【作者】 董双石；

【导师】 任南琪； Xiaotao Bi；

【作者基本信息】 哈尔滨工业大学 ， 环境工程， 2009， 博士

【摘要】 二氧化钛光催化在环境污染物治理过程中的研究已经较为深入,但绝大多数仍处于实验室阶段,其主要的原因是悬浆式光催化剂暴露出易失活、易凝聚、难回收的缺点和负载后的光催化剂催化效率不够理想。而对于反应器的研究,只是基于实验需要制作的简易装置,由于进一步的研究涉及到相对复杂的多相光催化机理和反应动力学原理,尚未有适用的反应体系与装置问世。因此,设计高效的光催化反应器并对反应器进行详细评价,为其实际应用提供理论依据势在必行。对于负载型TiO2/UV/O3光催化降解有机物的反应动力学,研究者所建立的模型大多以悬浆型TiO2光催化常用的Langmuir-Hinshelwood模型为主。由于引入了臭氧,反应过程复杂得多。因此需要建立耦合多个反应的动力学模型,用以解释TiO2/UV/O3光催化降解苯酚的动力学过程。综合考虑流化床中固相和液相流化状态的观察结果,以及对液-固流化床、气-液-固三相流化床的初始流化速度测量和预测的结果和系统运行的经济性、维护管理等方面的因素,认为采用小粒径球形活性炭颗粒(Kureha 0.64mm)作为TiO2载体更为合理。以Sol-gel法将TiO2负载到球形活性炭颗粒表面。结果表明,该方法可以将TiO2均匀地负载到颗粒表面,负载量为6.51 wt%。表面负载没有改变原活性炭表面的多孔结构,制备的光催化剂对苯酚的吸附能力并没有比原活性炭明显下降。两种颗粒对苯酚的吸附/脱附是可逆的,可以用Freundlich等温吸附模型较好地描述。在此基础上采用朗伯-比尔定律并结合流化床中固相运动规律建立了床层径向光强分布模型,为流化床光催化反应器处理苯酚废水模型建立提供依据。结果表明,流化床中光强度沿径向呈指数衰减,并且随着床层固相含率的升高而加速。流化床操作条件对降解效率的影响研究表明,当通气量为3 L/min液相流量为13.8 L/min时,流化床光催化氧化苯酚的效率最高。将通气位置移至床层上方,在同样通气量的条件下,上部通气有更高的光催化效率。建立了流化床光催化反应器床层内液相苯酚浓度径向分布模型,模型的结果解释了液相流量对光催化氧化苯酚的影响。通过对模型积分计算出的平均液相苯酚浓度与试验测量值的比较,大部分误差在30%以内,说明建立的模型可以较好地描述流化床光催化氧化苯酚的过程。其它运行条件一致的前提下,5种氧化过程对苯酚降解总量的顺序为:TiO2/UV/O3 > UV/O3/AC > TiO2/UV/O2 > UV/O3 > O3。光催化-臭氧氧化的作用机制为臭氧俘获了光生电子而后生成羟基自由基,这种协同作用使得其反应效率最高。据此,建立了UV/O3/AC和UV/O3/TiO2-AC氧化过程的动力学模型,与实验数据对比,所建立的模型误差在5%以内,可以很好的描述各种多相反应的动力学过程。根据实验数据计算各均相和多相反应表观反应速率常数的结果表明,TiO2/UV/O3与TiO2/UV/O2过程中多相光催化反应速率常数之比为3.7,说明臭氧比氧更适合作为光生电子的俘获剂,减少电子空穴的复合,提高光催化反应的效率。更多还原

