Packed-tower design, scale

Figure 9-21H. Updated generalized pressure-drop correlation rearranged version of earlier Eckert and Leva, using linear scale for the ordinate and use of capacity factor, Cg. Used by permission of Strigle, R. F., Jr., Packed Tower Design and Applications Random and Structured Packings, 2nd ed. Gulf Publishing Co. p. 21 (1994). Note G = gas, Ib/ft -hr, L = liquid, Ib/ft -hr.

References 77 through 90 provide additional information on packed tower wet scrubbers, design and scale-up principles, as well as operational guidance. 

The height of the transfer unit has not been satisfactorily correlated for application to a wide variety of systems. If pilot plant or other acceptable data are available to represent the system, then the height of packing can be safely scaled-up to commercial units. If such data are not available, rough approximations may be made by determining Hg and Hl as for absorption and combining to obtain an Hqg (Ref. 74, pg. 330). This is only very approximate. In fact it is because of the lack of any volume of data on commercial units that many potential applications of packed towers are designed as tray towers. 

Carey, W. F. and Williamson, G. J. Proc. Inst. Mech. Eng. (Steam Groupi 163 (1950) 41. Gas cooling and humidification design of packed towers from small scale tests. 

In the design of industrial-scale packed towers, an economic balance is important between the capital cost of the column, its ancillary equipment, and the operating cost. If the diameter of the tower is reduced, the capacity cost will also be reduced, but the cost of pumping the gas through the column will increase because of the higher AP. 

The evolution of chemical processes and process equipment is closely related to the methods and apparatus used in the chemistry laboratory. At the early stage of evolution of chemical industries, process steps in the manufacture of a chemical mimicked the steps used in the chemistry lab in its preparation. Most of these processes were batch processes. Some of these evolved into continuous processes as the production volumes increased. Batch processes occupy the preeminent position, even today, in the pharmaceutical and fine-chemical industries. Some of the process equipment - stirred vessels, packed towers, filters, and so on - are the up-sealed versions of the apparatus used in the chemistry laboratory of yesteryear. Process intensification (PI), which represents a paradigm shift in equipment as well as in process design, takes advantage of advances in reaction engineering and transport phenomena in the design of equipment and processes (as opposed to the mere scale-up of the apparatus of the chemistry lab and mimicking the step in the laboratory preparation). 

Figure 8.1.35. Schematic of various large-scale liquid-liquid extraction devices, (a) Packed tower for solvent extraction (b) sieve-plate extraction column (c) an early Scheibel column extraction design (d) Karr column, in which the plates have reciprocating motions (e) centrifugal extractor (f) porous hollow fiber membrane solvent extraction device (see Figure 8.1.13(a) for a detailed design).

Figure 28.3 Gas-liquid contacting tower designs commonly used for acid plant tail gas scrubbing. Large arrows represent gas flow in each figure. Packed towers are not commonly used for high scaling potential solutions such as those containing lime or limestone. Copyright 2013 MECS Inc. All rights reserved. Used with permission of MECS Inc.

The scale-up and design configurations of fluid-bed chemical reactors have evolved rapidly and empirically. An example is fluid catalytic cracking (FCC) [13]. The general fluid-bed concepts developed early. However, the correlations describing the various rate processes and other operational phenomena developed slowly because they could not easily be related back to already established data bases developed for other systems in the case of trickle-bed reactors, data developed for packed-bed absorption towers were utilized. 

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