Effective design of photocatalytic reactors: influence of radiative transfer on their performance

The role of radiative transfer on a photocatalytic reaction occurring in an annular reactor has been investigated by modeling the degradation of chloroform contaminating the processed aqueous stream. From the analysis of the resulting effects it is apparent that the crucial factor in assessing the performance of the reactor is the distribution of radiant energy absorption rate which, in turn, depends on the mass of catalyst and on the way it is dispersed within the reactor. An optimization of the reactor is conceivable through a proper selection of the catalyst dispersion.     

旋转薄膜浆态光催化反应器

在分析现有光催化反应器特点的基础上,提出了一种新型的旋转薄膜式浆态光催化反应器（RFFS）。用商品化光催化剂Degussa P25,以苯酚模型有机物为降解对象,对比了RFFS与传统鼓泡浆态光催化反应器（TBS）的性能,研究了RFFS的光催化性能。结果表明,与传统浆态光催化反应器相比,RFFS具有较高的光催化性能,尤其是能够在较高的光催化剂浓度下运行。在光催化体系的循环流速大于2.7 L·min-1、供气流量为1.0 L·min-1、催化剂浓度为3.0 g·L-1的条件下,RFFS比传统浆态鼓泡光催化反应器的降解速率提高1.6倍。RFFS利用旋转浆态薄膜强化了光催化反应体系的传质,同时提高了体系中光催化剂对光能的利用率,较好地解决了光在传统浆态体系中的传递问题,为开发新型的具有工业应用前景的光催化反应器提供了方案。苯酚在RFFS中的降解动力学符合表观一级动力学模型,理论值与实验结果吻合较好。     

First-principles modeling, scaling laws and design of structured photocatalytic oxidation reactors for air purification

First-principles, predictive engineering models provide a sound theoretical basis for quantifying the inherent light energy utilization capabilities and performance limitations of candidate commercial photocatalytic oxidation reactor configurations. In particular, these models provide insight into the similarities and differences between photoreactors based on structured honeycombed monoliths, and those based on reticulated foams or other random catalyst supports.

For honeycombed monoliths, a deterministic first-principles radiation field model provides the channel wall light intensity profile down the length of a single channel in the monolith. A three-dimensional developing flow convection–diffusion reaction model employing this radiation field submodel predicts the velocity and concentration fields. The model shows that light intensity gradients in a monolith of typical dimensions are severe, that only a fraction of the monolith can be effectively photo-activated, and as a consequence process performance is largely controlled by light distribution. For a given light source and photocatalyst combination, reactor performance scales according to the aspect ratio of the channeled monolith, the Reynolds number, and the Dahmköhler number.

For randomly structured monoliths, the radiation field must be determined by probablistic methods. Monte Carlo simulations show that the radiation field in such random porous structure scales according to the pore size distribution and the void fraction, and the photocatalyst film thickness. Reactor performance scales by the radiation field, the Peclet number, the Stanton number, and the Dahmköhler number. The complex interrelationship between the random structure of the monolith and the resulting radiation field and mass transfer behavior makes scaling of these reactor types particularly difficult.     

Similar effect of carbon and silica catalyst support on the hydrogenation reaction rate in organic slurry reactors

This paper investigates the influence of the catalyst support type on mass transport and reaction rate for the case of hydrogenation of α-methylstyrene to cumene in a gas inducing stirred slurry reactor and in a slurry bubble column. The reaction is carried out in the presence of 3% Pd/carbon and 3% Pd/silica catalyst particles. The lyophobicity of the two catalyst supports in the cumene slurry is found to be similar. The overall rate of the hydrogenation reaction is described by the classical transport and reaction resistances-in-series model. The rate of gas-to-liquid mass transfer is somewhat larger during reaction than without reaction. This enhanced mass transfer points to particle-to-bubble adhesion as a result of the relative affinity of both catalyst supports to the gas phase. The observed reaction enhancements are similar for both Pd/carbon and Pd/silica catalyst/cumene slurries.     

