9th International Seminar on Polymer Science and Technology Iran Polymer and Petrochemical Institute, Tehran, Iran 17-21 October 2009
Simulation of conversion and particle size distribution in a continuous emulsion polymerization reactor Melissa Barazandegan, Mohammad Shahrokhi1*, Hossein Abedini2 1.
Chemical and Petroleum Engineering Department, Sharif University of Technology, P.O. Box: 11365-9465, Tehran, Iran
2.
Department of Polymerization Engineering, Iranian Polymer and Petrochemical Institute, P.O. Box 115-14965, Tehran, Iran
Abstract A detailed bulk model has been used for prediction of conversion and particle size distribution of vinyl acetate emulsion polymerization. Finite volume (FV) and moment technique are applied for solving population balance equation under continuous operation. It is found that the finite volume method can predict sustained oscillations and match experimental data satisfactorily, while the moment method does not provide oscillations.
Keywords: Continuous emulsion polymerization- Particle size distribution- Population balanceSustained oscillation- Vinyl acetate
Introduction In emulsion polymerization, a continuous flow operation has more advantageous over batch process from economical point of view. However, oscillations in conversion and polymer particles number are usually observed when water-soluble monomers are polymerized in a continuous stirred tank reactor [2]. In the present work conversion and particle size distribution are simulated using a detailed bulk model. Finite volume and moment methods are used for solving system equations and their results are compared with experimental results [1].
Process Model For process modeling, the general structure of the bulk model of Immanuel et al [3] is adapted. To match the experimental data, some parameters have been tuned. Particles were assumed to be colloidally stable, and therefore coagulation is not considered. The population balance equation (PBE) is described by: ∂ ∂ dr F ( r ,t ) + ( F ( r ,t ) ) = Rnucδ ( r − rnuc ) ( 1 ) ∂t ∂r dt
*Corresponding author, E-mail:
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The particle density F ( r ,t )dr is defined as mole of particles of unswollen size between r and r + dr at time t . Rnuc is rate of nucleation and δ ( r − rnuc ) is the dirac delta function which is unity at r = rnuc and zero elsewhere.
Results and Discussion The simulation results of conversion for two different methods of solving PBE are shown in Fig. 1. As can be seen, there are relative good agreements between experimental data and simulation results for finite volume method. The FV method predicts sustained oscillations in the reactor. However, the moment method can not predict sustained oscillations in the reactor. Such oscillations, which are undesirable for a stable continuous operation, are well known to be due to periodic (on-off) particle nucleation. The particle numbers decline due to washout in the outlet stream when the nucleation is off. This causes the surfactant concentration to increase and eventually exceed the cmc barrier, thereby restarting particle nucleation. As the total number of particles
9th International Seminar on Polymer Science and Technology Iran Polymer and Petrochemical Institute, Tehran, Iran 17-21 October 2009 0.2
0.16 0.14 0.12 0.1 0.08 0.06 0.04 0.02
Conclusion In the present work, dynamic behavior of vinyl
[1] R. K. Greene, R. A Conzalez, and G. W. Poehlein, in Emulsion Polymerization, I. Piirma and J. L. Gardon, Eds .,ACS Symposium Series 24,Washington, D. C.,1976, p. 341. [2] S. J. Fang, W. Xue, and M. Nomura, Polymer Reaction Engineering, 11 (4), 815-827, 2003. [3] Immanuel, C. D., Cordeiro, C. F., Sundaram, S. S., Meadows E. S., Crowley, T. J., and Doyle, F. J., III, Comput. Chem. Eng., 26, 1133-1152 ,2002. [4] Zeaiter, J., Romagnoli, J. A., Barton, G. W., Gomes, V. G., Hawkett, B. S., Gilbert, R. G., Chem. Eng. Sci., 57, 2955-2969 ,2002. [5] Charles D. Immanuel, Mark A. Pinto, John R. Richards, and John P. Congalidis, Chem. Eng. Res. and design., 86, 692-702(2008).
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Fig 1. CSTR conversion vs. dimensionless time, θ = 30 min , T=40 °C, Sf=0.01 mol/l , fiw=0.7, If=0.005; -4
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Free Surfactant Concentration (mol/litre)
References
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Fig2. Emulsifier concentration vs. dimensionless time ( t
θ = 30 min , T=40 °C, Sf=0.01 mol/l , fiw=0.7, If=0.005;
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acetate emulsion polymerization in a continuous reactor has been simulated using the bulk model. Finite volume and moment techniques have been used for solving the population balance equation. The results indicate that the FV method predicts sustained oscillations in the reactor while moment method does not. Comparing the experimental result with the one obtained by FV method shows acceptable accuracy of the model and solving technique.
Finite volume method moment method Experimental results of Greene
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increases again, the surface area increases and the free surfactant concentration begins to drop, eventually leading to the stoppage of nucleation [5]. Free surfactant concentration and particle nucleation rate versus time, obtained by two different methods of solving PB equation, are shown in Fig. 2 and 3. Again no oscillation is observed in simulation results obtained by moment method.
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Fig3. Particle nucleation rate vs. dimensionless time ( t
θ = 30 min ,T=40 °C, Sf=0.01 mol/l , fiw=0.7, If=0.005;
*Corresponding author, E-mail:
[email protected]
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