Showing papers by "DECHEMA published in 1979"

The kinetics of the reduction of chromium oxide by hydrogen

Abstract: The reduction kinetics has been studied as function of hydrogen partial pressures,pH2O/pH2 ratio, gas flow rate, and temperature. The reduction follows a linear time law and is dependent on the gas flow rate below a value of approximately 10 cm · s-1, since the rate is determined by the removal of the water vapor being formed. In this range the reduction rate may be calculated from gas dynamical data. At sufficiently high flow rates the phase boundary reaction is rate determining. The activation energy is 123 kJ · mol-1. The reduction rate is proportional to the square root of hydrogen pressure and decreases with increasing water vapor content.

Oxidation und Aufkohlung hochlegierter Werkstoffe für Crackrohre. Teil 1: Das Oxidationsverhalten in Luft

Abstract: Es wurde das Oxidationsverhalten der Guswerkstoffe 1.4848, 1.4857, 2.4813 und einer 25/20/5-CrNiSi-Versuchsschmelze sowie der Knetwerkstoffe 1.4301 und 1.4841 in trockener synthetischer Luft zwischen 800 und 1300° C untersucht. Nur die Oxidation der Werkstoffe 1.4848 und 1.4841 erfolgte angenahert parabolisch, die der anderen Werkstoffe dagegen nach komplexen Zeitgesetzen. Die Oxidationsgeschwindigkeiten aller Guswerkstoffe sind sehr ahnlich. Es bildet sich eine Schichtenfolge Werkstoff/Cr2O3/MnCr2O4 aus. Die MnCr2O4-Schicht weist deutliche Fe-Gehalte auf. Die Cr2O3-Schicht enthalt besonders nahe der Grenze zum Metall SiO2-Einlagerungen. Oberhalb etwa 1000°C tritt deutliche innere Oxidation des Siliziums auf. Eine Ausnahme macht die 25/20/5-CrNiSi-Versuchsschmelze, die aufgrund ihres hohen Si-Gehaltes stets eine geschlossene SiO2-Schicht zwischen Werkstoff und Cr2O3-Schicht bildet. Der Werkstoff 1.4301 zeigt oberhalb 1050°C ortlich verstarkte Oxidation unter Pustelbildung. Der Werkstoff 1.4841 oxidiert oberhalb 1100°C im feinkornigen Gefugezustand erheblich schneller als im grobkornigen, weil oberhalb dieser Temperatur ein starkes Kornwachstum erfolgt, das die Schutzschichtbildung beeintrachtigt. Die Deckschicht enthalt hohe Gehalte an Eisen und der Cr-Gehalt im Werkstoff an der Grenze Metall/Oxid sinkt mit steigender Temperatur wesentlich starker ab als bei dem grobkornigen 1.4841 und den Guswerkstoffen mit rd. 25% Cr. Unterhalb 1100°C sind die Unterschiede gering, und die Oxidationsrate ist vergleichbar der der Guswerkstoffe. Ein Ubergang von innerer Si-Oxidation zu auserer SiO2-Deckschichtbildung erfolgt bei den Werkstoffen 1.4848 und 1.4841 bei 1100°C in H2-H2O-Gemischen bei Unterschreiten des O2-Partialdrucks des Cr/Cr2O3-Gleichgewichts. Oxidation and Carburization of High Alloyed Materials for Cracking Tubes. Part 1: The Oxidation Behaviour in Air The oxidation behaviour has been studied of cast materials (German Materials Numbers 1.4848, 1.4857, 2.4813), an experimental melt of 25/20/5 CrNiSi steel and wrought materials (German Materials Numbers 1.4301 and 1.4841) in dry synthetic air between 800 and 1300°C. Only the oxidation of the materials 1.4848 and 1.4841 follows an approximately parabolic law, with the other materials more complex kinetic laws are found. The oxidation rates of all the cast materials are rather similar; the scale layers successively formed on the base material are Cr2O3 and MnCr2O4. The latter is characterized by clearly distinguishable iron contents. The Cr2O3 layer contains SiO2 inclusions, in particular near the metal boundary. Above 1000°C internal oxidation of silicon is found. An exception to this is the 25/20/5 CrNiSi experimental melt where, because of the high Si content, a continuous SiO2 layer is formed consistently between base material and Cr2O3 layer. The material 1.4301 exhibits locally increased oxidation at temperatures above 1050°C; in this case oxide nodules are formed. The oxidation rate of material 1.4841 at temperatures above 1100°C is higher for the fine grained than for the coarse grained condition; this phenomenon is attributed to the fact that at these temperatures pronounced grain growth occurs which impairs the formation of a protective layer. The scale layers formed contain considerable amounts of iron and the Cr content in the metal at the metal/oxide boundary decreases at a considerable higher rate with increasing temperature than in the case of the coarse grained material 1.4841 and of the cast materials containing about 25% Cr. Below 1100°C the differences in the behaviour are rather small and the oxidation rate is comparable to that of the cast materials. A transition from internal silicon oxidation to external SiO2 scale formation is found with materials 1.4848 and 1.4841 at 1100°C in H2/H2O mixtures below the partial pressure of oxygen corresponding to the Cr/Cr2O3 equilibrium.

