Pergamon
Chemical Engineering Science. Vol. 51, No. I I, pp, 2739-2744, 1996 Copyright © 1996 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0009-2509196 $15,00 + 0.00
S0009-2509(96)00145-5
HYDRODYNAMICS OF SEMIBATCH SLURRY BUBBLE COLUMNS WITH POLYMER SOLUTIONS M.G. G6mez, Z. Alarctn, E. Parra, S. Siquier, F. Pironti and A.E. S ~ z Departarnento de Termodimhnicay Fentmenos de Transferencia Universidad Simtn Bolivar, Caracas, Venezuela Abstract - In this work we perform an experimental study of the hydrodynamicsof semibatch slurry bobble columns in which the liquid phase is a solution of a high molecular weight hydrolyzed polyacrylamide. Experiments were conducted in bubble columns with two different diameters (10 and 29 cm.) In gas-liquid operation, the gas holdup decreases as the polymer concentrationincreases due to the formation of large stable bubbles. In the presence of high solids concentration, this trend is reversed in the larger column due to an increase in the number of small bubbles and an appreciable increase in the residence times of the large bubbles brought about by an increase in slun'y circulation. The distribution of solid phase becomes more uniform as the polymer concentration increases. This trend is quantified by means of the application of a modified form of the sedimentation-dispersion model. The macromolecularconformationof the polymer in solution is altered by adding NaCI, whose positive ions screen negative charges in the polymer moleculeproducing a contraction of the coil. At the concen~ation levels used in this work, the addition of salt decreases the ability of the polymer to produce a more uniform solids distribotion.
INTRODUCTION Slurry bubble columns in which the liquid phase has non-Newtonian behavior find application in the field of biotechnology as fermenters where the liquid might contain high molecular weight species in solution. In previous works, solutions of carboxymethyl cellulose (CMC) have been usually employed as a typical case of a pseudoplasfic liquid for which the shear viscosity decreases as the shear rate increases. Examples of the analysis of bubble column hydrodynamics and gas-liquid mass transfer with CMC solutions are the works of Schumpe and Deckwer (1982), Godbole et al. (1982,1984), Haque et al. (1987), Vatai and Tekic (1987), Bukur and Patel (1989), and Philip et al. (1990). Other high molecular weight polymers that have been used in aqueous solutions in bubble columns are xanthan gum (Zaidi et al., 1990), and polyacrylamide (Kawase and Moo-Young, 1986). Polymer solutions exhibit relatively large values of the shear viscosity, when compared to the solvent. The viscosity increase leads typically to a reduction in gas holdup for both Newtonian and non-Newtonian liquids. This reduction in gas holdup is a consequence of the increase observed in the mean bubble size as the viscosity is increased. Larger bubbles are present at high viscosities as a consequence of two effects: fast, the possibility of achieving a larger stable bubble size, a fact that has been obsexved in polymer solutions (Terakasa and Tsuge, 1991), and, second, the reduction of the level of turbulence in the liquid, which decreases bubble breakup rates (Philip et al., 1990). Exceptions to this behavior have been observed. For instance, Godbole et al. (1982) obtained a maximum in gas holdup as the CIVIC solution viscosity increased (i.e., as the polymer concen~tion increased). The initial increase in gas holdup at low polymer concentrations might be a consequence of an inhibition of bubble coalescence due to interfacial activity of the dissolved polymer. The presence of small bubbles in CMC solutions has been well established. For instance, Muller and Davidson (1992) observed bimodal bubble size distributions in which the small bubbles contributed up to 50% to the gas-liquid mass transfer rates. The available empirical correlations for predicting gas holdups in bubble columns with non-Newtonian liquids rely on the use of power-law parameters to characterize the theology of the liquid phase (see, for example, Kawase et al., 1992). The power-law parameters are obtained from shear viscosity measurements as a function of shear rate performed in a Couette-type viscometer. Shear rates in bubble columns are sometimes evaluated as the ratio of liquid centexline velocity to column radius. Even though these representations might be of practical relevance, one has to keep in mind that the actual behavior of a polymer solution in the bubble column might differ from that observed in a rotational viscometer: the motion of the liquid phase in bubble columns is locally turbulent. This implies that the flow has important elongational components at the local level, caused by the rapid spatial and temporal change of the velocity vector in the turbulent eddies. It is well known that the molecular conformation of a macromolecuie in solution can be substantially altered in a sufficiently strong elongational flow field (see, for example, Keller and Odell, 1985). For instance, a molecule adopting a random coil configuration in static equifibrium might undergo a coil-stretch transition and thus increase dramatically its hydrodynamic interaction with the solvent and, therefore, the apparent solution viscosity. Macromolecules can also form transient molecular networks when subjected to elongation in semi-dilute solutions (Keller et al., 1987). These effects are thought to play a role in the phenomenon of drag reduction, which consists of a decrease of pressure drop in turbulent pipe flow as polymer is added to a Newtonian fluid. The conformational characteristics of the polymer molecule should play an important role in the hydrodynamics 2739
2740
M.G. GOMEZet
al.
