RICERCA ITALIANA     

Research program

INNOVATIVE CATALYTIC PROCESSES FOR THE SELECTIVE OXIDATION AND REDUCTION OF GLYCEROL IN WATER: STUDIES OF REACTION MECHANISMS AND KINETICS FOR THE PROCESS OPTIMISATION

University Co-ordinator

Università degli Studi di BOLOGNA - INGEGNERIA CHIMICA, MINERARIA E DELLE TECNOLOGIE AMBIENTALI - ()

Research Unit Leader

Giovanni Camera Roda

Description

The main objective of the present research is to optimize the yield in photocatalytic reactors for the partial oxidation of glycerol. The approach is experimental with the aid of mathematical models for the design of the apparatus, for the study of the relevant phenomena and for the analysis of the results.
Several experimental tests will be carried on in photocatalytic reactors designed on the basis of mathematical models which take into account the various phenomena.
Then a pervaporation process will be coupled to the reactor in order to study the integrated process reaction-separation.
It must be noted that in view of the fact that the reaction rate depends also on the local values of the concentration of the reagent and of the intensity of the radiations, the distribution of these two quantities inside the reactor will be taken in particular account in the design of the reactors.
In the study of the reactor many phenomena must be taken into account and analyzed, namely:
-the radiant energy transfer, since it affects the distribution of the intensity of the radiation
-the mass transport , since it affects the distribution of the concentration of the reagent
-the reaction, which causes the local disappearance of the reagent.

In most of the experiments the photocatalytic material will be immobilized as a film on suitable solid transparent supports, such as beads, or plane surfaces. Photocatalyst powders in a slurry will be utilized only in few test, principally to characterize the photocatalytic material, since some problems arise in practical applications with the slurries, such as difficulties in the separation of the very small particles at the end of the process or the problematic manipulation of the powders. The deposition of the photocatalytic films will be made by the other research units that participate to the present research national program. A reciprocal feedback is needed between the units in order to efficiently and mutually develop the materials and the process.

In particular the experiments which will be carried on are:
-test of photocatalysis by using photocatalytic films and possibly powders in a “photodifferential” [1] reactor
-test of photocatalysis in a continuous annular reactor with particles immobilized in a fixed bed (artificial light)
-test of photocatalysis in a continuous plane reactor with photocatalytic films on the surfaces of the reactor (solar or artificial light)
-test of photocatalysis in a continuous plane reactor with particles immobilized in a fixed bed (solar or artificial light)
In the different runs the concentration of the reagent, the intensity of the radiation, and the thickness of the zone occupied by the catalyst inside the reactor will be changed in order to investigate their effect on the yield.
Furthermore the influence of the thickness of the photocatalytic films will be studied, following the procedure suggested in [2]. In fact it has been demonstrated that the thickness of the film may affect significantly the observed rate of reaction.
The main parameters will be measured during every test: intensity of the radiation entering and leaving the reactor and concentration of the various chemical compounds.
Concerning the integrated process photocatalysis-pervaporation, first of all the pervaporation properties (separation factor and permeate flux) will be measured in a pervaporation module where each single compound is fed. Different temperatures will be tested (25, 30, 40, 50 °C) to choose the more appropriate temperature for the process.
The choice of the membranes will be made on the basis of their capability in permeating preferentially the products obtained by the partial oxidation of glycerol.
The membrane will not be present inside the reactor, so that it is not required that the membrane is resistant to the radiation. Organophylic (hydrophobic) membranes will be tested. At least one will be a polymeric membrane (probably with polydimethylsyloxane) and at least one will be an inorganic membrane (probably with silicalite). It is expected that the inorganic membrane is more resistant to chemicals and this property could be important.
Then pervaporation will be coupled to photocatalysis. The outlet stream from the reactor will enter the pervaporation module and the retentate stream from the pervaporation module will be recirculated to the reactor after passing through a thermostated tank. In this way the system will operate as a batch process. The “integration” of the two process is effective only if they operate simultaneously and not sequentially. The integration can be obtained even if the reaction and the pervaporation work in different apparatuses, provided that the flow rate is such to have a residence time much shorter than the characteristic time of a significant change of the concentrations in the system. The details can be found in [3].
The study of the integrated process will be completed by examining the effects of the variation of the ratio between the characteristic time of reaction and of pervaporation [4]. This ratio can be changed by using different numbers of pervaporation modules (the characteristic rate of pervaporation changes) or by modifying the intensity of the entering radiation (the characteristic rate of reaction changes). In fact it is sufficient that the pervaporation is capable to remove to a certain extent and good selectivity the products of the reaction. An higher capability is not necessary and could even be deleterious since it is more costly and it could contribute to remove also some amount of the reagent since the membrane is not perfectly impermeable to the reagent. Of course a perfect balance of the two coupled processes can be accomplished only by an experimental investigation of the integrated process, since the quantitative predictions on the basis only of the nominal properties of the membrane and of the kinetics of the reaction would be very inaccurate.
After having completed the experimental investigation the cost of the process will be estimated.

The expected results of the present research program can be summarized in the following points:
-information on the “intrinsic” kinetics of the reaction
-optimization of the geometry of the reactor
-effect of the concentration of the photocatalyst
-evaluation of the optimal thickness of the photocatalytic film
-identification of the more suitable membranes
-optimization of the operative conditions (temperature, flow rate, number of modules per unit volume of the reactor)
-knowledge about the intervening mechanisms
-development of mathematical models for the design of the apparatus, the prediction of the performances and the scale-up of the process.


References
[1] G. Camera-Roda, F.Santarelli and C.A.Martin, Design of photocatalytic reactors made easy by considering the photons as immaterial reactants, Solar Energy, 79, 343-352 (2005).
[2] G. Camera-Roda and F .Santarelli, Optimization of the thickness of a photocatalytic film on the basis of the effectiveness factor, in press on Catal. Today (2007), doi: 10.1016/j.cattod.2007.06.062
[3] G. Camera-Roda and F.Santarelli, A rational approach to the design of photocatalytic reactors, in press on Ind. Eng. Chem. Res. (2007), doi:10.1021/ie070302a
[4] G. Camera-Roda and F.Santarelli, Intensification of water detoxification by integrating photocatalysis and pervaporation, J. Solar Energy Engineering (2007), 129, 68-73.