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FIGURE 9.3 CfiPfiClTy COMPARISON OF irr?^' PACKING BND SIEVE TRP/S &P,SED ON STRlGlc ¿ SOLAN (s, ii)

FIGURE 9.3 CfiPfiClTy COMPARISON OF irr?^' PACKING BND SIEVE TRP/S &P,SED ON STRlGlc ¿ SOLAN (s, ii)

I EFFICIENCY COMPARISON The efficiency of packings and trays is j compared in Figure 9.4. This comparison is even less reliable thar. ; that in Figure 9.3. While the curves for trays are exact, these for 1 the packings are based or. the rules of thumb by Porter and Jenkins (4) and Frank (5) for packings KETP. A similar diagram shewing Icwei packing KETP's has been presented in Reference (3), and this woulJ. probably be more suitable if the HE TP for the system is well-know n. The author believes that the values plotted in Figure 9.4 include a larger safety margin, and are more suitable for this comparison, because cf the greater uncertainty associated in packed column efficiency prediction. This uncertainty has been discussed elsewhere . (7,8).

Figure 9.4 makes no allowance for the vertical height needed for distribution and support equipment such as distributor, redistributors and liquid collectors. These generally occupy much more vertical space in packed towers than in tray towers. In terms of Figure 9.4, this can simply be allowed for by raising the packing curves by a constant value which depends on the height required for this equipment compared to the bed height. This value may be quite high when the packed bed is shallow.

Figure 9.4 shows that small packings can achieve better HETP's than sieve trays. With larger packings, the HETP is roughly equivalent to that of sieve trays.

OVERALL COMPARISON Looking at both Figures 9.3 and 9.4, it appears that replacing sieve trays with large packings (2") is most suitable for capacity revamps, but in general is not likely to save a great deal of energy.

Replacing sieve trays with small packings (1") trades off the capacity advantage for improved efficiency, and therefore lowers energy consumption. Using an intermediate size packing (1 1/2") achieves both capacity improvements and energy savings.

Pressure drop in packings is considerably lower than pressure crop on < trays (1-3), and packings (particularly the larger types) are most suitable for pressure drop revamps. U

Under vacuum conditions the low pressure drop obtained with packings can be traded off for lower column bottom pressure and.therefore higher relative volatility, lower reflux ratios and lower energy f consumption. This cannot be evaluated from charts similar to Figures I I 9.3 and 9.4, and must be considered separately. !

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2* PACkiNG-S

2* PACkiNG-S

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figure Efficiency comparison of TRAyS

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