Page 230 - Packed bed columns for absorption, desorption, rectification and direct heat transfer

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to predict values of a/a greater then 1. That is why this form is not useful for
high liquid superficial velocity, when the influence of effective surface of the
drop and jets trickling in the free volume of the packing is significant.
Values of tr c, taken from [110], are represented in Table 13.
When using the data for plastic material from this table it must be taken
into account that because of slow partial oxidation of the surface area, the value
of a c increases with time. The same effect can be observed also due to the
influence of industrial gases and liquids [112, 113], Linek, Krivsky and Hudec
[111] have not observed such an effect after 3 weeks irrigation of
nonhydrophilized plastic packings with pure water. Our investigations [105]
with PVC Pall rings not in use for a long time show that their effective surface
is close to the effective surface of ceramic packings. Experiments [114] for
determination of the effective surface area using the Danckwerts method show
also that the wettability of the packings slowly increases with time.
Puranik and Vogelpohl [109] propose the equation:

41
^ = 1.045Re™ We OJS3 \ S t (103)
a {a-)

Table 13. Values of critical surface tension taken from [110]
z
Material <r exl0- kg/s 2
Glass 7.3'
Ceramics 6.1*
PVC 4.0*
Carbon 6.0 - 6.5*
Steel 7.1*
Paraffin 2.0*
Hydrophilteed polypropylene 5.4"
and polyethylene
* Taken from [110]
"Taken from [111]