Impact of Mixing Parameters on Homogenization of Borax Solution and Nucleation Rate in Dual Radial Impeller Crystallizer

Interaction between mixing and crystallization is often
ignored despite the fact that it affects almost every aspect of the
operation including nucleation, growth, and maintenance of the
crystal slurry. This is especially pronounced in multiple impeller
systems where flow complexity is increased. By choosing proper
mixing parameters, what closely depends on the knowledge of the
hydrodynamics in a mixing vessel, the process of batch cooling
crystallization may considerably be improved. The values that render
useful information when making this choice are mixing time and
power consumption. The predominant motivation for this work was
to investigate the extent to which radial dual impeller configuration
influences mixing time, power consumption and consequently the
values of metastable zone width and nucleation rate. In this research,
crystallization of borax was conducted in a 15 dm3 baffled batch
cooling crystallizer with an aspect ratio (H/T) of 1.3. Mixing was
performed using two straight blade turbines (4-SBT) mounted on the
same shaft that generated radial fluid flow. Experiments were
conducted at different values of N/NJS ratio (impeller speed/
minimum impeller speed for complete suspension), D/T ratio
(impeller diameter/crystallizer diameter), c/D ratio (lower impeller
off-bottom clearance/impeller diameter), and s/D ratio (spacing
between impellers/impeller diameter). Mother liquor was saturated at
30°C and was cooled at the rate of 6°C/h. Its concentration was
monitored in line by Na-ion selective electrode. From the values of
supersaturation that was monitored continuously over process time, it
was possible to determine the metastable zone width and
subsequently the nucleation rate using the Mersmann’s nucleation
criterion. For all applied dual impeller configurations, the mixing
time was determined by potentiometric method using a pulse
technique, while the power consumption was determined using a
torque meter produced by Himmelstein & Co. Results obtained in
this investigation show that dual impeller configuration significantly
influences the values of mixing time, power consumption as well as
the metastable zone width and nucleation rate. A special attention
should be addressed to the impeller spacing considering the flow
interaction that could be more or less pronounced depending on the
spacing value.

Authors:

• A. Kaćunić
• M. Ćosić
• N. Kuzmanić

Keywords:

• Dual impeller crystallizer
• mixing time
• power consumption
• metastable zone width
• nucleation rate.

References:

