Showing papers by "DECHEMA published in 1995"

Fundamentals of TiAl oxidation: a critical review

Abstract: The paper critically reviews the state of the art on the oxidation of TiAl. At the beginning differences compared to the oxidation of conventional Fe, Ni and Co alloys are discussed. The Knowledge of the TiAlO phase diagram as a basis for an understanding of the processes occurring during oxidation is very incomplete. Three different scale structures can be distinguished as functions of time or scale thickness. In the sub-surface zone two phases with high oxygen solubilities can be identified after a certain time. These phases do not follow from the existing phase diagram. Nitrogen can have two different effects depending on the time it is added to the oxidation environment. The micro-structure also affects the oxidation behaviour significantly, so this effect can lead to a misinterpretation of the influence of other parameters such as alloying elements. The influence of alloying elements is not yet understood. Mechanical loading leads to scale cracking, however, the system has a significant scale repair capacity, though about twothirds of the scale grows by inwards transport. Grundlagen der TiAl-Oxidation – Eine kritische Ubersicht Die Arbeit gibt eine kritische Betrachtung der Kenntnisse uber die Oxidation von TiAl. Einleitend wird auf Unterschiede im Vergleich zur Oxidation konventioneller Fe. Ni und Co-Legierungen hingewiesen. Die derzeitigen Kenntnisse uber das Phasendiagramm TiAlO als Basis fur das Verstandnis der bei der Oxidation ablaufenden Prozesse sind sehr unvollstandig. Im Aufbau der Oxidschicht konnen drei Grenzfalle unterschieden werden, die als Funktion der Zit nacheinander durchlaufen werden. Stickstoff kann offensichtlich zwei Effekte auslosen, abhangig vom Zeitpunkt der Zugabe zum Reaktionsgas. Die Mikrostruktur kann einen starken Einflus haben und andere Einflusse wie die von Legierungselementen verdecken. Der Einflus von Legierungselementen wird derzeit nicht verstanden. Damit sind auch keine sinnvollen Vorhersagen moglich. Mechanische Belastungen verursachen ein Aufreisen der Oxidschicht, die jedoch eine beachtliche Ausheilfahigkeit fur Oxidschichtrisse hat.

Mechanical properties of oxide scales

Abstract: At high temperatures most alloys rely on the protective effect of oxide scales formed by the reaction between oxygen from the environment and components of the alloy. The protective effect of these scales may, however, be impaired if stresses lead to cracking or spalling. Therefore, the mechanical properties of the scales play a vital part in protection, in particular under service conditions of the materials where the presence of stresses cannot be excluded. The paper provides a survey of the existing models that describe the mechanical properties of scales (the emphasis being on mechanical scale failure), the measuring techniques and the data. It will be demonstrated that, in the case of tensile conditions, the situation is relatively well understood and that a fair amount of data exists. With regard to compressive conditions scale failure consists of a combination of different failure steps, which can each be described by a quantitative model, but there is still clearly a need for experimental verification. While the measuring techniques for the properties under tensile stresses are fairly well developed, those for the behavior under compressive stresses, in particular those characterizing scale adhesion, are still under discussion and relatively few experimentally determined data are available in this case. Nevertheless, existing knowledge can be put to good use in assessing scale behavior under the effect of stresses, based on the models and data given in the paper.

Catalytic properties of mesoporous crystalline MCM-41 and related materials forhydrocarbon cracking

Abstract: The catalytic properties of extra-large pore aluminosilicate MCM-41 and a silica-containingVPI-5 were investigated by MAT ( M icro A ctivity T est) using n-hexadecane as a model feed and were then compared with the results obtained from a commercial FCC catalyst (FCC: F luid C atalytic C racking) It could be demonstrated that, by optimizing structural and chemical properties, the new mesoporous crystalline MCM-41 and related materials are suitable as active components in cracking catalysts for “deeper” cracking of high boilding hydrocarbons

Conversion of ethane on modified zsm‐5 zeolites: A study of aromatization as a function of reaction media

Abstract: Ethane was converted into aromatic hydrocarbons on H-ZSM-5 and Zn-ZSM-5 at 773 K in a flow reactor system using nitrogen and hydrogen as carrier gases. Residence time was varied by changing catalyst weights to obtain information about primary and secondary reaction steps. On H-ZSM-5 ethane conversion proceeds via formation of ethene and subsequent oligomerization. Oligomers undergo fast isomerization, cracking and cyclization leading to a broad spectrum of aromatics. Methane is a secondary product. Dehydrogenation reactions proceed irreversibly. Introduction of zinc species significantly enhances the aromatization activity of H-ZSM-5 on the one hand and causes a change of reaction mechanism on the other. Oligomerization of olefinic intermediates can take place at zinc sites. In nitrogen, aromatization of C 6 olefins is faster than isomerization and cracking of these oligomers. Dehydrogenation steps proceed reversibly. On using hydrogen as the carrier gas over Zn-ZSM-5, the conversion of ethane as well as the formation of aromatics decrease. Furthermore, aromatics richer in hydrogen (especially C 8 aromatics) are then formed preferentially.

Biological film on solid surface activity determining method

Abstract: This method involves using a miniature electrode with a head consisting of a platinum wire being 0.1mm in diameter, e.g. no thicker than the biofilm, in a 0.5mm diameter glass casing as a sensor. This is set through a solid body, and is thereby insulated, to the point of contact between the surface and the biological organism. The redox potential changes from negative to positive according to whether the biofilm is living or dead. This may be necessary whether the biofilm is undesirable, as in corrosion or desirable, as in may biotechnology applications.

