Analyzing pellet agglomeration in underwater polymer extrusion pelletizers: a numerical simulation study

Bebhash S. Raj; Abhilash J. Chandy

doi:10.1515/ipp-2023-4404

Article

Analyzing pellet agglomeration in underwater polymer extrusion pelletizers: a numerical simulation study

Bebhash S. Raj and Abhilash J. Chandy

Published/Copyright: September 29, 2023

Published by

Become an author with De Gruyter Brill

Submit Manuscript Author Information

From the journal International Polymer Processing Volume 39 Issue 1

Abstract

The production of thermoplastic pellets using underwater die-face pelletizers is a widespread process in the thermoplastics compounding industry. One major challenge in this process is pellet agglomeration, which occurs when the polymer is pliable or easily deformed under heat. To tackle this issue, the non-Newtonian flow of a polymer, along with the turbulent flow of heating oil and heat transfer through the die, are modeled using three-dimensional (3D) computational fluid dynamics (CFD) calculations in ANSYS Fluent. The computational model is validated by comparing its predictions of temperature and pressure using two models with and without a slip method, to experimental measurements from an industrial-scale pelletizer, resulting in a maximum error of <3 % for temperature and <16 % for pressure. The efficiency of the underwater die pelletizer is typically evaluated based on the rate at which it produces pellets. Minor variations in operational parameters, such as the inlet mass flow rate and temperature of the polymer, the temperature of the heating oil, and the water temperature, can greatly affect the quality of the final product. Firstly, contours of pressure, velocity and temperature are presented to understand their impact on pellet agglomeration. However, to more specifically link pellet quality, i.e. pellet agglomeration rate, to the input conditions, the study develops a non-dimensional parameter called the pellet agglomeration number (PAN), as a non-linear function of three other non-dimensional numbers: Reynolds number, Euler number, and a non-dimensional temperature. The values of PAN at the exit and inlet are shown to correlate well with the experimentally measured pellet agglomerations, thereby demonstrating the usefulness of PAN in not only differentiating between good and bad pellet quality but also determining apriori the appropriate operating conditions leading to fewer pellet agglomerations in commercial pelletizers.

Keywords: CFD; extrusion; polymer melt; pelletizer

Corresponding author: Abhilash J. Chandy, Department of Mechanical Engineering, Indian Institute of Technology Bombay, Mumbai, Maharashtra 400076, India, E-mail: achandy@iitb.ac.in

Acknowledgements

The authors gratefully acknowledge the financial support provided by Ansys Software Private Limited, as part of the Ansys PhD Fellowship program.

Research ethics: Not applicable.
Author contributions: The authors have accepted responsibility for the entire content of this manuscript and approved its submission.
Competing interests: The authors state no conflict of interest.
Research funding: None declared.
Data availability: The raw data can be obtained on request from the corresponding author.

References

BartosÌ, O. (2004). Fracture of polymer melts at high shear stress. J. Appl. Phys. 35: 2767–2775, https://doi.org/10.1063/1.1713838.Search in Google Scholar

Bird, R.B. (2002). Transport phenomena. Appl. Mech. Rev. 55: R1–R4, https://doi.org/10.1115/1.1424298.Search in Google Scholar

Bird, R., Armstrong, R., and Hassager, O. (1987). Fluid mechanics, dynamics of polymeric liquids, Vol. 1. Wiley, New York.Search in Google Scholar

Christopher, R.H. and Middleman, S. (1965). Power-law flow through a packed tube. Ind. Eng. Chem. Fundam. 4: 422–426, https://doi.org/10.1021/i160016a011.Search in Google Scholar

Delgadillo-Velázquez, O., Georgiou, G., Sentmanat, M., and Hatzikiriakos, S.G. (2008). Sharkskin and oscillating melt fracture: why in slit and capillary dies and not in annular dies? Polym. Eng. Sci. 48: 405–414, https://doi.org/10.1002/pen.20939.Search in Google Scholar

Dixit, R., Wilson, L., and Marks, M. (1988). Modeling the effect of polymer rheology on the performance of underwater pelletizers. ACS Publications, Washington D.C.Search in Google Scholar

