Modeling the photocatalytic degradation of formic acid in a reactor with immobilized catalyst

A kinetic model for the photocatalytic degradation of formic acid in an immobilized system is presented, including the dependency of the reaction rate on the concentration of formic acid and oxygen, the catalyst layer thickness and the light flux. In the system some external mass transfer limitation occurs which is included in the modeling with experimentally determined values for the mass transfer coefficient of both formic acid and oxygen. The model describes the measurements well. The degradation rate appears to depend linearly on the light intensity. The adsorption of formic acid and oxygen on the catalyst layer appears to play an important role in the degradation rate.     

Design consideration of photocatalytic oxidation reactors using TiO2-coated foam nickels for degrading indoor gaseous formaldehyde

In the design process of the photocatalytic oxidation (PCO) reactor using TiO₂-coated foam nickels, the optimum of catalyst film thickness, light intensity and flow velocity were considered. A model was developed to study the effect of catalyst film thickness on photocatalytic degradation of formaldehyde by a TiO₂-coated foam nickel at continuous flow mode. In this model, external mass transfer and internal molecule diffusion-reaction were considered. A first-order kinetics equation was used to account for the photocatalytic reaction. Two exponential equations were employed to describe the distribution of light intensities in foam nickels and catalyst films, respectively. Validated with experimental data, the model can be used to predict the optimal thickness of catalyst films. A method for determining appropriate light intensities was proposed and discussed. The appropriate light intensity can be obtained by giving a margin, regarded as an excess coefficient, to the light intensity calculated based on the assumption of complete use of excited electron–hole pairs. The excess coefficient needs to be determined experimentally. In addition, the optimal flow velocity of PCO reactors could be consistent with the required one by changing the windward area of foam nickels. Based on the theoretical analyses, a novel PCO reactor containing 15 parallel-connected cells was designed. Each reaction cell was composed of an UV lamp and a TiO₂-coated tubular foam nickel. The performance of the reactor was tested by degrading gaseous formaldehyde at an indoor concentration level. The results showed that the reactor had low pressure loss and good degradation capability.     

Radiation absorption and degradation of an azo dye in a hybrid photocatalytic reactor

Results are presented for the photocatalytic degradation of an azo dye, reactive blue 69, in a novel hybrid photocatalytic reactor, illuminated by solar radiation and artificial light, under different experimental conditions. A radiative transfer model, based on the P1 approximation, is proposed to evaluate the distribution of local volumetric rate of photons absorption (LVRPA) in the reaction space of the hybrid photocatalytic reactor. This radiation transfer model, together with a first order kinetic model, is used to fit the experimental results. The model correlates well with the experiments, and values for an apparent first order kinetic constant for the degradation of RB69 are obtained. The proposed radiative transfer model (P1 approximation) is simple enough to allow for an analytical solution, yet complex enough to take into account scattering of radiation in all directions and to all orders. Simulations show a distribution of LVRPA that varies smoothly at small catalyst concentration, and is very quickly attenuated for high concentrations. Around of 70% of photons supplied by both illumination sources to the hybrid photocatalytic reactor are absorbed by the catalyst. The experimental results show the decolorization degree increases as catalyst concentration increase. In relation to mineralization process, the removal of total organic carbon is nearly complete after 5 hours irradiation. This indicates that not only the azo bond breakage is carried out, but also that the intermediate species are mineralized. The apparent kinetic constant has a dependence on catalyst concentration which is described by an adsorption model. Addition of oxygen by means of an air diffuser proves to be beneficial to the process.     

以十二烷基苯磺酸钠为模型反应物的Y2O3／TiO2复合催化剂的制备及光降解催化性能

采用浸渍法制备了Y2O3/TiO2复合氧化物催化剂，并用紫外可见光谱、SEM、BET等手段对其进行了表征，以水相十二烷苯苯磺酸钠（DBS）溶液的光催化降解反应为实验模型，考察了TiO2掺杂Y2O3后的光催化氧化活性，并探讨了Y2O3掺杂量、吸附性、焙烧温度及时间对Y2O3/TiO2复合氧化物催化剂光催化活性的影响，实验结果表明：复合氧化物催化剂Y2O3/TiO2存在某一最佳组分比值，当两者重量比为1：200时，其催化活性是同样条件下前体催化剂TiO2的2.4倍。     

Mass transfer effects in hydrogenations in slurry reactors

The effects of external and internal particle mass transfer resistance on rate for hydrogenations carried out on finely divided catalyst in slurry reactors are treated. A method is developed for estimating external particle mass transfer resistance from rate measurements and is demonstrated experimentally. The existence of reactions with negligible internal diffusion resistance and significant external diffusion resistance is modeled theoretically and demonstrated experimentally.     

