Fluidization with hot compressed water in micro-reactors

In this paper the concept of micro-fluidized beds is introduced. A cylindrical quartz reactor with an internal diameter of only 1 mm is used for process conditions up to and 244 bar. In this way, fast, safe, and inherently cheap experimentation is provided. The process that prompted the present work on miniaturization is gasification of biomass and waste streams in hot compressed water (SCWG). Therefore, water is used as fluidizing agent. Properties of the micro-fluid bed such as the minimum fluidization velocity (U_mf), the minimum bubbling velocity (U_mb), bed expansion, and identification of the fluidization regime are investigated by visual inspection. It is shown that the micro-fluid bed requires a minimum of twelve particles per reactor diameter in order to mimic homogeneous fluidization at large scale. It is not possible to create bubbling fluidization in the cylindrical micro-fluid beds used. Instead, slugging fluidization is observed for aggregative conditions. Conical shaped micro-reactors are proposed for improved simulation of the bubbling regime. Measured values of U_mf and U_mb are compared with predictions of dedicated 2D and 3D discrete particle models (DPM) and (semi)-empirical relations. The agreement between the measurements and the model predictions is good and the model supports the concept and development of micro-fluid beds.     

Catalytic partial oxidation of methane to synthesis gas in a ceramic membrane reactor

Methane has been selectively converted to synthesis gas using a two-zone fixed bed of a Ni/Al₂O₃ catalyst inside a modified ceramic membrane. The first zone of the reactor was surrounded by an impervious wall, and therefore behaved as a conventional fixed bed reactor. In the second zone, some of the reaction products could preferentially diffuse out of the reactor, which yielded higher than equilibrium methane conversions. The influence of the different operating conditions has been studied, and the performance of the membrane reactor has been compared to that of a fixed bed reactor. The membrane reactor has also been used at pressures above atmospheric (2 bar), with good conversions and selectivities.     

Reaction of YBa2Cu3O7-δ Powder with NOx Gas in an Aerosol Reactor

Thermodynamic calculations predict, and experiments verify, that YBa₂Cu₃O₇-₈ (123) powder is unstable in the presence of NO_x-containing aerosol reactor exhaust gases at temperatures below about 600°C. Powders collected above the stability temperature are single-phase 123, while powders collected at lower temperature contain Ba(NO₃)₂ formed by reaction of the powder with NO_x, after exit from the hot zone.     

气固下行流化床反应器Ⅱ气固两相的流动规律

气固下行流化床反应器气固两相流动过程是比较复杂的，沿轴向气固两相运动可分为第一加速、第二加速和恒速３个运动段，沿径向局部气体速度、颗粒速度和颗粒浓度都具有不同程度的不均匀性。而这种不均匀性是由气固两相顺重力场湍动运动所决定的。和循环床提升管相比，下行管反应器气固两相沿径向分布的不均匀性得到有效地改善，气固可以实现超短接触操作，因而是一种新型高效气固超短接触反应器     

Mechanosynthesis of complex oxides and preparation of mixed conducting nanocomposites for catalytic membrane reactors

Nanopowders of LaGaO₃- and LaMnO₃-based complex perovskites (P) and ceria-based fluorites (F) were prepared by mechanosynthesis. Compatible nanocomposites F + P and P + P with mixed ion and electron conducting (MIEC) properties were prepared and sintered at moderate temperatures up to dense ceramics. The obtained materials were studied by means of XRD, SEM, TEM, electrical conductivity measurements, temperature programmed (TP) reduction/oxidation and preliminary estimations of permeability were obtained. A new strategy based on the advantages of the mechanochemical ceramic approach is proposed to design multilayer ceramic membranes for CMR. Casting technology and one-step sintering were used for the production of thin film membranes with MIEC properties on porous substrates. The coarse fraction of as-milled powders from agglomerates with density 70% was used for the porous substrate, and fine fractions of aggregates with sizes <1 μm were used in preparation of composites for thin dense films. Ceria-based composites prepared by the Pechini route and/or mechanochemical method are proposed as materials for protecting thin films.     