of the system. Polymers such as CMC, xanthan gum and hydrolyzed polyacrylamide in aqueous solutions have an expanded coil conformation and the macromolecules are semi rigid. On the other hand, aqueous poly(ethylene oxide) and hydrolyzed polyacrylamide in a salt solution adopt a coiled conformation. Other aspects related to structural characteristics might play an important role. One of these factors which has been mostly overlooked in the bubble column literature is the propensity of the polymer to degrade in the flow field. Polymer degradation leads to a reduction in polymer molecular weight, and thus to a reduction in the ability of the polymer to increase the apparent viscosity of the solution with respect to that of the solvent. Degradation can be induced by a variety of causes, such as free radicals, microbial activity, and rupture of the macromolecule due to mechanical stresses. We will be concerned in this work with the last cause, i.e., mechanical degradation. This is the degradation mechanism that cannot be externally controlled, since it is a result of the nature of the local flow field. Recently, Muller and Davidson (1994) performed a detailed study of degradation of CMC in bubble columns. Their results suggest that CIVICdoes not undergo significant mechanical degradation under the usual conditions employed in bubble columns, except perhaps at very high CMC concentrations and after long experiment times. In this work we perform an experimental study of the hydrodynamics of slurry bubble columns in which the liquid phase is a solution of a high molecular weight polyacrylamide. We analyze how the conformational characteristics of the macromolecules in solution affect hydrodynamic parameters such as gas holdup and solids distribution. EXPERIMENTAL The experimental set up and measurement procedures have been described in detail elsewhere (Pino et al., 1990a, 1990b, 1992). The set up consists of cylindrical bubble columns with conical inlet distributors through which atmospheric air flows upwards through a liquid or solid-liquid suspension (semibatch operation). The columns used have internal diameters of 10 and 29 cm. The 10 cm-diameter column has a conical distributor with an entrance pipe diameter of 1.5 cm and a cone apex angle of 30°. In the 29 cm-diameter column, the entrance pipe has a diameter of 2.7 cm, and the cone apex angle is 30°. The global gas holdup (eg) was determined by the bed expansion method as the ratio between the air volume in the column during operation,ineasured after complete disengagement of the gas phase at the end of the experiment, and the total volume of gas-slurry suspension. The columns were provided with sampling probes distributed axially which consist of tubes with an internal diameter of 0.64 cm through which slurry samples were withdrawn. The probes could be displaced radially. The solid concentration in the slurry was measured by determining the weight of solids after complete drying of the samples. It was determined that the solids concentration did not exhibit radial variations in the cylindrical section of the bubble columns. The effect of local fluctuation in the solids concentration due to turbulence was minimized by taking three consecutive samples at each point once the steady state was achieved. The solids concentrations reported in this work correspond to the mean value of the three samples. Except when indicated, the gas velocity was always set by increasing the gas flow rate from zero to the desired value. All the experiments were conducted at ambient temperature (23-25°C). The solid phase used was siliceous sand with the following particle size distribution: <63 pan: 8.4 wt%, 63150 pan: 2.8 wt%, 150-177 jam: 10.4 wt%, 177-212 larn: 6.8 wt%, 212-250 larn: 22.2 wt%, 250-354 p.m: 34.0 wt%, >350 I.tm: 15.4 wt%. The liquids used were aqueous solution of hydrolyzed polyacrylamide (HPAA) (ALCOFLOOD 1175A, provided by Allied Colloids Inc.) with a weight-average molecular weight of 18.2x10°. The polymer solutions were prepared by dispersing in a beaker the polymer in powder form on the surface of deionized water whilst stirring with a magnetic stirrer to produce a deep vortex. After dispersing the powder, gentle stirring followed for periods between 12 and 24 hours to assure complete