High-pressure Tubular Reactor Research Articles

Modeling and Simulation of a Multizone Circulating Reactor for Polyethylene Production with Internal Cooling.

Polyolefins play a role in industries and are typically manufactured using two types of reactors: high-pressure tubular reactors and fluidized bed reactors. An innovative technology called the Multizone Circulating reactor (MZCR) has emerged, which introduces an innovative approach with interconnected polymerization zones creating a continuous loop of polymer flow. This study focuses on modeling and simulating ethylene gas phase polymerization within the MZCR in the presence of internal cooling to gain insights into its behavior. To achieve this, a comprehensive computational fluid dynamics (CFD) simulation was developed. It considered momentum, material, and energy balance aspects. The model equations were solved using the finite difference method in COMSOL Multiphysics version 6.1. The investigation primarily focused on studying the impact of incorporating a cooler into the riser section on the temperature profile within the reactor and ethylene conversion. The presence of this cooler resulted in a reduction in temperature change along the riser from approximately 8.0 °C to 4.0 °C. Moreover, it led to an increase of 7%, in ethylene single-pass conversion.

Low-density polyethylene simulated production in high pressure tubular reactor

The importance of the LDPE polymerization process has spawned a slew of LDPE tubular reactor modelling and simulation projects. Through adjusting input parameters, the sensitivity study of input parameters in an industrial low density polyethylene tube reactor was addressed in this paper. This work's innovation was highlighted with the incorporation of the LDPE melt flow index in the reactor output. The highest temperature peak and monomer conversion were obtained when initiator concentration were increased by 1.06 factor, whereas the highest melt flow index was obtained when the inlet temperature was increased by 1.07 factor. The monomer flow rate, initiator flow rate, and solvent flow rate all had a substantial impact on the LDPE process performance in a tubular reactor.

Mathematical Modelling of Rheological Properties of Low-density Polyethylene Produced in High-Pressure Tubular Reactors

Abstract Low-density polyethylene (LDPE) is a commodity polymer widely used in a variety of applications. Due to the complexity of its molecular structure, which includes molecular weight distribution and randomly distributed long-chain branching, its rheological behavior is varied. In the present work, a mathematical model is used to predict molecular and rheological properties of LDPE produced in a high-pressure tubular reactor under different operating conditions. The agreement between flow curve predictions and experimental data is very good, demonstrating the potential of this model as a predictive tool for industrial producers.

Scaling Up of Continuous Mineral Carbonation Reactor for the Production of High Value PCC Products

The concept of supercritical CO2 mineral carbonation reaction was firstly proven and demonstrated in the laboratory scale using a 100cc high pressure PVT recombination chamber. Successful proof of concept led PETRONAS to scale up this reaction by developing a working prototype mineral carbonation reactor with the intention of demonstrating the continuous operation of the supercritical CO2 mineral carbonation process. The setup comprising of a feedstock tank, a high pressure liquid pump, a 0.8L high pressure tubular reactor, injector spray nozzle was thoroughly tested to understand the feasibility and robustness of the prototype reactor design and configuration. Another purpose was to study the effect of varying parameters such as reactor pressure, feedstock injection flow rate and compositional effect of methane in carbon dioxide gas reactant to the properties of the precipitated calcium carbonate produced from this process scheme. In summary, the setup was successful to demonstrate the concept of continuous supercritical mineral carbonation reaction up to 1.5 litre/min feedstock injection flow rate. The effect of reactor working pressure and feedstock injection flow rate was the main factors determining the properties and quality of precipitated calcium carbonate produced. Particle size distribution of the calcium carbonate precipitates produced from this setup averaged approximately 6 microns in size. Other parameters such as gas composition and recycling of carrier fluid sucrose have less effect on the properties of the product.

