Pressure drop, packings

For ordered, or structured, packings, pressure-drop estimation methods have been reviewed by Fair and Bravo [Chem. Eng. Progr, 86(1), 19 (1990)]. It is not common practice to use the packing factor approach for predicling pressure drop or flooding. For operation below the loading point, the model of Bravo et [Hydrocarbon... 

Capacity and separation gains due to lower pressure drop of packing. Pressure drop of packing is typically 3 to 5 times lower than that of trays. 

When considering pressure drop models based only on water, hydrocarbons system capacity can be significantly overstated. For Nutter random ring packings the pressure drop/capacity models fit the data within +10% over the range of commercial interest, i.e., 0.1 to 1.0 in. water/ft of packing. Pressure drop alues for design operation should... 

Diunped Packing Pressure Drop Below and at Flood Point, Liquid Continuous Range... 

Read, F, for the approximate calculated Cs, ft/sec from Table 9-37 at calculated liquid rate, Ib/hr-ft. Then read Y from IMTP Packing Pressure Drop chart. Figure 9-2IG and then read curves showing pressure drop, (may require interpolation). 

Norton Intalox Structured Packing Pressure Drop Coefficient F for Type 2T Packing... 

Hsu, Shih-liang, Packing Pressure Drop Estimated, Hydro. Processing, V. 64, No. 7 (1985) p. 89. 

Leva, M. Chem. Eng. Prog. Symp. Ser. No. 10, 50 (1954) 51-59. Flow through irrigated dumped packing Pressure drop, loading, flooding. 

Pressure Drop The GPDC discussed above (Figs. 14-55 and 14-56) and the Kister and Gill interpolation charts provide popular methods for calculating packing pressure drops. An alternative popular method that is particularly suitable for lower liquid loads was presented by Robbins (below). 

For gas flow through dry packings, pressure drop may be estimated by use of an orifice equation. For irrigated packings, pressure drop... 

Vacuum systems. Packing pressure drop is much lower than that of trays because the packing open area approaches the tower cross-sectional area, while the tray s open area is only 8 to 15 percent of the tower cross-sectional area. Also, the tray liquid head, which incurs substantial pressure drop (typically about 50 mm of the liquid per tray), is absent in packing. Typically, tray pressure drop is of the order of 10 mbar per theoretical stage, compared to 3 to 4 mbar per theoretical stage with random packings and about one-half of that with structured packings. 

A packing pressure drop of 1.5 in/ft is approximately 95% of column packing flood. At 2.0 in/ft of pressure drop DP, most random packed towers are at the flood point. It is thus prudent and good practice to keep the limiting D, at 1.5 in/ft or less. 

Kister and Gill (60,60a) demonstrated that despite differences in definitions, flood-point data compared quite well to correlation predictions. Both Kister and Gill (60,60a) and MacDougall (53) show that flood data from various sources (using various definitions) can be correlated to with 10 to 15 percent accuracy. It was also demonstrated that the flood point can be predicted far more reliably than packing pressure drop (55,58) and maximum operational capacity (60). 

Pressure drop This is often used to specify packed tower capacity. The application of this criterion, and the interpretation of packing pressure drop data are not trouble-free. Some inherent limitations and traps are... 

The Kiefer and Gill correlation. Zenz (76) discovered that packing pressure drop at the flood point decreases as the packing capacity increases. A similar observation was reported by Strigle and Rukovena (15,77) and Ma kowiak (736). Kister and Gill (60) applied this principle to derive a simple flood point correlation... 

A weak link in the correlation is the packing pressure drop prediction. Inaccurate pressure drop prediction procedures will lead to inaccurate flood-point predictions using this correlation. For best results, the author recommends applying Eq. (8.1) together with pressure drop predictions by interpolation (Sec. 8.2.9). 

Mersmann s correlation and Madkowlak s correlation. Mersmann (73) postulated that a thin liquid film forms in the flow channel of the packing. The ratio of film thickness to equivalent packing diameter is a function of the liquid load. Mersmann combined this function with a trickle flow model to yield an expression for dry packing pressure drop at flood as a function of liquid rate. Mafikowiak (78a) surveyed sources that followed up and improved on Mersmann s initial model. 

An excellent statistical fit to data is therefore insufficient to render a packing pressure drop correlation suitable for design. In addition to a good fit to data, the correlation limitations must be fully explored. Most published packing pressure drop correlations fail miserably here their limitations are often unknown, and if known, are seldom reported. 

Strigle (15) and Kister and Gill (60) compared predictions from the latest version of the Eckert correlation (Fig. 8.19a and b) to thousands of random packing pressure drop measurements. The Eckert correlation was shown to give good predictions for most pressure drop data (15,60). It generally works well for the alr-wator system for flow parameters as low as 0.01 and as high as 1 (60). For nonaqueous systems, it works well for flow parameters of 0.08 to 0.3 (typical of atmospheric distillation). 

The channel model This model attributes packed-column pressure drop to the resistance to flow in a multitude of parallel channels. The channels may have bends in them or may have contractions and enlargements. Liquid flows down the walls of the channel, thus consuming some of the available cross-section area. This in turn increases the pressure drop. The channel model has been applied both for random and structured packings (e.g., 3,62,736,78,91,92,93a), A popular application of this model is the Bravo et al. (91) correlation for structured-packing pressure drop ... 

The suitability of the GPDC interpolation charts as a basis for interpolation is not accidental. Packing pressure drops correlate extremely well with GPDC coordinates, i.e., the fiow parameter and the capacity parameter. The dependence does not always follow the correlation contours, but always appears to exist. Further, the correlation coordinates are essentially a performance diagram, i.e., a plot of a vapor load against liquid load, a tool commonly used for analysing column performance. 

Which method to use. Section 8.2.8 draws attention to the systematic nature of the limitations of packed-tower pressure drop correlations. Due to this systematic nature, the author warns against basing packing pressure drop calculations on any correlations whose limitations are not well known. Section 8.2.8 presents three correlations and elaborates on their limitations and application boundaries. Within their boundaries, these correlations should give reliable predictions. UBe of any other correlation is dangerous unless its limitations are explored. 

Interpolation of packing pressure drop data Is superior in accuracy and reliability, and should always be preferred to correlations. Section 8.2.9 presents two interpolation procedures The GPDC interpolation charts, and the Robbins interpolation. 

Vapor maldistribution Packing pressure drop places a resistance in the vapor path that helps spread the vapor radially. If pressure drop is too low, vapor will tend to channel through the bed, leading to poor mass transfer. 

Keep in mind that if you reached this step, you are in an uncertain region. The uncertainty is even greater if the system nature has a considerable effect on packing pressure drop. Proceed with extreme caution, recognizing that your calculation in this case will at best be only an educated guess. It may pay to consider using the reference packing instead of the one you had in mind because it offers more confidence in the reliability of the performance prediction. 

Boston inside-out method. 172-177, 198 Bravo, et al. rfructured packing pressure drop, 447, 499, 500 Bravo Fair et al. mass transfer random packing, 528-530 structured packings, 474, 529, 531, 532 Brown-Martin method, 109 Bryoden method, 161, 162, 175, 176. 179... 

LANPAC , 432. 434,621, 646 Lessing ring, 423, 424 Leva packing pressure drop equation, 497... 

Smoker method. 123-126, 192 Snap-Grid , 466-467, 637, 650 Souders and Brown constant. 276 equation, 276, 480 Spiegel and Meier flood. MeUapak , 488, 490 mass transfer. 474, 532 ST-100 packing, 444 Standart efficiencies, 365 Stichlmair et al. packing maldistribution, 547 packing pressure drop model, 501 Strigle ... 

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