Hariri, M.H., Arastoopour, H., and Rehmat, A.G. (1993). Agglomeration of polyolefin particles in a fluidized bed with a central jet part i. Experimental study. Powder Technol. 74: 231–238, https://doi.org/10.1016/0032-5910(93)85031-4.Search in Google Scholar

Henderson, L.S.III. (1985). The effect of die plate temperature gradients on pellet size distribution from underwater die-face pelletizers. Adv. Polm. Technol. J. Polym. Process. Inst. 5: 203–207, https://doi.org/10.1002/adv.1985.060050304.Search in Google Scholar

Hussain, M.A., Kar, S., and Puniyani, R.R. (1999). Relationship between power law coefficients and major blood constituents affecting the whole blood viscosity. J. Biosci. 24: 329–337, https://doi.org/10.1007/bf02941247.Search in Google Scholar

Hyvärinen, M., Jabeen, R., and Kärki, T. (2020). The modelling of extrusion processes for polymers: a review. Polymers 12: 1306, https://doi.org/10.3390/polym12061306.Search in Google Scholar PubMed PubMed Central

Isaac Ghebre-Sellassie, F.Z.J.D. and Martin, C.E. (2020). Pharmaceutical extrusion technology, 2nd ed. CRC Press, Boca Raton, FL.Search in Google Scholar

Kalika, D.S. and Denn, M.M. (1987). Wall slip and extrudate distortion in linear low-density polyethylene. J. Rheol. 31: 815–834, https://doi.org/10.1122/1.549942.Search in Google Scholar

Kast, O., Geiger, K., Grünschloss, E., and Bonten, C. (2014a). Analysis of pellet shaping kinetics at the die opening in underwater pelletizing processes. Polym. Eng. Sci., 55, 1170–1176, https://doi.org/10.1002/pen.23988.Search in Google Scholar

Kast, O., Musialek, M., Geiger, K., and Bonten, C. (2014b). Influences on particle shape in underwater pelletizing processes. AIP Conf. Proc. 1593, 20–23.Search in Google Scholar

Kiyasatfar, M. (2018). Convective heat transfer and entropy generation analysis of non-Newtonian power-law fluid flows in parallel-plate and circular microchannels under slip boundary conditions. Int. J. Therm. Sci. 128: 15–27, https://doi.org/10.1016/j.ijthermalsci.2018.02.013.Search in Google Scholar

Markarian, J. (2004). Pelletizing: choosing an appropriate method. Plastics, Addit. Compd. 6: 22–26, https://doi.org/10.1016/s1464-391x(04)00235-1.Search in Google Scholar

Memon, R., Solangi, M., Khokhar, R., Baloch, A., and Sayed, K. (2016). Computational analysis of rotating mixing of bird carreau model fluid. Sindh Univ. Res. J.-SURJ (Science Series) 47: 15–18.Search in Google Scholar

Mirzazadeh, M., Shafaei, A., and Rashidi, F. (2008). Entropy analysis for non-linear viscoelastic fluid in concentric rotating cylinders. Int. J. Therm. Sci. 47: 1701–1711, https://doi.org/10.1016/j.ijthermalsci.2007.11.002.Search in Google Scholar

Moffat, R.J. (1988). Describing the uncertainties in experimental results. Exp. Therm. Fluid Sci. 1: 3–17, https://doi.org/10.1016/0894-1777(88)90043-x.Search in Google Scholar

Molenaar, J. and Koopmans, R.J. (1994). Modeling polymer melt-flow instabilities. J. Rheol. 38: 99–109, https://doi.org/10.1122/1.550603.Search in Google Scholar

Molenaar, J., Koopmans, R.J., and Doelder, C.F.J.den. (1998). Onset of the sharkskin phenomenon in polymer extrusion. Phys. Rev. E 58: 4683–4691, https://doi.org/10.1103/physreve.58.4683.Search in Google Scholar

Nam, S. (1987). Mechanism of fluoroelastomer processing aid in extrusion of lldpe. Int. Polym. Process. 1: 98–101, https://doi.org/10.3139/217.870098.Search in Google Scholar

Nithi-Uthai, N. and Manas, I. (2003). Numerical simulation of sharkskin phenomena in polymer melts. Appl. Rheol. 13: 79–86, https://doi.org/10.1515/arh-2003-0006.Search in Google Scholar