Modelling of unsteady-state hydrodynamics in periodically operated trickle-bed reactors:Influence of the liquid-phase physical properties

Process intensification using periodic operation of trickle bed reactors (TBRs) is still a long way from replacing conventional steady-state operation in industrial use, despite the numerous benefits described in the literature. Complex interactions between hydrodynamics, mass transfer and reaction phenomena make the design of periodically operated TBRs an almost insurmountable challenge. The development of hydrodynamic models able to provide reliable quantitative predictions of flow behaviour and possessing a sound physical basis, is an essential prerequisite for obtaining the necessary insights into this complexity. In this work, the two-phase pressure drop and dynamic liquid hold-up during max/min and on/off periodical operation were predicted using a model based on the relative permeability concept. In order to demonstrate the utility of this approach, a systematic investigation of the quantitative influence of the liquid-phase physical properties was carried out. The results obtained show that the modelling of the hydrodynamics in periodically operated TBRs using the relative permeability concept is feasible. By selecting suitable permeability parameters, unsteady-state hydrodynamics for different periodic operating modes can be predicted successfully.     

Microstructured reactors and supports for ionic liquids

Different types of microstructures and their applications with respect to the synthesis and the use of ionic liquids are presented. Microstructured reactors are suitable for reactions with fast intrinsic kinetics, requiring high mass and heat transfer performances. Chemical synthesis can be performed safely under operating condition (e.g. high temperature, pressure, etc.) difficult to obtain in traditional reactors. The examples presented clearly indicate that microstructured reactors offer superior performance for the synthesis of ionic liquids in comparison to conventional equipment. For the use of ionic liquids as reaction media, existing ionic liquids show some limitations due to their higher viscosity compared to conventional solvents. Therefore, future research should be focused on the development of low viscosity ionic liquids.The approaches to use ionic liquids in microstructured reactors and in combination with microstructured supports for catalytic reactions show many advantages in view of high product selectivity and yield. The use of supported ionic liquids on microstructured materials seems to be particularly promising for gas phase as well as for gas/liquid reactions.     

A comparison between chemical and sputtering methods for preparing thin-film silver electrodes for in situ ATR-SEIRAS studies

Stable silver thin films were prepared either by chemical deposition or by argon sputtering on germanium and silicon substrates, respectively, and used as electrodes for in situ infrared spectroscopy experiments with a Kretschmann internal reflection configuration. The spectra obtained for acetate anions adsorbed from neutral solutions showed a noticeable intensity enhancement (SEIRA effect). This enhanced absorption has been related to the surface structure of the films that have been characterized by ex situ STM and in situ electrochemical measurements (lead underpotential deposition, UPD). STM images of the chemically deposited silver films show mean grain sizes ranging from ca. 20 to 90 nm for deposition times between 2 and 20 min, and the absence of flat domains. On the other hand, STM images of the films deposited by argon sputtering show mean grain sizes around 30 nm for a film growth rate of 0.05 nm s⁻¹ and 70 nm for a film growth rate of 0.005 nm s⁻¹. In this latter case, atomically flat domains up to 50 nm wide have been observed. This observation is consistent with a more defined voltammetric profile for lead UPD, that indicates a higher degree of surface order. Moreover, the roughness factor obtained from the charge density involved in lead UPD in the case of the sputtered silver film is lower than that measured for the chemically deposited silver film. All these structural data can be connected with the observations on the effect of deposition conditions of the silver film on the SEIRA effect for adsorbed acetate. Maximum enhancement is observed for chemically deposited films and sputtered films at high deposition rate for which the grain size is around 40-60 nm. The increase of the grain size for the sputtered silver films deposited at decreasing deposition rates can be related to the observed decrease in the SEIRA effect.     

Computational fluid dynamics modeling of immobilized photocatalytic reactors for water treatment

A computational fluid dynamics (CFD) model for the simulation of immobilized photocatalytic reactors used for water treatment was developed and evaluated experimentally. The model integrated hydrodynamics, species mass transport, chemical reaction kinetics, and irradiance distribution within the reactor. The experimental evaluation was performed using various configurations of annular reactors and ultraviolet lamp sizes over a wide range of hydrodynamic conditions (350 < Re < 11,000). The evaluation showed that the developed CFD model was able to successfully predict the photocatalytic degradation rate of a model pollutant in the analyzed reactors. In terms of hydrodynamic models, the results demonstrated that the laminar model performs well for systems under laminar flow conditions, whereas the Abe‐Kondoh‐Nagano low Reynolds number and the Reynolds stress turbulence models give accurate predictions for photoreactors under transitional or turbulent flow regimes. The performed analysis confirmed that degradation rates of organic contaminants in immobilized photocatalytic reactors are strongly limited by external mass transfer; as a consequence, the degradation prediction capability of the CFD model is largely determined by the external mass transfer prediction performance of the hydrodynamic models used. © 2010 American Institute of Chemical Engineers AIChE J, 2011     

