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On the local gas holdup and flow pattern in standard-type bubble columns
Chemical Engineering and Processing, 33 (1994) 7-21
On the local gas holdup and flow pattern in standard-type columns
bubble
J. Fischer*, H. Kumazawa-f and E. Sada Department (Received
of Chemical Engineering,
Kyoto
University,
July 15, 1993; in final form October
606-01 Kyoto
(Japan)
4, 1993)
Abstract Local gas holdups and bubble size distributions have been measured simultaneously in a standard-type bubble column equipped with a perforated plate as a gas disperser. High values in the time series of the local gas holdup which appear on a periodic basis could be detected and analyzed. The phenomenon of the bubble hose, defined as a zone of high bubble number density moving helically in a bubble column, has been recognized visually using photographs and by means of the recorded data. A formation mechanism for the bubble hose dependent on the gas disperser geometry capable of explaining the observed asymmetric liquid and gas flow pattern is suggested. The existence of a relationship between the bubble hose, the average bubble diameter and the superficial gas velocity has been demonstrated.
1. Introduction
the originally sampled data are then symmetrized fitted to the well-known equation
Local gas holdup in bubble columns is still an important subject in modern chemical engineering. Whilst, on one hand, the sensitivity of reaction components to local conditions, e.g. the sensitivity to the local oxygen concentration in bioengineering, increases the need for detailed information about the system, on the other hand, detailed modeling of the systems under investigation appears to have become feasible recently due to an increasing knowledge of each step in the transport phenomena and chemical reactions and the increasing capacities of even desktop size microcomputers. Thus, the application of existing models of bubble column phenomena has become more elaborate and sophisticated. However, the description of some physical properties, such as local gas holdup and local liquid flows, bubble size distributions and others, still remains unsatisfactory. When values of local gas holdup are reported, they are usually radial profiles taken along the diameter of the bubble column as shown in Fig. 5 below. A single point value, E~+(Y = const.), of the profile is obtained by time-averaging the data at that radial position. Using EG,I&)
= $2,
ICC+ r) + EG,loc( -r))
(1)
*On leave from Abteilung Chemietechnik, Lehrstuhl Technische Chemie B, UniversitLt Dortmund, 44221 Dortmund, Germany. tTo whom correspondence should be addressed.
0255-2701/94/$7.00 SSDI 0255-2701(93)00486-N
k,
,oc(~)= &G,,oc(~= O) x
and
[I -u(;>“]
or one of its modifications [l-3], yielding a parabola which represents the symmetrized data with a satisfying accuracy. The parabola drawn in Fig. 5 was calculated using the equation for the turbulent flow regime proposed by Menzel et al. [4]: &G,,oc(r)= 1.33 x
EG.,oc(r
=
O>x L_1.05(;>2]
(3)
According to ref. 4, the value of the parameter &G&r = 0) in Fig. 5 can be set equal to the measured value of the integral gas holdup in the bubble column, (~o,~,,~), since the profile is taken in the equilibrium zone. The results fit the intuitive concept of bubbles rising in the center of the bubble column and thus inducing a circular liquid flow: at the top of the column, the majority of bubbles leave the liquid medium which streams down in the downcomer region close to the column wall. All main hydrodynamic models for bubble columns are based on this concept [ 5,6]. Although for this ‘circulation cell model’ the height of one cell has been reported independently to equal approximately one column diameter by different authors using different methods [ 5, 71, a satisfactory description of the flow situation in regions where two or even more cells contact each other is still lacking. On the other hand, some
0
1994 -
Elsevier Sequoia.
All rights reserved
8
authors have reported deviations from the ideal symmetrical profiles or flow pattern even for the water/ air system [4,8,9], especially when working with liquids of higher viscosity [4, lo]. But for obvious mathematical reasons, the symmetrized profiles are commonly used in complex models. A more complete introductory overview of the literature on bubble column modeling has recently been published by Tzeng et al.
ill]. The second characteristic of standard models is that the bubble column is divided vertically into three regions: . the gas inlet zone directly above the gas disperser, . the equilibrium zone, where the shape of the radial gas holdup profiles are independent of the column height, and . the degassing zone at the top with its higher local gas holdup values. The intention of this article is to extend the different approach to the hydrodynamics in bubble columns first described by Franz et al. [9]. Focusing on the gas-phase behavior, special attention will be paid to the gas inlet zone in order to explain the origin of the observed asymmetry of the profiles of local gas holdup, and the related gas and liquid flows. Whereas the model suggested by Franz et al. [9] was based on average values calculated after a certain macroscopic measuring time interval AT, the purpose of this study is to clarify the events during this very interval.
A compressor supplied a constant volumetric flow of air which was measured using a rotameter. Since no significant loss of water due to mass transfer (liquid to gas) could be observed during the period of measurement, a special saturation column was not provided. The local gas holdup as well as the bubble size distributions were measured using a laser detector employing the Doppler effect (Laser Doppler Anemometry, LDA). Both the optical device and the evaluation electronic were provided by Nihon Kagaku Kogyo Co., Ltd. The sensor’s operation principle is shown in Fig. 1. The light of a laser beam is partly reflected at the gas/fluid interface. The fluid can be both water (A, C) or gas (B). A gas bubble hitting the tip of the sensor causes a change in the intensity I of the reflected light from I,_ to Zo capable of measurement via a photodetector. Thus, the time period t, during which a bubble is present at the tip can be determined and the local gas holdup can be calculated. Simultaneously, the approaching bubble causes a frequency shift Af of the laser light that can be used to determine its velocity. Knowing the speed and the time of the bubble, its diameter can also be calculated.
3. Results and discussion 3.1. Visual observation 3.1.1.
2. Experimental The experimental setup to investigate the ionexchanged water/air system in a tlrst approach was kept as simple as possible. It consisted of a standard-type bubble column made of acrylic resin with a 0.10 m inner diameter and a 0.88 m liquid height (H/D = 8.8) without any additional inner parts except the gas-phase detector described below. The column was operated batchwise with respect to the liquid phase and continuously with respect to the gas phase. All experimental data shown in this paper were taken at a height h, of 0.6 m (h,/D = 0.68) unless otherwise stated. Two different types of perforated plates were used as gas dispersers. The first had a perforated area diameter d,,,, = 0.4 m (d,,,,/D = 0.4) and is called a partial area disperser (PAD), whereas the perforated area of the second type covers the whole cross-section of the bubble column (dful,/D = 1.0) and is therefore called a full area disperser (FAD). They contained 14 and 88 holes of 0.8 mm diameter, respectively, with an equilateral triangular pitch L, = 11 mm. The center of their perforated area was in the center of the column in all the experiments performed.