LDPE Production in Tubular Reactors: Comprehensive Model for the Prediction of the Joint Molecular Weight-Short (Long) Chain Branching Distributions

Good control on molecular properties, and as a consequence on end-use properties, is very important for low-density polyethylene (LDPE) manufacturers. However, the connection between the architecture of polymer chains and the kinetic mechanism and polymerization conditions is still a subject of study. In this work, we present a comprehensive model of the polymerization of ethylene in high-pressure tubular reactors. In addition to the usual predictions of conversion, temperature profiles, and average molecular properties, this model also provides bivariate distributions such as molecular-weight–long-chain branching distribution and molecular-weight–short-chain branching distribution of LDPE produced under different operating conditions. The 2D probability-generating function technique is applied to obtain the bivariate distributions. This is a deterministic technique that allows the calculation of the distributions without any prior assumption of their shape.

Mathematical Tools and Approaches for Polymerization Reaction Engineering II— Statistical Modeling Tools and Approaches

Mathematical Tools and Approaches for Polymerization Reaction Engineering II— Statistical Modeling Tools and Approaches

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.

Optimal Design of Inlet Structure of High-Pressure Tubular Reactor Jacket

Flow characteristic of the desalted water influenced by three inlet structures in different diameter nomal tube, equal diameter nomal tube and tangential tube inlet of the jacketed tube in high-pressure tubular reactor was investigated based on computational fluid dynamics. The results show that the desalted water flow forms a region where the flow velocity is close to zero in the different diameter nomal tube structure and temperature of the desalted water in this region raises notablely. It is also found that the peaks of the shear stress occur at wall inside near inlet of the jacket. The other structures avoid in large extent advent of the former two phenomena and obviously reduce the shear stress value at wall. From the results, the second inlet structure is better than the third and the third better than the first.

Prediction of the molecular and polymer solution properties of LDPE in a high-pressure tubular reactor using a novel Monte Carlo approach

In the present work, a novel kinetic/topology Monte Carlo algorithm is developed for the prediction of molecular, topological and solution properties of highly branched low-density polyethylene (LDPE), produced in a high-pressure multi-zonal tubular reactor. It is shown that the combined kinetic/topology MC algorithm can provide comprehensive information regarding the distributed molecular and topological properties of LDPE (i.e., molecular weight distribution, short- and long-chain branching distributions, joint molecular weight-long chain branching distribution, branching order distribution, seniority/priority distributions, etc.) The molecular/topological results obtained from the MC algorithm are then introduced into a random-walk molecular simulator to calculate the solution properties of LDPE (i.e., the mean radius of gyration, R g , and the branching factor, g) in terms of the chain length of the branched polyethylene. The validity of the commonly applied approximation regarding the random scission of highly branched polymer chains is assessed by a direct comparison of the average molecular properties of LDPE (i.e., number and weight average molecular weights), calculated by the combined kinetic/topology MC algorithm, with the respective predictions obtained by the commonly applied method of moments (MOM). Through this comparison it is demonstrated that the ambiguous implementation of the random scission reaction in the MOM formulation can result in erroneous predictions of the weight average molecular weight and MWD of LDPE. Finally, the effects of two key process parameters, namely, the polymerization temperature profile and the solvent concentration, on the molecular, topological and polymer solution properties of LDPE produced in a multi-zonal tubular reactor are investigated.

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.

Residual Stress in an Autofrettaged Tube Taking Bauschinger Effect as a Function of the Prior Plastic Strain

Autofrettage is a practical method for increasing the elastic carrying capacity and the fatigue life of thick-walled cylinders such as cannon and high-pressure tubular reactor. Many analytical and numerical solutions for determining the residual stress distribution in an autofrettaged tube have been reported. It is still difficult to model the Bauchinger effect, which is dependent on the prior plasticity in an analytical solution. The reduced Young’s modulus during unloading affects residual stress distribution. However, until now this effect has not been considered in any analytical model. In this paper, an autofrettage analytical solution considering Young’s modulus and the reverse yield stress dependent on the prior plasticity, based on the actual tensile-compressive curve of the material and the von Mises yield criterion, has been proposed. New model incorporates the Bauschinger effect factor and the unloading modulus variation as a function of prior plastic strain, and hence of the radius. Thereafter it assumes a fixed nonlinear unloading profile. The comparison of predicted residual stress distribution by the present solution with that of fixed unloading curve model, and test results shows that the present solution gives accurate prediction of residual stress distribution of an autofrettaged tube. This analytical procedure for the cylinder permits an excellent representation of various pressure vessel steels.

