Ind. Eng. Chem. Process Des. Dev. 1904, 23, 151-154
151
Performance of the Gas Bubble Column in Molten Salt Systems Elzo Sada, Shlgeo Katoh, Hldehuml Yoshll, Toshlhlko Yamanlshl, and Aklo Nakanlshl Chemical Engineering Department, Kyoto Universiw, Kyoto 606, Japan
Experimental data on the gas holdup and the mean bubble size in a bubble column with a single nozzle was obtained for gas-molten salt systems of a eutectic mixture of LiCl (58 mol %)-KCI (42 mol %) and molten NaNO,. The liquid-phase mass transfer coefficient k L was evaluated from the specific surface area a and the volumetric coefficient k,a data for oxygen and carbon dioxide absorption into molten NaNO,. The dimensionless correlations of the performance of bubble columns for aqueous solutions can be extended to the gas-molten salt systems.
Introduction The interest in molten salts can be found in their potentiality as reaction media at elevated temperatures. The advantages such as high thermal conductivity, high heat capacity, ease of gaseous product recovery, low vapor pressure, and catalytic activity shown by molten salts open new technologies (Hatt and Kerridge, 1979). With the program of MSBR and other applications of molten salt systems, a number of recent investigations have been made concerning mass transfer of gases into molten salts: for example, gasification and desulfurization of coal, hydrocracking of coal extracts, chlorination of hydrocarbons, and so on. Although there are many projects using molten salts, little is known about mass transfer rates and reaction mechanisms in gas-molten salt systems. Having the disadvantage of being a strong corrosive, it is preferred to use a simple type of reactor for gas-molten salt systems. Therefore, gas bubble columns are considered a feasible reactor in these systems. However, there is very limited information concerning the performance of the gas bubble column in molten salt systems. The main purpose of the present work is to study the effects of the physical properties of molten salts on the gas holdup, the bubble size, and the liquid-phase mass transfer coefficient in the gas bubble column.
Experimental Section Figure 1shows schematically the bubble column used in the present work. The bubble column, made of transparent Pyrex glass, was 7.3 cm in inside diameter and 95 cm in height. It was equipped with four polished glass windows to measure bubble size at heights of 7,17,32, and 52 cm from the bottom plate of the column. The gas spargers used were of a single nozzle type, made of Pyrex glass or Pyrex glass with a carbon nozzle tip, and their inside diameters were 1.5,2.7, and 5.7 nun. The nozzle was located 5.5 cm above the bottom of the column. The molten salts used were a eutectic mixture of LiCl (58 mol %)-KC1 (42 mol %) and NaN03. The salts were of reagent grade and were dried in a vacuum desiccator for a few days. Carbon dioxide and oxygen were super pure grade, 99.96% and 99.995%, respectively. Figure 2 shows a schematic diagram of the experimental apparatus. All gas supply and outlet lines were of Pyrex glass and copper construction. The temperature of the molten salt was measured with a chromel-alumel thermocouple inserted into the salt through a sheath and kept constant within i 3 "C. The bubble column was operated continuously with respect to the gas flow and batchwise with respect to the molten salt. A weighed amount of the salt was transferred to the bubble column, melted at a desired temperature in an electric furnace, and dried again by bubbling dried nitrogen through the nozzle for 2 h. The gas was preheated 0198-430518411123-0151$0 1.5010
through a heater and a copper tube wound with a flexible insulated heater after measurement of the flow rate with rotameters and was fed through the nozzle from the bottom of the column. The gas holdup was obtained by measuring the height of the aerated liquid during operation, zF, and that of clear liquid, zL. Thus, the average fractional gas holdup EG is given as
