Power consumption for non-aerated Na-CMC solutions in multiple bioreactors agitated

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1 Power consuption for non-aerated Na-CMC solutions in ultiple bioreactors agitated Proceedings of European Congress of Cheical Engineering (ECCE-6) Copenhagen, Septeber 2007 Power consuption for non-aerated Na-CMC solutions in ultiple bioreactors agitated L. Broniarz-Press, S. Woziwodzki, M. Ochowiak Departent of Cheical Engineering and Equipent, Poznan University of Technology, pl. M. Sklodowskiej-Curie 2, PL Poznan, Poland Abstract Multiple ipeller vessels are generally used in any industries. Aong the criteria usable for characterization of ixing processes the ixing power requireent and relation between hydrodynaics of agitators and power requireents are of first iportance. The experiental obtained data showed that in double ipeller systes the power consuption depends on a distance between ipellers. The optial distance between ipellers with the sallest power consuption has been proposed. For three and ore ipellers it has been shown that ixing power depends on nuber of ipellers. Keywords: ixing, ultiple ipellers, power consuption, Na-CMC 1. Introduction Mixing process is one of the coonly used processes in industries as well in huan life. Especially ultiple agitators are used in any industries, because of the advantages of the longer residence tie, lower decrease of heat exchange area in scale-up treatent, lower power consuption per ipeller as copared to single ipeller systes. These systes are in the point of interest of biocheical industry, because aong the others in such systes the ass transfer coefficient is higher in coparison with single ipeller vessels. In the ultiple-ipeller syste a nuber of independent circulation loops occurs with ass exchange between the. Each loop can be considered as the single tank, and the exchange flow rate between these tanks has a ain influence on ixing tie. Mixing in bioreactors is very coplicated because in such systes the rheological paraeters depend on shear stress. By reason of rheological behavior of systes the accuracy of the proposed odels is liited. The power law fluids, especially carboxyethylcellulose sodiu salt solutions (Na-CMC), are widely used in the

2 L. Broniarz-Press et al. broths siulations in biotechnology. The rheological properties of these liquids can be deterined using the power law equation, which is given as: while τ = Kγ η p = Kγ 1 (1) (2) γ = kn (3) where K is the consistency factor, the flow index, γ the shear rate, τ the shear stress, η p the apparent viscosity and k is the Metzner and Otto (1957) constant. Mixing of such fluids (Metzner and Otto, 1957; Wasser and Hungenberg, 2005) is very coplicated. It has been already reported that elasticity increases the torque required for ixing (Collias and Prud hoe, 1985). It was found that fluids with pseudoplastic behaviour create sall circulation paths around blades and the circulation flow rate decreases (Takahashi et al., 1989). The power consued by the agitator is usually deterined by torque easureents either with the help of a strain gauge (Linek et al., 1996; Davies et al., 1985) by a torque eter ounted on the ipeller shaft (Singh et al., 1986; Nocentini et al., 1988) or by balancing the rotational oveent of the vessel. Arjunwadkar et al. (1998) used an ac/dc transducer working on the principle of Hall s effect. Pinelli, Nocentini and Magelli (1994) used a siple technique of easuring the restraining torque necessary to prevent the vessel fro rotating with respect to a stationary vessel by eans of a load cell. In any situations the correct knowledge of the power consuption is needed for better design and operation of the agitated vessel with ultiple ipellers. The vessel with ultiple ipellers has any unknown factors, i.e. the best set-up positions of ipellers, the best cobinations of ipellers, etc. The ai of this work was to investigate the power characteristics of turbine ipeller systes in carboxyethylcellulose sodiu salt (Na-CMC) solutions for ultiple ipeller (two to five) vessels. Moreover, for double systes the relation of distance between ipellers and power consuption was taken into account. 2. Experiental part The experiental set-up (Fig. 1) consisted of otor, inverter, speed sensor, PC coputers, interface, torqueeter. The vessel with diaeter T = 0.19 was equipped with flat botto. The four baffles of 0.1T width were used and the clearance fro the vessel wall was 0.02T. The height of liquid level was taken 2.3T. The three types of ipellers were used: Rushton turbine (RT), six flat blade turbine (FBT) and six pitched down blade turbine (PBT). Ipellers were ounted on coon shaft (D S = ) in two, three, four and five cobinations of one type ipeller. The ratio of ipeller diaeter (D = ) to vessel diaeter was D/T = The diensions of ipellers (Fig. 2) used are given in Table 1. 2