Partial
area disperser
Figure 2 illustrates typical photographs of rising bubbles in the column equipped with a PAD as a gas disperser at three levels of the superticial gas velocity was. The three photographs in the top row were taken from the middle part of the bubble column, whereas the bottom three were taken in the gas inlet zone directly above the gas disperser. In the gas inlet zone, as shown in Fig. 2(l), the bubbles are already pushed towards the wall by the downcoming liquid that flows horizontally inward at the bottom. This creates a zone of higher bubble number density in the liquid flow direction. Because of the reduced mass density in that small reactor volume, the bubbles rise faster than in regions with a lower bubble number density. This again induces a faster liquid circulation and thus a feedback process is generated. Since the direction of the liquid flow is not constant, the position of the high bubble number density zone is also not constant but changes very rapidly, which is substantially different from the stationary phenomenon described by Rietema [ 121. Once formed, this bubble hose can be easily recognized at any height in the bubble column as shown in Fig. 2(2) and, furthermore, it appears to be helical as can be seen clearly in Fig. 2(2c). According to the
I
Gas
Liquid
Liquid level >
Fig. 1. Operating
principle
of the optical
sensor equipped with a laser beam.
photographs of Fig. 2(2), the height of the turns of the helix seems to be a function of the superficial gas velocity: the higher gas velocity, the smaller the height of the turn. (This caused some problems in the numerical analysis of the data as will be discussed below, but an analysis of this phenomenon is beyond the scope of this paper.) Figure 3 shows an attempt to draw the two-dimensional projection of the three-dimensional bubble hoses (dot-filled areas) and the suggested corresponding downward liquid flows (thick lines). Additionally, as an example, one eddy moving along a streamline is inserted in Fig. 3(2b). This detail is presented once more in Fig. 4, which shows the superposition of the rotational and translational movement of the eddy. The helical rise of the bubbles implies a helical liquid flow
upwards and as well as downwards. This type of flow pattern was first suggested by Franz et al. [9], who measured local liquid velocities in all three directions. It gives a natural and self-evident explanation for the bubble drift in the gas inlet zone and the fast changes in its direction. Furthermore, it supports the observed asymmetry of the profiles of the local gas holdup such as that shown in Fig. 5. 3.1.2. Full area disperser (FAD) Figure 6 illustrates representative photographs of rising bubbles in the column equipped with an FAD at three levels of the superficial gas velocity. In a similar manner to Fig. 2, the top three photographs were taken from the middle part of the column, and the bottom three were taken just above the gas disperser. Figure 7
wq = 3.43 cm/s
W,‘ = 0.66 cm/s (a)
W
(c>
Fig. 2. Photographs of the rising bubbles in the column equipped with a PAD. Top row (2a-c): (20-60 cm). Bottom row (la-c): gas inlet zone (O-20 cm). shows the corresponding outlines of the bubble hoses with the assumed main liquid flows. Using an FAD, the bubble hose can still be clearly identified, although the gas is now fed over the whole cross-section of the column. However, the situation directly above the gas disperser is different, as depicted in Fig. 6( 1). Since there is no longer a pre-fixed location for the bubble origin, the liquid can flow downwards at any position in the cross-section before it flows horizontally. As a result, the cross-stream above the disperser can occur from any position in any direction, for example: . from the back to the front (c), . from right to left (b), . or even from the center to the front and back simultaneously (a) in Fig. 6( 1). The resulting flow structure directly above the gas disperser can be very stable - even at higher gas velocities - and creates extremely asymmetric profiles of local gas holdup in that region as shown in Fig. 8. The differences between the flow above both
middle part of the bubble colmnn
disperser types and the creation mechanisms bubble hose will be discussed in detail below.
of the
3.2. Recorded data
3.2.1. Local gas holdup as a time series From Figs. 2 and 6, the following question arises. What does the gas-phase detector here detect at a fixed position over a certain macroscopic time interval AT? Obviously, there will be periods of no or very low gas holdup and of very high gas holdup as well - corresponding to the momentary absence or presence of the bubble hose. To clarify this, the time interval AT has been split into short sampling intervals of equal length A? and the corresponding data recorded. Figures 9 and 10 show the local gas holdup as a function of time at the radial position r = 0.04m in the PAD and FAD cases respectively. The sampling time interval AT, for each point of the data was chosen to be 1 s. Distinctive peaks much higher than the average value EG ,oc calculated after AT = 900 s (indicated by the straight line)
11
(I)
3.&
=
0.66
cm/s
was = 2.14
WGS= 3.43 cm/s
(b)
(4 Fig. 3. Outlined
cm/s
bubble
hoses of Fig. 2 and assumed
main liquid flows (thick
can be clearly identified. Tn addition, these peaks occur periodically at the positions marked by ‘. . .’ in Figs. 9 and 10.
\ I
Fig. 4. Detail of Fig. 3(2b); superposition tion movement of an eddy.
of rotation
and transla-
line).
3.2.2. Bubble size distribution Although the optical sensor described in Fig. 1 is not suitable for recording true bubble size distributions, since the detected bubbles need to be hit centrally twice to obtain evaluable signals, it can be used to compare distributions recorded at different positions or under different operating conditions. Figure 11 compares the normalized bubble size distributions obtained for various superficial gas velocities using an FAD. Whereas at the column center (Fig. II(a)) for all gas velocities mentioned below the same narrow Gaussian shape, mean diameter d,, and Sauter diameter ds2 were calculated, noticeable changes were observed at the peripheral radial position r/R = 0.8. With increasing gas velocity the mean diameters decrease, apparently reflecting a higher proportion of small bubbles. However, the number of signals that could not be identified as ‘bubbles’ because of the sensor’s operating principle (denoted as n lo in the
12
‘l. :.. ‘:. :., ‘..
4!
-1
^'" -u.a
'_
-“.O
1.
-“.‘I
^'_
-UL
I
0
X=r/R Fig. 5. Experimental
Fig. 6. Photographs (20-70 cm). Bottom
and calculated
radial
profiles
0.2
0.4
0.6
0.8
I 1
(-) of the local gas holdup
of the rising bubbles in the column equipped row (la-c): gas inlet zone (O-20 cm).
with a PAD
with an FAD.
used as a gas dispe,ser.
Top row (2a-c):
middle
part
0 Original
of the bubble
data,
column
3
13
w_ = 4.17 cm/s (a)
(b)
wO*= 5.71 cm/s
(c)
Fig. 7. Outlined bubble hoses of Fig. 6 and assumed main liquid flows (thick
also increases at the same time. Comparing the distributions at both positions, it seems justified to conclude that larger bubbles are less often detected at the periphery because of their relatively high tangential velocity. This confirms the visual observation of a moving bubble hose. Furthermore, a comparison of Figs. 1 l(a)-(d) shows that over the range of superficial gas velocities investigated no bubble coalescence or bubble breakdown occurs. Both would be indicated by a significantly greater number of larger and smaller bubbles, respectively. Figure 12 shows a similar comparison for the PAD case. Again, Fig. 12(a) depicts a typical histogram obtained at the column center. It is noticeably wider than that calculated for an FAD. This refle;cts the fact that at the same superficial gas velocity a higher number of larger bubbles are formed for a PAD. At the peripheral position, the mean diameters again depend figures)
line).
on the superficial gas velocity, clearly showing the opposite trend to that in case of an FAD: with increasing gas velocity, the mean diameters ‘increase. It is thought that this occurs because 6f a higher rate of bubble coalescence as indicated by the increased proportion of large bubbles due to the liquid cross stream above the disperser (to be discussed below in connection with Fig. 14). Figure 13 summarizes the dependency of the mean diameters by illustrating the diameter &, as a function of the superficial gas velocity. Although correlation of the data via a least-squares fit of a straight line to yield the ‘-.-’ lines of Fig. 13 is obviously justified, it is also possible to interpret the plotted data as lyind on .a step-like curve and .a bent clvve for a PAD and an FAD, respectively. This unusual interpretation is suggebted because of another result presented below. ,. :
14
-1
-0.8
-0.6
-0.4
-0.2
0
0.2
0.4
0.6
0.8
1
X=r/R (-) Fig. 8. Profile of the local gas holdup directly (8 cm) above the gas disperser in case of an FAD.
