Theoretical prediction of the minimum stirrer speed in mechanically agitated suspensions

The objective of this article is to derive theoretical equations for the minimum stirrer speed in vessels containing suspensions. There are a variety of equations for the prediction of this stirrer speed (Zwietering, Chem. Eng. Sci. 8 (1958) 244–253; Kneule and Weinspach, Verfahrenstechnik 1 (12) (1967) 531–540; Nienow, Chem. Eng. Sci. 23 (1968) 1453–1459; Einenkel, VDI-Forschungsheft Nr. 595, Düsseldorf, 1979; Einenkel and Mersmann, vt-Verfahrenstechnik 11 (2) (1977) 90–94; Niesmak, Thesis TU Braunschweig, 1982; Molerus and Latzel, Chem. Eng. Sci. 42 (6) (1987) 1423–1437; Zehner, Chem. Ing. Technol. 58 (10) (1986) 830–831). However, nearly all of them are derived from experimental results obtained in a limited range of vessel size and suspension properties [1, 2, 3]. Some relationships based on experiments carried out in small vessels are contradictory and not useful for scale-up. Reliable models are necessary not only for the design of stirred vessels but also for the reduction of power consumption. It is important to distinguish between two decisive processes: the ‘avoidance of settling’ and ‘off-bottom lifting’.     

Methanol in microporous materials from first principles

Zeolites are amongst some of the most important heterogeneous catalysts in use commercially today, combining acid–base catalysis due to the presence of Brønsted acid sites with shape selectivity resulting from the microporous environment [1]. Despite this, there is still a great deal of uncertainty concerning the mechanisms of many of the processes which are known to occur and the way in which the zeolite accelerates them. While much information has been obtained from experimental techniques, including infra-red spectroscopy and magic angle spinning NMR [2], there is presently a need for models and reaction pathways to aid in their interpretation. Here theoretical methods are playing a major role in the field of microporous materials.     

Estimating correlations for the effective thermal conductivity of granular materials

Based on the well-known solution of Maxwell [1] for the thermal conductivity of dilute dispersions of spheres and the improved form of Maxwell’s equation developed by Chiew and Glandt [2], two general correlation for k_eff estimation are proposed. One of them apply to medium dense dispersions (0.15≤φ≥0.85) and the other one for dense porous materials (φ>0.9). Both correlating equations encompass a wide range of phase conductivity ratio values. The comparison with experimental measurements shows very good agreement.     

Mossgas “gas-to-liquid” diesel fuels—an environmentally friendly option

This paper presents a brief summary and comparison of heavy vehicle emissions using Mossgas synthetically derived diesel as opposed to a US Regular Federal 49-state number 2 diesel fuel. A series of engine dynamometer and heavy-duty chassis dynamometer tests were performed at West Virginia University early in 1999.

The Mossgas gas-to-liquid (GTL) low sulphur diesel fuel is produced primarily by the conversion of olefins to distillate (COD) process in conjunction with a high temperature Fisher–Tropsch technology process.

Findings were that there were significant emission benefits when using Mossgas GTL diesel fuel. Oxides of nitrogen (NO_x) and particulate matter (PM) emissions were substantially lower when compared to normal fossil fuel-derived diesel. Additionally the benefits have been found to cut across different engine technologies, various test cycles, engine age (1991–1999), as with or without engine aftertreatment catalysts.

The Mossgas GTL diesel meets the world-wide challenge to produce environmentally friendly transportation fuels and sets the benchmark for future diesel specifications. Accordingly Mossgas has petitioned the US Department of Energy to register this fuel as an alternative fuel under its EPACT program [1 and 2].     

Classification of the asphalts and their source rock from the Dead Sea Basin (Israel): the asphaltene/kerogen vanadyl porphyrins

The asphaltenes of the asphalts from the Dead Sea Basin were examined for the occurrence of vanadyl porphyrins. These examinations demonstrate that the asphalts fall into two broad class types: one which exhibits a relatively high vanadyl porphyrins content (>300 ppm), and the other characterized with no vanadyl porphyrins (<10 ppm). It is concluded that these asphalts belong to two distinct types and have separate origins. The kerogens isolated from the petroleum-source rock of the Dead Sea Basin were also analyzed. Two genetic types of kerogen appeared to exist: a marine one highly enriched with these pigments and a terrestrial one with no vanadyl porphyrins. For comparison, the asphaltenes of typical asphaltic crude oils from Western Venezuela and kerogen from their La Luna source rock were also examined. Previous detailed geochemical studies[1] indicated that the La Luna kerogen is derived from marine organic matter source. These materials have higher concentrations of vanadyl porphyrins than the Dead Sea asphaltenes and kerogens are enriched with these compounds. Our results strengthen the potential of vanadyl porphyrins of both petroleum asphaltenes and source-rock kerogen for use in genetic and correlation studies.     