The height of the clear liquid was kept at about 60 cm. Bubble sizes were measured by taking photographs through the polished glass window at 52 cm from the bottom of the column. Enlarged prints were made and several hundred to few thousand bubbles were scaled in 10 to 15 prints for a single run. The size of individual bubbles was determined by an arithmetic mean of the maximum and minimum dimensions scaled on the prints. The volumetric liquid-phase mass transfer coefficient kLa was obtained from experiments on the absorption of oxygen and carbon dioxide into molten NaN03. Dissolved gas was first removed from the molten salt in the column by sparging nitrogen. To start a run, the gas was sparged into the molten salt through the nozzle for a given time. The gas concentration in the salt was measured with use of a sampler shown in Figure 3 as follows. The sampler, which had been previously flushed with nitrogen, was immersed in the salt, the bubbling tube was then raised, and the salt entered it to the same level inside and out. When the salt filled it, the bubbling tube was placed back on the opening at the base of the sampler so that the salt in it was completely isolated from the rest of the salt in the column. The sampling volume of the salt was kept constant with use of an automatic sampler elevator. The absorbed gas in the sample salt was eluted by bubbling nitrogen through the bubbling tube at a flow rate of 92 cm3/min. The concentration of carbon dioxide in the eluted gas was measured with an infrared carbon dioxide analyzer (Shimadzu, Type URA-2s) and the amount of carbon dioxide in the salt was obtained by graphical integration. In the experiments for the absorption of oxygen, a zirconia oxygen analyzer (Toray, Model LC-7OOL) was used to measure the concentration of oxygen in the eluted gas. Since the vapor pressures of the salts were very low in the temperature range employed, the effect of the vapor pressure was negligible. The values of kLa were obtained from the following equation with the assumption of complete mixing in the liquid phase (Shah et al., 1978)
kLa =
1-cG c*-ci In t c*- Cf
0 1983 American Chemical Society
(2)
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Ind. Eng. Chem. Process Des. Dev., Vol. 23, No. 1, 1984
Table I. Physical Properties of Moleten Salts and Aqueous Solutions
solns or molten salts water methanol 40 wt % glycerol aqueous solution 0.015 M Na,SO, aqueous solution LiC1-KCl ( 5 8 mol %-42 mol %) NaNO, NaNO,
temp, "C 25 25 22
density, viscosity, g ~ m - ~ g cm-'s-' 0.997 0.90 x 0.788 0.45 X 1.099 3.65 X
25
1.006
0.91 x
450
1.647
2.55
X
350
1.888
2.32
X
400
1.853
1.90
X
surface tension, diffusivity of g s-z 0, (CO,), emz s-' 72 2.29 x 10-5 21.5 5.48 X loT5 70.0 6.61 X 2.28 x 10-5
72
lo', lo-'
130.0
(2.4
114.7
3.48 X (2.53 x 10-5)
112.7
lo-')
X
-
NBo 723 1914 820
4.67 x 109 11.69 X l o 9 0.345 X l o 9
729
4.65 x 109
662
1.59 x 109
860
2.52 x 109
859
3.59 x 109
NGa
r
7
Bu bMing tu be
Vertical seclioml view taken on line A - B
Figure 1. Shape and size of bubble column. Salt inlet irtusion
DUW
Autcmat IC sampler w/elevater
T
1
,
Figure 3. Salt sampler. co2 i
I
Recorder, Controller
20.1
Figure 2. Experimental apparatus.
where t is the gas sparging time, Ci and Cf are the initial and final concentrations of carbon dioxide or oxygen in the molten salt, and C* is the solubility at 1 atm. Pure water, methanol, 40 w t % glycerol solution, and 0.015 M Na2S04 solution were used as the solvents in comparison with the molten salt systems. In the absorption of oxygen into these systems, the liquid was first bubbled with nitrogen to remove oxygen. Then, oxygen was fed through the nozzle and the concentration of oxygen dissolved in the liquid was continuously measured by the oxygen analyzer (Beckman,Model-123301). The kLavalue was determined from eq 2. Errors in the kLa values due to the response lag of the oxygen electrode were negligible. Table I shows the properties of the liquid used in the experiments. Solubilities and diffusivities of carbon dioxide and oxygen in molten NaNO, were measured by
0.01
F -
NaN0,-He 400
0.
A LiCI-KCI-N,450
0
I
I
I l l
'C'C
A LiCI.KCI-He 4 5 0 'C
I
I 1
Ind. Eng. Chem. Process Des. Dev., Vol. 23,No. 1, 1984
153
nozzle diameter. 2.7"
?
