Mass transfer and agitation in the emulsion of PVDF

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High impact polypropylene: Influence of copolymerization conditions on powder and polymer properties

Isotactic Polypropylene (iPP) is more rigid than polyethylene (PE) but less tough, especially at lower temperatures. Incorporation of an elastomer of ethylene-propylene helps with this issue. Industrially this is typically done in two reaction steps. Initially iPP particles are polymerized, and afterwads a dispersed elastomeric phase is copolymerized in the still-reactive homopolymer matrix, always in the gas phase. Please click Additional Files below to see the full abstract

A new approach to stopped-flow reactions for slurry and gas-phase olefin polymerization

Given the industrial relevance of polyolefin production, it is certainly of great interest to understand the phenomena taking place during the reaction start-up. The first few seconds, or fractions of a second, are a determining moment in which morphology formation takes place and where the risk of particle overheating is at its highest. Nevertheless, the main drawback to studying these early stages is the lack of adapted apparatus that allow collecting accurate experimental data. It is, therefore, useful to have specifically designed tools for different processes. In this work, a combined approach of slurry and gas-phase systems was taken to evaluate the initial conditions of catalyst treatment for nascent olefin polymerization in supported catalysts. A rapid-quench slurry system was set up to perform polymerization reactions. Unlike previous works1, in which the stopped-flow technique was applied using separate vessels interconnected by tubes, our set-up was conceived in a single vessel under mechanical agitation and the reaction quenching was done by pressurized CO2. In parallel, a new stopped-flow gas-phase reactor has been developed for olefin polymerization, aiming to improve certain aspects of previous versions developed over the past few years in the C2P2 lab2–5. Our aims were to increase the polymer production capacity and reduce risks of temperature gradients formation inside the reactor, allowing for more accurate kinetic studies in the nascent polymerization phase. A solid porous membrane was incorporated inside the reactor, aiming to improve the gas convection and reduce the possibility of temperature gradients inside the reactor. The main goal of this study was to compare the impact of initial conditions of catalyst treatment in slurry and gas-phase under industrially relevant conditions and collect experimental data on important phenomena such as catalyst fragmentation and particle morphology development. Following steps include building up a physical interpretation of the impact of the startup conditions on initial particle overheating, reaction rate and morphology (fragmentation). (1) Liu, B.; Matsuoka, H.; Terano, M. Stopped-Flow Techniques in Ziegler Catalysis. Macromol. Rapid Commun. 2001, 22 (1), 1–24. (2) Olalla, B.; Broyer, J.-P.; McKenna, T. F. L. Heat Transfer and Nascent Polymerisation of Olefins on Supported Catalysts. Macromol. Symp. 2008, 271 (1), 1–7. (3) Silva, F. M.; Broyer, J. P.; Novat, C.; Lima, E. L.; Pinto, J. C.; McKenna, T. F. Investigation of Catalyst Fragmentation in Gas-Phase Olefin Polymerisation: A Novel Short Stop Reactor. Macromol. Rapid Commun. 2005, 26 (23), 1846–1853. (4) Tioni, E.; Broyer, J. P.; Spitz, R.; Monteil, V.; McKenna, T. F. L. Heat Transfer in Gas Phase Olefin Polymerisation. Macromol. Symp. 2009, 285 (1), 58–63. (5) Tioni, E.; Spitz, R.; Broyer, J. P.; Monteil, V.; McKenna, T. Packed-Bed Reactor for Short Time Gas Phase Olefin Polymerization: Heat Transfer Study and Reactor Optimization. AIChE J. 2012, 58 (1), 256–267

Condensed mode cooling for PE: Importance of thermodynamics in reactor and particle modelling

Heterogeneously-catalyzed gas-phase polymerization of ethylene in fluidized bed reactors (FBRs) is the single type of reactor used for polyethylene (PE) production. Due to an ever-increasing demand for PE, it is possible to substantially improve the heat removal efficiency, and consequently increase the production capacity of an FBR by exploiting the so-called condensed mode cooling method. In this method, compounds referred to as induced condensing agents (ICAs), typically C3-C8 alkanes, are added to the reactor recycle stream, (partially) liquefied in an external heat exchanger and then injected into the reactor in the form of small droplets along with the gaseous components. The fast evaporation of the liquid phase inside the reactor absorbs additional polymerization heat and thus enhances the production capacity of the unit. ICAs are chemically inert in the sense that they do not have any influence on the behavior of the active sites. Nonetheless, recent papers from our group have clearly demonstrated that the rate of ethylene polymerization (Rp) increases in the presence of ICAs. As the partial pressure of the ICA in the gas phase increases, Rp also increases. The normalized rates of polymerization with the one without any n-hexane reveals that the increase in the Rp is more pronounced at the beginning of reaction and decreases as the reaction progresses, approaching a steady-state value at the later steps. A new process model is developed based on the thermodynamics of sorption coupled with the dynamics of mass and heat transfer in a growing polymer particle. The thermodynamic simulations, performed by employing the Sanchez-Lacombe EoS, show that by increasing the partial pressure of n-hexane in the gas phase, the equilibrium concentration of ethylene in the amorphous phase of PE increases (cosolubility effect). However, at the beginning of the reaction, Rp is promoted much more strongly than would be expected from the cosolubility effect alone. According to the free-volume theory, in addition to the solubility, the diffusivity of ethylene in PE is also expected to increase by increasing the partial pressure of n-hexane (codiffusion effect). The simulations of polymer flow model (PFM), adapted to describe the dynamics of mass and heat transfer in a single growing particle, indicate that it is possible to appropriately describe the observed influence of n-hexane on Rp by taking into account the both cosolubility and codiffusion effects; during the early stages of the reaction, a specific magnitude of the enhancement in the ethylene diffusivity due to the codiffusion effect of n-hexane causes a more significant impact on the ethylene concentration gradient and therefore the efficiency (η) and overall rate of polymerization. Furthermore, the PFM simulations suggest that the addition of n-hexane has a negligible effect on the thermal behavior of the growing particles under the operating conditions used in the experiments. The presented process model is a new development in the field of modeling the process of gas phase ethylene polymerization on supported catalysts. This framework not only enables us to describe the observed polymerization rates, but also serves as a useful intellectual tool to develop improved understanding about the dynamics of the polymerization reaction in the presence of n-hexane as the ICA compound

