Structured packed towers

A typical amine system is shown in Figure 7-4. The sour gas enters the system through an inlet separator to remove any entrained water or hydrocarbon liquids. Then the gas enters the bottom of the amine absorber and flows counter-current to the amine solution. The absorber can be either a trayed or packed tower. Conventional packing is usually used for 20-in. or smaller diameter towers, and trays or structured packing for larger towers. An optional outlet separator may be included to recover entrained amines from the sweet gas. 

To allow for the vertical height required for packed tower distributors and redistributors—and in tray towers the vertical height used by additional trays—typically using 10%-20% of the vertical packed height (10% for 2-in. random packing and 20% for structured packing) [136] the analysis indicated ... 

Figure 9-21F. Strigle s latest generalized pressure drop correlation. Note G = gas mass velocity, Ib/ft -sec. Used by permission of Strigle, R. F. Jr., Packed Tower Design and Applications Random and Structured Packings, 2nd ed. Gulf Publishing Co., (1994) p. 19.

Grids have an open structure and are used for high gas rates, where low pressure drop is essential for example, in cooling towers. Random packings and structured packing elements are more commonly used in the process industries. 

Dullien, F. A. L. Porous Media Fluid Transport and Pore Structure (Academic Press, New York, 1979). Leva, M. Tower Packings and Packed Tower Design, 2nd edn. (U.S. Stoneware Co., 1953). 

Structured packing has about 50 percent more open area than Raschig rings and two or three times their wetted surface area. Hence, structured packing has largely replaced packing in the form of rings, in many packed towers. 

Maintaining Functional and Structural Efficiency in Packed Towers... 

A packed tower can successfully fractionate with a very small pressure drop, as compared to a tray. For a modern trayed tower, to produce one single theoretical tray worth of separation (that s like a single, 100 percent efficient tray), a pressure drop of about 6 in of liquid is needed. A bed of structured packing can do the same job, with one inch of liquid pressure drop, even when allowing for the vapor distributor. In low-pressure fractionators, especially vacuum towers used to make lubricating oils and waxes, this can be of critical importance. 

The key properties of mixtures of air and water vapor are described in Section 9.1. Here the interactions of air and water in packed towers under steady flow conditions will be analyzed. The primary objectives of such operations may be to humidify or dehumidify the ait as needed for particular drying processes or other processes, or to cool process water used for heat transfer elsewhere in the plant. Humidification-dehumidification usually is accomplished in spray towers, whereas cooling towers almost invariably are filled with seme type of packing of open structure to improve contacting but with minimum pressure drop of air. 

Refurbishment of cooling tower structures to include replacement of wood with plastic or metal, fitting of new drift eliminators, or fill-packs, and epoxy painting of exposed metal surfaces. 

FIG. 14-51 Illustrative cutaway of a packed tower, depicting an upper bed of structured packing and a lower bed of random packing. (Courtesy of Sulzer Chemtech.)... 

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.)... 

Figure 8.16 (Continued) Typical efficiency characteristics of packed towers, (c) Typical efficiency characteristics for wire-mesh structured packings (d) example of efficiency characteristics, measured for randem packing, that deviate from those in parts a and c. (Fart d from J. S. Eckert and L. F. Walter, Hydrocarb. Proc. Pet. Ref, February 1964. Reprinted courtesy of Hydrocarbon Processing.)...

High liquid rates. Multipass trays effectively lower the liquid load seen by each part of the tray. A similar trick cannot be applied with packed towers the capacity of packings, especially structured, tends to rapidly fall off at high liquid rates. It is often more economical to handle high liquid rates in tray columns. 

Redistributors and wall wipers counteract the spread of liquid toward the wall. There is uncertainty regarding their effectiveness in keeping the liquid off the walls of randomly packed towers (144). It appears that their main value is in promoting lateral mixing (Sec. 9.2.3). In structured packings, well-designed wall wipers were shown (67) to effectively keep liquid off the wall. 

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