4.5.3 Validation of the model in the combination of micro
photoreactor and adsorption column
The results in Figure 7 demonstrate that the conversion ofcis -cyclooctene as a function of the process time obtained from
solving the model discussed in section 3.3. This model has been solved
and sketched for different total flow rates and compared with each
experimental case. Here, time is the total operational time.
In case (a) after 400 min almost 70% conversion is achieved. This time
is the total required time of operation since this is a closed-loop
system operating in a recycling mode. In the case (b), as stated earlier
the total flow rate is doubled to 0.4 ml/min, while the residence time
in each photoreactor is the same as in case (a). Figure 7b shows that
there is a small deviation between the results of the model calculations
and the experimental results. Besides, upon increasing the total
operational time, the deviation increases and at some point the
experimental results tend to reach a plateau earlier than with the
predicted model. Since the total flow rate is doubled, the flow rate
inside the packed bed is also doubled. Therefore, the packed bed reaches
saturation earlier in case (b) than in case (a). With prolonging time,
the packed bed becomes saturated with TCO and TCO molecules are not
adsorbed anymore and recycled in the system which also affects the
photoreaction process to reach equilibrium faster. As a result, the
total conversion of cis -cyclooctene is not improved anymore and
therefore the experimental points tend to reach a plateau and deviate
from the model.
In case (c), there is less deviation between the experimental points and
the results of the model calculations (Figure 7c). The first switching
towards a fresh packed bed was at 75 min operational time and the second
switch was at 180 min. As indicated in Figure 7c, after 50 min the
packed bed tends to reach saturation. Therefore there is a plateau
before the fresh packed bed was in use (75 min) which explains the small
deviation between the experimental results and the results of model
calculations. Also, it is important to note that this is a closed system
and the concentration of the cis -isomer is getting less with
respect to the operational time. Therefore, the first packed bed is
saturated in less time than the ones used later, so as time increases
the packed bed saturation time also increase due to the smaller
concentration of cis -isomer in the flowing fluid. In case (c)
90% conversion is achieved after 250 min.
According to the results, case (c) shows a massive improvement compared
to the first two cases. Primarily, using the packed bed with smaller
volume gives the chance to quicker recycle the cis -isomer. This
results in an improvement of the total conversion at any certain time.
Also, changing the packed bed as soon as it reaches its saturation
point, improves the process performance in terms of an increasing rate
of production of the trans -isomer. Table 3 summarizes the
performance of the different design cases. To give better comparison, we
calculate the total conversion divided by the total time of operation
for each case which can be correlated to the overall reaction rate of
the closed-loop system. As it is expected this number is much higher in
case (c), 0.36 (min-1) compared to the case (a) and
(b), which are 0.175 and 0.2 (min-1), respectively.
Table 3 Summary of different configuration’s performance