Continuous countercurrent cloud point extraction of cinnamic acid

The transfer from batch to continuous mode enhances the productivity, as well as the extract purity in the liquid-liquid extraction process. For instance, a higher efficiency is obtainable in a countercurrent stirred column due to the higher number of theoretical stages (see chapter 2.3). Continuous countercurrent extraction of cinnamic acid was conducted as described in chapter 5.3 to demonstrate the potential of the food-grade and cosmetic-grade surfactant systems.

The parameters were set as summarized in Table 5.1. The choice of the process parameters was based on the LLE data and the batch experiments. Hence, the surfactant-rich phase (solvent) during the Silwet L-7230 experiments was pumped in at the top of the column as it was the heavy phase. During the ROKAnol NL5 experiments, the direction of the solvent was the opposite. The temperature was fixed at 39°C for Silwet L-7230 and 45°C for ROKAnol NL5. The ratio of the feed

(cinnamic acid solution) to the solvent (surfactant solution) was set in such a way, that a nominal surfactant concentration of approx. 3 wt% was ensured in the mixing zone. A lower stirring speed (15 rpm) was applied with the ROKAnol NL5 system due to the lower density difference.

The stability of the continuous extraction with both surfactants was compared based on the relative CA concentration (Equation 5-5) over the time. Hence, it was possible to observe normalized values, which were not influenced by the absolute concentration in the feed and raffinate. The corresponding plots are presented in

Figure 6.14.

Figure 6.14: Time profiles of the relative cinnamic acid concentration (crel.) in the raffinate during the continuous extraction with Silwet L-7230 (39 °C) and ROKAnol NL5 (45 °C). Error bars indicate the standard deviation within each experiment. N=2.

As shown in Figure 6.14, the tracer’s relative concentration in the raffinate initially decreased over time. That was due to the fact that the column was filled with feed at the beginning of all experiments. Subsequently, the tracer’s relative raffinate concentration tended against a constant value (reaching the steady state). The steady state was reached after three hours at the experiments with Silwet L-7230.

On the other hand, it took six hours to reach the steady state when performing the cloud point extraction with ROKAnol NL5. In comparison, the stationary state with the Triton X-114 system was reached after four hours at the operating temperature of 40 °C [43]. However, the lower density difference, as well as the opposite flow direction in case of ROKAnol NL5 could have resulted in the slower process stabilization. Fellechner et al. confirmed that observation, while observing a stationary state after 8 hours in the ROKAnol NL5 system [176]

0.0

Furthermore, the tracer’s relative raffinate concentration in steady state was higher in the Silwet L-7230 raffinate than in the one of ROKAnol NL5. This was contrary to the expectation since the food-grade surfactant was denoted with a higher accumulation capacity for cinnamic acid. The yield, enrichment factor, and the number of theoretical stages were calculated as described in chapter 2.3 to evaluate the performace of the CPE. The corresponding results of the continuous cloud point extraction with Silwet L-7230 and ROKAnol NL5 is shown in Table 6.2.

Table 6.2: Performance of the cloud point extraction of cinnamic acid with food-grade and cosmetic-grade surfactant systems (Ycont.): extraction efficiency, TCA: enrichment factor, Ntheo: number of theoretical stages)

surfactant ¤¥¦§¨. [%] ©ª«¦¬-®¯ °¨±²¦

Silwet L-7230 96.6 ± 0.1 2.35 ± 0.01 1.51 ± 0.01 ROKAnol NL5 103 ± 7 2.75 ± 0.06 5.6 ± 0.4

As shown in Table 6.2, the extraction efficiency was high enough to reach a yield of approx. 100% with both surfactants. Hence, the yield could be improved in comparison to the batch experiments. The enrichment of cinnamic acid in the micellar extract also attested for a pronounced accumulation of the solute in the extract. However, the number of theoretical stages reached with Silwet L-7230 was much lower than the value for ROKAnol NL5. Almost identical values describing the extraction performance in the system containing ROKAnol NL5 at 45 °C were reported by Fellechner et al [176]. On the one hand, this result illustrated the mass transfer limitations in the Silwet L-7230 system due to the high viscosity of the micellar phase. On the other hand, one observed 5.6 theoretical stages during the extraction with ROKAnol NL5. That illustrates the improvement of the separation in the ROKAnol NL5 cloud point extraction by the continuous process.

These results are comparable to the efficiency of the Triton X-114 continuous cloud point extraction of salicylic acid reported by Ingram et al. [43]. Moreover, the reached yield was higher than the result, achieved with Triton X-114 (see chapter 6.1.3). That is related to the lower total capacity, maintained during the experiments with Silwet L-7230 and ROKAnol NL-5.

Overall, the applicability of Silwet L-7230 and ROKAnol NL5 for a continuous extraction of aromatic compounds was confirmed at temperatures suitable for an

extraction of sensitive substances from biological materials. Further, the potential of the food-grade and the cosmetic-grade system was demonstrated in a batch and continuous extraction.

Ultimately, the results in chapters 6.1 and 6.2 prove the concept for a stable cloud point extraction in continuous mode, which is operated at mild conditions. Hence, the systems based on Triton X-114, Silwet L-7230 and ROKAnol NL5 are suitable for the direct recovery of sensitive solutes. Moreover, in case of good biocompatibility, the cloud point systems can be attractive as two-phase media for the direct ISPR from cell cultures as well.

However, such processes are beneficial, when the subsequent treatment of the extract stream is also not damaging the biomaterials. Therefore, several approaches for extract processing are discussed in the next chapter.