Contact Equipment Packed Towers

Towers filled with the various types of packings described in Chap. 6 are frequently competitive in cost with trays, and they are particularly useful in cases where pressure drop must be low, as in low-pressure distillations, and where liquid holdup must be small, as in distillation of heat-sensitive materials whose exposure to high temperatures must be minimized. There are also available extremely effective packings for use in bench-scale work, capable of producing the equivalent of many stages in packed heights of only a few feet [40]. 

For packed towers, rates of flow are based on unit tower cross-sectional area, mol/(area)(time). As for absorbers, in the differential volume dZ, Fig. 9A9a, the interface surface is a dZ, where a is the of Chap. 6. The quantity of substance A in the vapor passing through the differential section is Gy mol/(area)(time), and the rate of mass transfer is d Gy) mol A/(differential volume)(time). Similarly, the rate of mass transfer is d Lx). Even where the usual simplifying assumptions are not strictly applicable, within a section of the column G and L are both sufficiently constant for equimolar counterdiffusion 

A similar expression, with appropriate integration limits, applies to the stripping section. 

For any point (jc,y) on the operating line, the corresponding point (a ,-,, ) on the equilibrium curve is obtained at the intersection with a line of slope — kyky - k a/kyO drawn from (x,y), as shown in Fig. 9.496. For /c ky so that the principal mass-transfer resistance lies within the vapor, y,- — y is more accurately read than x - x-,. The middle integral of Eq. (9.152) is then best used, evaluated graphically as the area under a curve of I/ a(y,. — y) as ordinate, Gy as abscissa, within the appropriate limits. For k kyy it is better to use the last 

For cases where the usual simplifying assumptions apply, G and L within any section of the tower are both constant, and the heights of transfer units 

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Process air in sulfur-burning plants is dried by contacting it with 93—98 wt % sulfuric acid in a countercurrent packed tower. Dry process air is used to minimise sulfuric acid mist formation in downstream equipment, thus reducing corrosion problems and stack mist emissions. 

Extraction (sometimes called leaching) encompasses liquid-liquid as well as liquid-solid systems. Liquid-liquid extraction involves the transfer of solutes from one liquid phase into another liquid solvent it is normally conducted in mixer settlers, plate and agitated-tower contacting equipment, or packed or spray towers. Liquid-solid extraction, in which a liquid solvent is passed over a solid phase to remove some solute, is carried out in fixed-bed, moving-bed, or agitated-solid columns. 

Packed towers are used as contacting equipment for gas-liquid and liquid-liquid systems. Figures -1A and B present a cross-section of a typical unit. The shell is usually q lindrical, although square wooden, light metal, or reinforced plastic towers are used. The basic unit consists of ... 

Efficient contact is produced between the phases in agitated gas-liquid contactors and, therefore, this type of equipment can also be useful for those absorption and stripping operations for which conventional plate or packed towers may not be suited. It may also be useful where the operation involves the contact of three phases—say, gas, liquid, and suspended solids. The latter application could be represented by the low-pressure polymerization of ethylene with solid catalysts (F5). 

As the potentialities of liquid extraction as a separation method were developed, the need for efficient, continuously operated, multistage equipment became apparent. It was natural therefore to turn to devices which had been so successful in other similar fluid-contacting operations, such as the bubble-tray tower and the packed tower of distillation. These devices have proved to be disappointing in liquid-extraction service, however for example, bubble-tray towers provide tray efficiencies in liquid-extraction operations of less than 5% (S7), and conventional packed towers show heights of transfer units of 10 to 20 ft. or more (T3). 

Continuous changes in compositions of phases flowing in contact with each other are characteristic of packed towers, spray or wetted wall columns, and some novel equipment such as the FHGEE contactor (Fig. 13.14). The theory of mass transfer between phases and separation of mixtures under such conditions is based on a two-film theory. The concept is illustrated in Figure 13.15(a). 

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). 

The objective of absorption equipment is to provide intimate contact between the liquid concerned and gas, thus facilitating the absorption process. When a large volume of gas is absorbed per unit volume of liquid, a marked rise in temperature may result and devices for the removal of the heat evolved are then an important feature of the equipment. Apart from packed towers, the types of equipment described here include agitated vessels, bubble-plate towers, cooler-absorbers and spray towers. 

Both absorption and stripping can be operated as equilibrium stage operations with contact of liquid and vapor. Since distillation is also an equilibrium stage operation with contact of liquid and vapor, we would expect the equipment to be quite similar. This is indeed the case both absorption and stripping are operated in packed and plate towers. Plate towers can be designed by following an adaptation of the McCabe-Thiele method. Packed towers can be designed by use of HETP or preferably by mass transfer considerations (see Chapter 16T... 

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. 

In order that hot condenser water may be re-used in a plant, it is normally cooled by contact with an air stream. The equipment usually takes the form of a tower in which the hot water is run in at the top and allowed to flow downwards over a packing against a countercurrent flow of air which enters at the bottom of the cooling tower. The design of such towers forms an important part of the present chapter, though at the outset it is necessary to consider basic definitions of the various quantities involved in humidification, in particular wet-bulb and adiabatic saturation temperatures, and the way in which humidity data are presented on charts and graphs. While the present discussion is devoted to the very important air-water system, which is in some ways unique, the same principles may be applied to other liquids and gases, and this topic is covered in a final section. 

Equipment for reactive absorption is of the same types as for physical absorption, towers of various kinds and stirred tanks. Packed or tray towers are the most common, but spray or bubble towers are used for their mechanical simplicity and when there is a likelihood of clogging. Thus S02 is scrubbed from air with a spray of lime slurry in a tower. Fluorine waste gases form solids on contact with water, so they are scrubbed by bubbling the gas through water in an empty tower. 

It certainly is apparent that today s countercurrent packed extraction tower is not the ultimate in contacting and separating equipment. These huge towers, 40 to 80 feet in height, are often equivalent to only 1.5 to 4 theoretical stages. The desirability of a more compact, economical, efficient device is self-evident. 

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