.6). 

The narrow-trough vapor distributor shown in Fig. 7.4 is intended to disperse the vapor evenly across the bottom of the packed bed. The width of the chimney does not exceed 6 in. The older-style chimney trays, which may have had a few large round or square chimneys, reduced the separation efficiency of the packing. To work properly, the vapor distributor has to have a reasonable pressure drop, in comparison to the pressure drop of the packed bed. For example, if the expected pressure drop of a 12-ft packed bed is 10 in of liquid, the pressure drop of the vapor distributor ought to be about 3 to 4 in of liquid. 

Flooding, holdup, and mass transfer rates are highly interdependent and are not simply related. Reissinger and Schroter (1978) state that tray towers in comparison with other types have good efficiencies at 60 nr /m2 hr at frequencies of 60-90/min and amplitudes of 10 mm. Packed towers have about 2/3 the capacities of tray towers. Also in comparison with unagitated towers, which are limited to interfacial tensions below lOdyn/cm, pulsed towers are not limited by interfacial tension up to 30-40 dyn/cm. Some... 

Capacity and Efficiency Comparison Kister et al. [Chem. Eng. Progr., 90(2), 23 (1994)] reported a study of the relative capacity and efficiency of conventional trays, modem random packings, and conventional structured packings. They found that, for each device optimally designed for the design requirements, a rough guide could be developed on the basis of flow parameter L/G (p( /p, )05 (abcissa in... 

Three parameters were identified and adjusted to validate the model against the experiments. The parameters are the heat losses, the nominal tray holdup and the Murphree tray efficiency (EM). Figure 4.16 shows how EM is adjusted to match the dynamic model prediction and experimental temperature profile measured on Plate 12. Figure 4.17 shows the comparison between the experimental and model prediction of ethanol composition in the reflux drum, middle vessel and in the bottom of the column. Figures 4.16-17 show a good match between the model prediction and experiments. 

Table 6.1 compares the main tray types. The comparison is general and assumes the trays are properly designed, installed, and operated. Sieve and valve trays have comparable capacity, efficiency, entrain-... 

Structured packings have replaced trays and random packings as their cost has decreased and more is known of their performance behavior. Initially thought to be appropriate only for high vacuum distillations, they are now used for absorbers, strippers, and pressure distillations. Because of their open structure (over 90% voids) and large specific surface areas, their mass transfer efficiency is high when proper distribution of liquid and gas over the cross section can be maintained. Table 13.15 shows a comparison of features of several commercial makes of structured packings. 

TCE), vinyl chloride, aldicarb (pesticide), and VOCs are listed in Table 9. The removal performances of a diffused aeration system for several organics (VOCs and carbofuran) are listed in Table 10. PTAs and multiple-tray aerators are effective in removing hydrogen sulfides and carbon dioxide in the range of 90-95%. Comparison of removal efficiencies for PCA (PTA) and diffused aeration is shown in Table 3. As this shown in this table, PCA is more efficient than diffused aeration, but diffused aeration can achieve a comparable removal of trans-1, 2 dichloroethylene and TCE. 

Predicted composition profiles are shown in Figure 14.20. Flow and temperature profiles are shown in Figures 14.21 and 14.22 respectively. The location of the feeds is easily seen in these profiles. Murphree efficiencies, computed from Eq. 13.2.1 using the results of a simulation, are shown in Figure 14.23. As has been the case in many of our comparisons, the Murphree efficiencies vary widely from component to component and from tray to tray as well. 

An attractive feature of packed beds for mass and heat transfer operations is that the bed itself offers better turndown characteristics than the equivalent tray column. The mass transfer efficiency of a packed column with a good distributor is about as good at both low and high loadings. This favorable comparison is evident from the data in Fignres 12.26 and 12.49 (same colnmn and test mixture). The liquid distribution is the key, as indicated in Figure 12.50. 

Large-scale tray column efficiencies can be predicted from measurements in laboratory columns as small as 25 mm (1.0 in) diameter. Comparative studies (laboratory vs. commercial) have shown that the use of a special laboratory sieve tray column, the Oldershaw column, produces equivalent separations. A glass Oldershaw column is shown in Figure 12.61. Each tray has a center downcomer that discharges to one side of the tray below. Comparisons between laboratory and commercial scale efficiencies are shown in Figure 12.62 ° and Table 12.11. The Oldershaw value of overall efficiency is equivalent to the point efficiency of the larger column. 

FIGURE 12.62 Comparison of tray point efficiencies for different column sizes. Cyclohexane/ -heptane system, 0.20-0.27 atm pressure, total reflux. (Separations Research Program, Univ. of Texas at Austin.)... 

FIGURE 5.8-3 Pressure drop/efficiency comparisons, trays and packings ethylbenzene-sly rene sysiem ai 100 nm Hg pmssure (Refs. 8 and 9),... 

FIGURE 6.4-4 Simplified comparison of sieve trays, random packing, and high-efficiency ordered packing. (From Meier et al,, reprinted with permission from Chemical Engineering Progress, November 1977.)... 

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