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     

The Adsorption of Pollutants by Peat,Lignite and Activated Chars

The ability of peat, lignite and activated chars made from peat and lignite to adsorb dyes and metals from wastewater and NO₂ from air was investigated. Equilibrium isotherms were determined to assess the maximum adsorption capacity of the adsorbents for the pollutants. Kinetic studies for the adsorption of dyes and metal ions onto the adsorbents were undertaken in agitated batch adsorbers. Mass transport models were tested to predict the concentration decay curves in batch adsorbers. The models tested were single resistance models based on the assumption of a single external mass transfer coefficient and two resistance models which included an internal diffusion coefficient and an external mass transfer coefficient. The surface phenomena which influence the extent and the rate of uptake have been studied. The equilibrium capacity data conform to Langmuir plots. A previously proposed model was used to evaluate the external single resistance mass transfer model and was successfully applied to predict the adsorption of metal ions in single component systems under batch conditions. It has been shown that the assumption of negligible intraparticle diffusion is valid and that external film diffusion is the rate limiting step in describing the adsorption processes at high sorbent loadings. The same type of result is not observed for the adsorption of coloured organic matter onto peat where the sorption processes cannot be successfully modelled by use of a single resistance model and a two resistance model incorporating internal diffusion is required. The surface phenomena which influence the extent and the rate of uptake of NO₂ have been studied. The type of chars produced and the activation processes affect the adsorption. As activation increases, micropore volume and surface area increase and the maximum capacity of the adsorbent increases. Surface area alone is not the only parameter which affects equilibrium uptake. © 1997 SCI.     

CNTs负载TiO2对茜素红溶液的降解研究

以钛酸四丁酯为前驱体,冰醋酸为水解抑制剂,采用溶胶-凝胶法制备纳米TiO₂/CNTs复合催化剂。测定复合催化剂对不同浓度茜素红溶液的光降解活性,并对光催化降解动力学进行研究。结果表明,在CNTs负载质量分数7.5%、焙烧温度450 ℃和焙烧时间3.5 h条件下制备的TiO₂/CNTs复合催化剂光催化活性得到提升。相同条件下,TiO₂/CNTs对150 mg·L^-1茜素红溶液的最终降解率达67%,而TiO₂的最终降解率只有56%。复合催化剂光催化活性提升的原因是CNTs负载后光催化粒子在紫外光照射下生成更多的氢氧根活性自由基。光催化降解动力学符合Langmuir-Hinshelwood方程,催化剂对污染物的表观吸附是整个反应的“瓶颈”。随着反应浓度的增大,光催化降解反应的反应级数由一级反应逐渐下降为零级反应。     

An evaluation of single resistance transfer models in the sorption of metal ions by peat

The adsorption of Pb²⁺, Cd²⁺, Cu²⁺, Zn²⁺ and Al³⁺ onto peat has been studied using a system of standardised batch adsorbers under equilibrium and transient rate conditions. Models based on the assumption of a single external mass transfer resistance to adsorption have been developed to attempt to predict the concentration decay curves obtained experimentally, for several variable system parameters, under transient batch conditions. In this instance the single resistance is taken as that of film diffusion of the sorbate through the liquid film surrounding the sorbent particle. Two methods of determining the external mass transfer coefficient have been used, one based on the Weber model and the other on the Furusawa-Smith model. Equilibrium data have been analysed and shown to conform to a linear Langmuir plot. The decay curves predicted by the models represented the experimental data very well, particularly at high sorbent masses. Under these conditions film diffusion appears to be the rate controlling step in the adsorption of metal ions by the peat. At low sorbent loadings, however, the influence of internal mass transfer is evident.     

