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Mass Transfer at Horizontal Surfaces in Bubble Columns.

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Mass Transfer at Horizontal Surfaces in
Bubble Columns
S.A. Nosier and M.I. El-Khaiary'
Department of Chemical Engineering, Faculty of Engineering,
University of Alexandria, Alexandria 21544, EGYPT
The solid-liquid mass transfer characteristics of a single cylindrical horizontal
surface immersed in a gas-liquid bubble column were investigated by measuring the
rate of dimion-controlled dissolution of a copper surface in acidified dichromate
solution. Variables studied were the diameter of the cylindrical surface, physical
properties of the solution, and airflow rate. The mass tran$er coefficient wasfound
to increase with increasing air flow rate. Increasing the diameter was found to
decrease the mass transfer coefficient. The mass tranSfer data were correlated by
the equation: J = 0.147 (Fr Re)Q2" (dlde)Q235. It was found that the diameter does
not have a significant eflecr on the gas holdup.
Introduction
Bubble columns are widely used in the chemical and biotechnological industries as
absorbers, fermentors, and gas-liquid reactors. This is because of their cheap and
simple construction, large effective interfacial area, and high heat and mass transfer
coefficients. These devices are preferred to other types of gas-liquid contactors.
' Authorfor correspondence.
223
S.A. Nosier and M.I. El-Khaiary
A number of gas-liquid reactions canied out in bubble columns are highly
exothermic, and temperature control plays an important role in such cases.
Moreover, cooling or heating of the bubble bed is often needed in absorbers and
fermentors. In all these situations, knowledge of the heat transfer rates between the
gas-liquid dispersion and the heating or cooling surface is of great interest.
Therefore, numerous authors have studied the problem of heat transfer in bubble
columns. For example, Lewis et al. [l] measured the rate of heat transfer from
vertical and horizontal surfaces immersed in a gas-liquid bubble column. They
found that the heat transfer coefficient decreases with increasing height of the
vertical heater, and also the diameter of the horizontal heater. Kubie [2] measured
the heat transfer coefficient for horizontal cylindrical heaters in bubble columns and
obtained.results in agreement with Lewis et al. [11. However, little work has been
done in the analogous field of mass transfer to or from immersed solid surfaces in
gas sparging systems which is the objective of the present work. Mass transfer from
a horizontal cylindrical copper surface has been studied by measuring the diffusion-
controlled dissolution of the copper surface in acidifred chromate solution [3].
The study of mass transfer at gas-sparged horizontal cylinders will enable
prediction of the rate of diffusion-controlled processes which can occur at the
cylinder surface, e.g. diffusion-controlled corrosion and mass-transfer controlled
liquid-solidcatalytic reactions.
Experimental Details
The experimental set-up is shown in Figure 1, and consists of a glass rectangular
cross-sectionalcolumn (1O.x 8 x 80 cm) fitted with a G4 sintered-glassdistributor at
the bottom. The average diameter of the pores of the sintered distributor was 5-10
microns. A copper tube was placed horizontally across the column and mounted
centrally in the front and back walls of the column (10 cm above the distributor).
Four copper tubes were used, each 8.7 cm long and with diameters of 0.6,0.95, 1.2,
and 1.8 cm. In each run, a new copper tube was used to avoid any change in the
surface area. Air was introduced into the column at the required rate by means of a
224
Mass transfer at horizontal sulfaces in bubble columns
variable-speed air compressor. Metallic parts were avoided in the manufacture of the
column to avoid corrosion problems and to avoid undue loss of chromate.
For each run, 2 liters of fresh K2Cr207/H2S04 solution were carefully introduced
into the column in order to avoid mixing, as far as possible, before air stirring. At
the Same time, air was allowed to pass through the column. The change of K2Crz07
concentration with time was determined by withdrawing 10 ml samples at 10 min
intervals from the column, concentrations were determined by titration against
standard ferrous ammonium sulfate using diphenylamine indicator [4].
The
superficial air velocity was measured and varied from 0.3117 to 4.7975 cm s-'.
