Packing open area

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. 

Filters for mists and droplets have more open area than those used for dry parhcles. If a filter is made of many fine, closely spaced fibers, it will become wet due to the collected liquid. Such wethng will lead to mathng of the fibers, retenhon of more liquid, and eventual blocking of the fiter. Therefore, instead of fine, closely spaced fibers, the usual wet filtrahon system is composed of either knitted wire or wire mesh packed into a pad. A looser filtrahon medium results in a filter with a lower pressure drop than that of the filters used for dry parhculates. The reported pressure drop across wire mesh mist eliminators is 1-2 cm of water at face velocihes of 5 m sec T The essenhal collechon mechanisms employed for filtrahon of droplets and mists are inertial impachon and, to a lesser extent, direct intercephon. 

This is an open area packing with multiple layers of lattice-type panels. This grid, as described by the manufacturer s bulletin, consists of vertical, slanted, and horizontal planes of metal. The vertical strips have horizontal flanges oriented alternately right and left. Due to the random overlap, the vapor path must zig-zag through the bed. 

Open area—the average percentage of the cross-sectional area of the tower not blocked by the packing, and hence available for the flow of vapor and liquid. 

Structured packing has about 50 percent more open area than Raschig rings and two or three times their wetted surface area. Hence, structured packing has largely replaced packing in the form of rings, in many packed towers. 

Packed-bed supports. A simple support for a bed of lVfc-in pall rings might consist of a bar grid, with Vi-in-thick bars, set on 1 inch center. The grid itself (see Fig. 7.7) would be supported by two 6-in-wide I-beams. The open area of each component is... 

Considering that the rings, the grid support, and the I-beam are all in close physical contact, what is the open area at their mutual interface (Answer 80 percent X 75 percent X 90 percent = 54 percent.) This means that there will be a restriction to vapor and liquid flow at the grid support, which will promote flooding at the bottom of the bed. Naturally, this would cause the entire packed bed to flood. 

To avoid such flooding, the designer uses the corrugated bed support shown in Fig. 7.8. This grid will have over 100 percent open area. Crushed packing laying on even this excellent type of bed support can, however, cause flooding. 

Foaming (and emulsion). The low gas and liquid velocities in packing suppress foam formation. The large open area of the larger random packing promotes foam dispersal. Both attributes make... 

Negligible pressure drop may be a reasonable assumption for an unpacked tubular or annular region especially for a reasonably shon reactor or for a packed region where the catalyst particle size is relatively large compared to the opening area for the fluid flow. Otherwise, pressure drop equations applicable to packed beds of catalyst particles are needed. 

Throttling valves must be selected for the high pressure drops encountered with maximum unrestricted open area to avoid clogging. Valve noise level must be investigated, with provisions made to reduce the noise level if necessary. As with other equipment choices, the location, design and type of seals, packing and gaskets must be... 

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