结构化5A分子筛吸附床结构及工艺参数对N2/H2吸附性能的影响

利用吸附等温线获得动力学参数,建立了CFD模型,模拟了氢气/氮气在结构化5A分子筛吸附床中的吸附过程,研究了吸附剂层片间距、吸附剂厚度等结构参数和吸附压力、进气流量等工艺参数对混合气吸附效果的影响。结果表明：减小层片间距和吸附剂厚度可显著提高传质系数和床层利用率。增大吸附压力可提高床层利用率,但会减小传质系数。进气流量对传质系数的影响不明显,但当流量较大时,吸附容量和床层利用率均呈减小趋势。结构化5A分子筛吸附剂吸附性能良好。     

Photocatalytic degradation of gaseous perchloroethylene in continuous flow reactors: Rate enhancement by chlorine radicals

The degradation of perchloroethylene (PCE) by UV/TiO₂ photocatalysis in gas phase was studied. The degradation efficiency has been compared in different continuous flow reactors: a photocatalytic tangential reactor (PTR) where the air flows tangentially over the catalytic medium and two photocatalytic filtering reactors (PFR) where the air flows through the porous catalytic medium. The degradation rate shows a linear dependence with the concentration of pollutants (up to 350 mg PCE/N m³) for the PTR, but the degradation was negligible for the PFR. The degradation rate was enhanced by accelerating the chlorine radicals’ formation (by adding HCl in catalytic quantity in the air flow or by PCE over-heating). In these conditions, the oxidation rate constant of PCE in the PFR was about five times higher than that in the PTR, although the mass of catalyst involved in the PFR was about 10 times lower and the contact time was about a 1000 times shorter than that of the PTR. Thus, the catalyst is globally more efficiently used in the PFR, as the mass transfer is not limiting. As a result, a degradation mechanism of PCE, involving the generation of free chlorine radicals, as the first limiting step, has been confirmed.     

Spatiotemporal patterns in a porous catalyst during light-off and quenching of carbon monoxide oxidation

A detailed numerical model was used to simulate the behavior of carbon monoxide oxidation within a porous platinum/alumina catalyst during temperature ramps. The model was validated in previous work by fitting step-response experiments which were performed over a range of temperatures and in which concentration gradients over the catalyst layer were directly measured. As a result of the low CO and O₂ concentrations used, the catalyst layer could be considered isothermal. The numerical experiments performed with the model in this work reveal complex spatial patterns of species and local reaction rate which change with time and temperature.As temperature is increased, CO desorbs and reaction rapidly increases, reacting adsorbed CO off the Pt surface and producing a peak in CO₂ production during catalyst light-off. Over a nonporous surface of the same material, the reaction rate would be an order-of-magnitude lower and no CO₂ peak would be produced. At steady state after reaction light-off has been obtained, reaction occurs in a narrow zone below the external face of the layer which is exposed to the constant feed gas composition. As temperature is then decreased, the CO₂ production rate decreases gradually as the front of the region covered with adsorbed CO penetrates further and pushes the reaction zone deeper into the catalyst layer. When the adsorbed CO front reaches the internal face, the CO₂ production rate drops abruptly as the reaction “quenches”.Catalyst layer thickness was changed over the range 0.06-1.0 mm at constant total Pt content. As the layer thickness was decreased, the steady-state CO₂ production rate after light-off increased, however the range of temperatures in which the catalyst was active decreased. Three qualitatively different sets of spatiotemporal patterns were obtained as the layer thickness was changed from relatively thin, to medium, to thick. Analysis of the patterns provides understanding of the temperature-dependent behavior of the catalyst and how this behavior varies with catalyst layer thickness.     

纳米TiO2/凹凸棒石光催化复合材料的制备及其动力学

采用溶胶-凝胶法制备了纳米TiO2/凹凸棒石光催化复合材料,并对其进行了表征. XRD结果表明,复合材料为锐钛矿相和金红石相的混晶结构,HRTEM和SEM图像显示,在凹凸棒石表面均匀负载了一层纳米TiO2颗粒. 实验考察了煅烧温度、催化剂用量及材料吸附性能对其光催化性能的影响,结果表明,600℃下煅烧制备的复合材料中TiO2晶粒尺寸为16 nm,锐钛矿相含量87%. 复合材料对甲基橙有较强的吸附性,吸附平衡常数Ka=0.00896 L/mg,催化剂添加量为3 g/L时光照2 h对甲基橙的降解率达92%. 光催化动力学方程符合Langmnir-Hinshelwood模型,二级动力学方程可很好地描述其降解规律：lnCt+0.00896Ct=0.418-0.0197t.     