Experiments were carried out at a temperature of 25+1"C.
Three solution
compositions were used in this study, namely 0.003M KzCr~a!+ 0.5M H2S04,
0.003M K2C&
+ 1.OM H2S04. and 0.003M KzCr2& + 2.OM H2S04.
Figure 1. Schematic diagram of apparatus.
1. Rectangular vessel
2. Sintered glass
3. Compressor
5. cylindrical tube
6. Elecnolyte level
7. Control valves
4. Rotameter
Viscosity and density of the solution used were measured by an Ostwald
viscometer and a density bottle respectively [5]. The diffusivity of dichromate was
225
S.A. Nosier and M.I. El-Khaiary
calculated from literature values [3], and was corrected for the temperature. The gas
holdup was calculated from the equation:
&
=
H-H,
H
,..(l)
Results and Discussion
The mass transfer coefficient was obtained from the &chromate concentration versus
time data,which are related by the equation:
dC
-Q-=KAC
dt
....(2)
which upon integration yields
Figure 2 shows a typical dependence of ln(CJC) against time. The mass transfer
coefficient(K) was obtained from the slope of ln(C&) versus .t
H2SOLsonsrnlr~tion:0.5 H
Inibal ancmiratmn of KzCrz07
Diameter of cylindrical lube
I
ij
0.003 H
0.6 cm
0. 0.6235
a 1.3009
V
-u3 0.100.w-
0
o‘
0.12.
0.00.
0.06.
0.04
0.02
v
.
10
20
30
t I
LO
50
610
mini
Figure 2. Typical In (C,IC)vs. t plot at different gas superficial velocities.
226
Mass transfer at horizontal sugaces in bubble columns
Figure 3 shows the effect of the superficial air velocity on the mass transfer
coefficient. The data are shown to fit the equation:
K = a,
v
.....(4)
0.242
Diameter of cylindrml tube z O . 6 c m
Solution composition
0
0.5 M H2SOL e0.003 M K Cr 0 ( S c = 9 6 0 1
2 2 7
A
1.OM H2SOL .0.003 M K2Cr20,1Sc
0
2.0M
k$SOL.0.O03
:11131
M %Cr2071Sc =13621
0.3
0.2
2.L
2.6
2.8
3.0
L2
3.L
3.6
3.8
~ ~ g i v ~ 1 0 ~ 1
Figure 3. Effect of superficial gas velocity on the mass transfer coefficient.
Table 1 summarizes the velocity exponent obtained by different authors who
studied the effect of gas sparging on the rate of mass and heat transfer.
Table 1. Effect of the supeflcial gas velocity on the mass and heat transfer
( K a V o r h a V).
Author
Present data
Bohm et al. [6]
Sharma et al. [7]
Ibl et al. [8]
Fair et al. [9]
Steiff and Weinspach [lo]
Hart et al. [113
Lewis et al. [l]
Deckwer et al. [12]
Kubie [2]
Experimental Method
Mass transfer
Mass transfer
Mass transfer
Heat transfer
Heat transfer
Heat transfer
Heat transfer
Heat transfer
Heat transfer
Heat transfer
n
0.242
0.250
0.290
0.360
0.220
0.220
0.250
0.172
0.250
0.175
227
S.A. Nosier and M.I.El-Khuiary
The increase of K with the air flow rate may be attributed to the following effects:
(i) the rising bubbles generate turbulence in their wakes by virtue of hydrodynamic
boundary-layer separation; (ii) bubble coalescence or breakdown in the vicinity of the
tube surface generates turbulence which penetrates the diffusion layer; (iii) collision
of the rising bubbles with the front surface of the tube disturbs the diffusion layer
with a consequent increase in the rate of mass transfer; (iv) the swarm of rising
bubbles impart radial momentum to the surrounding fluid [ 131,
Figure 4 shows the effect of the tube diameter on the mass transfer coefficient,
the data correspond to the equation:
....(5)
"I
0.3
1
2.7
2.8
2.9
3.0
3.1
3.2
3.3
Figure 4. Effect of cylindrical tube diameter on the mass transfer coeficient at
different superficial gas velocities.
