Maldistribution, trays liquid

When straight or serrated segmental weirs are used in a column of circular cross section, a correction may be needed for the distorted pattern of flow at the ends of the weirs, depending on liquid flow rate. The correction factor from Fig. 14-33 is used directly in Eq. (14-112) or Eq. (14-119). Even when circular downcomers are utilized, they are often fed by the overflow from a segmental weir. When the weir crest over a straight segmental weir is less than 6 mm (V4 in), it is desirable to use a serrated (notched) weir to provide good liquid distribution. Inasmuch as fabrication standards permit the tray to be 3 mm hi in) out of level, weir crests less than 6 mm (V4 in) can result in maldistribution of liquid flow. 

Fouling and corrosion. If the service is a fouling one, dirt and pol3oner may accumulate under the downcomer and restrict the flow area. This may cause excessive backup, premature flooding, and maldistribution of liquid to the tray. Clearance under the downcomer should never be less than 2 in (38, 86, 123, 172, 192) in order to avoid blockage. It is best to avoid clearances smaller than 1 in, particularly if fouling may occur. 

In principle, a colrrrrm tray can be operated even with very small liquid loads because the necessary height of the two-phase layer (froth) on the tray is provided by the exit weir. At extremely low liquid loads, however, the liquid will flow in an uneven pattern across the tray resulting in some degree of maldistribution of liquid. Accordingly, it is recommended to ensure a ttunimum liquid flow rate over the exit weir larger than Fl/ w 2 m /(m h). In small diameter columns, however, the liquid load can be considerably lower. 

The above results are based on data obtained for optimized designs and under ideal test conditions. To translate our findings to the real world, one must factor in liquid and vapor maldistribution, which is far more detrimental to the efficiency of packings than trays. In addition. one also must account for poor optimization or restrictive internals, which are far more detrimental to the capacity of trays than packings. We also have cited several other factors that need to be considered when translating the findings of our analysis to real-world towers. ... 

Mechanism 2 of Figure 8-122B becomes apparent when the flow recirculation on the tray increases with increasing underflow clearance. The curvature of the column wall influences the movement of the liquid toward the center. High underflow clearance does not even out maldistribution due to backup where the irregular flow pattern enters into the tray below. This allows flow separation to occur on the downcomer floor, and leads to enhanced retrograde flow. 

Dual-Flow Trays These are sieve trays with no downcomers (Fig. 14-27b). Liquid continuously weeps through the holes, hence their low efficiency. At peak loads they are typically 5 to 10 percent less efficient than sieve or valve trays, but as the gas rate is reduced, the efficiency gap rapidly widens, giving poor turndown. The absence of downcomers gives dual-flow trays more area, and therefore greater capacity, less entrainment, and less pressure drop, than conventional trays. Their pressure drop is further reduced by their large fractional hole area (typically 18 to 30 percent of the tower area). However, this low pressure drop also renders dual-flow trays prone to gas and liquid maldistribution. 

Low liquid rates. With the aid of serrated weirs, splash baffles, reverse-flow trays, and bubble-cap trays, low liquid rates can be handled better in trays. Random packings suffer from liquid dewetting and maldistribution sensitivity at low liquid rates. 

Liquid flow patterns and maldistribution on large trays... 

Most popular theoretical models (such as the AlChE and the Chan and Fair models, Sec. 7.2.1) postulate that liquid crosses the tray in plug flow (Fig. 7.7a) with superimposed backmixing, and that vapor is perfectly mixed. Increasing tray diameter promotes liquid plug flow and suppresses backmixing. This should enhance efficiency in large-diameter columns, but such enhancement has not been observed (147,148). Liquid maldistribution is the common explanation to the observation. 

Case studies were reported (170,174) of large-diameter (> 15-ft) towers with sieve trays not reaching the expected efficiency. Maldistribution was cited as the culprit or at least one of the causes. Improving liquid flow patterns, often among other modifications, was the fix. The only other evidence that channeling adversely affects tray efficiency comes from the above-mentioned theoretical models. 

Vapor maldistribution. Most popular theoretical models (such as the AIChE and the Chan and Fair models, Sec. 7.2.1) postulate perfectly mixed vapor flow. In larga-diameter columns, vapor is more likely to rise in plug flow. Modeling work showed (143,179,180) that in the absence of stagnant zones on the tray, vapor flow pattern has generally little effect on tray efficiency. When column efficiency exceeds 30 percent (143), or when stagnant liquid zones exist (171,173,180), vapor plug flow reduces tray efficiency. 

A good liquid distributor is necessary to prevent maldistribution. The existing liquid distributor consists of a tray and a number of short chimneys. Liquid is collected on the tray and flows onto the bed from the chimneys through the small holes on the side. A cold flow model test showed that liquid flow rate from the short chimneys was extremely sensitive to the level of the tray. It was also pointed out that liquid dispersion from the short chimneys was poor. Therefore, we developed a new liquid distributor, which improved the defects of the existing one. The new liquid distributor with tall chimneys can achieve uniform liquid distribution, even if the tray is declined. Each chimney also has a feature to well disperse liquid onto the bed. 

Large liquid head gradients reduce tray capacity and may damage separation by causing maldistribution of vapor flow across the tray. In fact, an extremely large head gradient can cause some caps to dump the liquid to the tray below. 

Whenever a product draw is taken from a collector pan, the liquid fiow through any orifice hole depends on the local height. This guarantees that some degree of liquid maldistribution will take place. Additionally, orifice-pan distributors are installed on tray rings welded to the tower wall. If the tray ring is not level, the orifice pan will have liquid maldistribution caused by the out-of-level installation. 

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