Distillation in packed towers

The transfer unit concept is also applicable to distillation in packed towers. Height of the packing required is ... 

Figure 13.15. Mechanism, nomenclature, and constructions for absorption, stripping and distillation in packed towers, (a) Two-film mechanism with equilibrium at the interface, (b) Sketch and nomenclature for countercurrent absorption or stripping in a packed tower, (c) Equilibrium and material balance lines in absorption, showing how interfacial concentrations are found, (d) Equilibrium and material balance lines in stripping, showing how interfacial concentrations are found, (e) Equilibrium and material balance lines in distillation, showing how interfacial concentrations are found.

The subscript W indicates water as the liquid. The holdup data are summarized in Table 6.5. Although these data are based on work with absorption, desorption, and vaporization, the evidence is that the same coefficients apply to distillation in packed towers [89]. In distillation, a s should be used. 

The problems relating to mass transfer may be elucidated out by two clear-cut yet different methods one using the concept of equilibrium stages, and the other built on diffusional rate processes. The selection of a method depends on the type of device in which the operation is performed. Distillation (and sometimes also liquid extraction) are carried out in equipment such as mixer settler trains, diffusion batteries, or plate towers which contain a series of discrete processing units, and problems in these spheres are usually solved by equilibrium-stage calculation. Gas absorption and other operations which are performed in packed towers and similar devices are usually dealt with utilizing the concept of a diffusional process. All mass transfer calculations, however, involve a knowledge of the equilibrium relationships between phases. 

When compared to conventional systems (such as strippers, scrubbers, distillation columns, packed towers, bubble columns, evaporators, etc.), membrane contactors present several advantages, as reported in Figure 20.3. However, some drawbacks have also to be taken into account, as shown in Figure 20.4. 

Distillation stage calculations are usually performed with ideal stages, The number of ideal stages required for the separation is divided by the overall column efficiency (Sec, 7,1,1) to obtain the required number of trays. In packed towers, the number of stages in the column is multiplied by the HETP (Height Equivalent of a Theoretical Plate, see Sec. 9.1,2) to obtain the packed height. 

Separations in Packed Towers 398 Mass Transfer Coefficients 399 Distillation 401... 

This latest edition covers the technical performance and mechanical details of converting chemical and petrochemical processes into the appropriate hardware for distillation and packed towers. It incorporates recent advances and major innovations in distillation contacting devices and features new generations of packing. In addition, this new edition reflects the significant progress that has been made in process design techniques in recent years. 

If a waste contains a mixture of volatile components that have similar vapor pressures, it is more difficult to separate these components and continuous fractional distillation is required. In this type of distillation unit (Fig. 4), a packed tower or tray column is used. Steam is introduced at the bottom of the column while the waste stream is introduced above and flows downward, countercurrent to the steam. As the steam vaporizes the volatile components and rises, it passes through a rectification section above the waste feed. In this section, vapors that have been condensed from the process are refluxed to the column, contacting the rising vapors and enriching them with the more volatile components. The vapors are then collected and condensed. Organics in the condensate may be separated from the aqueous stream after which the aqueous stream can be recycled to the stripper. 

Packed vs Plate Columns. Relative to plate towers, packed towers are more useful for multipurpose distillations, usually in small (under 0.5 m) towers or for the following specific appHcations severe corrosion environment where some corrosion-resistant materials, such as plastics, ceramics, and certain metaUics, can easily be fabricated into packing but may be difficult to fabricate into plates vacuum operation where a low pressure drop per theoretical plate is a critical requirement high (eg, above 49,000 kg/(hm ) (- 10, 000 lb/(hft )) Hquid rates foaming systems or debottlenecking plate towers having plate spacings that are relatively close, under 0.3 m. 

Use of HETP Data for Absorber Design Distillation design methods (see Sec. 13) normally involve determination of the number of theoretical equihbrium stages or plates N. Thus, when packed towers are employed in distillation appRcations, it is common practice to rate the efficiency of tower packings in terms of the height of packing equivalent to one theoretical plate (HETP). 

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