cCA: concentration of the cinnamic acid in the micellar (M) phase or the feed(0); V: volume of the micellar phase (M) or the initial sample (0)
B‚ CI„„I= <‡=ˆ‰∙ ‰‹‹
<‡=<Œ•V− C7C
Whereby, the aggregation number was assumed to be equal to 220 [161].
Additionally, C7C. was calculated by applying the cmc value for Triton –X 114.
<‡=<Œ•V and <‡=ˆ‰ were calculated using the concentrations of the surfactant and the tracer, and the volume of the micellar and aqueous phases. Since micelle formation starts only above the cmc, all surfactant molecules needed to maintain the cmc were excluded from the estimation [162].
Continuous experiments
Three surfactants, namely Triton X-114, Silwet L-7230 and ROKAnol NL5, were applied for the continuous cloud point extraction of cinnamic acid in technical scale.
To that purpose, the set-up described in chapter 4.6 was applied. For all continuous experiments, CA solution in distilled water (approx. 200 mg·l-1) was set as feed.
The continuous extraction was carried out in countercurrent mode. The temperature at the column and the tanks was the same as the one during the batch experiments (Triton X-114 - 40 °C, Silwet L-7230 - 39°C, ROKAnol NL5 - 45
°C).
Firstly, Triton X-114 was utilized for the extraction of cinnamic acid in the stirred column. Each extraction experiment lasted 8 hours to ensure that the steady state was reached (around four hours) [43]. The solvent was introduced at the top of the column (heavy phase) via the peristaltic pump. An aqueous solution containing 20 wt% Triton X-114 was used as extracting agent. The concentration of the surfactant in the solvent flow was chosen in accordance with the tie-line at 40 °C (see LLE Triton X-114/water in appendix A 9, the mass fraction of Triton X-114 in the micellar phase= 0.20). The feed flow was introduced at the bottom of the column (light phase) by a gear pump. The calibration curves for both pumps are depicted in appendix A 10. The following parameters were varied according to the experimental design (see chapter 5.4): speed (n), feed-to-solvent ratio (ν) and column capacity (b).
Equation 5-4: Calculation of the micelle loading (YMicelle) in batch experiments
nsysCA: amount of CA molecules in the system; nsysCA: amount of surfactant molecules in the system; nAgg: aggregation number = 220; nAgg: aggregation number
Subsequently, continuous cloud point extraction of cinnamic acid was conducted using Silwet L-7230 and ROKAnol NL5. The solvent was a surfactant solution, containing 20 wt% Silwet L-7230 or 15 wt% ROKAnol NL5. Those values corresponded to the surfactant mass fraction in the micellar phase at the chosen temperature. Further, in case of Silwet L-7230, the solvent represented the dense phase and was therefore pumped in at the top of the column, whereas the feed as a lighter phase entered the column at its bottom. In case of ROKAnol NL5, the procedure was vice versa. The duration of each continuous experiment was 5 or 7 hours for Silwet L-7230 and ROKAnol NL5, respectively. The extraction parameters temperature, agitation speed, feed flow, and solvent flow for the latter two systems are presented in Table 5.1.
Table 5.1: Parameters for the continuous cloud point extraction with Silwet L-7230 and ROKAnol NL-5
In addition, a recirculation of the collected extract was investigated for Triton X-114, Silwet L-7230 and ROKAnol NL5 at 40°C, 39°C, and 45 °C, respectively.
Hence, several experiments with the surfactants were conducted, whereby after 150 minutes the solvent stream was switched from fresh solvent solution to the collected extract. The conditions for the continuous extraction were set as shown for Silwet L-7230 and ROKAnol NL5 in Table 5.1. The parameter combination for Silwet L-7230 was applied with Triton X-114. The solvent composition was set at 20 wt% surfactant amount in water, for the Silwet L-7230 and Triton X-114 experiments. The solvent for the ROKAnol NL-5 extraction contained 15 wt%
surfactant in water.
During all experiments, samples were collected from the feed, raffinate and extract phases. Hence, profiles of the CA concentration in the raffinate and extract were used to evaluate the performance. Moreover, to compare the time profiles of the CA amount in the raffinate phase during different sets of experiments, the relative raffinate concentration (Crel) was defined as shown in Equation 5-5:
•I„. = ˆ‰A
ˆ‰D
The experiments with Triton X-114 were evaluated using the cinnamic acid yield (BC* >.) and productivity (5ˆ‰) according to Equation 2-14 and Equation 2-15, respectively.
The performance of the continuous extraction with Silwet L-7230 and with ROKAnol NL5 was assessed by the enrichment factor ( ˆ‰) and the cinnamic acid yield (BC* >.). Those were calculated according Equation 2-13 and Equation 2-14, respectively. The number of theoretical stages (G>HI*) was also calculated according Equation 2-16 with Equation 2-17.
5.4 DESIGN OF EXPERIMENT:PARAMETER OPTIMIZATION PROCEDURE
The software “Stat-Ease Design-Expert Version 8” was used for the experimental design of the optimization for the continuous CPE of the tracer CA with the model surfactant Triton X-114. A response surface method was applied to reflect the chosen responses depending on the factor combinations. The responses were yield and productivity of cinnamic acid (Equation 2-14 and Equation 2-15). The studied factor combinations consisted of column capacity, feed-to-solvent ratio and agitator speed.
Moreover, the continuous cloud point extraction of cinnamic acid was restricted by the surfactant concentration in the raffinate. A surfactant fraction higher than 0.2 wt% in the raffinate led to the accumulation of Triton X-114 in the raffinate. That resulted in a high loss of surfactant or accumulation of the amphiphile in the surfactant-lean stream. Thus, an additional separation of the surfactant from the raffinate could be needed. A maximal concentration limit of 0.2 wt% Triton X-114 in the raffinate was restricted to minimize the surfactant loss. The reaching of a surfactant concentration in the raffinate equal to 0.2 wt% was defined as “stress limit.”
The influence of the agitation speed from 20 to 80 rpm on the Triton X-114 concentration in the raffinate was tested at capacities in the range of 1 – 2 l·h-1 at