3 Power consuption for non-aerated Na-CMC solutions in ultiple bioreactors agitated Fig. 1. Measureent set-up: 1 - otor, 2 inverter, 3 turning sensor, 4 - IBM PC coputer, 5 transitter, 6 torqueeter a) b) c) Fig. 2. Ipellers used: a) RT, b) FBT, c) PBT Table 1. Geoetric paraeters of ipellers Type Diaeter D Height of blade Length of blades Angle or pitch of blade RT FBT PBT As non-newtonian fluids the aqueous solutions of carboxyethylcellulose sodiu salt (Na-CMC, delivered by Siga-Aldrich Copany) were used. The characteristics of non-newtonian fluids agitated have been shown in Table 2. An electric power P required to drive an electrootor coupled with the shaft of the agitated vessel has been calculated on the basis of the knowledge of power characteristics which should be deterined for a given type of ipeller. 3

4 L. Broniarz-Press et al. Table 2. Characteristics of the non-newtonian fluids used Mediu Molecular weight Na-CMC 250,000 Aqueous solution concentration C p Teperature T Flow behaviour index Consistency coefficient K Density % [K] - [Pa s ] [kg/ 3 ] ρ The experiental results of the power consuption are usually presented in the for of a diensionless equation Ne = ARe where diensionless nubers are defined as follows: Re B Fr C (4) P Ne = (5) N 3 D 5 ρ = 2 1 ND ρ 4 (6) K ( π ) N D = (7) g Fr 2 and power consuption P is deterined fro the equation P = 2πNM (8) In turbulent flow regie in the agitated vessel equipped with baffles, exponents B and C are equal to zero, then Eq. (4) siplifies to Ne = const (9) In the transitional flow regie the power nuber is dependent on Reynolds and Froude nubers. 3. Results and discussion The investigation has been perfored in the transitional flow regie (100 < Re < 10000). Figs. 3 and 4 show the plot of power nuber Ne vs. Reynolds nuber for double RT and PBT systes. In the transitional flow regie 100 < Re < 3000 it is interesting that the power nuber values reain practically unchanged. This relation is valid for radial and axial double ipeller systes. The results of Nienow et al. (1995) support these observations. It has been shown that power nuber values for RT-RT, FBT-FBT and PBT-PBT are lower in transitional 4

5 Power consuption for non-aerated Na-CMC solutions in ultiple bioreactors agitated flow regie copared with these in turbulent flow. Ibrahi and Nienow (1995) attribute this to the reduction in the rate of shedding of trailing vortices in the transitional flow regie, which reduces the pressure difference between the front and backside of the blades, and to the lower power dissipation. Hudcova et al. (1989) showed that in dual ipeller syste the power nuber increases and reaches the individual additive power as the ipeller spacing increases. a) b) c) d) Fig. 3. The power characteristics of double RT syste in function of distance between ipellers: a) h/d = 1, b) h/d = 2, c) h/d = 3, d) h/d = 4 If the ipeller spacing is in the range of 0.5 < h/d < 1.5, one circulation loop per ipeller has been found with the discharge fro the lower ipeller leaving at 45º upwards fro the horizontal and that fro the upper leaving at 45º downwards, with solid body rotation of the liquid between the. These observations are partially confired. The power nuber values increase with a distance between ipellers, 5