Aluminous cements containing magnesium aluminate spinel from Egyptian dolomite

Three mixes of calcium aluminate cements containing MA spinel were prepared using appropriate mixtures of Egyptian dolomite (MgO, 20.16% and CaO, 31.32%) with active alumina (99.50% A)¹. The cement mixes were prepared at 1600°C using the sintering method. The products were finely ground and their chemical and mineralogical compositions were investigated using the appropriate techniques. Also, their physicomechanical and refractory properties had been determined. The results indicated that their mineralogical compositions were refractory MA spinel, in addition to CA and/or CA₂ phases depending on the composition of the starting materials. The prepared cements exhibited a compromise between considerable strength and higher refractoriness. When 10% of such cements were added to refractory grade magnesia aggregate, in the presence of 0.1% Li₂CO₃ as a strength modifier, refractory castable bodies with improved hot-strength and thermal shock resistance had been achieved.     

Design of coating resins by changing the macromolecular architecture: solid and liquid coating systems

An increased demand for new and improved coating systems, both due to environmental as well as performance reasons, have appeared during the last decades. Techniques such low temperature curing powder coatings, radiation curable systems, and high solids have gained an increased interest and obtained significant market shares. Although improved in many aspects, these systems still have limited use in certain applications due to technical reasons.

One way to change the properties of thermoset resins that has obtained significant interest during the last decade is by changing the molecular architecture of the resin. An example of polymers which exhibit different properties compared to conventional linear structures are highly branched, dendritic, polymers [1, 2 and 3]. These polymers, for example, exhibit a higher solubility and lower melt viscosity compared to their linear counterparts. They can also be tailored with respect to functionality and polarity to adjust the properties for certain applications. Coating resins based on hyperbranched polymers have been described both for liquid UV-curable systems as well as powder coatings [4, 5 and 6].

This presentation will focus on how properties of resins based on dendritic polymers can be tailored and how this can applied to coating systems such as powder coatings, radiation curable resins and no-solvent liquid systems. The correlation between resin structure and its properties both before and after cure will be discussed.     

Studies on segregation of binary mixture of solids in continuous fast fluidized bed: Part II. Effect of particle size

Experimental investigations on the continuous fast fluidized bed were extended for size segregation of binary mixture of solids in the column with air as the separation medium [1]. The feed is either jetsam-rich or flotsam-rich binary mixture of particles of different size, but same density. The variables tested were superficial gas velocity, solids feed rate and feed characteristics. At steady state, there exists physical equilibrium between the evolved flotsam and the residual jetsam when the granular solids are in fluid-like state as in the case of density segregation of solids [1]. Using the analogy of the binary liquid mixtures separation by distillation, the phase diagram was constructed from the experimental observations. The effect of solids feed rate, feed composition, particle size ratio and particle size on equilibrium distribution of the flotsam and jetsam were presented.     

Desulphurization of coals and chars by treatment in various atmospheres between 400 and 600 °C

Desulphurization of fifteen US coals of rank ranging from anthracite to sub-bituminous B and five high-temperature chars by carbon monoxide and other gases and gas mixtures between 400 and 600 °C has been studied. The sulphur content of the parent coals ranged between 3.0 and 7.3% and that of the chars between 1.3 and 3.8% by weight. A comparison between air, nitrogen, carbon monoxide, and steam-carbon monoxide mixtures as desulphurizing gases shows the order of desulphurizing ability as

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.     

Syngas in perspective

Syngas manufacture is an important part of a Fischer–Tropsch (FT) plant as it is responsible for ca. 60% of the investments. The developments of syngas technologies are summarized in Section 1. The present choice for large-scale FT plants based on natural gas is autothermal reforming. The requirements to the syngas and the state of art are summarized. The main costs of an ATR unit are related to the manufacture of oxygen. Air-blown technologies are discussed and compared with ATR.     

