The role of clusters in gas-solids reactors. An experimental study.

This PhD-work is meant to determine the contact efficiency experimentally for fluidization of fine particles over a wide range of superficial gas velocities (dp<200 mm and 0.1<ug<4.5 m/s) in bench scale units. A special measuring method has been developed based on the application of rapid oxidation of carbon monoxide over a Pt-catalyst, as a model reaction. For this reaction, it can be recognized immediately whether the conversion is determined by the intrinsic reaction kinetics or by any mass transfer processes. A change of the conversion rate controlling mechanism corresponds to a large change in the reaction order of CO, viz. from minus one to plus one. As a second tool of this study, dilution of the active catalyst with similar but inert particles has been used to investigate the influence of the local reaction rate on the contact efficiency

Mechanistic studies of first-row homogeneous transition metal catalysts

In our day-to-day lives, we are reliant on catalytic processes to produce a wide range of products. Due to the paramagnetic nature of the involved intermediates, the mechanism through which these catalysts operate is often poorly understood. In this thesis, using a variety of spectroscopic techniques (e.g. XAS and EPR), a deeper understanding of two important catalytic reactions is obtained. The first part of this thesis focusses on the chromium-catalyzed selective oligomerization of ethene. Typically, a trivalent chromium precursor is activated with excess alkylaluminium reagent. In Chapter 3 , a chromium-pyrrolyl system developed by Chevron Phillips Chemical is studied. The activation and deactivation pathways are described in detail. In Chapter 4 , the oxidation state of the active species in the [(R-SN(H)S-R)CrCl3] system is investigated. Using three different activators, the (electronic) structure of chromium after activation in the absence and presence of a suitable substrate (ethene, other alkenes and alkynes) is elucidated. Plausible models of the active species are studied through DFT calculations. In Chapter 5 , the roles of the ligand and activator in the activation process for chromium complexes containing phosphine ligands are studied. Complexes are prepared with and without an ortho-methoxy substituent within the ligand framework and their activation with AlMe3 and MMAO is studied. The second part of this thesis focusses on the copper-catalyzed azide-alkyne cycloaddition. In Chapter 6 , novel cationic copper complexes containing iminophosphorane ligands are detailed. Using a combination of spectroscopy, kinetics and DFT calculations, the resting state and the rate-determining step are identified

Processing in intractable polymers using reactive solvents: 3. Mechanical properties of poly(2,6-dimethyl-1,4-phenylene ether) processed by using various epoxy resin systems

The rather intractable polymer poly(2,6-dimethyl-1,4-phenylene ether) (PPE) can easily be processed by using epoxy resin as a reactive solvent. In this reactive solution processing technique, PPE is dissolved in epoxy resin at elevated temperatures and processed. After processing, the epoxy resin is polymerized and phase separation accompanied by phase inversion is initiated and the reactive solvent is subsequently integrated in the final material. In this paper, attention was focused on the possibility of tuning the properties of the in situ polymerized dispersed epoxy phase. A solvent system was studied which consisted of epoxy resins and diamine curing agents, based on bisphenol and poly(propylene oxide). Both resins could be used as a solvent for PPE and the resulting processable solutions exhibited upper critical solution temperature behaviour. Upon increasing the poly(propylene oxide) content in the reactive solvent system the properties of the dispersed phase could be varied gradually from non-ductile glassy to completely rubbery, and consequently the properties of the PPE/epoxy could be controlled over a broad range. The presence of a non-ductile glassy dispersed phase (with yield stress yield stress of PPE) resulted in an increase in the yield stress of the material and was shown to constrain yielding of the PPE matrix. Reduction of the yield stress of the dispersed phase facilitated ductile deformation of PPE in tensile loading but resulted additionally in a reduction in toughness. After changing the properties of the dispersed epoxy phase to completely rubbery a substantial increase in toughness was obtained. Interestingly, the rubber with the lowest level of adhesion proved to be the most efficient impact modifier for PPE

Processing of intractable polymers using reactive solvents: 1. Poly(2,6-dimethyl-1,4-phenylene ether)/epoxy resin

A new processing route for poly(2,6-dimethyl-1,4-phenylene ether) (PPE), an intractable polymer on account of its thermal and oxidative sensitivity, was explored. PPE can be dissolved at elevated temperatures in epoxy resin and these solutions can then be processed at temperatures as low as 175°C. For solutions of PPE with a molecular weight of 10, 20 and 30 kg mol-1, the phase diagram and the flow curves in the homogeneous region were determined. The upper critical solution temperature (UCST) cloud point curves intersect the glass transition-composition lines at a PPE content of 70 wt%. Below this composition, thermoreversible gelation is observed upon cooling which prevents complete phase separation. Curing of the homogeneous solutions, using diethyltoluene diamine, resulted in virtually complete phase separation. In the composition range that was studied (30–70 wt% PPE), the chemically induced phase separation is accompanied by phase inversion, yielding a final morphology of epoxy spheres dispersed in a PPE matrix. Thus, after processing, the (reactive) solvent is converted into a dispersed phase. The mechanical and thermal properties of the final materials, such as toughness and glass transition temperature, are dominated by the continuous PPE matrix

Mass transfer and influence of the local catalyst activity on the conversion in a riser reactor

Gas-solids contacting in risers has been studied based on measurements of mass transfer controlled CO oxidation over a Pt/γ-alumina catalyst, and on experimental results published by Ouyang et al. (1995) for the kinetically controlled ozone decomposition. In the present experiments, the catalyst activity was varied by mixing the active catalyst particles with similar, but inert γ-alumina particles (in ratios from 150 to 2500 m3inert/m3cat), whereas Ouyang and co-workers varied the operating temperature (de 300 à to 500 K). Mass transfer controlled CO oxidation occurs at temperatures above 750 K. A negative square root dependency has been observed for the relationship between the Sherwood number and the solid hold-up. Increasing the gas velocity always improves the gas-solids contacting. The local catalyst activity appears to be an important parameter. As an important conclusion of the present work, it can be stated that at a high local activity, the conversion rate per unit volume of catalyst decreases significantly due to local depletion of reactant

Gas solid contacting measurements in a turbulent fluidized bed by oxidation of carbon monoxide

The conversion rate of the mass transfer controlled oxidation of CO over a Pt/gamma-alumina catalyst (d(p) = 65 mu m) has been studied in a fluidized bed (internal diameter = 0.05 m) operated close to and in the turbulent fluid bed regime. The objectives were to investigate the gas-solids contacting efficiency to evaluate the conversion data in terms of overall mass transfer coefficients and define the apparent contact efficiency. At high superficial gas velocities, the concept of formation of particle agglomerates and voids is more realistic than the two-phase model considering discrete bubbles and a dense phase. The two-phase model is not useless but has hardly any relation with the real flow pattern in the turbulent regime