Theoretical performance of countercurrent reactors for production of enantiopure compounds

Irreversible reactions are being applied in enzymatic kinetic resolution to obtain enantiomerically pure compounds from racemic mixtures. Using model calculations for situations without mass transfer limitation, we show that reversible reactions might also be useful for enzymatic kinetic resolution, provided that countercurrent systems are used rather than batch or cocurrent systems. The required reaction time or enzyme amount in a countercurrent system is much lower than in an analogous cocurrent system or its batch equivalent. More importantly, often the calculated yield and enantiomeric excess are better in countercurrent systems. Racemization can also be favorably used in countercurrent systems. Consequently, to achieve with a reversible reaction a particular enantiomeric excess and yield, a countercurrent system needs less dilution or activated co-reactant and less enantioselective enzyme than a cocurrent system.     

A hybrid CFD—reaction engineering framework for multiphase reactor modelling: basic concept and application to bubble column reactors

The coupling of turbulent mixing and chemical phenomena lies at the heart of multiphase reaction engineering, but direct CFD approaches are usually confronted with excessive computational demands. In this hybrid approach, the quantification of mixing is accomplished through averaging the flow and concentration profiles resulting from a CFD flow field calculation and a computational (“virtual”) tracer experiment. Based on these results, we construct a mapping of the CFD grid into a generalised compartmental model where the chemistry calculations can be efficiently carried out. In contrast to the empirical models used in the residence time distribution (RTD) approach, the compartmental model in this methodology, owning to its CFD origins, retains the essential features of the equipment geometry and flow field. A procedure for extracting the mixing information from k-ε based CFD codes is outlined, but the main concept of the approach is not restricted to any particular type of turbulence modelling, and will therefore benefit from future developments. A phenomenological model of mass transfer and chemical reaction, based on the penetration theory, is employed to simulate the interfacial phenomena in gas-liquid reactors, and a study of CO₂ absorption into alkali solution is presented to demonstrate the method.     

Preparation and characterization of three-dimensional tin thin-film anode with good cycle performance

Three-dimensional tin thin-film anode was prepared by electroless plating tin onto three-dimensional (3D) copper foam (which served as current collector), and characterized physically by SEM, EDS and XRD. Its electrochemical property and mechanism were studied by charge-discharge test, cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). The SEM and EDS results indicated that tin film with 500 nm thickness was formed over the whole surface of copper branches. The XRD results suggested that a new phase of Cu₆Sn₅ was formed between copper and tin. Besides the tin microflake structure of 500 nm thickness, the interaction effects of the copper foam and Cu₆Sn₅ phase formed between copper and tin resulted in good cycle performance with first discharge capacity of 737 mAh g⁻¹, 97% capacity retention after 20 cycles and still 84% after 40 cycles.     

Modelling of the coupling hydrodynamic transfer for a gas-liquid countercurrent flow on a wavy surface

This paper concerns laminar countercurrent gas-liquid flow over a wavy wall column, in the case of a falling liquid film. The modelling concerns the coupling of hydrodynamic and heat and mass transfer for an absorption as an example of application. The falling liquid film interacts, through the free interface, with the gas phase. The wavy surface generates particular hydrodynamic conditions with the presence of a vortex in both phases. The consequence of these vortices is an increase of transfers compared to the smooth wall.     

Performance limits of isothermal packed bed and perforated monolithic bed reactors operated under laminar flow conditions. Part II: performance comparison and design considerations

The maximally attainable productivity of perforated monolithic bed reactors has been compared to that of the traditional packed bed of spheres for the case of laminar flow conditions and first-order isothermal reaction kinetics. Using the E number established in Part I, it could be shown in a very condensed, yet fully quantitative way that the maximal gain in total reactor productivity which can be obtained by switching from the tortuous pore system of the packed bed of spheres (large flow resistance) to the straight-running flow-through pores of a perforated monolithic bed (minimal flow resistance) typically is 25-40%. Much larger gains in total productivity can be obtained by using highly open-porous beds. Whether this high porosity is achieved using a perforated monolithic bed, a packed bed of hollow extrudates or a structured fibre-mesh bed is then only of secondary importance. The E number also allows to quantify the potential gain and the range of applicability of the more advanced reactor designs proposed in the past years (e.g., the parallel passage reactor). It could now be shown that the productivity gain of these advanced concepts can even amount up to a factor of 100. As these gains are to be realized in beds with ultra-small flow-through pores, the present study also provides a strong quantitative argument for the current research on (micro-)structured reactor beds.     