Difference points in reactive and extractive cascades: IV—Feasible regions for multisection columns with kinetic reactions and side streams

The feasibility of three component distillation columns containing kinetically controlled chemical reactions and side streams is addressed through the use of difference points. We decompose complex columns into sections and identify feasible regions in composition space where each section operates. These regions enclose sectional profiles for the full range of operating parameters and are bounded by profiles constructed under extreme conditions. We efficiently determine the bounding profiles without computing interior compositions. We also develop a graphical feasibility test to connect sections and construct full columns using feasible regions. Finally, we present a technique to characterize the feasible regions and quantify how much reaction or side stream may be placed on any stage of a column section. The first three papers in this series [Hauan et al., 2000. Difference points in extractive and reactive cascades. I—Basic properties and analysis. Chemical Engineering Science 55 (16) 3145–3159; Lee et al., 2000. Difference points in extractive and reactive cascades. II—Generating design alternatives by the lever rule for reacting systems. Chemical Engineering Science 55 (16) 3161–3174; Hoffmaster and Hauan, 2004. Difference points in extractive and reactive cascades. III—Properties of column section profiles with arbitrary reaction distribution. Chemical Engineering Science 59 (17) 3671–3693] derive the fundamental properties of difference points, develop a lever rule to facilitate full column design, and analyze the properties of sectional profiles in reactive cascades. This fourth paper develops the necessary feasibility analysis tools for systematic identification of reactive distillation design alternatives.     

The effects of pH on the molecular distribution of water soluble ruthenium(II) hydrides and its consequences on the selectivity of the catalytic hydrogenation of unsaturated aldehydes

The effect of pH on the formation and equilibrium distribution of the water soluble ruthenium hydrides [HRuCl(TPPMS)₂]₂, [HRuCl(TPPMS)₃] and [H₂Ru(TPPMS)₄] (TPPMS=(3-sulfonatophenyl)diphenylphosphine sodium salt) was studied in aqueous solution by pH-potentiometric and

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and

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NMR methods. Depending on the pH, [RuCl₂(TPPMS)₂]₂ and its hydrido-derivatives hydrolyse extensively, giving rise to formation of hydroxo-ruthenium complexes. It was established that at pH≤3.3 the dominant ruthenium(II) species was [HRuCl(TPPMS)₃], while at pH≥7 it was [H₂Ru(TPPMS)₄]. While [HRuCl(TPPMS)₃] catalyzed the slow, selective hydrogenation of the C=C bond in trans-cinnamaldehyde, [H₂Ru(TPPMS)₄] was found an active and selective catalyst for C=O reduction. Consequently, the selectivity of the hydrogenation of trans-cinnamaldehyde could be completely inverted by minor changes in the solution pH, shifting the equilibrium between [HRuCl(TPPMS)₃] and [H₂Ru(TPPMS)₄].     

Palladium(0) allylic alkylation in a two-phase system or with a supported aqueous phase catalyst

The reaction of allylic carbonates with various acyclic and cyclic carbonucleophiles is catalyzed by the system Pd(OAc)₂ and P(C₆H₄-m-SO₃Na)₃ (or tppts) in a two-phase liquid medium H₂O-nitrile, the activity of the catalyst depending mainly on the nature of the nitrile, the temperature of the reaction and the ratio palladium/tppts. The same system Pd(OAc)₂ and P(C₆H₄-m-SO₃Na)₃ supported on silica catalyzes also this reaction. The formation of the active palladium species in the two cases is followed by

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NMR spectroscopy and discussed.     

The sorption and reduction of nitrogen oxides by 12-tungstophosphoric acid and its ammonium salt

Nitrogen dioxide is removed from the gas phase by 12-tungstophosphoric (HPW), 12-tungstosilicic and 12-molybdophosphoric acids, with the two solid acids containing tungsten sorbing significantly larger quantities of NO₂ than that containing molybdenum, indicating the dependence on acidic strength. NO₂ interacts with the water contained on and in HPW to form HNO₃ which desorbs. On depletion of the water additional NO₂ remains held on HPW as

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.