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。     

Adsorption of acid dye onto woodmeal by solid diffusional mass transfer

The adsorption of Telon Blue (Acid Blue 25) dye onto wood has been studied using an agitated batch adsorber. The variables studied include agitation, initial dye concentration, wood mass, wood particle size and dye solution temperature. Isotherms were measured and the isotherm parameters were determined.

A mathematical model has been developed using the basis of the model proposed by Mathews and Weber Jr. This model is based on external mass transfer and solid-phase diffusion, and has been used to generate theoretical concentration—time decay curves. The results of the model were adjusted to the experimental data using a ‘best fit’ approach. The external mass transfer coefficient was found to vary with the degree of agitation, and consequently all other variables were considered at a constant agitation speed of 400 rev min⁻¹. A good agreement between the theoretical generated and the experimental concentration—time decay curves was achieved using a constant external mass transfer coefficient, 0.30 × 10⁻³ cm ⁻¹, and a constant solid-phase diffusivity, 0.200 × 10⁻⁸ cm² s⁻¹, for varying initial dye concentrations as well as wood mass. In experiments where the particle diameter was varied, a constant external mass transfer coefficient was sufficient to describe the system, but a decreasing diffusivity was required with increasing particle size. To simulate the effect of varying temperature, both external mass transfer coefficient and diffusivity were varied.     

Competitive adsorption of two dissolved organics onto activated carbon—III: Adsorption kinetics in fixed beds

Adsorption breakthrough curves for bisolute systems of dissolved organics on activated carbon are measured in fixed beds.Results for strongly adsorbable species indicate that at low liquid concentrations (X<0.1 mmol/l.) only external mass transfer resistance is rate determining.However, at higher liquid concentrations internal mass transfer becomes increasingly significant. Breakthrough behaviour is predicted using alternatively three different models with different assumptions about diffusion in the liquid filled pores and diffusion on the surface in series with external film diffusion.Multi-solute adsorption equilibria are predicted from single-solute data using the ideal adsorbed solution theory developed by Myers and Prausnitz, while the single-solute equilibria are represented by Freundlich isotherms. The external mass transfer coefficient for each component is calculated by a general correlation for heat and mass transfer in fixed beds. The internal diffusion coefficient for each component is determined in batch reactor tests with the single-solute system.Systematic deviations between measured breakthrough curves and those calculated from different models using only single-solute data are observed in all experiments with mixed solutes if there is significant internal diffusional resistance and marked displacement of one component inside the carbon particles. The deviations may be due to mutual interference of diffusing molecules. A better agreement between calculated and observed breakthrough curves can be obtained using an extended model in which mixture data are required.     

On the use of internal mass transfer coefficients in modeling of diffusion and reaction in catalytic monoliths

We utilize the recently developed concept of internal or intraphase mass transfer coefficient to simplify the problem of diffusion and reaction in more than one spatial dimension for a washcoated monolith of arbitrary shape. We determine the dependence of the dimensionless internal mass transfer coefficient (Sh_i) on washcoat and channel geometric shapes, reaction kinetics, catalyst loading and activity profile. It is also reasoned that the concept of intraphase transfer coefficient is more useful and fundamental than the classical effectiveness factor concept. The intraphase transfer coefficient can be combined with the traditional external mass transfer coefficient (Sh_e) to obtain an overall mass transfer coefficient (Sh_app) which is an experimentally measurable quantity depending on various geometric and transport properties as well as kinetics. We present examples demonstrating the use of Sh_app in obtaining accurate macro-scale low-dimensional models of catalytic reactors by solving the full 3-D convection-diffusion-reaction problem for a washcoated monolith and comparing the solution with that of the simplified model using the internal mass transfer coefficient concept.