Catalytic wall reactor: Catalytic coatings of stainless steel by VOx/TiO2 and Co/SiO2 catalysts

Catalytic wall (structured) reactors and structured supports are suitable to study the catalytic properties of nanosized materials. The coating of metallic (aluminum and stainless steel) plates by thin layers of active phase is presented in two cases, VO_x/TiO₂ and Co/SiO₂, catalysts used in the oxidative dehydrogenation (ODH) of propane and in Fischer–Tropsch synthesis (FTS) of clean fuels, respectively. The preparation of coated plates and their characterisation by various methods of physicochemical analysis are described. Both chemical and physical methods were used for coating. VO_x/TiO₂ layers were obtained by grafting of Ti (on Al or stainless-steel plates) and V (on TiO₂) alkoxides and use of sol–gel media or suspension. A silica primer was deposited (on stainless-steel plate) by plasma-assisted chemical vapour deposition (PACVD) onto which Co oxide and silica were coprecipitated from sol–gel. The catalytic experiments in the respective reactions were carried out in special plate reactors and compared with those of catalytic powders. The study shows that the coating of a metallic substrate by a catalyst is not straightforward and requires specific studies dealing with both chemistry (chemical affinity between substrate and catalytic layers) and catalytic engineering (catalytic performance in taylor-made reactors).     

Mass transfer from Taylor bubbles rising in single capillaries

Gas-liquid mass transfer from Taylor bubbles rising in 1, 2 and 3 mm diameter capillaries of circular and square cross-sections was investigated for air-water system. The liquid-phase volumetric mass transfer coefficient k_La was obtained from experimental oxygen absorption dynamics. The experimental k_La values are in good agreement with the model developed by van Baten and Krishna (2004. Chemical Engineering Science 59, 2535-2545), with the additional assumption that the dominant mass transfer contribution is to the film surrounding the bubble.     

ON THE CHEMICAL OSCILLATIONS IN DIFFERENTIAL REACTORS: A CASE STUDY OF AMMONIA OXIDATION ON PLATINUM

Trying to identify the mechanism(s) responsible for isothermal oscillatory behavior of heterogeneous catalytic reaction systems, many investigators have assumed low conversions with respect to the limiting reactant(s) so that the bulk phase concentrations of the reactants may be considered constant. In this study, it is shown that even in a differential catalytic CSTR, in which conversion of the reactants may be less than %, the changes of the gas phase concentrations of the reacting.species may play a key role in the overall dynamic behavior of a reaction system; for instance, they may drive the oscillations observed. The reduction of nitric oxide by ammonia on polycrystalline platinum is discussed in detail within this context. Several other oxidation reactions are also discussed     

Gas-phase residence time distribution in a falling-film microreactor

Industrial-scale performance of gas-liquid reactors can be difficult to optimise for very rapid or highly exothermic reactions. Microstructured reactors for laboratory measurements offer new opportunities for the study of these reactions by enabling precise heat management and fine control of reactor operating conditions. For accurate experimental study, characterisation of the flow conditions within these new reactor devices is essential.The present study examines experimental residence time distributions for the gas phase through a microstructured falling-film reactor, in order to develop an appropriate flow model for further study of gas-phase mass-transfer characteristics in the system. For the gas-phase residence time distribution experiments, the detection system involves a flow of oxygen containing ozone as a tracer gas with continuous monitoring of the concentration by UV-light absorption. The experimental results are used to model the flow behaviour in the gas volume over the gas-liquid contact zone as a series of continuous stirred tank reactors whose number is a simple function of the gas Reynolds number.The experimental results are compared with computational fluid dynamics calculations of the gas flow within the reactor. The comparison indicates a clear correlation of the flow model behaviour with the appearance of recirculation loops in the reaction chamber and the effect of the gas jet at the entrance of the gas-liquid contact zone.