Low-density Polyethylene Production Research Articles

A computational approach to partial least squares model inversion in the framework of the process analytical technology and quality by design initiatives

In the context of the paradigms founding the Quality by Design and Process Analytical Technology initiatives, the work herein presents a computational approach to support the decision-making process, in particular, about the feasibility of a product defined for some a priori given quality characteristics.The approach is based on the computation of the pareto-optimal front when simultaneously minimizing the expected differences between the predicted and the desired characteristics. Thus, the feasibility is tackled as an optimization problem with the novelty of doing so simultaneously for all the characteristics, preserving the correlation structure, but by separately handling each individual characteristic.With data from a low-density polyethylene production process, with fourteen process variables and five measured characteristics of the final polyethylene, solutions are found to define the Design Space for targeted quality characteristics on the product, and without the need of explicitly inverting the PLS (Partial Least Squares) prediction model fitted to the process.

Mathematical Modeling of the Molecular Weight Distribution in Low Density Polyethylene. I. Steady‐State Operation of Multizone Autoclave Reactors

AbstractA comprehensive mathematical model and an efficient strategy for its solution are presented to describe the complete molecular weight distribution (MWD) in the free‐radical polymerization for the production of low density polyethylene (LDPE) in continuous stirred tank reactors at steady state. The kinetic scheme includes all the relevant reactions that affect the MWD. The solution for the complete MWD is given as explicit algebraic expressions that are solved sequentially taking advantage of the algebraic structure of the equations; the strategy can simulate LDPE multizone autoclave reactors in a simple and straightforward manner. No approximation for the moment's closure is used. Model simulations reasonably match average molecular weight data from an industrial plant, as well as results from the POLYRED software. A parameter sensitivity study for key kinetic constants shows that the MWD polydispersity is predominantly affected by the transfer to polymer rate constant. The solution is applied to linear, topological and mechanical chain‐scission mechanisms.

Optimization of Energy Consumption in Gas-Phase Polymerization Process for Linear Low Density Polyethylene Production

Gas-phase polymerization is a method that is widely used in the polymer production process. The heterogeneous reaction between monomer gas and solid catalyst takes place in the fluidized bed reactor to produce the polymer product. The design and production planning of this process is a difficult task due to the complicated fluid behavior and polymerization reaction inside the fluidized bed reactor. In this paper, a model of the fluidized bed polymerization reactor for linear low density polyethylene (LLDPE) production is developed using the Aspen Custom Modeler (ACM) to predict the polymer characteristics and flow behavior. The developed model is then integrated to the main process flow simulation in order to optimize the energy consumption in the polymer production process. The results show that the total energy requirement is reduced while maintaining the same polymer characteristics.

Polyolefins, a Success Story.

This paper reports the principal discoveries which have played a major role in the polyolefin field and have positioned polyolefins as the most produced plastics. The early development of polyolefins covering the production of LDPE (Low density polyethylene) at ICI (Imperial Chemical Industries) and the discovery of Phillips or Ziegler-Natta catalysts are highlighted in the first section. In the second part, the impact of the implementation of molecular catalysts on the research in polyolefins is discussed together with the most recent advances leading to high-performance tailor-made resins.

Design of packing cup interference fit value of hypercompressors for low density polyethylene production

The hypercompressor is one of the core facilities in low density polyethylene production, with a discharge pressure of approximately 300 MPa. A packing cup is the basic unit of cylinder packing, assembled by the interference fit between an inner cup and an outer cup. Because the shrink-fitting prestresses the packing cup, serious design is needed to gain a favorable stress state, for example, a tri-axial compressive stress state. The traditional method of designing the interference fit value for packing cups depends on the shrink-fit theory for thickwalled cylinder subject to internal and external pressure. According to the traditional method, critical points are at the inner radii of the inner and external cup. In this study, the finite element method (FEM) has been implemented to determine a more accurate stress level of packing cups. Different critical points have been found at the edge of lapped sealing surfaces between two adjacent packing cups. The maximum Von Mises equivalent stress in a packing cup increases after a decline with the rise of the interference fit value. The maximum equivalent stress initially occurs at the bore of the inner cup, then at the edge of lapped mating surfaces, and finally at the bore of the outer cup, as the interference radius increases. The traditional method neglects the influence of axial preloading on the interference mating pressure. As a result, it predicts a lower equivalent stress at the bore of the external cup. A higher interference fit value accepted by the traditional method may not be feasible as it might already make packing cups yield at the edge of mating surfaces or the bore of the external cup. Along with fatigue analysis, the feasible range of interference fit value has been modified by utilizing FEM. The modified range tends to be narrower and safer than the one derived from the traditional method, after getting rid of shrink-fit values that could result in yielding in a real packing cup.

