Ind. Eng. Chem. Res. 2009, 48, 8237–8243
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Bubble Column with Electrolytes: Gas Holdup and Flow Regimes Sandra Orvalho,* Marek C. Ruzicka, and Jiri Drahos Institute of Chemical Process Fundamentals, Department of Multiphase Reactors, Academy of Sciences of the Czech Republic, RozVojoVa 135, 16502 Prague, Czech Republic
Experiments were performed in a laboratory scale bubble column, to investigate the effect of electrolytes (inorganic salts) on the behavior of bubbly mixtures. Aqueous solutions of three salts were studied, Na2SO4 (p.a. grade), NaCl (p.a. grade), and NaCl (kitchen quality). The gas holdup was measured, and the prevailing flow regime was determined. A simplified method for determining the regime transition based on the spline function was suggested and successfully tested. The main goal was to find whether the dual effect of the electrolyte on the gas holdup and homogeneous regime stability reported earlier for CaCl2 occurs also with other compounds. We confirmed this dual effect for two more solutions. The measurements with NaCl (p.a. grade) were most likely corrupted by salt crystallization inside the orifices of the perforated plate. 1. Introduction The application of gas-liquid reacting and contacting systems in a large number of technologies is well-known and documented.1-4 They work with both aqueous and organic media. With the former, the effect of inorganic contaminants (salts, electrolytes) always becomes an issue, since rarely do we use absolutely chemically pure water in industrial settings. The presence in water of both inorganic (nutrient salts) and organic (substrate) additives is typical for the biochemical reactor processing in biotechnology (bioreactors, fermenters). The occurrence in water of a broad spectrum of compounds of both of these kinds is typical for wastewater technologies. For instance, the sewage water aerated in huge tanks is far from being chemically “pure”. The importance of the contaminants on the hydrodynamic and transport phenomena in multiphase mixtures is well-recognized. In this contribution, we report on further experimental results obtained in measurements of the effect of inorganic salts on the behavior of the gas-liquid mixtures in bubble columns; see our previous paper5 (denoted as R15). The salts are electrolytes that act as so-called “negative surfactants” since they are usually repelled from the gas-liquid interface, and typically, they slightly increase the surface tension (e.g., section 5.4 in ref 6). In electrolyte solutions, we measured the voidage (gas holdup) and observed the prevailing flow regimes (homogeneous/ heterogeneous). More information on the flow regimes can be found in the literature.7,8 Aqueous media with salts are of common occurrence in technological applications. It is known that gas-liquid systems are very sensitive to the presence of electrolytes whose effects are multiple and complex, not fully understood until now. It is usually agreed that the surfactants suppress bubble coalescence, the bubble size is reduced (both due to the formation of smaller bubbles and lower coalescence), and the bubble rise speed is lowered (both due to smaller bubbles and Marangoni stresses at the interface). Many different results on the effect of surfactants (both positive and negative) on the behavior of the gas-liquid systems occur in the open engineering literature, not fully consistent and even contradictory. In R1,5 an extensive introduction into the problem and referencing can be found and, therefore, are not repeated here. An additional search revealed the following titles, relevant for the subject under study. * To whom correspondence should be addressed. E-mail: orvalho@ icpf.cas.cz. Tel.: +420 220 390 384. Fax: +420 220 920 661.
