Packed towers liquid distribution

The actual liquid-to-gas ratio (solvent-circulation rate) normally will be greater than the minimum by as much as 25 to 100 percent and may be arrived at by economic considerations as well as by judgment and experience. For example, in some packed-tower applications involving veiy soluble gases or vacuum operation, the minimum quantity of solvent needed to dissolve the solute may be insufficient to keep the packing surface thoroughly wet, leading to poor distribution of the liquid stream. 

Liquid distribution probablv phtys the most important part in the efficient o]veration of a packed tower. good packing from the process viewpoint can be rednced in effectiv eness bv poor liquid distribution across the to ) of its upper surface or the packing sections below mv feed inlet(s) oi rellitx inlets. 

Ov Crall, the careful design of a distributor for liquid in the top of a packed tower, and for the redistribution of liquid flowing dow n multi-section packing in the tower, is essential to good consistent tower performance. However, the liquid flow is not alone, the uniformity of vapor distribution is likewise essential, because non-uniform vapor distribution can cause non-uniform liquid downflow. Then, there is the selection of the packing itself and its characteristics and requirements/sensitivity to the uniform distribution of the liquid and vapor. As earlier emphasized, the level of the distributor tray or trough can be critical to the consistent uniform liquid distribution. 

For stacked packing the liquid usually has little tendency to cross-distribute, and thus moves down the tower in the cross-sectional area that it enters. In the dumped condition most packings follow a conical distribution down the tower, with the apex of the cone at the liquid impingement point. After about 12 ft vertical height, the liquid flow s vertically downward unless redistributed. For uniform liquid flow and reduced channeling of gas and liquid with as efficient use of the packing bed as pos.sible, the impingement of the liquid onto the bed must be as uniform as possible. 

Tour, R. S. and Lerman, F. Trans. Am. Inst. Chem. Eng. 35 (1939) 709-18. An improved device to demonstrate the laws of frequency distribution. With special reference to liquid flow in packed towers. 

Traditionally, packings have not been used in distillation duties, except for small columns of less than 1 m diameter, and even less for high pressure service. The situation has changed over the last 10-15 years, partly through a better understanding of how packed towers work, particularly with respect to liquid distribution, but also through the development of new proprietary packings. 

The problem we have just discussed—poor fractionation efficiency due to inadequate vapor and liquid initial distribution—is rather similar to tray deck dumping in trayed fractionators. And, just like trays, packed towers are also subject to flooding. 

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. 

Complex towers. Interreboilers, intercondensers, cooling coils, and side drawoffs are more easily incorporated in trays than in packed towers. In packed towers, every complexity requires additional distribution and/or liquid collection equipment. 

Having an HETP and the number of theoretical stages required, you are ready to set the packing height in the tower. Please realize that it is critical to ensure that good and effective liquid distribution occurs at the top of each packed tower section. You may ask how many packed sections are necessary. Industry worldwide practice has been to limit the height of each packed section to preferably 6 ft, and to no more than 10 ft in almost every case. 

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). 

Figure a. 16 Typical efficiency characteristics of packed towers, (a) Typical efficiency characteristics for random packings and for most corrugated-sheet structured packings (6) effect of liquid distribution on the efficiency characteristics of part o. (Part b from H. Z. Kieter, Distillation Operation. Copyright C by McGhawnMf, Me. Reprinted by permission.)... 

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