Modelling industrial scale high-pressure-low-temperature processes

The existence of metastable phases of ice I and liquid in the domain of ice III permits the use of optimized paths for pressure shift freezing (PSF) and pressure induced thawing (PIT) processes, crossing the ice III domain, without actual crystallization of this ice polymorph. The use of higher pressures and lower temperatures before pressure release, in the case of PSF, and the use of higher pressures for PIT, increases the real temperature gradients of both processes, therefore reducing the total processing time. In this paper, a computational model has been developed to simulate heat transfer phenomena during the precooling (in PSF) and the heating (in PIT) steps of these optimized processes. The high-pressure-low-temperature (HPLT) processes are assumed to be conducted in a high-pressure tubular reactor designed to work in a semi-continuous mode on an industrial scale. Results showed that important vessel lengths (40–370 m) are needed when the sample velocity ranges from 1 to 6 m/min. These vessel lengths are feasible thanks to the modular structure of the suggested high-pressure system.

A comprehensive dynamic model for high-pressure tubular LDPE reactors

A comprehensive dynamic model for high-pressure tubular LDPE (low-density polyethylene) reactors (HPTLR) is presented. The model is capable of predicting molecular weight distribution (MWD) characteristics of the polymer and the influence of process conditions on the same. The physico-chemical phenomena considered in the model include polymerization kinetics, monomer (ethylene) decomposition kinetics, turbulent micromixing and heat transfer. A two-environment (2E) model is used to describe micromixing following the approach proposed originally by Fox (1998). Since the model considers the effect of ethylene decomposition on the dynamic behavior of the reactor, dynamic changes in the process conditions leading to ethylene decomposition (decomp) and reactor runaway are studied. Furthermore, a consistent dynamic model for pressure-pulsing due to kick-valve operation (Zabisky, 1992) has been included, making the model more relevant to plant operations. Model parameters have been fit to plant data in order to satisfactorily emulate plant behavior. The model can thus be reliably used in off-line studies and should prove to be an effective tool to the plant engineer in making critical operational decisions.

Mathematical Modeling of Free-Radical Ethylene Copolymerization in High-Pressure Tubular Reactors

A comprehensive mathematical model is developed for the free-radical copolymerization of ethylene with various comonomers (e.g., vinyl acetate, methyl or ethyl acrylate, and acrylic or methacrylic acid) in high-pressure tubular reactors. Polar copolymers usually exhibit lower crystallinity and yield strength than low-density polyethylene grades and are used for applications requiring flexibility, toughness, stress-cracking resistance, and adhesion to coatings. In the present study, a detailed kinetic mechanism is proposed to describe the molecular and compositional developments in the free-radical copolymerization of ethylene with a comonomer. On the basis of 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. The model equations are coupled with a set of algebraic equations for estimating the thermodynamic and transport properties of the reaction mixture. The number ...

Modeling, Simulation and Optimal Control of Ethylene Polymerization in Non-Isothermal, High-Pressure Tubular Reactors

Low-density polyethylene (LDPE) is a very important polymer, which is usually produced through a free radical polymerization process in high-pressure tubular reactors. In this study, the mathematical model of this process is developed on the basis of a comprehensive reaction mechanism to accurately determine polymerization rate, and polymer properties under extreme temperature conditions. The model comprises of (i) the mass balances of monomer, initiator, solvent, live radicals and dead polymer chains, and (ii) the energy balances of a tubular LDPE reactor, and its jacket under steady state condition. The model considers the variations in the density and viscosity of reactants along reactor length. Computer simulation of the model is successfully carried out to accurately describe the performance of a non-isothermal, high-pressure, industrial LDPE reactor with multiple initiator injections. Based on the developed non-linear model, the optimal control of the industrial reactor is carried out using genetic algorithms to maximize monomer conversion using jacket temperature as a control function of reactor length. The application of optimal control leads to significant improvements in the final monomer conversion, and capacity utilization of the reactor.