【Abstract】 Photocatalysis by titanium dioxide on treatment of environmental contaminants has been studied by many researchers for several decades. Most of the researchers were laboratory scale while not the application in industry. There are two reasons addressed: (1) Requirement of post-process separation of slurry TiO2 from aqueous media is an important obstacle to its practicable applications, although it posesses higher photocatalytic activity; (2) Requirement of study on reaction kinetics and mechanism for the appropriate photoreactors. Thus, it is necessary to promote the photoactivity of immobilized titamium dioxide and to design the photoreactor to give a better performance, and then to set up a suitable model to predict the performance and provide the theoretical basis for application. Langmuir-Hinshelwood model was mostly used to describe the photocatalytic reaction. The presence of ozone could make many reaction steps involved and much more complicated than the slurry TiO2 system. Therefore, it is meaningful to set up a model coupled with all the reaction steps to have a better understanding of TiO2 photocatalysis process.The following three kinds of particles were used to select the appropriate substrate for coating TiO2 and the bed particles base on the hydrodynamics characteristics. The Kureha 0.64 mm spherical activated carbon was chosed according to the observation on fluidization of solid and liquid phase, the prediction of minimum velocity of liquid-solid fluidized bed and gas-liquid-solid fluidized bed and the feasibility of maintenance and operation of the fluidized bed system. The results can be used to select the power equipment and accessories and be the basic knowledge for the reactor design and scale-up.In this study, immobilized TiO2 on spherical activated carbon particles prepared by sol-gel method were selected as the bed particles. The SEM and AES analysis indicated that the TiO2 were coated uniformly on the surface of the activated carbon and the coated mass was about 6.51 wt%. The photocatalytic activity of the prepared catalyst was evaluated by the degradation of phenol in a bench scale test unit before applied to an annular fluidized bed reactor. The results showed that the optimal calcination temperature for TiO2/AC was 500 oC. The coating had a negligible effect on the structure of the original activated carbon, because the adsorption properties did not change obviously after the coating. It was found that the adsoption/desorption of the prepared photocatalysts and original activated carbon can be reasonablely fitted by Freundlich model and the adsoption/desorption was reversible.The performance of the photoreactor is closely related to the light intensity distribution in the reactor. The result of time-series signals of light intensity in the liquid-solid fluidized bed showed that the bed particles moved randomly within a specified zone and the uniform fluidization was realized in the bed. The light intensity distribution model was developed based on the above result and the Lambert-Beer law. It was shown by the model that the light intensity decayed along the radical distance and the decay would accelerate with the increase of solid hold-ups.Effects of both liquid and air flow rates on the phenol degradation rate were examined. The experimental results showed that the increase of liquid and air flow rates may enhance the phenol degradation rate. However, very high liquid flow rate (over 13.8 L/min) and air flow rate (over 3.0 L/min) could lead to the decrease in performance of the three-phase fluidized bed photoreactor. The results on the effect of initial phenol concentration on degradation rate indicated that the photocatalytic reaction in the fluidized bed followed the first order kinetics and could be reasonably fitted by the Langmiur-Hinshelwood kinetics model. Compared to the three-phase fluidized bed in which air is introduced into the bed from the distributor, the liquid-solid fluidized bed in which oxygen is provided by injecting air into the freeboard region of the reactor showed a better phenol destruction performance and is thus preferred for photodegradation of water contaminants.For the photocatalytic oxidation of phenol in the liquid-solid fluidized bed photoreactor, the modeling of the liquid phase phenol concentration distribution along the radical distance was conducted. The integration methodology for the model was used to calculate the average liquid phase phenol concentration. Compared with the experimental data, the relative errors was within 30%, which indicated that the photocatalytic oxidation process in the liquid-solid fluidized bed can be well discribed by developed model.Five oxidation processes, namely O3, UV/O3, UV/O3/AC, TiO2/UV/O2 and TiO2/UV/O3, for phenol degradation in fluidized bed were evaluated and compared, and the photocatalytic ozonation was found to give the highest phenol conversion because of the combined actions of homogenous ozonation in the liquid phase, heterogeneous ozonation on the surface of the catalyst support, i.e. activated carbon, and heterogeneous photocatlytic oxidation on the TiO2 catalyst surface. With the simplified kinetics model, photolytic ozonation was confirmed to predominantly take place on the particle surface comparing the heterogeneous and homogeneous photolytic ozonation. The photocatalytic reaction on the catalyst surface with ozone (TiO2/UV/O3) is much higher than that with oxygen (TiO2/UV/O2), with an enhancement factor ( ) of 3.7, which confirmed that ozone, as a more effective scavenger than oxygen, promoted the reaction process by reducing the recombination of generated holes and electrons. k r ( O3 ) /kr ( O2)更多还原