6 L. Broniarz-Press et al. while the change of these values is about 50% for radial ipellers and about 100% for axial ipellers. According to the literature data, the ixing power reduction occurs in range of Re (100, 200) (Metzner and Otto, 1957; Calderbank and Moo-Young, 1961; Broniarz-Press et al., 1997; Paul et al., 2003). For double Rushton turbine systes the relation between power nuber and Reynolds nuber can be described by Ne Re -0.1 while for FBT s and PBT s this relation is Ne Re a) b) c) d) Fig. 4. The power characteristics of double PBT syste in function of distance between ipellers: a) h/d = 1, b) h/d = 2, c) h/d = 3, d) h/d = 4 For vessels equipped with three, four and five ipellers it has been found that the ixing power reduction is saller in coparison with double ipeller systes (Fig. 5). Arenante and Mazzarotta (1999) have also shown that the power 6

7 Power consuption for non-aerated Na-CMC solutions in ultiple bioreactors agitated consuption in the ultiple-ipeller systes is inversely proportional to the ipeller spacing. They have shown that the power nuber for the ipeller increases with the decrease in the clearance of the botto ipeller and increase in the ipeller spacing. The effect of ipeller clearance can be explained by the throttling effect which indicates that there is a change in the direction of the flow when the downward jet discharged by the ipeller ipinges on the tank botto. If the clearance is saller, the change in the flow direction is ore abrupt, generating ore turbulence resulting in increased power consuption of the lower ipeller. a) b) Fig. 5. The exeplary relation Ne = f(re ) for: a) three ipeller systes, b) five ipeller systes The enlargeent of nuber of ipellers causes the reduction of distance between the. The hydrodynaic conditions in such systes have the ajor effect on ixing power. It has been shown that ixing power for three and ore ipellers is not proportional to the nuber of ipellers. The hydrodynaic interactions between particular ipellers causes the reduction in total power requireents. Acknowledgeent: This research was supported by Ministry of Science and High Education of Poland through grant PUT DS ICt 32/113. References Arjunwadkar, S. J., Sarvanan, K., Pandit, A. B. and Kulkarni, P. R., (1998) Biocheical Engineering Journal, 1, Arenante, P. M. and Mazzarotta, B., (1999) Industrial & Engineering Cheistry Research, 38, Broniarz-Press, L., Borowski, J. and Agacinski, P., (1997) Cheical Engineering and Apparatus, 36,

8 L. Broniarz-Press et al. Calderbank, P. H. and Moo-Young, M. B., (1961) Transactions of the Institiution of Cheical Engineers, 39, Collias, D. J. and Prud hoe, K., (1985) Cheical Engineering Science, 40, Davies, S. N., Gibilaro, L. G., Middleton, J. C., Cooke, M. and Lynch, P. M., (1985) Proceedings of 5 th European Conference on Mixing, Hudcova, V., Machon, V. and Nienow, A. W., (1989) Biotechnology and Bioengineering, 34, Ibrahi, S. and Nienow, A. W., (1995) Transactions of the Institution of Cheical Engineers, 73, Linek, V., Moucha, T. and Sinkule, J., (1996) Cheical Engineering Science, 51, Metzner, A. B. and Otto, A. B., (1957) Aerican Institute of Cheical Engineering Journal, 3,3-10. Nienow, A. W., Kendall, A., Moore, I. P. T., Ozcan-Taskin, G. N. and Badha, R. S., (1995) Cheical Engineering Science, 50, Nocentini, M., Fajner, D., Pasquali, G. and Magelli, F., (1988) Cheical Engineering Journal, 37, Paul, E. L., Atieo-Obeng, V. A. and Kresta, S. M., Handbook of Industrial Mixing, Willey-Interscience, US (2003). Pinelli, D., Nocentini, M. and Magelli, F., (1994) Proceedings of 8 th European Conference on Mixing, 8, Singh, V., Hensler, W., Fuchs, R. and Constantinides, A., (1986) Proceedings of International Conference of Bioreactor Fluid Dynaics, Takahashi, K., Iwaki, M., Yokota, T. and Konno, H., (1989) Journal of Cheical Engineering of Japan, 22, Wasser, K. H. and Hungenberg, K. D., (2005) Macroolecular Materials and Engineering, 290,