Comparison of catalytic ethylene polymerization in slurry and gas phase

Polyethylene (PE) with the annual consumption of 70 million tones in 2007 is mostly produced in slurry, gas-phase or combination of both processes. This work focuses on a comparison between the slurry and gas phase processes. Why does PE produced in theses two processes can show extremely different properties and extremely different reaction behaviour even if the same Ziegler-Natta (ZN) catalyst is used? Generally, it is known that the reason can be found in the differences of local conditions near active sites of ZN catalysts – the question is: which conditions are relevant? How do they interact? To answer these questions, a large number of single- and multi-stage experiments using TIBA as co-catalyst has been carried out in a 1.6-L reactor varying the following parameters: - amount of hexane (going from gas phase to slurry) - pre-contacting time catalyst – cocatalyst - hydrogen pressure (0 to 10 bar) - temperature (40 to 90°C) - ethylene pressure (1 to 12 bar) Isothermal-isobaric polymerization rate-profiles were analyzed in terms of activation and deactivation behaviour and the PE products were characterized by molecular weight distribution (MWD), particle size distribution (PSD), crystallinity and in some special cases by TEM and SEM images. This combination of methods allowed us to identify and explain a number of significant differences between slurry and gas phase processes. Finally, all these findings were concentrated in a new theoretical contribution, which we called GRAF (i.e. “Growth Rate Acceleration by Fragmentation”): 1- Gas-phase rate-profiles show rapid initiation followed by rapid decay (decay type), whereas slurry profiles show the “build up type curve” with long-lasting constant activity after initiation. 2- It was shown that hexane is not at all “inert” – it affects all the relevant transport and equilibrium conditions. Varying the amount of solvent could dramatically change the reaction rate profiles. 3- The higher the temperature, the lower the molecular weight and consequently the lower the molecular weight – common for both processes. 4- By increasing the temperature in slurry with the presence of hydrogen; the higher mobility of freshly produced polymers leads to faster crystallization. More and larger lamellae increase the brittleness of the particle. This promotes the fragmentation that can lead – in an extreme case – to fines generation. 5- Internal and external particle fragmentation, as a physical effect, generates new active sites, which in turn leads to a faster chemical reaction. 6- Faster fragmentation accompanied by faster generation of new active sites (GRAF) at a higher ethylene pressure leads to a higher initial slope of the rate curves. 7- Varying ethylene pressure either in slurry or gas phase experimentally confirmed the first order ethylene pressure dependency. 8- It was shown that increasing ethylene pressure might increase the solubility of hydrogen in the polymer structure leading to termination of more chains by hydrogen transfer. By introducing a “solubility function”, it was explained why the hydrogen concentration increases with increasing ethylene pressure. The change of the molecular weight as function of the ethylene pressure can be described by the following equation in which X is the hydrogen: ethylene pressure ratio: ........][12222mCHCptSCptAptMnPXkPkkPkAkkkM+++≈ 9- One of the most spectacular results was the “counter effect of hydrogen”. In the gas-phase, the reaction rate decreases with increasing hydrogen pressure; but the opposite effect was found in the slurry phase. 10- Hydrogen shows a similarly strong effect on the molecular weight of the polymer produced in either gas or slurry. In the absence of hydrogen, we found slightly lower molecular weights in slurry compared to the gas-phase. 11- DSC results confirm that hydrogen addition increases the level of crystallinity coupled with a simultaneous decrease in the melting temperature. This correlates with the higher chain mobility of shorter chains. Increasing the level of crystallinity can dramatically increase the production of fines in both phases and can change the particle size distribution accordingly if the brittleness of the crystalline particles and the growth stress reach critical levels (i.e. a crystallinity degree of 75%) 12- The polymer mobility is influenced by many variables such as: - temperature - chain length of the polymer produced - chain length of the dead polymer that surrounds the active sites (“matrix”) - hexane content in the amorphous part of the polymer matrix that changes the micro-viscosity. This different chain mobility leads to differences regarding the in-situ crystallinity, which has a direct impact on the particle brittleness. As a result, the particle can break at a critical growth stress that increases with the polymerization rate. This was the core result for the GRAF development. It is now very clear that this effect can affect the polymerization rate profiles in slurry and gas-phase polymerization differently due to different sorption, swelling and micro conditions around the active centres. 13- Two-stage experiments in different phases were carried out by varying the ethylene and hydrogen pressures to prove the GRAF hypothesis. A quick change of polymerization conditions (in the 2nd step) does not always lead to the same results of the one-stage experiments performed in the same conditions, since the history of the particle (defined by the 1st polymerization step) must determine the response – an effect that is explainable with GRAF. 14- Depending on which kind of PE – ductile or brittle - is produced in which step of the two-stage polymerization, one can produce particles with identical crystallinity and MWD, but with absolutely different fragmentation behaviour. 15- The hydrogen enhancement effect – in combination with the disintegration of particles leading to new active site generation – happens if hydrogen is introduced at the beginning of the polymerization. Producing ductile polymer in the 1st step decreases the fragmentation-controlled enhancement effect of hydrogen. 16- In general, the presence of ductile PE does not suppress particle fragmentation and the resulting rate enhancement completely, but the particle disintegration can still be reduced dramatically. This is a useful tool for optimizing a catalyst. 17- Removing hydrogen increases the reaction rate by the well-known “chemical effect”, for which different explanations exist. 18- The activity during the 2nd step depends strongly on what degree of fragmentation was reached in the 1st stage. However, for activation of new sites after fragmentation, the presence of the co-catalyst is required – “back-diffusion limitation”, and the “dilution effect” can partially compensate the rate accelerating fragmentation effect. 19- The lowest fines generation was found in a two-stage gas phase polymerization for bimodal PE production: the 1st step without hydrogen (making ductile PE) and the 2nd step with high hydrogen pressure (crystalline PE distributed within the ductile phase). 20- Changing the polymer matrix properties during switching from 1st to 2nd step conditions (by means of cooling, pressurizing, depressurizing, hexane evaporation, re-pressurizing) can influence both rate profiles and PSD. This is especially the case when performing the 1st step in slurry under high hydrogen pressures and the 2nd step in the gas-phase. 21- It is useful to analyze the MWD by deconvolution in terms of the GRAF hypothesis. The chain mobility plays an important role. In multi-stage polymerizations, the MWD is a fingerprint of the polymerization rate of each step: the amount of polymer produced in each step can be predicted from the MWD.