Fatigue strength and crack propagation life of in-service high pressure tubular reactor under residual stress

In the production of a tubular reactor for a high pressure polyethylene plant the autofrettage process was used to produce a compressive residual stress at bore, which will increase the fatigue strength of the reactor. The autofrettage residual stress in the reactor will, however, gradually relax during operation. It is necessary to investigate the influence of residual stress relaxation on the fatigue strength of the reactors. In the design only fatigue crack initiation life was considered as its fatigue strength. What the fatigue crack propagation life is was not known. In this paper, fatigue crack propagation life of a reactor with residual stress was analysed using Paris' equation. Based on the results of an internal pressure fatigue test on a reactor with inner and outer surface artificial notches, the effects of residual stress and suitable SIF expression were also analysed. The autofrettage residual stress can greatly increase crack initiation life of plain reactors. However, since crack propagation life of the reactors is short, effort should be taken to prevent crack initiation on the inner or outer surface of the reactor caused by other factors.

Heat transfer coefficient in a high pressure tubular reactor for ethylene polymerization

AbstractHeat transfer in tubular reactors for the high pressure polymerization of ethylene is very complex, since these tubular reactors are usually divided into several zones that exhibit different flow patterns and critical fouling behavior. The correct estimation of the overall heat transfer coefficient along the reactor axial distance is a major issue when assessing the predictive capabilities of a mathematical model for the process. In general, previous models employed either constant heat transfer coefficients or the usual correlations for the Nusselt number. Neither of these two approaches is accurate enough to allow a correct prediction of the reactor behavior with respect to temperature profiles and product molecular properties. The present work performs a more comprehensive estimation of the heat transfer coefficient in these reactors. At a first stage the overall heat transfer coefficients were estimated by using approapriate energy balances and a good set of experimental data. Then, a predictive model was proposed for the overall heat transfer coefficient. All flow regimes, as well as fouling effects, were taken into account, and the parameter estimation was based on temperature profiles obtained from an industrial reactor. The temperature profiles, conversions, pressures and molecular properties calculated by means of the experimentally fitted heat transfer coefficients or with the predictive model showed good agreement with plant data.

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.

Product design through multivariate statistical analysis of process data

A methodology is developed for finding a window of operating conditions within which one should be able to produce a product having a specified set of quality characteristics. The only information that is assumed to be available is that contained within historical data on the process obtained during the production of a range of existing product grades. Multivariate statistical methods are used to build and to invert an empirical model of the existing plant operations to obtain a window of operating conditions that are capable of yielding the desired product, and that are still consistent with past operating procedures and constraints. The methods and concepts are illustrated using a simulated high pressure tubular reactor process for producing low density polyethylene

On-line optimization of a high-pressure low-density polyethylene tubular reactor

Low-density polyethylene (LDPE) is a high-volume commodity polymer that is produced in high-capacity tubular and autoclave reactors. The extreme operating conditions employed in the high-pressure free-radical polymerization of ethylene, and the tight profit margins of the LDPE market provide a strong economic incentive to seek the optimal operation of a high-pressure LDPE reactor. In the present work, an on-line multi-level optimization procedure is developed for a high-pressure tubular LDPE reactor based on a comprehensive steady-state mathematical model of the process. Following the well-established separation hypothesis, the problem is hierarchically decomposed in two phases: (i) the parameter estimation phase where certain key model parameters are adjusted, based on selected process measurements, in order to eliminate any process/model mismatch; (ii) the optimization phase where the adapted model is optimized by minimizing a specified objective function defined in terms of the ethylene conversion and the “quality” of the final polymer product. The proposed optimization algorithm is successfully tested against the operation of a large-scale industrial reactor. The most prominent features of the procedure are highlighted. These include the development of a comprehensive mathematical model of the process, the disturbance oriented selection of the adjustable model parameters, the appropriate spatial decomposition of the parameter estimation problem into a number of subproblems and the conditional structuring of the optimization problem.