Support, packing open area

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. 

In large-diameter packed colxunns where I-beams support the packing support plate, a larger space between the reboiler return nozzle and the packing support plate may be required. The prime consideration here is allowing sufficient open area between the bottom of the beam and the high liquid level for adequate vapor distribution. Detailed discussion is in Sec. 8.2. 

Packing supports therefore must provide both a high fraction of open area and the required mechanical strength. 

Open areas provided by modem packing supports are usually of the order of 70 to 100 percent of the column cross-sectional area (74, 257, 305). The open area depends on the materials of construction and the column diameter some common designs in ceramic, carbon, and plastic have open areas smaller than 65 percent (214, 257, 305). 

Grid supports are generally less expensive than gas-injection supports and can provide open areas as high as 70 percent of column cross-sectional area in ceramic applications (305) and 95 to 97 percent in metallic applications (74). Grid supports are commonly used for structured packing, where gas-injection supports are usually unsuitable, and where most of the disadvantages listed below do not apply. 

Grid supports often provide insufficient open area with small packing sizes. The space between adjacent bars, and therefore the grid... 

If packing is to he installed in a column that previously contained trays, it is best to remove all internal support rings to within % in of the column shell (74, 299a). Horizontal support rings left in the column would interfere with liquid distribution and reduce the available open area, thus reducing column capacity and possibly its efficiency. 

Vertical bars (e.g., downcomer supports) are generally far less detrimental to distribution and open area but would hinder structured packing installation. Usually, these are only removed if they are likely to interfere adversely with the new internals or with liquid flow through them (386). Some designers (299a) prefer to always have these removed. 

Provide sufficient open area at the packing supports. 

Very important for all types of packings is a uniform liquid distribution at the top of the bed and a limitation of bed height to 6 or 8 m. Beneath each bed, the liquid has to be collected, mixed, and redistributed. These measures intend to suppress the so-called maldistribution of liquid because it strongly affects mass transfer rates. The design of liquid distributors, liquid collectors, support grids, etc., should provide a large open area not to hinder the countercurrent flow of gas and liquid. 

The first packing support plates for random dumped packings were slotted or perforated, flat ceramic plates. The use of 1-in. to 2-in. size ceramic Raschig ring packings, common in those days, required small holes or narrow slots that gave the plate a low open area of only 15% to 25%. Even... 

The packing support consists of a floor grating material with bar openings spaced to give 57.5% free void area of cross-section. 

An open-tubular column is a capillary bonded with a wall-supported stationary phase that can be a coated polymer, bonded molecular monolayer, or a synthesized porous layer network. The inner diameters of open-tubular CEC columns should be less than 25 pm that is less than the inner diameters of packed columns. The surface area of fused silica tubing is much less than that of porous packing materials. As a result, the phase ratio and, hence, the sample capacity for open-tubular columns are much less than those for packed columns. The small sample capacity makes it difficult to detect trace analytes. 

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