The decrease in the mass transfer coefficient with increasing tube diameter may
be attributed to the growth of a hydrodynamic boundary layer and a diffusion layer,
whose thicknesses increase with increasing tube diameter. The diameter exponent in
the present work (-0.251) is in fair agreement with the values obtained by Kubie [2]
for vertical (-0.325) and horizontal (-0.375)cylinders in bubble columns. The
diameter exponent obtained by Lewis et al. [l] for vertical cylindrical heaters was
(-0.5). Most other workers did not vary this parameter, e.g. Kolbel et al. [14],
Ruckenstein and Smigelschi [15], and Burke1 [16].
228
Mass transfer at horizontal surfaces in bubble columns
An overall mass transfer correlation was envisaged using the dimensionless
groups: J, Re, and Fr which are often used to correlate heat and mass transfer data
in gas sparged systems. From the present finding that the tube diameter significantly
affects the mass transfer coefficient,an extra dimensionless term was used to account
for this effect. This is the ratio between tube diameter (d) and the equivalent
diameter of the rectangular column
(a).
Tube diameter (d) was used as the
characteristic length in calculating Re. Figure 5 shows that for the conditions:
960 c Sc c1362, 0.00283 c Fr.Re c 11.819, and 0.064 <
ad,< 0.193
the appropriateequation is:
....(6)
J = 0.147 (Fr Re)".% (d/&)023s
with an average deviation of k2.85 of the experimental results.
Figure 5. Overall mass transfer correlation; 0.003M KzCr20, + OJM H2S04
[SC= 9601.
The existence of the
(a&)ratio in
the overall mass transfer correlation may
throw some light on the mass transfer mechanism: (i) the bubble swarm induces
radial momentum transfer which brings a fresh supply of the liquid reactant to the
solid surface (surface renewal model) [12]; (ii) the turbulence which may result from
229
S.A. Nosier and M.1. El-Khaiary
bubble coalescenceor breakdown, collision of bubbles with the tube surface, or in the
wake regions of rising bubbles lead to disturbance of the diffusion layer
(hydrodynamic model). According to the first model (i) the tube diameter (d) does
not affect K, but according to the second model (ii) the diameter has a significant
effect on K. Thus it seems that the two mechanisms contribute to enhancing the rate
of mass transfer at the tube surface.
Similar results to our findings were obtained by Bohm et al. [6] in their study of
liquid to wall mass transfer in a bubble column, all the experimental data were
correlated by introducing the geometric factor (L/D,) where L is the electrode length
and D, is the column diameter. Also Zarra et al. [17] correlated the experimental
data in their study of solid-liquid mass transfer in a packed bubble-column by
introducing two geomerric factors, namely (Wd) and (Vd) where d is the sphere
diameter and 1 is the bed height.
Gas holdup measurements for the different tube diameters used in this
investigation are shown as a function of the superficial gas velocity in Figure 6. It
was found that the tube diameter does not significantlyaffect the gas holdup.
H2SOL concentration D 0.5 M
Initial concentration of K2Cr20,
Diameter of cylindrical tube
: 0.003
:0.6
M
cm
0.160.1L9.12
-
~0.10.
0.08.
0.060.04.
0.02
1
05
1.0
1.5
2.5
2.0
V Icm/s
3.0
I
Figure 6. Effect of superficial gas velocity on gas holdup.
230
3.5
Mass transfer at horizontal suljfaces in bubble columns
Conclusions
Mass transfer measurements from horizontal cylindrical surfaces of different
diameters to gas-liquid systems with a range of physical properties are reported.
The dependence of the solid-liquid mass transfer coefficient on superficial gas
velocity, diameter of the cylindrical surface, and the physical propemes of the liquid
was established experimentally. It was shown that the diameter of the cylindrical
surface is of considerable importance in determining the solid-liquid mass-transfer
coefficient. Therefore, it is reasonable to include a length parameter in the overall
mass-transfer correlation (see Equation 6).
