Weirs picket fence

Picket fence weirs were installed on other inboard downcomers also for shielding any blowing. 

Adjustable weirs (Fig. 14-22 ) are used to provide additional flexibility. They are uncommon with conventional trays, but are used with some proprietary trays. Swept-back weirs (Fig. 14-22b) are used to extend the effective length of side weirs, either to help balance liquid flows to nonsymmetric tray passes or/and to reduce the tray liquid loads. Picket fence weirs (Fig. 14-22c) are used to shorten the effective length of a weir, either to help balance multipass trays liquid flows (they are used in center and off-center weirs) or to raise tray liquid load and prevent drying in low-liquid-load services. To be effective, the pickets need to be tall, typically around 300 to 400 mm (12 to 16 in) above the top of the weir. An excellent discussion of weir picketing practices was provided by Summers and Sloley (Hydroc. Proc., p. 67, January 2007). 

For picket-fence weirs and for rectangular notched weirs, and tray liquid level below the top of the teeth, QL in Eq. (6.49) is based on the weir length less the total weir length occupied by the teeth. Fw is based on the total weir length (including the length of the teeth). 

Picket fence weirs are used in low-liquid-rate applications (Fig. 8). Picket fence weirs can serve two purposes at low liquid rates. First, they reduce the effective length of the weir for liquid flow increases the liquid height over the weir. This makes tray operation less sensitive to out-of-level installation. Second, pickets can prevent liquid loss (blowing) into the downcomer by spraying. This occurs at low liquid rates when the vapor is the continuous phase on the tray deck. Picket fence weirs should be considered if the liquid load is less than 1 gpm per inch of weir (0.0267 ft /sec/ft, 0.00248 m /sec/m). At liquid rates lower than 0.25 gpm per inch of weir (0.00668 ft / sec/ft, 0.000620 m /sec/m) even picket fence weirs and splash baffles have a mixed record in improving tray efficiency. Operation at liquid rates this low strongly favors the selection of structured packing. 

Picket fence weirs are also used to balance flow to the different passes on trays that have more than two flow paths. Splash baffles are an alternative to picket fence weirs when the major problem is blowing. 

Obviously, the weir loading is too small and less than the lower limit of 2 gpm/in. This could cause fluidization resulting in poor distillation efficiency. To prevent this from happening, picket fence weir is used to artihcially increase the weir loading. Let 60% of the weir flow length be blocked by teeth and thus the effective weir length is 0.8 ft. In this case, the weir loading becomes... 

Downcomer and Weir Type Straight downeomer and chordal weir are selected for simplicity. Picket fence weir (blocked weir type) is employed to increase the wen-... 

Maximum Liquid Leading Limit Based on Table 12.2, the maximum weir loading allowed for a single pass is 8 gpm/in. Keep in mind that the effective weir length iwe = 0.8 ft because a picket fence weir is used, although the physical weir length L is 2 ft. Thus, the maximum liquid loading can be calculated as... 

FIG. 14-22 Unique outlet weir types, (a) Adjustable, (b) Swept back, (c) Picket fence. (Parts a, c,from H. Z. Kister, Distillation Operation, copyright 1990 by McGraw-HiU reprinted by permission. Part b, courtesy of Koch-Glitsch LP.)... 

Multipass Balancing There are two balancing philosophies equal bubbling areas and equal flow path lengths. Equal bubbling areas means that all active area panels on Fig. 14-21 d are of the same area, and each panel has the same hole (or open-slot) area. In a four-pass tray, one-quarter of the gas flows through each panel. To equalize the L/G ratio on each panel, the liquid needs to be split equally to each panel. Since the center weirs are longer than the side weirs, more liquid tends to flow toward the center weir. To equalize, side weirs are often swept back (Fig. 14-22b) while center weirs often contain picket fences (Fig. 14-22c). 

The alternative philosophy (equal flow path lengths) provides more bubbling and perforation areas in the central panels of Fig. 14-21d and less in the side panels. To equalize the L/G ratio, less liquid needs to flow toward the sides, which is readily achieved, as the center weirs are naturally longer than the side weirs. Usually there is no need for swept-back weirs, and only minimal picket-fencing is required at the center weir. 

Two methods are used to prevent tray liquid blowing. The simplest is probably the picket-fence exit weir (see Fig. 13). In this arrangement, a high exit weir interrupts the trajectory of flying liquid droplets. The slots control liquid depth on the tray. 

Fig. 13 Picket-fence exit weir 1) spray-catching baffle, 12 in. or higher 2) nominal exit weir height 3) and open slots to allow liquid passage.

Picket fence splash baffles. These are placed on top of downcomers or weirs to break up foam entrainment or froth. 

According to Summers and Sloley (2007), when actual liquid loading is small, the most effective way to avoid spray regime is using picket fence outlet weirs to artificially increase weir loading. 

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