Goodloe Packing Hetp

11.2 Structured Packings 11.2.1 Efficiency data plots

Figures 11.1 to 11.10 and Table 11.2 contain published efficiency data for structured packings. Section 11.2.2 presents a procedure recommended by the author for applying these data. Section 11.2.3 is a legend for the comments in the right-hand column of Table 11.2.

Unlike random packings, structured-packing HETP often rises with loads (Fig. 8.16c). HETP is measured at a constant reflux ratio (L/V), usually at total reflux, so that vapor and liquid loads are raised and lowered simultaneously. It is therefore difficult to state whether HETP rises due to the increase in the vapor load or that in the liquid load (Sec. 8.2.1), but there are suggestions (Sec. 8.1.10) that the liquid load plays the prime role.

During scaleup, the user must judge whether the load effect is vapor- or liquid-related, or select the more conservative of those. Consider a service to be designed for a C-factor of 0.2 ft/s and a liquid rate of 15 gpm/ft2 using the Koch-Sulzer BX® packing. Figure 11.2a suggests an HETP between 4 and 7 in, assuming the load effect is vapor-related. Figure 11.26 suggests an HETP between 12 and 18 in, assuming the load effect is liquid-related. The author knows of a case where the designer judged the effect to be vapor-related in circumstances similar to that example. The result was a column that did not achieve its separation.

To provide the designer with all the needed information, Figs. 11.1 to 11.10 have HETPs plotted both against the vapor and liquid loads. The liquid load is expressed as gpm/ft2 of empty column cross-section area. The vapor load is expressed as the C-factor, Cs (ft/s), given by where us is the superficial vapor velocity, ft/s, based on the empty column cross-section area, and pG and pL are the vapor and liquid densities, respectively.

11.2.2 Interpolation procedure

The procedure below is based on the axiom that while data interpolation is generally quite reliable, extrapolation seldom is. Extrapolation is best avoided. When unavoidable, it must be performed conservatively and with extreme caution. The following steps are recommended.

1. Examine Sees. 8.1.10 and 9.1.3, and determine what constitutes a system similar to the system under consideration.

2. Look at the data for the packing under consideration. Check whether there is sufficient data for a system similar to yours with the considered packing. If so, proceed to step 3. If not, proceed to step 7.

3. Using the vapor and liquid loads in your design, read test HETPs from parts a and b of the diagram for the relevant packing. Pick the more conservative of the values you read. If extrapolation is required to obtain the test HETP, your value may be unreliable. If extrapolation is excessive, it is best to terminate the calculation (step 7).

4. Look at data for the packing considered and for similar packings, using Figs. 11.1 to 11.10 and Table 11.2. Pay special attention to column diameter and height effects and to system effects that may be apparent in the data. Also, look for any weird dependencies of HETPs on vapor or liquid rates or any other odd behavior of HETP. All these effects (if they occur) need to be allowed for in the design. Judgment is required, and the author advocates a conservative approach.

5. Examine Sec. 9.3.3, and use its guidelines to scale up the HETP from the test data to your column. Pay attention to the effects of diameter, height, and underwetting. Judgment is required here. It pays to look at the original reference from which the data was extracted in order to check whether distribution, data scatter, or test procedure could have influenced the data.

6. Check that you have not violated any of the criteria recommended in Sec. 8.1.10. Your calculation is now complete; skip step 7.

7. If you reached this step, interpolation of the data in this chapter cannot answer your problem. Check if proprietary data banks or the packing supplier have the data you are missing. If you cannot obtain these data, consider an alternative packing for which HETP can be predicted with confidence for your system.

11.2.3 Legend for Table 11.2 comments

1. At "75 percent of flood."

2. Material is chemically oxidized phosphor bronze.

3. AEC stands for the Atomic Energy of Canada.

5. EG stands for ethylene glycol; DEG stands for diethylene glycol; TEG stands for triethylene glycol.

6. Where two values are shown under column height, the first describes the packed height above the feed, the second the packed height below the feed.

8. High purity {>99 percent) of both top and bottom products.

9. Pressure cited is at bottom of tower.

10. Value of bed height is the "total packed height" specified by the source. Presumably, there were a few packed beds.

11. DEA stands for diethanol amine; TEA stands for triethanol amine.

12. Underwetting?

13. No distributor was used, but it was stated (45,46) that with a distributor, the same efficiency is obtained.

14. Obtained with high-surface-area (>600 ft2/ft3) Goodloe® packing, not with the standard Goodloe®. Material was copper-bronze (49).

15. Flexipac® data.

16. C-factor of about 0.3 fVs.

17. Concentration range 10 percent methanol at bottom to 98 percent at top.

18. Water content 60 to 99.5 percent; stainless steel packings.

19. This is an isotope exchange system rather than a bona-fide distillation system. Separation is part of the GS process, and runs at molar LP/ of 0.5.

20. Naphtha wash section in a fluid catalytic cracker (FCC) main fractionator.

21. Below the feed there are three packed beds.

22. The pressure is the average pressure in this section of column.

23. Atmospheric crude tower, naphtha/light fuel oil (LFO) fractionation section.

24. Atmospheric crude tower, kerosene/gas oil fractionation section.

25. Atmospheric crude tower, naphtha/kerosene fractionation section.

26. Atmospheric crude tower, gas oil/resid fractionation section.

27. Atmospheric crude tower, light fuel oil <LFO)/atmospheric gas oil (AGO) fractionation section.

28. Atmospheric crude tower, stripping section.

29. Based on Norton's analysis of FRI's data.

30. ITdC stands for Instituto Technologico de Celaya, Mexico.

31. Maldistribution? Compare Fig. 8.166.


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