Distillation packed towers

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. 

TABLE 5-28 Mass Transfer Correlations for Packed Two-Phase Contactors—Absorption, Distillation, Cooling Towers, and Extractors (Packing Is Inert)... 

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). 

The HETP of a packed-tower section, valid for either distillation or dilute-gas absorption and stripping svstems in which constant molal overflow can be assumed and in which no chemical reactions occur, is related to the height of one overall gas-phase mass-transfer unit Hqc by the equation... 

Na+fl) + K(g). Since K is the more volatile (p. 75), it distils off more readily, thus displacing the equilibrium and allowing the reaction to proceed. By fractional distillation through a packed tower, K of 99.5% purity can be obtained but usually an Na/K mixture is drawn off because alloys with 15-55% Na are liquid at room temperature and therefore easier to transport. 

UK. = Light key component in volatile mixture L/V = Internal reflux ratio L/D = Actual external reflux ratio (L/D) ,in = Minimum external reflux ratio M = Molecular weight of compound Mg = Total mols steam required m = Number of sidestreams above feed, n N = Number of theoretical trays in distillation tower (not including reboiler) at operating finite reflux. For partial condenser system N includes condenser or number theoretical trays or transfer units for a packed tower (VOC calculations) Nb = Number of trays from tray, m, to bottom tray, but not including still or reboiler Nrain = Minimum number of theoretical trays in distillation tower (not including reboiler) at total or infinite reflux. For partial condenser system,... 

Figure 9-1B. Typical packed tower with internals for improved distillation. Used by permission of Jaeger Products Inc., Bull. 1100.

Most packed towers are used for mass transfer operations such as absorption, distillation, and stripping however, there are other uses such as heat transfer quenching and entrainment knockout. 

Packed towers are used in some distillation operations in preference to plate towers. Usually the selection requires an understanding of the fouling characteristics of fluids of the system. These towers have been used even in polymer forming operations. However, other contacting devices can be cleaned easier. For some processes the packed tower is much more effective as well as cheaper than a tray tower. 

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

Covers distillation and packed towers, and shows how to apply techniques of process design and interpret results into mechanical equipment details... 

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. 

It is doubtful if any design is entirely novel. The antecedence of most designs can usually be easily traced. The first motor cars were clearly horse-drawn carriages without the horse and the development of the design of the modern car can be traced step by step from these early prototypes. In the chemical industry, modem distillation processes have developed from the ancient stills used for rectification of spirits and the packed columns used for gas absorption have developed from primitive, brushwood-packed towers. So, it is not often that a process designer is faced with the task of producing a design for a completely novel process or piece of equipment. 

Bubble-cap columns or sieve trays, of similar construction to those described in Chapter 11 on distillation, are sometimes used for gas absorption, particularly when the load is more than can be handled in a packed tower of about 1 m diameter and when there is any... 

As Sherwood and Pigford(3) point out, the use of spray towers, packed towers or mechanical columns enables continuous countercurrent extraction to be obtained in a similar manner to that in gas absorption or distillation. Applying the two-film theory of mass transfer, explained in detail in Volume 1, Chapter 10, the concentration gradients for transfer to a desired solute from a raffinate to an extract phase are as shown in Figure 13.19, which is similar to Figure 12.1 for gas absorption. 

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). 

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