Turndown packings

Liquid maldistribution lowers packing turndown. The 2-in Pall rings curve in Fig. 14-59 shows HETP rise upon reaching the distributor turndown limit. 

Liquid maldistribution tends to lower packing turndown (221, 386, 387). The "standard distributor curve in Fig. 3.1 depicts typical variation of packing HETP (height equivalent of a theoretical... 

Random packing has traditionally been used in small diameter ( t in.) towers. This is because it is easier and le.ss expensive to pack t small diameter towers. However, random packed beds are prone to c neling and have poor turndown characteristics when compared i trays. For these reasons, trays were preferred for tower diameters greater than 20 in. In recent years an improved understanding of the impact of... 

Packed beds also seem to have a better turndown capability than valve or sieve trays, at low vapor flows. On the other hand, many packed fractionators seem quite intolerant of reduced liquid or reflux flow rates. This is typically a sign of an improperly designed distributor in the packed fractionator. 

In comparison with tray towers, packed towers are suited to small diameters (24 in. or less), whenever low pressure is desirable, whenever low holdup is necessary, and whenever plastic or ceramic construction is required. Applications unfavorable to packings are large diameter towers, especially those with low liquid and high vapor rates, because of problems with liquid distribution, and whenever high turndown is required. In large towers, random packing may cost more than twice as much as sieve or valve trays. 

Turndown. Moving valve and bubble-cap trays normally give better turndown than packings. Unless very expensive distributors are used, packed tower turndown is usually limited by distributor turndown. 

Packed-tower efficiency and turndown are strongly dependent on the quality of initial liquid distribution. Uneven distribution may cause local variations in the liquid/gas ratio, localized pinch conditions, and reduced vapor-liquid contact. Figure 14 shows two common liquid distributor types, the ladder type (shown as the top distributor) and the orifice type (shown as the redistributor). The ladder type is a horizontal header of pipes, which are perforated on the underside. The orifice type is a flat perforated plate equipped with round or rectangular risers for gas passage. Other common types of distributors are a header equipped with spray nozzles (spray distributor) and a header of horizontal channels, with V notches cut in the vertical walls of the channels (notched-trough distributor). 

Generally, the minimum wetting rate is at 0-5 to 2 gpm/ft2 for random packings, and 0.1 to 0.2 gpm/ft2 for structured packings (Sec. 8.2.15). It follows that point A is usually a distributor turndown limit. Regardless of which limit point A represents, it is extremely sensitive to maldistribution (Fig. 8.16b). When liquid distribution is poor, it will take more liquid to wet the entire bed, and point A will shift to the right. If distribution is very poor, point A may never be observed, and the curve will have no flat region at all. A V-shaped curve is not uncommon, and is indicative of poor distribution. 

Packed towers are used increasingly in small fractionating towers and absorbers where accessibility is restricted. Their use is also advantageous where corrosion control is essential. Because of liquid-distribution concerns, packing still is usually not recommended for large-diameter columns or where a high turndown ratio is required. All packings tend to spread liquid as it flows downwards. This characteristic is adequate in... 

An attractive feature of packed beds for mass and heat transfer operations is that the bed itself offers better turndown characteristics than the equivalent tray column. The mass transfer efficiency of a packed column with a good distributor is about as good at both low and high loadings. This favorable comparison is evident from the data in Fignres 12.26 and 12.49 (same colnmn and test mixture). The liquid distribution is the key, as indicated in Figure 12.50. 

For distillation columns Add sufficient trays to account for disturbances and anticipated expansion of production. The expected turndown ratios affect the choice of internals in a distillation column with a large turndown ratio, suggesting the use of bubble caps a low turndown ratio may point to structured packings. At the same time, safety and hazard analysis indicates that we want a minimum of liquid holdup in the system. 

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