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.