Raj, B.S. and Chandy, A.J. (2022). Numerical investigations of flow and heat transfer of polymer melt in underwater extrusion pelletizers. Int. J. Heat Mass Transfer 192: 122899, https://doi.org/10.1016/j.ijheatmasstransfer.2022.122899.Search in Google Scholar

Ramamurthy, A. (1986). Wall slip in viscous fluids and influence of materials of construction. J. Rheol. 30: 337–357, https://doi.org/10.1122/1.549852.Search in Google Scholar

Salema, A.A. and Afzal, M.T. (2015). Numerical simulation of heating behaviour in biomass bed and pellets under multimode microwave system. Int. J. Therm. Sci. 91: 12–24, https://doi.org/10.1016/j.ijthermalsci.2015.01.003.Search in Google Scholar

Shende, T., Niasar, V.J., and Babaei, M. (2021). An empirical equation for shear viscosity of shear thickening fluids. J. Mol. Liq. 325: 115220, https://doi.org/10.1016/j.molliq.2020.115220.Search in Google Scholar

Shojaeian, M. and Koşar, A. (2016). Convective heat transfer of non-Newtonian power-law slip flows and plug flows with variable thermophysical properties in parallel-plate and circular microchannels. Int. J. Therm. Sci. 100: 155–168, https://doi.org/10.1016/j.ijthermalsci.2015.09.024.Search in Google Scholar

Shore, J.D., Ronis, D., Piché, L., and Grant, M. (1997). Sharkskin texturing instabilities in the flow of polymer melts. Phys. Stat. Mech. Appl. 239: 350–357, https://doi.org/10.1016/s0378-4371(96)00491-8.Search in Google Scholar

Sornberger, G., Quantin, J., Fajolle, R., Vergnes, B., and Agassant, J. (1987). Experimental study of the sharkskin defect in linear low density polyethylene. J. Non-Newtonian Fluid Mech. 23: 123–135, https://doi.org/10.1016/0377-0257(87)80014-9.Search in Google Scholar

Thompson, M., Xi, C., Takacs, E., Tate, M., and Vlachopoulos, J. (2004). Experiments and flow analysis of a micropelletizing die. Polym. Eng. Sci. 44: 1391–1402, https://doi.org/10.1002/pen.20134.Search in Google Scholar

Todd Baker, D.B. (2011). Pelletizing. In: Encyclopedia of polymer science and technology. John Wiley and Sons, New Jersey, pp. 802–810.Search in Google Scholar

Tomé, M., Grossi, L., Castelo, A., Cuminato, J., McKee, S., and Walters, K. (2007). Die-swell, splashing drop and a numerical technique for solving the Oldroyd b model for axisymmetric free surface flows. J. Non-Newtonian Fluid Mech. 141: 148–166, https://doi.org/10.1016/j.jnnfm.2006.09.008.Search in Google Scholar

Venet, C. and Vergnes, B. (2000). Stress distribution around capillary die exit: an interpretation of the onset of sharkskin defect. J. Non-Newtonian Fluid Mech. 93: 117–132, https://doi.org/10.1016/s0377-0257(00)00105-1.Search in Google Scholar

Vergnes, B. (2015). Extrusion defects and flow instabilities of molten polymers. Int. Polym. Process. 30: 3–28, https://doi.org/10.3139/217.3011.Search in Google Scholar

Yang, Y., Arastoopour, H., Hariri, M., and Rehmat, A. (1993). Agglomeration of polyolefin particles in a fluidized bed with a central jet part II: theory. Powder Technol. 74: 239–247, https://doi.org/10.1016/0032-5910(93)85032-5.Search in Google Scholar

Zill, D.G. (2020). Advanced engineering mathematics. Jones & Bartlett Publishers, Burlington, MA.Search in Google Scholar

Received: 2023-06-21

Accepted: 2023-09-01

Published Online: 2023-09-29

Published in Print: 2024-03-25

You are currently not able to access this content.

Articles in the same Issue

https://doi.org/10.1515/ipp-2023-4404

Keywords for this article

CFD; extrusion; polymer melt; pelletizer