Oels et al.9 studied the effect of additives on the behavior of a bubble column bioreactor. Both the organic compounds (substrate: alcohols C1-C4) and inorganic substances (cultivation salt media, a solution of Na2SO4) were considered. They found that the additives enhance the gas holdup, by reducing the bubble rise speed. The effect of the alcohols increased with the chain length. They also evaluated the bubble stability in the parametric plane Weber number-Bond number (We-Bo). Keitel and Onken10 measured the bubble size in a number of aqueous solutions of organic (n-alcohols C1-C8, diols, ketones, carboxylic acids, detergents, sacchrose, sodium carboxymethyl cellulose (CMC)) and inorganic (NaCl, Na2SO4, Al2SO4, NaOH) contaminants, at a fixed value of the gas input. The bubble size was considerably reduced by the surface active agents. The gas holdup increased in the alcohol solutions with their concentration, the longer carbon chains acting more effectively. The ionic strength of the electrolytes was a promising parameter to reduce the scatter of the data from different salt solutions. At the conditions of their experiment, the gas holdup was practically unaffected by the concentration of the electrolyte and also of some organic compounds (which is rather counterintuitive). Nicol and Davidson11 studied the response of a pilot-plant circulating bubble column to the presence of two surface active agents (octanol and bovine serum albumin (BSA)-protein). With the disengagement technique, they obtained the distribution of the bubble sizes and the bubble velocities. These distributions were sensitive to the additive content. Jamialahmadi and Muller-Steinhagen12 investigated the effect of several surfactants on the bubble size, bubble speed, and holdup in two bubble columns. The organic compounds had different polarity (C1-C3 alcohols and carboxylic acids). The inorganic compound (KCl) at different concentrations provided the effect of the ionic strength. The bubble size on formation was strongly reduced by the organic substances, with a larger impact of the longer carbon chain. On the contrary, the bubble size was slightly enhanced by the salt KCl. The authors explain these opposite trends with help of the formula for the equilibrium bubble size that depends on the surface tension as ∼σ1/3. The organic compounds reduce the surface tension considerably and the electrolytes increase it slightly, hence the answer. The other difference was in bubble shape. Organic solutions yielded roughly spherical bubbles while the salt caused an ellipsoidal shape. The former bubbles were stable in the We-Bo diagram, while the latter were unstable, with a tendency to break. At
10.1021/ie900263d CCC: $40.75 2009 American Chemical Society Published on Web 07/20/2009
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higher gas inputs, these indeed broke into smaller bubbles. With all the additives, the bubble speed was reduced and the gas holdup generally increased. Subtle differences were found in the KCl solutions in the holdup graphs, for low and high salt contents and gas inputs. Muller and Davidson13 studied the effect of a surfactant (octanol) on the mass transfer in a viscous batch (1% CMC in water). The surfactant enhanced the transport process substantially. Using the intermittent bubbling technique (on/off gas input; engagement/disengagement), they evaluated the holdup structure in the bubble column. The octanol produced a trimodal distribution of the bubble sizes, with a large population of small bubbles. The respective contributions of the three bubble classes to the total transport rate were also estimated. Muthukumar and Velan14 measured the effect of three organic additives (propanol, iso-amyl alcohol, benzoic acid) on the behavior of an internal airlift reactor. They reported an increase in the gas holdup, both in total and in the two reactor compartments, presumably due to the coalescence suppression and the reduction of the bubble speed. They found a lower liquid circulation speed, caused by the presence of the additives. The above referred sources are concerned with the effect of various kinds of surfactant on the overall holdup, holdup structure, bubble size and speed, and the mass transport rate. They provide a good coverage on the resulting output of the surfactant action, as well as a useful