_.. 0.9 0.8 t 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0 0
50
0.6 0.5 0.4 0.3 0.2 0.1 0.0 m
350
150
200
250
300
450
500
550
600
time[sl
400
the [sl
il.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 nn
I
I.1
6w
650
700
750 Lime[sl
800
Fig. 9. Local gas holdup, zG,,_, as a function of time in the case of a PAD.
900
15
0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0 3m
350
400
650
700
450
500
550
600
800
850
900
“..,
600
750 limeIs]
Fig. 10. Local gas holdup, zG,,oc as a function of time in the case of an FAD.
3.3. Bubble hose 3.3.1. Basic assumptions The fundamental assumptions based on the visual observation and the recorded data are that (a) the bubble hose exists and (b) its movement is flexibly periodic. However, the expression periodic should not really be used in this context, since it is related to mathematical expressions, e.g. sin X, which relate to precise periods which cannot be expected at all in a bubble column. The term quasi-periodic has a special connotation in chaos research and clearly should also not be used in this context since the necessary relation has not yet been reported. The term which we will assign is flexibly periodic, which expresses the fact that after a certain time interval called a period, another shorter time interval follows in which the local gas holdup is expected to have an especially- high value relative to the previous period. Furthermore, because of
the disturbances caused by the liquid or gas flow, the peak values are not expected to ‘appear at every predicted location although the system tries to resume its periodicity as quickly as possible. This is thought to occur in order to minimize the friction loss between the rising and downcoming streams. As a consequence one particular period should be attributed to every superficial gas velocity since only one state of the mentioned energy minimum is expected to be reached. This concept does not contradict previous results obtained after time averaging, since time averaging over a circular motion might result in stationary ‘circulation cells’. On the basis of the above definition of a period, a semi-numerical algorithm was designed in order to try to analyze the signals and predict the peak occurrences. Its numerical part consisted of three steps: (i) creating artificial data (AD) by summing up the measured data over a short time interval; (ii) eliminating all AD lower
diameter
* (4 Fig. 11. Bubble
d (mm)
diameter
(h)
d (mm)
3
diameter size distributions
d (mm) at the radial
diameter
Cd) positions
(a) r/R = 0 and (b-d)
than a given threshold value; and (iii) classifying the distances between the remaining AD in a histogram. The resulting histogram depends on the values selected for both the time interval and the threshold. The subsequent non-numerical procedure comprises the extraction of the period out of a variety of histograms. Finally, the determined periods are applied to the originally measured data. The ‘. .’ intervals in Figs. 9 and 10 mark the positions at which high values of local gas holdup were predicted and resulted in an extremely high degree of correspondence with respect to the turbulent system under investigation. 3.3.2. Formation mechanism A probable creation mechanism for the bubble hose in case of a PAD has already been briefly discussed above. The liquid flowing downwards mainly along the wall does not affect bubble formation, since it simply does not occur in this area. On changing to a horizontal cross flow over the gas disperser, the liquid pushes the
d (mm)
r/R = 0.8 in the case of an FAD.
newly formed bubbles aside as shown in Fig. 14(a). The tangential velocity component rq of the liquid has not been drawn in the figure, since its effect is the same because it is also perpendicular to the axial component v, . For an FAD the situation is different. The liquid still flows downwards, but it always has one vertical velocity component perpendicular to the gas disperser directly above its perforated area. The arrangement of these factors increases the pressure on the disperser in the axial direction so that bubble formation and rise is hindered at this position (see Fig. 14(b)). The bubbles rise at the location of lower pressure and are again pushed aside by the cross flow. Since the increase in pressure Apr is a function of the liquid velocity according to Apr = ;pvz2
(4)
it is thought that the higher the value of the superficial gas velocity wGs which induces the liquid flow, the
3
diameter
4
5
6
7
8
9
10
diameter d (mm)
d (mm)
r
I
L
*
Cc)
5
6
7
8
9
*
10
Fig. 12. Bubble size distributions
at the radial positions (a) r/R = 0 and (b-d)
higher is the stability of the phenomenon. Of course, for a particular was value bubble formation is expected to occur over the whole perforated area, but when this occurs bubble rise will still be hindred so that the phenomenon does not change qualitatively. Although only two perforated plates have been used in this study, the creation mechanism proposed so far is also expected to be valid for other two-dimensional gas disperser types. Here the expression two-dimensional refers to gas dispersers that feed the gas over a certain area as opposed to the other types listed in Table 1, which is a simple attempt to classify various disperser constructions according to their geometry in relation to the resulting operation conditions. 3.3.3. Period as a function
of the super-cial
3
45
6
7
8
9
10
diameter d (mm)
diameter d (mm)
gas
velocity
To illustrate the correlation of the period with one of the main operating parameters of a bubble column, the semi-numerically determined periods have been plotted
r/R = 0.8 in the case of a PAD.
as a function of the superficial gas velocity in Fig. 15. For a PAD three different levels of the periods at 18, 15 and 12 s were extracted, whereas in case of an FAD two periods of 20 and 15 s were identified. So far, no explanation for these data values can be given and they are therefore presented here without further discussion. For low gas velocities the identification of the periods was relatively evident, but at higher gas velocities an additional phenomenon complicates the evaluation. As can be seen from Fig. 2, the height of one spiral turn decreases with increasing gas velocity. Thus, the superposition of two periods, one rotational and one translational, suggests a challenging research topic, since it is not yet possible to distinguish between both from data analysis using our algorithm. However, an additional significant observation can be made. The levels of the periods in Fig. 15 correspond to the plateaus of the step-like curve and parts of the bent curve described in Fig. 13 for a PAD and an FAD, respectively. This suggests that subtle changes in the
18
2.2
1
1
0.6
0.8
1.2
1.0
1.6
1.4
2.0
1.8
2.4
2.2
2.6
superficial gas velocity (cm/s) Fig. 13. Calculated PAD +.
mean
bubble
diameter
d,, as a function
of the superficial
gas velocity
P +AP,
Fig. 14. Plausible
TABLE
sketches
1. Dimensions
of the flow above
of different
Example
zero one two three
single orifice ring of perforated tube perforated plate gas fed at different heights
the gas dispersers:
into the bubble
position
r/R = 0.8. FAD
0,
- P +Ap,
(a) partial
area disperser;
(b) full area disperser.
where the integral gas holdup (.zG+) is shown as a function of the superficial gas velocity wGs for both types of disperser. For a PAD, the slope of this curve changes during the velocity interval where the second step of the diameter increases and the second change in the period level occurs.
