Average pressure drop

Static holdup is liquid remaining on the packing after it has hem fully wetted and drained for a long time. The contribution of static holdup to mass transfer rates is limited (99). Static holdup can be estimated using the relationship of Shulman et al. (100), as recommended (14). Shulman s correlation was derived during the first generation of random pairing but the author is not aware of any updated alternatives. 

Mjadkowiak (736) evaluated liquid holdup predictions from several recent correlations. His evaluation selected a simplified version of the M mann and Deixler correlation over alternative methods (92a), and demonstrated that it fitted experimental holdup data to within 20 to 25 percent. This correlation has a sound theoretical basis and can he expressed in a dimensionless form. It has been ractensive tested for random packings, but the author has no information on how it works for structured packings. The simplified version of the Mersmann and Deixler correlation (736, 92a) is 

Re in Eq. (8.32) is obtained from Eq. (8.8). and Op are obtained from Table 8.2. q. (8.31) is only valid in the preloading regime (i.e., at less than 65 percent of the flood vapor velocity). The range of plication of the correlation is 

The effect of liquid and vapor rates on the operating holdup is shown in Fig. 8.21. In the preloading regime holdup is essentially indq[ en-dent of vapor flow (100,101), but is a strong ftinction of liquid flow rate and packing size. Smaller-i e packings and high liquid rates tend to have more holdup. 

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One means to obtain a desired uniform distribution is to make the average pressure drop across the holes Ap large compared to the pressure variation over the length of pipe Ap. Then, the relative variation in pressure drop across the various holes will be small, and so will be the variation in flow. When the area of an individual hole is... 

Farooqi, S. 1. and Richardson, J. F. Trans. Inst. Chem. Eng. 60 (1982) 323. Horizontal flow of air and liquid (Newtonian and non-Newtonian) in a smooth pipe Part II Average pressure drop. 

Average pressure drop reduction as a function of flow rate for a series of different... 

Fig. 3.17 Average pressure drop reduction as a function of flow rate for a series of different surfaces in a micro-channel having dimensions W = 2.54 mm, H = 127 pm, and L = 50 mm. The experimental data include a series of ultrahydrophobic surfaces with a regular array of square micro-posts with d = 30 pm with a spacing between micro-posts of w = 15 pm represented by triangles (A), <7 = 30 pm and w = 30 pm represented by squares ( ), J = 30 pm and w = 60 pm represented by circles ( ), and d = 30 pm and w = 150 pm represented by diamonds ( ). Reprinted from Ou et al. (2004) with permission...

Instead of using Eq. (8.30), the specific pressure drop is sometimes taken as the arithmetic average of APtop and AP. This gives a slightly conservative estimate of the average pressure drop (15). 

In this relationship the capillary resistance is directly proportional to the number of lamellae in the flow path and the average pressure drop across a single lamella. 

Using the methods discussed in Chapter 14, the average pressure drop predicted by simulation is 0.1 kPa per tray, giving a total column pressure drop of (0.1) (22) = 2.2 kPa. 

This means that the average pressure drop per tray in the wash oil section was 6.74 in. of hot gas oil. Now the tray spacing (i.e., the vertical dimension between the tray decks) for the wash oil trays was 24 in. The pressure drop per tray as a percentage of tray spacing is an important measure of the capacity of a sieve tray (or common valve tray). To calculate this value, divide the observed pressure drop by the tray spacing ... 

To make this decision, we must calculate the average pressure drop of the liquid flowing through the orifice holes ... 

Provide a difference in head between the liquid level in the reflux drum (or reflux accumulator) and point of reflux return to the column at least ten times as large as the average pressure drop across the vapor piping and condenser. 

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