C
water-Nz
25°C
e water-C02 25°C 0
methanol-N2 25'C m A
NaN0,-He 400 O C LiCI-KCI-N, 450°C
a 0.0015' 0 01
4
I
I
l
l
I
1
01
I
-
I
1.0
EG
Figure 7. Correlation for specific interfacial area.
Figure 5. Correlation for gas holdup.
100 100
c
NaN03-He 400°C 400°C with carbon nozzle LiCI.KCI-NZ 450°C
0 NaNq-Nz A
0.01 UG
0 .I
m
Figure 6. Correlation for average bubble size.
has a lower gas holdup. This decrease of the gas,holdup with gas density has been report& by Koetaier et al. (1976) and others. Although the effect of gas density on the gas holdup is relatively small, this effect should be considered in gas-molten salt systems because of the low values of gas-liquid density ratio at elevated temperatures. Thus, the gas holdup is expressed by the following equation modified from the equation of Akita and Yoshida (1973) with the addition of the term of gas-liquid density ratio.
where NBo = gD2pL/r, NGa = gD3/vL2,and NFr = UG/ In Figure 5, the measured values of EG were plotted on log-log coordinates. These values agree with the correlation curve shown by the solid line within *30%. Bubble Size. The volume-surface mean diameter of the swarm bubbles in the column was calculated by the equation (4)
UG , mhf'
Figure 8. Plots of k ~ vs. a U, for various systems.
(Akita and Yoshida, 1974) may be extended to molten salt systems. The wettability of the carbon nozzle might affect the size of bubbles in the molten salts, since the size of bubbles just after leaving nozzles changes depending on the wettability (Irons and Guthrie, 1978). As shown in Figure 6, however, the bubble size did not depend on the nozzle wettability above the superficial gas velocity of 10 m/h. This is reasonable because the bubble size is controlled by a balance between bubble coalescence and breakup rates. The specific interfacial area a can be calculated from the measured values of the gas holdup and the volumesurface mean diameter. a = - 6% (6) dV, Figure 7 shows the close agreement of Akita and Yoshida's correlation with the experimental data on the molten salts (Akita and Yoshida, 1974). aD = y3NB00.5N 0.1~~1.13 (7) Ga
The distribution of bubble size was described by the logarithmic normal distribution law as in the case of aqueous solutions. The bubble size was not affected by the nozzle diameter. Figure 6 shows the correlation for average bubble size. The straight line in this figure is represented by the correlation with eq 5. -0.12N -0.12 d VB / D = 26NB0-0.50N (5) Ga Fr
Mass Transfer Coefficient. Figure 8 shows the kLa values for the absorption of oxygen and carbon dioxide. The kLa values for carbon dioxide absorption into molten NaN03 are almost equal to those for oxygen absorption into water. According to Akita and Yoshida's correlation i 3 1 ) t~ (1973), plots of Ns,(aD)/ ( N s > 5 N ~ 2 6 2 N ~ fagainst are shown in Figure 9. The straight line in Figure 9 shows
Viscosities, surface tensions, and liquid densities of molten salts are higher than those of water, but the values of the dimensionless groups in eq 5 in molten salt systems are close to those in water, as shown in Table I. Therefore, the correlation proposed for aqueous and organic solutions
Nsh(aD)= 0.51Nsc0.5N~00.62N~a0.31€~1,1 (8) where NSh= kLD/DL and Ns, = vL/DL. Although the coefficient in eq 8 is slightly smaller than the value of 0.60 in Akita and Yoshida's correlation, the kLa values of the molten salt systems and aqueous and
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Ind. Eng. Chem. Process Des. Dev., Vol. 23, No. 1, 1984 0.06r
I
I
I l l ,
I
-1 o water-02 2 5 C ' o methanol-02 25°C
mass transfer coefficient for both molten salts and aqueous and organic solutions can be correlated by the same equations, which were modified from the dimensionless equations proposed for aqueous systems. The correlation for the gas holdup should contain the factor of the gasliquid density ratio, especially a t elevated temperatures as in the case of molten salts.
