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