In contrast the ammonium salt of HPW reduces NO₂ to N₂ , NO and N₂O at an optimal temperature of 175°C for the reduction to N₂. The interaction process occurs between NO₂ and the reductant NH₃ while it is bound as

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on the solid. Thus NH₃ is not lost to the gas phase during the reduction process, and ammonia slip is effectively eliminated. The ammonium salt can be regenerated by exposure of HPW to gaseous NH₃ or by precipitation from aqueous solution.     

Structures of H3PO4/SiO2 catalysts and catalytic performance in the hydration of ethene

The hydration of ethene was carried out over H₃PO₄/SiO₂ having various amounts of H₃PO₄. The rate of the ethanol formation increased markedly with the increasing H₃PO₄ loadings, in particular above 60–70 wt%. By X-ray diffraction (XRD), and

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and

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MAS NMR methods, it was revealed that various silicon phosphates were produced in the preparation of the catalysts. The structures of the phosphates depended on the H₃PO₄ loadings. It was suggested that Si(HPO₄)₂·H₂O species which formed at higher H₃PO₄ loadings were hydrolyzed to H₃PO₄ and SiO₂ during the course of the reaction, yielding the catalysts with high performance. The bulk phase of the H₃PO₄ was involved in the reaction.     

Epoxide–tertiary amine combinations as efficient catalysts for methanol carbonylation into methyl formate in the presence of carbon dioxide

The influence of CO₂ on the carbonylation of methanol into methyl formate was investigated with two classes of catalytic systems: MeO⁻ and epoxide–amine combinations. In the absence of CO₂, NaOMe is the more active, but its efficiency is rapidly destroyed by CO₂. On the other hand, the epoxide–amine system is much less sensitive to the presence of CO₂. Different classes of amines and epoxides have been tested. Tertiary alkyl amines and terminal epoxides are the most efficient. Phosphines associated with an epoxide have also been found to be able to catalyse this reaction. On the basis of

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NMR studies, a plausible mechanism which can also be applied to amines is proposed for the reaction.     

Catalytic dehydrofluorination of CF3CH3(HFC143a) into CF2CH2(HFC1132a)

The catalytic dehydrofluorination of CF₃CH₃ was studied over various metal phosphate catalysts in a fixed-bed reactor. The Mg₂P₂O₇ catalyst exhibited the moderate activity and high selectivity of CF₂CH₂, and it is the most suitable catalyst for the dehydrofluorination of CF₃CH₃. Deactivation did not take place during the 100 h reaction over the Mg₂P₂O₇ catalyst, and XRD patterns of the catalyst were unchanged after 100 h reaction. However, small amounts of F⁻ ions were present on the surface of the catalyst from results of XPS. The active sites for CF₂CH₂ formation are weak acid sites of the catalysts, and carbon deposition and/or polymerization take place on strong acid sites. Results of CF₃CH₃-TPD indicated that the dehydrofluorination proceeds through a

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carbonium-ion mechanism over Mg₂P₂O₇ catalyst, and the rate-determining step is the cleavage of the C–F bond.     

Oxide ion concentration dependence of the potential of stabilized zirconia-air electrode in a molten LiF-KF system

This paper describes oxide ion concentration dependence of the potential of stabilized zirconia-air electrode in a molten LiF-KF eutectic system at 823–973 K. Experimental results can be explained well by the theoretical equation of the zirconia-air electrode potential,

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derived by the authors.

As an example of the application of the equation, solubility product of Al₂O₃ in a molten LiF-KF eutectic system has been determined to be (1.0–3.0) × 10⁻¹⁷ at 823 K, by the use of the zirconia-air electrode.     

Sulfonic acid functionalized periodic mesostructured organosilica as heterogeneous catalyst

Periodic mesostructured organosilicas (PMO) were first synthesized using 1,2-bis(triethoxysilyl)ethylene (BTENE) under acidic conditions using Pluronic 123 as surfactant. The ethylene bridges were then arylated with benzene using AlCl₃ as catalyst. These materials were further treated with sulfuric acid for the sulfonation of the phenyl moieties yielding a new preparation of sulfonic acid functionalized PMO. Ordered hexagonal mesostructures with surface areas up to 440 m²/g and narrow pore size distribution (around 5.3 nm) were obtained. This work thus provides a new example of chemical modification for the conception of functionalized PMO catalysts. Liquid phase self-condensation of heptanal was performed at 75 °C in the presence of these catalysts and the results were compared with those obtained with several other heterogeneous acid catalysts.

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