4.5.2 Process configurations
Case (a) . This case concerns the basic continuous flow setup in which a photo-microreactor of 5 m length and an adsorption column of 5 cm length packed with AgNO3/SiO2 powders are used (Figure 6a). The flow rate is set to 0.2 ml/min. As reported earlier in section 2.4, in all cases the outlet flow from the packed bed is recycled to the vessel with the reactor feed solution.
Case (b). Since, our system is a closed-loop recycle flow system, it is important that as soon as photoreaction takes place the molecules are transferred to the separation unit and from there quickly recycled to the feed vessel .Therefore, the key in this design (case (b)) is to reduce the residence time of the flowing fluid in the connecting tubes and parts between the feed vessel, the photo-microreactor(s) and the packed bed.
In this case, we applied the concept of symmetrical flow splitting by using a T-micromixer placed in reverse mode to act as flow splitter in a bifurcate fashion the feed flow rate is directed towards two photo-microreactors of 5 m length which are wound around one UV light and placed in parallel (Figure 6b). With this design, it is possible to double the main flowrate while keeping the residence time in the photo-microreactor the same as in case (a).
Therefore, the total flow rate is raised to 0.4 ml/min which speeds up the flow in connecting parts, and by symmetrical flow splitting, the flow rate is kept 0.2 ml/min in each photo-microreactor.
Case (c). This case is similar to the case (b). Again, two parallel photo-microreactors are used. But, the main difference is that in this configuration more than one packed bed, each with a length of 2.5 cm, are installed in parallel, see Figure 6c. This adds the function of refreshing the adsorption column under use, by switching between two columns set in parallel – one for the closed-loop flow and the other in a wait position, being re-activated. Such an integrated flow configuration for purification following micro-flow reaction was already proposed by us in a recent paper, using a fixed-bed column packed with Amberlyst resin for an ion-exchange based purification of the product mixture [26].
In the operational mode, the parallel packed beds are connected to the photoreactor by a switching valve. While one packed bed is in use to adsorb the trans -isomer, the other is in the waiting mode. As soon as the first column reaches saturation, the flow is switched to pass through a fresh packed bed. Based on the amount of the inletcis -cyclooctene, in this case, three parallel packed beds are used. Also, the flow rate is increased to 0.66 ml/min.