G. Camera-Roda

Heterogeneous Photocatalysis and Photoelectrocatalysis: From Unselective Abatement of Noxious Species to Selective Production of High-Value Chemicals

Heterogeneous photocatalysis and photoelectrocatalysis have been considered as oxidation technologies to abate unselectively noxious species. This article focuses instead on the utilization of these methods for selective syntheses of organic molecules. Some promising reactions have been reported in the presence of various TiO2 samples and the important role played by the amorphous phase has been discussed. The low solubility of most of the organic compounds in water limits the utilization of photocatalysis. Dimethyl carbonate has been proposed as an alternative green organic solvent. The recovery of the products by coupling photocatalysis with pervaporation membrane technology seems to be a solution for future industrial applications. As far as photoelectrocatalysis is concerned, a decrease in recombination of the photogenerated pairs occurs, enhancing the rate of the oxidation reactions and the quantum yield. Another benefit is to avoid reaction(s) between the intermediates and the substrate, as anodic and cathodic reactions take place in different places.

Combination of ozonation and photocatalysis for purification of aqueous effluents containing formic acid as probe pollutant and bromide ion.

The treatment by advanced oxidation processes (AOPs) of waters contaminated by organic pollutants and containing also innocuous bromide ions may generate bromate ions as a co-product. In the present work heterogeneous photocatalysis and ozonation have individually been applied and in combination (integrated process) to degrade the organic compounds in water containing also bromide anions. The results show that: i) the sole photocatalysis does not produce bromate ions and in the case of its presence, it is able to reduce bromate to innocuous bromide ions; ii) the integration of photocatalysis and ozonation synergistically enhances the oxidation capabilities; and iii) in the integrated process bromate ions are not produced as long as some oxidizable organics are present.

Non-Fickian mass transport through polymers: A viscoelastic theory

Abstract The non-Fickian mass transport behaviour observed in polymers is described based on a non linear viscoelastic constitutive equation for the diffusive flux, containing a single relaxation time. The resulting non linear hyperbolic mass transport problem is numerically solved and the influence of the diffusive Deborah number is analyzed. The proposed model has the ability to represent both Case I1 and anomalous diffusion, as well as the frequently observed overshoot and oscillations in the weight uptake around the final equilibrium value.

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

Abstract 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.

Heat and mass transfer boundary layers in radial creeping flow

Abstract Heat and mass transfer in radial flow between two parallel disks are analysed. This kind of problem is sometimes encountered in chemical engineering and we faced this situation in membrane separation processes in which the fluid is fed into the center of the cell and flows towards zones at higher radii. In the examined case heat and/or mass are exchanged across one of the two surfaces which confine the flow region. An analysis of the transport phenomena of such a geometry is performed by resorting to two different mathematical models. As a result the concentration and/or temperature profiles, the transport coefficients and the influence of the relevant dimensionless groups are obtained. The capability of the models to predict the experimental behavior of the vacuum membrane distillation process is demonstrated.

Study and optimization of an annular photocatalytic slurry reactor.

The experimental results obtained for the photocatalytic degradation of a model organic dye in an annular slurry reactor are analyzed with the aid of a mathematical model. The model is used also to study the effects on the performances of many operative conditions: flow rate, photocatalyst concentration, power of the lamp, size of the photocatalytic particles, dimensions of the reactor. The investigation demonstrates that the rate of the process is often limited by the radiant energy transfer and that some simple rules can be followed in order to optimize different yields and the observed rate of reaction.

Improvement of membrane performances to enhance the yield of vanillin in a pervaporation reactor.

In membrane reactors, the interaction of reaction and membrane separation can be exploited to achieve a “process intensification”, a key objective of sustainable development. In the present work, the properties that the membrane must have to obtain this result in a pervaporation reactor are analyzed and discussed. Then, the methods to enhance these properties are investigated for the photocatalytic synthesis of vanillin, which represents a case where the recovery from the reactor of vanillin by means of pervaporation while it is produced allows a substantial improvement of the yield, since its further oxidation is thus prevented. To this end, the phenomena that control the permeation of both vanillin and the reactant (ferulic acid) are analyzed, since they ultimately affect the performances of the membrane reactor. The results show that diffusion of the aromatic compounds takes place in the presence of low concentration gradients, so that the process is controlled by other phenomena, in particular by the equilibrium with the vapor at the membrane-permeate interface. On this basis, it is demonstrated that the performances are enhanced by increasing the membrane thickness and/or the temperature, whereas the pH begins to limit the process only at values higher than 6.5.

Pervaporation membrane reactors

Abstract: Pervaporation is a peculiar membrane separation process which is currently being considered for integration with a variety of reactions in promising new applications. Indeed, pervaporation membrane reactors have some specific uses in sustainable chemistry, which is an area currently growing in importance. The fundamentals of this type of membrane reactor are presented in this chapter, along with the advantages and limitations of different processes. A number of applications are reviewed with particular attention given to potential future developments.

A model of black-liquid solar collector☆

Abstract The concept of a black-liquid flat-plate solar collector, in which the working fluid can absorb directly the solar radiation, has been studied. Mixed laminar convection and radiative transfer have been taken into account in setting the local balance equations for mass, momentum, and energy together with the relevant boundary conditions. The resulting system of equations has been solved numerically to obtain the temperature and velocity profiles and to compute the heat loss and the collection efficiency. On this basis, the investigation consisted of (1) evaluating the importance of radiative exchange with the sky, (2) examining the effects that some parameters, namely the spacing between the covers, the optical thickness of the working fluid, and the bottom plate reflectivity, have on the collector performance, and (3) assessing the capability of two simplified models to reproduce the results given by the full model. From the analysis of the results, some important design criteria for collectors of such a type have been determined.

Heterogeneous Photocatalysis: A Promising Advanced Oxidation Process

Abstract This chapter briefly presents the fundamentals of the processes of heterogeneous photocatalysis (PC). In particular, Langmuir–Hinshelwood and Eley–Rideal models that are often used to model the kinetics of heterogeneous photocatalytic reactions are described together with other models and equations derived. Moreover, some considerations about the dependence of the rate of PC on the light intensity, pH, and photocatalyst modification are done. Methods for photoreactor modeling are described, and the coupling of PC with other advanced oxidation processes (AOPs) using H2O2, O3, and peroxydisulfate is discussed. It is highlighted that coupling different technologies results in a real intensification of the global efficiency only for specific ratios of the rates of the single processes. The suitability of the practical application of the AOPs must be based on a precise process efficiency assessment, which, as the results show, is a challenging but necessary task.