holds for metal Pall rings, metal IMTP, sheet metal structured packings, and metal gauze structured packings。 Equation 63 can be taken to imply that a form of the Chilton–Colburn analogy holds for turbulent liquid film   flow。

Analysis Methodology

The  variation  of  the  hHETPi  with  liquid  and  vapor  flow

Penetration/surface renewal theory, on the other hand, pre- dicts that the liquid-side mass-transfer coefficient is propor- tional to the square root of the liquid diffusivity, so that

ShL / Re1 Sc1=2f ðReLÞ (56)

rates can be quite complex。 Figure 4 is an illustration of ‘‘typical’’ hHETPi behavior for a binary distillation at total reflux。 For low liquid rates, there is  a  deterioration  in col- umn efficiency due to underwetting of the packing。 The effi- ciency also  deteriorates near flood。 Sometimes, there is    also a  region  near  loading  with  enhanced  column    efficiency。38

L     L

The  simple  power-law  expressions  that  we  have   proposed

Assuming that the friction factor is nearly independent of Reynolds number, Eq。 56 reduces  to

for ky, kx, and am cannot reproduce the complexity of this typical hHETPi behavior。 Therefore, we have limited our analysis to the intermediate regions of the hHETPi   function,

where  the  variations  of  the  hHETPi with  flow  rate  are  more

L     L amenable  to  description  with  simple  power-law expressions

for ky, kx, and am。

Turning now to the experiment, Sherwood and Holloway33 correlated absorption and  desorption  data  for  various gases in water with the  expression

Because the contribution of the liquid-side resistance has been found to be small in the great majority of binary distil- lations, we will proceed by setting  the  front  factor,  AL, in Eq。  44  to  unity  (this  is  equivalent  to  using  Eq。  45)。   This

then fixes the contribution of the small liquid-side resistance to the hHETPi。 As the small contribution of the liquid-phase resistance is now completely specified, we can curve fit ex-

with the exponent ‘‘n’’ ranging from 0。22 to 0。46 for Raschig rings and  saddles。  Koch  et al。34   measured  liquid-film  mass-

perimental hHETPi data to Eq。 45 to determine the power- law  exponents  on  the  various  dimensionless  groups  and to

Figure 4。 Schematic showing the potentially complicated variation   of   hHETPi   with  density-corrected

exponents in the fits。 For all but the BX style gauze struc- tured packing power-law exponents were limited to the   range

—0。2 to 0。2。 In the case of BX, the range was expanded to be —0。25 to 0。25。 The reasons for limiting the ranges of the power-law coefficients are difficult to elucidate。 Part of the explanation lies in the fact that data fitting becomes more problematic when the same physical  quantity  appears  in more than one dimensionless grouping。26 For  example,  fit- ting the data with no imposed restrictions would often lead power-law exponents with large magnitudes but whose net effect on some variable,  usually  the  liquid  velocity, would be small to negligible。 A further consideration was the fact that many other reported mass-transfer correlations have power-law exponents on dimensionless groupings whose magnitudes are less than 1。64 The final reason was  a prag- matic one—limiting the range on these coefficients resulted in correlations whose predictions were generally better in ev- ery situation, and which turned out to be more robust in the simulator。