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Abstract The design of gas liquid processes requires a detailed knowledge of bubble size distributions, since they determine the mass transfer. In this contribution the influence of operating conditions and physical properties of gas and liquid phase on initial and final (‘‘stable’’) bubble sizes is discussed. The measurements were performed in a lab scale bubble column for different liquids sparged with nitrogen for pressures up to 50 bars and temperatures up to 175 C. Bubble size distributions were determined by image processing.
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Bubbles tend to become smaller with decreasing surface tension, increasing gas density and decreasing liquid viscosity, resulting in reduced stable bubble sizes at increased pressure and also at increased temperature as long as evaporation can be neglected.Impurities in aqueous and organic liquids can severely influence bubble sizes by restraining coalescence. For such systems bubble size distributions in a column mainly depend on the size of the primary bubbles, which are determined by the sparger design.
2002 Elsevier Science Inc. All rights reserved.23982
Keywords: Bubble breakage; Bubble column; Bubble formation; Coalescence; High pressure and temperature; Stable Sauter-diameter
1. Introduction
Most investigations in bubble columns have been
performed at ambient conditions and for aqueous sys-
tems in spite of the fact that industrial bubble columns
are often operated at increased temperature and pres-
sure using organic liquids.
Recent findings on bubble size distributions at fixed
positions in the reactors show that bubble sizes are
influenced significantly by pressure and temperature
[13,23]. Investigations on primary bubble sizes show,
that for aqueous systems the initial bubble size decreases
with increased pressure [7,9,10,21,24]. Luo et al. [14]
measured bubble size distributions in a bubble column at
0.3 m above the sparger for nitrogen bubbles in Para-
therm NF heat transfer fluid and also found decreasing
bubble sizes with increasing pressure. Similar results
have been obtained by Lin et al. [13], who determined bubble size distributions for the same gas liquid system
0.4 m above the sparger. They further measured physical
properties in situ and found decreasing bubble sizes with
rising temperature. In contrast to Lin et al. [13], Poho-
recki et al. [17] found no dependence on temperature and
pressure. They published bubble size distributions of
nitrogen bubbles in water for different height levels of a
bubble column determined from photographs for vary-
ing pressure, temperature, superficial gas velocity and
for different spargers, stating that the average bubble
diameter is practically constant to a value of 6.67 mm
(12%). Recently Pohorecki et al. found considerably
smaller bubble sizes for N2/cyclohexane [18], which
they correlated with superficial gas velocity by dS ¼
0:001658usup 0:12
G .
These contradictory examinations probably result
from differences in the column design, sparger configu-
rations or measuring techniques. In order to quantify
the effect of physical properties such as gas density,
liquid viscosity, surface tension and of operating con-
ditions such as pressure, temperature, superficial gas
velocity and of liquid impurities on bubble size distri-
butions, detailed measurements have been performed.2. Experimental
Bubble size distributions were determined by non-
invasive image processing. Due to the transparent bub-
ble column design with windows from 4 sides (Fig. 1,
left) measurements across the complete cross section at
different levels could be performed. Bubbly flow in a
glass tube inside the reactor casing with an inner dia-
meter of 54 mm was investigated at pressures up to 50