Packed towers Random packings

Figure 9-95. HETP and HTU correlation for tower random packings. Used by permission of Whitt, F. R., British Chemical Engineering, July (1955) p. 365 all rights reserved.

Grids have an open structure and are used for high gas rates, where low pressure drop is essential for example, in cooling towers. Random packings and structured packing elements are more commonly used in the process industries. 

In comparison with tray towers, packed towers are suited to small diameters (24 in. or less), whenever low pressure is desirable, whenever low holdup is necessary, and whenever plastic or ceramic construction is required. Applications unfavorable to packings are large diameter towers, especially those with low liquid and high vapor rates, because of problems with liquid distribution, and whenever high turndown is required. In large towers, random packing may cost more than twice as much as sieve or valve trays. 

The fluid mechanics in the packed tower is dominated by the inert material in the packed tower. This material can be small pieces dumped randomly or larger structures carefully stacked inside the tower. Random packing is cheaper and common structured packing is more expensive but more efficient. The efficiency is typically improved by around 30%, a significant gain when producing commodity chemicals at low margins. 

Tower Internals and Equipment Modification. Tower capacity expansion can be achieved through the use of random or stmctured packing, or through the use of higher capacity trays such as the UOP multiple downcomer tray. Packing has been used in the gasoline fractionator, water quench tower, caustic and amine towers, demethanizer, the upper zone of the deethanizer, debutanizer, and condensate strippers. Packing reduces the pressure drop and increases the capacity. 

SOURCE Strigle, R. L., Random Packings and Packed Towers, Gulf Publ. Co., Houston, 1987. 

Bed limiters commonly are used with metal or plastic tower packings. The primary function of these devices is to prevent expansion of the packed bed, as well as to maintain the bed top surface level. In large diameter columns, the packed bed will not fluidize over the entire surface. Vapor surges fluidize random spots on the top of the bed so that after return to normal operation the bed top surface is quite irregular. Thus the liquid distribution can be effected by such an occurrence. 

Random packing has traditionally been used in small diameter ( t in.) towers. This is because it is easier and le.ss expensive to pack t small diameter towers. However, random packed beds are prone to c neling and have poor turndown characteristics when compared i trays. For these reasons, trays were preferred for tower diameters greater than 20 in. In recent years an improved understanding of the impact of... 

The diameter of packed towers may differ depending upon parameters developed by the packing manufacturers and random packing. Conventional packing will require appro.ximat.ely the same diameter as bubble... 

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 ... 

Eckert [125] provides some basic guidelines to good packing selection for various system performance requirements. Kunesh [126] illustrates the often-determined pressure drop advantage of random packed towers over the usual valve tray. See Figure 9-19 [126],... 

As a guide with only rough experimental backing, the ratio of maximum random packing size to tower diameter is... 

Figure 9-21F. Strigle s latest generalized pressure drop correlation. Note G = gas mass velocity, Ib/ft -sec. Used by permission of Strigle, R. F. Jr., Packed Tower Design and Applications Random and Structured Packings, 2nd ed. Gulf Publishing Co., (1994) p. 19.

Updated by permis.sion from R.F. Strigle, Jr., Random Packings and Packed Towers, Gulf Publishing Co. (1987), added Snowflake data by permission Norton Chemical Process Products Corp., Bull. ISPP-IR, E/90 by permission Jaegar Products Co. 

Robbins new equation for generalized pressure drop for random tower packings ... 

Figure 9-47. Effective interfacial areas for random ceramic tower packings. Note for gases or vapors other than air, use abscissa, G, as G/(pgas/0.075) -°. Based on the detta of Fellinger [27]. Used by permission of the Amedcan Institute of Chemical Engineers A.I.Ch.E. Jour., Shul-man, H. L, Ulldch, C. F., Proulx, A. Z., and Zimmerman, J. O., V 1, No. 2, (1955) p. 253. All rights reserved.

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