Effect of initiator concentration to low-density polyethylene production in a tubular reactor

Low-density polyethylene (LDPE) is one of the most widely used polymers in the world, which is produced in high-capacity tubular and autoclave reactors. As the LDPE industry turn into more competitive and its market profit margins become tighter, manufacturers have to develop solutions to debottleneck the reactor output while abiding to the stringent product specification. A single polyolefin plant producing ten to forty grades of LDPE with various melt flow index (MFI), therefore understanding the reaction mechanism, the operating conditions as well as the dynamic behavior of tubular reactor is essential before any improvement can take place. In the present work, a steady state mathematical model representing a tubular reactor for the production of LDPE is simulated using MATLAB R2015a®. The model developed is a function of feed inlet, reactor jacket, single initiator injector and outlet stream. Analysis on the effect of initiator concentration (CI) shows sudden declining trend of initiator's concentration which indicates that all of the initiators are exhausted after polymerization reaction and no further reaction occur from this point onwards. Furthermore, the results demonstrate that the concentration of initiator gives significant impact on reactor temperature's profile and monomer conversion rate, since higher initiator concentration promotes greater polymerization rate, and therefore leads to higher monomer conversion throughput.

Simulation Studies of Low-density Polyethylene Production in a Tubular Reactor

This paper deals with the model based of an industrial low-density polyethylene (LDPE) tubular reactor. As the LDPE industry becomes more competitive, manufacturers have to come out with solutions to debottleneck the reactor output while abiding to the stringent product specification. In other words, they have to deal with maximization of the production (maximization of the monomer conversion for a given feed flow rate) and minimization of unwanted product (ethyl, butyl, vinyl and vinylidene groups), while maintaining the polymer product quality with regards to its molecular weight distribution (MWD). To achieve these goals, understanding the effects of operating variables manipulation as well as the dynamic behavior of tubular reactor is essential to develop high performance tubular reactor. In this study, the dynamic model of a tubular reactor for the production of low-density polyethylene (LDPE) is simulated using MATLAB R2015a® in order to predict the temperature profile and monomer conversion percentage along the tubular reactor. The model consists of feed stream, reactor jacket, initiator injector and outlet stream. Several operating variables are involved, notably the feed flow rates, the inlet pressure and temperature with varying parameters to analyze the effect on the reactor productivity. Plots of reactor temperature profile and monomer conversion percentage are presented and from the result, it can be concluded that feed temperature gives significant impact as it effects monomer conversion rate.

Identification of Polymer Deposits in the High-pressure Recycling System of Low-density Polyethylene Production

This paper outlines the results of analysis of Low-Density Polyethylene (LDPE) depositions on filter elements of upstream II cascade compressor by means of X-ray and Fourier transform infrared spectroscopies, gel permeation chromatography and viscometry, differential scanning calorimetry and 13C NMR spectroscopy. Based on comparison with experimental data of process designers the supposition has been given to the cause of the formation.

Dynamic simulation of high-pressure tubular reactor for low-density polyethylene production

Low-density polyethylene, one of the most important polymer products, is commonly produced in high-pressure free radical polymerization processes. A dynamic model of the high-pressure polymerization of ethylene initiated by oxygen in tubular reactor is introduced, and a dynamic optimization problem is formulated for process start-up strategies. The present study proposes a kinetic model based on an assumed reaction mechanism. The model describes the rates of oxygen decomposition and propagation of free radical ethylene polymerization. The mass and heat balance equations in an adiabatic tubular reactor operated at a constant pressure of 2.4 kbars and a temperature range of 110–300°C are presented. Simulations of polymerization process predict temperature of the reaction mixture, response time for cooling water, and also conversion along the reactor length. Response time was obtained using different inputs of controlled variables. Values obtained from these simulations are compared with real data from the process unit ( Polietilen, Dioki®, Zagreb, Croatia) and a model validation is confirmed. Improvement in reactor productivity and better understanding of few different start-up procedures is achieved.

