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Abstract A heterogeneous magnesium oxide catalyst is a good alternative for homogeneous catalysts for the transesterification of alkyl esters for the production of fine-chemicals as well as for the production of biodiesel. The transesterification of ethyl acetate with methanol was used as a model reaction to simulate fine-chemical production in a batch slurry reactor at industrial conditions. The transesterification of triolein with methanol to methyl oleate was chosen to simulate continuous production of biodiesel from rapeseed oil. A kinetic model based on a three-step ‘Eley–Rideal’ type of mechanism in the liquid phase was used in both process simulations. The transesterification reaction occurs between methanol adsorbed on a magnesium oxide free basic site and ethyl acetate or the glyceride from the liquid phase. Methanol adsorption is assumed to be rate-determining in both processes. Activity coefficients were required to account for the significant non-ideality of the reaction mixture in the simulations of both processes. The simulations indicate that a production of 500 tonnes methyl acetate per year can be reached at ambient temperature in a batch reactor of 10 m 3 containing 5 kg of MgO catalyst, and that a continuous production of 100,000 tonnes of biodiesel per year can be achieved at 323 K in a continuous stirred reactor of 25 m 3 containing 5700 kg of MgO catalyst. Although various assumptions and simplifications were made in these explorative simulations the assumptions concerning the reaction kinetics used, the results indicate that for both processes a heterogeneous magnesium oxide catalyst shows promising potential as a viable industrial scale alternative.

Abstract A Gas–Solid Vortex Reactor (GSVR), in literature also referred to as Rotating Fluidized Bed in Static Geometry (RFB-SG), is a promising reactor type for Process Intensification (PI) with respect to reactor volume reduction. Although replacing gravitational by centrifugal force has been considered since the seventies, the hydrodynamics of the reactor flow and the bed behavior remain largely unknown. In the present work experiments have been carried out in a cold flow GSVR with diameter of 0.54 m, length of 0.1 m and 36 inlet slots of 2 mm. Gas injection velocities of 55–111 m/s and particles with densities of 950–1800 kg/m 3 and diameters of 1–2 mm have been applied. Depending on solids density, particle diameter and gas injection velocity the bed behavior can be considered as stable, semi-stable or unstable. For semi-stable and stable flow regimes the effect of the above process conditions on the maximum solids capacity was investigated. With increasing solids density, the bed stability considerably decreases, while the maximum solids capacity increases. By increasing the gas injection velocity, both the bed stability and the maximum solids capacity increase. Once bed stability is accomplished, a further increase of the gas injection velocity does not affect the maximum solids capacity. With increasing particle diameter, the bed stability decreases while the maximum solids capacity increases.

Supercritical heat transfer has already been applied for decades, as it has several benefits such as improved thermal efficiency of the thermodynamic cycle. Accurate knowledge about supercritical heat transfer and pressure drop of the different working fluids is required to design the heat exchangers and other components used in these systems. In literature, supercritical heat transfer of water and CO2 has already been widely investigated, the research involving refrigerants (for their application in low-temperature heat conversion systems) is however rather scarce. This paper gives an overview of the existing research on supercritical heat transfer. An overview of the applications, general characteristics and the main findings for water and other fluids are summarized. Due to the sharp variations in thermophysical properties, heat transfer and pressure drop cannot be accurately predicted on a single-phase based approach only. An in-detail review of the current research and status of knowledge about supercritical heat transfer of refrigerants is presented. The effect of the different investigated refrigerants and operating parameters on heat transfer and pressure drop, both for heating and cooling applications, is discussed. The remaining gaps in literature are highlighted, which include studies involving larger diameter tubes, horizontal flow, cooling heat transfer and pressure drop estimations and creation of a wider database for a more general correlation development and measurements on newer refrigerants (with low Global Warming Potentials) as these will become increasingly important in the near future. In addition, advances in numerical research should focus on development of suitable turbulence models. Overall, further improving the basic understanding of the fluid structure and occurrence of deteriorated heat transfer, as well as forming reliable models for the thermophysical properties are key in future efforts.

Abstract An experimental study on the surface texturisation of monocrystalline wafers with solutions containing sodium-hydroxide and isopropanol was carried out. Both the composition and the temperature of the etching solution were optimised on the basis of etch-rate measurements on silicon samples having different crystallographic directions. It was found that the density and the size of the pyramids are influenced by the etch-rate of silicon in the 〈1 0 0〉 direction and also by the anisotropy factor of the solutions being the quotient of the etch rate in the 〈1 0 0〉 to 〈1 1 1〉 directions. Design of experiments and response surface methods were used to extract the etch rate as a function of different input parameters, such as the sodium hydroxide and isopropanol concentrations and the temperatures of the solutions. Optimum texturing conditions were found at a temperature of 80°C and a composition which causes a relatively high etch rate in the 〈1 0 0〉 direction with an anisotropy factor of 10. At the starting point of the etching process, the inhomogeneity of pyramid nucleation can be avoided by mixing an additive to the texturing solution. With such a solution, the pyramid size can be tuned by varying the etching time in order to obtain a low reflectivity from the textured silicon wafers. Based on our results the texturing process could be stabilised with respect to the reproducibility on a large scale of wafers.

Abstract In the present tutorial, solar energy conversion is described in the framework of endoreversible thermodynamics, i.e. a recently developed subset of irreversible thermodynamics. From a general thermo-chemical endoreversible engine, we deduce the photovoltaic, the photothermal and the photochemical solar energy converter, besides of a few other, non-solar devices.

Efforts to develop a more precise definition and measurement of cogenerated electricity than those contained in the European Union's 2004 Directive have made real progress, but additional improvements are needed to yield a better-founded, more transparent methodology. The author offers suggestions on how to complete this important job.

Thermokinetics of Biochar production.