Recirculation of the Micellar Phase *

* The results in this chapter were part of the Master thesis by Dennis Wenzel [173].

The recirculation of gathered extract was investigated to increase the concentration of cinnamic acid in the extract and to save surfactant. The surfactants Triton X-114, Silwet L-7230 and ROKAnol NL5, were used for the experiments as described in chapter 5.3. The recirculation was switched on after 150 min, as marked by the dotted line. However, the chosen process conditions for the continuous extraction with Triton X-114 did not fully correspond to the parameter combination, which was obtained from the response surface methodology (see chapter 6.1.2 - 6.1.4). Namely, the feed-to-solvent ratio was decreased from 6 to 5, the capacity – from 1 to 0.8, and the agitation speed – from 43 to 25. Hence, no maximum of the productivity was expected.

Nevertheless, the new parameters allowed a comparison to Silwet L-7230 as well.

The conditions for the CPE with recirculation of the solvent using Silwet L-7230 and ROKAnol NL5 were analogous to the experiment without recycling of the micellar phase.

In Figure 6.15, the continuous extraction of cinnamic acid with extract recirculation using Triton X-114 at 40°C is compared to the corresponding Silwet L-7230 and ROKAnol NL-5 process.

Figure 6.15: Time profiles of the relative raffinate concentration of cinnamic acid during the continuous extraction with Silwet L-7230 at 39 °C ROKAnol NL-5 at 45 °C and Triton X-114 at 40 °C, with recirculation of the extract. Error bars indicate the standard deviation within each experiment;

N=2. The dotted line indicates the time of the recirculation start (in black for Silwet L-7230 and Trion X-114 and in red for ROKAnol NL-5). Dashed lines are added to guide the eyes.

As can be obtained from Figure 6.15, the tracer’s relative raffinate concentration first decreased with time. The time profile of the cinnamic acid concentration in the raffinate was steeper for Triton X-114 and Silwet L-7230 than the one corresponding to ROKAnol NL-5. That was in accordance with the results shown in chapters 6.1.2 and 6.2.6 and could be explained with the lower density difference, the opposite flow direction, and the resulting slower separation in the settling zone in case of ROKAnol NL5. However, the cinnamic acid amount in the raffinate started increasing, as soon as the extract recirculation was initiated. Cause for that effect was the lower concentration gradient between feed and recirculated solvent than between feed and fresh solvent since the recirculated solvent was already loaded with cinnamic acid. Consequently, the time profile of the relative raffinate concentration showed an increasing trend, as the recirculated extract could take up less cinnamic acid with each recirculation run. That effect was more pronounced in case of Silwet L-7230 and Triton X-114. Again, due to the moderate reduction of the concentration gradient, the system based on ROKAnol NL-5 was reacting

cinnamic concentration in the ROKAnol NL-5 micellar phase could be registered within that limited experiment duration.

Based on these results, it can be assumed that mass transfer of solute into the extract would get continously worse for longer process times. Therefore, the tracer’s relative raffinate concentration would eventually increase up to the initial value of 100%, when no further cinnamic acid could be extracted. That observation is in good agreement with the theory concerning extraction processes in chapter 2.3.

Further, in Figure 6.15 can be observed, that the tracer’s relative raffinate concentration decreased less pronounced for Silwet L-7230 than for Triton X-114 and ROKAnol NL-5, before extract recirculation was switched on. That was expected based on the better performance of the Triton X-114 and ROKAnol NL-5 in comparison with Silwet L-7230. However, the tracer’s c profiles showed an equal incline after the beginning of extract recirculation for both Silwet L-7230 and Triton X-114. That may be due to the higher affinity of the Silwet L-7230 micellar phase to load cinnamic acid. Hence, when the mass-transfer is limited due to the lower concentration gradient, then the higher capacity of the Silwet L-7230 system is compensating for that limitation.

As shown in chapter 6.2.4, Silwet L-7230 extract turned to a liquid crystalline phase at room temperature. This phenomenon played a crucial role in the experiments with recirculation. Hence, after some time, recirculated extract was stuck in the solvent feed tube or re-entered the column in a liquid crystalline state.

By this means, it was immiscible at the applied agitation speeds. Concerning that critical aspect, Silwet L-7230 was described as not suitable for the extraction with recirculation.

In addition, the constant change in the solute concentration in the extract led to elevated surfactant concentration as well. The plots in Figure 6.16. illustrate the concentration of the tracer and the surfactant over the time during the continuous extraction of cinnamic acid with Triton X-114.

Figure 6.16: Cinnamic acid and surfactant concentration of recirculated extract phase as a function of time for the continuous extraction with Triton X-114 at 40 °C. Lines are added to guide the eyes.

As can be obtained from Figure 6.16, the concentration of cinnamic acid and Triton X-114 within the recirculated extract increased over time. That can be explained as follows:

Every run of recirculated extract phase was higher concentrated with cinnamic acid than the previous one. Therefore, also the extract surfactant concentration continuously increased due to the cloud point reduction caused by cinnamic acid at such high concentration levels. Consequently, the volume of extract phase at the bottom of the column, which could be recirculated, steadily decreased until the column bottom ran empty. For solving this problem in further experiments, it was necessary to either adjust the process temperature or to lower the recirculated extract’s surfactant concentration by dilution. The dilution by mixing the extract with fresh solvent in order to minimize the latter effect was applied during the experiments with authentic feed solutions.

Overall, the recirculation of the extract phase until reaching its saturation with solute is beneficial due to the lower amount of surfactant for the extraction.

Additionally, a higher loading with the target compound is also achievable.

However, when further applying the approach, the operation of the process has to be adapted correspondingly.