insight into the possible mechanisms behind it. On the other hand, the more delicate issue of the hydrodynamic flow regimes of the bubbly mixture in bubble columns is not considered. We have only found some studies directly related to the flow regimes and their transitions. Kelkar et al.15 performed a thorough experimental study on the effect of five aliphatic alcohols on the behavior of two bubble columns (liquid phase: batch/continuous). In both columns, the surfactants produced a considerable increase in the gas holdup, in the following order: water < C1 < C2 < iso-C3 < n-C3 ct) in experiments on pairwise coalescence of bubbles under well-defined conditions (small cell, no liquid flow, etc.). Two open questions arose. What are the underlying mechanisms of the salt effect on the bubbly mixture? Are our experimental findings particular to CaCl2 or of a more
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a
Table 1. Relevant Properties of the Three Experimental Solutions Used in This Study
DW
concentration c [mol/L]
conductivity γ [S/m]
surface tension σ [N/m]
density F [kg/m3]
viscosity µ [Pa · s]
ratio c/ct [-]
0.00001
0.00027
0.0705
999
0.00091
∼1 × 10-4
999 1001 1005 1016 1059
0.00090 0.00091 0.00093 0.00096 0.00112
0.02 0.33 1 3 9.8
997 999 1005 1017 1076 1186
0.00092 0.00091 0.00093 0.00095 0.00111 0.00172
0.007 0.34 1.03 3.1 13.8 41.4
Na2SO4 A B C D E
0.001 0.017 0.051 0.153 0.5
0.022 0.31 0.82 2.08 5.3
0.0709 0.0710 0.0715 0.0716 0.0726
A B C D E Fb
0.001 0.05 0.15 0.45 2 6
0.011 0.49 1.39 3.82 13.4 22.4
0.0708 0.0710 0.0710 0.0717 0.0743 0.0795
NaCl
a DW: distilled water. Electrolyte one: Na2SO4 (p.a. grade). Electrolyte two: NaCl (p.a. grade). Electrolyte three: NaCl (kitchen quality). Both NaCl salts had the same material properties, within the detection limits. Properties of CaCl2 discussed in this study are shown in Table 1 of our previous paper; see R1.5 b Solution F of NaCl was too concentrated to produce the uniform regime.
general validity, covering also other electrolytes? The latter question is addressed in this study. We tested solutions of three salts for their effect on bubble column behavior. The salts commonly used in applications were chosen. One set of solutions was prepared from Na2SO4. The other two sets contained NaCl of two different qualities, chemical grade p.a. and common kitchen salt, to see the possible effect of the presence of impurities. We now have results for compounds with three different ratios of the cation/anion valence: 2:1 for CaCl2 as reported in R1,5 1:1 for NaCl, and 1:2 for Na2SO4. We found that the dual effect reported for CaCl2 in R15 was observed in this study also for the other salts. Therefore, it is very likely of common nature and can thus generally be expected when we have electrolytes in bubble columns and airlifts. 2. Experiments The experiments were performed in a lab scale plexiglas cylindrical bubble column, 0.14 m inner diam and 2 m total height. The 3 mm thick brass perforated plate with circular orifices of 0.5 mm diam and relative free area 0.2% was used to produce both types of flow regime. The clear liquid height was 0.4 m. The typical bubble size in the homogeneous regime was 4-5 mm. Compressed air from laboratory lines was the gas phase, and great care was taken to clean it properly. Distilled tap water was used for preparing the solutions. The solutions of the following three salts in distilled water (denoted DW) were employed in the measurements, Na2SO4 (p.a. grade), NaCl (p.a. grade), and NaCl (kitchen quality). The kitchen salt had the following composition: NaCl (98.78% wt), Ca2+ (0.159%), Mg2+ (0.034%), SO42- (0.89%), insoluble matter (0.028%), moisture (0.1%). The iodine is added by the producer, in the form of KIO3, in an amount of 45 mg/kg. For each salt, several solutions of different concentration were used, starting from c ) 0 (pure water) and finishing at some larger concentration cm, to have the transient coalescent concentration ct somewhere in the middle of each range, at least 1 order of magnitude from both ends. The physicochemical properties of the solutions are shown in Table 1. Both NaCl salts had identical physical properties, within the error of determination. The physicochemical properties are published in standard handbooks.26,27 The transient concentrations ct can be found, e.g., in refs 28 and 29.