dissolution of the polymer. One of the aims of this study was to assess the effect of the macromolecular conformation in solution on the hydrodynamics of the system. In deionized water solutions, the hydrolyzed polyacrylamide has a semi-rigid structure characterized by a highly-expanded coil conformation. This is due to the ionic repulsion among the hydrolyzed groups in the molecule. When an electrolyte is added to the solution, the ions with positive charge exert a screening effect on the hydrolyzed groups of the polymer molecule, and the molecule adopts a contracted coil conformation. Macroscopically, this contraction leads to a decrease in the shear viscosity of the solution since friction between the macromolecules and the solvent is diminished. In some of the experiments, the polyacrylamide was dissolved in a concentrated sodium chloride solution (0.5 M). Independent tests performed in an elongational flow apparatus (an opposed-jets device) showed that this salt concentration produced a maximum effect with regards to coil contraction. RESULTS AND DISCUSSION
Figure 1 shows the gas holdups obtained for polyacrylamide solutions in the 29 cm-diameter column as a function of gas superficial velocity, V~ (C s represents the average solids concentration in the slurry). Straight lines have been drawn between data points for a better visualization of the trends. In the absence of solids (Fig. la) and solutions without salt, an increase in polymer concentration leads to a decrease in the gas holdup. This effect is due to the increase in the viscosity of the solution and the consequent formation of larger stable bubbles. In the presence of solid phase, Fig. lb, or when 0.5 M NaCI is used as solvent, Fig. la, the trend changes: adding polymer to a concenWation of 50 ppm increases the gas holdup, and further increases of concentration lead to a decrease of this parameter. Notice that the addition of solid phase to water (compare curves in Figs. la and b) leads to a decrease of the gas holdup whereas addition of solids to the 100 ppm polymer solution results in an increase in that parmneter. Visual observation of the experiments confirmed that large stable bubbles were present with and without solids when the liquid was a polymer solution. However, in the gas-liquid flow case, these large bubbles moved rapidly upwards whereas in the presence of solids a strong recirculating motion of the slurry phase increased the residence times of the large bubbles in the column. Furthermore, a substantial increase in the number of small bubbles were
Sere|batch slurry bubble columns with polymer solutions
2741
present in gas-liquid-solid operation and in gas-liquid operation with salt solutions. These effects led to gas holdups larger than expected. This change in the flow pattern of the liqulut in the presence of solid phase does not occur for water. In order to explore the nature of the gas holdup increase in solid-liquid suspensions when polymer is added, we performed gas-liquid mass transfer experiments. In these experiments, we inmaduced a step change in the oxygen concentration of the air at the inlet of the bubble column and measured, by means of polarogruphic oxygen electrodes located at various point in the system, the evolution of the oxygen concentration in the liquid phase in the bubble column. A study of oxygen concentration l~of'flcs revealed that the sys~m behaved as pca-fcctly mixed. The volumetric mass transfer coefficients, kla, were determined from the oxygen concentration vs. time curves. The results are presented in table 1. In gas-liquid flow, the addition of polymer reduces slightly the mass transfer coefficient whereas at high solids concentration the reduction is appreciably larger. Furthermore, the presence of solid phase causes a reduction in kla. It is interesting to note that the 50 ppm solution with solids has a value of kla that is lower than the same solution without solids and also lower than water with solids, even though its gas holdup is larger than those of the two mentioned cases. This result implies that the gas phase is distributed differently. This is consistent with the observation reported above of large gas bubbles with a long residence time. This leads to a higher gas holdup but a lower overall gas-liquid interracial area.
