Design of a gas-liquid unbaffled stirred tank with a concave blade

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1. DESIGN OF A GAS–LIQUID UNBAFFLED STIRRED TANK WITH A CONCAVE BLADE IMPELLER

2. CUPRINS INTRODUCTION EXPERIMENTAL SETUP EXMERIMENTAL RESULTS CONCLUSION REFERENCES

3. INTRODUCTION Multiphase (gas–liquid) stirred tanks are widely used in industrial processes where complete mass transfer takes place in a turbulent regime of a fluid flow in sparging (gassed) conditions. T he use of unbaffled stirred tanks may be desirable, for example, in crystallization where the presence of baffles may promote particle attrition , in precipitation processes where baffles can lead to the encrustation problems , in biological operations where cell damage may be intensified by the presence of baff l es , and in food and pharmaceutical industries where tank cleanliness is of primary importance . Unbaffled stirred tanks give higher values of the mass transfer coefficient than baffled tanks at the same power consumption [16, 17]. Some studies on standard impellers have already been carried out to analyze the fluid flow hydrodynamics (velocity and turbulence field) of unbaffled stirred tanks [15, 18] for their better performance on mixing and power consumption

4. EXPERIMENTAL SETUP Experimental and CFD setups

5. Mass Transfer Coefficient. The standard dynamic method was used to measure the volumetric mass transfer rate. For implementing this method, the liquid phase was deoxygenated by flushing with nitrogen. Then, after replacing nitrogen by air, the variation in the dissolved oxygen concentration with time was measured until reaching saturation. This technique is widely described in the literature. The Thermo Orion® DO meter, which is used to measure the concentration, is accurate to within 0.1 mg/l. Under the assumption that liquid phase is well mixed and gas absorption is liquid phase controlled, the volumetric mass transfer coefficient at the temperature T can be determined from the following equation :

6. EXPERIMENTAL RESULTS Fig. 2. The values of KLaT vs. N for D = 150 mm at d/D = 0.2 (a) and 0.3 (b) with different values of C/D and at C/D = 0.3 (c) and 0.6 (d) with different values of d/D.    

7. CONCLUSION The influence of geometrical parameters of both the tank and concave blade impeller in unbaffled gas–liquid stirred tank (the clearance between the impeller and tank base as well as impeller diameter) on the process characteristics has been studied. The strong influence of these parameters on the mass transfer rate, power consumption, and gas holdup was observed. The mass transfer coefficient increases with decrease in C/D at constant d and N, and it also increases with impeller diameter. The design criteria for the mass transfer coefficient, gassed power consumption, and gas holdup have been developed. The results agreed with the predicted values to within the experimental error. The liquid fl ow pattern was not significantly influenced by the gas inflow from the sparger , but when the impeller was placed nearly half way along (C/D = 0.5 and 0.6), a pronounced influence was observed.

8. REFERENCES Y. S. Liu, J. Y. Wu, and K. P. Ho, Characterization of oxygen transfer conditions and their effects on phaffi a rhodozyma growth and carotenoid production in shake- fl ask cultures, Biochem . Eng. J., 27, 331–335 (2006) P. F. F. Amaral, M. G. Freire, M. H. M. R. Leao , I. M. Marrucho , J. A. P. Coutinho, and M. A. Z. Coelho, Optimization of oxygen mass transfer in a multiphase bioreactor with perfl uorodecalin as a second liquid phase, Biotechnol . Bioeng ., 99, 588–598 (2008) A. Karimi , F. Golbabaei , M. Neghab , M. R. Mehrnia , K. Mohammad, M. R. Pourmand , and A. Nikpey , Investigation of oxygen transfer in a two-phase partition stirred tank bioreactor in the presence of silicone oil, Chem. Biochem . Eng. Q., 25, 209–219 (2011)