types of gas disperser
Dimension
at the
column
‘, flow structure occur which are reflected by variations of both quantities. Furthermore, this hypothesis appears to be partly confirmed by the data depicted in Fig. 16,
3.3.4. Comparison of two-dimensional and three-dimensional bubble columns A different approach towards clarifying flow structures in bubble columns was made by Chen et al. [13]. In that study, a two-dimensional column was used in
19
13 12 11 10 0.6
0.8
1.0
1.2
1.4
1.6
1.8
2.0
2.2
2.4
2.6
superficial gas velocity (cm/s) Fig. 15. Determined periods as a function of the superficial gas velocity at the radial position r/R = 0.8. FAD 0, P.&D +. which staggered circulation cells with an asymmetric flow pattern were obtained. In addition, it was pointed out that the HID ratio is a very important factor in pattern formation, since at values below H/D = 1 the resulting patterns are significantly different. Using a numerical model for two-dimensional columns, a remarkable good computer simulation of the observed pattern reported by Chen et al. [13] has
been achieved by Webb et al. [ 141 for H/D values below 2. Moreover, the computer simulation shows that two bubble jets which are already off-centered are immediately pushed further towards the wall, creating one single stable and stationary vortex. On the other hand, the staggered vortices are not stationary with respect to a fixed position, but form steadily at the column top and move downwards during which
superficial gas velocity (cm/s) Fig. 16. Integral gas holdup (se. ,nt) as a function of the superfkial gas velocity. FAD 0, PAD +
course they can even disintegrate
at higher gas velocities
[111. As demonstrated by a comparison of the results reported in refs. 8 and 13, qualitatively similar phenomena are to be expected in three-dimensional columns but with a higher degree of complexity., According to the formation mechanism proposed here for the bubble hose in three-dimensional columns, off-centered rising bubbles are encountered as previously described by Webb et al. [ 141 for two-dimensional columns, and thus similar asymmetric flow is to be expected. Hence, in Figs. 2 and 6, which depict flow structures similar to those of a two-dimensional column, the three-dimensional equivalent of the wave-like pattern reported by Tzeng et al. [ 1 l] can be readily recognized. In twodimensional columns, the tight walls of the columns strongly limit the movement of the rising bubbles and thus ‘trap’ the bubbles between the walls ‘and the equally restricted vortices. As a consequence, the fast bubble flow region with its large coalesced bubbles, as described by Tzeng et al. [ 1 l] will be formed. However, as can be concluded from the photographs depicted in Figs. 2 and 6 and the recorded bubble size distributions of Figs. 11 and 12, this limitation does not exist in three-dimensional columns and, consequently, mobil helical bubble rise is observed. 1
4. ~opclusions
’
This article reports measurements of the local gas holdup, SG, ,a:, anh bubble size distributions carried out in a standard-type bubble column equipped with a perforated plate as a gas disperser. As an extension of the standard presentation of time-averaged values of the local gas holdup EG,,=, time-dependent sG, ,oc(t) values are also presented to illustrate the dynamic behavior of this parameter. Both visual observation and electronically sampled data reveal that a zone of high bubble number density, called a bubble hose, moves in a flexibly periodic manner in the bubble column. A formation mechanism for this bubble hose is suggested that takes into account the geometry of both the gas disperser and the gas inlet zone of the bubble column. Furthermore, this mechanism can explain the flexible periodic movement of the bubble hose without being in contradiction with results obtained using the time-averaging approach of the ‘circulation cell model’, as well as aiding an understanding of some non-symmetrical phenomena reported by other researchers. The reported data and the analysis provided in this article indicate. that, in order to obtain more detailed knowledge of some aspects of the phenomena occurring in bubble columns, further investigations need to be undertaken not only in local ‘positions’ but also in local ttimes’,
leading to a more dynamic description structures.
of the phase flow
Acknowledgement
The authors gratefully appreciate the fundamental support of Professor Emeritus Dr. U. Onken and his interest in this work.
Nomenclature a d d 10 d 32 dpart d full
P
Af Z
k H 4 n n
ok
n
lo hi
n
N NM P APf x tG
factor in eqn. (2) referring to associated references bubble diameter mean diameter of bubble Sauter diameter of bubble diameter of a partial area gas disperser diameter of a full area gas disperser diameter of the bubble column frequency frequency shift light intensity measuring height above gas disperser liquid height in the bubble column length of pitch of the perforated plate / exponent in eqn. (2) number of bubbles within diameter range OI *lOmm number of signals not identified as a bubble number of bubbles larger than 10 mm number of buubbles in one size class total number of bubbles used for the size distribution pressure pressure increase radial coordinate radius of the bubble column time a fluid phase is present at the tip of the sensor macroscopic measuring time interval microscopic measuring time interval radial component of liquid velocity tangential component of liquid velocity axial component of liquid velocity superficial gas velocity dimensionless radius
Greek letters &G, lxx SG, lw
(eG, int) P
local gas holdup time .averaged local gas holdup integral gas holdup’ density
21
Subscripts 0 reference value gas phase G L liquid phase
References and S. Morooka, Distribution of gas 1 Y. Kato, M. Nishinaka holdup in a bubble column, Kagaku Kogaku Ronbunshu, 1 (1975) 530-533. 2 T. Miyauchi and C. N. Shyu, Flow of fluid in gas bubble columns, Kagaku Kogoku, 34 (1970) 958-964. 3 K. Ueyama and T. Miyauchi, Properties of recirculating turbulent two-phase flow in gas bubble columns, AIChE J., 25 (1979) 258-266. 4 T. Menzel, T.i.d. Weide, 0. Staudacher, 0. Wein and U. Gnken, Reynolds shear stress for modeling of bubble column reactors, Ind. Eng. Chem. Res., 29 (1990) 988-994. 5 J. B. Joshi and M. M. Sharma, A circulation cell model for bubble columns, Trans. Inst. Chem. Eng., 57 (1979) 244251. 6 P. Zehner, Impuls-, Staff- und Warmetransport in Blasensaulen. Teil 1. Striimungsmodell der Blasensaule und Fliis-
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9
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11
12 13
14
sigkeitsgeschwindigkeiten, ut >> uerfahrenstechnik <<, 16 (1982) 347-351. M. Millies and D. Mewes, Liquid circulation in columns due to bubble drift, Chem. Eng. Technol., 15 (1992) 2588264. N. Devanathan, D. Moslemian and M. P. Dudukovic, Flow mapping in bubble column using CARPT, Chem. Eng. Sci., 45 (1990) 2285-2291. K. Franz, T. Menzel, H. J. Kantorek and R. Buchholz, Einfluss der Reaktorgeometrie und des Gasverteilers auf das Strijmungsfeld in Blasensaulen, Chem.-Trig.-Tech., 57 (1985) 474-475. Z. Yang, U. Rustemeyer, R. Buchholz and U. Onken, Profile of liquid flow in bubble columns, Chem. Eng. Commun., 49 (1986) 51-67. J.-W. Tzeng, R. C. Chen and L.-S. Fan, Visualisation of flow characteristics in a 2-D bubble column and three-phase fluidized bed, AIChE J., 39 (1993) 733-744. K. Rietema, Science and technology of dispersed two-phase systems, Chem. Eng. Sci., 37 (1982) 1125- 1150. J. J. J. Chen, M. Jamialahmadi and S. M. Li, Effect of liquid depth on circulation in bubble columns: a visual study, Chem. Eng. Rex Des., 67 (1989) 203-207. C. Webb, F. Que and P. R. Senior, Dynamic simulation of gas-liquid dispersion behaviour in a 2-D bubble column using a graphics mini-supercomputer, Chem. Eng. Sci., 47 (1992) 3305-3312.