For the voidage measurements, the column was properly washed with distilled water and with the solution to be measured. The gas flow was gradually increased to obtain the gas flow rate (superficial velocity) q from 0 to about 0.1 m/s. The bubble bed expansion was measured by a ruler, which is the simplest and most reliable method for determining the steady mean value of e. The precision of the measurement in the uniform regime is very good, since the liquid surface is welldefined, horizontal, and steady (a skilful person can reach (1-2 mm variance only). In the nonuniform regime, the surface oscillates and the measured value is taken to be the mean of the lowest and highest surface positions over certain number of oscillations, typically 10-20, depending on the complexity of the motion. The oscillations develop gradually, and their amplitude grows with the gas input, beyond the critical point. Since the data relevant for the stability are those before the critical point, and possibly slightly after it, the detailed statistics of the measurements in the violent heterogeneous regime at very large q (which is not the case of this study) was not performed. Each measurement was done three-times and then averaged. The error of our results is expected to be below 5%. In the strongest solutions, the uniform regime might not develop and the corresponding data could not be evaluated for the critical point. Further details about the experimental setup, solution analysis, measurements, data evaluation, and errors can be found in R1.5 3. Results and Discussion 3.1. Simplified Method for Flow Regime Transition Point. The mean data e(q) for each salt solution were used for further treatment. In R1,5 the critical point [qc, ec] of the homogeneous-heterogeneous flow regime transition was determined by two different yet related ways, confronting the data with two modeling concepts: the hindrance model and driftflux model (see R15). For a given closure equation for the bubble slip speed, u ) u(e), we have two plots for the slip speed method, u(q) and u(e), and two plots for the drift-flux method, j(q) and j(e). The resulting numerical values of the criticals, qc and ec, then are the averages of the four numbers. With two closures, as in R1,5 we have to find eight numbers to yield either critical value, qc and ec. This approach (approach I) is reliable and accurate. On the other hand, it is demanding and timeconsuming.
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Figure 1. Two evaluation procedures for the flow regime transition point: approach Isthe complicated procedure ((); approach IIsthe simplified procedure (×). Both methods were tested on the large set of CaCl2 data reported in R1.5 The vertical line corresponds to ct ) 0.056 M. Maximum values are the following: qc ) 0.063 m/s and ec ) 0.47 [-].
To find a reasonable compromise between the cost and precision, in this study we employed a relatively simple way of how to obtain the transition point (approach II). Simultaneously with the elaborate evaluation described above, we fitted the experimental data u(e) on CaCl2 from R15 with a spline function and looked for the first apparent minimum. The data on the steady mean bubble speed u are obtained from the measured voidage e and the set gas input q simply, using the basic formula e ) q/u, which is the mass balance of the gas phase in the column (but not a definition of voidage). After some practice, this point corresponded well to the correct one. We tried several splines, and the cubic spline was considered to be suitable.30 The two evaluation procedures for the critical data are compared in Figure 1, with good agreement. It proves that the simpler approach can be used in routine evaluation of the data on the flow regime transition, assuming certain level of awareness and practice of the person in charge. This common salt used in R1,5 CaCl2, is a representative of the class with the valence ratio 2:1. It is highly soluble and strongly hygroscopic material. The most concentrated solution employed, cm ) 5.3 M, has 31% higher static surface tension than water. The transient concentration ct ) 0.056 M is about one hundredth of the maximum one, cm/ct ≈ 95. In the case of CaCl2, the value of cm was also the saturation value (Table 1 in R15). 3.2. Voidage and Flow Regimes in Na2SO4 Solutions. This common salt Na2SO4 is a representative of the class with the valence ratio 1:2. It is relatively soluble. Our strongest solution of 0.5 M exerts only a 3% increase in the static surface tension, compared to water. The transient concentration 0.051 M is about one tenth of the maximum one, cm/ct ) 9.8. The results for the six solutions DW and A-E from Table 1 are shown in Figure 2. The voidage data display a strong increase, typical for the addition of an electrolyte into a bubble
Figure 2. Results for electrolyte one: Na2SO4 (p.a. grade). (a) Gas holdup at increasing salt content. (b) Mean bubble slip speed and the critical point (O). (c) Stability diagram in critical values qc (9) and ec (0). The concentration range for plots in a and b is the following: c ≈ 0 (water DW, 0), c ) 0.001 (9), c ) 0.017 ((), c ) 0.051 (2), c ) 0.153 (×), c ) 0.5 M (*). The vertical line in c corresponds to ct ) 0.051 M. Maximum values are the following: qc ) 0.070 m/s and ec ) 0.66 [-].
