Packed-Tower Height

The film mass-transfer coefficient and and effective wetted area of packing are calculated using the correlation developed by Onda et al. [11], which can be used to calculate and Hl 

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Determine the number of transfer units, and the packed tower height. 

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

Sometimes the height equivalent to a theoretical plate (HETP) is employed rather than and to characterize the performance of packed towers. The number of heights equivalent to one theoretical plate required for a specified absorption job is equal to the number of theoretical plates,... 

Computation of Tower Height The required height of a gas-absorption or stripping tower depends on (1) the phase equilibria involved, (2) the specified degree of removal of the solute from the gas, and (3) the mass-transfer efficiency of the apparatus. These same considerations apply both to plate towers and to packed towers. Items 1 and 2 dictate the required number of theoretic stages (plate tower) or transfer units (packed tower). Item 3 is derived from the tray efficiency and spacing (plate tower) or from the height of one transfer unit (packed tower). Solute-removal specifications normally are derived from economic considerations. 

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... 

Some performance data of plants with DEA are shown in Table 23-11. Both the absorbers and strippers have trays or packing. Vessel diameters and allowable gas and liquid flow rates are estabhshed by the same correlations as for physical absorptions. The calciilation of tower heights utilizes data of equilibria and enhanced mass-transfer coeffi-... 

Many operating data for carbonate plants are cited by Kohl and Riesenfeld (Gn.s Purification, Gulf, 1985) but not including tower heights. Pilot plant tests, however, are reported on 0.10- and 0.15-m (4- and 6-in) columns packed to depths of 9.14 m (30 ft) of Raschig rings hy Benson et al. (Chem. Eng. Prog., 50, 356 [1954]). 

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 ... 

The HETP (Height Equivalent to a Theoretical Plate (stage or plate)) is the tray spacing divided by the fractional overall tray efficiency [82]. The transfer unit concept has been useful for generalized correlations [89]. Because packed towers operate with continuously changing compositions through the packed height, the concept... 

Because the packed tower is a continuous contacting device as compared to the step-wise plate tower, performance capacity is expressed as the number of transfer units, N, the height of the transfer unit, H.T.U., and mass transfer coefficients K a and Kj a. Figure 9-68 identifies the key symbols and constant flow material balance. 

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. 

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