On the local gas holdup and flow pattern in standard-type columns
bubble
J. Fischer*, H. Kumazawa-f and E. Sada Department (Received
of Chemical Engineering,
Kyoto
University,
July 15, 1993; in final form October
606-01 Kyoto
(Japan)
4, 1993)
Abstract Local gas holdups and bubble size distributions have been measured simultaneously in a standard-type bubble column equipped with a perforated plate as a gas disperser. High values in the time series of the local gas holdup which appear on a periodic basis could be detected and analyzed. The phenomenon of the bubble hose, defined as a zone of high bubble number density moving helically in a bubble column, has been recognized visually using photographs and by means of the recorded data. A formation mechanism for the bubble hose dependent on the gas disperser geometry capable of explaining the observed asymmetric liquid and gas flow pattern is suggested. The existence of a relationship between the bubble hose, the average bubble diameter and the superficial gas velocity has been demonstrated.
1. Introduction
the originally sampled data are then symmetrized fitted to the well-known equation
Local gas holdup in bubble columns is still an important subject in modern chemical engineering. Whilst, on one hand, the sensitivity of reaction components to local conditions, e.g. the sensitivity to the local oxygen concentration in bioengineering, increases the need for detailed information about the system, on the other hand, detailed modeling of the systems under investigation appears to have become feasible recently due to an increasing knowledge of each step in the transport phenomena and chemical reactions and the increasing capacities of even desktop size microcomputers. Thus, the application of existing models of bubble column phenomena has become more elaborate and sophisticated. However, the description of some physical properties, such as local gas holdup and local liquid flows, bubble size distributions and others, still remains unsatisfactory. When values of local gas holdup are reported, they are usually radial profiles taken along the diameter of the bubble column as shown in Fig. 5 below. A single point value, E~+(Y = const.), of the profile is obtained by time-averaging the data at that radial position. Using EG,I&)
= $2,
ICC+ r) + EG,loc( -r))
(1)
*On leave from Abteilung Chemietechnik, Lehrstuhl Technische Chemie B, UniversitLt Dortmund, 44221 Dortmund, Germany. tTo whom correspondence should be addressed.
0255-2701/94/$7.00 SSDI 0255-2701(93)00486-N
k,
,oc(~)= &G,,oc(~= O) x
and
[I -u(;>“]
or one of its modifications [l-3], yielding a parabola which represents the symmetrized data with a satisfying accuracy. The parabola drawn in Fig. 5 was calculated using the equation for the turbulent flow regime proposed by Menzel et al. [4]: &G,,oc(r)= 1.33 x
EG.,oc(r
=
O>x L_1.05(;>2]
(3)
According to ref. 4, the value of the parameter &G&r = 0) in Fig. 5 can be set equal to the measured value of the integral gas holdup in the bubble column, (~o,~,,~), since the profile is taken in the equilibrium zone. The results fit the intuitive concept of bubbles rising in the center of the bubble column and thus inducing a circular liquid flow: at the top of the column, the majority of bubbles leave the liquid medium which streams down in the downcomer region close to the column wall. All main hydrodynamic models for bubble columns are based on this concept [ 5,6]. Although for this ‘circulation cell model’ the height of one cell has been reported independently to equal approximately one column diameter by different authors using different methods [ 5, 71, a satisfactory description of the flow situation in regions where two or even more cells contact each other is still lacking. On the other hand, some
0
1994 -
Elsevier Sequoia.
All rights reserved
8
authors have reported deviations from the ideal symmetrical profiles or flow pattern even for the water/ air system [4,8,9], especially when working with liquids of higher viscosity [4, lo]. But for obvious mathematical reasons, the symmetrized profiles are commonly used in complex models. A more complete introductory overview of the literature on bubble column modeling has recently been published by Tzeng et al.
ill]. The second characteristic of standard models is that the bubble column is divided vertically into three regions: . the gas inlet zone directly above the gas disperser, . the equilibrium zone, where the shape of the radial gas holdup profiles are independent of the column height, and . the degassing zone at the top with its higher local gas holdup values. The intention of this article is to extend the different approach to the hydrodynamics in bubble columns first described by Franz et al. [9]. Focusing on the gas-phase behavior, special attention will be paid to the gas inlet zone in order to explain the origin of the observed asymmetry of the profiles of local gas holdup, and the related gas and liquid flows. Whereas the model suggested by Franz et al. [9] was based on average values calculated after a certain macroscopic measuring time interval AT, the purpose of this study is to clarify the events during this very interval.
A compressor supplied a constant volumetric flow of air which was measured using a rotameter. Since no significant loss of water due to mass transfer (liquid to gas) could be observed during the period of measurement, a special saturation column was not provided. The local gas holdup as well as the bubble size distributions were measured using a laser detector employing the Doppler effect (Laser Doppler Anemometry, LDA). Both the optical device and the evaluation electronic were provided by Nihon Kagaku Kogyo Co., Ltd. The sensor’s operation principle is shown in Fig. 1. The light of a laser beam is partly reflected at the gas/fluid interface. The fluid can be both water (A, C) or gas (B). A gas bubble hitting the tip of the sensor causes a change in the intensity I of the reflected light from I,_ to Zo capable of measurement via a photodetector. Thus, the time period t, during which a bubble is present at the tip can be determined and the local gas holdup can be calculated. Simultaneously, the approaching bubble causes a frequency shift Af of the laser light that can be used to determine its velocity. Knowing the speed and the time of the bubble, its diameter can also be calculated.
3. Results and discussion 3.1. Visual observation 3.1.1.
2. Experimental The experimental setup to investigate the ionexchanged water/air system in a tlrst approach was kept as simple as possible. It consisted of a standard-type bubble column made of acrylic resin with a 0.10 m inner diameter and a 0.88 m liquid height (H/D = 8.8) without any additional inner parts except the gas-phase detector described below. The column was operated batchwise with respect to the liquid phase and continuously with respect to the gas phase. All experimental data shown in this paper were taken at a height h, of 0.6 m (h,/D = 0.68) unless otherwise stated. Two different types of perforated plates were used as gas dispersers. The first had a perforated area diameter d,,,, = 0.4 m (d,,,,/D = 0.4) and is called a partial area disperser (PAD), whereas the perforated area of the second type covers the whole cross-section of the bubble column (dful,/D = 1.0) and is therefore called a full area disperser (FAD). They contained 14 and 88 holes of 0.8 mm diameter, respectively, with an equilateral triangular pitch L, = 11 mm. The center of their perforated area was in the center of the column in all the experiments performed.
