CFD based evaluation of polymer particles heat transfer coefficient in gas phase polymerization reactors ☆ Mohammad A. Dehnavi a , Shahrokh Shahhosseini a, ⁎, S. Hassan Hashemabadi a , S. Mehdi Ghafelebashi b a Simulation and Control Research Laboratory, Department of Chemical Engineering, Iran University of Science and Technology, P.O. Box 16765-163, Tehran, Iran b Petrochemical Research and Technology Company, Tehran, Iran abstract article info Available online 9 August 2008 Keywords: Convection heat transfer coefficient Computational fluid dynamics (CFD) Olefin polymerization Gas phase reactor In this work, computational fluid dynamics (CFD) has been employed to compute convection heat transfer coefficient (h) that is the key parameter in calculation of heat transfer rate between particles and fluids in an ethylene polymerization fluidized bed reactor. In addition, the effects of various parameters such as free stream fluid velocity, particles size, and particles interactions with different configurations on heat transfer coefficient were studied. Simulation results are in agreement with common engineering knowledge of the process. The results also indicate that particle interactions, particle size and fluid velocity have more significant impacts on h. © 2008 Elsevier Ltd. All rights reserved. 1. Introduction Polyolefins, polymers and copolymers of ethylene and propylene, represent more than 40% of plastics produced every year. Today, world polyolefin production exceeds 100 million metric tons. The current polymerization processes for production of polyolefin's are conven- tional high-pressure polymerization, solution polymerization, slurry or suspension polymerization and gas phase polymerization. Slurry (suspension) polymerization is the oldest and most widely used method due to its maturity and flexibility. Gas phase technology, the newest technology, is increasingly utilized since it requires low investments and operating costs. Among them, gas phase polymer- ization process is the most versatile and recently developed process. Gas phase technology has been used commercially to produce polymers since 1970s and has several advantages over high-pressure technology such as operation at lower pressures and temperatures, no need for solvents and a better heat removal system [1–5]. Gas phase polyolefin production usually takes place in continuous fluidized bed reactors (FBR). During this process, small catalyst particles (e.g. dimensions of 20 to 80 mm) are continuously fed into the reactor at a point above the gas distributor and react with the incoming fluidizing monomer mixture to form a broad distribution of polymer particles in the size range of 100–5000 mm. At the early stages of polymerization, the catalyst fragments into a large number of small particles, which are subsequently encapsulated by the growing polymer phase. During their stay in the bed, the polymer particles grow in size due to polymerization, and can be entrained by the fluid- izing gas or undergo particle agglomeration, if the reactor operates close to the polymer softening temperature. Due to the differences in the polymer particle sizes, segregation occurs and fully-grown polymer particles migrate to the bottom where they are removed from the reactor [1,5,6–10]. Given that the polymerization of olefins is a highly exothermic reaction, with heats of polymerization on the order of 100–110 kJ/mol, heat production rates are extremely high inside a polymerization particle. Large temperature excursions in and around growing particles can be very dangerous, since the reaction is typically carried out at a temperature of 80–90 °C, and the melting point of the polymer varies from 105 °C to 135 °C depending on its composition [1,5,9–11]. Detailed modeling of such a reactor is a highly complex task involving reactor design, complex multiphase flows, interphase mass transfer, particle–particle and particle–reactor wall interactions, intraparticle heat and mass transfer, and nano-scale phenomena such as kinetics of catalyst active sites and polymer crystallization [9,10]. The research groups of Chiovetta at the University of Buenos Aires and of Ray at the University of Wisconsin Madison were the first to perform an in-depth investigation of heat transfer phenomena in and around fresh catalyst, and growing polymer particles. They pointed out in a number of articles that heat transfer problems can be crucial in gas phase reactions and probably not so much in slurry or liquid- phase reactions [12–14]. Most of the previous works have been done without including particle–particle interactions into estimates of the heat transfer coefficient and also non polymerizing systems. Brodkey et al. took particle–particle and particle–wall interactions into account and provided expressions that calculate higher values of Nu and thus h compared to those from Ranz–Marshall correlation [15]. Martin has also investigated particle–wall interactions, and claimed that this type of heat transfer can become dominant for particles smaller than one International Communications in Heat and Mass Transfer 35 (2008) 1375–1379 ☆ Communicated by W.J. Minkowycz. ⁎ Corresponding author. E-mail address: Shahrokh@iust.ac.ir (S. Shahhosseini). 0735-1933/$ – see front matter © 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.icheatmasstransfer.2008.07.017 Contents lists available at ScienceDirect International Communications in Heat and Mass Transfer journal homepage: www.elsevier.com/locate/ichmt