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Polyolefins: improved property control and reactor operability

Exploitable results

Quench flow, or stop flow reactor is extremely useful in looking at very short-term (< 1s in some cases) kinetics and morphology issues. Benefits include the possibility for project partners (and others outside consortium after 11.2005) to do original testing on different types of catalysts in wide range of conditions - conditions close to industrial T and P! Dissemination will take place in the form of scientific papers, presentations at international conferences.
As part of the PhD work of Y. Banat (IPP University of Twente Enschede), Sorption models must be used in the particle growth and morphology models. The expected result is a review of the existing models and the selection of the best option for usage in the final model. This includes literature survey, model testing vs. existing data and data generation where some data are missing. Collaboration with other POLYPROP partners will allow minimising experimental work for the benefit of all partners. Detailed Result: A review of the equilibrium sorption of ethylene and a-olefins in semi crystalline polyethylene is done. The influence of crystallinity, type of polyethylene, and type of catalyst used, branching, and polyethylene structure on penetrant sorption is discussed. The Sanchez- Lacombe equation of state with temperature-dependent binary interaction parameters has been used for quantitative comparison of the studied systems. The predicted polymer swelling from S-L EOS using the optimised binary interaction parameters is compared to the measured swelling isotherms for the polyethylene samples studied. Moreover, the concentration of sorbed ethylene/a-olefins in the amorphous part of polyethylene is also predicted. An excellent match between the swelling isotherms obtained from the dilation isotherms and the values predicted by S-L EOS are obtained at all temperatures studied and for all the systems reviewed. It is found that no general correlation can be used for a precise description of the concentration or solubility of ethylene/a-olefins, even in the simplest form of polyethylene represented by HDPE. It is concluded that several structural factors, such as crystallinity, branching, type of comonomer, comonomer composition, molecular weight, molecular weight distribution and comonomer composition distribution, contribute significantly to the sorption process. Furthermore, the equilibrium sorption of gaseous propylene in semi crystalline polypropylene particles of different diameters of 100-425, 600-800 micron, and 2.4 mm was measured. The sorption measurements were performed at temperatures of 320.9 and 344.5 K and pressures up to the vapour pressure of propylene using an electromagnetic thermal balance. The swelling of polypropylene caused by the sorption of propylene was estimated from the corresponding sorption isotherms and the specific volumes of the crystalline and the amorphous polypropylene fractions, assuming ideal mixing. The measured solubilities were correlated with the Sanchez-Lacombe equation of state (S-L EOS) with temperature-dependent binary interaction parameters. The predicted polymer swelling from S-L EOS coupled with the optimised binary interaction parameters was compared to the estimated swelling based on ideal mixing. The concentration of sorbed propylene in the amorphous part of polypropylene was estimated. Unexpectedly, Henry’s law with a temperature-dependent constant could describe propylene concentration. Furthermore, the results were extrapolated to represent liquid propylene sorption in polypropylene at different temperatures using Flory-Huggins theory (FH) with temperature-dependent F-H binary interaction parameters. The results obtained were compared with well-known trends found in the literature. The measured sorption data were used for the comparison of gas and liquid phase kinetics of propylene polymerisation with a highly active 4th generation Ziegler-Natta catalyst. It was found that the same catalyst system at the same temperature and hydrogen concentration has the same relative activity in liquid pool and gas phase polymerisation.
A software module for the simulation of the olefin polymerisation in a Multizone Circulating Reactor (MZCR) was developed. The system is modelled as a series of two interconnected polymerisation zones working with different gas phase compositions. The model takes into account particle and reactor levels, as well as the particle population balance. The outputs of the model include polymer productivity, particle size distribution, molecular weights and polydispersity. Simulations show that a wide range of product characteristics can be achieved by varying the operation conditions in the reactor.
Powder and pellets behave more difficult than gases or liquids when transported or stored in chemical plants. Wrong design or operation of transport lines and storage facilities may cause blockage of these devices, resulting in plant shutdown (for cleaning) and production losses. Especially, elastomeric (rubber-like) materials tend to stick together and form agglomerates. Hence, the ¿flowability¿ of particulate solids has to be investigated already in the stage of product development. However, the ¿flowability¿ is not a physical property, such as density, but an empirical quantity containing influences of the applied measurement method. The currently available methods are more or less empirical tests, which give rather qualitative trends than quantitative indications with predictive power. The measurement method (ring-shear test) evaluated and refined within this project overcomes known limitations of other methods; e.g. undefined force fields on the flowing particles. It is expected that results obtained with this method can predict the flow behaviour of particulate solids in large-scale equipment much better. Product and plant design converge if product properties critical for plant operation (such as flowability) can be determined reliably in advance. This leads to decreased investment costs for new plants and increased on-stream time of existing plants.
