Vertically packed towers

The Chlorex process utilizes simple countercurrent mixing and settling tanks (four to seven stages) or modern vertical packed towers. Solvent recovery involves conventional flash columns and strippers operated under vacuum (26 to 28 inches of mercury) at about 300° to 325° F. Low temperatures are desirable to minimize decomposition and formation of hydrochloric acid. 

An extractor column is generally a tall, vertical packed tower that has two or more bed sections. Each packed bed section is typically limited to no more than 8 ft tall, making the overall tower height about 40 to 80 ft. Tower diameter depends fully upon liquid rates, but is usually in the range of 2 to 6 ft. Liquid-liquid extractors may also have tray-type column internals, usually composed of sieve-type trays without downcomers. These tray-type columns are similar to duoflow-type vapor-liquid separation, but here serve as contact surface area for two separate liquid phases. The packed-type internals are more common by far and are the type of extractor medium considered the standard. Any deviation from packed-type columns is compared to packing. 

The extraction operation proper consists of a series of countercurrent contacts between oil and solvent as in Figs. 11-8 and 11-9 however, vertical packed towers are more widely used than settling tanks. 

Packed-bed scrubbers m be construc ted for either vertical or horizontal gas flow. Vertical-flow units (packed towers) commonly use countercurrent flow of gas and liquid, although cocurrent flow is sometimes used. Packed scrubbers using horizontal gas flow usually employ cross-flow of liquid. 

In a trayed absorber the amine falls from one tray to the one below in the same manner as the liquid in a condensate stabilizer (Chapter 6, Figure 6-4). It flows across the tray and over a weir before flowing into the next downcomer. The gas bubbles up through the liquid and creates a froth that must be separated from the gas before it reaches the underside of the next tray. For preliminary design, a tray spacing of 24 in. and a minimum diameter capable of separating 150 to 200 micron droplets (using the equations developed in Volume 1 for gas capacity of a vertical separator) can be assumed. The size of packed towers must be obtained from manufacturer s published literature. 

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 ... 

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). 

Absorption can take place in a countercurrent, cocurrent, or cross-flow device. Vertical countercurrent towers are either built with a metal, plastic, or ceramic packing or constructed as plate towers with various types of plates. This chapter will discuss the solvents used to carry out absorption and the various types of absorption equipment. 

No column is perfectly vertical. Several reasons for column slant were described in Sec. 7.9. In this section, the effect of slant on packed towers is reviewed. 

Conventional packed towers are used for different mass transfer operations such as distillation and absorption. The tower consists of a vertically positioned cylindrical shell filled with packing. Liquid enters at the top of the packed column and flows downward, and gas enters at the bottom and flows upward through the packing. Thus, a countercurrent process takes place, and the packing provides for a high area for mass transfer contact (see Fig. 6.18). The efficiency of the packed tower for mass transfer depends upon the specific area of packing and liquid irrigation rates. It has... 

Packed towers, used for continuous contact of liquid and gas in both counter-current and cocurrent flow, are vertical columns which have been filled with packing or devices of large surface, as in Fig. 6.27. The liquid is distributed over, and trickles down through, the packed bed, exposing a large surface to contact the gas. 

Refer to Fig. 8.23, a schematic representation of a differential section of the packed tower [41). For convenience, the phases are shown separated by a vertical surface. Enthalpies are referred to the pure unmixed components in their normal state of aggregation at some convenient temperature /q and the tower pressure. Liquid enthalpies are given by Eq. (8.12), expressed per mole of solution... 

The gas and liquid are often placed in contact in a large vertical cylindrical tower. Several tower designs are shown in Fig. 28.3. Packed towers, spray towers, tray towers, and reverse jet-type scrubbers have aU been used to treat acid plant tail gas. They are usually constructed of fiber reinforced plastic with a suitable corrosion-resistant liner. Unlined stainless steel and rubber-lined carbon steel are also used. 

Other types of wet scrubbers in common use include spray towers, vertical and horizontal packed towers, jet ejectors, extended surface scrubbers, fan spray scrubbers and various individual designs, some of which may be described as cleaners. 

Equipment Absorption, stripping, and distiUation operations are usually carried out in vertical, cylindrical columns or towers in which devices such as plates or packing elements are placed. The gas and liquid normally flow countercurrently, and the devices serve to provide the contacting and development of interfacial surface through which mass transfer takes place. Background material on this mass transfer process is given in Sec.. 5. 

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