Packing random

The mass transfer requirement stipulates that the patella (connecting strip) cannot be larger than 5 mm in width. If the patella is larger than that, a droplet will form on the surface, which will effectively reduce the wetted surface available for the reaction. If the patella is less than 5mm in width, the liquid is capable of moving over the surface without accumulating. The result is that the operating range is widened. The downside is that as the width is decreased, the pressure drop increases. This increase may limit the economics of the process. 

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There are two classes of solids that are not crystalline, that is, p(r) is not periodic. The more familiar one is a glass, for which there are again two models, which may be called the random network and tlie random packing of hard spheres. An example of the first is silica glass or fiised quartz. It consists of tetrahedral SiO groups that are linked at their vertices by Si-O-Si bonds, but, unlike the various crystalline phases of Si02, there is no systematic relation between... 

Fig. 2. Packing materials for packed columns, (a)—(f) Typical packing elements generally used for random packing (g) example of stmctured packing.

The situation is very much poorer for stmctured rather than random packings, in that hardly any data on Hq and have been pubHshed. Based on a mechanistic model for mass transfer, a way to estimate HETP values for stmctured packings in distillation columns has been proposed (91), yet there is a clear need for more experimental data in this area. 

Fig. 23. Pressure drop and flooding correlation for various random packings (95). ip = p- o IP-l (standard acceleration of free fall) = 9.81 m/s, p, = liquid viscosity ia mPa-s numbers on lines represent pressure drop, mm H2O /m of packed height to convert to ia. H2O /ft multiply by 0.012. Packing...

Eor randomly packed spherical particles, the constants M and B have been deterrnined experimentally to be 150 and 1.75, respectively. Eor nonspherical particles, equivalent spherical diameters are employed and additional corrections for shape are introduced. 

Fig. 21. Random packing elements for distillation columns (a), Raschig ring (metal) (b). Bed saddle (ceramic) (c), Intalox saddle (ceramic) (d), PaH ring...

Typical experimental values of HETP for a random packing such as 50-mm PaH rings, and a stmctured packing, such as Intalox 2T of Norton Co., under the same system conditions, are shown in Figure 25. Many designers of packed columns prefer the use of HETP instead of but the latter is more fundamental and discrirninates between Hquid- and vapor-phase resistances. It should be noted that terms such as H and N are based on... 

Corrosivity. Corrosivity is an important factor in the economics of distillation. Corrosion rates increase rapidly with temperature, and in distillation the separation is made at boiling temperatures. The boiling temperatures may require distillation equipment of expensive materials of constmction however, some of these corrosion-resistant materials are difficult to fabricate. For some materials, eg, ceramics (qv), random packings may be specified, and this has been a classical appHcation of packings for highly corrosive services. On the other hand, the extensive surface areas of metal packings may make these more susceptible to corrosion than plates. Again, cost may be the final arbiter (see Corrosion and corrosion control). 

R. P. Strigle, Random Packings and Packed Tomr Design, Gulf Pubhshing, Houston, Tex., 1987. 

A. Absorption, counter-current, liquid-phase coefficient Hi, Sherwood and Holloway correlation for random packings... 

B. Absorption counter-current, gas-phase coefficient Hq, for random packing... 

E. Distillation and absorption, counter-current, random packings, modification of Onda correlation, Bravo and Fair correlation... 

F Absorption, co-current downward flow, random packings... 

FIG. 14-50 Pressure drop correlation for random packings, as presented hy Robbins. [Cbem. Eng. Progr., 87(1), 19 (1990). Reproduced with peimission of the Ameiican Institute of Chemical Engineers. Copytight 1990 AlChE. All fights reseroed. ] To convert inches H20/ft to mm H20/m, multiply by 83.31. 

FIG. 14-58 Typical holdup data for random packings and the air-water system. The raschig rings are of ceramic material. To convert pounds per hour per fr to Idlograms per second per m , multiply hy 0.001.356 to convert inches to millimeters, miinltiplyhy 25.4. [Shulman etal., AIChE J. i, 247 (I.9.5.5).]... 

The coefficients are usually corrected to a hydroxide conversion of 25 percent at 24°C. For other conversions. Fig. 14-15 may be used. Reported values of Kog< for representative random packings are given in Table 14-8. The effect of liquid rate on the coefficient is shown in Figs. 14-70 and 14-71. 

Distillation Applications Packings are now routinely considered for distillation columns with diameters up to 10 m or more. The pressure drop advantages of the modern, through-flow random pack-... 

SOURCE Strigle, R. L., Random Packings and Packed Towers, Gulf Publ. Co., Houston, 1987. 

As indicated above, packed column internals include hqiiid distributors, packing support plates, redistributors (as needed), and holddown plates (to prevent movement of packing under flow conditions). Costs of these internals for columns with random packing are given in Fig. 14-80, based on early 1976 prices, and a Marshall and Swift cost index of 460. 

Plates and random packings have much the same efficiency and capacity. 

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. 

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