Nomenclature a = specific gas-liquid interfacial area, L-' Ci, Cf = initial and final concentrations of dissolved gas in liquid, respectively, ML-3 C* = dissolved gas concentration at saturation, ML-3 dbl = bubble diameter, L d,, = volume-surface diameter, L D = column diameter, L DL = diffusivity of gas in liquid, L 2 T 1 g = gravitational constant, L T 2 k L = liquid phase mass transfer coefficient, L T ' kLa = volumetric liquid phase mass transfer coefficient, T1 N,= number of bubbles NBo = Bond number = gD2pL/y,dimensionless N F =~ Froude number = UG/(gD)1/2, dimensionless NG, = Galileo number = gD / v L 2 , dimensionless Nsc = Schmidt number = v L / D L , dimensionless Nsh = Sherwood number = kLD/DL,dimensionless t = time, T UG = superficial gas velocity with respect to the total cross section of column, L T ' ZF = height of aerated liquid, L zL = height of clear liquid, L
-1 o4Owt*/.giycerol+ 2 2 " ~ 0 NaNQ-C02
NaNQ- 02
0 001 0.01
0.1
350'C
350 "C
06
EG
Figure 9. Correlation for kLa.
d
0 NaNQ-COz 350 "C NaNQ-02 350 "C
0 01
01
10
EG
Figure 10. Correlation for kL.
Greek Letters y = surface tension, M T 2
= gas holdup, dimensionless = liquid viscosity, ML-lT' v~ = kinematic viscosity of liquid, L 2 T 1 p G = gas density, ML-3 pL = liquid density, ML-3 CG
organic solutions can be correlated with the same equation. The slight difference in the coefficient might be due to the small column diameter used in the present work. With use of eq 6, the kLa values can be split in factors. The group N~dv,c4~5N~~12iV~a4.21 containing the kL thus obtained are plotted against EG in Figure 10. The straight line shows the correlation derived from eq 7 and 8. Since all the kL data on aqueous, organic, and molten salt systems can be correlated by the same equation, the contamination of aqueous solutions has little effect on the k , with high turbulence a t free gas-liquid interface as in the gas bubble column in contrast with the kL in a stirred vessel with a known gas-liquid interfacial area (Sada et al., 1982). It is concluded that the higher surface tension effects of the molten salts on the kL are not likely to dominate the mass transfer process. Conclusion In the bubble column with a single nozzle, data on the gas holdup, the mean bubble size, and the liquid phase
fiL
Literature Cited Akita, K.; Yoshida, F. Ind. Eng. Chem. Process Des. Dev. 1973, 12, 76-80. Akita, K.; Yoshida, F. Ind. Eng. Chem. Process Des. Dev. 1974, 13, 84-90. Calderbank, P. H. Trans. Inst. Chem. Eng. 1959, 37, 173-185. Han, B. W.; Kerridge, D. H. Chem. Br. 1979, 15, 78-81. Irons, G. A.; Guthrie, R. I.I . Met. Trans. 8 1978, 9, 101-110. Janz, G. J.; Krebs, U.: Siegenthaler, H. F.; Tomkins, R. P. T. J. Chem. Eng. Ref. Data 1972, I , 581-744. Janz, G. J.; Tomkins, R. P. T.; Allen, C. B.;Downey, J. R., Jr.; Gardner, G. L.; Krebs, U.; Singer, S. K. J. Chem. Eng. Ref. Data 1975, 4 , 871-1161. Koetsier, W. T.; Van Swaaij. W. P. M.; Van Der Most, M. J. Chem. Eng. Jpn. 1976, 9 ,332-333. Sada, E.; Katoh. S.; Yoshii, H.; Yasuda, K. Ind. Eng. Chem. Fundam. 1982, 21, 43-46. Shah, Y. T.; Stiegel, G. J.; Sharma, M. M. AIChE J. 1978, 24, 369-400
Received for review October 28, 1982 Accepted May 24, 1983