column; see Figure 2a. The drop in voidage at larger salt content is much less pronounced, as compared with that for CaCl2 reported in R1.5 Note however, that the drop in R15 begins at about c ) 0.5 M (solution no. 15 in R15), and this value corresponds to the strongest solution of Na2SO4 in this study (Table 1). The transition point evaluation from u ) q/e using the simple method of the spline function (approach II) is shown in Figure 2b. The minimum could easily be located, in these particular cases. The graph u(e) decreases first due to the hindrance effect, which is characteristic for the behavior of uniform, dispersed layers. Then, with an increasing bubble concentration, the uniformity starts to break and liquid circulations develop gradually, increasing the rise speed of the gas phase. The regime transition begins where these two opposite trends meet, in the minimum of the u(e) graph. The stability diagram is depicted in Figure 2c, where the “amount of stability” of the uniform regime is expressed by the numerical values of the critical quantities. First, both the criticals, qc and ec, do possess maxima. Second, these maxima occur near the transient
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concentration ct. This means that the salt progressively stabilizes the homogeneous regime when the system is “coalescent”, by a gradual suppression of the coalescence. The maximum stability is reached where the coalescence is eliminated; it is near ct. Beyond the value ct, the system is “noncoalescent” and various phenomena can contribute to reducing the holdup. Note that, at a given value of the gas input q, the voidage e depends only on the mean bubble speed u, e ) q/u. The quantity u reflects all the complexity of the hydrodynamics of the gas-liquid mixture. It depends, e.g., on the bubble size, single bubble speed, collective bubble speed, liquid flow pattern, and others. Consequently, many phenomena have to be understood thoroughly, e.g. bubble formation, single bubble rise, hydrodynamic interaction forces, dynamics of mesoscale clusters, coalescence and breakup, and coupling between the gas and liquid phases, namely in surfactant solutions. At present, we are far from the ultimate knowledge. The current data only show that the uniform regime is stabilized by the surfactant in the coalescent region and destabilized by the surfactant in the noncoalescent region. This finding seems to be in a contradiction with the commonly adopted opinion that the bubble coalescence is the primary source of the homogeneous regime collapse. Further investigation is needed to explain this discrepancy. The results of this section bring strong support for the hypotheses that the above specified behavior (dual effect) is typical for electrolytes generally and is not particular only to CaCl2. 3.3. Voidage and Flow Regimes in Pure NaCl Solutions (p.a. Grade). This common salt is a representative of the class with the valence ratio 1:1. It is relatively soluble, with the anomaly that the solubility is almost independent of the temperature. Our strongest solution of 6 M increased the surface tension by ∼12%. The transient concentration 0.145 M is about 1/40 of the maximum one, cm/ct ) 41. The results for the six solutions DW and A-E from Table 1 are displayed in Figure 3. This salt behaves differently, as compared with the previous two. The voidage does not rise gradually with the salt addition, and the increase is smaller; Figure 3a. The voidage also does not drop at larger salt content. Before the critical point, the bubble slip speed varies in a narrow range; Figure 3b. The stability curve displays a minimum, not a maximum, near the transient concentration; Figure 3c. A speculation for explanation of this anomaly is suggested below, in section 3.5. We consider this anomaly not to be particular for the given salt (pure NaCl) but to be a potential problem typical for working with strong electrolyte solutions where the salt crystallization in the orifices of the gas sparger can corrupt the holdup whence stability data, of which Figure 3 is a nice example. 3.4. Voidage and Flow Regimes in Contaminated NaCl Solutions (Kitchen Quality). Since we were puzzled with the strange behavior of the pure sodium chloride described above, we bought a common kitchen salt in a foodstore, whose detailed composition was not known. Its physical properties were not detectably different from the pure NaCl species. The results for the six solutions DW and A-E from Table 1 are displayed in Figure 4. There is no anomaly, and the kitchen salt behaves like CaCl2 and Na2SO4. The gradual increase and then drop in the voidage is seen in Figure 4a. The usual ordering of the critical points is shown in Figure 4b. The typical maximum of the uniform regime stability near the transient salt concentration is documented in Figure 4c. An attempt has been made to compare our results with data of other authors, which is done in Figure 5. The only data we
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Figure 3. Results for electrolyte two: NaCl (p.a. grade). (a) Gas holdup at increasing salt content. (b) Mean bubble slip speed and the critical point (O). (c) Stability diagram in critical values qc (9) and ec (0). The concentration range for plots in a and b is the following: c ≈ 0 (water DW, 0), c ) 0.001 (9), c ) 0.05 ((), c ) 0.15 (2), c ) 0.45 (×), c ) 2 M (*). The vertical line in c corresponds to ct ) 0.145 M. The data are corrupted by the salt crystallization inside the sparging orifices (see section 3.5).
