Structural packing

CMPs pack to maximise van der Waals interactions between the polymer chains. This packing can be fmstrated by topological considerations 

Within a singular framework, stiff networks are less able to respond to the local environment by adapting their molecular configuration than more flexible polymers. This means that, for the individual framework, a stiff polymer is more likely to be open and the nodes well dispersed. A flexible polymer will be able to pack more efficiently, even if no network interpenetration is taken into account. 

Network interpenetration is energetically favourable as it increases the van der Waals interactions between frameworks and therefore allows space to be packed more efficiently. Frameworks where the node-to-node distance is comparatively large compared to the width of the strut will have space for network interpenetration to occur. Those that have comparatively short struts may inhibit network interpenetration simply due to geometric space considerations. 

The flexibility of the network can also directly influence the degree of network interpenetration. CMPs with long flexible node-struts are able to respond to the local environment through bending of the strut and twisting around the node to allow individual frameworks to find space to pack efficiently in to. 

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Foi structured packings, values correspond to crimp height. 

D. Rectification in vertical wetted wall column with turbulent vapor flow, Johnstone and Pigford correlation =0.0.328(Wi) Wi P>vP 3000 < NL < 40,000, 0.5 < Ns. < 3 N=, v,.gi = gas velocity relative to R. liquid film = — in film -1 2 " [E] Use logarithmic mean driving force at two ends of column. Based on four systems with gas-side resistance only, = logarithmic mean partial pressure of nondiffusing species B in binary mixture. p = total pressure Modified form is used for structured packings (See Table 5-28-H). 

Counter-current flow. Structured packings. Gauze-type with triangular flow channels, Bravo, Rocha, and Fair correlation... 

Refitting of a tray-type column is desired to increase loading, increase efficiency, and/or decrease pressure drop. Structured packing is pariicularly applicable in this case. 

Liquid rates are very low and/or vapor rates are high, in which case structured packing may be particularly desirable. 

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... 

Proc., 65(3), 45 (1986)] is preferred. To use this and alternate models, dimensional characteristics of structured packing must be defined. Figure 14-51 shows nomenclature and definitions of key dimensions. Not shown, but also important, is the angle the corrugations make with the horizontal (usu y 45 or 60°). Then the Rocha et al. predictive equation is ... 

Sepn. Purif., 3, 19 (1989)] takes holdup into account and applies to random as well as structured packings. It is somewhat cumbersome to use and requires three constants for each packing type and size. Such constants have been evaluated, however, For a number of commonly used packings. A more recent pressure drop and holdup model, suitable for extension to the flood point, has been pubhshed by Rocha et al. [Jnd. Eng. Chem. Research, 35, 1660 (1996)]. This model takes into account variations in surface texturing of the different brands of packing. 

Representative pressure drop data for random and structured packings are given in Figs. 14-52-14-54. 

FIG. 14-59 Typical vendor data for liquid holdup of a structured packing, Gempak 2A. [Cou7tesy Glitsch, Inc., Dallas, Texas.]... 

FIG. 14-60 Comp arison of measured and calculated values of liquid holdup for Gempak 2A structured packing, air-water system. [Rocha et al., Ind. Eng. Chem., 32, 641 (1.9.93).] Reproduced with permission. Copyright 199.3 American Chemical Society. 

Efficiency data for a representative structured packing at two column diameters are shown in Fig. 14-74. The Max-Pak packing has a surface area of 246 m /m (7.5 ft /fE). The same test mixture (cyclo-hexane//i-heptane) and operating pressure was used for both tests. It would appear that column diameter does not have an influence in this range of values (0.43 to 1.2 m). 

Efficiency and pressure drop data for Siilzer BX metal gauze structured packing and for three test mixtures are shown in Fig. 14-7.5. For the ethyl benzene/styrene test mixture, the effect of operating pressure is shown. The high viscosity mixture, propylene glycoL/ethylene... 

FIG. 14-74 HETP values for Max-Pak structured packing,. 35 kPa (5 psia), two column diameters. Cyclohexane/n-heptane system, total reflux. For 0.4.3 m (1.4 ft) column perforated pipe distributor, 400 streams/m2, 3.05 m (10 ft) bed height. For 1.2 m (4.0 ft) column tubed drip pan distributor, 100 streams/m ,. 3.7 m (12 ft) bed height. Smaller column data. University of Texas/Austin Larger column data. Fractionation Research, Inc. To convert (ft/s)(lb/ft ) to (m/s)(kg/m ) , multiply by 1.2199. (Couiiesy Jaeger Troducts, Inc., Housion, Texas.)... 

Structured packing efficiency is about 1.5 times that of plates or random packing. 

At a parameter of 0.02, the structured packing has a 1.3-1.4 capacity advantage over random packing and plates. This advantage disappears as the parameter approaches 0.1. 

Structured packing has about the same capacity as plates and random packings. 

The efficiency advantage of structured packing over random packings and plates decreases from 1.5 to 1.2 as the parameter increases from 0.1 to 0.3... 

The loss of capacity of structured packing is greatest in this range. 

The random packing appears to have the highest capacity and efficiency, and structured packing the least capacity and efficiency. 

Table 7 show s ranges of pressure drop for design. Pressure drop sets the allowable vapor flow rate. The flood pressure drop, for random or structure packings, is given in Reference 15 as ... 

Generalizations are not as easy for structured packings as for random packings. See the following subsection, Comparison to Trays, for study type structured packing data. 

Reference 13 is recommended for its large database of structured packing infonnation. One parameter for structured packing within the narrative of Reference 13 is that the minimum w etting rate is 0.1 to 0.2 gpm/ft compared to 0.5 to 2gpm/fT for random packings. 

Tlie flood pressure drop for structured packing, already shown in the subsection on random dumped packings, is repeated here ... 

In addition, Kqa data are given for some structured packing in Chapter 4 Absorbers—Inorganic Type. 

The structured packing efficiency is about 50% higher than either the trays or the random packing. 

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