Micro reactor trials on Ziegler Natta Catalyst for HDPE polymers. On a set of catalysts provided by BP (Ziegler natta, Metallocene), several experiments have been carried out on micro reactor. Shape replication of the catalysts particle was observed and quantified. Furthermore, it was proved that catalyst particles with the same size and under the same conditions show different activities. Moreover, due to catalyst/polymer particle fragmentation, it was found that this fragmentation led to formation of sub-grains, which were kept attached to the big particle. As this phenomenon is considered as being one basic step in the formation of PE agglomerates in commercial scale reactor, this experiments offers the possibility to identify the ways of overcoming the effect of fragmentation for a given catalyst. These preliminary results are quite encouraging. Next steps will be to get a temperature map to identify and quantify the particle overheating and fragmentation as a function of operating conditions.
A suite of Population Balance Models has been developed for predictions of dynamic changes in population due to effects of particle nucleation, growth, agglomeration and breakage. The module includes models as Discretization Method, Standard Method of Moments and Quadrature Method of Moments. The Discretization method predicts the changes in Particle Size Distribution directly. In order to be accurate, the number of discrete zones/classes needs to be large. This in turn will increase computational cost. If concerns are more about changes of mean values of a particle population, the latter two methods are more appropriate, especially the Quadrature method of moments has put no restrictions on forms of functions of nucleation, growth, agglomeration and breakage. The above models are fully coupled with Fluent’s various multiphase models and are capable of describing detailed distributions in both 3-Dimensions and time spaces, of particle population dynamics. The models will be offered as an important feature in the coming Fluent main software code release.
A software module was developed for 2/3 dimensional and transit simulations of localised micro mixing in Fluidised Bed Polymerisation Reactors. The module was built on the basis of CFD (Computational Fluid Dynamics) and Eulerian Multiphase techniques. Effects of mass, heat and species transfer due to mixing and polymerisation were accounted for. The models incorporated into the module are capable of describing detailed distributions of various variables/parameters involved in polymerisation both in 3-dimensions and time spaces. This includes predictions of instantaneous behaviours (instability effects) and averaged values. The validation of the models were conducted by comparing the predicted results with the data obtained on industrial/pilot-scale fluidised bed reactors, which were supplied by BP and SABIC and the satisfactory agreement was obtained. The model can be used to analyse effects of operating conditions (e.g. operating pressure, temperature, super-facial carrier gas velocity, and duration of time, etc), catalysts properties and geometrical structure of reactors, on output and properties of polymer products. The model can also be used to aid the design and maintenance of fluidised bed reactors with other types of chemistry processes.
Software Module for the Prediction of a-Olefin Diffusion Coefficient in Semi-Crystalline Polyolefins: A software module was developed for the prediction of the a-olefin diffusion coefficient in semi-crystalline polyolefins. The model was built under the Studio Fortran environment and is based on free-volume concepts in which the components in the system are envisioned to migrate by jumping into free-volume holes formed by natural fluctuations. The predictive capabilities of the free volume diffusion model were demonstrated by comparing the predictions of ethylene diffusion coefficient in PE of different crystallinities with the corresponding experimental data obtained by the Borealis BORSTAR process.
At this moment five (5) Invention Disclosures have been made covering the achievement described in Result 1. These invention disclosures are now being transformed into Patent Applications. These Patent Application will protect the work done and will give Borealis the opportunity to stay in the forefront particularly in polyethylene pressure pipe applications.
Repsol’s interest is to develop new supported metallocene catalysts to produce Polypropylene, homo and copolymers, with the typical characteristics of metallocene polyolefins but without to modify the existing commercial plants. Exploitation on pilot plant studies should be immediate. Evaluation of methods for increased/improved production will be validated on a new pilot plant starting 2004. This information will be used to test the feasibility of construction of a new commercial gas phase plant in the near future. Several supported catalysts have been prepared based on Repsol’s Metallocene Technology, using different supports and different metallocene compounds. Experimental data have been obtained to validate the models proposed by other groups in the Consortium: Kinetic data and, Evolution of catalytic particle shape and aggregated state, particle size and distribution and pore volume and distribution. Some chosen supported catalysts, due to the better morphological behaviour, were studied in polymerization. Polypropylene homopolymers were obtained at 60 and 70°C and their molecular characterization were carried out in order to know the evolution of the material properties during the polymerisation process. The performance of the developed supported metallocene catalysts was demonstrated to be better than the previous described in the state-of-the-art due to the homogeneous characteristic of the produced polypropylenes, independently of the residence time.
Sorption, Diffusivity and Swelling Measurements of a-Olefins in Polyolefins by Using a New Experimental Setup: The experimental set-up consists of a high-pressure, high-temperature feeding tang, where the desired monomer temperature is obtained, the suspension micro-balance (SMB), where sorption measurements are carried out and a high-pressure, high-temperature view cell (VC), where elongation measurements are conducted. The view cell is connected to a stereomicroscope, equipped with a high-resolution camera for continuous frame grabbing. Sample photos with respect to time are analysed using the Image Analysis-Pro software. Sample length and width are measured versus time, whereas sample thickness is measured during the measurement. The new experimental set-up is capable of working at high temperatures (up to 120oC) and high pressures (up to 300 bar), obtaining simultaneously sorption and swelling curves under isobaric and isothermal conditions.