Model-Based Optimization of the Cooling System of an Industrial Tubular LDPE Reactor

The model-based optimization of the cooling system of an industrial tubular reactor for the production of low-density polyethylene (LDPE) is studied in this paper. First a detailed reactor simulator is presented. Second, a series of well-posed optimization problems are formulated and solved. To this end, the optimization problems are presented with an increasing number of degrees of freedom. Moreover, economic cost functions consisting of conversion and energy terms are derived and constraints due to operational and safety reasons are added. The degrees of freedom involve parameters such as the initiator feed rates, the cooling water temperatures, and the switching position between hot and cold cooling water. The optimization results indicate that a dual water circuit operating at a low and a high temperature allows significant improvements in conversion with respect to the reference case of a single temperature circuit, while maintaining similar molecular characteristics.

Modernization of “Polimir” Process of High-Pressure Polyethylene Production at “Tomskneftekhim” Company

Results of development of high-pressure process of low-density polyethylene production at “Tomskneftekhim” Company have been presented. New technology allowed increasing productivity, product quality, extending polymer assortment. Comparison with designed indexes and the ones at other Russian and world companies has been done.

Dynamic Modeling and Simulation of Low Density Polyethylene Production – A Comparative Study

This study deals with the modelling and simula- tion of low density polyethylene (LDPE) production. LDPE is one of the most widely produced polymers which ap- pears in the form of simple goods in our everyday life (e.g. food and pharmaceutical packaging, carrier bags). It is produced in a complex industrial process which takes place under extreme operating conditions (at pressure of ca. 2000-3000 bar) and may lead to nonlinear dynamics due to highly exothermic addition polymerization reac- tions. In principle, the process is represented by a distrib- uted system with an external coordinate (the reactor length > 1000m) and various internal coordinates (the chain length of the polymer molecules, short and long chain branching and the number of double bounds), which can have strong effect on product properties. In this con- tribution a detailed reference model is introduced and possible model simplifications are discussed systematically from an on-line optimization and control point of view.

Improvements in Energy Efficiency of Polyethylene Autoclave Reactor using Advanced Process Control

This work presents how advanced process control can improve energy efficiency of an industrial plant employed to production of low density polyethylene. First, using industrial data, a phenomenological-based model is developed to infer temperature and component concentrations profiles along high-pressure autoclave reactor where polymerization occurs. Then, an advanced control strategy based on feedback linearization technique is proposed for automatic control of reactor variables. Besides promoting operational safety and productivity improvements, use of advanced control strategy allows the operations of autoclave manipulated variables closer to their constraints and, consequently, a remarkable reduction in energy consumption associated to the process is obtained.

Model based optimisation of tubular reactors for LDPE production

This paper presents a study on the model based optimisation of an industrial tubular reactor for the production of low-density polyethylene (LDPE). First a detailed reactor simulator is presented. Second, a well-posed optimisation problem is formulated. To this end, an economic cost function consisting of conversion and energy terms is derived and constraints due to operational and safety reasons are added. The degrees of freedom involve parameters such as the initiator feed rates, the cooling water temperatures and the switching position between hot and cold cooling water. The optimal design of a dual water circuit operating at a low and a high temperature achieved in this work allows significant improvements in conversion with respect to the reference case of a single temperature circuit, while maintaining similar molecular characteristics.

Detailed steady-state simulation of tubular reactors for LDPE production: Influence of numerical integration accuracy

Simulators are widely used for analyzing and optimizing the production of low density polyethylene in tubular reactors under steady state conditions. This steady state is in practice often simulated by chemical engineers using a series of CSTRs type model due to its stable behavior with respect to spatial discretization and smooth convergence for the underlying stiff model equations. Although already a large number of CSTRs is used, this number appears to be too low for the physical reality. Here, this traditional cascaded CSTR approach is compared with a plug flow type approach for a highly detailed reaction model describing the free radical copolymerization. Additionally, the influence of the discretization is rigorously investigated and quantified. It is shown that the discretization does not significantly affect the temperature and the conversion profile, but has a major impact (deviations up to 30%) on the properties which determine the end product. However, this impact of discretization is in practice often overlooked.

