Turndown distributor

A perforated plate can be flat, concave, convex, or double-dished. The main advantages of the perforated plate are that it is simple, inexpensive, easy to modify, and easy to clean. The disadvantages of a perforated plate are the possibiUty of soflds leaking, ie, weeping through it into the plenum lower turndown capabiUty than other distributors the requirement of a peripheral seal and a relatively high pressure drop requited for good distribution. 

Several cap-type distributors are shown in Figure 12. These minimize weeping and have good turndown, but are difficult to clean and modify, and are more expensive than perforated plates. A peripheral seal is also requited as for a perforated plate. 

Figure 13 shows two pipe distributors, one in a branched and one in a ring configuration. These distributors minimize weeping, have good turndown, may requite the lowest pressure drop, and avoid the need for a plenum chamber. They are also well suited to multiple-level fluid injection. The disadvantages of these distributors are that there are defluidized soHds beneath the distributor and the mechanical design is more complex. 

Turndown on a PAN type tray should be limited to 2 1 (ratio of high to low flow rates), which results in a reasonable design. Bonilla [131] points out that it is cosdy to design for short-term high turndown rates such as start-up, shutdown, or for other short term periods, because it is better to increase reflux ratio to increase internal loads for such periods rather than design the distributor for large turndown. 

Wide turndown ratio limited only by distributors... 

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. 

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

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. 

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. 

The main consideration for introducing reflux or intermediate feed into a packed tower is adequately distributing the incoming stream to the packing. Unlike most tray columns, packed towers are sensitive to distribution. Maldistribution is detrimental to packing efficiency and turndown. The main devices that set the quality of distribution in a packed column are the top (or reflux) distributor, the intermediate feed distributor, the redistributor, and sometimes the vapor distributor. Adequate hydraulics in the inlet area is also important failure to achieve this can affect distributor performance and can also cause premature flooding. 

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... 

The common gravity distributors are the weir type and the orifice type. Both types can handle large liquid flow rates. The weir tjq)e is generally one of the least troublesome distributors and has an excellent turndown, but it can usually provide only a limited number of drip points and is extremely sensitive to levelness and liquid surface agitation. The orifice type may suffer from corrosion and plugging, but it can be designed with a large number of drip points to provide superior liquid distribution. 

The perforated-pipe distributor has a relatively low turndown ratio, roughly 2 1 to 2.5 1 (111, 305). Excessive liquid flow rates may generate fine mist, while deficient liquid flow rates may generate uneven irrigation (Fig. 2.5d). The turndown can be enhanced by using a dual liquid distributor (Fig. 3.65 Sec. 3.8). 

Like perforated-pipe distributors, the spray distributor turndown ratio is about 2 1 (111). Excessive liquid flow rates may create a mist problem, while low liquid flow rates reduce the cone diameter and may create poorly irrigated areas. For this reason, oversized spray nozzles must be avoided (237). As with perforated pipe distributors, a dual liquid distributor can be used to enhance turndown (Sec. 3.8). 

Within the above limits and the turndown requirements, there is often an incentive to minimize the maximum liquid depth. The lower the maximum liquid depth, the greater is the number of drip points that can be incorporated, the smaller is the vertical space consumed by the distributor, and the lesser is the... 

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