could find in the literature, in the required form qc(c) and ec(c), are those of Krishna et al.17,18 on ethanol and those of Ribeiro and Mewes20 on the three electrolytes (NaCl, Na2SO4, NaI). Despite the fact that the agreement is not quantitative, the plot indicates that the trends roughly agree. Therefore, in properly normalized coordinates, the variety of data on the surfactant effects could possibly reduce some degrees of freedom and eventually tend to a common master curve. Note the absence of published experimental data obtained for c/ct > 1 (at least to our best knowledge). 3.5. Experimental Problems with Electrolytes. We have no sound hydrodynamic explanation for the different behavior of the pure and kitchen NaCl salts. We observed, however, that the pure NaCl solutions had a strong tendency to produce fine crystals in the orifices drilled on the perforated plate. The crystal growth produced variation in the orifices size in time, during the measurement. The variation was unpredictable, presumably random over the plate. As a result of this randomly distributed spatiotemporal orifice blockage, the uniformity of the flow
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Figure 4. Results for electrolyte three: NaCl (kitchen quality). (a) Gas holdup at increasing salt content. (b) Mean bubble slip speed and the critical point (O). (c) Stability diagram in critical values qc (9) and ec (0). The concentration range for plots in a and b is the following: c ≈ 0 (water DW, 0), c ) 0.001 (9), c ) 0.05 ((), c ) 0.15 (2), c ) 0.45 (×), c ) 2 M (*). The vertical line in c corresponds to ct ) 0.145 M, found for the pure salt. Maximum values are the following: qc ) 0.074 m/s and ec ) 0.65 [-].
regime likely suffered. The final output of this ill-defined process is the anomalous behavior of the pure salt. Similar crystallization inside the orifices was detected when we worked with other salts, at large concentrations. The physical picture likely is as follows. A thin liquid film of the concentrated salt solution is drawn into the orifice by the capillary forces, to cover the wettable area of the metal plate material. The falling liquid film is exposed to the countercurrent stream of the dry air used for sparging the bubble column. The air speed inside the orifice can be large, of the order of magnitude ∼q/(free plate area), which in our case gives values up to 0.1/0.002 ) 50 m/s. The corresponding value of the Reynolds number is 104. The film thickness is expected to be much smaller than the orifice lateral dimension (bore). With our 0.5 mm diameter orifice, the liquid film could be of micrometers. Under these circumstances, the evaporation of the solvent (water) can be quite effective. The film temperature decreases by the evaporation heat. The film concentration increases by the water escape. The crystallization can begin. The speed of the solute precipitation depends on the properties of the given salt.26,27 First, the salt reduces the water vapor
Figure 5. Comparison of our data with results of other authors in reduced coordinates (c/ct and qc/qc_max, ec/ec_max): (our data) lines without marks (bold full) CaCl2 from R1,5 (medium thick dashed) Na2SO4, (thin dotted) NaCl kitchen quality; (data of Ribeiro and Mewes)20 marks without lines (9) Na2SO4, (0) NaCl, (×) NaI; (data of Krishna et al.)17,18 marks without lines (2) ethanol. The data of other authors were read manually from enlarged figures of their papers. Krishna present four data points at c/ct ) 0, 0.043, 0.72, 1.43. Ribeiro claims to present four plus one data points at c/ct ) 0, 0.059, 0.118, 0.354, 1.000, but there are a few minor inconsistencies (e.g., in their Figure 5, some data lie off the declared values of c/ct, only one datum is shown at c/ct ) 1).