Gas holdup in a bubble column was measured for a range of liquid physical
properties and different tube diameters, as holdup varies directly with superficial air
velocity but the tube diameter does not significantly affect the gas holdup.
Nomenclature
A
Surface area of the heating element
C
Dichromate concentrationat time t
CCl
INtialdichromate concentration
d
Tube diameter
d,
D
Equivalent diameter of the column
g
Acceleration due to gravity
h
Heat transfercoefficient
H
Height of gas-liquid dispersion
H,
Clear liquid height
K
Mass transfer coefficient
L
Copper tube length
Q
Volume of solution
t
Time
V
Superficial gas velocity
Dichromate diffusivity
231
S.A. Nosier and MI.El-Khaiar).
Greek letters
P
Solution viscosity
P
Solution density
&
Gas holdup
Dimensionless groups
W2&d)
Fr
J
Froude number
Mass m s f e r coefficient
(St SCO.66)
Re
Reynoldsnumber
(PWP)
sc
Schmidt number
St
Stanton number
References
1. Lewis, D.A.; Field, R.W. Xavier, A.M.,and Edwards, D. 1982. Heat transfer m bubble colunms. Trans.
IchemE., 60.4047.
2. Kubie. J. 1974. Multiphase flow systems. I c h d . Symposium Series. 38-H1.
3. Gregory, D.P., and Riddiford,A.C 1960. Dissolution of copper in sulfuric acid solution. J. Electrochrm.
Soc, 107,950-956.
4. Vogel, A.I. 1961. A Textbook of Quantitative Inorganic Analysis, 3rd Edn, Loagmaru.London.
5. Findly, A., and Kitchener. AJ. 1%5. PracticalPhysical Chemistry. 8th Edn, Longmano, LonQn.
6. Cavatorta, O.N.,and Bohm. U. 1988. Heat and mass transfer m gas sparging systnns: empxical
comlarionsand theomical models. Cl~em.Eng. Res. Dev.. 66,265-274.
7. Patil, V.K.. and Sharma, M.M. 1983. Solid-liquidoms transfer coefficients in buwle colunms up U) me
meter diameter. Chem. Fhg. Res. Dev.. 61,21-28.
8. lbl, N.; Kind, R. and Adam. E. 1975. Mass msfer at elearodes with gas stirring. Arm Quha. 71,
1008-1016.
9. Fair. J.R; Lambright, AJ., and Anderson, J.W. 1%2. Heat transfer and gas holdup m a sparged
contactor. Ind Eng. am.P~OC
D ~ sDev.,
.
1,33-36.
10. S M . A, and Weinspa&, P.M 1978. Heat transfer in s
t
e
i and non-stimd gas-liquid nactors.
German chm. Eng., 1,150-157.
11. Hart, W.F. 1976. Heat tmsfer m W l e agitated systpn. General codation. Ind. Eng. C~ICUI. Roc.
Des. D ~ v .15,
, 109-114.
12. Dedrwer, W.D. 1980. On the mechanism of heat transfer in bubble column na~ors.chcm.Fhg. Sci.,
35,1341-1346.
232
Mass transfer at horizontal surfaces in bubble columns
13. Kas,W. 1%3. Zintersuchungenzum warmeubergang in blasensaulen. Chm.Ing. Tech.. 3.5,785-788.
14. Kolbel, H.:Siemes, W.;Maas, R, and Muller, K 1958. Warmeubergmg and blasmaular.
(hem. Ing.
Tech., 30,400-404.
15. Ruckenstein, E., and Smigelschi, 0. 196.5. Heat transfer to bubble beds. Trans. IChemE., 43,7334"338.
16. Burke], W. 1972. Der waxmeubergang an heimmd kuhlflach in begastm flussigkeiten. chem Ing.
Tech.,44,265-271.
17. Zarra, M A ; EI-Abd, M.Z.; El-Tawil, Y.A.;Farag, H.A..and Sadahmed, G.H. 1994. Liquid to solid
mass uansfer in a batch packed bubble column.
Chem. h g . J., 54,51-55.
Received: 27 March 19%; Accepted after revision: 30 September 1996.
233
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