Sorption, Diffusivity and Swelling Measurements of a-Olefins in Polyolefins by Using a New Experimental Setup: The experimental set-up consists of a high-pressure, high-temperature feeding tang, where the desired monomer temperature is obtained, the suspension micro-balance (SMB), where sorption measurements are carried out and a high-pressure, high-temperature view cell (VC), where elongation measurements are conducted. The view cell is connected to a stereomicroscope, equipped with a high-resolution camera for continuous frame grabbing. Sample photos with respect to time are analysed using the Image Analysis-Pro software. Sample length and width are measured versus time, whereas sample thickness is measured during the measurement. The new experimental set-up is capable of working at high temperatures (up to 120oC) and high pressures (up to 300 bar), obtaining simultaneously sorption and swelling curves under isobaric and isothermal conditions.
An improved micro reactor set-up that enables in situ optical observations of a single polymer particle during its growth is developed. The optical microscopy is used for direct measurement of polymer growth in gas-phase ethylene polymerization and ethylene/1-hexene copolymerization with prepolymerized Ziegler-Natta and supported metallocene catalysts. Different experiments were performed on a particle level to achieve better understanding of the catalyst performance, particle growth, fragmentation process, and the change in surface morphology during the particle growth. It was found that performing such experiments and under industrial conditions is quite possible showing good reproducibility and high activity of the two catalyst used. Shape replication of the two catalysts was observed. Furthermore, it was proved that catalyst particles with the same size and under the same conditions show different activities. Moreover, due to catalyst/polymer particle fragmentation, it was found that this fragmentation led to formation of sub-grains, which were kept attached to the big particle.
Software Module for the Prediction of Particle Collisions in a FBR. Development of a New Agglomeration Kernel: A software module of force balance was developed to predict the agglomeration of two particles in the FBR. The forces involved in the collision were compared to determine whether the particles stayed agglomerated after the collision or not. The model was built under the Studio Fortran environment and proposes the use of force comparison as the criterion for particle agglomeration by collision.
Software Module for the Simulation of the Single Particle Growth in Heterogeneous Olefin Polymerisation: A software module was developed for the simulation of the single particle growth in heterogeneous olefin polymerisation. The mathematical model, namely the random pore polymeric flow model, was built under the Studio Fortran environment and is based on the well-known polymeric flow model as well as on the Stefan-Maxwell generalized diffusion equations. The predictive capabilities of the single particle model were demonstrated by simulating: (i) the effect of initial catalyst size, (ii) the effect of the active site concentration, (iii) the effect of catalyst morphology (e.g., extent of polymer crystallinity, etc.), (iv) the effect of the hydrodynamic conditions on the particle polymerisation rate, particle growth and particle overheating of highly active Ziegler-Natta catalyst particles (e.g., fresh or prepolymer) in gas-phase olefin polymerisations.
A Software using Delphi TM 6 was developed for processing raw data of solubility and for predicting the swelling of the polymer and the concentration of a penetrant in a polyolefin. The equilibrium sorption of gaseous propylene in PP particles of different diameters was measured using an electromagnetic thermal balance. The swelling of polypropylene caused by the sorption of propylene was estimated assuming ideal mixing. The measured solubilities were correlated with the S-L EOS. The predicted polymer swelling from S-L EOS coupled with the optimised binary interaction parameters was compared to the estimated swelling based on ideal mixing. The concentration of sorbed propylene in the amorphous part of polypropylene was estimated. Unexpectedly, Henry's law with a temperature-dependent constant could describe propylene concentration. The measured sorption data were used for the comparison of gas and liquid phase kinetics of propylene polymerization with a highly active 4th generation Ziegler-Natta catalyst. Precise 'isoperibolic' polymerization rate profiles were measured in liquid pool polymerisation, while gas phase polymerisation profiles were measured using the method of 'reactive bed preparation' under isoperibolic conditions. It was found that the same catalyst system at the same temperature and hydrogen concentration has the same relative activity in liquid pool and gas phase polymerization. Furthermore, a review of the sorption of ethylene and α-olefins in PE is done. The influence of crystallinity, type of polyethylene, and type of catalyst used, branching, and polyethylene structure on penetrant sorption is discussed. The S-L EOS has been used for quantitative comparison of the studied systems. The predicted polymer swelling from S-L EOS using the optimised binary interaction parameters is compared to the measured swelling isotherms for the polyethylene samples studied. An excellent match between the swelling isotherms obtained from the dilation isotherms and the values predicted by S-L EOS are obtained for all the systems reviewed. It is found that no general correlation can be used for a precise description of the concentration or solubility of ethylene/α-olefins. It is concluded that several structural factors, such as crystallinity, branching, type of comonomer, comonomer composition, molecular weight, molecular weight distribution and comonomer composition distribution, contribute significantly to the sorption process.

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