[1] K. Shimizu, H. Nagasawa and K. Takahashi, “Effect of off-Bottom Clearance of a Turbine Type Impeller on Crystal Size Distribution of Aluminum Potassium Sulfate in a Batch Crystallizer”, J. Crystal Growth, 1995. 154: p. 113-117.
[2] K. Shimizu, T. Nomura and K. Takahashi, “Crystal Size Distribution of Aluminum Potassium Sulfate in a Batch Crystallizer Equipped with Different Types of Impeller,” J. Crystal Growth, 1998. 191(1-2): p. 178-184.
[3] K. Shimizu et al., “Effect of Baffle Geometries on Crystal Size Distribution of Aluminum Potassium Sulfate in a Seeded Batch Crystallizer,” J. Crystal Growth, 1999. 197(4): p. 921-926.
[4] L. Marmo et al., “Influence of mixing on the particle size distribution of an organic precipitate,” J. Crystal Growth, 1996. 166: p. 1027-1034.
[5] Z. Sha and S. Palosaari, “Mixing and Crystallization in Suspension,” Chem. Eng. Sci., 2000. 55: p. 1797-1806.
[6] E. L. Paul, V. A. Atiemo-Obeng and S. Kresta, Handbook of Industrial Mixing, Hoboken, New Jersey: John Wiley and Sons, Inc., 2004, pp. 1027-1029.
[7] I. Fort and T. Jirout, „A study on blending characteristics of axial flow impellers,“ Chem. Proc. Eng., 2011. 32 (4): p. 311-319.
[8] J. Landau and J. Procházka, “Experimental methods for following the homogenation of miscible liquids by rotary mixers,” Collect. Czech. Chem. Commun., 1961. 26: p. 1976-1990.
[9] G. Delaplace et al. „Determination of mixing time by colourimetric diagnosis - Application to a new mixing system,“ Exp. Fluids, 2004. 36: p. 437-443.
[10] R. Mann et al., „Computational Fluid Mixing for Stirred Vessels - Progress from Seeing to Believing,“ Chem. Eng. J. Biochem. Eng. J., 1995. 59: p. 39-50.
[11] O. Visuri, M. Laakkonen and J. Aittamaa, „A digital imaging technique for the analysis of local inhomogeneities from agitated vessels,“ Chem. Eng. Technol., 2007. 30: p. 1692-1699.
[12] N. Otomo et al., „A novel measurement technique for mixing time in an aerated stirred vessel,“ J. Chem. Eng. Jpn., 2003. 36: p. 66-74.
[13] W. M. Lu, H. Z. Wu and M. Y. Ju, „Effects of baffle design on the liquid mixing in an aerated stirred tank with standard Rushton turbine impellers,“ Chem. Eng. Sci., 1997. 52: p. 3843-3851.
[14] E. A. Fox and V. E. Gex, „Single-Phase Blending of Liquids,“ AIChE J., 1956. 2: p.539-544.
[15] I. Houcine, E. Plasari and R. David, “Effects of the Stirred Tank's Design on Power Consumption and Mixing Time in Liquid Phase,” Chem. Eng. Technol., 2000. 23: p. 605-613.
[16] P. Sayan, S. Titiz Sargut and B. Kiran, “Effect of impurities on the microhardness of borax decahydrate,” Powder Technology, 2010. 197: p. 254-259.
[17] W. D. Einenkel and A. Mersmann, "Erforderliche Drehzahl zum Suspendieren in Rührwerken," Verfahrenstechnik, 1977. 11 (2): p. 90-94.
[18] T. N. Zwietering, "Suspending of Solid Particles in Liquid Agitators," Chem. Eng. Sci., 1958. 8: p. 244-253.
[19] M. Jahoda et al., “CFD modelling of liquid homogenization in stirred tanks with one and two impellers using large eddy simulation,” Chem. Eng. Res. Des., 2007. 85: p. 616–625.
[20] O. Sahin, “Effect of Borax on the Crystallization of Boric Acid,” J. Crystal Growth, 2002. 236: p. 393-399.
[21] F. Jones et al., “The Effect Of Calcium Cations on the Precipitation of Barium Sulfate 2: Calcium Ions in the Presence of Organic Additives,” J. Crystal Growth, 2004. 270: p. 593-603.
[22] D. C. Y. Wonh, Z. Jaworski and A. W. Nienow, “Effect of Ion Excess on Particle Size and Morphology During Barium Sulfate Precipitation: An Experimental Study,” Chem. Eng. Sci., 2001. 56: p. 727-734.
[23] J. C. Givand, A. S. Teja and R. W. Rousseau, “Manipulating Crystallization Variables to Enhance Crystal Purity,” J. Crystal Growth, 1999. 198/199: p. 1340-1344.
[24] S. Ramalingom, J. Podder and S. Narayana Kalkura, “Crystallization and Characterization of Orthorhombic MgSO4∙7H2O,” Crys. Res. Technol., 2001. 36(12): p. 1357-1364.
[25] A. Nokhoddchi, N. Bolourtchiana and R. Dinarvand, “Crystal Modification of Phenytoin Using Different Solvents and Crystallization Conditions,” Int. J. Pharm., 2003. 250: p. 85-87.
[26] A. G. Jones, Crystallization Process Systems, London: Butterworth-Heinemann, 2002, pp. 58-141.
[27] J. W. Mullin, Crystallization, 4th ed., Oxford: Butterworth-Heinemann, 2001, pp. 86-403.
[28] A. Myerson, Handbook of Industrial Crystallization, Boston: Butterworth-Heinemann, 2002, pp. 1-218.
[29] M. Akrap, N. Kuzmanića and J Prlić-Kardum, ”Effect of mixing on the crystal size distribution of borax decahydrate in a batch cooling crystallizer,” J. Crystal Growth, 2010. 312(24): p. 3603-3608
[30] K. J. Kim and A. Mersmann, “Estimation of Metastable Zone Width in Different Nucleation Processes,” Chem. Eng. Sci., 2001. 56(7): p. 2315-2324.
[31] A. Schubert and A. Mersman, “Determination of Heterogeneous Nucleation Rates,” Trans. Inst. Chem. Eng., 1996. A 74(1): p. 816-821.
[32] A. Mersmann, “Supersaturation and Nucleation,” Trans IchemE, 1996. 74(A): p. 812-819.
[33] A. Mersmann, „General prediction of statistically mean growth rates of a crystal collective,“ J. Crystal Growth, 1995. 147: p. 181-193.
[34] Y. H. Cheon, K. J. Kim and S. H. Kim, „A study on crystallization kinetics of pentaerythritol in a batch cooling crystallizer,“Chem. Eng. Sci., 2005. 60: p. 4791-4802.
[35] R. Kuboi and A. W. Nienow, “The power drawn by dual impeller systems under gassed and ungassed conditions,“ In: Proc. 4th Eur. Conf. Mix., Noordwijkerhout, the Netherlands, 1982, pp. 247.
[36] Z. X. Weng, “The effect of the distance between multiple impellers in the turbulent tank,” Chem. Eng., 1983. 6: p. 1–6.
[37] V. Mishra and J. Joshi, “Flow generated by a disc turbine. IV: Multiple impellers,” Chem. Eng. Res. Des., 1994. 72: p. 657-668.
[38] K. Rutherford et al., “Hydrodynamic characteristics of dual Rushton impeller stirred vessels,” AIChE J., 1996. 42: p. 332–346.