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Limitations in Packing Efficiency Tests

Interpretation of efficiency test results and applying to different operations

Structured packing in a vacuum tower stripping section.

To: asloley@distillationgroup.com
Subject: Better packing for a stripping service
Date: Fri, 29 Jun 2001 07:24:00 -0700

Andrew,

What are your thoughts on installing XXX structured packing in our vacuum stripping section? The vendor claims some recent test unit work has shown this packing has more efficiency and capacity than other packings of similar size, especially in vacuum service.

S., North American Refiner

(Edited to remove confidential information.)

To: S.
Subject: Interpreting test unit results
Date: Sat, 19 July 2001 09:46 -0500

S.,

The first paragraph is a quick summary. To abridge the summary, the vendor's claims are meaningless for your service. If you want more details, dig into the second section of the reply.

Summary

It is very unlikely that the vendor's claims will be supported by the results from the unit operation. For any given packing size, efficiency (HETP) is nearly completely defined by the total surface area of the packing per unit volume. However, there are packing types that allow for higher liquid and vapor loads for a given service. If the vendor's proposal is that they can use a smaller packing at a given capacity and this gives more theoretical stages, that may be true. If their claim is that their packing has a higher efficiency for a given size, it is unlikely to be true for your service.

The major factors in determining packing efficiency for your service are:

Operating data has failed to show any substantial benefits from 'more efficient packing' in vacuum stripping service.

Details

Differences between structured packing's gross geometry and surface treatment

Packing efficiency depends upon many factors, including the system involved, wetting characteristics of the liquids, and both absolute and relative vapor and liquid loads. This is assuming that the liquid and vapor distribution are effectively achieved. If improper vapor and liquid distribution exist, all packings perform poorly and differences between types of packing are irrelevant.

To understand how structured packing responds to different conditions, we must first understand how roughly similar size structured packing can have different efficiencies. Structured packing is fabricated from sheets of bent material (usually metal) joined together. Liquid and vapor flow along channels in the packing. Figure 1 shows an exploded view of a structured packing element with liquid and vapor flow channels indicated.

Figure 1
Fluid flow in structured packing [1]

Packing works by spreading the liquid in a thing layer to maximize surface area for vapor contact. Obviously, with shallower the crimp height more packing surface area fits into a given volume. This spreads the liquid in a thinner film and over a larger area: the efficiency of small-crimp packings is higher. Figure 2 illustrates the difference between a large-crimp size and a small-crimp size structured packing. The differences between packing sizes are normally referred to as the gross packing geometry or macrostructure.

Figure 2
Comparison between large-crimp and small-crimp structured packing

Structured packing may also differ in the packing surface treatment or microstructure. Figure 3 illustrates the difference between the gross packing geometry and the surface treatment.

Figure 3
Comparison of gross packing geometry to surface treatment

Different packings have different surface treatments. Dimples, slots, vanes, tabs, troughs, holes and many other types of mechanical structures have been used. Surface treatments aim to achieve several possible purposes:

The affect of surface treatment usually increases at low liquid rates. At low liquid rates, the liquid conforms closely to the surface treatment (Figure 4). At high liquid rates, a thick liquid film forms that may dramatically reduce the effectiveness of the surface treatment.

Figure 4
Liquid on packing at low liquid rates

Figure 5
Liquid on packing at high liquid rates

Purported differences in packing performance for the same specific surface area are usually attributed to surface treatment differences. Benefits of surface treatment fall with high liquid loading.

Test unit conditions

The importance of the liquid film becomes apparent when we compare the liquid and vapor rates and conditions for test unit operations to your specific application. Test operating units come in two major configurations:

Figure 6 shows the layout for a total reflux unit. Figure 7 shows the operation with a product recycle unit. Efficiency data is gathered nearly exclusively from total reflux operation.

Figure 6
Total reflux test unit

Figure 7
Product test unit

Depending on the test operation desired (temperature and pressure) many different mixtures can be used. Most often, standard mixture combinations are used to make comparison between devices straightforward [2].

For any section of the column, the vapor and liquid traffic must have the same mass flow rates for total reflux operation. This implies that for any given operating pressure and loading factor (Souders-Brown coefficient, or Csb) the liquid rate must fall in a very narrow operating range. In recycle operation, the recycle stream conditions allow you to vary the liquid to vapor ratios above and below the feed. The standard operation used by most test facilities recycles a liquid feed stream very close to its bubble point. It may be either slightly subcooled or slightly vaporized, however, the amount of deviation from the bubble point is small. With a bubble point feed entering a tower close to the optimum feed tray, the L/V ratios in the top of the tower will be less than total reflux operation and the L/V ratios in the bottom of the tower will be more than total reflux operation.

Figure 8 plots total reflux and recycle operation liquid loading for a constant vapor loaded tower at different operating pressures. Figure 9 shows the same data as a function of vapor density. Your vacuum stripping operating conditions have also been plotted on both figures.

Figure 8
Liquid loading versus test unit operating pressure (constant Csb)

Figure 9
Liquid loading versus test unit vapor density (constant Csb)

The vacuum stripping operation has considerably higher liquid loads than the total reflux test conditions. Your operation clearly has a liquid rate where any benefit from surface treatment would be minimal. An efficiency benefit claimed by comparing only the pressure of your operation to the test unit results and ignoring the liquid loading is highly likely to be wrong.

Andrew Sloley
DGI

References

[1] From Chen, G.; Kitterman, B. L.; Axe, J. R. US Patent 4,604,247 Tower packing material and method. 5 August 1986.
[2] Onken, U.; Arlt, W. Recommended test mixtures for distillation columns, second edition. Institution of Chemical Engineers, Rugby, England: 1990.

Disclaimer

No performance, suitability for use, or lack of suitability for use for any given process service is implied to any particular model or brand of packing by these comments. Figures used have been used as illustrative of generic classes of equipment.

Images have been sized for full screen display on an 800x600 monitor.

This page updated 19 July 2001.
© 2001 Andrew W. Sloley. All rights reserved.