Process intensification in a photocatalytic membrane reactor: Analysis of the techniques to integrate reaction and separation

Different methods to integrate reaction and separation in a membrane reactor are studied in the present work, with the aim being to highlight the pros and cons of the different alternatives and the effects of the intervening parameters. The coupling of the two processes can take place inside a single apparatus or using separate units. If a single apparatus is utilized, the coupling is more direct, but separate units offer higher degrees of freedom for the design with more opportunities to optimize the system without constraints. However, when using separate units, the integration of the two unit operations depends largely on the parameters intervening in the coupling procedure. These parameters are the recycle ratio, R, if a recycle stream is used to backmix part of the exiting stream, or the number, N, of blocks if the volume of the reaction and the membrane area are fractionated into multiple reaction-separation blocks. The possibility of different combinations of these two basic methods is also presented. The case study to illustrate the effect of the various parameters (R, N, the Damköhler number and the Péclet number) and to find the proper operating conditions is the photocatalytic green synthesis of an aromatic aldehyde, which is recovered from the reaction solution by a pervaporation process to avoid further oxidation. The results show that process intensification is the result of appropriately choosing the reaction system and operating conditions because these factors may substantially increase the yield.