eg
0.35
|
0.30
eg
•
|
•
0.12
O.25
0.10
0.20
0.08
0.15
0.06
0.10
0.04
O-13-x
0.02
0.05 ..j
0.00 4
6
8
10
i
0.00
12 14 Vg (cm/s)
4
wate~ 50 ppm 100 ppm
l
6
8
10
I
12 14 Vg (cm/s)
(a) Gas-liquid Co)Gas-liquid-solid, Cs=600 kg/m3 Fig. 1. Gas holdups for HPAA solutions in distilled water in the column with diameter 13=29cm. Open symbols represent solutions without sail filled symbols represents solutions in 0.5 M NaC1. Table 1. Volumetric mass ~ansfer coefficients in the column with diameter D=29 cm, Vg=9 cm/s.
Gas-liquid water 50 ppm HPAA
eg
kla (s "1) ~s=600 kg/m3
0.045 0.043
0.6
eg
0.027 0.015 0.6
0.5
0.5
0.4
0.4
0.3
0.3
0.2
0.2
0.1
O.1
0.0
0.0 0
2
4
6
8
10 12
14 16 18 Vg (cm/s)
0
2
4
6
8
10 12
14 16 18 Vg (cm/s)
(a) Aqueous solutions Co)Solutions in 0.5 M NaCI Fig. 2. Gas holdups for hydrolyzed polyacrylamide solutions for gas-liquid flow in the column with diameter D=10 crn. Figure 2a shows results for polyacrylamide solutions in the 10 era-diameter column. Notice that the gas holdup levels are larger in this smaller column (compare to Fig. la). The effect of the oolvmer concentration is similar to
2742
M.G. GOMEZet al.
that observed in the 29 cm-diameter column, although the reduction of gas holdup is more noticeable at higher velocities. The same solutions were prepared using an aqueous 0.5 M NaCI solution as solvent. The results are presented in Fig. 2b. The presence of the electrolyte slightly increases the gas holdup of water at low and intermediate velocities (compare to Fig. 2a). This is consistent with trends observed in previous works where experimental results have indicated that the electrolyte tends to inhibit bubble coalescence (Kelkar et al., 1983). In the salt solutions the presence of the polymer does not lead to appreciable changes in the gas holdup. The fact that in the presence of salt the H P A A molecules have a contracted coil conformation indicates that since, at a given concenlration level, the size of the macromolecule is smaller in the presence of the electrolyte than in water, then the viscosification produced by the polymer is lower. However, as we have pointed out, the flow field in the bubble column is turbulent and it therefore has important elongational components. The results presented in Fig. 2b seem to indicate that possible uncoiling of the macromolecules and local flow modification are not playing a role with regards to global gas holdup. The gas holdups obtained in the 10 era-diameter column in the presence of solid phase did not change significantly with respect to those presented in Fig. 2, up to an average solids concentration of Cs--400 kg/m 3. In this case, the presence of solids seems not to alter significantly the flow field of the liquid phase as it happened in the larger column. This is a consequence of the appreciable wall effects present in the small column. One important aspect of subjecting polymer solutions to turbulent flows is mechanical degradation. We have investigated the extent of H P A A degradation in the experiments. For this purpose we took samples of the polymer solution from the bubble column after steady state was achieved and measured the viscosity of the solution. This was done in the experiments carried out in the 29 cm-diameter column. The kinematic viscosity (v) was measured by using a Ubbelohde viscometer that produced shear rates in the ranges 45-50 s -1 (50 ppm solution) and 50-55 s"-I (100 ppm solution). The experiments were conducted as follows. Starting from the stagnant solution in the bubble column, the superficial gas velocity was increased in steps. After each step, when steady state was achieved, a sample was taken and its kinematic viscosity was measured. This was done up to a gas superficial velocity of 12 cm/s. After reaching this gas velocity, this parameter was decreased in steps and the kinematic viscosity of samples was determined. The results are shown in Fig. 3. As the gas velocity is increased, the viscosity of the solution decreases. An explanation consistent with this result is the occurrence of polymer degradation: at a given gas velocity, local strain rates in the liquid phase are in excess of those required for breaking high molecular weight fractions of the dissolved polymer. As the gas velocity increases, the increase of the local strain rates induces seission of chains with lower molecular weight. When the gas velocity is reduced from its maximum value, the viscosity remains approximately constant since no further mechanical degradation can occur. It is interesting to point out that gas holdups were larger at the same gas velocity in the decreasing velocity part of the experiments but without reaching the water values. This hysteresis in gas holdups is then a consequence of the varying degrees of polymer degradation. V
