A Dynamic Simulator for Slurry‐Phase Catalytic Olefin Copolymerization in a Series of CSTRs: Prediction of Distributed Molecular and Rheological Properties

Abstract

in low-pressure catalytic (e.g., Ziegler-Natta, metallocenes) solution, slurry and bulk phase reactors. Despite the success of metallocene catalysts the majority of commercial polyolefins are still produced by Ziegler-Natta and Philips chromium catalysts. [1] Continuous slurry-phase polymerization, in the presence of a heterogeneous Ziegler-Natta (Z-N) catalyst, is one of the most commonly employed processes in the production of polyolefins, including high-density polyethylene (HDPE), isotactic polypropylene (IPP) as well as their copolymers with higher olefins. [1][2][3][4][5][6][7][8][9][10] The Z-N slurry-phase olefin polymerization in a series of continuous stirred tank reactors (CSTRs) and loop-reactors continue to dominate the HDPE production, accounting for almost 70% of the total installed capacity. Polymerization in a series of reactors is often employed to control the comonomer incorporation rate and copolymer distribution, which is important for polyolefin grades and applications (e.g., pipes) that require excellent environmental stress crack resistance. The attractive combination of good flowability and improved mechanical properties is achieved by tailoring MWDs and controlling comonomer branching distribution. The slurry-phase HDPE process technology (see Figure 1) employs two or more stirred-tank reactors in series and utilizes a Z-N catalyst system composed of a titanium chloride compound and an alkyl aluminum cocatalyst. The process uses hydrogen as a chain-termination agent to control the molecular weight of the product and a comonomer to control the density of the polyethylene grade. [11] The process operates in a continuous mode using a cascade of two or three autoclave-type vessels. Each reactor can operate under a different hydrogen partial pressure, thereby allowing the control of the molecular weight distribution produced in each reactor of the series. Low molecular-weight polyethylene has a low viscosity and a fast crystallization rate, accounting for high polymer stiffness, whereas high molecular-weight polyolefins exhibit significantly higher toughness, but a much slower crystallization rate and a lower crystallinity together with a high melt viscosity due to extensive polymer chain entanglement. Typical polymerization conditions are: temperatures of 70-90 °C, pressure less than 10 bar and a reactor mean residence time of 45 min.Modeling A comprehensive mathematical model is developed to simulate the dynamic Ziegler-Natta ethylene-α-olefins copolymerization in a series of slurry-phase continuous stirred tank reactors. A generalized multisite kinetic mechanism is considered to describe the molecular and compositional developments (joint molecular weight-copolymer composition distribution) in a series of continuous stirred tank reactors (CSTRs). Dynamic macroscopic mole balances are derived to calculate the dynamic evolution of all species concentrations in three phases (i.e., gas, liquid, and polymer) of the multiphase system. The polymer molecular properties (i.e., molecular weight dist...

Modeling of Slurry Phase Olefin Polymerizations in a Semibatch Well-Mixed Stirred Tank: Effect of Mass Transfer

“…9,32 The equilibrium concentration of monomer in the semicrystalline polymer phase can be computed using the sorption data 33 and EOS. 9 Alizadeh et al 32 gave the detailed solution process by the Sanchez−Lacombe equation of state (SL EOS) for gas olefin polymerization. However, it is worth noting that the thermodynamic equilibrium in the slurry polymerization process is relatively complicated since the component fractions in three phases (gas, liquid and solid) have never been measured because of experimental difficulty.…”

Scale up of reactor recipes for bimodal high-density polyethylene production in cascade process

“…The reaction medium comprises a coordination catalyst (Ziegler-Natta or metallocene), the growing polymer particles, and the gases ethylene, 1-olefin comonomer, and hydrogen dissolved in a hydrocarbon solvent. [9][10][11][12] As the polymerization proceeds, the gaseous reagents must be transferred from the gas phase to the liquid diluent, and from there to the catalyst sites embedded in the growing polymer particles, where the polymerization reaction takes place.…”

“…As a result, the similarity of the relative gas-phase composition between pilot and industrial reactors is not a sufficient condition to successfully scale up slurry polymerization recipes. Based on this background, and despite a long history of papers reporting the development of polyolefins with precisely controlled microstructure for tailormade properties, 9,10,[21][22][23] no specific guidance is provided on how to scale up the polymerization recipes from pilot to industrial reactors.…”

Steady-State and Dynamic Modeling of the Solution Polyethylene Process Based on Rigorous PC-SAFT Equation of State

“…Polyethylene (PE) is one of the most important polymers, which can be produced by various technologies, such as suspension (slurry) polymerizations, solution polymerizations, gas-phase polymerizations, bulk high-pressure polymerizations, etc. , To date, a large number of models for ethylene polymerization processes have been developed. − Khare et al presented both steady-state and dynamic models for a commercial slurry high-density polyethylene (HDPE) process using a polymerization process simulator. They provided a detailed modeling methodology, including thermodynamic properties, phase equilibrium, reaction kinetics, and polymer properties.…”

“…Chen et al developed another rigorous model for industrial successive supercritical slurry-phase and gas-phase catalytic-polymerization reactors of the Borstar bimodal polyethylene process. In addition, a series of comprehensive mathematical models were presented to simulate the behavior of the industrial slurry ethylene polymerization process. − In addition to these ethylene polymerization models, several steady-state and dynamic models were established for commercial polypropylene processes. − Embirucu et al ,− developed a comprehensive mathematical model for the copolymerization of ethylene/1-butene in solution with Ziegler–Natta catalysts, whereas detailed descriptions of thermodynamic and physical properties of the multisystem and phase behavior in the polymerization reactors were absent.…”