Packing Hydraulics

S.2 Packing Hydraulics 8.2.1 Pressure drop flow regimes... 

Currently, interpolation of experimental HETP data is the most reliable means of obtaining packed-tower design HETPs. Due to our poor understanding of packing hydraulics and mass transfer, rules of thumb outperform theoretical models, while data interpolation outperforms both (Secs. 9.1.4 to 9.1.6). 

The complexities of packing refits into an existing tower are well beyond the current scope of discussion. Here, we will concentrate on the simplest element of packed tower design, selection and verification of packing hydraulic performance, along with an introductory discussion of packing efficiency. 

Across the usable loading range, packing hydraulics go through two major zones. First, as vapor load increases the packing pressure drop increases close to... 

Low vapor liquid density difference Random packing Known cause of structured packing hydraulic failures... 

Another parameter in the design of a packed column is the column diameter, which is determined on the basis of hydraulic considerations, such as pressure drop and flooding conditions. The hydraulic behavior of the column depends on the packing type and operating conditions. Correlations based on experimental data are available for calculating column packing hydraulics, and are discussed in this chapter. 

This flow parameter is the square root of the ratio of liquid kinetic energy to gas kinetic energy. The ordinate of this correlation includes the gas flow rate, the gas and liquid densities, the a/e ratio (which is characteristic of the particular tower packing shape and size), and a liquid viscosity term. Lobo et al proposed the use of a packing factor to characterize a particular packing shape and size [17]. They determined that the a/e ratio did not adequately predict packing hydraulic performance. Eckert further modified this correlation and calculated the packing factors from experimentally determined pressure drops [18]. 

It can be seen, that packing No 1 is practically completely wetted at liquid superficial velocity about 7.10 m /(m s). At this liquid velocity the ratio of effective to total surfece of packing No 2 is about 0.78. At =10 m /(m s) it is about 0.8 and is not increasmg with fiirtiier incrc ing of L The ct that the effective area of this packing cannot reach the specific area is explained [64] with the small packing hydraulic diameter at which the influence of the thickness of the liquid film on the hydraulic diameter, respectively on the effective surface area, is not to be eliminated. That is, at =10 mV(m s) this packing is also completely wetted. 

A duplex outside-end-packed plunger pump with pot valves, of the type used with hydraulic presses and for similar service, is shown in Fig. 10-49. In this drawing, plunger A is direct-connec ted to rod B, while plunger C is operated from the rod by means of yoke D and tie rods. 

The Asahi process (Fig. 16-63) is used principally for high-volume water treatment. The hquid to be treated is passed upward through a resin bed in the adsorption tank. The upward flow at 30-40 m/h [12-16 gal/(min ft")] keeps the bed packed against the top. After a preset time, 10 to 60 min, the flow is interrupted for about 30 s, allowing the entire bed to drop. A small portion (10 percent or less) of the ion-exchange resin is removed from the bottom of the adsorption tank and transferred hydraulically to the hopper feeding the regeneration tank. 

Special Arrester Types and Alternatives Several types of unlisted arresters (water seals, packed beds, velocity-type devices, and fast-acting valves) mentioned in API 2028 are described more fully in Howard (1982). There are few design or test data for hydraulic and packed-bed arresters some types are designed and used by indiviciual companies for specific applications, while others are commercially available. Figure 26-27 shows some special arrester types. 

Decomposition Flame Arresters Above certain minimum pipe diameters, temperatures, and pressures, some gases may propagate decomposition flames in the absence of oxidant. Special in-line arresters have been developed (Fig. 26-27). Both deflagration and detonation flames of acetylene have been arrested by hydrauhc valve arresters, packed beds (which can be additionally water-wetted), and arrays of parallel sintered metal elements. Information on hydraulic and packed-bed arresters can be found in the Compressed Gas Association Pamphlet G1.3, Acetylene Transmission for Chemical Synthesis. Special arresters have also been used for ethylene in 1000- to 1500-psi transmission lines and for ethylene oxide in process units. Since ethylene is not known to detonate in the absence of oxidant, these arresters were designed for in-line deflagration application. 

FIG. 26-27 Some special arrester designs (a) liquid seal arrester (h) Linde hydraulic seal arrester (c) wetted packed-bed acetylene decomposition arrester. (Howai d, 19S2.)... 

Crushing, selective particle size packing or hydraulic compaction can be used to reduce interparticle void space and increase the bulk density within the storage tank to approach the particle density of the carbon. Even with these extreme methods of packing, the fraction of the vessel which is micropore is never greater than 0.50 for any commercial carbon, considerably short of the 0.70 which is necessary for 170 V/V storage. 

On initial start-up and shut-down the heat exchanger can be subjected to damaging thermal shock, overpressure or hydraulic hammer. This can lead to leaky tube-to-tubesheet joints, damaged expansion joints or packing glands because of excessive axial thermal, expansion of the tubes or shell. Excessive shell side flowrates during the "shake down can cause tube vibrations and catastrophic failure. 

The pressure switch that permits squeezing of the membranes only when the plate pack is compressed with the hydraulic closing system. 

Acetylene may propagate decomposition flames in the absence of any oxidant above certain minimum conditions of pressure, temperature, and pipe diameter. Acetylene, unlike most other gases, can decompose in a detonative manner. Among the different types of flame arresters that have proven successful in stopping acetylene decomposition flames are hydraulic (liquid seal) flame arresters, packed beds, sintered metal, and metallic balls (metal shot). 

Tests are needed to verify the design criteria of nonstandard flame arresters (hydraulic flame arresters, packed bed arresters, etc.). 

Strigle [82, 94] describes the hydraulics and HETP performance of a packed column by referring to Figure 9-22. As noted, the HETP values are essentially constant over a wide range of Cg alues shown as B-C on the figure. Note that Cg can be expressed ... 

Kaiser [140] presents a correlation analysis for flooding in packed towers that is more analytical in the performance approach. It is based on single phase hydraulics. It would have been helpful for the article to present a comparison of results tvith the other more conventional techniques. 

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