An Experimental Apparatus for Measurement of Elongational Flow Properties for LDPE Melt and the Effect of Testing Conditions

An experimental rig was designed and constructed for melt strength measurement and was assembled at the end of a single screw extruder used for the production of low-density polyethylene (LDPE) melt. The experimental rig was coupled with a high speed data logging system and a personal computer for the real-time measurement of melt strength. The molten LDPE was extruded through a capillary die, forming a continuous filament before being pulled down by speed-adjustable mechanical rollers until the filament failed. A digital camera was used for measuring the actual extrudate size at failure point. The drawdown forces as a function of volumetric flow rate from the extruder, roller speed, die temperature and take-up style were of interest in this study. It was found that the experimental rig could be used for accurate measurement of the mechanical strength for the LDPE melt. The experimental results suggested that the melt strength of LDPE was dependent upon the volumetric flow rate through the die from the screw extruder, roller speed, and the take-up style. For ladder-step take-up, increasing roller speed resulted in non-linear increases in the drawdown forces, the drawdown force changes being associated with the molecular disentanglement and elastic resistances of the branched LDPE melt. The drawdown forces of the LDPE melt measured under the rapid speed take-up method were 40–60% greater than those tested under the ladder-step speed take-up method, depending on the volumetric flow rate used in the screw extruder. The tensile viscosity of the LDPE melt was found to decrease slightly with strain rate and die temperature in the testing conditions used in this work.

Innovative design of an LDPE reactor bay against gas explosion using a risk-based approach coupled with high fidelity computer tools

High pressure tubular reactors for the production of Low Density Polyethylene (LDPE) are, for safety reasons, enclosed in a protective bay open towards the sky. They are becoming longer and larger due to production capacity increases. TECHNIP has historically designed these reactor bays based on Licensor information, which include more or less empirical correlations. Further capacity developments require the design of these concrete enclosures to be challenged. With this in mind, the present study shows the interest of implementing an innovative methodology based upon a risk-based approach to define new design criteria for the enclosure walls. The method, which combines both risk analysis with computational fluid dynamics modelling (3D CFD) and non-linear finite element analysis (NLFEA) tools, allows a clear determination of the design overpressure loads applied on walls and offers an optimisation of the concrete wall thickness. The CFD study also leads to a better understanding of the ventilation and efficiency of safety systems (gas detection, depressurisation) with possibility for improvement of the hazardous area classification rating.

The effect of polyethylene glycol on the characteristics of kenaf cellulose/low-density polyethylene biocomposites

Toward the development of biocomposites for packaging applications, the possibility of using kenaf cellulose (KC) was investigated in the production of low-density polyethylene (LDPE)/KC/polyethylene glycol (PEG) biocomposites. First, cellulose was extracted from the cell walls of kenaf-bast fibers. Then, different weights of LDPE, KC, and PEG were blended, and the effects of varying the concentrations of KC and PEG on the synthesis process were evaluated, and the resulting composites were characterized with respect to their mechanical, thermal, biodegradability and water-absorption properties. A scanning electron microscope (SEM) was also used to observe the surface morphology of the samples before and after biodegradation tests. The results showed that the mechanical properties of the biocomposites decreased slightly as the KC content was increased from 0 to 50 wt% in the biocomposite formulation; however, there was a good homogeneity between samples with added PEG. The addition of KC improved the thermal resistance of these biocomposites; PEG also had positive role in the thermal behavior of the composites. Based on a soil-burial test, the biodegradability of the composites showed a clear trend of increase degradation with increasing KC content in the formulation. While water-absorption values for the composites were higher than that of pure LDPE polymer, the addition of PEG to the formulation reduced the water absorption of the composites.

Parameter estimation for LLDPE gas-phase reactor models

Product development and advanced control applications require models with good predictive capability. However, in some cases it is not possible to obtain good quality phenomenological models due to the lack of data or the presence of important unmeasured effects. The use of empirical models requires less investment in modeling, but implies the need for larger amounts of experimental data to generate models with good predictive capability. In this work, nonlinear phenomenological and empirical models were compared with respect to their capability to predict the melt index and polymer yield of a low-density polyethylene production process consisting of two fluidized bed reactors connected in series. To adjust the phenomenological model, the optimization algorithms based on the flexible polyhedron method of Nelder and Mead showed the best efficiency. To adjust the empirical model, the PLS model was more appropriate for polymer yield, and the melt index needed more nonlinearity like the QPLS models. In the comparison between these two types of models better results were obtained for the empirical models.

Decompositions in the production of low density polyethylene: Reasons, consequences and prevention

In the production of low density polyethylene by free radical polymerization of ethylene at high pressure, ethylene and polyethylene occasionally decompose to carbon, hydrogen and methane resulting in an enormous increase of pressure and temperature. Huge explosions occur in a polymerization reactor and in other parts of the installation. In addition to the well known reasons of decompositions, in this work it is pointed out that the polymerization under critical entropy conditions is an additional cause of explosions, which has not been recognized and elaborated in scientific literature and industrial practice.