Analysis of the coupled transport, reaction, and deactivation phenomena in the immobilized glucose oxidase and catalase system.

Abstract: In a previous paper, the overall or macrokinetics of the immobilized glucose oxidase--catalase system has been presented. In this paper a detailed analysis of the interaction of diffusion and reaction in this system will be presented. The mathematical treatment includes two consecutive reactions with two-substrate kinetics. Furthermore, the deactivation of both enzymes due to the intermediate product peroxide is taken into account. The predicted results suggest that the efficiency of the glucose oxidase reaction depends on the concentration ranges of the two substrates. Furthermore, the external mass-transfer rate may cause a shift from glucose limitation to oxygen limitation. The efficiency of the coupled system is always higher than that predicted for the uncoupled reaction path. The calculations show that the economics of the coupled system depend a great deal on the deactivation of the enzymes.

Investigation of hydrodynamic test systems for the selection of high flow rate resistant materials

Abstract: Flow-dependent corrosion phenomena can be studied in the laboratory and on a pilot plant scale by a number of methods, of which the rotating disc, the rotating cylinder, the coaxial cylinder and the tubular flow test are the most important. These methods are discussed with regard to mass transfer characteristics and their applicability to flow-dependent corrosion processes and erosion corrosion. To exemplify the application of such methods to materials selection for seawater pumps, corrosion data of non-alloyed and low alloy cast iron are presented.

Inhomogeneous distribution of fixed enzymes inside carriers

Abstract: Waterinsoluble catalysts with high activity and homogeneous enzyme distribution show low efficiency when using low substrate concentrations, high molecular weight substrates of unbuffered systems. This paper presents an optimised.catalyst design by introducing inhomogeneous enzyme distribution throught the matrix which leads to higher efficiencies.

Non uniform enzyme distribution inside carriers

Abstract: For catalyst optimization, enzyme immobilization may be controlled in such a way that most of the protein is fixed in the outer shells of a porous particle. Calculation of the profiles of fixed enzymes predicts efficiencies similar to those found experimentally.

Electrolytic gas scrubber - having conducting graphite packing through which wash liquor flows and across which a potential is applied to oxidise or reduce gas

Abstract: Gaseous components in gas streams are removed by contact with a solvent liquor, such as water, in the presence of an electrical potential. One electrode is formed from a fixed bed or conducting packing held in a column through which a wash liquor, in which the gaseous impurity is soluble, is passed.

Untersuchungen des Stofftransports und der chemischen Reaktionen im System Polyäthylen‐weich/Schwefelsäure–Zur Frage der Spannungsrißkorrosion in Kunststoffen–

Abstract: Untersuchungen des Stofftransports und der chemischen Reaktionen im System Polyathylen-weich/Schwefelsaure zeigen, das sowohl an eigenspannungsbehafteten Vollproben ohne ausere Belastung, als auch an im Zeitstandinnendruckversuch gepruften Rohrproben verschiedene Schadens- und Reaktionsablaufe zu registrieren sind. Licht- und rasterelektronenmikroskopische Ergebnisse fuhren zu dem Schlus, das sowohl die werkstoffspezifische Mikrostruktur als auch die herstellungsbedingte Makrostruktur in Verbindung mit inneren und auseren Spannungen und einem kritischen Medium zu einem Schadensbild fuhren, das man bei Polymerwerkstoffen als „Spannungsriskorrosion” bezeichnet. Investigations on mass transport and chemical reactions in the system LDPE/H2SO4–The question of stress corrosion cracking in plastic materials– Investigations on mass transport and chemical reactions in the system LDPE/H2SO4 reveal different kinds of damage and chemical attack both with bulk specimen containing internal stresses only and with pipes examined in creep tests under pressure. Evaluating light microscopy and electron scanning microscopy pictures it was found that the microstructure specific to the material as well as the macrostructure determined by the production process influence the character of damage. These structural parameters combined with internal or external stresses and a critical medium lead to a kind of failure which for polymers is called “stress corrosion cracking”.