1.5
(cm2/s) 1.4 1.3 ( 1.2 1.11 1.0 0.9 0
5
10
15 Vg (cm/s)
Fig. 3. Kinematic viscosity at 250C of HPAA solutions in the bubble column, I:)=29cm. The arrows indicate direction of change of superficial gas velocity in gas-liquid flow operation. We have also analyzed solids concentration profiles with and without polymer in the bubble columns. The solid employed has a wide enough particle size distribution so that it cannot be considered monodisperse. The solids concentration profiles were well fitted by a modified form of the sedimentation-dispersion model in which we consider the solid to be bidisperse and applied the model to each of the two particle size ranges. The ranges employed were: range 1:0-250 gm, with an average particle size dpl=169 lJm, and range 2:>250 l~m, with an average particle size dp2=316.5 pro. Each range contributes approximately 50% by weight to the original distribution. The modified form of the sedimentation-dispersion model included the effect of high solids concentration. The model is based upon the application of the original sedimentation-dispersion model (see, for example, Smith et al., 1986 and Jean et al.,1989) to each particle size range, considexing the hindering effect on the solid sedimentation velocity exerted by a high solids content in the slurry. The differential equation that governs the axial distribution of solids is
2744
M.G. GOMEZet al.
The ability of the polymer solution to suspend solids is best illustrated through the parameters cti. A lower value of this parameter implies a more uniform solids concentration profile since this would be a consequence of either a lower sedimentation velocity or a higher dispersion coefficient. Figure 5 shows the dependence of ot1 on polymer concentration for a fixed gas superficial velocity. In the range of gas superficial velocities between 4 and 12 cm4s, the value of the sedimentation-dispersion parameter did not change appreciably. Figure 5 shows that the effect of the polymer is more appreciable for the 10 cm-diameter column in the absence of NaCI. The decrease of ct1 with polymer concentration is due to the increase in viscosity which leads to lower sedimentation velocities. Notice that, in the presence of NaCi, solutions of up to 100 ppm of HPAA do not exert a large influence on the solids concentration profiles. Figure 5 also shows that the sedimentation-dispersion parameter decreases as the column diameter increases. This result is a consequence of the higher levels of dispersion in the 29 cm-diameter column. CONCLUSIONS The hydrodynamics of bubble columns using hydrolyzed polyacrylamide as liquid phase exhibits aspects that differ from those observed in bubble columns with Newtonian liquids. First of all, polymer degradation depends on gas superficial velocity and on the manner that operating conditions are reached. This fact might lead to hysteresis in the determination of hydrodynamic parameters. The effect of the presence of the polymer on gas holdup depends on the diameter of the column and the solid concentration. The results have shown that, even though the addition of polymer leads to larger stable gas bubbles, flow modification might result in increased values of the gas holdup. Finally, the quality of the solvent plays a crucial role in the magnitude of the effect that the polymer has on the hydrodynamics. We have shown that increasing the ionic strength of the solution through the addition of sodium chloride leads to a reduction in the effect of the polymer on gas holdup and solids distribution.