The influence of gaseous impurities in air on the high temperature corrosion of coated and uncoated nickel-based superalloys

Abstract: The influence of small concentrations of SO2, HCl or Na2SO4 vapour in air on the corrosion of the uncoated nickel-based superalloys IN 792 DS + Hf, CMSX-6 and MA 760 ODS and the coatings RT 22 and LCO 22 has been investigated at 1000 and 1100°C. A level of 1% SO2 in air somewhat increased the oxidation of IN 792 DS + Hf and favoured the scale spallation of CMSX-6 at 1000°C. Some precipitates of Ti sulphide were found in the subsurface zones of both alloys. No influence was observed at 1100°C. The oxidation of MA 760 ODS and the two coatings was not affected by SO2 at 1000 and 1100°C. The addition of 100ppm HCl to air favoured the spallation of the scales of IN 792 DS + Hf and CMSX-6 at 1000°C. Again, no influence could be observed with these alloys at 1100°C nor with MA 760 ODS or the coatings at either of these temperatures. Contrary to expectations, no synergistic effect was found in air with 1% SO2 and 100ppm HCl, but SO2 reduced the negative effect of HCl. Catastrophic corrosion occurred with IN 792 D...

A New Concept of an Electrochemical Heat Pump System: Theoretical Consideration and Experimental Results

Abstract: A new concept of an electrochemical heat pump system is described which consists of two identical electrochemical cells operating at different temperatures in opposite directions. The principle is based on the exchange of reversible heat (TΔRS) of the reaction. The thermodynamics and efficiency of such a system are considered for different cell reactions. Calori-metric measurements, using commercial accumulators and non-commercial redox cells, were carried out to prove the validity of the theoretical predictions with respect to heat exchange and efficiency. The efficiency of a heat pump with the redox system Cu/CuSO4//Na3[Fe(CN)6], Na4[Fe(CN)6]/Pt was found to be comparable to that of conventional heat pump systems.

Titanium@-aluminium@ alloy powder with improved high temperature corrosion resistance

Abstract: A titanium-aluminium alloy part is treated with a gaseous halogen-containing compound which reacts with the workpiece surface.

Towards a Cleaner Environment Using Electrochemical Techniques

Abstract: Electrochemical processes can contribute considerably to environmental protection by means of waste purification processes and production integrated waste minimization. As examples of waste purification techniques, electrochemical processes for gas purification and removal of toxic metal ions from waste water will be described. For production integrated processes, the examples given are fluidized bed electrolysis for metal recovery in cellulose acetate production, the membrane process for alkali chloride electrolysis, and the electroreduction of dichloroacetic acid.

Synthesis and catalytic properties of extra-large pore crystalline materials for n-hexadecane cracking

Abstract: Publisher Summary The Fluid Catalytic Cracking process (FCC) is of paramount importance in petroleum refining. At present, besides an active matrix, the active component of the cracking catalyst is zeolite Y and/or ZSM-5, both of which are limited in the processing of heavier oil fractions due to the restriction caused by their pore dimensions. The new zeotype/mesoporous materials VPI-5, Cloverite, and MCM-41 open up interesting perspectives for the conversion of heavier feedstocks because of the accessibility of long-chain and/or bulky molecules to the active centers. This chapter presents the synthesis of and discusses the results obtained by using Si-VPI-5 and MCM-41 as catalysts in n-hexadecane cracking in comparison with a commercial FCC catalyst.

Secondary Synthesis and Characterization of Boron‐Containing ZSM‐5 Zeolites

Abstract: Under conditions of secondary synthesis and to varying extents, boron can be incorporated into the tetrahedral framework sites of a ZSM-5 zeolite which has been synthesized template-free or in the presence of organic template compounds and subjected to calcination or hydrothermal treatment. Isomorphic incorporation of boron is verified by means of 11B MAS NMR measurements and it is reflected in a contraction of the unit cell volume of the zeolite lattice. A comparative study of the catalytic properties of the boron-containing ZSM-5 in acid catalysed n-hexane cracking revealed that samples based on template synthesis product are more active and show less tendency to deactivation. Sekundarsynthese und Charakterisierung Bor-haltiger ZSM-5-Zeolithe In einen templatfrei und in Gegenwart von Templat synthetisierten und einer thermischen oder hydrothermalen Behandlung unterworfenen ZSM-5-Zeolith kann Bor unter den Bedingungen der Sekundarsynthese auf die tetraedrischen Gitterplatze in unterschiedlichem Mase eingebaut werden. Der isomorphe Boreinbau wurde mittels 11B-MAS-NMR-Messungen verifiziert und spiegelt sich in der Kontraktion des Elementarzellenvolumens des Zeolithgitters wider. Vergleichende Untersuchungen der katalytischen Eigenschaften praparierter borhaltiger ZSM-5-Zeolithe in der saurekatalysierten n-Hexanspaltung zeigen, das die ausgehend vom Templat-Syntheseprodukt erhaltenen Proben eine hohere katalytische Aktivitat und eine geringere Desaktivierungsneigung haben.

Asymmetric Hydrogenation of Ethyl Pyruvate Using Ptcontaining Zeolites Modified With (-)-Cinchonidine

Abstract: Platinum loaded, (-)-cinchonidine modified zeolites of the types Y, ZSM-35, β and ZSM-5 manifest high catalytic activity for the asymmetric hydrogenation of ethyl pyruvate to ethyl lactate. The catalysts based on Y and ZSM-35 zeolites show the highest enantioselectivity. To obtain the highest reaction rates and the optical yields the Pt content should be around 5 wt%