Pervaporation-photocatalysis

Pervaporation - photocatalysis In the described systems the membrane usually permeates water and rejects the reactants, enhancing their residence time in the photoreactor. However, it is known that some intermediate products of the photo-catalytic degradation of organic compounds can negatively affect the reaction rate, therefore, in some cases it is useful to eliminate these by-products in order to improve the thermodynamic and/or the kinetics of the reaction. 

Due to the fact that the photocatalyst is typically a non-toxic substance (titanium dioxide in most of the applications) and that no chemical additive is needed, alongside the low energy demand which can be satisfied by the solar radiation, it is evident that an integrated pervaporation-photocatalysis process can absolutely satisfy the requirements of sustainable chemistry. 

To this purpose, in a study on the photocatalytic degradation of 4-chlorophenol, Camera-Roda and Santarelli [89] proposed an integrated system in which photocatalysis is coupled with pervaporation as process intensification for water detoxification. Pervaporation represents a useful separation process in the case of the removal of VOCs and in this study it is used to remove continuously and at higher rate the organic intermediates that are formed in the first steps of the photocatalytic degradation of the weakly permeable 4-CP. 

The feasibility of an integrated photocatalysis-pervaporation process can be preliminarily evaluated by taking into account some characteristics of photocatalysis. In the typical appHcations of photocatalysis, the reagents, which are termed substrates in the common terminology of photocatalysis (Braslavsky et al., 2011), should be present in low concentrations in order to ensure the economy of the process. Therefore the system is not as chemically aggressive for the membrane, particularly if the pervaporation modules are kept in separate equipments. In this way, the membrane is not exposed to the radiation or to the photoactivated radicals, which are present only close to the surface of the photocatalyst, as long as it is illuminated, since the lifetime... 

With photocatalysis the main problems encountered in the previously examined applications of PVRs are avoided. It is also worth noting that (i) the coupling of photocatalysis with the pervaporation process allows for a complementary exploitation of the solar spectrum. In fact, photocatalysis is normally only able to use its ultraviolet component, while the remaining thermal part of the spectrum can be utilized to heat the fluid and to evaporate the permeate (ii) photocatalysis and pervaporation have common operative conditions liquid solutions (often aqueous solutions), low concentrations of the reactants and consequently of the products, low temperature and atmospheric pressure and (iii) once the type of light source is chosen, photocatalysis is a modular process like pervaporation. Therefore the integration of the two processes is straightforward and advantageous. 

Photocatalytic membrane reactors have been analysed over the years (see e.g., Damodar and You, 2010 Grzechulska-Damszel et al, 2009 Molinari et al, 2002,2008,2009,2010 Mozia et al, 2008,2010), but only more recently has photocatalysis been coupled with pervaporation in an integrated process. 

The integration of photocatalysis and pervaporation shows positive effects on both the processes thus establishing a real synergy and a process intensification. 

Indeed, it is observed that the selectivity of the membrane towards some intermediates is high. So pervaporation removal takes advantage of the presence of photocatalysis which transforms the poorly permeable 4-CP into more permeable compounds. At the same time the rate of photocatalysis degradation of 4-CP is enhanced by PV which removes intermediate compounds that would hinder this reaction. 

Various experiments, carried out with different photocatalytic reactors and commercial pervaporation membranes, by varying the relative weight of pervaporation with respect to photocatalysis, have demonstrated that the intensification factor depends solely on the parameter 5 previously defined, as is apparent in Fig. 3.7. 

From the definition of it is obvious that, both when 5- 0 (only photocatalysis) and when (only pervaporation), the intensification factor... 

Camera-Roda G and Santarelh F (2007), Intensification of water detoxification by integrating photocatalysis and pervaporation , / Sol Energy Eng-T ASME, 129, 68-73. 

The pervaporation operation integrated with heterogeneous photocatalysis has been demonstrated as a very promising method to improve the detoxification efficiency of water streams containing organic pollutants at low concentration (Camera-Roda and Santarelli,2007). 

The most common configurations of PMRs with suspended photocatalyst are those utilizing pressure driven membrane processes, such as microfiltration (MF), ultraflltration (UF) and NF. In other types of PMRs, photocatalysis is combined with dialysis, pervaporation (PV) or direct contact membrane distillation (DCMD, MD) (Mozia, 2010). 

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