tension. The larger the reduction, the slower the evaporation and solution thickening. For instance, CaCl2 reduces the tension roughly twice, as compared with NaCl. Second, the higher the tendency to dissolve (large negative value of heat of solution Qs), the lower the tendency to crystallize (large positive value of heat of crystallization, -Qs). For instance, Na2SO4 · 10H2O used in our measurements has Qs ) -18.7 kcal/mol, while NaCl only has -1.2. Third, hygroscopic materials tend to contain bound moisture (“crystallic water”) and have a tendency to deliquescence. One of them is CaCl2, with a strong affinity to water, resulting in apparent unwillingness to crystallize readily. The hygroscopy can be increased substantially by the presence of a small amounts of contaminants. This can be the reason for the observed difference in behavior of the pure and contaminated NaCl salts. The difference in the salt properties may explain, why in the certain case, the problem with the crystallization can either begin soon (NaCl p.a.) or may not be detected within the whole concentration range measured, up to the fully saturated solution (CaCl2). One more point is how much the salt (pure NaCl) deposition inside the orifices could affect the properties of our solutions in the column. We carried out mass balance calculations to estimate this effect. Considering the upper bound of this effect (all orifices are completely packed with the salt), the solution concentration would decrease by less than 1% (except for the weakest solution that, however, behaved as the kitchen salt).
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This section shows that different salts as well as the same salt of different purity can produce very different behavior of a bubble column. In our particular case, the anomalous behavior of pure NaCl was due to the orifice blockage by the crystal growth. This should be attributed to the gas maldistribution, but not to the hydrodynamics of the bubbly mixture itself. Preliminary experiments with different spargers where the crystallization is suppressed indicate that also NaCl p.a. can behave like the other salts reported here. We believe that our experience may be useful for those working both in research and application. 4. Conclusions Laboratory experiments have been performed aimed at the effect of inorganic salts on gas holdup and flow regimes in a bubble column. Three salt solutions were used Na2SO4 (p.a. grade), NaCl (p.a. grade), and NaCl (kitchen quality). Our observations can be summarized as follows. • The simple method of spline-function data-approximation can be used for evaluation of the flow regime transition point. • The dual effect of the salt on the gas holdup and homogeneous regime stability probably is of general validity. • The degree of salt purity can affect the bubble column behavior considerably, and some salts can cause different auxiliary effects that can corrupt the measurements (e.g., crystallization inside the orifices on the plate). Acknowledgment The financial support by GACR (Grant Nos. 104/07/1110, 104/09/P255), by GAAV (Grant No. IAA200720801), and by MSMT (Project LA319) is gratefully acknowledged. This contribution is dedicated to Prof. J. B. Joshi, from whom we all can learn about bubble columns. Nomenclature c ) electrolyte (salt) concentration [M ) mol/L] e ) gas holdup (voidage) [-] j ) drift flux [m/s] q ) gas flow rate [m/s] Qs ) heat of solution [kcal/mol] u ) mean bubble slip speed, u ) q/e [m/s] Indexes c ) critical value (flow regime transition begins) m ) maximum value t ) transient coalescence concentration of a surfactant
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ReceiVed for reView February 17, 2009 ReVised manuscript receiVed June 12, 2009 Accepted June 29, 2009 IE900263D
