Apostolos Baltsas

Application and validation of the pseudo-kinetic rate constant method to high pressure LDPE tubular reactors

The present study is concerned with the application of the pseudo-kinetic rate constant method to high pressure LDPE tubular reactors. The utilization of multiple monomers is of profound importance to the polymer manufacturing industry since it leads to the production of an unlimited number of polymer types with a wide range of properties. However, by increasing the number of comonomers the kinetic rate expressions describing the appearance / disappearance of the various molar species become fairly complex. In order to simplify the kinetic treatment of the resulting expressions, the pseudo-kinetic rate constant method can be employed, in combination with the method of moments, to model the multicomponent free-radical polymerization of ethylene in a high pressure tubular reactor.

A comprehensive investigation on high-pressure LDPE manufacturing: Dynamic modelling of compressor, reactor and separation units

Abstract A comprehensive mathematical model is developed for the simulation of high-pressure Low Density Polyethylene (LDPE) plants. Correlations describing the thermodynamic, physical and transport properties of the ethylene-polyethylene mixture are presented and compared with experimental data. Energy balances around the compression units are derived to calculate the energy requirements. A detailed kinetic mechanism is proposed to describe the molecular and structural developments of the free-radical polymerization of ethylene. Based on the postulated kinetic mechanism, a system of differential mass balance equations are derived for the various molecular species, total mass, energy and momentum in the polymerization system. Simulation results show that the proposed mathematical model can be successfully applied to the real-time prediction of reactor temperature profile and polymer melt index. Moreover, model predictions are compared with industrial measurements on reactor and coolant temperature profiles, reactor pressure, conversion, and final molecular properties for different polyethylene grades. Finally, various equations of state (e.g., Sako-Wu-Prausnitz, SAFT, PC-SAFT) are employed to simulate the operation and phase equilibrium in the flash separation units.

A Comprehensive Model for the Simulation of Ethylene Decomposition in High-Pressure LDPE Autoclaves

Abstract In the present study, a comprehensive mathematical model was developed to simulate the dynamic behaviour of multi-zone, multi-feed high pressure ethylene polymerization autoclaves and assess the risk of ethylene decomposition under different scenarios. To describe the complex flow patterns occurring in low density polyethylene (LDPE) autoclaves, a user-specified multi-segment, multi-recycle model representation of the actual multi-zone reactor is established. A suitable micro-mixing model has been applied taking into account initiator segregation phenomena due to inefficient mixing conditions at the initiator injection points. A general reaction mechanism (that includes a comprehensive ethylene decomposition kinetic scheme) is employed to represent the kinetics of ethylene polymerization. The present model is capable of predicting accurately the dynamic behaviour of industrial LDPE autoclaves (reactor temperature profile, polymerization rate, monomer conversion, molecular weight properties and MWD) and, thus, can be employed in the design, optimization and control of these reactors. Decomposition phenomena due to operation condition changes (e.g., feed impurities and disturbances, excess initiator, controller failure, poorly tuned controller, etc.) have been analyzed and simulated.

Calculation of the Optimal Distribution of the Active Metal Site Concentration in a Ziegler-Natta Catalyst to Maximize Polymer Yield in Olefin Polymerizations

Abstract It is well-known that depending on the catalyst preparation conditions the concentration of active metal sites in Ziegler-Natta catalysts can vary considerably with particle radius. This non-uniform concentration of active metal sites can result in non-uniform polymerization conditions in the catalyst particle that can affect the particle morphology and polymerization rate. In the present study, a comprehensive single particle growth model is developed to investigate the effect of the active metal site concentration distribution on particle growth, particle overheating and polymer yield in heterogeneous Ziegler-Natta catalytic olefin polymerizations. Following the original work of Kanellopoulos et al. (2004) a random pore polymeric flow model for a single particle was developed to describe the spatial-temporal monomer concentration and temperature profiles in a growing catalyst/particle. To assess the effect of the active metal site concentration distribution in a Ziegler-Natta catalyst/particle on particle growth and particle overheating, a number of numerical simulations were carried out, using the developed random pore polymeric flow model, by varying the initial catalyst diameter, monomer partial pressure, particle morphology (e.g., porosity) and catalyst active metal site distribution. It was shown that depending on the initial active metal site distribution, the predicted polymerization rate, particle overheating and polymer yield can vary considerably. Finally, a non-linear optimization problem was formulated to calculate the optimal active metal sites spatial distribution in a Ziegler-Natta catalyst in to maximize the polymer productivity or/and minimize the total active metal concentration. In particular, via the combined solution of the single particle’s PDEs and a non-linear optimizer, the optimal spatial distribution of active metal sites in the catalyst was calculated that minimized the total active metal concentration for a given value of polymer yield and productivity.

Dynamic Modeling and Optimization of Flash Separators for Highly-Viscous Polymerization Processes

In the present study, a multi-phase, multi-zone mathematical model is developed to describe the dynamic operation of industrial high-pressure separators (HPSs) for highlyviscous polymer systems. The proposed multi-phase, multi-zone description of the highpressure separator takes into account the complex gas carry-under and liquid droplets carryover phenomena. Moreover, the model takes into account the mass transfer rate from the liquid droplets to the gas phase as well as the bubble formation in the liquid zone. Extensive numerical simulations are carried out to determine the optimal operating conditions (i.e., temperature, pressure, feed composition and mass flowrate, etc.) on the dynamic performance and the separation efficiency of the HPS for highly-viscous fluids. It is shown that the proposed model is capable of simulating the dynamic operation of industrial-scale HPSs over a wide range of operating conditions (i.e., pressures 200-260 bar and temperatures 220-260 0C) and copolymers of different copolymer composition and viscoelastic properties (i.e., melt index in the range of 2-50 g/10min). Finally, it is shown that industrial HPSs do not operate near the thermodynamic equilibrium conditions. Therefore, their non-ideal behaviour should be taken into account when simulating their dynamic operation. Subsequently, model-based optimization and control studies are carried out to optimize the dynamic operation and performance of an industrial HPS.