Partial
area disperser
Figure 2 illustrates typical photographs of rising bubbles in the column equipped with a PAD as a gas disperser at three levels of the superticial gas velocity was. The three photographs in the top row were taken from the middle part of the bubble column, whereas the bottom three were taken in the gas inlet zone directly above the gas disperser. In the gas inlet zone, as shown in Fig. 2(l), the bubbles are already pushed towards the wall by the downcoming liquid that flows horizontally inward at the bottom. This creates a zone of higher bubble number density in the liquid flow direction. Because of the reduced mass density in that small reactor volume, the bubbles rise faster than in regions with a lower bubble number density. This again induces a faster liquid circulation and thus a feedback process is generated. Since the direction of the liquid flow is not constant, the position of the high bubble number density zone is also not constant but changes very rapidly, which is substantially different from the stationary phenomenon described by Rietema [ 121. Once formed, this bubble hose can be easily recognized at any height in the bubble column as shown in Fig. 2(2) and, furthermore, it appears to be helical as can be seen clearly in Fig. 2(2c). According to the
I
Gas
Liquid
Liquid level >
Fig. 1. Operating
principle
of the optical
sensor equipped with a laser beam.
photographs of Fig. 2(2), the height of the turns of the helix seems to be a function of the superficial gas velocity: the higher gas velocity, the smaller the height of the turn. (This caused some problems in the numerical analysis of the data as will be discussed below, but an analysis of this phenomenon is beyond the scope of this paper.) Figure 3 shows an attempt to draw the two-dimensional projection of the three-dimensional bubble hoses (dot-filled areas) and the suggested corresponding downward liquid flows (thick lines). Additionally, as an example, one eddy moving along a streamline is inserted in Fig. 3(2b). This detail is presented once more in Fig. 4, which shows the superposition of the rotational and translational movement of the eddy. The helical rise of the bubbles implies a helical liquid flow
upwards and as well as downwards. This type of flow pattern was first suggested by Franz et al. [9], who measured local liquid velocities in all three directions. It gives a natural and self-evident explanation for the bubble drift in the gas inlet zone and the fast changes in its direction. Furthermore, it supports the observed asymmetry of the profiles of the local gas holdup such as that shown in Fig. 5. 3.1.2. Full area disperser (FAD) Figure 6 illustrates representative photographs of rising bubbles in the column equipped with an FAD at three levels of the superficial gas velocity. In a similar manner to Fig. 2, the top three photographs were taken from the middle part of the column, and the bottom three were taken just above the gas disperser. Figure 7
wq = 3.43 cm/s
W,‘ = 0.66 cm/s (a)
W
(c>
Fig. 2. Photographs of the rising bubbles in the column equipped with a PAD. Top row (2a-c): (20-60 cm). Bottom row (la-c): gas inlet zone (O-20 cm). shows the corresponding outlines of the bubble hoses with the assumed main liquid flows. Using an FAD, the bubble hose can still be clearly identified, although the gas is now fed over the whole cross-section of the column. However, the situation directly above the gas disperser is different, as depicted in Fig. 6( 1). Since there is no longer a pre-fixed location for the bubble origin, the liquid can flow downwards at any position in the cross-section before it flows horizontally. As a result, the cross-stream above the disperser can occur from any position in any direction, for example: . from the back to the front (c), . from right to left (b), . or even from the center to the front and back simultaneously (a) in Fig. 6( 1). The resulting flow structure directly above the gas disperser can be very stable - even at higher gas velocities - and creates extremely asymmetric profiles of local gas holdup in that region as shown in Fig. 8. The differences between the flow above both
middle part of the bubble colmnn
disperser types and the creation mechanisms bubble hose will be discussed in detail below.
of the
3.2. Recorded data
3.2.1. Local gas holdup as a time series From Figs. 2 and 6, the following question arises. What does the gas-phase detector here detect at a fixed position over a certain macroscopic time interval AT? Obviously, there will be periods of no or very low gas holdup and of very high gas holdup as well - corresponding to the momentary absence or presence of the bubble hose. To clarify this, the time interval AT has been split into short sampling intervals of equal length A? and the corresponding data recorded. Figures 9 and 10 show the local gas holdup as a function of time at the radial position r = 0.04m in the PAD and FAD cases respectively. The sampling time interval AT, for each point of the data was chosen to be 1 s. Distinctive peaks much higher than the average value EG ,oc calculated after AT = 900 s (indicated by the straight line)
11
(I)
3.&
=
0.66
cm/s
was = 2.14
WGS= 3.43 cm/s
(b)
(4 Fig. 3. Outlined
cm/s
bubble
hoses of Fig. 2 and assumed
main liquid flows (thick
can be clearly identified. Tn addition, these peaks occur periodically at the positions marked by ‘. . .’ in Figs. 9 and 10.
\ I
Fig. 4. Detail of Fig. 3(2b); superposition tion movement of an eddy.
of rotation
and transla-
line).
3.2.2. Bubble size distribution Although the optical sensor described in Fig. 1 is not suitable for recording true bubble size distributions, since the detected bubbles need to be hit centrally twice to obtain evaluable signals, it can be used to compare distributions recorded at different positions or under different operating conditions. Figure 11 compares the normalized bubble size distributions obtained for various superficial gas velocities using an FAD. Whereas at the column center (Fig. II(a)) for all gas velocities mentioned below the same narrow Gaussian shape, mean diameter d,, and Sauter diameter ds2 were calculated, noticeable changes were observed at the peripheral radial position r/R = 0.8. With increasing gas velocity the mean diameters decrease, apparently reflecting a higher proportion of small bubbles. However, the number of signals that could not be identified as ‘bubbles’ because of the sensor’s operating principle (denoted as n lo in the
12
‘l. :.. ‘:. :., ‘..
4!
-1
^'" -u.a
'_
-“.O
1.
-“.‘I
^'_
-UL
I
0
X=r/R Fig. 5. Experimental
Fig. 6. Photographs (20-70 cm). Bottom
and calculated
radial
profiles
0.2
0.4
0.6
0.8
I 1
(-) of the local gas holdup
of the rising bubbles in the column equipped row (la-c): gas inlet zone (O-20 cm).
with a PAD
with an FAD.
used as a gas dispe,ser.