Disentangled UHMWPE - Control of crystallization, chain entanglement and rheology via process conditions

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Disentangled UHMWPE - Control of crystallization, chain entanglement and rheology via process conditions

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Mass transfer phenomena in vinylidene fluoride emulsion polymerization

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Structural analysis of the spiroplasma virus, SpV4: implications for evolutionary variation to obtain host diversity among the Microviridae

AbstractBackground: Spiroplasma virus, SpV4, is a small, non-enveloped virus that infects the helical mollicute Spiroplasma melliferum. SpV4 exhibits several similarities to the Chlamydia phage, Chp1, and the Coliphages α3, φK, G4 and φX174. All of these viruses are members of the Microviridae. These viruses have isometric capsids with T = 1 icosahedral symmetry, cause lytic infections and are the only icosahedral phages that contain single-stranded circular DNA genomes. The aim of this comparative study on these phages was to understand the role of their capsid proteins during host receptor recognition.Results: The three-dimensional structure of SpV4 was determined to 27 å resolution from images of frozen-hydrated particles. Cryo-electron microscopy (cryo-EM) revealed 20, ∼54 å long, ‘mushroom-like’ protrusions on the surface of the capsid. Each protrusion comprises a trimeric structure that extends radially along the threefold icosahedral axes of the capsid. A 71 amino acid portion of VP1 (the SpV4 capsid protein) was shown, by structural alignment with the atomic structure of the F capsid protein of φX174, to represent an insertion sequence between the E and F strands of the eight-stranded antiparallel β-barrel. Secondary structure prediction of this insertion sequence provided the basis for a probable structural motif, consisting of a six-stranded antiparallel β sheet connected by small turns. Three such motifs form the rigid stable trimeric structures (mushroom-like protrusions) at the threefold axes, with hydrophobic depressions at their distal surface.Conclusions: Sequence alignment and structural analysis indicate that distinct genera of the Microviridae might have evolved from a common primordial ancestor, with capsid surface variations, such as the SpV4 protrusions, resulting from gene fusion events that have enabled diverse host ranges. The hydrophobic nature of the cavity at the distal surface of the SpV4 protrusions suggests that this region may function as the receptor-recognition site during host infection

Impact of geometric properties of silica supports on metallocene catalyst behavior

The objective of this work was to evaluate the effect of the physical properties of several different commercial silicas on the performance of metallocene catalysts when used in gas and slurry phase polymerization. A lot is known about how the chemistry of the silica effects the polymerization and the final product, but very little is described in the literature concerning parameters such as pore volume and pore diameter. This work dealt with these issues by using two different metallocenes in homo and copolymerization of ethylene and ethylene 1-hexene respectively. In terms of silica porosity, the metallocene/MAO catalyst supported on the silica with lower pore volume appears to polymerize faster than the one which is supported on the silica with higher pore volume. This behavior can be attributed to [email protected] the fact that the fragmentation of the growing catalyst/polymer particle with lower pore volume will be faster than its counterpart. In terms of mean particle size, if other physical properties like pore volume, pore diameter and surface area of the silica supported metallocene/MAO catalysts are kept similar along with the metal loadings, the smaller catalyst particles are more active than their bigger counterparts. This effect of particle size on instantaneous activity seems to be the same at different monomer pressures and in the presence and absence of a comonomer (like 1-hexene). Finally, the effect of pore diameter is very complex. The normal trend would be the smaller the pore diameter the faster the polymerization should be, due to the reasons explained for the pore volume. However, by using the technique we employed for the previous parameters, it was not possible to draw a valid conclusion. It seems that MAO penetration depends on the pore size, and that it might not penetrate into particles with small pore diameters

XX/XY Sex Chromosomes in the South American Dwarf Gecko (\u3cem\u3eGonatodes humeralis\u3c/em\u3e)

Sex-specific genetic markers identified using restriction site-associated DNA sequencing, or RADseq, permits the recognition of a species’ sex chromosome system in cases where standard cytogenetic methods fail. Thus, species with male-specific RAD markers have an XX/XY sex chromosome system (male heterogamety) while species with female-specific RAD markers have a ZZ/ZW sex chromosome (female heterogamety). Here, we use RADseq data from 5 male and 5 female South American dwarf geckos (Gonatodes humeralis) to identify an XX/XY sex chromosome system. This is the first confidently known sex chromosome system in a Gonatodes species. We used a low-coverage de novo G. humeralis genome assembly to design PCR primers to validate the male-specificity of a subset of the sex-specific RADseq markers and describe how even modest genome assemblies can facilitate the design of sex-specific PCR primers in species with diverse sex chromosome systems