REFERENCES Bukur, D.B. and Patel, S.A., 1989, Hydrodynamic studies with foaming and non-Newtonian solutions in bubble columns, Can. J. Chem. Engng. 67, 741-751. Godbole, S.P., Honath, M.F. and Shah, Y.T., 1982, Holdup structure in highly viscous Newtonian and nonNewtonian liquids in bubble columns, Chem. Engng. Comm. 16, 119-134. Godbole, S.P., Schumpe, A., Shah, Y.T. and Carr, N.L., 1984, Hydrodynamics and mass transfer in non-Newtonian solutions in a bubble column, A/ChE J. 30, 213-220. Haque, M.W., Nigam, K.D.P., Viswanathan, K. and Joshi, J.B., 1987, Studies on gas holdup and bubble parameters in bubble columns with (carboxymethyl)cellulose solutions, Ind. Engng. Chem. Res. 26, 86-91. Jean, R.-H., Tang, W.-T. and Fan, L.-S., 1989, The sedimentation-dispersion model for slurry bubble columns, AIChE J. 35, 662-665. Kawase, Y. and Moo-Young, M., 1986, Liquid phase mixing in bubble columns with Newtonian and nonNewtonian fluids, Chem. Engng. Sci. 41, 1969-1977. Kawase, Y, Umeno, S. and Kumagai, T., 1992, The prediction of gas hold-up in bubble column reactors: Newtonian and non-Newtonian fluids, Chem. Engng. J. 50, 1-7. Kelkur, B.G., Phulgaonkar, S.R. and Shah, Y.T., 1983, The effect of electrolyte solutions on hydrodynamics and backmixing characteristics in bubble columns, Chem. Engng. J. 27, 125-133. Keller, A. and Odell, J.A., 1985, The extensibility of macromolecules in solution; a new focus for macromolecular science, Colloid Polym. Sci. 263, 181-201. Keller, A., Miller, AJ. and Odell, J.A., 1987, Entanglements in semi-dilute solutions as revealed by elongational flow studies, Progress Colloid Polym. Sci. 75, 179-200. Muller, F.L. and Davidson, J.F., 1992, On the contribution of small bubbles to mass transfer in bubble columns containing highly viscous liquids, Chem. Engng. Sci. 47, 3525-3532. Muller, F.L. and Davidson, J.F., 1994, Rheology of shear thinning polymer solutions, Ind. Engng. Chem. Res. 33, 2364-2367. Philip, J., Proctor, J.M., Niranjan, K. and Davidson, J.F., 1990, Gas holdup and liquid circulation in internal loop reactors containing highly viscous Newtonian and non-Newtonian liquids, Chem. Engng. Sei. 45, 651-664. Pino, L.Z., Y6pez, M.M., S~iez, A.E. and De Drago, G., 1990a, An experimental study of gas holdup in two-phase bubble columns with foaming liquids, Chem. Engng. Commun. 89, 155-175. Pino, L.Z., Y6pez, M.M. and Sttez, A.E., 1990b, Hydrodynamics of a semibatch slurry bubble column with a foaming liquid, AIChE J. 36, 1758-1762. Pino, L.Z., Solari, R.B., Siquier, 8., Est6vez, L.A., Y6pez, M.M. and S~tez, A.E., 1992, Effect of operating conditions on gas holdup in slurry bubble columns with a foaming liquid, Chem. Engng. Commun. 117, 367-382. Schumpe, A. and Deckwer, W.D., 1982, Gas holdups, specific inteffacial areas, and mass transfer coefficients of aerated carboxymethyl cellulose solutions in a bubble column, ind. Engng. Chem. Process Des. Dev. 21,706-711. Smith, D.N., Ruether, J.A., Shah, Y.T. and Badgujar M.N., 1986, Modified sedimentation-dispersion model for solids in a three-phase slurry column, A1ChE J. 32, 426-436. Terakasa, K. and Tsuge, H., 1991, Bubble formation at a single orifice in non-Newtonian liquids, Chem. Engng. Sci. 46, 85-93. Vatal, G.Y. and Tekic, M.N., 1987, Gas hold-up in bubble columns with non-Newtonian liquids, Chem. Engng. Sei. 42, 166-169. Zaidi, A., Deckwer, W.-D., Mrani, A. and Benchekchou, B., 1990, Hydrodynamics and heat transfer in three-phase fluidized beds with highly viscous pseudoplastic solutions, Chem. Engng. Sci. 45, 2235-2238.