Top row (2a-c):
middle
part
0 Original
of the bubble
data,
column
3
13
w_ = 4.17 cm/s (a)
(b)
wO*= 5.71 cm/s
(c)
Fig. 7. Outlined bubble hoses of Fig. 6 and assumed main liquid flows (thick
also increases at the same time. Comparing the distributions at both positions, it seems justified to conclude that larger bubbles are less often detected at the periphery because of their relatively high tangential velocity. This confirms the visual observation of a moving bubble hose. Furthermore, a comparison of Figs. 1 l(a)-(d) shows that over the range of superficial gas velocities investigated no bubble coalescence or bubble breakdown occurs. Both would be indicated by a significantly greater number of larger and smaller bubbles, respectively. Figure 12 shows a similar comparison for the PAD case. Again, Fig. 12(a) depicts a typical histogram obtained at the column center. It is noticeably wider than that calculated for an FAD. This refle;cts the fact that at the same superficial gas velocity a higher number of larger bubbles are formed for a PAD. At the peripheral position, the mean diameters again depend figures)
line).
on the superficial gas velocity, clearly showing the opposite trend to that in case of an FAD: with increasing gas velocity, the mean diameters ‘increase. It is thought that this occurs because 6f a higher rate of bubble coalescence as indicated by the increased proportion of large bubbles due to the liquid cross stream above the disperser (to be discussed below in connection with Fig. 14). Figure 13 summarizes the dependency of the mean diameters by illustrating the diameter &, as a function of the superficial gas velocity. Although correlation of the data via a least-squares fit of a straight line to yield the ‘-.-’ lines of Fig. 13 is obviously justified, it is also possible to interpret the plotted data as lyind on .a step-like curve and .a bent clvve for a PAD and an FAD, respectively. This unusual interpretation is suggebted because of another result presented below. ,. :
14
-1
-0.8
-0.6
-0.4
-0.2
0
0.2
0.4
0.6
0.8
1
X=r/R (-) Fig. 8. Profile of the local gas holdup directly (8 cm) above the gas disperser in case of an FAD.
_.. 0.9 0.8 t 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0 0
50
0.6 0.5 0.4 0.3 0.2 0.1 0.0 m
350
150
200
250
300
450
500
550
600
time[sl
400
the [sl
il.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 nn
I
I.1
6w
650
700
750 Lime[sl
800
Fig. 9. Local gas holdup, zG,,_, as a function of time in the case of a PAD.
900
15
0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0 3m
350
400
650
700
450
500
550
600
800
850
900
“..,
600
750 limeIs]
Fig. 10. Local gas holdup, zG,,oc as a function of time in the case of an FAD.
3.3. Bubble hose 3.3.1. Basic assumptions The fundamental assumptions based on the visual observation and the recorded data are that (a) the bubble hose exists and (b) its movement is flexibly periodic. However, the expression periodic should not really be used in this context, since it is related to mathematical expressions, e.g. sin X, which relate to precise periods which cannot be expected at all in a bubble column. The term quasi-periodic has a special connotation in chaos research and clearly should also not be used in this context since the necessary relation has not yet been reported. The term which we will assign is flexibly periodic, which expresses the fact that after a certain time interval called a period, another shorter time interval follows in which the local gas holdup is expected to have an especially- high value relative to the previous period. Furthermore, because of
the disturbances caused by the liquid or gas flow, the peak values are not expected to ‘appear at every predicted location although the system tries to resume its periodicity as quickly as possible. This is thought to occur in order to minimize the friction loss between the rising and downcoming streams. As a consequence one particular period should be attributed to every superficial gas velocity since only one state of the mentioned energy minimum is expected to be reached. This concept does not contradict previous results obtained after time averaging, since time averaging over a circular motion might result in stationary ‘circulation cells’. On the basis of the above definition of a period, a semi-numerical algorithm was designed in order to try to analyze the signals and predict the peak occurrences. Its numerical part consisted of three steps: (i) creating artificial data (AD) by summing up the measured data over a short time interval; (ii) eliminating all AD lower
diameter
* (4 Fig. 11. Bubble
d (mm)
diameter
(h)
d (mm)
3
diameter size distributions
d (mm) at the radial
diameter
Cd) positions
(a) r/R = 0 and (b-d)
than a given threshold value; and (iii) classifying the distances between the remaining AD in a histogram. The resulting histogram depends on the values selected for both the time interval and the threshold. The subsequent non-numerical procedure comprises the extraction of the period out of a variety of histograms. Finally, the determined periods are applied to the originally measured data. The ‘. .’ intervals in Figs. 9 and 10 mark the positions at which high values of local gas holdup were predicted and resulted in an extremely high degree of correspondence with respect to the turbulent system under investigation. 3.3.2. Formation mechanism A probable creation mechanism for the bubble hose in case of a PAD has already been briefly discussed above. The liquid flowing downwards mainly along the wall does not affect bubble formation, since it simply does not occur in this area. On changing to a horizontal cross flow over the gas disperser, the liquid pushes the
d (mm)
r/R = 0.8 in the case of an FAD.
newly formed bubbles aside as shown in Fig. 14(a). The tangential velocity component rq of the liquid has not been drawn in the figure, since its effect is the same because it is also perpendicular to the axial component v, . For an FAD the situation is different. The liquid still flows downwards, but it always has one vertical velocity component perpendicular to the gas disperser directly above its perforated area. The arrangement of these factors increases the pressure on the disperser in the axial direction so that bubble formation and rise is hindered at this position (see Fig. 14(b)). The bubbles rise at the location of lower pressure and are again pushed aside by the cross flow. Since the increase in pressure Apr is a function of the liquid velocity according to Apr = ;pvz2
(4)
it is thought that the higher the value of the superficial gas velocity wGs which induces the liquid flow, the
3
diameter
4
5
6
7
8
9
10
diameter d (mm)
d (mm)
r
I
L
*
Cc)
5
6
7
8
9
*
10
Fig. 12. Bubble size distributions
at the radial positions (a) r/R = 0 and (b-d)
higher is the stability of the phenomenon. Of course, for a particular was value bubble formation is expected to occur over the whole perforated area, but when this occurs bubble rise will still be hindred so that the phenomenon does not change qualitatively. Although only two perforated plates have been used in this study, the creation mechanism proposed so far is also expected to be valid for other two-dimensional gas disperser types. Here the expression two-dimensional refers to gas dispersers that feed the gas over a certain area as opposed to the other types listed in Table 1, which is a simple attempt to classify various disperser constructions according to their geometry in relation to the resulting operation conditions. 3.3.3. Period as a function
of the super-cial
3
45
6
7
8
9
10
diameter d (mm)
diameter d (mm)
gas
velocity
To illustrate the correlation of the period with one of the main operating parameters of a bubble column, the semi-numerically determined periods have been plotted
r/R = 0.8 in the case of a PAD.
as a function of the superficial gas velocity in Fig. 15. For a PAD three different levels of the periods at 18, 15 and 12 s were extracted, whereas in case of an FAD two periods of 20 and 15 s were identified. So far, no explanation for these data values can be given and they are therefore presented here without further discussion. For low gas velocities the identification of the periods was relatively evident, but at higher gas velocities an additional phenomenon complicates the evaluation. As can be seen from Fig. 2, the height of one spiral turn decreases with increasing gas velocity. Thus, the superposition of two periods, one rotational and one translational, suggests a challenging research topic, since it is not yet possible to distinguish between both from data analysis using our algorithm. However, an additional significant observation can be made. The levels of the periods in Fig. 15 correspond to the plateaus of the step-like curve and parts of the bent curve described in Fig. 13 for a PAD and an FAD, respectively. This suggests that subtle changes in the
18
2.2
1
1
0.6
0.8
1.2
1.0
1.6
1.4
2.0
1.8
2.4
2.2
2.6
superficial gas velocity (cm/s) Fig. 13. Calculated PAD +.