Preparation and photocatalytic properties of macroporous honeycomb alumina ceramics used for water purification

In this work, porous alumina ceramics with highly ordered capillaries were successfully fabricated by ionotropic gelation process of alginate/alumina suspensions. By varying the initial solid loading (10–30 wt%) of slurries, the porosity of alumina ceramics ranged from 60.4% to 79.5% with controlled pore size (180–315 μm). Due to the well-crosslinked macroporous structure and large specific surface areas, the porous ceramics were utilized as the photocatalyst supports of TiO₂ catalysts whose photocatalytic activity was characterized by degrading methyl blue under UV irradiation. TiO₂ coatings prepared by sol–gel method demonstrated excellent adhesion to the substrates. When the solid loading of supports reached 15 wt%, the TiO₂ coatings showed the highest photocatalytic efficiency of 79.52%. Besides, TiO₂ films possessed nearly the same photocatalytic activity as titania/water suspension. Thus, the honeycomb ceramic prepared by self-organization process holds promise for use as photocatalyst supports in water purification without recycling process of powders.     

Fabrication of ZnO:Ag/GO composite thin films for enhanced photocatalytic activity

Zinc Oxide (ZnO), ZnO/GO (Graphene Oxide) and ZnO:Ag/GO nanocomposite thin films were deposited on glass substrates using a simple cost-effective automated jet nebulizer spray pyrolysis technique for photocatalytic degradation of organic dyes. The ZnO:Ag/GO film exhibits superior photocatalytic activity when compared with ZnO and ZnO/GO films. The degradation rate constant values of pristine ZnO and ZnO/GO films are found to be 0.0143 and 0.0176 min⁻¹, respectively whereas that of ZnO:Ag/GO is 0.0567 min⁻¹. The possible mechanism involved in the enhanced photocatalytic activity of ZnO:Ag/GO films for the degradation of Methylene Blue dye is proposed with the help of structural, optical and photoluminescence studies. The powder XRD profile confirms the hexagonal wurtzite structure of the synthesized catalysts. The results of Raman, XPS and EDX studies confirm the presence of GO in the ZnO/GO and GO and Ag in the ZnO:Ag/GO films.     

Application of a three-layer modeling approach for solids transport in horizontal and inclined channels

The three-layer model concept developed previously for solid-liquid flow has been adapted to model solids transport in inclined channels. The present model predicts the pressure loss and transport rate of solids in Newtonian and power-law fluid suspensions by assuming stratified flow conditions. Sets of stationary sand bed transport rate tests were performed to verify the predictions of the model. A 70-mm flow loop was constructed to measure the average transport rates and critical flow rates, which are required to initiate the motion of solids bed particles. The tests were carried out by eroding stationary sand beds with water and an aqueous solution of poly anionic cellulose (PAC) in a transparent pipe. Four sand beds with different particle size ranges were used. The average transport rates of the beds were predicted using the model. The model predictions show a satisfactory agreement with experimentally measured results when the grain Reynolds number is between 15 and 400 and the flow rate is sufficiently higher than the critical flow rate. Therefore, with some degree of limitation, the three-layer model can be applicable for predicting the transport rates of stationary solids beds in inclined channels for both Newtonian and power-law fluids.     

Design and optimization of a new photocatalytic reactor with immobilized ZnO for water purification

The objectives of this work were to study the degradation of the niflumic acid (NIF) with a new solar photocatalytic reactor using immobilized ZnO. The residence time distribution and the reactor efficiency for the NIF removal were studied. Two models for Retention Time Distribution (RTD) have been summarized and compared with the experimental data. Effects on the photocatalytic degradation process owing to various operating parameters conditions were also studied in continue flow. The repetitive operation performance indicates the good reproducibility of the new reactor for NIF degradation. Concerning the energy, the reactor was designed to ensure its energy autonomy through solar energy.     

溶胶-凝胶法制备二氧化钛薄膜的研究进展

光催化净化空气是一项造福人类的绿色环保技术。TiO₂薄膜化是光催化技术实用化的关键。介绍了近几年来溶胶-凝胶法制备TiO₂薄膜光催化净化空气的研究进展,对溶胶-凝胶法制备TiO₂薄膜的各种影响因素进行了分析,并探讨了提高TiO₂薄膜光催化能力的途径及其相关的光催化反应系统。