HYDRODYNAMIC MODELLING IN AN AIR-LIFT LOOP REACTOR

A simple hydrodynamic model is proposed for use in the design, scale-up and characterization of external loop air-lift reactors. The approach is based upon a momentum balance for the flow loop coupled with a drift-flux equation for the reactor riser and establishes a rational basis for a predictive model relating gas throughput to induced liquid flow and gas hold-up in a range of air-lift reactors. The effective resistance of the reactor, k, defined in terms of the total loss coefficients of the reactor and the aerated height of the two-phase riser, was identified to allow for the quantification of the influence of reactor design on the hydrodynamic variables. An extensive body of data for the air-water system, collected on two reactors with active volumes of 0·055 m³ and 0·3 m³, is presented and used, in conjunction with literature data encompassing a wide range of reactor geometries and flow conditions, to define a unique relationship between flow behaviour and reactor configuration. The model, which accounts for the prevailing flow regime in the reactor, provides a direct method of predicting hydrodynamic behaviour in relatively non-viscous systems.     

L2 disturbance attenuation for highly dissipative nonlinear spatially distributed processes via HJI approach

For many practical industrial spatially distributed processes (SDPs), their dynamics are usually described by highly dissipative nonlinear partial differential equations (PDEs). In this paper, we address the L₂ disturbance attenuation problem of nonlinear SDPs using the Hamilton–Jacobi–Isaacs (HJI) approach. Firstly, by collecting an ensemble of PDE states, Karhunen–Loève decomposition (KLD) is employed to compute empirical eigenfunctions (EEFs) of the SDP based on the method of snapshots. Subsequently, these EEFs together with singular perturbation (SP) technique are used to obtain a finite-dimensional slow subsystem of ordinary differential equation (ODE) that accurately describes the dominant dynamics of the PDE system. Secondly, based on the slow subsystem, the L₂ disturbance attenuation problem is reformulated and a finite-dimensional H_∞ controller is synthesized in terms of the HJI equation. Moreover, the stability and L₂-gain performance of the closed-loop PDE system are analyzed. Thirdly, since the HJI equation is a nonlinear PDE that has proven to be impossible to solve analytically, we combine the method of weighted residuals (MWR) and simultaneous policy update algorithm (SPUA) to obtain its approximate solution. Finally, the simulation studies are conducted on a nonlinear diffusion-reaction process and a temperature cooling fin of high-speed aerospace vehicle, and the achieved results demonstrate the effectiveness of the developed control method.     

Voidage characteristics and prediction of bed expansion in liquid-solid inverse fluidized bed

Studies on voidage fluctuations, axial voidage profile and bed expansion are carried out by measuring the local void fraction using particles of wide ranging characteristics in liquid-solid inverse fluidized bed. The quality of fluidization is elucidated by the local voidage fluctuations. The RMS voidage fluctuation depicts a maximum with respect to average bed void fraction and increases with increase in Archimedes number. The fluidization quality has been quantified using average normalized RMS voidage fluctuation in terms of Transition number. The axial void fraction is almost uniform throughout the bed except for particles with size distribution. All the literature and present experimental data on bed expansion are unified in terms of Richardson and Zaki equation using experimental terminal velocities. A new correlation is proposed for predicting the wall effect corrected experimental terminal velocities, as a substitute for standard drag equation. The bed expansion data are also predicted using the drift flux model.     

Mass transfer coefficients of styrene and oxygen into silicone oil emulsions in a bubble reactor

The absorption of oxygen and styrene in water-silicone oil emulsions was independently studied in laboratory-scale bubble reactors at a constant gas flow rate for the whole range of emulsion compositions (0-10% v/v). The volumetric mass transfer coefficients to the emulsions were experimentally measured using a dynamic absorption method. It was assumed that the gas phase contacts preferentially the water phase. In the case of oxygen absorption, it was found that the addition of silicone oil hinders oxygen mass transfer compared to an air-water system. Decreases in k_La_oxygen of up to 25% were noted. Such decreases in the oxygen mass transfer coefficient, which imply longer aeration times to transfer oxygen, could represent a limiting step in biotechnological processes strongly dependent on oxygen concentration. Nevertheless, as the large affinity of silicone oil for oxygen enables greater amounts of oxygen to be transferred from the gas phase, it appears that the addition of more than 5% silicone oil should be beneficial to increase the oxygen transfer rate. In the case of styrene absorption, it was established that the volumetric mass transfer coefficient based on the emulsion volume is roughly constant with the increase in the emulsion composition. In spite of the relatively high cost of silicone oil, water-silicone oil emulsions remain relevant to treat low-solubility volatile organic compounds, such as styrene, in low-concentration gas streams.     

Numerical study on the turbulent reacting flow in the vicinity of the injector of an LDPE tubular reactor

Three-dimensional (3-D), transient numerical simulations of the turbulent reacting flow in the vicinity of the initiator injection point of a low-density polyethylene (LDPE) tubular reactor using a large eddy simulation (LES) approach combined with a filtered density function (FDF) technique are presented. The numerical approach allows for detailed predictions of the turbulent flow field and the associated (passive and reactive) scalar mixing. The aim is to study the influence of the injector geometry and initiator injection temperature on the LDPE process in terms of product quality (average polymer chain length, and polydispersity) and process efficiency (such as initiator consumption).     

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.     

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.     

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.