Packed towers structured packings

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. 

The broad classes of packings for vapor-liquid contacting are either random or structured. The former are small, hollow structures with large surface per unit volumes that are usually randomly dumped into the tower. Structured packings are normally layers of elements fabricated form expanded metal, woven wire screen, or sheet metal, and are stacked into the tower very carefully. 

Some other representative cost relationships can also be found in this reference (12), but these cannot be used as a substitute for data from the supplier. In general, the equipment costs include the following components fans, water pumps to dehver the water to the top of the tower, tower structure and packing, exterior covering, and electrical installation. The principal operating and maintenance costs are electrical power for pumping water and air, operating and maintenance labor, and chemicals to raise the pH. 

Solid adsorbents must also be structurally capable of being packed into a tower, resistant to fracturing, and capable of being regenerated and reused after saturation with gas molecules. Although some small units use throwaway canisters or charges, the majority of industrial adsorbers regenerate the adsorbent to recover not only the adsorbent but also the adsorbate, which usually has some economic value. 

Earliest type, usually cheaper per unit cost, but sometimes less efficient than others. Available in widest variety of materials to fit service. Very sound structurally. Usually packed by dumping wet or dry, with larger 4-6-inch sizes sometimes hand stacked. Wall thickness varies between manufacturers, also some dimensions available surface changes with wall thickness. Produce considerable side thrust on tower. Usually has more Internal liquid channeling, and directs more liquid to walls of tower. Low efficiency. 

We conclude this subsection with a bravo to Bravo" who provides useful rules of thumb for tower retrofits. These are intended for engineers comparing structured packing to high-performance trays for replacing older conventional trays. 

Problem A structured packing vacuum tower had too much heavy key in a vapor side-draw between the feed and the column bottom. The side-draw heavy key concentration was several times the design value. 

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. 

Some recent advances in structured packing are being used by some operators in larger diameter towers where they would have normally used trays. The structured packing is said to allow both smal " ter and less height of tower. 

Figure 9-6CC. York-TwisP structured tower packing. Note insert showing weave and sections fabricated to fit through manway for larger towers. Used by permission of Otto H. York Co., Inc., a division of Glitsch, Inc.

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.

The heavy naphtha-Ught gas oil fractionation zone of a crude tower has to be revamped to handle 25% more capacity. Because trays would be working at high percent flooding, Gempak structured packing is condensed (Figures 9-56A-D). 

A commercial 4-ft diameter refinery depropanizer unit s performance after replacement of the lower half of a trayed tower with Intalox-2T (R) structured packing is described in Reference 118. 

Water quality is important, not only from an environmental point of view but also in relation to the type of packing to be specified. Analysis of the circulating water is simple to obtain, but it is very seldom offered to the cooling tower designer. The quality, or lack of it, will determine the type of pack to be used, the selection of structural materials and whether the tower should be induced or forced draft, counterflow or crossflow. Water treatment, in the shape of chemicals to control pH and to act as counter-corrosion agents or as biocides, all has a bearing on tower selection. 

Packings of random and structured character are suited especially to towers under 3 ft dia and where low pressure drop is desirable. With proper initiai distribution and periodic redistribution, voiumetric efficiencies can be made greater than those of tray towers. Packed internals are used as replacements for achieving greater throughput or separation in existing tower shells. 

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. 

CUORE is a proposed tightly packed array of 988Te02 detectors, each being a cube 5 cm3 on a side with a mass of 750 g. The array consists of 19 vertical towers with 52 crystals each, arranged in a cylindrical structure as shown in the Fig. 16.6. 

There are a few other kinds of industrial plants. An air-lift tower fermenter was developed by ICI for the production of single-cell protein from methanol which employs an external loop for heat removal. Trickle beds for the treatment of waste waters employ packing structures or packing elements of 4-6 cm dia and porosities above 50%. Microbial films are formed on the packing that react with the organic substrate and the air as the liquid flows down and the air up. 

Carbon nitride, 27 214—215 conductivity of, 2 7 204-206 field emission properties of, 27 221 theoretical band structure calculations for, 27 203-204 Carbon nitride films, 2 7 205 Carbon nitride solids, 2 7 203 Carbonochloridic esters, 6 290 Carbon paper, 4 738-739 Carbon profile, in gas carburizing, 26 204 Carbon Raschig-ring tower packing, 22 745 Carbon reduction, of ferrovanadium, 25 518... 

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). 

Adding a downstream gasoline splitter can help when reducing gasoline sulfur by minimizing octane loss. Other towers may be expanded by using structured packing. 

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