mean
bubble
diameter
d,, as a function
of the superficial
gas velocity
P +AP,
Fig. 14. Plausible
TABLE
sketches
1. Dimensions
of the flow above
of different
Example
zero one two three
single orifice ring of perforated tube perforated plate gas fed at different heights
the gas dispersers:
into the bubble
position
r/R = 0.8. FAD
0,
- P +Ap,
(a) partial
area disperser;
(b) full area disperser.
where the integral gas holdup (.zG+) is shown as a function of the superficial gas velocity wGs for both types of disperser. For a PAD, the slope of this curve changes during the velocity interval where the second step of the diameter increases and the second change in the period level occurs.
types of gas disperser
Dimension
at the
column
‘, flow structure occur which are reflected by variations of both quantities. Furthermore, this hypothesis appears to be partly confirmed by the data depicted in Fig. 16,
3.3.4. Comparison of two-dimensional and three-dimensional bubble columns A different approach towards clarifying flow structures in bubble columns was made by Chen et al. [13]. In that study, a two-dimensional column was used in
19
13 12 11 10 0.6
0.8
1.0
1.2
1.4
1.6
1.8
2.0
2.2
2.4
2.6
superficial gas velocity (cm/s) Fig. 15. Determined periods as a function of the superficial gas velocity at the radial position r/R = 0.8. FAD 0, P.&D +. which staggered circulation cells with an asymmetric flow pattern were obtained. In addition, it was pointed out that the HID ratio is a very important factor in pattern formation, since at values below H/D = 1 the resulting patterns are significantly different. Using a numerical model for two-dimensional columns, a remarkable good computer simulation of the observed pattern reported by Chen et al. [13] has
been achieved by Webb et al. [ 141 for H/D values below 2. Moreover, the computer simulation shows that two bubble jets which are already off-centered are immediately pushed further towards the wall, creating one single stable and stationary vortex. On the other hand, the staggered vortices are not stationary with respect to a fixed position, but form steadily at the column top and move downwards during which
superficial gas velocity (cm/s) Fig. 16. Integral gas holdup (se. ,nt) as a function of the superfkial gas velocity. FAD 0, PAD +
course they can even disintegrate
at higher gas velocities
[111. As demonstrated by a comparison of the results reported in refs. 8 and 13, qualitatively similar phenomena are to be expected in three-dimensional columns but with a higher degree of complexity., According to the formation mechanism proposed here for the bubble hose in three-dimensional columns, off-centered rising bubbles are encountered as previously described by Webb et al. [ 141 for two-dimensional columns, and thus similar asymmetric flow is to be expected. Hence, in Figs. 2 and 6, which depict flow structures similar to those of a two-dimensional column, the three-dimensional equivalent of the wave-like pattern reported by Tzeng et al. [ 1 l] can be readily recognized. In twodimensional columns, the tight walls of the columns strongly limit the movement of the rising bubbles and thus ‘trap’ the bubbles between the walls ‘and the equally restricted vortices. As a consequence, the fast bubble flow region with its large coalesced bubbles, as described by Tzeng et al. [ 1 l] will be formed. However, as can be concluded from the photographs depicted in Figs. 2 and 6 and the recorded bubble size distributions of Figs. 11 and 12, this limitation does not exist in three-dimensional columns and, consequently, mobil helical bubble rise is observed. 1
4. ~opclusions
’
This article reports measurements of the local gas holdup, SG, ,a:, anh bubble size distributions carried out in a standard-type bubble column equipped with a perforated plate as a gas disperser. As an extension of the standard presentation of time-averaged values of the local gas holdup EG,,=, time-dependent sG, ,oc(t) values are also presented to illustrate the dynamic behavior of this parameter. Both visual observation and electronically sampled data reveal that a zone of high bubble number density, called a bubble hose, moves in a flexibly periodic manner in the bubble column. A formation mechanism for this bubble hose is suggested that takes into account the geometry of both the gas disperser and the gas inlet zone of the bubble column. Furthermore, this mechanism can explain the flexible periodic movement of the bubble hose without being in contradiction with results obtained using the time-averaging approach of the ‘circulation cell model’, as well as aiding an understanding of some non-symmetrical phenomena reported by other researchers. The reported data and the analysis provided in this article indicate. that, in order to obtain more detailed knowledge of some aspects of the phenomena occurring in bubble columns, further investigations need to be undertaken not only in local ‘positions’ but also in local ttimes’,
leading to a more dynamic description structures.
of the phase flow
Acknowledgement
The authors gratefully appreciate the fundamental support of Professor Emeritus Dr. U. Onken and his interest in this work.
Nomenclature a d d 10 d 32 dpart d full
P
Af Z
k H 4 n n
ok
n
lo hi
n
N NM P APf x tG
factor in eqn. (2) referring to associated references bubble diameter mean diameter of bubble Sauter diameter of bubble diameter of a partial area gas disperser diameter of a full area gas disperser diameter of the bubble column frequency frequency shift light intensity measuring height above gas disperser liquid height in the bubble column length of pitch of the perforated plate / exponent in eqn. (2) number of bubbles within diameter range OI *lOmm number of signals not identified as a bubble number of bubbles larger than 10 mm number of buubbles in one size class total number of bubbles used for the size distribution pressure pressure increase radial coordinate radius of the bubble column time a fluid phase is present at the tip of the sensor macroscopic measuring time interval microscopic measuring time interval radial component of liquid velocity tangential component of liquid velocity axial component of liquid velocity superficial gas velocity dimensionless radius
Greek letters &G, lxx SG, lw
(eG, int) P
local gas holdup time .averaged local gas holdup integral gas holdup’ density
21
Subscripts 0 reference value gas phase G L liquid phase
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sigkeitsgeschwindigkeiten, ut >> uerfahrenstechnik <<, 16 (1982) 347-351. M. Millies and D. Mewes, Liquid circulation in columns due to bubble drift, Chem. Eng. Technol., 15 (1992) 2588264. N. Devanathan, D. Moslemian and M. P. Dudukovic, Flow mapping in bubble column using CARPT, Chem. Eng. Sci., 45 (1990) 2285-2291. K. Franz, T. Menzel, H. J. Kantorek and R. Buchholz, Einfluss der Reaktorgeometrie und des Gasverteilers auf das Strijmungsfeld in Blasensaulen, Chem.-Trig.-Tech., 57 (1985) 474-475. Z. Yang, U. Rustemeyer, R. Buchholz and U. Onken, Profile of liquid flow in bubble columns, Chem. Eng. Commun., 49 (1986) 51-67. J.-W. Tzeng, R. C. Chen and L.-S. Fan, Visualisation of flow characteristics in a 2-D bubble column and three-phase fluidized bed, AIChE J., 39 (1993) 733-744. K. Rietema, Science and technology of dispersed two-phase systems, Chem. Eng. Sci., 37 (1982) 1125- 1150. J. J. J. Chen, M. Jamialahmadi and S. M. Li, Effect of liquid depth on circulation in bubble columns: a visual study, Chem. Eng. Rex Des., 67 (1989) 203-207. C. Webb, F. Que and P. R. Senior, Dynamic simulation of gas-liquid dispersion behaviour in a 2-D bubble column using a graphics mini-supercomputer, Chem. Eng. Sci., 47 (1992) 3305-3312.